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This paper is an introduction to selected topics in geology and hard rock mining. It is an overview, and is intended to help the legal professional to learn basic information concerning these topics. References are provided to allow the lawyer or landman to delve more deeply into the subjects covered.
The broad goal of the Rocky Mountain Mineral Law Foundation is to educate professionals in natural resources law. The Foundation serves practitioners, academics, and others who work with natural resources law. The objective of the Foundation's Science and Technology Series is to assist lawyers, landmen, legislators, and teachers to understand the basic technology, economics and science underlying the practice of natural resources law. This paper is the first in that Series.
PRINCIPAL REFERENCES CONCERNING THE SCIENCE AND TECHNOLOGY OF GEOLOGY AND HARD ROCK MINING
The practitioner seeking additional information about the topics discussed in this paper will find many technical and scientific references available. The following principal references are listed in order of importance.
1) SME Mining Engineering Handbook, 1992, H.L.Hartman, ed., Society for Mining, Metallurgy and Exploration, Inc., Littleton, CO, 2260 p.
2) Nonfuel Mineral Resources and Public Lands, 1969, Prepared for The United States Public Land Law Review Commission by: The University of Arizona, G.F.Leaming, W.C.Lacy Investigators, 5 Volumes.
3) Anatomy of a Mine, From Prospect to Production, 1977, U.S.D.A. Forest Service, General Technical Report INT-35, Prepared by H.Banta, 69 p.
4) Minerals - Foundations of Society, Ann Dorr, 1984. League
of Women Voters of Montgomery County, M.D.Inc. 56 p.
Geology is the study of the earth, its surface configurations, and the physical and chemical processes acting upon its surface and its interior. It is the study of the earth's composition and physical and chemical processes which act upon it. Geology has developed into several areas of special interest (Figure l).
1. PHYSICAL GEOLOGY deals with the physical behavior of the earth, how it was formed, and the processes which have, or have had, effects upon it.
2. GEOPHYSICS is a more specialized study of the physical properties of the earth (e.g. its vibrations, density, magnetism), the basic physical forces which affect it (e.g. gravity), and the effects of these forces.
3. GEOCHEMISTRY is the specialized study of the composition of the earth, its components, and the chemical processes taking place at the surface (e.g. reactions of the lithosphere with the atmosphere and hydrosphere), and reactions at depth under the influence of heat, pressure and deformation forces, radioactive decay and isotopic relations.
4. HISTORICAL GEOLOGY deals with the history of the earth, its changing environment and the development of life forms.
5. GEOCHRONOLOGY involves the study of the ages of rocks, minerals, plants, fossils and events as measured by ratios of radioactive decay elements found in existing rocks, analyses of tree rings and sediment layers.
6. MINERALOGY AND PETROLOGY are concerned with the chemical composition, physical characteristics, nature of formation, occurrence, and atomic structure of minerals and rocks. Minerals are the component materials of all rocks and soils. Petrology involves investigation of the mineral and chemical composition of rocks, and the various chemical and physical processes which led to their formation.
a. MINERAL is any naturally occurring element or inorganic substance having a definite (or variable within fixed limits) chemical composition and crystalline structure. Deposits of coal, petroleum and naturally occurring brines do not fit such a rigorous definition, but have in generic terms also been considered minerals, principally because they are not readily identifiable as either animal or vegetable. (This is an area of controversy.) Supplies of groundwater have been described by some to be mineral in character, because water is a naturally occurring substance found in the earth's crust that is not organic in origin. (Figure 4.)
b. ROCKS are solid, cohesive aggregates of one or more types of minerals, which have formed as a result of various geological processes. Rocks are classified not only according to their mineral content, but also in accordance with their mode of formation (igneous, sedimentary, metamorphic), chemical composition, grain-size, and physical appearance.
c. UNCONSOLIDATED SEDIMENTS consist of loose rock fragments of all sizes which have been transported by water, air, ice, gravity and accumulated on floodplains, and in valleys, lakes and oceans.
d. SOILS, from a geological point of view, constitute a surficial mantle over rock in which physical and chemical processes of weathering cooperate in close association with biological and agricultural processes.
7. SEDIMENTOLOGY is that branch of geology concerned with the transport and deposition of sediments and sedimentary rocks.
8. HYDROLOGY is concerned with all aspects of the earth's hydrosphere - water in the atmosphere, surface and subsurface water, and the effects of the hydrosphere on climatic changes and the water balance.
9. STRUCTURAL GEOLOGY is that branch of geology concerned with the attitudes and positions of rock formations relative to each other, the sequence of events that caused these formations to arrive at their existing configurations, and the forces responsible for these events. The geological map is the principal tool of the structural geologist. It shows existing rock relationships and permits interpretation of past relations and forces.
10. ECONOMIC GEOLOGY involves the theories of formation, the physical and chemical characteristics, the environments favorable for formation, and different methods and techniques for discovery of potentially economic mineral deposits. It involves a familiarization with mining and metallurgical technology, and the economics of the mineral industries.
Figure 2 is an overview of geological/geochemical cycle.
Pressure and temperature within the earth increase gradually with distance below the surface. At depths of several tens of miles the material which makes up the earth's crust (depending upon its composition) may become partially molten. This molten material is MAGMA, and is normally less dense than the overlying solid rock. As the magma is subjected to tectonic forces, it may be squeezed upward along weak zones in the crust. This process by which magma penetrates the crustal rock is called INTRUSION. The intrusive magma which has cooled and solidified is known as IGNEOUS ROCK (granite, granodiorite, diorite, gabbro).
Some magma reaches the surface of the earth and pours out, explosively or slowly, from volcanoes or fissures in the earth's crust in a process known as EXTRUSION. A magma which pours out over the surface is called LAVA, as long as it is molten, and VOLCANIC ROCK when solidified. When the magma erupts rapidly and explosively into the air, it is fragmented into small particles which solidify into VOLCANIC ASH. Larger particles called CINDERS, VOLCANIC BOMBS, and angular rock fragments comprising BRECCIA and AGGLOMERATE. When the ash settles to the ground and solidifies it may in time be compacted into TUFF, or if welded by contained heat in GLOWING AVALANCHES, it is called IGNIMBRITE.
Heat source for the formation of magmas may be residual heat from the earth's formation, heat generated by tectonic movements, or 'hot spots' from local tectonic heating and/or concentrations of radioactive materials.
Decomposition and disintegration of rocks at or near the surface by physical and chemical processes is called WEATHERING. The products of weathering are normally carried off by EROSION. Water percolating into the ground dissolves some minerals from the rocks and forms acidic or basic solutions which further attack the rock and break it down chemically.
The action of groundwater is selective under certain conditions, and may leave some elements or minerals in place while altering or leaching others. The weathering and subsequent erosion of aluminum or nickel bearing rocks may leave residual or secondary deposits (LATERITES) of materials rich in these metals.
Downward slope movement of soils and rock fragments impelled by gravity, MASS WASTING, is an important part of erosion. Decomposed rock materials are transported by wind (as sand and dust), by water (in streams, rivers, and ocean currents) and by ice (in glaciers).
The process of sedimentation entails the physical and/or chemical movement of the products of weathering to sites of deposition -- through chemical precipitation, evaporation or physical settling. The accumulated weight of sediments, over time, results in compaction and chemical action (DIAGENESIS) that may cement particles together so that sediments are lithified and converted into SEDIMENTARY ROCKS (shale, sandstone, limestone, etc.). Sediments are usually deposited in nearly horizontal layers. Changes in character and rates of deposition may cause development of distinct planes called BEDDING PLANES, which separate the sedimentary layers.
Heat and fluids emanating from magma may alter the adjacent rock creating a halo of CONTACT METAMORPHIC ROCKS. Rocks which become deeply buried are subjected to heat and pressure, which with the aid of contained or introduced fluids, and tectonic forces are metamorphosed. Original bedding may be destroyed and a FOLIATED structure developed.
The earth is a dynamic body, undergoing constant movement of both continental and oceanic crust, driven by convection currents and readjustments in the earth's mantle. Different portions of the earth have, at different times, been uplifted above or depressed beneath the seas. Other areas have crumpled rock layers into FOLDS, FAULTS, OVERTHRUSTS, while other areas have been pulled apart forming RIFTS, HORSTS AND GRABENS. The contact between drifting continental masses and spreading oceanic crust is particularly subject to deformation and intrusive/volcanic activity resulting from the SUBDUCTION of the oceanic crust under the continental crust at CONVERGENT MARGINS. Where DIVERGENT MARGINS occur, RIFTS form in the MIDOCEANIC RIDGE (Figure 2).
Groundwater solutions (METEORIC WATER), and/or solutions emanating from a cooling magma (HYDROTHERMAL FLUIDS), and/or fluids ejected from compressing sediments (POREWATER or CONNATE WATER) penetrate along fractures and tiny pore spaces between mineral grains in the rock. Under certain conditions these solutions, which contain different compounds, may dissolve, deposit other minerals, or alter the rock-forming minerals (ROCK ALTERATION).
Leaching, or the decomposition, dissolution and removal of soluble minerals from the rocks may be caused by hydrothermal or meteoric solutions. The dissolved mineral products may be dispersed or redeposited elsewhere as the same or different minerals, leaving often greater concentrations (ENRICHED). For example, relative content of gold may be increased by the removal of originally associated minerals, leaving an enriched residual gold deposit. And, portions of a copper sulfide deposit in the weathered zone may become enriched through the leaching of the copper content from the surface rocks by downward percolating meteoric waters and redeposition at the watertable.
Any intrusive igneous body is a PLUTON. Intrusives occupying a small area, having an irregular to cylindrical shape, and cutting across the intruded rock is called a STOCK. If such an intrusion occupies an extremely large area, it is referred to as a BATHOLITH. Some cross-cutting plutons gradually lose their penetrating force during the latter stages of intrusion, and merely push the overlying rocks upward rather than cutting through them, thus form DOMES or LACCOLITHS. DIKES are tabular intrusions which cut across enclosing rock, while intrusions that penetrate parallel to bedding or foliation are called SILLS. (Igneous structures are shown in Figure 3a.)
Extrusive structures result from volcanic activity. During such activity the extruded material (lava, ash or cinders) may spread outward in gently-dipping layers, or may flow from a fissure or vent. Often, however, this material accumulates adjacent to the volcanic crater in the form of a CONE. CALDERAS are enlarged craters, formed through partial collapse of underlying rock, or through explosive activity. The subterranean conduit for a volcano is called a PIPE, PLUG, or NECK.
Igneous intrusions often brecciate the rock along their margins and the path in advance of their path forming STOCKWORKS and BRECCIA PIPES, and a complex of mineralized fractures.
A VEIN is a relatively narrow tabular mineralized structure. A LODE is either a single vein or a system of related roughly parallel vein. A STOCKWORK is a mass of intersecting veins. All are related to late magmatic hydrothermal fluid deposition, or deposition from circulating ground waters.
Horizontal and/or vertical movements of the earth's crust and the effects of igneous intrusions may crumple layers of sedimentary rocks into folds. Downwarped beds form SYNCLINES and upwarped beds form ANTICLINES. Long, relatively narrow, and very large warps of the earth's crust are designated as GEOANTICLINES or GEOSYNCLINES. Where the downwarping is large, significant portions of the continental crust may become flooded and become the site of widespread deposition of sedimentary rocks, often attaining great thicknesses -- the environment of great thicknesses of limestone. Smaller rounded downwarps are designated as BASINS. (Sedimentary structures are shown in Figure 3a.)
Formations of sedimentary rocks are seldom uniform in thickness. The deeper portions of sedimentary troughs or basins receive thicker sediments, and the layers of sediments thin and pinch out at the margins of depositional areas. Shifting currents in the water or air or obstructions may result in beds deposited at angles to each other (CROSS-BEDDED).
Silt, sand and gravel deposited in and on the margin of streams or rivers (FLOODPLAIN DEPOSITS) form discontinuous, serpentine-shaped deposits. Melt-waters from melting glaciers deposit a variety of OUTWASH GRAVELS, ESKERS, MORAINES, TILL, etc., that form distinctive land forms.
Sedimentary beds deposited on an older erosion surface are said to unconformably overlie the older rock. The older and younger beds may be parallel (DISCONFORMITY) or the younger beds may rest on tilted older beds (ANGULAR UNCONFORMITY).
Where forces exerted by the earth movements exceed the strength of the rock or its ability to bend into folds, the rock is broken and segments on opposite sides of the break may be moved relative to each other. The rock is designated as FAULTED. Relative movement along FAULTS may be vertical, lateral or diagonal (or all three at various times) with relative displacement from less than an inch to hundreds of miles laterally, and tens of miles in depth. Different faults range in attitude from nearly horizontal to vertical.
Rock caught between opposing walls of a fault may be ground completely to rock flour (GOUGE) or crushed into angular fragments (FAULT BRECCIA).
Erosional processes (air, water, ice) etch the land surface revealing underlying rock structures and rock character. In addition, surface processes (fluvial, glacial, volcanic, etc.) leave deposits having characteristic forms, and mass-wasting leaves evidence of landslides, mud flows, etc. This evidence facilitates geological analysis using aerial photographs and satellite imagery.
The history of the earth falls into distinct ERAS (Figure 7). The oldest era, the PRECAMBRIAN (ARCHEOZOIC and PROTEROZOIC), was by far the longest and is the period about which we know the least. Rocks formed during the Archeozoic resulted from extensive intrusive and extrusive igneous activity and have been affected by profound metamorphism. They comprise the BASEMENT COMPLEX on which younger sedimentary rocks rest. The younger Precambrian (Proterozoic) sediments can hardly be distinguished in places from the overlying PALEOZOIC SEDIMENTS.
The more recent eras of geologic time (PALEOZOIC, MESOZOIC, CENOZOIC eras) have many subdivisions and are partly defined by mountain-building OROGENIES or REVOLUTIONS, and partly by changes in the types of plant and animal life evidenced by fossil remains.
The earth's crust is not a homogeneous rock mass, and, although every element may have an average crustal concentration, in very few specific areas does any element exist in exactly that average concentration. Geologic processes, past and present, may result in concentration or depletion of certain elements. Some crustal regions show concentrations of certain elements and are identified as METALLOGENIC PROVINCES. Also, certain periods in the earth's evolution favored the concentration of certain metal components. These periods are identified as METALLOGENIC EPOCHS. For example, most banded iron formations were formed between 3500Ma and 1800Ma, and porphyry copper-moly-tin deposits were formed between 130Ma to the present. Identification of metallogenic provinces and epochs are important in broad guidance of mineral exploration programs (Figure 6).
The useful elements in the earth's crust do not normally occur in sufficient concentrations and in the proper chemical combinations to allow for them to be commercially extracted from the earth for man's use at the present time. They must be found in a relatively concentrated state and in a specific chemical form in order to be utilized. Such concentrations of the proper chemical compounds, enriched within the GEOCHEMICAL CYCLE in the earth's crust we refer to as VALUABLE MINERAL DEPOSITS. Concentration is brought about through various geological processes.
Chemical elements, including the ore metals, are unevenly dispersed through the lithosphere and are continuously being cycled and redistributed under the influence of the earth's dynamic geological processes. The geochemical cycle represents the complex physiochemical changes and varieties of processes that earth materials and their contained elements follow in response to those processes. It entails both deep-seated and surficial geological environments (Figure 2).
a. ORE
ORE is a concentration of minerals that can be mined processed and marketed at a PROFIT. It is economically defined.
(A distinction must be made between ORE and ORE MINERALS. A deposit of ore minerals in geological terms is not always an ore deposit.) While an ore mineral (Table 1) is a mineral from which a metal can feasibly be extracted, an ore deposit (or an orebody) is a mass of rock from which a metal or mineral can be profitably produced. What is, or is not, becomes dependent upon economic, technological, and political factors as well as geological criteria.
b. WASTE
Within a given mineral deposit ore minerals are normally associated with other minerals which are less valuable or lack value. These are termed GANGUE MINERALS (Table 2). The rock which does not contain an adequate percentage of ore minerals to be economically valuable as a source of these minerals is called WASTE.
Waste, like ore, is an economic rather than a geologic term, and changing technology, economic, or political conditions may change waste to ore, or back again, many times. For example, the Mount Morgan gold/copper mine in Australia underwent four life cycles. The first was when gold was easily extracted by gravity separation. The second was when flotation processes were introduced and copper sulfide could be recovered. The third was when mining shifted from selective underground mining to open pit bulk mining. The fourth was hydrometallurgical processing of "waste" materials.
c. CUT-OFF
Many factors: cost of mining and metallurgical treatment, percentage of recovery of metal values during treatment, deleterious elements present, cost of transport and marketing, metal or mineral pricing, taxes and royalties, etc. all influence the ore/waste transition. The transition from ore to waste is known as the CUT-OFF.
2. ORE-FORMING PROCESSES (Figure 2)
a. WEATHERING AND EROSION. Weathering results in the breakdown of rock minerals. Elements may be concentrated as resistant elements left behind, or in the mobile elements removed and transported in solution, or carried and concentrated during the erosional processes.
b. Igneous/volcanic process may result in concentrations through crystallization and differential gravity settling, concentration in end phases of crystallization, evolved fluids, or as a consequence of heat energy introduced by the intrusives.
c. Sedimentation/diagenesis. Gravity separation and concentration may occur within clastic sediments in streams, lakes, and by ocean shore currents. As pore-waters are expelled from compacting sediments they may selectively leach, carry and deposit specific elements.
d. Tectonic/metamorphic processes may result in the breakdown and transformation of rock minerals and concentration of certain elements by expelled fluids or diffusion.
Whether or not a deposit containing a valuable element or mineral is likely to become a VALUABLE MINERAL DEPOSIT depends upon engineering, economic and political factors as well as geological conditions and concentrations. In general terms - a valuable (or potentially valuable) mineral deposit contains some commodity (rock or mineral) which can be, or has the potential of being removed from the earth and marketed either before or after some form of processing.
3. CLASSIFICATION OF MINERAL DEPOSITS
Under existing technological conditions, certain minerals are more valuable than others because a particular element (usually a metal or chemical material) can be readily prepared from them or because they have useful physical properties. It has been customary to classify useful mineral deposits according to whether they were chiefly valuable as a source of metal (METALLIC DEPOSITS), as a source of chemical, building materials, etc. as NONMETALLIC DEPOSITS or INDUSTRIAL MINERALS AND ROCKS (Tables 3,4), or as a source of energy materials (FUELS). This subdivision has become inadequate and confusing. For example: uranium may fall into two categories, as a metallic deposit or as a fuel. Salt (NaCl) may be utilized as the chemical, rock salt, or as a source of sodium metal. Even the mineral bauxite is used as the principal ore from which aluminum is derived, or as an industrial mineral used in refractories and abrasives.
The simple three-way classification of metallic, industrial minerals and rocks (non-metallic), and fuels is no longer adequate. (Table 5 provides a list of commodities classified according to the traditional system.) Some of the problems with such a classification may be readily apparent in the overlapping of classes and the extreme heterogeniety within the non-metallic/industrial minerals and rocks category.
In many respects, classifications such as those shown in Table 5 are irrelevant and should not be used as a basis for legislation. Table 6 and its continuation, Table 6a, classify the most significant economic minerals according to the environments in which they commonly occur. Under this method of classification the overlapping is apparent. In sum, any system of classifying mineral deposits which proposes mutually exclusive categories is contrary to geologic reality.
The recoverable value within an ore deposit and the cost of extraction and sale determine the profit margin. Deposits with a high profit margin, generally small with a high gold-silver content and selectively mined, are commonly referred to as BONANZA DEPOSITS. Those deposits with large tonnage and low profit margin, mined in bulk are referred to as BULK LOW-GRADE DEPOSITS and include such deposits as the porphyry copper/gold deposits.
a. Titley (1992) proposed a geological classification of mineral deposits based upon genetic considerations as to environment of formation (Tables 7, 8).
Primary mineral deposits may be created as a result of rock-forming processes, such as intrusion, extrusion, and sedimentation, that result in the concentration of specific elements or minerals. Or, they may be created by the introduction into existing rocks by fluids (hot or cold) containing elements and compounds which crystallize upon cooling or reaction with the rock. During the crystallization and cooling of some igneous rocks specific minerals may be trapped as widely disseminated grains, or they may be more or less segregated and concentrated within the rocks as zones or bands (copper, nickel, platinum, chromium, iron).
Primary mineral deposits often result from sedimentation, diagenesis, and metamorphism. Beds and lenses of limestone, potash, diatomite, phosphate, gypsum and sodium carbonate (TRONA) are the result of sedimentary rock forming processes. High pressure and temperature may convert shale to slate and garnet, organic matter to graphite, limestone to marble. The evicted fluids may carry and concentrate gold values.
Primary mineral deposits formed through the introduction of hydrothermal fluids may occur as fillings in previously open spaces in rocks as replacement of the rock's original minerals, or in the adjacent fractures, breccias and faults. Many deposits of base and precious metals and uranium were formed in this manner.
Secondary mineral deposits are formed at the contemporary surface by the action of geologic processes on primary mineral deposits. Weathering and erosion, for example, cause the formation of bauxite (aluminum ore) deposits by residual concentration after weathering has broken down aluminum-bearing minerals, and subsequent erosion has removed the non-aluminum-bearing minerals. Iron and nickel may be similarly enriched in laterite ores.
Placer gold, platinum, tin, and diamonds are also secondary in nature. Ore mineral-bearing rock fragments are broken down as they move down slope and transported in streams and rivers. Resistant and heavier minerals become separated from the gangue minerals, and, because they are heavier, sink to the bottom of the stream. Placer deposits are usually formed where currents decrease in velocity, permitting the heavier minerals to come to rest.
Other secondary mineral deposits form at or near the surface by chemical leaching of primary mineralization by groundwater. This action results in lateral and downward transportation of copper, silver, zinc, lead, gold and uranium compounds in solution. Redeposition from solution below the water table or in localities of organic material form secondary deposits of these elements.
4. STRUCTURES OF MINERAL DEPOSITS (Figures 8, 9, 10, 11, 12 13, 14, 15, 16, 17, AND 18)
a. SEDIMENTARY DEPOSITS. Mineral deposits formed as a consequence of sedimentary processes occur generally as lenses or beds which parallel enclosing sedimentary rocks, and may extend for thousands of feet or tens of miles, but are rarely more than a hundred feet thick. These deposits occur in sedimentary basins, along ancient slopes and stream channels, and in ancient lagoons. However, similar deposits may occur as a consequence of replacement of reactive beds of limestone or dolomite - usually at the base, or as impregnation of a permeable strata unit, giving the impression of initial deposition.
b. STRUCTURAL DEFORMATION. Structural deformation may alter the form and attitude of some of these deposits. For example, salt domes along the coast of the Gulf of Mexico result from the squeezing of the salt from flat-lying beds, intruding upward along zones of weakness. Many limestone beds have been severely tilted and even overturned.
c. METAMORPHIC PROCESSES. Lenticular masses, veins, lodes and zones of disseminated mineralization result from metamorphic processes. These deposits may conform to the attitudes of the enclosing rocks or they may be cross-cutting (pegmatite veins).
d. VEINS/LODES. Veins and lodes consist of aggregates of minerals containing base and/or precious metals, uranium, etc. which have been deposited in fractures in the enclosing rock mass, or have replaced the rock immediately adjacent to the fracture. The veins are roughly tabular, but usually thicken and thin at irregular intervals. Quartz, calcite and pyrite constitute the common gangue minerals in most metallic vein deposits. Lengths and widths of veins are usually in the order of hundreds to thousands of feet in length, less than a foot to a hundred feet in width, up to several thousand feet in depth. Veins formed in the deep volcanic environment generally have good depth continuity of values, while those formed in the epicrustal volcanic environment generally give out, or become uneconomic, at depths of 1000 to 1500 feet.
e. REPLACEMENT DEPOSITS may be disseminated or massive. BEDDED REPLACEMENTS, lead, zinc, silver, copper deposits in limestone, generally occur in more or less flat-lying (MANTOS) clusters having lateral dimensions of a hundred to several thousand feet, but are usually less than a hundred feet thick. The basal limestone bed in a sequence tends to be the favored horizon, particularly where it is adjacent to an intrusive, forming a CONTACT METAMORPHIC or PYROMETASOMATIC assemblage. These mineralized bodies are generally irregular in shape and variable in size, but may border porphyry-type deposits and may be mined as a part of the adjacent deposit.
Other host-rocks containing carbonate as primary or alteration minerals, or organic material are favorable hosts for replacement by ore minerals as disseminations. Sizes and shapes are dependent upon the favored host.
f. PORPHYRY-RELATED deposits are formed in the epicrustal volcanic environment and constitute a principal source of copper-gold-molybdenum. Shallow porphyry intrusives, stockworks and breccia systems form large tonnage (up to several billion tons) low-grade deposits amenable to low-cost bulk mining and treatment. They are generally one to ten miles in diameter and may extend to a mile in depth. Commonly thy are capped by a barren leached capping which is underlain by a zone of SECONDARY ENRICHMENT of values.
g. Deposits formed at the surface by WEATHERING CONCENTRATION (bauxite, iron and nickel laterites) have more or less tabular forms with large lateral dimensions (several thousand feet to several miles, and thicknesses to a few hundred feet.
h. STREAM PLACER deposits are usually not more than a few tens of feet thick, with valuable minerals (gold, platinum, tin, diamonds) occurring in relatively narrow and relatively long horizontal patterns. BEACH PLACERS tend to be roughly rectangular in outline and may have adjacent, related dune concentrations.
The structures of common mineral deposits are shown in the accompanying illustrations. Figure 8 depicts structural controls affecting the localization of ore shoots. Figures 9, 10, 11, and 12 illustrate the environments of saline deposits, phosphate deposits, calcium carbonate deposits, and iron mineral deposits, respectively. Figure 13 shows the beneficiation process for iron ores, and Figure 14 illustrates advanced processing of iron ores. Figure 15 shows the environments for copper ores, and Figure 16 depicts the beneficiation of copper ores. Finally, Figure 17 shows the environments of gold, tin and tungsten mineral deposits, and Figure 18 illustrates the environments of lead, zinc, and silver mineral deposits.
Recognition of the environment and existence of potentially economic mineral deposits may be based upon a variety of geological criteria:
a. Association with specific types of igneous rocks -- e.g., copper with quartz-monzonite porphyry, diamonds with kimberlite pipes, tin with granites, etc.
b. Host rock association -- e.g. lead and zinc with carbonate rocks.
c. Wallrock alteration -- e.g. a concentric pattern of feldspathization, sericitization and propylitization around porphyry copper deposits, and dolomitization around lead-zinc replacement deposits.
d. Age of mineralization -- e.g. banded iron formation deposits are characteristic of Precambrian age rocks.
e. Gangue mineral association -- e.g. gold associated with quartz-ankerite veins.
f. Trace metal association -- e.g. gold associated with arsenic and mercury in trace amounts.
g. Structural controls -- e.g. laterite deposits associated with unconformities, replacement deposits associated with crests of anticlines.
h. Physiographic associations -- e.g. silicified breccias often stand up as isolated hills; oxidized pyritic bodies in limestone generally form low covered areas.
i. Weathering effects -- e.g. oxidation of pyrite leaves a residue of iron oxide gossan marking possible underlying deposits.
j. Ore and gangue mineral in fresh or oxidized states in outcrop of
derived sediments may give surface evidence of underlying or adjacent deposits.
A mining operation begins with prospecting and exploration -- stages with long periods of investment and high risk of failure. However, prospecting and exploration are necessary forms of investment and insurance for the future of any mining company. Success in mineral exploration or the acquisition of high-potential mineral properties by negotiation determines the survival of mining companies and industrial nations.
Prospecting and exploration may discover evidence of a mineral occurrence and outline its size and character, but ore deposits that support a mining operation are "made" through the collective efforts of project geologists, geophysicists, geochemists, metallurgists, engineers, chemists, lawyers, and even politicians. Some deposits may go through multiple stages of rejection and recommendation, discovery and development, decline and abandonment, rediscovery and development, etc., as economic, technological or political conditions change or geological understanding is improved. The gold/copper deposit at Mt. Morgan in Australia illustrates this: stage l - gravity separation of high-grade oxidized gold ores in near surface workings; stage 2 - discovery of the froth flotation technique enabling recovery of sulfide copper minerals with contained gold from underground workings; stage 3 - transition to bulk mining open pit operation; stage 4 - retreatment of dumps and tailings by hydrometallurgical leaching methods.
Ore deposits occupy a small space (in the U.S. 0.3% of the land area), yet produce 4.25% of the U.S. G.N.P. and 1.34% of wages. They are generally concealed, offer complex metallurgy, and produce large quantities of waste material so are highly visible. They are capital intensive, and are faced with environmental problems. Discovery of new deposits in the U.S. is becoming more difficult in spite of improved technology, particularly since more and more of areas sparsely explored in the U.S. are being withdrawn from mineral location, and permitting procedures delay operations to the point that they are uneconomic. Exploration efforts are moving to countries with fewer restraints.
An orebody, strictly speaking, is that part of a mineral deposit which can be mined and marketed at a profit under contemporary technological, economic and legal conditions. Economic conditions and technology are constantly changing, as are the laws, taxation and restrictive policies of governments. All of these factors dictate whether a deposit of a specific mineral is or is not an orebody. Hence, mineral exploration is the search for, and evaluation of mineral deposits which have the POTENTIAL of becoming orebodies under expected conditions at some favorable date in the future.
The principal objective of mineral exploration is to find economic mineral deposits that will appreciably increase the value of a mining company's stock to the shareholders on a continuing basis, or to yield a profit to the explorer. For an established mining company this may entail discovery or acquisition of new ore reserves and mineral resources to prolong or increase production or life of the company, to create new assets and profit centers by product and/or geographic diversification. Or, in the case of individuals or exploration companies, an objective may be to seek a deposit for sale to, or joint venture with, a major operating company, or to serve as a basis for stock issue and formation of a new company. On occasion manufacturing companies will seek sources of critical metals to insure a supply.
Each organization involved in exploration must define its own objectives in terms of mineral commodities, geographic locations, acceptable size, life, profitability, and acceptable risk. The exploration geologist must be aware of these limits.
Prospecting and exploration strategies vary widely dependent upon the mineral commodity sought, the geologic and climatic environment, political and social restrictions, and the explorer's experience and available resources.
Bailly (1972) outlines possible strategies for the acquisition of mineral deposits: (1) acquire a producing mine, (2) acquire developed reserves, (3) develop a known deposit, (4) explore known deposits, and (5) explore for new deposits - (a) near known deposits, (b) in a mining district, (c) in a mineral belt, or (d) in a favorable virgin area.
Acquisition of land or ownership position may be by: staking claims , lease/option, joint venture, royalty or purchase, and will be determined by land ownership, local customs, and the level of confidence in economic feasibility.
Most exploration programs focus progressively on areas of decreasing size, using methods increasing in cost per unit area, with declining risk of failure. Table 9 illustrates the mineral exploration process in detail, and Table 10 shows techniques for detecting non-ferrous metallic mineral deposits. Table 11 summarizes geophysical exploration methods. Figure 19 outlines the exploration, development and operation of a copper deposit. Mineral exploration tactics are described in this section.
a. RECONNAISSANCE
"Conventional Prospecting," consisting of field search for directly observable geological structures and minerals commonly associated with ore mineralization, as well as evidence of former prospecting activity.
Aerial geophysical coverage for selected minerals with diagnostic geophysical signatures (radioactivity, magnetic).
Literature and geologic research involving both satellite imagery and aerial photo analysis with the selection of geological favorable areas.
Historical notations of former production and/or observations of evidences of mineralization.
Selection of target areas with the potential of meeting company objectives.
b. TARGET INVESTIGATION
Investigation of a selected target area entails:
1) Determination of land status, staking of ground, obtaining lease/option, or prospecting permit, and obtaining required licenses for access and testing procedures.
2) Multistage coverage of selected target areas involving detailed geological mapping, geochemical and/or geophysical coverage, and/or use of special techniques (fluid inclusions, isotope ratios, etc.)
i. GEOLOGICAL MAPPING
a) Geological indications of a possible mineral body include: presence of gossans or leached capping, rock alteration.
b) Structural intersections, breccias, fold axes (Figure 5).
c) Favorable rock types.
d) Topographic features suggesting anomalous rock conditions.
ii. GEOCHEMICAL SAMPLING
Exploration geochemistry, or geochemical prospecting, includes any method of mineral exploration based on a systematic measurement of one or more chemical, or chemically influenced, properties of naturally occurring material. The property measured is most commonly the trace concentration of some chemical element or group of elements. It may also include molecular and isotopic compositions and bacterial counts. The naturally occurring material may be rock, soil, stream sediment, glacial sediment, surface water, ground water, vegetation, micro-organisms, animal tissue, particulates, or gases including air. (Coope, 1992).
iii. GEOPHYSICAL PROSPECTING
Exploration geophysics or geophysical prospecting includes not only geophysical surveying for the purposes of mineral discovery, but also the subsurface mapping of geological units. Direct mineral deposit detection is not readily achievable, except in detection of magnetic minerals, electrical conductors, or those giving response to specific electrical charges. However, geophysical methods can provide critical information about the mineral deposit environment, even without direct detection capabilities. Table 11 summarizes geophysical exploration methods.
Planning of the geophysical survey is one of the most important aspects. First, one must determine the most probable geologic model of the target zone in terms of depth, body geometry, and physical property contrasts of minerals and rocks. Other factors such as terrain, survey costs, land ownership, condition of the environment, and equipment availability may be important. It may turn out that blind drilling is more cost effective than geophysical prospecting (Sumner, 1992).
Analysis of satellite imagery and computer modeling are not currently direct ore-finding tools, but they may be integrated with geologic, geophysical and geochemical data to improve the efficiency of the exploration program. They can lead to a better understanding of the relationships and controls of mineralization and increase the probability of making a discovery.
4. SURFACE AND UNDERGROUND TESTING
A trenching, drilling and sampling program, and/or underground sampling by shafts, drifts and crosscuts, is essential.
It is essential in the evaluation of a mineral deposit to have, as accurately as possible, a model of the mineralized zone geometry -- shape, size, quality, variability, and limits. Physical, chemical and geological characteristics may vary greatly within a single deposit and from deposit to deposit. Critical data can be collected in a variety of ways, including drilling, surface and/or underground mapping, geophysical or geochemical surveys, or studies of rock mechanics properties, mineralogical types and relations.
Underground geological data are costly to obtain but critical for proper evaluation and mining.
a. DRILLING
Sampling the subsurface may involve one or more types of drilling, determined by the nature of the material to be sampled, rock conditions, and the objective of the sampling. Present exploration and development programs utilize DIAMOND CORE DRILLING or ROTARY PERCUSSION drilling with reversed circulation (RC).
i. ROTARY PERCUSSION is fast and the least expensive method. A hammer transmitting its force through drill rods to a rotating drill bit which does the penetration. Air or water is circulated through the drill rods to cool the bit and carry out the rock cuttings to the collar of the hole, where they are collected and prepared for study and assay. The method works well where the wallrock is competent, dry and impermeable. It has a practical depth limit of 200 to 300 feet.Metal values may be lost by seepage into the wallrock, or added or diluted by caving or seepage into the drill hole. Reverse circulation of the drill water down the hole and up the drill rods greatly improves the accuracy of the sample.
Unfortunately, the rock fragments from the rotary-percussion drilling provide little information as to the rock-mineral relations, and are best used after this type of information has been obtained by diamond core drilling.
ii. DIAMOND CORE DRILLING is slower and more expensive than rotary percussion(RC) drilling, but provides more useful and accurate samples of a mineral deposit as to the rock-mineral types and relations, and rock structures and characteristics. WIRELINE techniques enable removal of core up through the hollow rods without pulling rods from the hole. This greatly speeds the drilling process and improves core recovery. However, it yields a smaller diameter core.
Earlier drilling techniques often produced poor core recovery, but with improved core barrels, bit design and wireline retrieval of the core, total core recovery is now the general rule except in very poor ground where recovery of sludge samples is advisable.
Disadvantages of diamond core drilling are its high cost, small size of sample and slow penetration rate.
Bulk sampling for metallurgical testing or placer deposit testing are generally obtained by the drilling of large diameter holes (plus 6-inches in diameter), or by sinking winzes.
b. TRENCHING, generally using a backhoe or bulldozer, enables shallow testing and sampling (bulk sampling or cut-channel samples) on a continuous basis across the mineralized zone. Most areas now require back-filling of the trenches, once the sample is taken.
c. LOGGING
Before the core, rock-chip, or cut samples are reduced for assay, they must be carefully logged. Careful logging of diamond drill core enables basic three dimensional understanding of the mineral body. Proper logging takes practice under informed supervision to recognize and record critical factors that relate to ore genesis, deposit structure, metallurgical and rock-mechanics aspects, etc. Any logging must include: location and hole attitude data, lithologic data, structural data, rock alteration and mineralization data -- shown in graphic as well as descriptive form.
Cuttings from rotary-percussion holes are more difficult to interpret. Microscopic examination of the rock fragments is necessary, with sludge boards constructed to record rock changes with depth of drilling and corresponding concentrations of heavy mineral content.
Unfortunately, logging is tedious and often relegated to the youngest least experienced member of the geological team. Important clues are overlooked and lost as the samples are prepared for assay.
Diamond cores are generally split, with one half saved for reference while the other half is prepared for assay. This splitting can be a serious source of error. Another approach is to retain a representative core sample from each unit in a core library.
d. SAMPLING
Samples to be analyzed may consist of rock chips, sludge and/or core from drill holes, cut channel samples or bulk samples from trenches, or underground workings. For assay, these samples must be reduced in volume and size of particles without dilution or enrichment of metal values. Errors may be introduced in many ways by careless handling and lack of cleanliness.
The desired end is preparation of homogeneous rock powder suitable for chemical analysis. The assay procedure entails the following stages:
DRYER (at oven temperatures of 220-285 degrees F, except in the case of mercury (212 degrees F).
CRUSHER (reduction to -8 mesh by JAW CRUSHER, CONE CRUSHER, ROLL CRUSHER, HAMMER MILL).
SPLITTER (to 1/4 - 1 pound by RIFFLE SPLITTER or ROTATING SECTORAL SPLITTER).
PULVERIZER (to 100-150 mesh by PLATE or VIBRATORY RING MILL.
Where samples may contain nuggety gold it is necessary to retain larger samples and to conduct multiple analyses, since the presence or absence of a gold particle will greatly affect the assay.
e. ASSAYING
Two basic assay methods are available: geochemical and quantitative. Geochemical methods are semi-quantitative but have very low levels of detection and are generally used during the exploration drilling phase. Quantitative procedures are used during exploration, target analysis, and the sampling and analysis for ore reserve estimation and subsequent stages of development and operation. These may be by classical volumetric and gravimetric methods, calorimetric methods or instrumental analysis, or fire assay methods.
Precision and accuracy are best established and maintained through the use of reference standard samples or replicates (3 of each 20). When the results from control samples do not agree within acceptable limits, the entire group of assays must be rejected until the differences are resolved.
Metallurgical testing of mineralized rocks is an essential step that must be carried out early in the investigation. Expenditures on a project should be curtailed when it becomes established that mineralization under investigation will not yield to current technology, or the treatment will result in unacceptable environmental problems.
Mineral deposits are detected by individuals, and the importance of the human resource cannot be overemphasized. Local knowledge, detection methods, and time and money expended are of little value if the explorationist fails to recognize or misinterprets favorable indications, fails to accurately record geological data, lacks accurate sampling data, or if management lacks the courage or economic or technical resources to proceed.
"Discovery" of valuable mineral is the foundation of the U.S. Mining Law of 1872. It is a prerequisite to mining claim validity. Initially, "discovery" was legally defined as the finding of sufficient quantity and quality of mineralization that a person with ordinary prudence, with reasonable hope of success, would be justified in further expenditure of labor and means. This was called the "prudent man" test. However, at present the Department of the Interior and the courts have modified their definition of discovery to a more stringent test, a "marketability" test. In order to establish discovery, the locator must demonstrate the existence of a mineral deposit that can currently be mined, treated, and marketed at a profit - a currently economic discovery. One cannot anticipate technology or the metal market fluctuations.
Lack of agreement by persons within the mineral industry as to the definition of "discovery" results from different objectives. What may be considered as a successful discovery for a small operation or company may not be acceptable to a large corporation.
An ECONOMIC DISCOVERY is achieved when 1) capital for development can be raised within a reasonable period of time; 2) tenure and ownership will be respected (the mining claims will qualify for patent); 3) a reasonable profit margin can be projected; 4) technology for mining and treatment exists or can be developed within a reasonable period of time; and 5) there is social and political acceptance of the mining activity. A geological discovery does not indicate an economic discovery.
Lag time between discovery and development and operation of deposits influences capital investment decisions. Between the time of initial detection of base metal deposits and development is an average period of ten years. Gold and uranium deposits on an average require shorter preproduction periods and lower capital investments. Albers (1977) suggests an average of a seven year preproduction period from geological, geochemical or geophysical "discovery" for all mine types; ten years for copper deposits, five years for uranium deposits, and three years for gold deposits. Deposits with complex metallurgical problems, or permitting problems, may be delayed for 20 or more years.
A breakdown on time requirements for various exploration and development
stages was proposed by (Allen, 1956):
| Stage | Cost | Time |
| Reconnaissance | - 1 unit | 1-20 years |
| Examination and evaluation | - 5 units | ½ - 3 years |
| Mine development | 30 units | 2 - 5 years |
| Plant construction | 80 units | 2 - 3 years |
| Initiating production | 7 units | 1 - 6 months |
| Production with continuing exploration and development |
An average target selection, lacking land or political complications, will cost in the vicinity of $150,000 to $250,000. To arrive at confirmation or rejection of the target will normally require an additional $500,000 to over one million dollars. Commonly it requires greater expenditure to arrive at a rejection conclusion than confirmation. In part this is psychological. The exploration manager or geologist in charge of the project may have become enamored with "his discovery" and is reluctant to abandon it. Also, in part it entails the difficulty in unraveling the complexities - geological, metallurgical, environmental, political and economic - of a deposit and the fear that another organization will discover an overlooked factor and make a major discovery.
Many companies change personnel and use outside consultants and mining and metallurgical engineers during the evaluation stage of a deposit to avoid the situation of "having a bear by the tail," and personnel afraid or embarrassed to let go. Some major mistakes can be avoided by having a mining engineer and a metallurgist assigned to the exploration team.
Some mineral deposits are more difficult and expensive to evaluate than others. Those deposits having good lateral continuity, such as base metal bedded deposits and mantos (iron, lead-zinc, uranium, industrial rocks), are relatively easy to evaluate. Those with spotty distribution of values, such as gold, silver, and tin deposits and many copper deposits, are very difficult to evaluate due to lack of correlation of values between test drilling and/or trenching.
A mineralized deposit should not be developed into an operating mine unless the estimated annual operating profit, after taxes, is judged to be sufficient to recover, with interest, the estimated capital and operating costs of developing the mine. The accuracy of estimation of capital and operating costs depends upon the quality of the technical assessment and knowledge of expected mining and mineral processing conditions.
a. EXPLORATION COSTS
Average costs of making a discovery range between 15% and 20% of the
gross revenue anticipated from the deposit. Some companies consistently
beat these averages, while others seldom achieve success despite substantial
expenditures. This appears to be a function of management of exploration
programs:
| GROSS REVENUE FROM DISCOVERY ($) | COST OF DISCOVERY (1984-86) ($) |
| 500 million | 60-100 million |
| 1,000 million | 120-200 million |
| 1,500 million | 200-300 million |
| Geological-Geochemical prospecting | 35% |
| Geophysics | 5-15% |
| Drilling | 30-35% |
Size of various types of deposits and land requirements for mining and concentrating facilities, tailings ponds and dumps are shown on Tables 12 and 13, and range from 1/4 square mile to over 5 square miles. Land requirements are dependent upon whether mining operations are underground or open-pit, lateral extent of the orebody, and percentage of gangue within the ore that must be discarded as waste, and the topography.
Records of exploration programs carried out in the U.S., Canada during
the 1970s yielded the following results:
| Location | Favorable Areas | Target Areas | Mines |
| U.S. | 352 | 23 | 2 |
| Australia | 600 | 12 | 1 |
| Canada | 1000 | 70 | 7 |
Expenditures for mineral exploration and development are wasted unless secure tenure on surface and mineral rights is obtained on all parcels vital and necessary for a mineral project.
Mineral land ownership in the United States resides with the Federal Government (FEDERAL LANDS), the State Governments (STATE LANDS) or with private individuals and business entities (PRIVATE LANDS).
The Mining Law of 1872 is being subjected to scrutiny and revision, and major questions may arise as to what permitting is required, who owns a particular type of mineral, and what type of surface and mineral rights can be transferred.
Federal lands can be categorized as: l) PUBLIC LANDS where both surface and mineral rights belong to the United States, and the Mining Law of 1872 governs acquisition by locating or leasing; 2) RESERVED or WITHDRAWN lands, which are permanently or temporarily withdrawn from mineral location and leasing; 3) ACQUIRED LANDS, acquired from state or private owners, are not available for location or leasing except by special provisions; and 4) SEVERED LANDS, lands for which the federal government has sold surface rights but retained mineral rights.
Each state has regulations which supplement federal mining laws, and should be consulted.
Private lands are owned by individuals, trusts, corporations, mining operations, railroads, forest product companies, etc. Ownership may be in fee or consist of separate ownership of surface and mineral rights. The mineral rights may be further subdivided based upon the type of mineral or geological factors, such as depth of mineralization, or may be confined to specific stratigraphic horizons.
Where private lands are involved, a variety of approaches can be utilized to secure adequate tenure during exploration and development phases. The approaches vary dependent upon l) local customs and expectations, 2) risk involved, 3) the financial position of the owner and explorer, 4) technological capabilities of the explorer, etc. The most important factors include the sequence and timing of exploration and development stages, and the importance of reducing risk factors and establishing potential of the prospect before each major payment is required.
A good agreement requires knowledge and cooperation of a lawyer experienced in current mining law and pending legislation; an engineer and/or geologist who understands the practical aspects of scheduling exploration, testing and mining; a tax specialist who understands tax implications of the agreement; and a financial analyst. Commonly, however, the entire responsibility falls on the lawyer.
Permission is required from land surface owners and owners of patented or operating mineral claims prior to conducting geological, geochemical or geophysical surveys, in order to avoid liability for trespass and to establish good relations with the owner. However, on inactive mining claims no trespass is committed by persons passing through the area on federal lands. Prospectors and geologists may examine the showing on a mineral claim without prior knowledge of its status as a mining claim. It is common practice to examine mineral showings and quickly map and sample the surface and underground geology of a prospect without contacting the owner of a claim. More extended investigations require inquiry as to ownership and owner permission.
In the situation of private lands and patented or operating mining claims,
permission of the surface and mineral rights owners must be obtained to
conduct geological, geochemical or geophysical surveys. When entry is made
for these purposes without permission of the owner, or where permission
is exceeded, courts have in some cases held the prospector liable for trespass
damages on the basis that the landowner has the right to have the mineral
potential of his land remain unknown unless he is paid a fee of gives permission
to someone to enter and gather such information.
The terms "valuation" and "evaluation" in a mining context are often used interchangeably, but there are subtle differences. VALUATION has a narrow meaning of placing a monetary value on the worth of the project as a whole. EVALUATION has a much broader meaning of determining all variables that are important in assessing the feasibility and worth of a project: the assessment of relative viability of the project; estimates of ore reserves and resources; mining methods and rates; revenues; costs; expected returns; risks, cost/benefit analyses, as well as the monetary worth.
Evaluation at each stage of tactics employed depends upon the character of the deposit under investigation, topographic and geologic conditions, access, the commodity sought, the human and economic resources available, previous experience of the explorer, and results obtained in earlier stages of the exploration program.
1. EVALUATION OF EXPLORATION PROGRAM
Initial exploration drilling tests likely sites for the presence and extent of ore-grade mineralization by the tracking of evidence, hoping that it will lead to an orebody. If successful, delimiting and defining the orebody is then done with closely spaced holes. It is in this phase that the continuity of ore-grade mineralization (or lack of it) is determined. At this stage the determination of the shape of the orebody is modified from the outline inferred from initial drilling (Figure 20), and geological mineral resources can be computed, and decisions made as to continuing the program.
If the exploration manager terminates the project too early, he may have missed the orebody. On the other hand, continued drilling where no orebody is to be found will cost excessive money, time and manpower. He may have a "bear by the tail" and lack courage to let go, embarrassed by expenditures already authorized on a loser.
Guidelines for justifiable expenditure can be presented as:
Expenditure = ( (Present Value of Reward) (Risk)) ÷ Factor
where "Factor" is 1 for governmental valuation, 2 or 3 for companies.
The evaluation is a continuing process applied to every step in the exploration and development process. Work continues so long as the risk of failure is progressively reduced.
Once the existence of a mineral deposit of current or potential value is demonstrated, the project is normally turned over to the mining and metallurgical engineers. They conduct bulk sampling (through shafts or other excavations), metallurgical testing generally utilizing pilot mills, and feasibility studies as to various mining methods. They decide HOW, WHEN, and sometimes IF the deposit should and can be mined on the basis of a preliminary feasibility evaluation.
a. BENEFICIATION
Metalliferous minerals are not generally found in pure form; they are mixed with rock and gangue minerals, and are usually found as compounds of several elements. Physical separation of the various mineral compounds is the first of many beneficiation steps that eventually provide the degree of concentration required. Crushing and grinding ore used to achieve liberation of mineral grains; then, if required, grains must be classified to a pre-determined particle size that best suits the particular concentration process to be used. Then, separation of the ore mineral by gravity methods, froth flotation, hydrometallurgical techniques, etc., can take place.
During the exploration phase, tests should be carried out on a laboratory scale to determine feasibility of concentration, grinding size required for liberation of grains, percent recovery of metal values, etc. During development stage, testing generally needs to be expanded to bulk samples of various mineralization types in a pilot concentrator.
b. RISK
Mining is a high risk venture in view of the multitude of unpredictable factors in the finding, acquisition, extraction, treatment and sale of the product. Risks may be geological, technological economic, political and/or social.
i. GEOLOGIC RISKS
We have no way of seeing directly what variations or discontinuities occur in rock beneath the surface. We depend upon projection of surface indications with projection of rock types and structures and/or interpretation of geophysical responses, and/or widely spaced drill holes.
A diamond drill core, for example, has its limitations - it provides a cylinder of rock one to two inches in diameter which represents a sample half the distance to the next drill hole (usually 100 to 250 feet). There is no guarantee of continuity of mineralization, or lack of mineralization shown in adjacent drill holes. Expected continuity varies with the type of mineralized body. For example, there is a greater probability of continuity in base metal veins, stockworks and mantos than in precious metal veins, stockworks and mantos. There is better chance with silver than with gold (Figure 21).
A grid of drill holes, shafts or adits and cross-cuts may encounter high-grade mineralization or completely miss the mineralization as a consequence of local controls (Figure 20). Mining history is replete with examples where ore-grade was encountered in as many as 20 adjacent holes on a grid pattern, only to find that there were no values or only low-grade values between. On the other hand, barren drill holes have missed a major orebody by a matter of a few feet. Murphy's Law applies --"If anything can go wrong, it will." Hidden faults may disrupt continuity of mineralization (Figure 5), or variations in the character of mineralization may reduce recovery.
Groundwater conditions may be critical. Encountering an aquifer horizon, permeable fault zone or breccia, or cavernous limestone can flood underground workings making mining uneconomic, requiring excessive pumping or expensive drainage facilities. (The Tombstone Mine in Arizona is an example of a mine experiencing these problems).
ii. METALLURGICAL RISKS
Mineral deposits are not uniform either in their physical character of the wallrock (hardness, toughness, stand-up time, required support), or in the nature of the ore and gangue minerals (intergrowths, variations in composition, intensity of oxidation and/or alteration processes acting on the ore minerals influencing their metallurgical behavior). Calculations of crushing and grinding characteristics, liberation size, metallurgical recovery, as well as variability of grade of mineralization may fail to meet expectations. Some portions of the mineralized body may contain deleterious elements (arsenic, mercury) that make them unacceptable to smelters, or they may contain unstable pyrite/marcasite that ignites spontaneously (PYROMORPHIC) within mine workings or during concentrate shipment.
Many of these risks can be anticipated with adequate geological logging of core samples and/or rock chip samples, and multiple metallurgical laboratory tests, but they are often overlooked.
iii. ECONOMIC RISKS
Unlike most commercial products, prices of metals are generally set internationally and low-grade national products must compete for markets with higher-grade foreign products. Discovery and development of large and/or high-grade deposits elsewhere in the world may depress the market prices, or substitution of alternate material that will perform the function of a particular metal cheaper or better will diminish the market, or environmental or health influences of the product may result in its being banned or limited in its usage (mercury, arsenic).
Many governments view mineral deposits as belonging to the state and feel free to apply and increase taxes and royalty assessments at will, and change permitting regulations and restrictions on operations. The rules for operation change after investment has been made, often causing cut-off levels to rise; destroying ore reserves or, in some cases, the entire orebody.
There is no substitute for experience in the assessment of various risk factors.
BENEFITS comprise all of the nice things that will flow from any project (economic, social or political) financial or non-financial. COSTS comprise all of the undesirable things, financial or non-financial. Unfortunately, what is a cost to one person or group may be a benefit to another person or group. (Taxes are an example.) A balance must be achieved between present and future interest groups.
V(f) = 
,
where
B(t) = benefits accruing in time t.
C(t) = costs accruing in time t.
r(t) = the discount rate for period t.
V(f) = net present value for group f.
Evaluation of costs and benefits of any planned project, by anybody, requires forecasting - no matter how it may be dressed up in formulae or in computer language. However sophisticated the analytic techniques, forecasting remains a "guessing game". It can never be classified as "correct" or "incorrect". The forecasting techniques might be classified as "incomplete" or "biased". The forecasting of values for particular benefits or costs can be assessed as "unduly optimistic" or "pessimistic"; but, in the end, one can only assess a forecast as: "wildly improbable," "unlikely," "likely," "essentially dishonest," or "honest, but misguided".
It is possible to manipulate forecasts to conform with one's prejudices. A few deft manipulations will bring the desired answer -- bring the benefits in a bit earlier or later; add or subtract from the list of secondary benefits or costs; change the discount rate, etc. The use of good technique is no guarantee against abuse. Regrettably, much of this kind of analysis is prepared by activist groups with strong biases, and are often, if not generally, dishonest.
Nevertheless, a cost/benefit analysis can, if honestly prepared, focus attention on all aspects of present and future impacts.
There is no question that development (extraction and processing) of mineral resources impacts the environment. However, with proper planning and precautions, these impacts can be minimized in terms of severity and duration. In the past there have been few environmental controls and there exists a legacy of environmental disturbances created by mining and mineral processing (acid drainage, subsidence areas and unreclaimed open mine workings). Over 60 billion tons of solid waste have been produced by hardrock mining in the U.S.: 50% from copper mining; 24% from iron mining; and 16% from phosphate mining. Sites containing "hazardous waste" from metal mining and processing pose problems from soil and water contamination to dust and "toxic" gas emissions. Most of these adverse impacts predate environmental consciousness, laws and regulations, and technology that now enables greater control. In general, environmental priorities accompany economic affluence.
4. ORE RESERVE/RESOURCE ESTIMATION
A RESOURCE ESTIMATE is based on the prediction of the physical and chemical characteristics of a mineral deposit through collection of data, analysis of the data, and modeling of the predicted size, shape and grade of the deposit. Physical characteristics of the mineralized zone must be predicted and include: 1) the size, shape and continuity of the mineralized zone; 2) the frequency distribution of the metal grade; 3) the spacial variability of the metal grade, and 4) recoverability of metal values. These characteristics are never completely known, but are inferred from sample data which consists of one or more of the following:
1) Physical samples taken by drilling, trenching, test pitting and channel and bulk sampling of underground workings;
2) Measurement of the quantity of mineral or metal in the samples through assaying or other analytical procedures;
3) Direct observation, such as geologic mapping and/or drill core logging.
4) Analysis and synthesis of these data to develop a resource model.
This procedure will produce a GEOLOGICAL RESOURCE estimate.
A MINING RESOURCE estimation procedure must be made with some knowledge of the proposed mining method, since different mining methods may affect the size and shape and/or grade of the potentially minable reserves. These estimates must include:
1) The range of likely cut-off grades;
2) The degree of selectivity and the size of selective mining units; and
3) Variations in the deposit that affect the ability to mine and process the mineralized material.
The mining factors often determine the degree of detail that is required for the resource model, and thus the degree of difficulty in developing that model. For example, a disseminated copper or copper/gold deposit may be continuous and regular in shape if mined by bulk, open-pit methods. The same deposit may be discontinuous and difficult to estimate, however, if mined by selective underground methods at a higher cut-off rate. Such large differences in deposit shape due to variations in cut-off grade and mining method may require different estimation procedures for different mining methods.
The geological interpretation of the orebody should be used in developing any resource model considering such features as:
1) Receptive vs non-receptive host rocks;
2) Alteration types that accompany mineralization and may create problems in beneficiation;
3) Faulting, folding and other structural modifications that may dislocate values, and/or may result in weakened rocks;
4) Multiple phases of mineralization that may modify mineral assemblages; and
5) Post-mineral deposition features processes such as oxidation, leaching, and enrichment which influence grade and amenability to beneficiation methods.
Estimated blocks should not cross geological boundaries, and a clear understanding of the genesis of the mineral body will give clues to the grade distribution. Estimations based on simple geometric forms, independent of geological units and structure can be misleading.
Compositing of assay data generally entails computing a weighted average over a mining unit.
RESOURCE ESTIMATES of different kinds of deposits all involve quality and quantity (tonnage and grade) of a usually concealed resource that must be amenable to profitable extraction. Estimates of resources are far from being precise: QUANTITY (tonnage) is generally the most precise because it is based mainly on measurement. GRADE to QUALITY is much more difficult to assess since it must consider not only variability in distribution, recoverable values, and greater probability of sampling errors. In general, the lower the proportion of the valuable constituent in the mineral body the greater the possible range of assay values and spottiness or variability in distribution (Figure 21 illustrates the geological continuity of mineral deposit types.) In general sedimentary deposits have greater continuity than structurally controlled deposits. For example: coal, limestone, evaporite and laterite deposits have high lateral continuity; skarn deposits, Mississippi Valley type lead zinc deposits in limestone, vein gold, and vein copper, lead, zinc, silver deposits tend to lack lateral continuity. (Figures 22a through 22j show the forms of vein deposits, irregular pockety deposits, high grade "bonanza" deposits, continuous high grade to low grade deposits, disseminated deposits, deep continuous high grade to low grade deposits, bedded disseminated deposits, long vertical deposits, pockety disseminated deposits, and disseminated deposits with vertical veins.)
Estimate procedures must adapt to the geometry of the mineralized body, to the pattern of the drill holes, adits and cross-cuts, trenches, etc. and to the continuity of values within the mineral body. A number of methods are in common use (Table 14):
a) AN OLD STYLE APPROACH (Figure 23h) in which samples are all lumped together for calculating average grade and width from underground drifts, raises, etc. without weighting for spacing. In exploration or evaluation of data from old workings, this method may be used in early stages. It assumes that each sample represents grade and width halfway to the next sample. Regular spacing of samples is essential to avoid bias.
b) The POLYGONAL METHOD (Figure 23c,d) in which each drill hole is in the center of a polygon bounded by median lines or angular bisectors. Within the polygon, it is assumed for purposes of the estimate that thickness and grade are uniform. It is essential that the polygons not cross geological boundaries. Assay values from the sample are used only once.
c) The TRIANGULAR METHOD (Figure 23e) in which each hole is taken to be at one corner of a triangle, or a number of them, with a width and grade assumed to be the average of its three corner holes. Some samples in the fringe area may be used an unequal number of times, introducing a possible bias.
d) A SECTIONAL METHOD (Figure 23f) in which dimensions and grade between two drilled sections are assumed to be equal to the mean of the sections. It assumes continuity between sections.
e) A CONTOURING METHOD (Figure 23g) assumes continuity of values between drill holes or blocks of similar grade and basically means deriving a plan of grade distribution from vertical holes. Contouring may be accomplished by use of a traveling circle that averages all values within the circle. This smooths out irregularities.
f) A SPHERE OF INFLUENCE METHOD in which the grade of a portion of a mineral body is derived from samples within the surrounding blocks (in two or three dimensions) giving greater weight to near samples and less to distant samples. Samples are used repeatedly in a manner only feasible using a computer.
g) GEOSTATISTICS also uses surrounding blocks to estimate grade of particular blocks with weighting calculated from a 'variogram' representing a distance/value relationship between samples.
In this progression from simple to complex mathematical procedures has both advantages and disadvantages:
1) It derives a grade for a particular portion of a deposit from more samples - in effect down-grading high assays and up-grading low assays.
2) One may become so wrapped up in computer mathematics and printouts that assumptions are lost sight of, and geological controls and characteristics and sampling errors are lost.
Computer-generated estimates must always be compared against geometric-generated block estimates superimposed on geological maps and sections and discrepancies explained.
The accuracy of estimation of capital and operating costs of a mining project is dependent upon the reliability and quality of data from exploration and development stages -the assessment and knowledge of expected mining and metallurgical conditions.
Estimates will need to be adjusted as more detailed information is acquired. Cost formulae can provide some guidance as to the order of magnitude of both capital and operating costs, but accurate estimation must be trusted to consulting engineering firms who employ civil, structural, mechanical, chemical, metallurgical and electrical engineers.
The most important factor affecting costs is the size of the mine and processing plant in terms of ore mined and milled per day of operation. Consequently, they are sensitive to the accuracy of ore reserve calculations - the size, shape, continuity, depth, and rock physical character. Uncertain forecasts of behavior of the metal market, equipment costs, future government actions and restrictions make estimates uncertain even with the best engineering.
Costs estimated in a preliminary feasibility study are unlikely to be more accurate than plus or minus 40%. These are usually based upon comparative costs with other operations of similar character, formulae and "rules of thumb". This degree of accuracy is not sufficient to provide a sound basis for major mine financing or assurance of a profitable mining operation.
Estimation of costs with an accuracy of plus or minus 10%, which is needed for a detailed feasibility study, requires completion of extensive technical work and studies on mine planning, general plant layout and design, environmental studies and assessment of supplies of labor, and equipment required for mining, milling and service operations (transport, housing, water and electrical, etc.). This type of feasibility study is normally required for obtaining long-term financing.
The subject of mine financing is complex and varies with ownership, risk, market conditions, taxation and governmental policies, and the stage of mine development. As a project evolves from prospecting and exploration to target examination, then to evaluation, development and operation, the mode of financing changes (Figure 41).
During exploration by an exploration company/syndicate, risk monies are sought, and the promoters seek to retain as much of the equity and as little of the financing obligations as possible. In general there are two types of exploration companies or syndicates: those looking to find and develop a mine, and those whose objective is to develop a mineral deposit to a promising stage and then sell out.
The exploration or independent company at the exploration stage generally has a capital structure consisting largely of equity or tax-sheltered funds. Equity can be raised from individuals, companies stock issues, junk (high-yield) bonds, and some 30 to 40 other sources of funds.
Development companies may have one or two exploration successes which require financing for drilling and feasibility studies before actual mine development can be financed. Some junior companies may specialize in redeveloping defunct mines or restructuring troubled operations as a way to finance mine development.
The venture capitalist seeks high returns, as follows:
Development funding may come from a variety of sources and the cost
of capital will depend upon tax laws which allow deduction of interest
payments, but not dividend payments. For example, the following is an approximation
of the cost of capital for a project:
| Finance Source | % New Capital | Cost %/Annum | After Tax Cost |
| Bank Loans, unsecured | 15.0% | 16.0% | 9.6% |
| Mortgage Bond | 20.0% | 14.0% | 8.4% |
| Equipment Leases | 15.0% | 16.0% | 9.6% |
| Preferred Shares | 4.0% | 17.0% | 17.0% |
| Common Shares | 41.0% | 19.0% | 19.0% |
As a "rule of thumb," the objective of most mining developments is to achieve a capital structure of 50% equity and 50% debt financing. A project with a highly predictable cash-flow, based on strong sales contracts can move leverage to 90 to 100%, while a mine with cash-flows difficult to predict, as with most industrial minerals (mercury, etc.) can expect to attract very little traditional debt financing. However, as the ratio of borrowed money to equity money increases, the interest payments and the rate of interest may become so large that profits may disappear.
(After C.R.Tinsley.)
A. FEASIBILITY ANALYSIS (E.S. Frohling, N.C. Hario, 1971) (TABLE 19)
A feasibility study entails a review of costs and potential earnings of a proposed mining venture. Throughout mining history, low-cost mines continue to be developed regardless of the impact of new production or technology on price, and, in times of oversupply, low-cost producers replace marginal producers. It is essential for new producers to know where their prospects stand economically, relative to other producers.
After a potential orebody has been discovered, a decision must be made as to specifically, what is to be produced and marketed (ores, concentrates, refined metals) and the cost of such production? A feasibility study must, for example, determine what production costs are required to upgrade the product and determine the additional investment required.
The first objective is a market survey. Prediction of demand/price, political atmosphere and regulations (taxes, royalties, risk of expropriation, etc.) is an assessment of risk. Comparative cost analysis is appropriate. After the capital cost estimates are determined, estimates can be made of operating costs, and these can be compared.
Companies use different internal criteria to determine basic viability: Net Present Value (NPV), Rate of Return on Investment (RRI), Payback Period, Project Life, etc. In politically unstable areas it is important to recoup investment as quickly as possible (perhaps 4 years), while in stable areas one might stretch the amortization period to 8-9 years or more (Figure 41).
2. DETERMINATION OF CAPITAL COSTS
Capital cost estimates may be required for a number of purposes. The purpose of the estimate dictates the accuracy required:
- whether more capital should be spent to develop more definitive data and the allocation of additional funds;
- information for private financing from individuals, banks, or other mining companies;
- to furnish data to go for public financing through stock issues;
- to obtain funds from governmental agencies (AID, EXIM, etc.).
The greater the degree of accuracy required, the higher the preparation cost and time required. For example, people experienced in estimating certain types of plants have data on relative capital costs depending upon ore grade and proposed feed rate. With minimal information of this type, accuracy might not be better than 40%.
Where a flow-sheet is established, with equipment lists, price of equipment, etc.; building costs based upon consumption of concrete, steel, etc.; electric costs based upon cost per horsepower; piping as a percentage of equipment costs, etc, the estimate might not be better than 30%. This could be refined to within 15%.
Finally, when about 40% of the final engineering is complete and the project is well defined, estimates should be within 10%.
Risk analysis entails a review of the sensitivity of the proposed venture to production market conditions, cost of production, selling price of products, grade variations, geological variations, etc.
B. MARKETING AND FINANCING (B. Nolk, 1992)
Sales and marketing of mineral products varies with individual companies. The entire system of exploration, discovery and evaluation, mining and processing depends upon sale of the product at a profit within a long preproduction and production time frame. Most major mining companies have their own marketing division, and marketing strategies can help compensate for the risks inherent in any mining venture (Figures 38, 39, 42).
Mineral material is frequently sold at several stages in its development. Copper, for example, may be sold as ores or concentrates, intermediate metal (BLISTER copper anodes), refined metal (electrowin or electrolytic copper cathodes), alloyed metal, semi-finished products or finished products, or, finally, as scrap. At each step the material is bought and sold, or if processed internally, at least priced for that purpose.
A copper company will need to know the market for concentrates as well as blister copper to determine whether it is economically worthwhile to build or up-grade smelter facilities.
Internal selling of ore to concentrator, concentrator to smelter, blister copper to refinery, etc., is a valid marketing exercise. It serves to evaluate economic efficiency of the system and guides marketing strategy to avoid gluts or bottlenecks in the chain.
Increasing internationalism of the minerals commodity market causes a shortage or surplus in one part of the world to influence the market worldwide.
Transactions include: 1) sales contracts at fixed prices and tonnages, or 2) multi-year contracts with periodic pricing linked to exchange price formulae for various tonnages.
Producer pricing is a straight forward approach by which the producer establishes a reasonable and competitive price for his product by announcing to key customers and media a fixed price for the day (or month or quarter). Consumers can contract set amounts of product in increments for delivery. It is possible for producers to maintain pricing control of their product when there are few competitors for the product or the quality required.
Today, much of the production of major metallic mineral commodities is integrated vertically, so that the large miner of metallic ore has considerable control over the sale of the finished metallic product. However, for the alloying metals only a small proportion of the production is carried through to the refined metal products. Ores and concentrates of these are generally sold either by the miners to the industrial consumer or through dealers. For the minor metals, dealers are important in marketing.
In times of price volatility, the producer may wish to tie pricing to a free market indicator, such as the London Metal Exchange (LME) or the New York Commodity Exchange (Comex) futures contract, or even to free market quotes, such as those published monthly in the Engineering and Mining Journal (E&MJ).
Both buyers and sellers must choose the same pricing mechanism.
Trading in futures and options contracts has become increasingly important and offers an opportunity for HEDGING. A futures contract enables producers to guard against the risk that prices will fall and protects the buyers from the risk that prices will rise. The need for hedging is important when several foreign currencies become involved. Hedges may involve metal commodities, currencies, or interest rates.
Most companies do not like to hedge 100% of their production, in hopes that they can take advantage of price spikes that take place periodically. A 'rule of thumb' for hedging is (C.R. Tinsley):
1) Cover 100% of mine operating cost as far forward as possible on the downside production profile and be sure local currency costs are matched in that currency.
2) Cover loan repayments by at least 50% where currency hedging is required. Change the percentage hedged progressively by roll-overs and perhaps leave unhedged from time to time.
3) Cover forward price volatility with floor-price programs and PUT options designed to insure a minimum cash position, after debt repayment and exploration costs.
4) Match debt obligations where possible by commodity-linked programs, e.g. gold loans, especially for volatile commodities.
Exchange trading of metals serves to establish PRICE DISCOVERY for commodities traded and, by extension, to the comparable materials both up and down the materials chain, by use of premiums and discounts from the exchange traded price.
Marketing of industrial minerals and rocks is normally local to regional, except for potash and sulfur. Marketing is done by the producers or through use of sales agents, and, in many instances, the producer is the seller of the finished product (i.e. gypsum as wallboard, asbestos, phosphate, etc.).
In the marketing of industrial minerals and rocks, the factors of primary concern are the location and size of the particular local markets (existing and potential), the chemical and physical specifications of the product, unit value and transportation costs. The product generally must be "sold" to the potential consumers. It will not normally "sell" itself, as is true of metal commodities (Table 4).
4. MINERAL PRICES (S.D. Strauss, 1992)
In any feasibility study for a metals or mineral project, an assumption must be made as to the prices at which the projects products can be sold. Forecasts of prices must cover the extended period of pre-production and production until investment is recouped.
Among virtually all the metallic minerals prices are highly volatile, although there have been some periods of relative stability for the published prices of some commodities.
C. FINANCING MINE DEVELOPMENT (C.P. Tinsley)
1. Post-feasibility study mine development funding offers a wide variety of options international in character. Once exploration has resulted in a viable mining project, the financial requirements for funding change in emphasis. Factors such as cash flow, impact of taxation policies, net income, etc. become dominant considerations.
A great variety of financing opportunities are available and selection of the optimum combination often determines the profitability of the project. Financing possibilities include: 1) equity financing, 2) commodity-linked financing, 3) contract mining, 4) supplies/buyers credits, 5) governments, 6)joint ventures, 7) institutions/insurance companies, etc. (Figure 43).
2. Present corporation financing policies tend toward PROJECT MULTI-LAYERED FINANCING rather than internal corporate financing, with each development standing on its own merits. Although project financing is usually
