GEOL

GEOL.363 MINERAL DEPOSITS

Hand out No. 1

(Semester 1/2001)

DEFINITION

Ore : a mineral or minerals that can be mined and extracted from a rock at a profit.

Gangue: Those minerals which occur with the ore minerals but which have no value, such as quartz, calcite or pyrite.

Mineralization: A general term which usually refers to ore minerals but which often may refer to other metallic minerals such as pyrite.

Waste: Rock which is not ore. Usually referred to that rock which has to be removed during the normal course of mining in order to get at the ore.

Grade: The concentration of each ore metal in a rock sample, usually given as weight percent. If concentrations are extremely low, as with Au, Ag, Pt and others, the concentration may be given in grams per tonne (g/t). The average grade of an ore deposit is calculated, often employing very sophisticated statistical procedures, as an average of the grades of a very large number of samples collected from throughout the deposit.

Cut-off: The cut-off grade is the arbitrarily defined lowest grade which will be mined from an ore ore deposit, and usually defines the boundary of the deposit. For example, if the average grade of a porphyry deposit is 0.5% Cu, the cut-off might be 0.2% Cu. any rock with a grade below 0.2% Cu would be waste.

Reserves: The amount of ore in a given deposit, usually quoted as the number of tonnes available at a specific average grade.

Host rock: The rock within which the ore deposit occurs.

Country rock: The rock which surrounds the ore deposit. Also referred to as wall rock , in particular that rock on either side of a vein.

Hydrothermal: Hot fluids, usually mainly water, sometimes acidic which may carry metals and other compounds in solution to the site of ore deposition or wall rock alteration.

Alteration: A change in the mineralogy of the country rock as a result of a chemical reaction with hydrothermal solutions. For example, mafic minerals such as hornblende or biotite may alter to chlorite and feldspars may alter to clay. An alteration zone describes rocks which have been altered to a specific group of secondary or alteration minerals, usually around the perimeter of a mineral deposit.Vein: A tabular deposit usually formed by deposition of ore and gangue minerals in open spaces within a fault or other structural environment.Replacement: A chemical process whereby hydrothermal fluids, passing through permeable rocks, react with the rocks to dissolve original minerals and relace them with ore and/or other gangue minerals.Massive sulphide: A stratiform (see below), usually lens-shaped mineral deposit consisting of at least 60% sulphide minerals.Skarn: A replacement of limestone (calcium carbonate) or other carbonate-rich rocks adjacent to an intrusive contact by calc-silicate minerals usually through the addition of Si and other elements.

Epigenetic: Mineralization which has been deposited later than its immediate host rocks, for example a vein. The ore is younger than the host rocks.

Syngenetic: Mineralization which has been deposited simultaneously with its host rocks, for example placer deposits. The ore is the same age as the host rocks.

Gossan: A rusty, surficial weathering zone which is caused by the oxidation of pyrite to produce secondary iron oxide minerals. Since pyrite is often associated with ore deposits, gossans can be a guide to ore. Reserves : Quantity of a resource that has been found and can be recovered economically using existing technology. Reserves increase as geological knowledge increases, making it possible to find more mineral deposits. Reserves also increase due to improved technology that allows minerals to be produced from previously uneconomic ores. The volume of reserves will always be less than the volume of resources. For example:

world resources of silver total approximately 2 million million tons in the upper 1 km of the earth's crust

Silver occurs with an average concentration of 0.07 ppm (ppm = parts per million; 0.07 ppm = 1 part silver for 14,285,714 parts of the crust)

Worldwide, we use about 10,000 tons of silver annually.

At first glance it might seem that we have more silver than we can ever use but the big number is resources, not reserves.

Silver reserves are estimated as 200,000 tons

Consequently, we currently have about a 20 year supply of silver on hand.

However, there is little chance that we will run out of silver after 20 years. Mining companies will find more silver deposits and prices will inevitably rise (limiting demand) as stocks dwindle. It is unlikely that we will exhaust the reserves of any key mineral for another 100 years.

Concentration Factor (CF) :The increase in the concentration of an economic mineral required to generate a metallic ore. The average concentration of minerals in the crust is insufficient to form an ore. Various geological processes concentrate minerals within the crust. The concentration factor necessary to generate an economic mineral deposit can be readily determined by dividing the economic concentration by the average concentration of the mineral in the crust. For example:

Aluminum is one of the most common elements in crustal rocks. On average, it makes up 8% of rock

An economic ore of Aluminum must have a concentration of ~35%

Concentration factor is 35/8 = 4.4.

Rare minerals have large CF values, e.g. gold, CF =2,500, whereas common elements have low CF values, e.g. silicon, CF = 2.

MORPHOLOGY OF MINERAL DEPOSITS

Concordant: Any geologic body, such as an ore deposit, which lies within or parallel to volcanic or sedimentary bedding and does not cut across the bedding structures. (Also conformable ).

Discordant: A geologic body, such as a dike or vein, which cuts across primary rock structures such as bedding.

Stratiform: A mineral deposit which occurs as a specific stratigraphic (or sedimentary) bed.

Stratabound: A mineral deposit which occurs within a specific stratigraphic bed or horizon, but which does not comprise the entire bed.

Footwall : The lower contact of an inclined vein, or the wall rock which lies on the lower side of a dipping vein.

Hangingwall: The upper contact of an inclined vein.

Fault: A planar feature or fracture zone along which displacement has occurred.

Shear zone: A planar zone of weakness, similar to a fault, but consisting of several parallel displacement zones usually over a greater width than a single fault.

Lode, shoot: All refer to mineralized zones within a fault or shear zone or a vein fissure, stringer structure.

Breccia: Angular fragments of rock produced by movement along a fault or explosive igneous activity. The material which surrounds the fragments and cements them together is called matrix and might be vein minerals, igneous material or very fine rock fragments.

Stockwork: A large number of small, closely spaced veins, often with many different orientations, is referred to as a stockwork and sometimes as a stringer zone.

Chimney : Also referred to as a pipe , this is a vertically oriented, cylindrical body, often a breccia, of vein or replacement mineralization.

Manto: This is a horizontally oriented chimney-like structure, usually of replacement mineralization.Reference: http://www.bc-mining-house.com/prospecting_school/dp_intr1.htm

GEOL.363 MINERAL DEPOSITS

Hand out No. 2

(Semester 1/2001)

MODES OF MINERAL DEPOSITION

Syngenetic - A deposit formed at the same time as the rocks in which it occurs. Ex. Banded Iron FormationEpigenetic - A deposit introduced into the host rocks at some time after they were deposited. Ex. Mississippi Valley-type Deposits

Current theories of ore deposit genesis can be divided into internal and external or surface processes. Keep in mind more than one mechanism may be responsible for the formation of an ore body. Example stockwork porphyry copper deposit at depth (epigenetic) with a syngenetic massive sulfide deposit at the surface. The Table at the end of the document summarizes the principal theories of ore genesis

Magmatic Segregation - Those deposits, not including pegmatites that have formed by direct crystallization from a magma. Two types:

Fractional crystallization - Any process whereby early formed crystals can not re-equilibrate with the melt. Includes 1) gravitative settling; 2) flowage differentiation; 3) filter pressing and 4) dilation. Number 1 is the most important and results from the settling of early formed crystals to the bottom of the magma chamber. Rocks formed in this manner are termed cumulates and are often characterized by rhythmic layering. In ore deposits the alternating layers are often magnetite and/or chromite between layers of silicate. Ex. Bushveld igneous complex. Immiscible liquid - Typical example is oil and water. In ore deposits we deal with silicate and sulfide magmas. As a magma cools sulfides coalesce as droplets and due to higher density settle out. Most common sulfides are iron sulfides, but nickel, copper and platinum also occur. Ex. Sudbury, Canada. The settling out of the heavier sulfides results in the peculiar net-textures ores often found in many of these deposits.

The billiard ball model is often used to explain the ore texture found in immiscible liquid ores (Figure). Imagine a beaker partly filled with billiard balls and water (A). These represent olivine grains and interstitial liquid. Mercury is added to represent the immiscible sulfide liquid. This will sink to the bottom and the balls will tend to float on it. However, the lower balls will be forced down into the mercury by the weight of the overlying ones. If the contents of the beaker are frozen before all the mercury percolates to the bottom then a situation such as (B) results. This is exactly analogous to the massive ore (mercury), net textured ore (mercury and billiard balls) and disseminated ore (some mercury, billiard balls and water) zones of these deposits.

a. Pegmatitic Deposition - Very coarse grained igneous rocks. Commonly form dike-like masses a few meters to occasionally 1-2 km in length. Economic ore deposits are associated with granitic pegmatites since felsic magmas carry more water. Residual elements such as Li, Be Nb, Ta, Sn and U that are not readily accommodated in crystallizing silicate phases end up in the volatile fraction. When this fraction is injected into the country rock a pegmatite is formed. Temperatures of deposition vary from 250-750 C. Pegmatites are divided into simple and complex. Simple consist of plagioclase, quartz and mica and are not zoned. Complex have a more varied mineralogy and are strongly zoned. Crystals in pegmatites can be large, exceeding several meters. Three hypotheses to explain their formation:

b. fractional crystallization

c. deposition along open channels from fluids of changing composition

d. crystallization of a simple pegmatite and partial to complete hydrothermal replacement

Hydrothermal Processes - Hot aqueous solutions are responsible for the formation of many ore deposits. Fluid inclusion research indicates most ore forming fluids range in temperature from 50C to 450C. Analysis of the fluid in inclusions has shown that water is the most important phase and salinities are often much greater than those of seawater. The chemistry of ore fluids and the mechanism of deposition of ore minerals remains a subject of hot debate. Arguments boil down to a) source and nature of the solutions b) means of transport of the metals and c) mechanism of deposition.a. Sources of the solutions and their contents - Earliest thought was that hydrothermal fluids represented the low temperature residual fluids left over from the crystallization of a magma. These fluids would contain the metallic elements which are not accommodated in silicate minerals. The solution were assumed to move upward along available channelways to sites of deposition. The main problem with this theory was that many ore deposits had no close spatial relationship to igneous rocks of appropriate age to supply the ore fluids. To overcome this problem we relied on ore fluids often traveling impossibly long distances or emanating from igneous intrusives beneath the level of subsurface mapping. The high salinities of fluid inclusions and studies of oil field brines has led to a more recent theory which favors connate brines trapped in sedimentary rocks as the source for the fluids. The fluids are driven out of the rocks during compaction and migrate up-dip in response to fluid/rock density differences. Research has shown that a typical sedimentary basin with 9 km of sediments will reach temperatures in excess of 300C. The hot brines expelled from theses basins then leach metals and perhaps sulfur from the rocks through which they pass ultimately precipitating their metallic component in near surface settings.

b. Means of transport - Sulfides have very low solubilities in water. Example amount of zinc in equilibrium with ZnS in a saturated solution at pH 5 and 100C (reasonable conditions) is about 10-5 g/l. To form an ore body of one million tons zinc would require 1017 liters of solution passing through the ore body. This requires a volume of water 10,000 km2 X 10km high (clearly an impossible situation). Figure further illustrates the problem. In order to have enough Pb and H2S to form an ore body requires a pH of 3 or less. This is highly unlikely in any rocks. Fluid inclusions and theoretical geochemistry suggest the pH of ore solutions is probably near neutral. Thus ionic solubility does not appear to be a viable mechanism of metal transport.

1. Complexes

2. S2- - would form complex sulfide ions like HgS22-. However, for to be a dominant anion requires a pH of 11 or greater. This is not possible under normal geologic conditions.

3. HS- - bisulfide complex is an important constituent of near neutral solutions which contain abundant H2S. It can form complexes such as PbS(HS)-. This can increase the solubility of galena by 10 to lOOX. Unfortunately, it also requires a high concentration of H2S and HS- to remain stable, much higher than that found in fluid inclusions.

4. Chloride - favored due to the high salinities of most ore fluids. Typical chloride complex would be AgCl2-.

5. Metal-organic - have not been extensively studied, but may be important for some ore deposits.

The transport of sulfur is also a problem. If we assume chloride complexes are the important transporting agent we must also assume S(r) must be less than 105M to transport sufficient metal. Thus S must be supplied at the site of deposition or abundant sulfate must be reduced at the site of deposition.

1. Two possible mechanisms of sulfide deposition

2. sulfate reduction

3. carrying reduced sulfur with the metals and precipitating ores in response to cooling and/or pH changes

1. Anderson Figure (see diagram) summarizes many of the problems:

2. only solutions above and to the left of the 10-5 Pb contour contain enough lead to make ore

3. 10-5 sulfur contour is the boundary below which sufficient reduced sulfur is present to make ore

4. ore forming area on the left side of the diagram is not realistic. Diagram appears to show that reduced sulfur must be added at the site of deposition.

Lateral Secretion - Formation of ores by leaching of metals and SiO2 from host rocks adjacent to open fractures. Difficult to distinguish from basement-derived hydrothermal fluids. However, geochemical patterns should be different (Figure). In Case A quartz is added both to the vein and the surrounding host rock and as such must be derived from a distant source (hydrothermal). In Case B silica has obviously been leached from the wall rocks adjacent to the vein and lateral secretion is a viable genetic model.

Case history Yellowknife Goldfields - Deposits occur in quartz-carbonate lenses in chloritic shear zones cutting amphibolites. Principle minerals are quartz, carbonates, sericite, pyrite, arsenopyrite, stibnite, chalcopyrite, pyrrhotite, sphalerite, galena and native gold. Alteration haloes of carbonate-sericite and chlorite-carbonate occur in the host rocks adjacent to the deposits. Quartz is the most important mineral. Figure shows that substantial quartz has been subtracted from the alteration haloes. Some subtraction of Fe, Mg, Ca, Ti and Mn has also occurred and this is doubtless the source for the ore minerals py-cp-po. Alumina is depleted in the outer halo and concentrated in the inner halo in the form of sericite. C02 and H20 seem to have been added from outside sources. Calculations indicate that the sheared amphibolites have 20X the concentration of metals necessary to produce all the ore in the deposit. Thus, in this case lateral secretion is a viable model.

Volcanic Exhalative Processes

Ores often, show spatial relationships to volcanic rocks. They are conformable with the host and frequently banded suggesting sedimentary processes. Principal constituent is pyrite with lesser cp, sp, gn, barite and Ag-Au. Thought until the late 60s to be epigenetic, but now realized they are syngenetic. Show a progression of types with three distinct end members:1. Cyprus type - Associated with mafic volcanics and ophiolite sequences. Found in spreading centers and back arc basins. Consist predominantly of pyrite with lesser chalcopyrite. Typified by the Cyprus pyrite-cu ores.

2. Besshi type - Associated with basaltic to dacitic volcanism. Thought to form during the initial stages of island arc formation. Many Besshi type deposits occur in Precambrian rocks and these may have been generated in entirely different tectonic settings. Pyrite dominant, but chalcopyrite and sphalerite very common. Typified by many of the volcanogenic deposits of Canada.

3. Kuroko type - Associated with dacitic to rhyolitic volcanics. Form during the waning stages of island arc volcanism. Pyrite occurs, but is not dominant Usually galena or sphalerite are predominate with lesser chalcopyrite and tetrahedrite. Also significant silver in this type. Typified by the Kuroko deposits.

Although it is agreed ores are associated with volcanism the source of the ore bearing solutions continues to be debated. Many feel ore fluids are of magmatic origin, but others feel they are merely convecting seawater. Stable isotopes have provided the best evidence to support the convecting seawater hypothesis. As can be seen in the Figure the Cyprus deposits have O/H isotopic values which plot exactly on the seawater field. Kuroko ores show some deuterium depletion relative to seawater, but still appear to have had a dominant seawater component. Figure shows the model suggested from the isotopic studies.Metamorphic Processes

1. Pyrometasomatic deposits (skarns) developed at the contact of plutons and host rock. Generally, host rock is a carbonate and new minerals formed are the calc-silicates diopside, andradite and wollastonite. Ts involved thought to be 300-500C, but pressure probably quite low. Three stage process:

2. Recrystallization

3. Introduction of Si, Al, Fe, Mg

4. Hydration and introduction of elements associated with volatile fraction

Other metamorphic processes are relatively unimportant, but hydration/dehydration during regional metamorphism may concentrate metals at the metamorphic front. Sodic metasomatism of K-spar is thought to have been important in the concentration of gold at Kalgoorlie. Conversion of feldspar from K-spar (1.33A) to Na plag (.97A) resulted in the expulsion of gold (1.37A) which could no longer be accommodated in the feldspar lattice. Skip over Mechanical-chemical sedimentary processes since they are covered in the other course.References: http://geology.csupomona.edu/drjessey/class/GSC433/Genesis.htm

PRIVATETHEORIES OF ORE DEPOSIT GENESIS

A: Origin Due to Internal Processes

Magmatic SegregationSeparation of ore minerals by fractional crystallization during magmatic differentiation.PtCr depositsBushveld, S.A.Titanium depositTahawas, N.Y.

Liquid immiscibility. Settling out from magmas of sulfide, sulfide-oxide or oxide melts which accumulate beneath the silicates or are injected into country rocks or extruded on the surface.Cu-Ni ores of Sudbury, Canada and the nickel extrusives of Kambalda, West Australia.

Pegmatitic DepositionCrystallization as disseminated grains or segregations in pegmatites.Li-bearing pegmatites of Kings Mtn. N.C.

HydrothermalDeposition from hot aqueous solutions of various sources.Porphyry Cu-Mo deposits of theW. Cordillera.

Lateral Secretion Diffusion of ore and gangue forming materialsfrom the country rocks into faults and other structures.Gold deposits of Yellowknife, B.C. and the Mother Lode, CA.

Metamorphic ProcessesPyrometasomatic (skarn) deposits formed by replacement of wall rocks adjacent to an intrusive.W deposits at Bishop, CA. Fe depositsIron Mtn UT.

Initial or further concentration of ore elements by metamorphic processes.Homestake Au Mine, Lead, South Dakota.

B: Origin Due to Surface Processes

Mechanical AccumulationConcentration of heavy minerals into placerPlacer Au deposits of Alaska and California.

Sedimentary PrecipitationPrecipitation of certain elements in sedimentary environments.Banded Iron Fm.of the CanadianShield.

Residual Processes Leaching of soluble elements leaving concentrations of insoluble elements.Nickel lateritesof New Caledoniaand Arkansas bauxite.

Secondary or Supergene EnrichmentLeaching of certain elements from the upper part of a mineral deposit and their reprecipitation at depth to produce higher concentrations.The upper portion of many porphyry copperdeposits.

Volcanic Exhalative ProcessExhalations of sulfide-rich magmas at the surface, usually under marine conditions.Mt. Isa, Aust., Sullivan and Kidd Creek,Canada, Kuroko,Japan.

GEOL.363 MINERAL DEPOSITS

Hand out No. 3

(Semester 1/2001)

CLASSIFICATION OF MINERAL DEPOSITS In order to more readily study mineral deposits and explore for them more effectively, it is helpful to first subdivide them into categories. This subdivision, or classification, can be based on a number of criteria, such as minerals or metals contained, the shape or size of the deposit, host rocks (the rocks which enclose or contain the deposit) or the genesis of the deposit (the geological processes which combined to form the deposit).Since there is considerable debate among geologists as to the exact mode of formation (genesis) of most mineral deposits, this is not a good classification criterion. It is best to stick to features we can all agree on, namely, the physical description of the deposit. We soon see that, even though no two mineral deposits are exactly alike, most of them fall into one or another of a small number of categories. We also see that each of these categories coincides with a generally accepted hypothesis as to how the mineral deposits formed. In other words, although we started out with a physically descriptive classification, we end up with a classification which also coincides with what we perceive to be unique genetic processes. It is therefore useful to define a small number of terms used in the classification which have a genetic connotation:

HydrothermalHot water or hydrothermal solutions have actually been observed forming mineral deposits, for example, the "black smokers" on the sea floor. The ore constituents, such as Cu, Pb, Au or other metals are dissolved in a hot aqueous solution along with other deposit constituents such as Si, S and Fe. These elements are deposited to form the ore and gangue minerals in response to a change in the solution, very often a sharp decrease in temperature. an example of this process would be if you dissolved as much table salt as possible in boiling water. If you then cool the solution in the fridge, much of the salt will precipitate or come out of solution.

MagmaticSome mineral deposits, particularly those containing Ni, Cr and Pt, form by the separation of the metal sulphide or oxides in the molten form, within an igneous melt before it crystallizes. These are known as magmatic deposits. They occur within the igneous rock from which they were derived, such as a gabbro. The ore metals concentrated as liquid in much the same manner as metals are purified in a smelter or blast furnace. The heavier metal-rich liquids sink and concentrate at the base of the intrusive body, while lighter silicate liquid and crystals tend to rise, the same as the slag in a blast furnace.

SyngeneticA syngenetic mineral deposit is a deposit which formed at the same time as the rocks that enclose it. Magmatic deposits are syngenetic in that the ore minerals crystallize from the same liquid that produces the silicate minerals which form the bulk of the intrusive - they crystallize more or less simultaneously as the melt cools. Deposits which form on the earth's surface in the form of a sedimentary layer are also syngenetic. The rocks which they lie upon were deposited just prior to the mineralizing event, while the overlying rocks were deposited just after - all three layers being deposited at essentially the same time in terms of the geological time frame.

EpigeneticIf a mineral deposit formed much later than the rocks which enclose it, it is said to be epigenetic. An example is a vein. The first step in the formation of a vein is the fracturing or breaking of rock along a fault zone, at a depth ranging from surface to several kilometers below surface. The rock must be solid (lithified) and brittle, creating open spaces when it breaks. Hydrothermal solutions pass along the fault zone and deposit or precipitate the ore and gangue minerals within the open spaces. Thus, the vein is necessarily younger than the rocks that contain it.

Since we are fairly certain which deposits are syngenetic and which are epigenetic (although there will always be some degree of uncertainty and overlap), it is convenient to begin the classification with this discrimination. Beyond this, the various categories are based on their physical description, including size and shape. A third level of subdivision is usually based on the metals contained. Here, then, is the classification:

1. EpigeneticA. 1. Porphyry

Large, low grade deposits usually associated with a porphyritic intrusive body.

B. Cu-MoB. Cu (-Au)C. Mo (-W)2. SkarnMineral deposits formed by replacement of limestone by ore and calc-silicate minerals, usually adjacent to a felsic or granitic intrusive body.

A. W-Cu (-Zn, -Mo)B. Zn-Pb-Ag (-Cu, -W)C. Cu (-Fe, -Au, -Ag, - Mo)D. Fe (-Cu, - Au)E. Sn (-Cu, -W, -Zn)F. Au (-As, -Cu)

3. VeinA. Fracture filling deposits which often have great lateral and/or depth extent but which are usually very narrow.

B. Hypothermal - Cu (-Au)B. Mesothermal - Cu-Pb-Zn-Ag-AuC. Epithermal - Au-Ag (-Hg)4. Mississippi ValleyNamed for the region where they were first described, these deposits formed within porous carbonate rocks (limestone reefs or caves). They are Pb-Zn deposits with low Ag values.

2. Syngenetic1. Volcanic Massive Sulphide (VMS)A. These deposits formed as massive (over 60% sulphide) lens-like accumulations on or near the sea floor in association with volcanic activity.

B. Felsic volcanic hosted - Cu-Pb-Zn-Ag-AuB. Mafic volcanic hosted - Cu (-Zn, -Au)C. Mixed volcanic/sedimentary - Cu-Zn (-Au)2. Sedimentary Massive Sulphide (Sedex)These are formed by hydrothermal emanations on or near the sea floor in association with the deposition of sedimentary rocks.

A. Pb-Zn-AgB. Ba

3. MagmaticA. During the crystallization of a magma, usually mafic or ultramafic, heavy, metal-rich liquids settle and accumulate at specific sites, often at the base, within the intrusion.

B. PGM (Platinum group metals)B. ChromiteC. Ni-Cu (-PGM)4. PlacerFormed within sediments by the concentration of heavy resistant minerals (Au diamond, cassiterite) by stream or wave action.

REFERENCESMcKinstry, H.E., 1948,Mining Geology: Englewood Cliffs, NJ, Prentice Hall, 680 p.Peters, William C., 1978, Exploration and Mining Geology: John Wiley & Sons Inc., 696 p.Guilbert, John M. and Park Jr., Charles F., 1986, The Geology of Ore Deposits: W.H. Freeman and Company, 985 p.Evan, Anthony M., 1993, Third Edition, Ore Geology and Industrial Minerals-An Introduction: Blackwell Scientific Publications Inc., 389 p.Eckstrand, O.R., 1984, Canadian Mineral Deposit Types: A Geological Synopsis, Geological Survey of Canada, Economic Geology Report 36: Canadian Government Publishing Centre, 86 p.Roberts, R.G. and Sheahan, P.A., 1988, Ore Deposit Models, Geoscience Canada, Reprint Series 3: Geological Association of Canada, 194 p.

Reference: http://www.bc-mining-house.com/prospecting_school/dp_intr2.htm

GEOL.363 MINERAL DEPOSITS

Hand out No. 4

(Semester 1/2001)PLATE TECTONICS AND MINERALIZATION

Many of the processes for mineral concentration that were described in the previous section depend upon the presence of magma bodies underground. In Unit 3, Plate Tectonics, you learned that magma often results from tectonic activity near plate boundaries, and so it should seem reasonable that there are links between the occurrence of mineral deposits and the plate tectonic history of a region. In particular, magma generation is associated with oceanic spreading ridges and with subduction zones. Let us examine the deposits that are likely to result from each of these settings.1. Mineral Deposits at the Oceanic RidgesWe have already discussed the process by which hydrothermal fluids can concentrate metal sulfide deposits at the oceanic ridges. Ocean water circulating through cracks in the new basaltic sea floor created at spreading ridges is heated by contact with hot rocks. Metals are dissolved in the saline water and precipitate near the discharge points of the hot springs when they combine with hydrogen sulfide to form metal sulfides. In this manner copper, iron, lead, and zinc sulfide minerals precipitate from solution. When the mineralized water encounters and mixes with cold ocean water, oxides of iron and manganese may precipitate.At the sites along the East Pacific Rise where the submersible Alvin was used to investigate the spectacular hot spring vents described in Unit 6, Dynamics of the Oceans, the black smoker chimneys were found to consist largely of metal sulfides. Chimneys have been discovered 15 meters (50 ft) tall resting on a metal sulfide mound of possibly equal height and up to 30 meters (100 ft) across. Such chimney-mound structures may weigh several thousand tons and one that was sampled was found to contain 14% iron, 0.7% copper, and 31% zinc, along with some cobalt, silver, and gold.Only a small portion of the oceanic ridge system has been investigated in detail to date, but on the basis of present evidence, it seems that the extremely active chimney-vent type of activity may be restricted to fast-spreading ridges such as the East Pacific Rise, where new sea floor is being added at a rate of about 18 cm (8 in) per year. This type of vigorous sea floor spreading activity is reflected in the hydrothermal regime present at the ridge crests. Sea-floor spreading rates are much slower in other parts of the world, and this seems to influence the style of hydrothermal activity as well. For instance, at sites investigated on the Mid-Atlantic Ridge, where the spreading rate is on the order of two to three centimeters (one inch) per year, surface deposits consist of nearly pure manganese oxide encrusting the ocean floor. Where deeper layers of the sea floor are exposed, metallic sulfide deposits are found, indicating that the hydrothermal fluids have had a chance to mix thoroughly with ocean water before emerging from cracks in the walls of the central rift valley at the crest of the ridge. Still another type of spreading boundary environment is encountered in the Red Sea mineral deposits. The Red Sea is a new ocean, formed by the rifting of the Arabian peninsula away from Africa. Its deposits were discovered in the 1960s during an international oceanographic expedition when echo sounders on the research vessels recorded an unusual reflection within the ocean water some distance above the seafloor. When the water was sampled at that depth, very high salinities were found, indicating that the sonar reflection marked a boundary between normal seawater and denser salt brine that was warm and rich in metals. The brine was collected in a series of pools located along the axis of the Red Sea. When cores were taken of the sea floor sediments, it was found that they contained layers of metal-rich sediment ten meters or more thick. The Red Sea deposits are rich in iron, copper, zinc, and small amounts of silver and gold. It is estimated that the largest of these pools, the Atlantis II Deep, contains three million tons of metals, not counting the iron minerals. The two countries flanking that part of the Red Sea, Saudi Arabia and Sudan, have formed a joint commission to study the feasibility of mining this deposit. If the venture appears to be profitable, the Red Sea may be the site for the first commercial application of deep-sea mining. The technique most likely to be used is a dredge combined with a powerful suction device controlled from a sea-surface vessel. The device would scoop up loose sediment from the ocean floor and pump it up as a slurry to a ship on the surface. In a few places, bits of oceanic crust have been scraped off the lithosphere at a subduction zone and added to continental crust, making them easily accessible. These hunks of displaced ocean floor are called ophiolites and have been mined for their stores of copper and other metals since antiquity. The best-known example is the Troodos Massif on the island of Cyprus in the Mediterranean Sea.2. Mineral Deposits Related to Subduction ZonesImportant occurrences of copper ore are found in what are called porphyry copper deposits. These form by magmatic concentration with hydrothermal alteration and replacement in magma intrusions into continental crust. Figure 11-4 shows the worldwide distribution of regions containing major porphyry copper deposits as shaded zones. Note that many occur in close association with subduction zones, such as those in South America, the Philippines, and the Middle East. In other cases, such as in western North America, eastern Australia, and in the Ural Mountains of Russia, the deposits possibly were associated with subduction that took place in the past.Recall that in a subduction zone, oceanic crust on the downgoing slab is heated, melted, and chemically differentiated, producing relatively felsic magma that is supplied to the volcanic arc above the subduction zone. The subduction process not only supplies the magma intrusions to fuel the hydrothermal process, but because the subducted ocean floor was already enriched in metal deposits by the processes described previously, it taps a source that has an abundant supply of metals. Porphyry copper deposits tend to be large and relatively low grade, containing from 0.2% to 2% copper. Fortunately, the refining process is relatively easy, and because of the size and grade of the deposits, they are often profitable to mine. A single porphyry deposit may contain up to several million tons of copper, though most are considerably smaller. Porphyry Copper Deposits in Relation to Plate BoundariesIn South America, the subduction zone associated with the Peru-Chile Trench is hard against the continental coastline, while in many places along the northwest margin of the Pacific Ocean, Back-Arc Spreading Shown in Relation to a Subduction Zonesubduction takes place some distance offshore from the continent of Asia, forming a volcanic island arc separated from the main continental landmass by an oceanic basin in which slow sea-floor spreading may be taking place. The Japan Sea is an example in which spreading has widened the separation of the Japanese islands from China. Figure 11-5 shows the relation between this spreading basin and the subduction zone.A number of different mineral deposits are associated with back-arc spreading, resulting from similar concentrating mechanisms to those active on the other oceanic spreading ridges. Chief among these are the Kuroko-type massive sulfide deposits found in northern Japan and elsewhere that contain copper, zinc, lead, gold, and silver concentrations. Although smaller than the porphyry copper deposits, these are often significantly higher-grade ores and are of economic value.

In some places, the extensional or pulling-apart forces that generate back-arc basins can operate on a continent as well, so long as active subduction takes place nearby. This forms continental rift systems much like the oceanic rifts, but within the continent. Where these rifts are flooded, either with inflow of seawater or as lakes, like the great lake system of the East African Rift, subaqueous hydrothermal activity may form other types of lead-zinc-silver-rich deposits in a manner like that described for the Red Sea. The western part of the United States was the scene of continuous subduction of Pacific Ocean floor along the west coast until about 26 million years ago. Continental rifting occurred inland from the subduction, similar to back-arc spreading and, where this was accompanied by certain types of igneous intrusions, resulted in hydrothermal emplacement of large molybdenum deposits in the front ranges of Colorado. These give the United States its dominant position in reserves of this important steel-making element, as shown in Figure 11-3.

F. SOME NOTABLE MINERAL DEPOSITSThis is a good time for us to put together the various concepts discussed so far in terms of the origin and setting for different kinds of deposits.

1. The Bushveld Igneous Complex, South AfricaThe Bushveld igneous complex in South Africa is magmatic in origin and consists of a series of layered igneous rocks that contain vast reserves of chromium along with platinum, nickel, and iron. It is the largest single deposit of chromium ore found anywhere in the world and has been estimated to contain reserves in the range of 6,000 million tons of chromite. The Bushveld complex is a good example of magmatic concentration as described earlier, in which chromite (chromium ore) accumulates toward the bottom of a layered magma intrusion due to its high density and tendency to crystallize before the rest of the magma. The metals are concentrated in a series of relatively thin layers ranging in thickness from centimeters to as much as one meter. They extend over an area of many thousands of square kilometers, however, accounting for large reserves. Radioisotope dating of the Bushveld complex yields ages on the order of 2 billion years, placing its origin nearly half way between the origin of Earth and the present. The complex occurs as a series of nearly circular igneous intrusions that are relatively isolated in that the surrounding rock is much older and essentially undisturbed. It does not appear to be associated with either subduction or sea-floor spreading and so its origin is still a bit of a mystery, although an ancient form of continental rifting may have been involved. Two interesting hypotheses have been advanced -- one is that the intrusion is due to hot spot activity, and the other is that the circular igneous complexes resulted from the impacts of fragments of an asteroid.

2. The Troodos Massif in CyprusThe Troodos Massif in the western part of the Mediterranean island of Cyprus contains the Troodos ophiolite segment of ocean floor produced by sea-floor spreading in Cretaceous time. It contains massive sulfide deposits of iron, copper, zinc, and cobalt that were concentrated by sea-floor hydrothermal activity at an ocean ridge, as well as magmatic nickel sulfide and chromium deposits that were formed in deeper intrusive rocks at the ridge. The copper deposits on Cyprus have been worked since antiquity, and indeed the metal derives its name from this locality, having been called the "Cyprian metal." Ophiolites around the world have their origins in the ocean floor and in addition to their mineral deposits are of interest to geologists because they provide dry-land exposures where the structure of what was once oceanic crust may be studied with relative ease.

3. Kuroko Massive Sulfide Deposits, JapanWe have already mentioned the Kuroko-type deposits in the previous section, relating their origin to back-arc sea-floor spreading. The Kuroko massive sulfide deposits are found in the northern portion of the main Japanese island of Honshu and contain zinc, copper, and lead along with minor amounts of precious metals. The concentrating mechanism was hydrothermal activity on the sea floor approximately 12 to 15 million years ago, making this a relatively young deposit, but there are many similar, much older deposits in more ancient rocks in other parts of the world.

4. The Noranda District of Quebec, CanadaThe Horne Mine and others at Noranda in northern Quebec is an example of a layered massive sulfide deposit of considerably greater age, dating back to the early Precambrian. It is a classic example of a submarine volcanic process forming layered sulfide deposits over the hot springs on the ocean floor, and is similar to the Kuroko deposits of Japan. It produces copper, zinc, gold, and silver.5. Uranium Deposits in Saskatchwan, CanadaUranium deposits are generally found in sedimentary environments and are of several types, but their exact origin is still a matter of debate. Some geologists believe that the concentration mechanism is chemical in nature, due to circulating groundwater that collects dispersed uranium from the sediments and deposits it in favorable settings.

G. SEARCHING FOR MINERALSOur greatly improved knowledge of how mineral deposits form has strongly affected the approach used in locating them. If we return to our grizzled prospector with his pan and mule, we can gain some perspective on how far we have come. For the most part, these hardy fortune-seekers had limited knowledge of geology or mineralogy. They were simply looking for a particular type or color of rock. Not that they expected gold or other mineral deposits to appear so obviously, but they knew that many of the minerals that they sought were to be found in rocks that had been hydrothermally altered and were rich in sulfides. The prospector, then, was looking for an environment favorable for mineralization, even though he knew little of the mineral concentration mechanisms. Modern prospecting is involved in similar searches, but in a more sophisticated manner. An important first step is in the production of geologic maps of a region, which identifies the rock types at the surface (or outcrop) in each area of the map. The geologic structure of an area can provide important clues as to where mineralization may occur. Faults and fracture zones in the rock can provide pathways for hydrothermal fluids and such linear features are often found in association with mineralized zones. But these are all surface features, and many mineral deposits are covered by unmineralized rocks or surface debris. How may these be found? Dense or magnetic mineral deposits may be located by using gravity or magnetic surveys, in which the Earth's gravity and magnetic fields are measured and mapped to high precision. For example, if the gravity field shows an anomalously high value over a portion of ground, this probably indicates that dense rock is buried below. Geochemical surveys provide other techniques for literally "sniffing out" buried minerals. The concentration mechanisms that we discussed may tend to act in a somewhat diffuse manner, placing most of the deposit in one restricted location, but spreading smaller amounts of the mineral throughout the rock layers over a much wider area. Leaching of minerals by groundwater subsequent to the formation of a deposit may also serve to provide widespread indications of its presence. Samples of rock and soil are chemically analyzed and examined for traces of compounds that indicate the presence of particular minerals. Even vegetation may yield important clues, since trace minerals in the ground may be taken up by the plant's roots and incorporated into wood and leaves. These analyses are usually carried out in sophisticated chemical laboratories, but new technology has provided portable and even airborne instruments that allow analyses to be done on the spot, with greater efficiency and less cost.

1. Mineral ExplorationThe use of genetic models-which relate the geologic processes such as have been discussed to the specific geologic environment that is most likely to host the particular kind of ore body being sought-is an important first step in the exploration process. Choosing this geologic environment -- "getting in close" -- is the "art" of exploration, the successful practice of which requires a broad knowledge of the science and of the ore deposits, the real-life ore models. Once the geologist has "gotten in close," largely by the use of general geologic principles, augmented by fieldwork, the target commonly can be narrowed by geochemical and geophysical surveys. Although a geochemical anomaly may be a direct reflection of a buried deposit, a geophysical anomaly is only an indirect reflection of mineral deposits. Geophysical methods, by and large, measure electrical responses and magnetic properties and responses that are not specific to a given mineral or ore. Some of the metallic mineral deposits have conductive properties that, when excited by an induced current, generate a magnetic field which can be recorded by sophisticated instrumentation. A number of electrical-magnetic prospecting methods and instruments are in wide use, both as ground and airborne systems, and many hidden massive sulfide ore bodies have been discovered around the world by their use. The natural radioactivity of rocks can sometimes be more valuable a clue to mineral concentration than those of the more "classical" physical properties. Apart from uranium and thorium and the products of their radioactive decay, the only naturally occurring radioactive element of any importance in prospecting is potassium. Most rocks contain some uranium, thorium, and potassium, so radioactivity surveys, both ground and airborne, can be of great use in general geologic mapping. Such surveys, of course, are of direct use in prospecting for uranium deposits. Radioactivity sensors can also be lowered into drill holes to detect various elements and properties in such holes not otherwise directly detectable. Since none of the geochemical and geophysical tools allow the geologist to see what is below the surface, his most important exploration tool is the drill. It alone allows him to "see into the ground" by taking samples from below the surface. Since a drill hole through rock is expensive, and "sees" only the rock penetrated by the hole, the geologist must place his holes judiciously. Moreover, since the hole may narrowly miss a deposit, he must study the samples closely and he must recognize the "alteration halo" around the hidden deposit, in the same -- but more sophisticated -- way as the prospector recognized the obvious altered rocks. In summary, recent advances in scientific knowledge of how and where mineral deposits form have given today's exploration geologist the intellectual tools unavailable to both his predecessor and the prospector. These tools have allowed the geologist to narrow the search area. Theoretical and technological advances in geochemistry and geophysics have allowed him to focus the search. However, no methods yet known enable us to remotely detect a hidden ore deposit. Only the expensive drill hole can make the discovery. The single most important element of exploration, therefore, has been and remains, the drill hole. Important as they are to the existence, well-being and improvement of modern society, mineral deposits occupy much less that 1% of the Earth's surface. Since they occur only where the vagaries of geologic processes placed them, if society is to continue to depend on an ever-increasing supply of minerals for its well-being, it must carefully weigh the benefits to be gained from exploration for and extraction of minerals against the benefits from other uses or designations for the same land that would exclude these activities.2. Remote SensingSome of the most intriguing new developments in mineral exploration do not even require the presence of a geologist at the time a site is being investigated. Remote sensing, the analysis of the surface of Earth from aircraft or spacecraft, has provided another method of exploration geology. The first application of remote sensing was apparently made by Galileo, who turned his telescope on the Moon as it rose during daytime from behind a stone wall in his garden. He noted that the wall, which was illuminated by sunlight, appeared brighter than the surface of the Moon. He concluded that the lunar surface must reflect less light than terrestrial rocks and so must be made of darker material. Remote sensing, as we have already seen, was developed to high levels of sophistication during the investigation of the solar system by spacecraft carrying a wide variety of instruments. That technology has been turned back on Earth itself in a series of satellites designed to provide a new look at our own planet from a much larger perspective than the ground-based observer ever could obtain. In 1972, the first in a series of Earth-resource satellites, called LANDSAT, was launched. Three additional Landsats have followed, providing unbroken surveillance for more than a decade. The first three Landsats carried a multispectral scanner, or MSS. This is a device that scans a narrow strip of the surface beneath the satellite as it moves along its orbit. The strip is 185 km (115 mi) wide, and the scanner is able to resolve features that are only 200 meters (660 ft) or less in size. The unique feature of the MSS, however, is its ability to record the image as seen through four different filters, each admitting radiation of a particular color or wavelength range. Two of these spectral ranges are in visible light (green and red) and two are in the infrared portion of the spectrum (see Figure 7-3). The images are digitized and radioed to Earth, where they are processed and reconstructed into pictorial images. Modern satellite images can now record 255 wavelengths of radiation with spatial resolutions of 10 meters.

A more recent version of these satellites contains a more advanced version of the MSS, called a thematic mapper that uses seven spectral bands instead of four, and is capable of even higher resolution (LANDSAT 7), approximately 20 meters per pixel side. When different materials reflect sunlight, they selectively reflect more or less of each wavelength of light in a pattern that is often unique to that material. This reflectance spectrum may then be used to identify the material that is illuminated. The four (or seven) spectral bands measured by the Landsats allows this kind of analysis to be made remotely from space. You have probably seen the brightly colored images of portions of Earth from Landsat, in which the four spectral images are superposed and printed in four false colors. The false color requirement arises from the fact that infrared is invisible to the human eye, and so these images are printed in arbitrary colors that have been chosen to make them stand out against the visible light images. A variety of computer enhancements have been applied to these images in order to emphasize the differences between rock and soil types, vegetative cover, and so on. Landsat images have been used to study the shape of landscape features, geologic structures such as folds and faults, and to produce rough geologic maps of broad areas that have not yet been mapped in detail by traditional methods. The exploration geologist can use Landsat images in a variety of ways to aid in the search. For instance, mineral concentration zones may reveal themselves via the distinctive colors and reflectance spectra of the altered rock that often surrounds them. Uranium deposition in sedimentary rocks is sometimes marked by distinctive colors in the rocks. Landsat's sensitivity to the reflectance of chlorophyll can also be used to detect characteristic vegetation patterns over mineralized zones. These kinds of data have provided geologists with new tools and new perspectives for mineral exploration that can be applied worldwide and, in some cases, at relatively little cost. Another new remote sensing technique is provided by Side-Looking Airborne Radar (SLAR). Radar is microwave radiation, with the unique capability of penetrating clouds. Unlike Landsat, which is dependent upon clear weather for its observations of Earth's surface, radar can function through overcast and view the surface without interference from the weather.SLAR imagery shows the shape of the landscape so clearly that many geological structures may be clearly seen. Linear alignments of valleys, cliffs, or other structures, called lineaments, commonly betray the existence of faults and fractures within the rocks that may be sites of mineralization. Synthetic Aperature Radar (SAR) remote sensing instruments use a similar microwave radiation source. However, later additional computer processing builds a "virtual antenna" equivalent to a kilometers-long space craft. Fully processed images, such as those from the European Space Agency ERS-1 satellite, have resolutions of approximately 10 meters. Related instruments have also been flown in the Space Shuttle. These Shuttle Imaging Radars (SIR-A,B,C) have been useful in studying geological and structures worldwide.RECOMMENDED READINGFor a basic description of resources check Skinner (1976) or Park and MacDiarmid (1976). For specific deposits refer to Dixon (1979) or Kunz (1967). The geology of these deposits and their relation to plate tectonic processes is well described in Bates (1969), Brookings (1990), and Sawkins (1990). Robert L. Bates, Geology of the Industrial Rocks and Minerals, Dover, 1969.

D. G. Brookins, Mineral and Energy Deposits, Merrill, 1990. C. J. Dixon, Atlas of Economic Mineral Deposits, Cornell University Press, 1979. George F. Kunz, Gems and Precious Stones of North America, Dover, 1967.

C. F. Park Jr., and R. A. MacDiarmid, Ore Deposits, W. H. Freeman, 1975.

F. J. Sawkins, Metal Deposits in Relation to Plate Tectonics, Springer-Verlag, 1990.

Brian J. Skinner, Earth Resources, Prentice Hall, 1976.

Reference: http://mac01.eps.pitt.edu/harbbook/c_xi/chap11.html