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Post by 1dave on Jan 21, 2019 15:16:21 GMT -5
4f2- Pegmatite Deposits
Mineral deposits of the pegmatite type have formed in material that solidified at a late stage during the cooling of the magma. They occur as dikes, sills, pipes, or irregular masses, formed as a result of injection of the still liquid portion of the magma into fractures in the invaded rock or into previously solidified portions of the parent intrusive mass.
The pegmatites are composed principally of quartz, feldspars, and micas as a coarsely crystalline aggregate. A variety of minerals are often present, including magnetite, tourmaline, topaz, cassiterite, apatite, ilmenite, rutile, and beryl. Some of the sulphide minerals, like pyrite, arsenopyrite, pyrrhotite, chalcopyrite, molybdenite, bornite, and sphalerite may be present.
Gem varieties of ruby, beryl, and tourmaline are of rare occurrence. Reaction with the wall rocks has produced some minerals otherwise foreign to this type of deposit.
Although the pegmatites in Nevada contain a variety of minerals, they have, with few exceptions, been of minor economic importance. Pegmatites in the Humboldt range contain beryl and have also yielded considerable tungsten in the form of scheelite.
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Post by 1dave on Jan 21, 2019 15:28:31 GMT -5
4f3- Pyrometasomatic Deposits
Pyrometasomatic deposits are a distinct type because of the predominance of characteristic minerals and because of their position along, or near, the contact between the intrusive rock and the invaded rock (thus the common term "contact-metamorphic deposits"), and where the invaded rocks are usually of calcareous composition, like limestone, dolomite, or calcareous shale.
The deposits have formed by replacement of the adjoining rock by reaction with solutions emanating from the magma. Permeable rocks are most extensively altered. Where limestones are present silicate minerals may occur hundreds of feet from the contact. Similar silicate minerals also form along fractures and in fragmented portions of the intrusive near the contact, indicating that at least a portion of the magmatic solutions permeated the intrusive after the upper portion had solidified.
Quartz, calcite, and metallic minerals often accompany the silicates, and the bulk of these minerals usually occur on the cooler, or invaded rock, side of the contact. The characteristic gangue minerals are garnet and epidote with other silicates of calcium, magnesium, iron, and aluminum. Quartz and calcite are subordinate.
The sulphide minerals are usually of simple composition such as pyrite, chalcopyrite, pyrrhotite, sphalerite, molybdenite, arsenopyrite, and galena. The oxides of iron, magnetite and specularite, are sometimes common and abundant. Scheelite occurs in important amounts in some deposits.
The forms of the deposits are variable. Tabular, as well as irregularly spaced lenticular bodies, on or near the contact, are common. Deposits may form through the irregular replacement of certain beds and have the same attitude as the sedimentary series, like the scheelite ores at Mill City.
Contact-metamorphic deposits a r e numerous in Nevada, particularly in the western part of the State, and yield important tonnages of scheelite ore. Some of the Nevada iron deposits are also of this type.
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Post by 1dave on Jan 21, 2019 20:19:34 GMT -5
4f4- Hypothermal Deposits
Hypothermal deposits have probably been formed by cooler solutions than those that formed the pyrometasomatic deposits.
They occur both as veins and as replacements deposits, both in the intrusive and in the adjacent rocks, indicating that deposition took place after the upper part of the magma had solidified.
The mineral composition of the hypothermal deposits is rather characteristic.
Quartz predominates as a gangue mineral, and the characteristic silicate minerals are tourmaline, mica, and topaz. Magnetite and specularite are common, and the other ore minerals most often present are pyrite, arsenopyrite, chalcopyrite, cassiterite, wolframite, and gold.
The distinguishing characteristics of the deposits of this type are the predominance of coarse-grained quartz, the lack of banded structure, except where layers of wall rock are included, and the presence of relatively high temperature minerals such as tourmaline.
Many vein deposits of this type occur in Nevada, but only a few of them have been economically important. Spurr described the gold-bearing deposits of the Silver Peak district and presented evidence to support his belief that some of the deposits were formed as a direct transition from alaskite dikes to quartz veins and lenses, indicating deposition at high temperature. The ores of Rochester and Majuba Hill are of this type.
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Post by 1dave on Jan 21, 2019 21:01:30 GMT -5
4f5- Mesothermal Deposits
Mesothermal deposits have formed by deposition from solutions emanating from solidifying magma. The escaping solutions Penetrated along fissures and deposited their load. Deposition was largely confined to fissures in siliceous or aluminous rocks, while silicification and replacement occur in calcareous rocks.
Alteration of the wall rock is usually pronounced, with feldspar and ferromagnesian minerals being altered to sericite, while pyrite is introduced into the country rock.
The identifying characteristics of the mesothermal deposits are their close association to deep-seated intrusives, comparatively massive appearance of the vein filling, lack of distinct colloform structure, absence of high temperature minerals, and the presence of a variety of complex metallic minerals.
The ore minerals are of less simple composition than those in the deposits formed at higher temperatures. Sulphide, arsenide, sulpharatimonide and sulpharsenide minerals are prevalent. The common ones are pyrite, chalcopyrite, arsenopyrite, galena, sphalerite, tetrahedrite, sulphantimonides and sulpharsenides of silver, and native gold. Oxides such as magnetite and specularite may be present in small amounts.
Quartz is the predominant gangue mineral. Calcite, siderite, and barite are subordinate.
High temperature minerals, such as tourmaline, garnet, biotite, and topaz are absent.
Many of the major metal producing deposits of the world are mesothermal and occur both as veins and as replacement bodies.
Many of the veins have been notably continuous in both length and depth and occur in both the intrusive and in the adjacent rocks. The replacement deposits are often extensive, particularly where the mineralizing solution permeated easily replaceable calcareous rocks.
The mesothermal type of deposits occur throughout Nevada and have been the source of much silver, copper, lead, zinc, and gold, as at Eureka, Pioche, and Belmont.
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Post by 1dave on Jan 21, 2019 21:18:52 GMT -5
4f6- Epithermal Deposits
Epithermal deposits have formed from hot solutions which had their origin in the deep-seated reservoirs where the magma supplied the various types of flow rocks. That they were not formed by solutions emanating directly from the flows is evident from the fact that they occur in fissures that cut a whole series of flows.
Their common occurrence near the center of volcanic activity has led to the belief that the ore-forming solution rose from the same deep-seated source as did the lavas.
Epithermal deposits have formed near the surface where open fissures and cavities could exist. Deposition was not entirely confined to simple fissures, but also took place in the shattered wall rocks, and formed irregular masses. Deposition in open spaces is evidenced by thinly-banded textures indicating deposition in successive layers. The quartz is usually fine grained and often has the appearance of unglazed porcelain. Small quartz
crystals lining cavities are common.
Here again, as in the mesothermal and hypothermal deposits, quartz is usually the most abundant gangue mineral. The texture of the quartz is distinct and it seldom appears as a glassy or milky aggregate. Calcite, dolomite, barite, and fluorite are common gangue minerals and predominate in some deposits. Rhodochrosite and rhodonite are plentiful in many occurrences.
Adularia, a vein-forming feldspar, is a particularly characteristic gangue mineral and often occurs intergrown with quartz and calcite. High temperature minerals are conspicuously absent.
The enclosing rocks may be of igneous or sedimentary origin, but most of the epithermal deposits of Nevada are found where Tertiary volcanics are the predominate rocks.
Successive stages of mineralization are prominent features of these deposits. Earlier gangue minerals, such as calcite and barite, may be wholly dissolved and replaced by later quartz and adularia, however, the original texture is often preserved. Where the calcite was dissolved, but only partially replaced, a cellular structure results.
Intense alteration accompanied mineralization, developing chlorite and pyrite far out into the wall rocks. Silicification is most intense adjacent to the fissures. In some cases this silicified rock contains sufficient gold and silver to constitute an ore of these metals.
A variety of metallic minerals are usually present, but only a few of them occur in sufficient abundance to be of economic importance. Massive ore bodies are rare. Among the metals, gold and silver are of major importance, while minor amounts of copper, lead, and zinc may be present. Small quantities of arsenic, antimony, tellurium, and bismuth are found in some ore bodies.
Pyrite is not particularly abundant. The commercially valuable quicksilver ores in Nevada are usually confined to more shallow deposits of this type.
Gold is commonly present as native metal, usually combined with silver. The gold often occurs in fine particles so completely enclosed in quartz and other minerals that extremely fine grinding is required to liberate it. Gold or gold-silver tellurides are not uncommon.
The most abundant silver mineral is argentite, however, the complex silver sulphantimonides and sulpharsenides are abundant in many districts. Native silver and silver chloride (horn silver), which are often present in the near surface portion of the deposit, are due to the alteration of primary silver minerals.
Epithermal deposits are numerous and widespread in Nevada and have furnished a major portion of Nevada's gold and silver.
Some of the more notable districts of this type are the Comstock Lode, National, Tuscarora, Goldfield, and Tonopah.
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Post by 1dave on Jan 21, 2019 22:26:45 GMT -5
4f7- Influence of the parent Magmas and Local Environment
Many of the metalliferous ore deposits of economic importance have been formed through deposition from aqueous solution given off by magmas during cooling and solidification. The composition, shape, and location of these deposits are not merely the result of chance, but have been influenced, or controlled, by the combined effects of physical and chemical factors.
Recognition of the effects of these factors is necessary to the systematic search for ore deposits, not only in and about operating mines, but also in unexplored areas.
Solutions that originate from the cooling magmas or magma basins find their may upward along channel ways such as fissures, faults, sheared or crushed zones, folds, contacts, or any other places of weakness. The solutions are guided by the accessible channels. and the largest volume of solution will flow through those openings offering the least resistance, so the shape, position, and open condition of the channels are reflected in the resulting deposits.
From these solutions the various ore and gangue minerals are deposited as a result of cooling, relief of pressure, influence of wall rocks, mingling of chemically different solutions, or chemical reaction with previously deposited minerals. Of the conditions that cause mineral deposition, lowering of the temperature and a decrease of the pressure on the solutions are probably the most effective.
However, a change in acidity or alkalinity of the solution by reaction with the wall rocks is effective where the solutions contact or permeate rocks which react chemically with the solutions.
Another factor which has a profound effect on the composition of the resulting deposit is the composition of the mineralizing solutions. Only those elements contained in the solution can possibly be deposited by that solution. Since the composition of the solution is a function of its source, the parent magmas and differentiation within them effect a primary control on the composition of the mineral deposits. The ores of certain element. show a definite tendency to occur in association with particular types of rock.
Chromium. platinum, nickel, and cobalt seldom occur in commercial quantities in other than basic plutonic rocks. Tin is most often associated with granitic rocks. Most of the deposits of gold, silver, copper. lead, and zinc occur associated with intrusions, but are not limited to rocks having some particular composition.
However, the majority of Nevada deposits valuable for their gold and silver content alone occur associated with dikes and flows of Tertiary igneous rocks. Within a given area the association of certain elements with particular rocks may be so apparent that this criterion serves as a guide in the search for ore.
That certain elements occur associated with particular types of rock can be definitely shown, but this does not mean that commercial deposits have formed in association with every exposure of that rock. On the contrary, many areas apparently exhibiting all of the conditions favorable to ore deposition contain no ore deposits.
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Post by 1dave on Jan 21, 2019 22:58:21 GMT -5
4f8- Influence of Zonal Deposition
Metals are deposited in a rather definite sequence. This is reflected in the changing composition of deposits in depth. Minerals deposited at higher temperature and pressure are found at greater depths than are those deposited at a lower temperature and pressure. Overlapping of deposition is common, and the occurrence of metals in reverse of the normal order is not unknown. Changing environment during deposition would probably account for this variation.
Spurr shows that the order of deposition of the metals upward from the magmatic source are similar in many of the major metal-producing districts of the world and that the usual sequence in the deposits associated with intrusive rocks in the order of deposition away from the source of the solutions are molybdenum, tungsten, gold, copper (silver), zinc, lead (silver).
Emmons shows that the metals are usually arranged in a rather definite sequence outward as well as upward from the source, and has listed the metals according to a zonal arrangement. His zonal sequence successively from the surface toward the parent Batholith is here reproduced.
A RECONSTRUCTED VEIN SYSTEM from SURFACE to NEAR BATHOLITH ROOF. (After W. H. Emmons.)
BARRON .... ..... 1 .... Barren zone, chalcedony, quartz, barite, fluorite, etc. Some veins carry a little mercury, antimony, or arsenic.
MERCURY... 2 .... Quicksilver veins, commonly with chalcedony, marcasite, etc. Earite-fluorite veins.
ANTIMONY... 3..Antimony ores, stibnite often passing downward into lead, with antimonates. Many carry gold.
GOLD, SILVER 4....Bonanza ores of precious metals. Argentite, antimony, and arsenic minerals common. Silver minerals, some copper, lead and zinc sulphides, quartz, calcite, rhodochrosite, adularia, alunite, etc.
BARREN... ... .. 5 .... Most nearly consistent barren zone, represents the bottoms of many Tertiary precious metals veins. Quartz, carbonates, etc., with pyrite and small amounts of other sulphides.
SILVER.. . .. 6....Argentite veins, complex antimony silver sulphides, stibnite, etc. Galena veins with silver. Commonly silver decreases with depth. Quartz gangue, siderite common, often increasing with depth.
LEAD ............. 7 ...Galena veins, commonly with some silver. Sphalerite, generally present, increasing with depth. Chalcopyrite common. Gangue is quartz and often carbonates (Fe, Mn, Ca) .
ZINC ....... ...... 8 .... Sphalerite veins with some lead and chalcopyrite, quartz gangue.
COPPER... ...... . 9 .... Tetrahedrite veins, commonly argentiferous, chalcopyrite present. Some pass downward into chalcopyrite. Enargite veins generally with tetrahedrite and tennantite.
COPPER.. .... 10. ...Chalcopyrite veins, generally with pyrite, often with pyrrhotite. The gangue is quartz and in some places carbonates. Some pass downward into pyrite and pyrrhotite, with a little chalcopyrite. Generally carry silver and gold.
GOLD. ...... ...... 11 .... Gold veins with quartz, pyrite, and commonly arsenopyrite and chalcopyrite. At places, 10 and 11 are reversed. '
BISMUTH... .... 12. ... Bismuthinite and native bismuth with quartz and pyrite, etc.
ARSENIC.... ..... 13 ....Arsenopyrite with chalcopyrite and often tungsten ores.
TUNGSTEN... . 14. ... Tungsten veins with quartz, pyrite, chalcopyrite, pyrrhotite, etc. Arsenopyrite is commonly present.
TIN ........-........ 15 .... Cassiterite veins with quartz, tourmaline, topaz, etc.
BARREN... ....... 16. ...Quartz with small amounts of other minerals.
There is a general agreement in the order of deposition of the metals as listed by these authors. Spurr and many other students of ore deposits have expressed the same general conclusions.
It must be emphasized, however, that in no single deposit are all of these changes observable. Erosion may have removed all but the lower zones or, as is the case with many deposits formed at shallow depths, mining operations have been carried to insufficient depth to prove or disprove the zonal deposition theory. That the metals have been deposited in an orderly sequence outward from the magmatic source is evident from the relative position of deposits in some areas.
It is not uncommon to find deposits of tungsten and copper near the intrusive, and those of zinc, lead, and silver several hundreds or thousands of feet from the intrusive.
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Post by 1dave on Jan 21, 2019 23:16:18 GMT -5
4f9- Influence of Structural Conditions
The effects of structural control limit the position, size, shape, and attitude of mineral deposits. Faults, sheared or crushed zones, or contacts guided the mineral-bearing solutions. Deposition occurred only where the solutions could penetrate so the resulting deposit conforms roughly with the preexisting fractures.
Since all of the minerals were not deposited at the same time, local concentrations of certain minerals formed in those parts of the channels open during the time that other conditions favorable to their deposition existed.
These local concentrations, when within a mineral deposit, are commonly called ore shoots.
The permeability and replaceability of the rocks adjacent to the channels are also factors that control the physical characteristics of mineral deposits. Where the wall rocks were permeable or replaceable, massive and irregular bodies occur, such as those formed by the replacement of limestone or dolomite.
Many mineral deposits are composed entirely of economically valueless minerals, some contain local concentrations which constitute the only valuable portions, and only a few are of uniform composition and valuable throughout their whole volume. This lack of uniformity in composition is partly due to a variation in the composition of the mineral-bearing solutions and conditions causing deposition, and partly due to the effects of structural control.
The structural conditions favorable to abundant and concentrated deposition are:
(1) the intersection of veins or veins with fissures, particularly where the intersection is at an acute angle,
(2) recurrent fault movement causing fracturing and reopening of a fissure or vein during mineral deposition,
(3) impervious layers or beds that confine solutions,
(4) the crests, troughs, or flanks of tight folds,
(5) the intersection of fissures with permeable or replaceable rocks,
(6) masses or zones of brecciated, fragmented, or closely jointed rock,
(7) open parts of a fissure formed by displacement along an undulating surface,
(8) fissures formed by branch faults, and (9) volcanic rocks, chimneys, or pipes.
Faulting and folding accompanying igneous activity form zones of weakness that later serve as channels for circulating solutions. In many areas faulting continued during and after mineral deposition, displacing the mineral deposits a s well as the rock masses.
Faults formed prior to mineral deposition are termed premineral faults, and those formed later than mineral deposition, postmineral faults.
Only premineral faults can serve as channels for the circulation of ore-bearing solutions.
Postmineral faults shatter or displace portions of the deposit, complicating the work of the geologist and miner in finding and extracting the ore.
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Post by 1dave on Jan 22, 2019 0:30:44 GMT -5
5- Supergene Enrichment
Many minerals are formed by ascending thermal waters and are termed hypogene. Deposits resulting from rearrangement of the hypogene deposits through oxidation and the action of descending surface waters are termed supergene.
The near surface portion of most deposits show evidence of some supergene action, and in many deposits only the supergene enriched portions have been economically valuable.
Supergene enrichment, commonly termed secondary enrichment, is brought about by oxidation, solution, and redeposition.
Oxidation converts many of the minerals, particularly the sulphides, into more readily soluble compounds. Surface water dissolves and removes the soluble compounds, leaving the insoluble minerals behind.
Redeposition occurs where the solutions encounter conditions that cause precipitation. Much of the dissolved material is dissipated, but the greater part of it descends in solution and is redeposited within the confines of the deposit, thus increasing the metal content.
Supergene processes cause both separation and concentration. Insoluble minerals, and those converted to the insoluble state by oxidation, remain in the oxide zone as a residual mass.
Enrichment is influenced by many factors, such as relative rates of erosion and oxidation, duration of exposure to weathering and erosion, permeability, mineral composition, depth of the water level, and climatic conditions.
If erosion proceeds more rapidly than oxidation and leaching, the metals are carried away and enrichment thereby prevented.
On the other hand, when erosion is feeble the rate of oxidation is much slower at considerable depth than it is near the surface, and enrichment is retarded.
Conditions for maximum enrichment exist when erosion just keeps pace with the rate of oxidation and leaching, as exemplified in many Nevada deposits.
Oxidation is such a relatively slow process that it must be active over at period of long duration to have appreciable effect.
Young deposits. and those only recently exposed by erosion, show but minor effects.
Old deposits that were soon buried by younger rocks likewise have been little effected.
Enrichment is most pronounced in old deposits that have been exposed during much of their geological history.
Permeability is essential for supergene enrichment. Oxidation and solution progress very slowly in relatively impermeable deposits. Intensely shattered and brecciated deposits are those most thoroughly affected. Oxidation and solution increase permeability, but openings must extend into the zone below water level for the formation of important quantities of supergene sulphide ores.
Magnesium concentration occurs where circulation of the solutions is confined to the mineral deposit, and the minimum where the solutions penetrate the wall rocks.
Mineral composition has a profound influence on the progress of enrichment. Solution of many of the minerals is dependent upon the presence of acid sulphate solutions which can form in the deposit only by the oxidation of sulphides, so the presence of appreciable quantities of sulphides, particularly iron sulphides, are necessary for intense leaching. Furthermore, any minerals that will quickly neutralize the acid will inhibit enrichment processes.
A shallow water level restricts oxidation to a small upper portion of the deposit, preventing the formation of important quantities of supergene ores. A deep water level permits extensive oxidation, but a large part of the metals may be precipitated on their long downward journey, and an important concentration will not form.
Supergene concentration is greatest when the water level is at moderate depth. In a warm climate oxidation and solution progress more rapidly than in a cold climate. Heat hastens chemical reaction and solution.
Where the ground is frozen throughout most of the year supergene action is slow. Since water is essential for solution, rain or snow are necessary. However, the annual rainfall need not be heavy. Many deposits in arid regions have been extensively enriched, as has been the case in Nevada.
As indicated previously, the metals taken into solution by supergene processes may be redeposited above ground-water level in the zone of oxidation, or below ground-water level where free oxygen is excluded. Redeposition may occur from any of several causes.
Descending solutions that dissolve metals are acid, and any conditions that neutralize the solutions will precipitate the metals.
Solutions coming in contact with carbonate compounds are quickly neutralized with precipitation of the metals as carbonates, consequently there is only a limited amount of migration of metals in sulphide mineral deposits containing plentiful quantities of calcite, dolomite, siderite, or limestone.
The feldspars also act as neutralizers, but their action is much slower than that of the carbonate compounds, and some metals are precipitated when their solutions pass through finely divided silicate minerals, kaolin in particular.
Deposition results from a chemical exchange. Powdered clay gouge often contains finely divided particles of metals and their compounds, indicating that finely powdered silicates do cause precipitation.
Metals in the descending solutions are deposited as sulphides below ground-water level. Here, also, as in the zone of oxidation, there are several causes of precipitation.
Precipitation is brought about by a diminution of the solvent power of the solutions due to a decrease in acidity. Acidity diminishes below water level because air is excluded and the rate of oxidation of the sulphide minerals is no longer rapid enough to replenish the acid as fast as it is neutralized.
Hydrogen sulphide, an active precipitant of some metals from sulphate solutions, is generated when acid sulphate solutions attack sphalerite or pyrrhotite. Hydrogen sulphide is quickly used up in the zone of oxidation by reaction with oxygen or ferric sulphate, so it can not precipitate important quantities of metals there. Even if it did, the newly formed compounds would be subject to attack and destruction by the agents of oxidation.
However, in the reducing environment of the zone of saturation, hydrogen sulphide is free to precipitate the metals and probably plays a part in the formation of the supergene sulphide ores of some deposits.
Deposition of metals below ground-water level is also brought about by chemical exchange between solutions and solids. In many cases this is probably the most important precipitating process. One metal replaces another according to the relative solubility of their sulphides. Each replacing those of higher solubility.
Thus, silver or copper would be precipitated by sulphides of lead, zinc, or iron; lead from solution by zinc and iron.
Supergene metallic sulphides, such as sulpharsenides and sulphantimonides form by the same process.
Supergene sulphide ores are often exceptionally rich because they contain the metals not only leached from the existing oxide zone, but also in part from the portion of the deposit that has been removed by erosion.
The history of a mineral deposit is not displayed conspicuously in its outcrop. The agents of erosion and decomposition have destroyed much of the evidence of its original mineral composition.
However, the prospector, geologist, or mining engineer must base his predictions of continuity or change of ore in depth upon this scant evidence if he is to appraise a newly discovered deposit.
Evidence indicative of its type, size, shape, and attitude can often be obtained from a study of the relation of the deposit to the general geology of the area. Such evidence will serve as a basis for estimating possible tonnage. The metal content of the outcrop can be ascertained by sampling and assaying. However, the results of surface samples are generally unreliable as a basis for predicting the persistence or change of values in depth.
The composition and environment of mineral deposits are so variable that rules for the identification of criteria in the outcrop indicative of supergene enrichment are not universally applicable. However, there are certain features that are most often evident, and that serve as the best guides.
The favorable features are summarized as follows :
1. Indications of a strong primary mineralization, together with a porous or cellular texture, and evidence of post-mineral fracturing.
2. Thorough oxidation and solution, that is, absence of sulphide minerals other than galena, and presence of limonite, quartz, and kaolin.
3. Indications that the primary material contained only minor amounts of the rapid neutralizers, calcite, siderite, limestone, or dolomite.
4. Moderate-to-strong relief and a moderate-to-deep groundwater level.
5. A moderate-to-rapid erosion rate.
6. Long exposure to weathering agents.
The unfavorable features are as follows:
1. Presence of sulphide minerals other than galena.
2. The presence of, or evidence indicating the prior existence of, abundant rapid neutralizers.
3. Base-leveled terrain, where erosion is slow and the water level shallow.
4. Evidence indicating that the deposit has been exposed to oxidation and leaching for only a short time.
5. Compact, impervious deposits through which solutions cannot permeate freely.
6. Evidence of regional glaciation, particularly if the glacial erosion has been deep. Small mountain glaciers have only local effect.
As previously stated, evidence in the outcrop of the extent of supergene enrichment is seldom conspicuous. Weathering has frequently so modified the exposed portion that diagnostic criteria are obscure. Even the limonite, manganese oxide, kaolin, and small particles of gold are often removed by running water leaving a bleached and uninviting siliceous or earthy mass on the surface. Shallow pits or trenches, however, are usually sufficient to expose material in which evidence of leaching is fairly well preserved.
A porous or cellular mass of earthy, quartzose, or jaspery material stained by iron oxide is the best evidence of a strong primary mineralization, since it indicates that the primary material contained sulphide minerals, part of which, at least, were iron-bearing. The most common sulphide carrying iron is pyrite, and it often occurs as the only sulphide, thus giving no chance of secondary enrichment of other metals.
It is asserted that the amount, the color, the form, and the position of the limonite in the leached outcrop all serve as indicators of the original sulphides which existed there.
Blanchard and Boswell make the very positive statement that "today not only is a limonite that has been derived from pyrite clearly distinguishable from one derived from the other sulphides named (chalcopyrite, chalcocite, sphalerite, or galena), but where the copper, lead, and zinc minerals are involved, the grade of the material prior to leaching can be judged in many instances as accurately as if the original sulphides were still visible in the outcrop," and to one skilled in the interpretation of the type of limonite "the various limonite products left by chalcocite, sphalerite, or galena, for example, are not much more difficult to distinguish from one another than would be the respective copper, zinc, or lead carbonate or sulphate minerals."
Much excellent material has since been written on this subject; but many authorities are much less positive in their statements as to the accuracy of the deductions and the value of this type of study.
The surface residue varies according to the mineral composition of the original deposit. Quartz, hematite, limonite, pyrolusite, and kaolin constitute its bulk. Cassiterite, wolframite, gold, cerargyrite, and often anglesite and cerussite remain. The soluble minerals and those converted to soluble salts are transported and precipitated as newly formed compounds, both in the zone of oxidation and at or immediately below ground-water level. Oxides, carbonates, sulphates, silicates, chlorides, and native metals predominate among the compounds precipitated in the zone of oxidation, and sulphides at or below water level.
The processes of supergene enrichment have been described by many investigators and summarized by Emmonsr. in particular. Investigations carried out in both the field and in the laboratory indicate that solution is aided and hastened by the products formed by the oxidation of pyrite. In the presence of water, oxygen attacks pyrite, forming sulphuric acid and ferric sulphate.
These products, in association with free oxygen, react with many of the primary minerals, converting them to sulphates. The solubility of these sulphates has a marked influence on their migration. The sulphate of iron is readily soluble. However, in the presence of water and ample free oxygen, iron sulphate is changed to the insoluble hydrous iron oxide, and sulphuric acid is liberated, thus much of the iron remains in the leached material.
Copper and zinc sulphides are particularly soluble in oxygen laden acid waters. Their removal from the zone of active oxidation may be nearly complete, which accounts for many barren outcrops above bodies of copper and zinc ores.
Silver sulphides are soluble and much of the silver taken into solution as the sulphate is quickly precipitated out by any alkaline salt as the relatively insoluble chloride, cerargyrite, commonly termed horn silver.
Since sodium chloride (common salt) is prevalent in meteoric waters, much of the silver is prevented from migrating far, and remains close to the surface.
Galena is to some extent soluble, but the resulting lead sulphate often forms a coating around a central core of galena, protecting it from further attack by acid solutions. The result is generally a near-surface enrichment.
Cinnabar is practically insoluble and remains unaltered, with no consequent mercury enrichment from reprecipitation.
Gold is not soluble in sulphuric acid or ferric sulphate. However, it is soluble in acid solutions containing uncombined chlorine.
Chlorine is liberated in the reaction between sodium chloride and sulphuric acid in the presence of the strong oxidizing agent, manganese oxide. Thus, an active solvent of gold is present in some deposits. The gold would be immediately precipitated by ferrous sulphate were it not for the fact that manganese oxide converts ferrous sulphate to ferric sulphate which does not precipitate gold. Thus manganese oxide plays a dual part in the supergene enrichment of gold ores, and its presence in outcrops may indicate a possible leaching and redeposition of gold with corresponding enrichment. Thorough oxidation and leaching are essential for enrichment.
Solution is dependent upon the conversion of the metals to soluble compounds by oxidation. Leaching is essential for their downward migration. Any metals fixed in the upper oxide zone will be removed and dissipated as erosion proceeds. The presence of sulphides, other than galena, denotes incomplete oxidation. The presence of abundant metals, the oxidation products of which are usually easily soluble, indicates that leaching has not been thorough. The metals most diagnostic of the extent of leaching are copper and zinc. If carbonates and silicates of these metals are abundant in the upper oxidized zone, leaching has not been intense, and insufficient quantities of the metals have migrated downward to appreciably increase the grade of ore in depth.
Evidence of abundant rapid neutralizers is usually apparent, though not invariably so. Since rapid neutralizers impede migration of the metals, it is essential that the results of their influence be recognized and given proper weight. Where the rocks of one or both walls are limestone or dolomite, acid solutions will be rapidly neutralized upon coming in contact with them. Carbonate wall rocks have impeded migration of the metals except in a few rare cases where a protective coating or seal of gypsum has formed along the boundaries of the deposit.
Evidence of carbonate wall rocks should be obvious, but evidence of rapid neutralizers within the deposit may not be particularly obvious, since these minerals are converted to soluble compounds by the very reactions that neutralize the acid solutions and are removed. The best evidence lies in the minerals that the rapid neutralizers have prevented from migrating. Iron remains practically in place as highly pulverulent or earthy limonite; copper and zinc are precipitated as carbonates. The significance of rapid neutralizers is this: downward migration of the metals is impeded, preventing their accumulation as enriched masses in depth.
The depth of ground-water level, though conforming roughly to the ground surface, is influenced by the general relief of the terrain. Ground-water level usually lies much deeper high up on steep slopes than it does near the edge of valleys or in a region of gentle slopes. Structures, such as impervious strata or fault zones, may impound the water and form local areas of shallow ground water where the water level would otherwise be deep.
A deep water level permits deep and extensive oxidation and downward migration of the metals, while a shallow-water level prevents this. The best criteria of a deep ground-water level are strong relief, steep slopes, low annual rainfall, and the absence of springs near the deposit. The present water level may be higher than in a previous period of leaching, with consequent oxide ore now extending below water level, or it may be lower with secondary sulphide ores above it.
The rate of erosion influences the progress of oxidation. Slow erosion retards its progress, whereas rapid erosion may remove material before oxidation and leaching are complete. For maximum effectiveness, erosion should just keep pace with thorough oxidation and leaching. The erosion rate varies with the gradient, amount of precipitation, climate, and resistance of the deposit and enclosing rocks. The rate of erosion is most conducive to sulphide enrichment in regions of moderate-to-strong relief.
Slow erosion is favorable to residual enrichment, hence deposits enriched by residual processes are most numerous in regions of low relief. Since oxidation and leaching are slow processes, long exposure to their action is essential if extensive enrichment is to result.
As young deposits, in general, have been subject to enrichment processes for a much shorter time than have the older ones, the degree of enrichment is correspondingly less.
Criteria by which to judge the age of mineralization are the age of enclosing rocks, the relation to rocks or old erosion surfaces of known approximate age, the depth of erosion since mineralization, and the mineralogy of the deposit.
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