The fineness of placer gold is generally greater than that of the lode gold from which it is derived, and its degree of fineness increases as distance from its source increases owing to the leaching action of streams that transport it; furthermore, the rare occurrence of minute, drusy crystals of gold on nuggets indicates the solution and precipitation of small amounts of gold.
Pure silica sand, derived during one or more cycles of erosion from granitic and other siliceous rocks, requires not only the extremely fine grinding but the complete decomposition of such minerals as the feldspars, whose specific gravities are so near to that of quartz that mechanical action alone can not produce the final results. Mechanical processes may greatly predominate in colder climates, but in warm humid climates the materials concentrated by mechanical processes have first been liberated by the thorough chemical decomposition of the rocks and of any mineral deposits contained in them.
These facts serve to emphasize the complexity of the geological processes involved, and to show the tolerance with which any classification must be accepted and used. In class IIA, deposits produced by chemical processes of concentration in bodies of surface waters, the impossibility of distinction between certain sedimentary rocks and mineral deposits is again apparent, as is the impossibility of recognizing any one process to the exclusion of all others; for example, although the phosphorus that enters guano is gradually concentrated by a succession of organic processes in sea water, the guano itself is eventually a dry-land deposit whose commercial value may be enhanced by the leaching action of surface waters, a process that should be included under class IIB1a.
Even if guano deposits, which are exceptions in the realm of mineral deposits, were grouped elsewhere, these would be similar obstacles to an absolutely consistent classification. The pebble phosphate deposits of Florida offer similar difficulties, as they are partly residual, partly attrital, partly marine, and partly fluviatile in origin.
There is also difficulty in distinguishing sharply between inorganic and organic reactions in surface waters; for example, the iron that spring water has dissolved from rock in its underground course may, on entering a stream and after coming in contact with the air, become oxidized and deposited, but it is common to find certain kinds of algae taking part in the process.
Limestones, whether formed in marine or fresh water, may owe their origin in part to the loss of carbon dioxide, which may be a simple inorganic process or more likely is influenced or controlled by organic agencies; in part they represent the direct secretions of mollusks and corals, but to a considerable extent they may represent the mechanical breaking down, redistribution, and recementation of those secretions.
Most dolomites, which occur interbedded with limestones, have been formed through the replacement of part of the original calcium of limestone by magnesium by prolonged contact either with the water of shallow sea-bottoms or with underground water. They may be grouped in part, therefore, with deposits formed by inorganic reactions in marine surface waters and in part by concentration by ground water of deeper circulation IIBb.
Certain light gray to buff colored dolomites in the West, for example, in the Yellow Pine district, Nevada,8 are most reasonably attributed to the reaction between limestone and waters of magmatic origin that have traveled far from their source.
Other deposits certain siderites, cherts, sulphides, and sedimentary iron and manganese ores discussed by Lindgren in his chapter on deposits produced by chemical processes in bodies of surface water are subject to similar discussion. From this discussion the reader may be led to infer that the tabulated classification is defective; but the point to be emphasized is that, although the processes indicated are distinct, they seldom act alone and thus may only account in part for the complex origin of certain rocks and mineral deposits.
The formation of deposits by evaporation is less subject to involved discussion. There may be many physico-chemical details as well as problems of sedimentation and earth movements to be studied regarding the process, but the deposition of the constituents of sea water or of undrained lakes by evaporation is readily appreciated. Even this process may be accompanied by some mechanical deposition of mud or silt and by some bacterial action; but the effects of these accompanying processes do not obscure the effects of evaporation in the least.
That subsequent changes in these deposits may confuse the student of mineral origin is illustrated more and more by recent investigations of the German potash deposits, whose history was formerly interpreted in such simple terms.
Class IIB which includes all deposits formed chemically within bodies of rock, comprises nearly all of the metalliferous deposits, except placers and the sedimentary iron and manganese ores, and also includes several important deposits of non-metallic minerals.
Discussion of the processes involved may be somewhat less disconcerting than that on the preceding pages, but few, if any, of the deposits represented can be attributed entirely to one process. Those deposits formed by concentration of substances contained within the geologic body itself may have more complicated origins than those formed by introduction of substances foreign to the rock; thus deposits resulting from rock decay, though dependent mainly on chemical processes, may be dependent, at least to a minor extent, on processes of mechanical disintegration.
Chert and barite nodules in the residual soils left by the solution of certain limestones, are simple insoluble residues, but kaolin and limonite that are commonly classed as residual may have a more complicated origin; indeed, even the simplest process imaginable requires that the constituent elements go into either true or colloidal solution and become redeposited with or without appreciable transfer from one point to another. In some deposits, both of kaolin and limonite, small and even large quantities of these materials have been carried in solution for short to considerable distances and redeposited by replacement of the adjoining rock.
Both of these minerals have replaced siliceous sandstones and quartzites to some degree and have replaced limestone and dolomite in relatively large quantity.
Some kaolin, though not of economic importance, is also attributed to ascending water derived directly or indirectly from volcanic sources.
The term, kaolin, as here used includes several minerals of the clay group, some of which may be more restricted in their modes of origin, so that the precise determination of the clay minerals, that is now in progress, together with a careful study of geological conditions, will doubtless result in a more refined interpretation of these deposits. Sufficient has been said here to emphasize the complexity of processes associated with weathering and rock decay.
The oxidation and enrichment of ore deposits involves similar processes. Gold in oxidized deposits remains largely as a residue and iron in pyrite, if converted promptly to an insoluble oxide, also remains largely in its original position; but both of these metals as well as others may under some conditions be largely dissolved, carried downward, and redeposited by reaction with certain rocks or minerals. These processes are discussed more fully in chapter IX.
The role played by deeply circulating waters in the concentration of mineral deposits is perhaps less clearly demonstrated than most other processes of mineral deposition. Years ago, when the theory of lateral secretion was more favorably regarded, the importance of deeply circulating waters was generally taken for granted; but now, since most metalliferous deposits that originated below the belt of weathering have been convincingly attributed to magmatic waters, the inadequacy of deeply circulating meteoric waters to form such deposits appears more and more probable, and one wonders just what their true functions are.
Anyone who has seen the thick efflorescences and stalactites in the deep workings of some mines is impressed with the amount of mineral matter contained in waters below the zone of oxidation. A striking example is afforded by the Ibex mine at Leadville, Colorado, in which the floor of a stope was covered by a layer of malachite 5 inches thick that had accumulated within a period of 10 years.
This mineral matter, however, was mainly derived by leaching in or just below the zone of oxidation and was redeposited only by the artificial tapping and evaporation of the water. Even in well mineralized districts, where the amount of dissolved material carried down from the oxidized zone is unusually great, the minerals deposited in quantity beneath that zone owe their existence largely to opportunities for reaction between the descending waters and certain minerals—for example, the precipitation of black copper sulphide chalcocite as a replacement of chalcopyrite or pyrite and of zinc carbonate by replacement of permeable limestone or other carbonate rock.
It is striking and even distressing when we realize how much zinc carried down from the oxidized zone has been scattered and lost in the vast sea of underground water because it did not come in contact with replaceable carbonate rock in or immediately below that zone. Lead-silver-zinc associations D. Silver-cobalt-nickel-bismuth-uranium associations E.
Tin-silver-tungsten-bismuth associations F. Antimony-mercury-arsenic-selenium associations G. Nonmetallic associations Fluorite-barite-quartz IV. So, they may be used in the field.
M , Economic mineral deposits New York ; wiley , pp. Buddington, A. High —temperature mineral associations. Clark L. A, The Fe —As-s system ; phase relations. Total views 17, On Slideshare 0. From embeds 0. Number of embeds 8. Downloads Shares 0. Comments 0. Likes You just clipped your first slide! Clipping is a handy way to collect important slides you want to go back to later. Now customize the name of a clipboard to store your clips.
Visibility Others can see my Clipboard. Cancel Save. Exclusive 60 day trial to the world's largest digital library. Translated from the original German edition by Dr. Revised and supplemented throughout by Dr. Niggli and Dr. London: Thomas Murby and Co. Van Nostrand Co. Reprints and Permissions. Classification of Ore Deposits. Nature , Download citation. Sata Ajjam.
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