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July, 1937

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The chemical composition of metalliferous deposits is ascertained directly by means of assays and chemical analyses, and indirectly by determinative mineralogy. The direct methods do not fall within the geologist's province, though their results are often of great value to him. Mineralogical studies, which yield not only qualitative information about the chemical elements present, but also a knowledge of the state of combination of those elements as minerals, play an essential part in the geological investigation of ore deposits.

Identification of minerals at sight is often possible after some experience, especially if they are coarsely crystalline, hut such identification is always open to suspicion, and the use of accurate methods is desirable. The blowpipe method is only satisfactory for fairly coarse, pure minerals, and has largely been superseded by the use of the microscope, an outline of which method is given in this article.

Microscopic technique

The microscope is used for the identification of minerals, and for the study of their relations in the ore deposit or rock. The technique varies according to whether the minerals transmit light or are, opaque. In general the economically valuable minerals, mostly metallic oxides or sulphides, are opaque. These are investigated by means of the reflecting or "metallographic" microscope. Highly polished surfaces of the ores about by ½ in. are prepared by grinding on carborundum wheels or emery paper, followed by polishing on rotating cloth laps on which a magnesia emulsion is used as an abrasive. In an improved method of polishing devised by L. C. Graton at Harvard University, the ores are mounted in bakelite and polished on a machine using lead laps, the specimen being rotated about two axes. The polished section is examined under the microscope by means of light reflected perpendicularly from its surface. Minerals are identified from the following criteria :– (1) Colour; (2) Reflection pleochroism and birefringence, seen respectively when plane polarised and doubly polarised reflected light is used; (3) Hardness, tested by scratching with a needle, or on a hardness machine ; (4) Etch tests, the following reagents being used in order: HNO₂, HCl, KCN, FeCl₃, KOH, HgCl₂. Where confirmatory evidence is needed highly sensitive micro-chemical tests are employed (see Short, M. N., reference 4).

The advantage of the microscopic method of determining minerals lies in the fact that very finely divided minerals can be determined with certainty. Magnifications of upwards of 1,000 diameters can be obtained when well polished specimens are used, and optical tests can be applied at very high magnification. Etch tests can be used at over 100 diameters. Diagnostic criteria are now available for almost all opaque minerals. (Refs. 3 & 4.)

Non-opaque minerals are studied by means of the petrographic microscope. Thin sections are prepared by mounting a slice on a glass slip with Canada Balsam and grinding until the thickness is of the order of 1/200 mm. The following are the principal criteria used for the determination of the non-opaque minerals under the microscope :– (1) Colour, (2) Crystal form and cleavage, (3) Pleochroism in polarised light, (4) Birefringence in doubly polarised light, (5) Optical sign and optic axial angle, (6) Refractive Indices, determined by immersion in liquids of known refractive index. In the study of ore deposits, petrographic methods are generally used as applied to "source rocks," to unaltered and altered "host" and wall rocks* and to gangue minerals.

Replacement

Replacement, or metasomatism (as now used, the terms are synonymous, and "metasomatic replacement" is a tautology) of a favourable host rock by ore minerals has long been recognised. Microscopic study has not only revealed some of the intimate details of this process, but has also shown that in many deposits replacement plays a leading part within the deposit itself. Most epigenetic deposits have owed their origin to solutions whose composition was continuously changing during the primary emplacement of the deposit. Further chemical alterations occurred when the circulating ground waters reached the primary deposit. A record of both primary and secondary chemical changes is to be found in the form of mineral replacements which can be deduced from the textural relations of the minerals.

The conditions which determine the susceptibility of a particular rock to replacement are not yet fully understood. It is well known that porous rocks such as limestones and sandstones are more susceptible than impervious shales and clays. Chemically reactive carbonate rocks are much more readily replaced than siliceous rocks. There are, however, some more subtle controls of this process which remain to be discovered. The selection of a particular horizon for replacement in a limestone series is not uncommon. In the Tn-State zinc-lead deposits of the Central U.S.A., it has been shown that in the Boone limestone formation the ore has replaced persistently six beds, though intervening beds, also of limestone, contain little or no ore. Similarly in the Katanga and Rhodesian copper belt, the ore has replaced particular dolomitic shale and dolomite beds in the Serie des Mines, which contains dolomites, shales and sandstones.

The first series of photomicrographs (Figs. 1 to 4), illustrate the replacement of limestone in the Northern Pennine lead mining field, where metasomatic "flat deposits" are associated with fissure veins where they cross certain of the Yoredale limestones.

Primary deposits

Returning now to the relations of the minerals in primary epigenetic deposits, it may be stated that deposits in which all the minerals were deposited simultaneously are very rare. Generally there is a definite order of mineral formation, though it is by no means always consistent. If deposition of minerals without reaction between solutions and earlier-formed minerals took place, simple banded textures which are known in many districts often resulted. If on the other hand the earlier-deposited minerals became unstable as the temperature or composition of the mineralising solutions flowing through the place of deposition changed, this led to replacement. Evidence of such replacement is best studied under the microscope. Fig. 5, for example, shows the replacement of pyrite by chalcopyrite, part of the pyrite remains as rounded "islands" enclosed by the chalcopyrite, which has in its turn been invaded and partly replaced by sphalerite. Similarly in Fig. 6, the sphalerite is traversed by veinlets of galena, the ragged edges of which suggest that this mineral has replaced the sphalerite.

The microscopic examination of complex primary ores may be very helpful in the mining of the deposits since it reveals the distribution of the valuable mineral or minerals in the ore. In the case of gold deposits, the metal is often associated with one of the more obvious minerals in the ore, and once this is established, selective mining may be possible. Complex ores should always be examined microscopically before the treatment process is decided upon, since the amount of grinding necessary to ensure maximum extraction may be determined from the grain size of the constituent minerals present. Such examinations may usefully be applied to ores like the tetrahedrite-sphalerite-chalcopyrite-enargite-pyrite-quartz ore shown in Fig. 7.

Oxidised and enriched deposits

Erosion of the cover over primary deposits gradually brings them within reach of the ground waters circulating above the water table. These waters, which carry atmospheric oxygen and carbon dioxide, oxidise the minerals of the primary deposits, sulphides being converted into hydrous oxides, sulphates and carbonates. The zone of oxidation extends from the surface to the permanent groundwater table ; the outcrops of ore deposits and the shallow levels below them therefore contain mainly secondary minerals. Some idea of the probable composition of the primary deposit below may be gained from the secondary minerals and from unoxidised remnants of primary minerals. Fig. 8 shows a gold ore in which the metal occurs in limonite. The presence of vestiges of pyrite suggests that the primary ore will prove to be a pyritic gold ore. Thus at the water table the character of the ore will change from free-milling to refractory.

Usually the oxidation products of the primary minerals replace them in situ; they migrate only in inert chemical environments. In many minerals, oxidation occurs along cleavages or cracks. Fig. 9 shows tetradymite, the bismuth telluride, being replaced by bismutospharite, its oxidation product. The replacement along cleavages of galena by cerussite is shown in Fig. 11, and of siderite by limonite in Fig. 12.

In copper and in certain silver deposits, all the metal is not fixed in oxidation minerals, some of it migrates to the bottom of the oxidation zone, where it is deposited by reaction with the primary sulphides there. In this way a zone of secondary sulphides is formed between the oxidation zone and the primary deposit. Chalcocite, the sulphide with the highest ratio of copper to sulphur, is the characteristic copper mineral of this zone, argentite the corresponding silver mineral. Values in the secondary enrichment zone are generally much higher in copper or silver than those in the primary deposit, so that a fall in the grade of the ore may be expected when the secondary enrichment zone is exhausted.

Primary mineral zones

Oxidation and enrichment are not the only causes of variation in the mineral content of ore deposits. In deposits well beyond the reach of secondary action, consistent changes in mineral composition have been observed, both in depth and along the strike. Zones in which a particular mineral or group of minerals predominates may be recognised in such cases. An outstanding example is the Dolcoath mine in Cornwall, where the upper levels yielded copper ore, the lower levels tin ore. A similar arrangement has been found in many mines in Cornwall, so that the effect is no local accident, but indicates a definite control at the time of primary mineralisation.

Mineral zoning operates not merely in individual deposits, but throughout whole districts. A concentric arrangement of zones about one or more centres is often found. In the Butte district, Montana, which covers an area of six square miles, there are three zones :– (1) Enargitepyrite-quartz ; (2) Bornite-enargite, with sphalerite, tetrahedrite and tennantite appearing towards the outer margin ; (3) Sphalerite-manganese minerals. On an even larger scale, in the Northern Pennine district, covering nearly 1,000 square miles, there are consistent zones arranged about two main centres, one at Tynehead, the other below Weardale. The zones are as follows :– (1) Fluorspar chalcopyrite; (2) Galena fluorspar-blende; (3) Galena-blende - fluorspar - barytes; (4) Galena-barytes; (5) Barytes, barren of sulphides.

The control of the primary zonal distribution of minerals is generally considered to be the fall in temperature of the mineralising solutions as they travel further and further from their source. The minerals of the outer zones are thus believed to be soluble at lower temperatures than those of the inner zones. Referring to Fig. 1 in the first article it will be noted that the deposits in the sedimentary rocks occur in the following order away from the igneous source rock (1) copper; (2) zinc ; (3) lead. This relation holds in. many districts.

Summary

Finally a brief review of the conclusions regarding the origin of metalliferous deposits which command general agreement may be given. Certain deposits, notably the bedded iron and manganese deposits, were concentrated by the ordinary processes of sedimentation. (Syngenetic deposits.) A few iron and nickel deposits may have originated by direct crystallisation from ingeneous magmas. (Syngenetic magmatic deposits.) A majority of the deposits formed later than the rocks which enclose them (Epigenetic deposits), were derived from fluids concentrated from igneous magmas during their crystallisation. These fluids may have left their source in the gaseous condition, but early in their history they became liquids. They travelled upwards and outwards from their source by way of structurally formed channels, among which fissures were the most important. Deposition took place in structural openings as a result of the falling temperature of the solutions, or as a result of reactions between solutions and host rocks, leading to the replacement of favourable host rocks by ore minerals.

The minerals of primary deposits both syngenetic and epigenetic, are oxidised when erosion brings the circulating ground waters to their level. Secondary enrichment may take place by the deposition of secondary sulphides at the bottom of the oxidation zone.

SELECTED REFERENCES

• 1. Holmes, A., "Petrographic Methods and Calculations," 2nd Ed., London, 1928.
• 2. Larsen, E. S., and Berman, H "The Microscopic Determination of the Non-opaque Minerals," 2nd Ed., U.S. Geol. Survey Bull. 848, 1934.
• 3. Schneiderhohn, H., and Rhamdor, P., "Lerhbuch der Erzmikroskopie," Bd. 11. Berlin, 1931.
• 4. Short, M. N., "The Microscopic Determination of the Ore Minerals," U.S. Geo. Survey Bull. 825, 1931.

Drawings and Photographs accompanying the article