Art. V.—Notes on the Formation of Zones of Secondary Enrichment in certain Metalliferous Lodes.
[Read before the Otago Institute, 8th August, 1905.]
It has been noted in many parts of the globe that masses of ore of exceptional richness often occur in the oxidized portion of the ore-body—commonly in that portion lying at the boundary of the oxidized and unaltered sulphides.
Microscopic investigation has proved that these rich masses are not of primary but of secondary origin. Their genesis is supposed to be due to the migration pf the valuable contents of the upper part of the vein to, and their concentration in the lower part of the oxidized zone.
In some cases the processes of dissolution, migration, and redisposition may have taken place over and over again, each cycle resulting in an increasing degree of concentration.
The veins in which secondary enrichment are most often seen are those of gold, silver, copper, lead, and zinc.
Gold-ores, in the zone of weathering, are often augmented in value by the long-continued disintegration of the vein and the enclosing rock, whereby the gold set free from its matrix is permitted to concentrate at the outcrop.
Extensive areas of the Australian Continent have been subject to subaerial denudation since the close of the Palæozoic period; and it is doubtless due to this cause that so many notable examples of mechanical enrichment of gold-bearing veins have been found in Victoria and Western Australia.
The migration of gold, copper, silver, lead, and zinc from the upper to the lower parts of the veins is effected by descending surface waters in the zone of vadose circulation. The processes involved in the migration are chemical dissolution and electrochemical deposition.
Chemical processes may operate in various ways to cause secondary enrichment, as follows:—
By the removal of worthless metals, thereby leaving the valuable contents in a purer form.
By removal of worthless metals, and their replacement by valuable metals removed from a higher part of the vein.
By deposition of valuable metals on primary sulphides in those portions of the vein subject to the influence of circulating surface waters. In this case the primary sulphides may form the nuclei for the deposit of the secondary sulphides.
Manifestly the first operation in the process of secondary enrichment is the chemical weathering and oxidation of the metallic contents of the vein.
The oxidation of base sulphides can be seen in operation every day. In a mass of mixed sulphides of iron, copper, zinc, and galena, the iron will be the first to be affected, from its affinity for oxygen. Iron-pyrites is decomposed and forms ferrous-sulphate, which is changed into Fe(OH)3, Fe2(SO4)3, and H2SO4. The H2SO4 attacks fresh iron-sulphide and forms ferrous sulphate, liberating H2S, which at once combines with free oxygen to form H2SO4. The ferrous sulphate changes to the ferric sulphate, which attacks gold and sulphides of copper, lead, zinc, and silver. The process of dissolution is necessarily slow, on account of the extreme dilution of the solutions.
For many years it was believed that the only secondary enrichment that could take place was the formation of rich bonanzas of carbonate ores and chloride of silver, in the zone above water-level. But careful investigation has shown that primary sulphides have been enriched by the deposition of secondary sulphides even in places below the present water-level.
It was proved experimentally by Skey* in 1870 and Liversidge† in 1893 that gold is more readily precipitated from its. solutions by metallic sulphides than by organic matter. Furthermore, Skey showed that sulphides of the base metals were readily precipitated from their alkaline sulphide solutions in a solid coherent form in the presence of iron-pyrites, galena, blende, stibnite, &c.
The descending acid solutions formed in the zone of oxidation will attack the constituents of the rocks through which they percolate, producing alkaline silicates and sulphides.
Gold dissolved by ferric sulphate would be also carried down and deposited as leaf, scale, or wire gold in cracks in sulphide ore, thereby causing local enrichment.
It is maintained by some writers that secondary sulphides have been found below water-level. The evidence on this question is not quite conclusive. Changes of water-level may have taken place since the secondary sulphides were deposited.
[Footnote] * Skey, Trans. N.Z. Inst, Vol. iii, 1879, p. 226.
[Footnote] † Liversidge, Proc. Roy. Soc. N.S.W., Vol. xxvii, 1893, p. 287.
The property possessed by silica, clay, and porous substances of absorbing metals from dilute aqueous solutions may be an important factor in the formation of zones of secondary enrichment in the oxidized portions of metalliferous lodes. In this we may have the explanation of the rich kaolin ores of silver at Broken Hill, of the concentration of gold in the talcose clays of the lode-formations of Kalgoorlie, and of the copper-bearing shales of Europe and America.
Impoverishment of Veins in Depth.
T. A. Rickard, when discussing Professor Posepny's paper on “The Genesis of Ore-deposits,” states that the general non-persistence of ore in depth is a fact capable of proof.‡ He contends that since heat and pressure are the two factors which increase the solubility of mineral substances, the deep region will favour solution the most, while the shallow zone will favour precipitation owing to the decrease of heat and pressure.
There is much in favour of this contention, and many examples could be adduced in its support in all parts of the globe.
Moreover, progressive poverty in depth below a certain point must be the natural corollary of the general law governing the orderly distribution of ores in horizontal zones in vertical distance through the agency of ascending waters.
In some cases impoverishment in depth is determined by the prevailing geological conditions. Ore-veins which are confined to a single overlying formation often die but or become exhausted on reaching the underlying rock.
A notable example of this is afforded by the hydrothermal veins of the Thames, Tapu, Coromandel, and Kimotunu mining districts in the Hauraki mining region of New Zealand, where the gold-bearing veins are contained in altered andesites which rest on a highly eroded surface of Lower Mesozoic slaty shales and sandstones.
Mining operations have in all cases shown that when the veins which occur near the borders of the andesite-flows reach the basement rock they die out completely, or end in small strings which soon disappear in depth.
[Footnote] * S. J. Emmons, Trans. Am. Inst. M.E, Vol xx, 1900.
[Footnote] † W. H. Weed, Bull. Geol. Soc. Am., Vol. xi, 1900, p. 179; and Trans. Am. Inst. M.E., Vol. xx, 1900, p.
[Footnote] ‡ T. A. Rickard, “Genesis of Ore-deposits”: Discussion, p. 190.
The principle of secondary enrichment precludes the continuance of the enriched portion of the vein downward in vertical distance.
When the values of secondary enrichment are added to ore already of a payable quality, the result is a rich shoot or bonanza; but when, as often happens, the secondary values are added to lean ore, then the net result is to render the lean ore just profitable. Hence below the zone of enrichment the ore will be lean and unprofitable.
Absorption of Metals by Clays in relation to Secondary Enrichment.
It has been noted in many mines that the ore in the zone of secondary enrichment is commonly associated with or contained in, a matrix consisting of clay or other finely divided mineral matter. Of this there are no better examples than the kaolin silver-ores of Broken Hill and the talcose gold-ores of Kalgoorlie.
Clays and clayey matter are the natural products of the alteration of rocks and ores in the zone of oxidation, hence their presence calls for no comment. But the frequent occurrence of rich ores in a clayey matrix in the zone of oxidation in certain lodes has led to much speculation as to the relation existing between the clay and its metallic contents.
It has been suggested by some writers that this association is not accidental, nor the result of paragenesis, but due to some quality of the clay. That clay and finely divided matter possess the property of absorbing or extracting metals from their aqueous solutions has long been known; and with this knowledge in mind it has been contended that the clayey matter acting as a porous filter in the lower part of the zone of oxidation has absorbed the metals from the descending solutions, thereby effecting a concentration of the valuable contents.
There is much to be said in favour of this view, but it has still to be determined whether clayey matter is a primary or merely a contributing cause in the formation of zones of secondary enrichment.
Walter Harvey Weed* early in 1905 described some. experiments made by himself and others in the laboratory of the United States Geological Survey on the absorptive property of clays, &c. The results obtained confirmed the researches of W. Skey
[Footnote] * W. H. Weed, “Absorption in Ore-deposition,” Engineering and Mining Journal, Feb. 23, 1905.
in 1869, and of E. Kohler* in 1903, who found that clays and porous substances such as gelatinous silica, carbonaceous and colloidal matter, possess the power of extracting metals from their dilute aqueous solutions. As the subject is one having an important bearing on the concentration of metals in the zone of oxidation, and perhaps also in bed-impregnation and vein-filling, it may be of interest to notice more fully the experiments made by Skey in the New Zealand Government Laboratory in 1869, 1871, and 1874. Skey, it should be noted, was one of the pioneers in this department of research, and it is a tribute to the marvellous ability and skill he displayed in his research-work to find his determinations so fully verified by investigators of later date.
In 1869 he proved experimentally† that finely pulverised massive quartz, rook-crystal, and silica possess the power of absorbing or extracting the oxide of iron from its acetate solution. He also found that prepared silica especially manifests this property if ignited at a low temperature; and, besides, takes oxides of copper and chromium from their acetate solutions. The more finely divided the silica the more apparent is the absorption.
In 1871 Skey‡ found that when a weak ammoniacal solution of copper containing a little caustic potash is poured upon a filter of Swedish paper (cellulose), the liquid which passes through the paper is quite or nearly colourless, and the filter is found to have retained all, or nearly all, the copper of such solution.
In 1874 he showed that clay possesses the property of absorbing and fixing natural petroleum in such a way as to form a substance resembling natural oil-shale, the oil being chemically combined with the clay.§ He does not appear to have tried to ascertain the absorptive power of clay upon solutions of the metals, but his discovery that silica and porous substances, such as cellulose, possess the property of absorbing metals from their solutions has an important bearing upon the chemistry of ore-deposition.
[Footnote] * E. Kohler, Zeitschrift fur Praktische Geologie, 1903, p. 49 et seg.
[Footnote] † W. Skey, “On the Absorptive Properties of Silica, and its Direct Hydration in Contact with Water,” Trans. N.Z. Inst., Vol. ii, p. 151: Wellington, N.Z., 1869.
[Footnote] ‡ W. Skey, “Absorption of Copper from its Ammoniacal Solution by Cellulose in Presence of Caustic Potash,” Trans. N.Z. Inst., Vol. iv, 1871, p. 332.
[Footnote] § W. Skey, “Notes on the Formation and Constitution of Torbanite and similar Minerals,” Trans. N.Z. Inst., Vol. vii, 1874, p. 387.