Art. VI.—The Deposition of Mineral Matter from Aqueous Solutions in its Relation to the Filling of Cavities and Vein-fissures.
[Read before the Otago Institute, 13th September, 1904.]
The deposition of metalliferous and mineral matter from underground solutions may be effected by one or more of the following causes:—
A decrease of temperature.
A decrease of pressure.
By contact with other mineralised solutions.
By gaseous emanations.
Absorption of metals from dilute solutions by silica, clays, and porous substances.
The dissolution and deposition of mineral matter from aqueous solutions must necessarily be governed by physico-chemical laws. It is therefore reasonable to assume that the prevailing geological conditions in each case will determine the forces or processes that will be brought into operation.
The dissolving-power of water is enormously increased by heat and pressure, and it has been proved experimentally that water and water-vapour at high temperatures and pressures are capable of dissolving almost all known rocks and metals. Hence water will possess its greatest solvent power at the greatest depth reached by it, whether it is disengaged from a cooling igneous magma, or exists as a deep-seated circulating current. In the first case the water and vapour will gather their mineral contents from the parent magma, either in whole or in part, and in the second case from the rocks through which the channels chance to pass.
The hot mineral-laden solutions will naturally tend to ascend, and in ascending will gradually part with heat and become subject to less pressure. The substances which were dissolved only at the greatest temperature and pressure will be the first to pass out of solution; and thereafter, as the solutions ascend, with decreasing temperature and pressure, the dissolved substances will be deposited in the inverse order of their solubility. The most difficulty soluble substances will be the first to go out of solution, and the most easily soluble the last. Thus when the ascending waters reach the surface we should only expect to find in solution the easily dissolved alkaline silicates, carbonates, and sulphates.
It is notorious that hot mineral springs do not deposit sulphides at the surface. The cinnabar which has been and is still being deposited in the sinters at Ohaeawai hot springs in New Zealand is being formed from gaseous emanations, and not from the mineral waters. This is also probably true of the cinnabar-deposits at Steamboat Springs and Sulphur Bank in America. At Ohaeawai, Rotorua, and everywhere through out the volcanic regions of New Zealand solfataric and fumarolic action are intermittent phases of the same pipe or vent. In many cases, however, hot springs and fumaroles exist side by side.
The weathering and oxidation of the outcrops of metalliferous lodes by meteoric waters, followed by the transference
and concentration of the valuable contents to a lower depth, forming zones of secondary enrichment, are the work of chemical dissolution and electro-chemical precipitation in which the primary sulphides probably play an important part.
The power possessed by clays, silica, and porous mineral substances to absorb or extract metals from dilute aqueous solutions may play a more important part in the formation of ore-deposits than generally supposed.
W. Skey,* as far back as 1869, proved experimentally that finely pulverised massive quartz, rock-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 appaént 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 possessed 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-formation.
E. Kohler,§ in 1903, experimenting on the line followed by Skey in 1869, showed that clays and porous substances such as gelatinous silica, carbonaceous and colloidal substances, possess the power of extracting metals from their dilute solution.
In this property of clay, silica, and porous substances we may have found the key to the concentration of gold in the
[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 Gaustic 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.
[Footnote] § E. Kohler “Zeitschrift für Praktisohe Geologie,” 1903, p.49.
clayey and talcose matrix of the remarkable lode-formations of Kalgoorlie; of the rich horn-silver and embolite found in the kaolin clay of Broken Hill Lode; of the silver in the silver-sandstones of Utah; and of the copper in the copper-bearing shales of Mansfield and elsewhere. Rock-impregnation by magmatic water in the zone of metamorphism connected with an igneous intrusion may also be traced to the same cause.