Page:History of West Australia.djvu/399

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To such deep-seated action of various degrees immense masses of archæan strata must have been subjected along the great divisional fractures of the interior gold region. Gases under pressure as well as overheated steam and water could easily pass through rocks in such a state. Soluble chemical combinations became dissolved, the waters became charged with them; and forced by their own steam and gas pressure those solutions entered upon their journey towards the surface.

If gold existed in archæan rocks which have suffered such subterranean solfatara leaching, it could enter into solution.

Alkaline sulphides, ferri-sulphate, solutions of sodium carbonate and sodium silicate, are solvents of gold in a higher or lesser degree.

Solfatara solutions rising out of silicate rocks generally contain alkaline carbonates and silica. Alkaline sulphides, in which metallic sulphides are slightly soluble, are often found to occur in conjunction with them. Probably the gold has entered such solutions as gold sulphide, and in some cases as telluride.

The solfatara solutions have risen to the surface through fissures, but certain channels within the fissures, where there was least resistance, have served as principal vents.

During their upward journey the solfatara solutions became gradually relieved of their steam and gas pressure, and they also were subject to gradual cooling. In consequence thereof, the less soluble silica was deposited within the fissures along with sulphides (chiefly iron sulphides) and metallic gold. Sulphide of gold, being a very unstable chemical compound, became dissociated at the moment of its precipitation, and metallic gold was deposited either as minute particles enclosed in sulphide of iron (pyrites) or as gold free in quartz.

Telluride of gold has probably played a similar role as sulphide, but being a stable chemical compound became deposited as such.

All the primary auriferous gold deposits within the interior gold region are more or less connected with fissure eruptions. Dyke fissures, extending sometimes for miles, have served as eruption channels, until the rising magma within became solidified by successive cooling. The igneous rock within such dyke fissures appears occasionally interrupted, and its place is taken by quartz. So-called quartz blows generally disclose such occurrences. Magma has not entered those portions of the fissures, and the open spaces became successively filled either by lateral secretion or by deposits from circulating solutions. If such solutions were auriferous gold deposits were formed.

Quartz lodes or reefs formed within such empty spaces of dyke fissures, where igneous magma did not enter, generally contain very low-grade ore; still phenomenally rich gold shoots have been found within some of those lodes or within their branches.

The country formations through which the dykes have broken are chiefly dioritic and chloritic schists, but along contacts of such schists with archæan rocks they occasionally enter the latter also.

In some instances portions of dyke rock (diorite, diabase, porphyrite, and felsite-porphyrite) are traversed by numbers of more or less irregular auriferous quartz veins. The fissures which have served for the formation of the latter are generally contraction fissures, due to the cooling of the dyke rock. Solfatara solutions rising along the dyke walls have permeated the dyke rock within such fractured portions, and have deposited auriferous quartz and auriferous pyrites.

A complete kaolinisation of such dyke portions is of frequent occurrence; it generally affects the adjoining country formation, and reaches down to depths of 200 and more feet.

Such complete kaolinous decomposition is generally confined to true fissures and to the vicinity of such. This kaolinisation is apparently a sequence to the cessation of activity within hydrothermal vents.

The general or surface alteration of the palæozoic greenstones has chiefly produced chloritic features.

Portions of dykes containing auriferous quartz veins can be considered as compound ore deposits, inasmuch as lodes of igneous origin (the dykes) contain within their body numerous veins of aquæous origin. In many instances the dyke rock, and in some cases the adjoining country formation, have been impregnated with gold and iron pyrites. The latter impregnate the rocks in form of countless small hexahedrons; in decomposed portions such hexahedrons are hematite pseudomorph after iron pyrites.

In some cases of similar occurrences not only the smaller auriferous veins within the dyke rock, but also the latter, and perhaps even some of the adjoining country rock, will constitute payable gold ore.

Eruption fissures and faults are generally accompanied by numbers of parallel, lateral, and branch fissures, which gave occasion for the formation of auriferous lodes and quartz reefs.

The various occurrences of auriferous ore in reefs are as follow: The auriferous quartz in some of the reefs forms a continuous body, the width of which does not vary considerably; cross-fissures and faults, as a rule, cause increased richness in gold.

The auriferous quartz appears in form of lenticular bodies along a fissure. In the intervening spaces the fissure narrows greatly, and contains only fractured country rock. In such cases the edges of the lenticular quartz bodies have been found to be of greater richness. The dip of such quartz bodies may follow the underlay of the lode fissure, or else it may be inclined along the course of the lode.

The so-called formation lodes generally occur in schistose country formation, and their course follows more or less the strike of the schistosity. They are almost confined to the vicinity of eruption fissures, and contain in their decomposed levels, besides chloritic talcose and serpentinous gangue, loose and porous gossan and silicious sinter. The fissures of the formation lodes are not clean open fissures, but rather plains along which rock movements have taken place. Solutions under steam and gas pressure rising through such fracture planes have altered the country rock for a certain distance; they have caused its ore impregnation, and their deposits in conjunction with crushed country rock have formed lode breccia. The country alteration and ore impregnation, even in the absence of lode breccia, retains a true lode appearance. Several such formation lodes have been opened up in the southern portion of the Kalgoorlie lode system, and their size and richness in gold have attracted considerable attention. The gold in the upper levels of the Kalgoorlie formation lodes occurs almost entirely as free gold, but in two allotropic modifications, namely, as the usual yellow gold with metallic lustre, and also as amorphous gold of brown, red, and purple colour. Samples of the latter assume by heating and also by rubbing in the agate mortar the usual gold colour. This modification is more resistive against amalgamation, but dissolves easily in chlorine and cyanide of potassium. In some of those lodes at lower levels very rich tellurides of gold are found. The