Chapter V.—The Examination of Various Constituents of Crystalline and Eruptive Rocks for Gold and Silver.

In the South Island of New Zealand an unusually favorable opportunity is offered for the analysis of the older crystalline rocks, underlying the sedimentary rocks which form the: "country" of the gold deposits. The Manipori formation of this island, comprising the greater part of the mountainous district west of lakes Manipori and Te Anau, in Otago, consists of an enormous thickness (estimated by Prof. Hutton^* at 160,000 feet) of crystalline schists, gneiss, syenite and syemtie gneiss, with associated masses of granite; the whole forming the most picturesque part of New Zealand. The famous West Coast sounds occur in this formation. No paying auriferous lodes have been discovered in it; and for this reason an examination of the rocks is specially interesting here. Stelzner, Posepny and others, who have criticized the conclusions of Sandberger, have laid great stress on the fact that all the silicates analyzed by him were taken from the vicinity of ore-

^* Geology of Otago, by Hutton and Ulrich, p. 28.

page 27 bodies containing those heavy metals which he notes as occurring in the silicates of the country-rock; their contention being that these metals, supposed to occur as silicates in gneiss and other crystalline rocks, were really contained as sulphides, and were therefore impregnations from the neighboring ore-bodies. The same objection, whatever be its weight, might be urged with equal justice against Mr. Becker's derivation of the mercury and gold of the lodes of the Pacific Coast from the granite underlying their country-rock. In the case of the Manipori formation, just mentioned, such criticism could hardly be made, as no auriferous lodes have been found within many miles of the district from which samples were taken for the present investigation.

In the South Island also, samples were taken from the granite abutting on the Carboniferous strata near Reefton, which latter are the chief carriers of auriferous lodes in the western part of the island.

Other samples of granite were collected from various parts of "Westland and Nelson, as far north as the granite quarries of Cape Foul wind.

Selwyn has pointed out^* that the auriferous Silurian rocks of Victoria probably rest everywhere on granite; and this conclusion has been supported by the later observations of Murray^† and others. This underlying granite has not been reached in any of the mines working in the stratified rocks about it; but it is exposed at the surface in many parts of the colony, both near to gold-bearing areas and distant from them. Numerous samples of it were taken for this investigation.^‡

The study of the country-rock from Gympie and Charters Towers, Queensland, gave a good opportunity for the analysis of silicates from crystalline igneous rocks. In the case of Charters Towers, the whole country-rock (locally termed granite) is quartz-diorite, with a little accessory mica; while at

^* Geology and Physical Geography of Victoria, by A. R. C. Selwyn and G. F. H. Ulrich.

^† Geology and Physical Geography of Victoria, by A. R. F. Murray, Melbourne, 1887.

^‡ In New South Wales, granite forms the country-rock of a number of auriferous lodes; and it is much to be desired that some investigator on the spot would take up this inquiry. The present paper contains no analyses of New South Wales granite.

page 28 Gympie a thick sheet of diorite aphanite (locally termed greenstone), containing a large percentage of carbonate of lime, is interbedded with the shales which bound the auriferous reefs, these kindly shales occurring both above and below it. Messrs. Alfred Lord and R. Steele, of Gympie, have furnished samples of the "greenstone."

Sandberger's results having been criticized on the ground that all the rocks he analyzed contained sulphides, and Posepny^* having urged the same objection to Becker's conclusions in the case of the Comstock lode, this point was carefully guarded. In every instance (even though an examination of a hand-specimen showed no sulphides), an analysis of the rock was first made, to determine whether sulphides were present; and whenever such was the case, the sulphides were isolated by panning, and separately assayed for gold and silver; but when sulphides were thus found, no separation of the other crystalline constituents of the rock was made.

It was deemed necessary to isolate and examine the following minerals: (1) Mica and (2) Hornblende, because to these Sandberger traces the silver and gold of many European lodes; (3) the Pyroxenes of the later eruptives, because Becker traces the gold and silver of the Comstock to the augite of the diabase, and both Hutton and Park (already cited) are inclined to refer the gold and silver of the Thames district, N. Z. to the pyroxenes of the andesite;^† and (4) Magnetite, because of Prof. Hutton's remark:^‡

"I would suggest that, as part, at least, of the pyrites has been formed from magnetite, the gold may have been originally in the magnetite, and have been released during the formation of the pyrites. I do not think that this has been the case, but it is a point worthy of investigation by the chemist. The pyrites is, no doubt, a secondary mineral, formed in the rock after consolidation; and if it should turn out to be generally auriferous, we must suppose either that the gold came from below with the sulphur, or that its source is the titaniferous magnetic which is one of the original constituents of the rocks."