GOLD IN GRANITE.

West Coast Times, Issue 357, 14 November 1866, Page 2 (Supplement)

GOLD IN GRANITE.

(feom the bendigo advertiser, SEPT. 28.) A correspondent, formerly resident in Sandhurst, and interested in its mines, writes to us from Glasgow : — " The subjoined report, extracted by me from the January number of the ' Geological Magazine,' may bo interesting to your readers. It propounds some theories in reference to the existence of gold somewhat a variance with those generally accepted as to the decrease of gold in depth. I brought under the notice of the Geological Society here the existence of gold at great depths in Australia, and particularly on Bendigo, and took occasion to differ from the generally expressed opinions ou this subject. Ilccent advices from your district have further confirmed my statements, and strengthened mo in my belief that it is only because of the laborious and expensive character of the search, and its thus necessarily limited extent, that the discoveries of gold aj; great depths have been hitherto so few. "In reference to the existence of gold in granite, and more particularly under the first division, it appears to me that — although entirely at variance with Australian workings up to this dale — the correctness of the theory might, to a moderate extent, be practically tested in your neighborhood. If I remember rightly, there are in the Mount Alexander district an older and a newer formation of granite ; and between the Big Hill and the Mount, somewhere near Havens wood, there is a, considerable extent of ground consisting entirely of decomposed granite, answering to Mr Forbes's description, of which I am not aware that any trial has been made. Whether this granite belongs to the nge referred to by him I caunot say ; but, as it is very easy stuff to work, and would wash readily, a trial would neither be difficult nor expensive. "The author believed that the gold deposits of South America had not as yet been studied with a view to determine the geological period at which the gold itself had made its appearance. The present communication was the resnlt of observations made during seven years' travels over a large part of South America, and which had enabled him to class all the deposits of gold which he had visited under two heads, both of which could be traced directly or indirectly to the intrusion or eruption of igneous rock. Under the first head belonged all gold derived from the disintegration of granite rocks ; later than much, if not all, of the silurian strata, biU probably not later than the diornian period. The large gold washings of South America, and probably of the whole world, he looked upon as derived from this source, as well as the auriferous quartz veins, as they could be traced to the proximity of the granite, which he believed to have originated iv or been injected from the granite into the neighboring strata, carrying the gold, which was a normal constituent of the granite itself, along with it. This granite wherever met with is invariably auriferous in itself ; and although it would not pay to grind down granite mountains, and work out the gold in them, yet in many parts of South America — iv Brazil, near Valparaiso, &c. — the granite, apparently solid, was frequently decomposed in situ to depths of even 200 feet, as shown frequently in railway cuttings, and then it sometimes repaid the labor of washbig the .whole mass for the sake of the gold in it. To this class also belong many metallic veins, injected also from the granite into the neighboring silurian strata, which contain gold, and are remarkable for the presence of other minerals very characteristic, as oxide and sulphates of tin, pyrites, copper pyrites^compounds of bismuth, tellurium, selenium, &c, many of which are seldom or never met with in later rocks. " The second appearance of gold is, however, totally distinct from the * On the Existence of Gold-bearing Eruptive Eocks in South America which havo made their appearance at two very distinct geological periods. By David Forbes, F. 8.5., F.G.S., &c.

above, in mineral character as well as iv geological age, and results from the eruption of dioritic (greenstone) rocks, composed of hornblende nnd felspar i (without quartz), which break the j strata, even as late as those containing oolitic fossils, and consequently must be regarded as younger than the oolitic period, but, as far as researches have yet shoAvn, arc probably not posterior to the deposition of the cretaceous strata. In this case, instead of quartz veins carrying the gold from the granite into the neighboring strata, veins of metallic sulphides and arsenides act in the same manner, and the gold is found embedded in its metallic state in the compounds of sulphur and arsenic, with iron, copper, &c. ; and from some unknown cause the more superficial parts of these veins appear as a rule to be much richer in gold, which by the miners is generally supposed to decrease in depth. The minerals commonly fouud in these veins arc not the same as in the metallic veins mentioned as occurring with the granite rocks under the first head, and as far as observations have gone, the metals tin, tellurium, tunstcn, titanium, selenium, &c, are never found in the auriferous veins of later dates. Nothing could be more conclusive than the totally distinct age of these two sets of auriferous eruptive rocks, which the author believes to represent the only j ages at which gold has been introduced I iuto the upper crust of the globe, and thinks it probable that this generalisaj tion may be carried into other parts of J the world, if it be not altogether universal."