THE GEOLOGICAL AGE OF GOLD DEPOSITS.

Hawke's Bay Times, Volume XIII, Issue 544, 20 January 1868, Page 3

THE GEOLOGICAL AGE OF GOLD DEPOSITS.

)TnK Rev. J. E. T. Woods has addressed i t he following interesting letter to the editor of the Australasian : Sin, —Your contributor, in bis “Novelties in Natural History,” lias mentioned a p iper of Mr Forbes’s in the Geological Magazine, on the probable age of gold, which I think deserves a little more attention in this colony. The lime is still distant in which we shall be able to establish positive laws of general application in this obscure matter, but all the researches of contemporary inquirers should be made accessible to those so much interested in gold-mining as the people of Victoria. I offer no apology for the few particulars 1 here subjoin, and regret that time will not allow me to enter more fully into the derails. 1 The geological survey of California has (established the immense development on the Pacific side of North America of rocks equivalent to the upper trias of the Alps, extending from Mexico to T?ritish Columbia, an area which is broken up, interrupted uid covered by volcanic and eruptive rocks, and usually highly metamorphosed. | Accompanying this triass'c formation in jthe Sierra Nevada is an extensive tract ol Jurassic rocks, usually much altered by j chemical or igneous action, and exfremaly ibarren of fossils. These two sedimentary (.areas and a small quantity of carboniferous !limestone form the great metalliferous or (gold bearing belt of the Pacific coast of iNoith America ; therefore a large portion of (he auriferous rocks of California consist of mctamorphic, trassic, and Juras-dc strata, and there is no evidence, says Professor Whitney, to uphold the theory so often maintained tiiat all or even a portion of the auriferous stratas are older than the carboniferous, not a trace of a Devonian or Silurian fossil ever having been dis covered in California. Indeed, he goes on to say that cretaceous rocks are as as much metamorphosed as (he the lower formations, and seem as rich in gold. The vast tertiary formation on the fluiks of the Sierra Nevada, which arc dug and washed for gold in the tunnel mines and hydraulic works of California, are not of marine origin, nor spread out uniformly, but are formed of materials b>ought down from the mountain heights in the later pliocene period, under the sclion of causes similar to those now existing, but probably of greater intensity (I sm quoting from Geo Mag., vol. ii., p. 207), and deposited in valleys, where they filled river-channels and lakes and often embedded whole fjrests, together with (h j remains of land and fresh-water animals. These great detrir.nl valley deposits were covered with volcanic ashes, pumice, and lava streams issuing from the higher parts of the Sierra during the tremendous outbreak of igneous energy ; and since then by the wear and tear of weather, the flanks of Sierra have been further degraded! and hollowed into new valleys, but pro-j tected by (heir covering of hard flat lavas These gold-bearing gravels, once lying lower than the surface of the channelled | mountain sides, have been left to form! high plateaus between the present rivers! and the ridges known as table mountains. Tunn.-l or placer diggings for gold are carried on both in the pliocene deposits and the latter post-tertiary gravel-beds. Professor Whitney thinks that the auriferous (alluvia!) deposits were accumulated in ancient valleys, either beds of ancient rivers or lake-like expansions of former watercourses during the latter pliocene period, and not at the quaterniary drift epoch. It is doubtful whether Mr Whitney means that the gold was formerly, or isl now, disseminated through the triissic. Jurassic, and cretaceous strata but if so, Mr Forbes is of opinion that the conclusion is incorrect, because his observations in Cltili, Peru, and the Bolivian Andes have convinced him that gold is really intrusive, and when found in the more modern sedimentary rocks, such as cretecoous, is always connected with the eruption ol ir.tusive and igneous dykes of diorita or greenstone.

With regard to the time of the introduction of gold, Mr Forbes gives as the result of his observations two periods of auriferous impregnation, which he designates respectively as Ist, the older or auriferous granite outburst; 2nd, the younger or auriferous diorifo out Hu ret,

x!:« 01-ier or auriferous granite uppers to have occurred at gome timP between the Silurian and periods; certainly not older than tha neper si'.urian nor younger than tha carboniferous strata. To this period belongs all the gold derivedfrom the disiutegra ioa of granitic rocks and the auriferous ninirts-T reins, which Mr Forbes looks upon as derived f ora the granite and injected from it into the neighboring strata, carrying gold, which U a normal constituent of the granite itself. The granite is deci.iej|jss auriferous, and thougu it would to grind down granite South America, where it is toundgßsjjjjßßS posed to depths of 200 feet, it repays the labor of washing it. To the same age also imetalic veins injected from granite neighboring strata which contain gold, are remarkable for the presence of minerals, us oxide of tin, tin aud pyrites, compound of bhmouth, selenium, and many of which are seldom or neve? met with in later rocks.

(void formations belonging to this period present themselves in Australia, Bohemia, Bolivia, Brazil, Buenos, Ayres, Chili, Cornwall, Eucador, Hungary, Mexico, New Granada, Norway, Fern, Sweden, Ural, Wicklow. (See Quar. Jour. Gaol. Soc., vol. xvii, Brit. Assoc. Eep. 1815; Geol. Mag. pp. 23 and 38(> vol. iii.) The second appearance of gold is totally distinct from the other in character as well as age. It results from the eruption of dioritic (greenstone) rocks composed of hornblende and felspar without quartz, which break through strata even as lato as those containing oolitic fossils, but probably not posterior to the deposition of the cretaceous strata. The strata are frequently much altered and metamorphosed by the contact of igneous thorite, and at such points often become auriferous, or are cut by auriferous veins proceeding from thadiorite head mass. In this case, however, instead of quartz veins carrying gold from the granite to tha neighboring strata, veins of metallic sulphides and arsenides act in the same manner, and the gold is found embedded in its metalic state in the compounds of sulphur and arsenic with iron, copper, Ac. From some unknown cause, the tnoro superficial parts of the veins appear, as a rule, to be much richer in gold, which by the miners is generally supposed to increase in depth. The minerals commonly found in these veins arc the same as those belonging to the first epoch, and, as far as observations have gone, the metals, tin, tellurium, tungsten, selenium, aro never found in these veins of later date.

Mr Forbes says that at first he supposed these peculiarities to confined to America, but ho has since seer, specimens collected from Italy, the Ural Mountains, California, and the gold districts of India, all of which correspond with what he observed in South America. Lastly, he has had an opportunity of examining a fine series of specimens sent to the Museum of Economic Geology by Mr Alpin, of the Victorian Geological Survey, and these, lie says, aro strikingly similar to those seen elsewhere. The localities for gold of the second epoch arc principally Australia, Bolivia, Chid Mexico, Peru, and Ural. I do not pretend to offer an opinion on these observations of Mr Forbes, which are considerably at variance with those of many Australian geologists. Whether wa agree withthem or not, however, we must admit that the present state of theory oa the subject is of the most unsatisfactory kind. I own that I cannot accept Mr Selwyn’s conclusion as to the pliocene age of all our rich auriferous quartz reefs, even backed as that opinion is by so wide a series of observations and such facilities fur comparison in Victoria. Jr, app f.v> to me to be too much at variance with the almost universal conclusions with regard to the age of quartz veins made elsewhere. Vet the arguments upon which Mr delwyn fourds his conclusions are very s!liking, and one certainly does not sea where to look for a more satisfactory solution. It lias occurred to me, however, in reading Mr Ulrich’s most interesting and valuable ur.ntralogieal report, that since lh-> members of the survey are inclined to regard geld in drift as more a chemical deposit than an alluvial one derived drum abrasion, probably chemical reasons may explain the barrenness of the miocena quartz gravels. It may be that, first, chemical causes were not in operation to cause the deposition of gold ; or, second, it may have been deposited and afterward# removed. The raiocene quartz reefs w T ould s! ill be a difficulty, but I do not gather from Mr JSelwyn’s report that their age was inferred from anything hut their non-auri-ferous character. To convince me I should require evidence of a quartz vein passing through mioceno beds. In conclusion, I may remark that diorite or greenstone is very extensively developed in Australia. The Wood’s Point reefs are in diorite, and I have seen dykes of the same mineral at Coleraine, in Western Victoria, at the junction of the Glenelg and Sleep Bank Uivuiet (where it is largely exposed), and in some few places in South Australia. In Tasmania toe whole island is intersected with veins of this mineral, and among some specimens lately for. warded to me irom Portli-west Australia (Nicoi Bay) F recognised some very freshlooking fragments of the same rock. The latter tact is curious, because the volcanic character of that portion of the continent had been affirmed by the French officer# of Admiral Bundle's expedition, and denied by Stokes, Ki'g Mr Frank Gregory’s volcanic and Iran mountains are ! probably all dioric dykes, for I have scea | no other igneous mineral among the many [ specimens I have examined. 1 J. E. Tbsisos Woods.