MINING GEOLOGY.

Otago Witness, Issue 3743, 8 December 1925, Page 80

MINING GEOLOGY.

M ITALLOGEN ETIC EPOCHS. By Professor James Park. V. Intense igneous activity s an expression of the organic movements that built up the inountuiu chains of past and present times. And since the origin of most oredeposits can be traced to igneous activity it follows that the great epochs of oreformation may be grouped around the major period of diastrophic movement, that is of crustal deformation by folding and faulting. As a consequence of their fjentic r. lationship with tectonics the metalogenetic provinces cointide with the geographical limits of the petrographic provinces. The periods of Mountain-building recognised in Europe are : 1. Tho Caledonian, a Devonian period of S.W.-N.E. folding that formed one of the greatest chains of the past. 2. Tho Armorican, a pre-Permain W.N.W. E.S.E. folding. 3. The Variscan, a pre-Permain W.S.WE.N.E. folding. 4. The Alpine, the early Cainozoic folding that formed existing apline chains, possibly a recrudescence of the late Palaeozoic foldings. Collectively the Armorican and Varisoan chains, of which, like the Caledonian, only the worn-down stumps now remain, are known as the Ilereynian system. The piles of marine and deltaic sediments, which accumulated in the periods of quiescence, show that the land during these periods was not stable, but on the contrary slowly sinking, this perhaps arising from the overloading of the sea-floor along the fringe of the ancient continents. When the overloading disturbed the isostatic equilibrium, diastrophic movement began and continued till equilibrium was once more established. Then followed another period of quiescence and sedimentation. accompanied as before by general subsidence. Tho main periods of sedimentation and sea-transgression were the Silurian. Carboniferious, Mesozoic, and Cainozoic. The orogenic foldings of the past, though first distinguished in Europe, are now believed to have been world-wide; hence the Caledonian. Ilereynian. and Cainozoic alpine foldings may be regarded as worldtypes of diastrophic movement. Such diastrophic movements are held to be in themselves major events in the earth’s history. Chamberlain* believes them to have been the direct cause of extensive changes in sedimentation ; and these, together with climatic and other effects that followed, influenced profoundly the development of life. It is certain that, in the main, the diastrophic periods were distinguished by the magnetic intrusions that originated the whole series of after-eruption processes by whioh many valuable ore-deposits were formed. De Launayt was the first to distinguish the various regional G-nes of ore, and to show their genetic relationship with orogenic movements and with petrographic provinces. The regional types he called metallogenetic provinces. Later, Maclarent applied the same method ir constructing his auriferous provinces. The major organic events were continental, if not world-wide, and as a consequence the periods of mineralisation exhibit a characteristic relationship. The Middle Tertiary movements, which ggave rise to the existing mountain systems, were followed by vast effusions of lavas that are metallogenetic in regions as wide apart as Hungary, Nevada, Central America, and New Zealand, where in similar lavae there occurs the same type of mineralisation. Propylitic quartz veins with gold and silver are characteristic of the Cainozoic or Alpine epoch of deformation. Along the western part of the North American continent, from Alaska to Mexico., following the Cordilleran region, occur piles of Tertiary andesites, dactites, and rhyolites, with quartz veins containing gold and silver, prominently at tho Comestock. Tonopah, Cripple Creek, Pachuca, and Elo Oro districts, where propvlitisation and mineralisation have mainly taken place at or near the centres of volcanic activity. In the Province of Banat. in Hungary, and at Waihi. Thames, and Coromandel, the Miocene andesites also contain vein# carrying gold and s.ilver, usually associated, as in the Cordilleran chains, with tellurium and sometimes silenium. The propylitic veins sometimes contain local concentrations of lead and zinc, antimony, or quicksilver. The Hercynian movements were accompanied by widespread igneous activity tvpically characterised bv intrusions of diorite. which in one phase grades into the younger granite, and in another phase into norite, gabbro, or peridolitc. The granitic nhasp generally oaves rise to the tin-copoer lead-zmc tvpe of mineralisation, as in Cornwall and Freiberg: the basic phase, to the gold-bearing veins of California. Victoria, and South Island of New Zealand, and se >n in Silurian and Ordovician rocks. Where Hercynian igneous intrusives have come in contact with older Palaeozoic shales or 9lates mineralisation with copper has not infrequently taken place, as at Parys Mountain, Anglesey, and in Northern Norway. Tlie Caledonian (Devonian) movements were everywhere characterised by granite intrusions. The most important, concentrations that can be assigned to this epoch of mineralisation are the pyrrhotite and chalcopyrite ore-bodies near Loch Fyne, in Western Scotland, the wolfram bearing pegmatites of the Lake Country of Northern England and the pyritic fahlbands of Norway and Sweden. The leadzinc ores of the Lake Country are postSilurian and possibly pre-Oarboniferous; but the information concerning this is not precise, and later research may show that after all they are Hercynian. The Pre-Cambrian epoch of mineralization is mainly characterised by bod-impreg-nation, and deposits of the dynamic and contact metaphoric types. Notably we have the iron-ore segregations of North America contained fn the sedimentary rocks associated with the Keewatin, tne magnetite and corundum ores in the sedimentaries of

the Grenville series in the Adirondack's and tions in the Lake Superior region; the gold-bearing quartz veins in the Porcupine district, Ontario; the pyritic ores of Rio Tinto; the lead-zinc ores of Broken Hill; the gold-bearing veins of Mysore; the mag ietic iron-ore segregations of Scandanavia, and the less important pvritic masses in the gneisses of the Scottish Highlands. The bedded iron-ores that occur in gneisses and other crystalline rocks are sedimentary ores altered and concentrated by dynamic* uietaphorism. Th.ev do not belong to the type of ore-body thta owes its existence to igneous activity. The great assemblage of life that exist.-d in the Cambrian, much of it highly organised. lend’ to the hielief that the p e Cambrian in which the ancestral fr .ms developed, must have covered a vist interval of time. It is, therefore, ro* a matter of surprise to find that the preCambrian, as tvpically seen in En.-uin Canada. contains repeated epochs of mineralisation. In the province of Ontario chemical deposition of iron has taken plate at three different, periods, the Grenville, Tirniskamian. and Animikian, -while metalliferous veins are associated with basic intrusions of post-Timisknmian age, with Algernon granite, and with the Keweenawan intrusions. Tn connection with the pre-Cambrian orebodies Miller and Knight ** summarise the result of their observations as shown in the following table. Keweenawan. - Basiu intrusions with sotno aci-lio facies. Silver, cobalt, nicjcel an<l arsenic at Cobalt. Nickel and cobalt at Sudburv. Gold in various Places. Animikian.— Chemical deposition of iron formation*. Algoman.—Granite intrusions. Gold at Porcupine and elsewhere. Galena, zinc blende and fluorspar. Basic intrusions of post-Timiskian age.—Nickel and chrooy'e Magnetite and titaniferous fitanite. Tirniskamian.— Chernicai depositions of iron formations. Laurentian.- Granite intrusions. Grenville.—Chemical deposition of iron formation*. Keewatin. -Basic volcanic eruptions **Gvol. Mag. in 1(5, p. 573. ffWaldeuiar Lingren, “Metallographic Enochs ” Econ. Geol. vol. 4. 1909. pp. 400-410. XXA. M. Fin lay s<m, “The Metallogeny of the British Isle?," Quart. Jour. Geo. Soc.,' vol. 68, 1910. p. 281. The -metallogenetic epochs of North America in their relation to the tectonic history of the country have been summarised by Lindgren*, and of the British Lies by Finlayson. ++ Lindgren sums up the available information relating to the epochs of tniner.disation in he Cordilleran belt in the following useful statement. Principal Principal rocks metals. associated with deposits. 1. Deposits of pro- gold and granites diroites, Cambrian period copper gabbro 2. Deposits of early basalt, diabase musozoic copper gabhTO 3. Deposits of late granodiorite tertiary gold quartz-monzxjnite gold, granodiorite . -n , silver, quartz-monzonite 4. Deposits of enrly • wm, tot,ar ' lead correspond,™ porzinc phyritic rocks 5. Deposits of late gold, andesi te tertiary silver rhyolite 6. Deposits of post- quiekpliaeene silver basalt 7. Cretaoeous or later sandstone concentrations In copper shale, ounsedimentary rook* glomerate The cooling of the crust and consequent contraction are possibly responsible for the mountain-building, but needless to say the origin of the mountain chains of past time is only of interest to the student of economic geolog}’ insofar as the building of these chains is responsible for the formation of cavities and ore concentration. According to Jeffreys*** the level of the line of no stress is more than 100 km from the surface, and the compression above this neutral line is, he thinks, sufficient to have reduced the rpper crust by 45 x 10 to the 15th power square cm. Folding and overthrusting are an expression of (he shortening arising from rock contraction. The data relating to the rate of cooling are meagre, but the experimental work of P. W. Biidgman+tt shows that at a depth of several hundred kilometres the thermal conductivity of rocks may be several times greater than hitherto assumed for the purpose of computation. This would permit cooling to a greater depth, and might easily increase the available compression. There might also be local expansions an:l contractions associated with isostatic adjustments, and contraction due to the change of the earth's rotation. The contraction at the surface arising from surface crystallisation ranges from 5 to 10 per cent., ami possibly the contraction due to crystallisation in depth from a viscous body may be as much or more. In whatever w r ay it has arisen contraction seems competent to explain all tho phenomena of mountain-building. Against this view it is urged that it does not explain the periodicity of mountainbuilding and its related metallogenetic epochs. But it must not be forgotten that the stresses are slowly accumulative till the limit of elastic deformation is reached, after which relief is obtained by folding and fracturing. With progressive cooling the process again begins, each cycle of quiescence being followed by a shorter Interval of intense diastrophic movement, with its consequent igneous activity and metallogenesis.