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• W45084306 abstract "The Darwin Granite is located in the south central portion of the Cambrian Mt. Read Volcanic Beltin western Tasmania. It has been dated at 510 +64, -21 Ma by Adams et al. (1985) and is ofparticular interest because of it's association with several nearby Cu-Au prospects and itsapparent contemporaneous timing with respect to ore formation at the Mt. Lyell mining field. Themain aims of this research are to determine the nature and origin of the Darwin Granite, itsrelationship to the various volcanic suites in the Mt. Read Volcanics, and its relationship to thehydrothermal alteration and copper-gold mineralisation in the district.The Darwin Granite is a highly fractionated phanerocrystalline 1-Type magnetite seriesequigranular granite with Suite I characteristics as defined by Crawford et al. (1992). The mainpink granite phase is intruded by smaller white leucocratic, phanerocrystalline, equigranular toporphyritic, medium to coarse-grained granodiorite and microgranite phases. GENMIX modelingof major elements and REE data supports a model that the granodiorite and microgranite formedfrom fractionation of the pink granite phase. Negative £Nd(sooMa) values indicate the DarwinGranite was derived from partial melting of a crustal source. REE patterns and abundances inhost Central Volcanic Complex (CVC) rocks have virtually identical Suite I characteristics, butREE and trace element data suggest the Darwin Granite was not comagmatic with the volcanicunits and they were sourced from a magma with REE and trace element characteristics similar tothe Murchison Granite. A Murchison-like parental granite has not been identified in the study area,although geochemical evidence suggests that such a granite occurs beneath the study area.Numerous small tonnage, relatively high grade copper-gold prospects are located along exposedflanks and subsurface projections of the Darwin Granite. With increasing distance from thegranite, ore styles are variable from iron-oxide veins and stockworks containing pyrite andchalcopyrite ± specular-hematite ± magnetite ± tourmaline and quartz-pyrite-chalcopyrite veins, todisseminated pyrite-chalcopyrite ± covellite, to veins containing quartz, bornite, neodigenite,chalcopyrite and hematite. At the Jukes Prospect, alteration and ore related assemblages arehosted in coherent dacitic volcanics of the CVC. Mineralisation occurs as disseminated pyrite andchalcopyrite, magnetite-pyrite ± tourmaline ± scheelite veins, chalcopyrite-pyrite-magnetite veins,magnetite± chlorite hydrothermal breccias, and quartz-chalcopyrite stringers.Hydrothermal sericite, chlorite and K-feldspar alteration styles occur throughout a 15 km X 3 kmzone that extends northward from the Darwin Granite to the Jukes Prospect and regionalaeromagnetic data suggests that the Darwin Granite underlies the entire northerly trending belt(Leaman and Richardson, 1989; Payne, 1991; Large et al., 1996). Hydrothermal alteration zonesaround the Darwin Granite and Jukes Prospect represent different parts of the samehydrothermal system. Hydrofracturing and phreatic brecciation of the cupola region releasedmagmatic-hydrothermal fluids which reacted with country rocks and resulted in a complex zonedalteration system. The inner zone is composed of intense secondary K-feldspar assemblagesassociated with copper mineralisation and grades outward to zones of chlorite and sericiteassemblages. Accessory minerals in the K-feldspar zone include sericite, chlorite, pyrite,magnetite, and chalcopyrite. In the chlorite zone accessory sericite, pyrite, magnetite occur aswell as chalcopyrite veins. Initial sericite ± chlorite alteration styles were associated withmicrofracture and vein formation around and above the granite.Total mass changes at the Jukes Prospect were minor and typically involved replacement of onemineral with another without significant nett mass changes. In sericite altered rocks, K20 gainseffectively balanced A(Na20 + GaO) depletions (from plagioclase destruction) and total masschanges were small. In K-feldspar altered rocks, K20 gains were larger and were accompaniedby minor Si02 and Fe20 3 gains, although the total mass changes were still small (av. 6.1gms/100gms). Total mass changes in chlorite altered rocks effectively balance A(K20 + MgO +Fe20 3) gains with A(Na20 + GaO + Si02) losses. In contrast to the minor mass change at theJukes Prospect, large mass changes occurred near the Darwin Granite. In the dacites, largegains in Si02 (+80 gms/100gms) accompanied smaller mass gains in Al20 3, K20, Ba and Sr,while depletions of Si02 and Al20 3 occurred in adjacent andesites. The mass changes observedare explained by invoking a magmatic hydrothermal model in which magmatic fluids exsolvedfrom the Darwin Granite. These fluids mixed with modified seawater in reaction zones around thehotter portions of the upflow or discharge zones and K20, and Fe20 3 rich alteration assemblagesresulted.Monomineralic vein phases suggest that close to the center of the hydrothermal system fluidswere buffered by water (high volume, water/rock ratio). Farther from the systems centre, polymineralicveins and alteration styles suggest rock buffering due to lower water/rock ratios. Thewidespread occurrence of tourmaline throughout the Jukes-Darwin area suggests significant B inthe mineralising fluids. Boron, in addition to H20, may have depressed the solidus temperature ofthe Darwin Granite allowing the ascending granite additional time for cooling and intrusion to ahigher crustal level (and a lower lithostatic pressure regime) than otherwise may have occurred.The higher level of intrusion allowed the granite to reach a level at which second boiling occurredreleasing enough mechanical energy to fracture host rocks at depths of 4-5 km (Burnham andOhmoto, 1980; Burnham, 1985) consistent with estimated depths of emplacement at Mt Darwin.Whole rock o180 data for the Darwin Granite is consistent with crystallisation from a magma witho180 values between 9%o and 10.5%o. Magmatic fluids in equilibrium with the granite had o180values around 9 ± 1o/oo. Limited quartz and K-feldspar data suggest that they formed from thesame fluid at 485 +90C, -60C and o180fluid value of -6 ± 1%o. Magnetite o180 values are 6 to 7.5± 1%o at temperatures of 460-550C. Quartz, K-feldspar and magnetite probably formed fromnearly pure magmatic fluids, the likely source of which was the Darwin Granite.Sulfur isotopes in the Jukes-Darwin system suggests that the initial sulfur budget was dominatedby magmatic sulfur (o34S = 6%o) and volcanic rock sulfur (o34S = 1 0-15%o) and the seawatercontribution was small (<25%). As the hydrothermal system developed, the contribution of rocksulfur and magmatic sulfur decreased and the contribution of reduced seawater sulfate increased,consistent with the arguments of Solomon et al. (1988). o34S values (average 7%o) from the PrinceLyell Cu-Au deposit at Mt. Lyell are interpreted to represent magmatic values but are not uniquelydefinitive of a genetic source of sulfur. Based on o34S values, either Suite II andesites (Crawfordet al., 1992) or Cambrian granites could have provided the magmatic sulfur in the Mt. Lyell pyritechalcopyriteores.REE patterns for the Prince Lyell Suite II andesites and Suite I Murchison diorite and granodioriteare nearly identical to REE patterns in apatite at Prince Lyell and the Garfield Cu-Au Prospect.However, apatite t:.Nd(soomal values from the two prospects are different and are strong evidencethat the Prince Lyell and Garfield apatites were derived from different source rocks. The PrinceLyell apatite t:.Nd1500maJ values suggest that the apatites were derived from a primitive Suite I dioriteor granodiorite precursor with a crustal signature similar to the Darwin and Murchison Granites.This evidence, combined with similar REE patterns, support the interpretation that magmaticfluids derived from a Suite I granite were directly responsible for apatite-magnetite ores at PrinceLyell. Although the magnetite-apatite was almost certainly derived from a Suite I graniteprecursor, the source granite is either too deep to be geophysically detectable or the fluidsmigrated from the east, probably along the Great Lyell Fault." @default.
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• W45084306 date "2001-02-01" @default.
• W45084306 modified "2023-09-26" @default.
• W45084306 title "Cambrian granite-related hydrothermalalteration and Cu-Au mineralisationin the southern Mt Read volcanics,Western Tasmania, Australia" @default.
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