Cambrian granite-related hydrothermal alteration and Cu-Au mineralisation in the southern Mt Read volcanics, Western Tasmania, Australia

The Darwin Granite is located in the south central portion of the Cambrian Mt. Read Volcanic Belt in western Tasmania. It has been dated at 510 +64, -21 Ma by Adams et al. (1985) and is of particular interest because of it's association with several nearby Cu-Au prospects and its apparent contemporaneous timing with respect to ore formation at the Mt. Lyell mining field. The main aims of this research are to determine the nature and origin of the Darwin Granite, its relationship to the various volcanic suites in the Mt. Read Volcanics, and its relationship to the hydrothermal alteration and copper-gold mineralisation in the district. The Darwin Granite is a highly fractionated phanerocrystalline 1-Type magnetite series equigranular granite with Suite I characteristics as defined by Crawford et al. (1992). The main pink granite phase is intruded by smaller white leucocratic, phanerocrystalline, equigranular to porphyritic, medium to coarse-grained granodiorite and microgranite phases. GENMIX modeling of major elements and REE data supports a model that the granodiorite and microgranite formed from fractionation of the pink granite phase. Negative £Nd(sooMa) values indicate the Darwin Granite was derived from partial melting of a crustal source. REE patterns and abundances in host Central Volcanic Complex (CVC) rocks have virtually identical Suite I characteristics, but REE and trace element data suggest the Darwin Granite was not comagmatic with the volcanic units and they were sourced from a magma with REE and trace element characteristics similar to the 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 exposed flanks and subsurface projections of the Darwin Granite. With increasing distance from the granite, ore styles are variable from iron-oxide veins and stockworks containing pyrite and chalcopyrite ± specular-hematite ± magnetite ± tourmaline and quartz-pyrite-chalcopyrite veins, to disseminated pyrite-chalcopyrite ± covellite, to veins containing quartz, bornite, neodigenite, chalcopyrite and hematite. At the Jukes Prospect, alteration and ore related assemblages are hosted in coherent dacitic volcanics of the CVC. Mineralisation occurs as disseminated pyrite and chalcopyrite, 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 km zone that extends northward from the Darwin Granite to the Jukes Prospect and regional aeromagnetic data suggests that the Darwin Granite underlies the entire northerly trending belt (Leaman and Richardson, 1989; Payne, 1991; Large et al., 1996). Hydrothermal alteration zones around the Darwin Granite and Jukes Prospect represent different parts of the same hydrothermal system. Hydrofracturing and phreatic brecciation of the cupola region released magmatic-hydrothermal fluids which reacted with country rocks and resulted in a complex zoned alteration system. The inner zone is composed of intense secondary K-feldspar assemblages associated with copper mineralisation and grades outward to zones of chlorite and sericite assemblages. Accessory minerals in the K-feldspar zone include sericite, chlorite, pyrite, magnetite, and chalcopyrite. In the chlorite zone accessory sericite, pyrite, magnetite occur as well as chalcopyrite veins. Initial sericite ± chlorite alteration styles were associated with microfracture and vein formation around and above the granite.Total mass changes at the Jukes Prospect were minor and typically involved replacement of one mineral with another without significant nett mass changes. In sericite altered rocks, K20 gains effectively balanced A(Na20 + GaO) depletions (from plagioclase destruction) and total mass changes were small. In K-feldspar altered rocks, K20 gains were larger and were accompanied by minor Si02 and Fe20 3 gains, although the total mass changes were still small (av. 6.1 gms/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 the Jukes Prospect, large mass changes occurred near the Darwin Granite. In the dacites, large gains 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 observed are explained by invoking a magmatic hydrothermal model in which magmatic fluids exsolved from the Darwin Granite. These fluids mixed with modified seawater in reaction zones around the hotter portions of the upflow or discharge zones and K20, and Fe20 3 rich alteration assemblages resulted. Monomineralic vein phases suggest that close to the center of the hydrothermal system fluids were buffered by water (high volume, water/rock ratio). Farther from the systems centre, polymineralic veins and alteration styles suggest rock buffering due to lower water/rock ratios. The widespread occurrence of tourmaline throughout the Jukes-Darwin area suggests significant B in the mineralising fluids. Boron, in addition to H20, may have depressed the solidus temperature of the Darwin Granite allowing the ascending granite additional time for cooling and intrusion to a higher 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 occurred releasing enough mechanical energy to fracture host rocks at depths of 4-5 km (Burnham and Ohmoto, 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 with o180 values between 9%o and 10.5%o. Magmatic fluids in equilibrium with the granite had o180 values around 9 ± 1o/oo. Limited quartz and K-feldspar data suggest that they formed from the same fluid at 485 +90\C -60\"C and o180fluid value of -6 ± 1%o. Magnetite o180 values are 6 to 7.5 ± 1%o at temperatures of 460-550\"C. Quartz K-feldspar and magnetite probably formed from nearly 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 dominated by magmatic sulfur (o34S = 6%o) and volcanic rock sulfur (o34S = 1 0-15%o) and the seawater contribution was small (<25%). As the hydrothermal system developed the contribution of rock sulfur 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 Prince Lyell Cu-Au deposit at Mt. Lyell are interpreted to represent magmatic values but are not uniquely definitive of a genetic source of sulfur. Based on o34S values either Suite II andesites (Crawford et al. 1992) or Cambrian granites could have provided the magmatic sulfur in the Mt. Lyell pyritechalcopyrite ores. REE patterns for the Prince Lyell Suite II andesites and Suite I Murchison diorite and granodiorite are 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 evidence that the Prince Lyell and Garfield apatites were derived from different source rocks. The Prince Lyell apatite t:.Nd1500maJ values suggest that the apatites were derived from a primitive Suite I diorite or granodiorite precursor with a crustal signature similar to the Darwin and Murchison Granites. This evidence combined with similar REE patterns support the interpretation that magmatic fluids derived from a Suite I granite were directly responsible for apatite-magnetite ores at Prince Lyell. Although the magnetite-apatite was almost certainly derived from a Suite I granite precursor the source granite is either too deep to be geophysically detectable or the fluids migrated from the east probably along the Great Lyell Fault."