University of Tasmania
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Gold bearing skarns from the Moina area northwest Tasmania

posted on 2023-05-26, 07:12 authored by Taylor, AC
Stormont, Fletcher's Adit and Ti Tree Creek are three mineralised, calc-silicate skarns in the Moina district, NW Tasmania. Stormont is a Au and Bi-bearing skarn, Fletcher's Adit contains minor Cu, Au, Bi, W, Sn and Zn, and Ti Tree Creek contains small amounts of Sn and Bi. The three skarns are hosted in the basal section of the Ordovician Gordon Limestone and the upper portion of the underlying Late Cambrian to Early Ordovician Moina Sandstone. Metasomatism and mineralisation occurred during the late Devonian intrusion of the ilmenite series, Dolcoath Granite. The granite-to-prospect distance increases to the west, from Ti Tree Creek to Fletcher's Adit, to the most distal (0.5 - 1 km) Stormont deposit. The three skarn deposits have a similar paragenesis during the metamorphic, metasomatic and early retrograde stages of skarn development Wollastonite and salitic clinopyroxene were formed during the metamorphic stage of skarn development. Andradite (and minor Cr-rich grossular) garnet, salitic clinopyroxene and minor vesuvianite were formed during the infiltration metasomatic phase. Epidote, actinolitic amphibole, quartz and fluorite were the major phases formed during the actinolite replacement stage of retrograde alteration, while chlorite, fluorite, muscovite, calcite and quartz were stable during the later mineralisation stage of retrograde alteration. Differences between the deposits in the late retrograde and mineralisation stages include the intensity of retrograde alteration, the abundance of minor retrograde phases, native gold, native bismuth, bismuthinite, galenobismutite, magnetite and base metal sulphides. Opaque minerals present in the three skarns are magnetite, pyrrhotite, pyrite, marcasite, arsenopyrite, native bismuth, bismuthinite, galenobismutite, a Bi-Te sulphide, chalcopyrite, native gold, hematite and goethite. Gold is associated with bismuthinite. Generally, Au, Bi, Pb and Sn mineralisation is restricted to the calcareous host rocks, Cu, W, As, Ag and Zn mineralisation occurs in the skarn or the footwall lithologies and Mo mineralisation occurs exclusively in the footwall. Stormont is a gold skarn, having a high pyroxene/garnet ratio, a deficiency of base metal sulphides, abundant retrograde alteration, late stage reduced mineralising fluids, statistically significant positive Au/Bi, Au/Pb and Bi/Pb metal correlation coefficients, a statistically insignificant positive Au/Cu metal correlation coefficient, locally high Au and Bi grades, very high Au(ppm)/Cu(%) ratios, and common bismuthinite and native bismuth. The compositions of major calc-silicate phases support Stormont's status as a gold skarn. Clinopyroxenes from Stormont belong to the diopside-hedenbergite series, ranging from Diso to Di76· although they are occasionally anomalously manganiferous (up to J09), and are not highly aluminous. Garnets from Stormont are grandites and range from A£49 to Ad6Q. The observed opaque mineral paragenesis and thermodynamic modelling indicate that gold was deposited (with bismuthinite) as a bisulphide complex late in the paragenesis, as a result of the mineralising fluids dropping in temperature and f02. A discrete population of high Au and Bi grades is unique to Stormont in the Moina area, indicating that the mineralising fluids that precipitated Au and Bi at Stormont were of a different character to those at Fletcher's Adit. The gold at Stormont is hypothesized to have originated from underlying Cambrian volcanics which were leached by late stage, circulating meteoric (and magmatic?) fluids. Stormont's increased granite-to-prospect distance, fracture/fault controlled permeability and inter-granular permeability of the skarn (due to massive actinolite replacement), as well as the regionally high permeability of the footwall arenites may have assisted the development of an efficient large-scale convection cell.


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