Wall-rock alteration and metamorphism of a volcanogenic massive sulfide deposit at Que River, Tasmania; petrology and mineralogy

Univ. Newcastle, Dep. Geol., Newcastle, N.S.W., Australia
CSIRO Div. Explor. Geosci., Australia

The rocks in the alteration halo surrounding the polymetallic massive sulfide deposit at Que River preserve primary hydrothermal mineral textures and assemblages, despite the imprint of later prehnite-pumpellyite facies regional metamorphism. Their textural preservation is attributed to the mechanical strength of many of the hydrothermal minerals such as pyrite, carbonates (except calcite), and quartz, the competency of some of the host rocks, and the low strain that has affected many rocks.At the margin of the halo, assemblages include quartz, white mica, carbonates, albitized plagioclase, pyrite, and chlorite. Closer to the ore, as intensity of alteration increases, albite disappears, pyrite, apatite, rutile, and anatase become more common; white mica, quartz, carbonates, and chlorite vary in abundance and are distributed irregularly. These minerals also occur in hydrothermal veins throughout the stockwork zone. Several generations of veins are commonly developed, with each generation having its own distinctive hydrothermal assemblage. Both hydrothermal and metamorphic phases are compositionally variable. Despite some overlap, there appear to be systematic differences in composition.X-ray diffraction and microprobe analyses show that the hydrothermal white micas are variable in composition, ranging from illites or muscovites to fuchsites. Their metamorphic counterparts contain less pyrophyllite and more celadonite. The hydrothermal carbonates are most commonly ferroan dolomite, magnesian siderite, and ferroan magnesite; the latter two are more abundant than dolomite where alteration is intense. Calcite and ferroan dolomite formed during metamorphism and the composition of the hydrothermal and metamorphic carbonates may be different in individual specimens. Both hydrothermal and metamorphic chlorites are ferro-clinochlores and magnesio-chamosites; metamorphic chlorites outside the halo are more Fe rich and more Al ^vi deficient than those within. The hydrothermal chlorites exhibit Fe = Mg and Al ^iv + Al ^vi = Si ^iv + Mg, Fe ^vi (tschermakite) substitutions and contain Al ^iv contents suggesting temperatures of crystallization of 290 degrees to 320 degrees C, similar to the temperature of 280 degrees C inferred from fluid inclusions.The chemistry of the hydrothermal phases varies throughout the halo. Overall, Fe and Mg do not show a systematic trend toward mineralization; however, Fe enrichment of hydrothermal phases in the ore is apparent, as is Mg enrichment of locally developed chlorite-carbonate rocks. Na and celadonite in white micas and Ca in carbonates, on the other hand, decrease with increasing alteration. Mg only became an important component in the fluid when seawater infiltrated into the hydrothermal system. The heterogeneous distribution of the hydrothermal phases is attributed to successive influxes of hydrothermal fluids of varying composition into the alteration pipe, variable water/rock ratios, and seawater entrainment.

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