Page 26 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

sulfide deposit containing 16 million tones grading 7% copper, 136 ppm silver and 2.4 ppm gold. The deposit
lead, 13% zinc, 0.4% copper, 160 ppm silver and 2.3 comprises a number of sheet-like ore lenses hosted in
ppm gold (McArthur and Dronseika, 1990). The massive to laminated, pumiceous, rhyolitic sandstone
massive sulfide body is located between a footwall and siltstone, above a footwall of very thick, mass-flow-
sequence of feldspar-phyric andesitic lavas, with minor emplaced pumice breccia (Allen and Cas, 1990; McPhie
volcaniclastic units, and a hangingwall sequence of and Allen, 1992). The footwall sequence is altered and
pillow basalt and black mudstone (Fig. 12). locally strongly deformed to quartz-sericite and chlorite
Immediately along strike, the ore position is represented schist containing disseminated pyrite. The hangingwall
by coarse, polymict, volcanic lithic-rich, mass-flow- sequence is dominated by variably crystal-rich or
emplaced breccia and laminated volcaniclastic pumiceous volcaniclastic sandstone, and in places
mudstone and sandstone (Waters and Wallace, 1992). includes thin intervals of black mudstone (Fig. 13).
The massive sulfide body is underlain by an alteration
pipe which displays a series of alteration zones and Both these deposits and others in the Mount Read
related stringer mineralization (Gemmell and Large, Volcanics were described by Large (1992) and in
1992). related papers in the Economic Geology Special Issue
(1992) on "Australian volcanic-hosted massive sulfide
The Rosebery deposit is a 25 million tones massive deposits and their volcanic environment".
sulfide ore body, grading 4.2% lead, 13.8% zinc, 0.6%

Fig. 11 Schematic facies architecture of submarine volcanic sequences, such as the Mount Read Volcanics, western
Tasmania. Volcaniclastic mass-flow deposits include resedimented hyaloclastite from intrabasinal lava flows and
domes, and thick, tabular units of pumice breccia that provide good markers for correlation. There are considerable
regional variations in relative proportions of lava flows, sills and volcaniclastic units, and in volcanic versus non-
volcanic facies. Modified from McPhie and Allen (1992).

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