The Scuddles volcanic-hosted massive sulfide deposit, Western Australia; geochemistry of the host rocks and evaluation of lithogeochemistry for exploration

CSIRO Div. Explor. Geosci., North Ryde, N.S.W., Australia
Univ. N. Engl., Australia

The present composition of host rocks from the Archean Scuddles volcanic-hosted massive sulfide (VHMS) deposit reflects superimposed hydrothermal alteration on primary lithologies ranging from andesite to rhyolite. Within both the hanging wall and footwall, Ti, Al, Zr, Nb, Y, and the rare earth elements (REE) appear to have been largely immobile during mineralization and subsequent regional metamorphism. Within the mineralized horizon there is evidence for some mobility of these elements. The hanging wall and footwall are most clearly distinguished geochemically by the depletion of alkalies and alkaline earths in the footwall. A similar depletion in the mineralized horizon appears to be due to hydrothermal alteration.The hanging wall and footwall have similar chondrite-normalized REE patterns; the mineralized horizon shows more variable REE patterns, with some of the massive sulfides showing pronounced positive Eu anomalies. Except for the mineralized horizon, observed Eu anomalies at Scuddles are smaller than those associated with several other VHMS deposits. The muted Eu anomalies may be reflecting relatively oxidizing alteration conditions during mineralization rather than primary magmatic processes.Whereas some of the geochemical parameters observed in the Scuddles host rocks match those in mineralized volcanic sequences from Canada, other characteristics could be interpreted to indicate little mineralization potential. Thus Zr/Y ratios in some of the footwall rocks are very low, a "fertile" signature, while in the same rocks Eu anomalies are either small or absent, a "barren" signature.The use of lithogeochemistry in the search for VHMS deposits requires an appreciation and understanding of both primary and secondary processes involved in the formation and subsequent evolution of volcanic belts and their contained ore deposits. The rare earth elements do not appear to provide unequivocal evidence about the "fertility" of the host rocks at Scuddles.

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