Lithogeochemical Exploration of Metasomatic Zones Associated with Volcanic-Hosted Massive Sulfide Deposits Using Pearce Element Ratio Analysis

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Abstract
Reactions between mineralizing hydrothermal fluids and host rocks produce distinct miner- alogically and geochemically zoned alteration haloes in the footwall of many volcanic-hosted massive sulfide (VHMS) deposits, often very much larger than the deposits themselves. Empirical alteration indices have been developed to exploit these features and are widely used as geochemical vectors in the search for new deposits, but only with limited success. The principal limitation of using these indices as lithogeochemical exploration vectors is that they cannot distinguish between the geochemical modifications produced by hydrothermal alteration and the geochemical heterogeneity present in host rocks before the onset of hydrothermal activity. To overcome this problem, a theoretically based lithogeochemical exploration method has been developed that (1) identifies the sources of geochemical variability in VHMS host rocks, (2) isolates the pre-existing geochemical heterogeneity in these rocks through use of linear fractionation and crystal sorting models, and (3) quantifies material transfer due to hydrother- mal metasomatism. This method has been applied to whole-rock geochemical data from footwall rhyolites, collected 250 m below the VHMS deposits at Rio Tinto, Spain. An alkali depletion anomaly is detectable 2.5 km from the deposits, increasing steadily in intensity from unaltered rocks at the margin to totally alkali-depleted rocks in the core of the footwall alteration halo. An Fe and Mg addition anomaly is detectable 1.5 km from the center of mineralization, also increasing toward the core of the alteration halo. The most intensively altered rocks in this set of samples also are the most evolved petrologically, suggesting the possible involvement of a late- stage magmatic hydrothermal phase in the formation of the VHMS deposits at Rio Tinto. Given adequate sample density, this lithogeochemical method may be used to vector toward the core of a footwall (and probably also a hanging wall) alteration halo, and perhaps to massive sulfide mineralization.