Geochemistry of the host rocks of the volcanogenic massive sulfide deposit at Que River, Tasmania

Abstract

Que River is a moderate-sized, high-grade polymetallic volcanogenic massive sulfide deposit located in the Cambrian Mount Read Volcanics of western Tasmania. Several subvertical lenses of massive sulfide occur in a sequence dominated by variably altered intermediate to silicic volcaniclastic rocks. The sequence was deformed and regionally metamorphosed to prehnite-pumpellyite facies during the Devonian. Approximately 240 host-rock samples have been analyzed in an attempt to determine their primary geochemical affinities and to evaluate the geochemical effects of alteration.On binary variation diagrams, elements such as Zr, Nb, Y, Ti, and Al define linear fields that characteristically project toward the origin. The linear behavior of these elements on such plots is in marked contrast to that of almost all other elements, which show very complex distribution patterns. The linear patterns are interpreted to reflect immobility of the components mentioned, with movement up and down the linear arrays being controlled by concentration or dilution due to subtraction or addition of other components. Thus, primary ratios between immobile components have been preserved and can therefore be used to infer the primary geochemical affinities of the altered rocks. The intersection point of the linear trends defined by the Que River rocks with a low-pressure fractional crystallization trend defined by Cenozoic orogenic lavas probably provides the best estimate of the initial composition of the rocks prior to alteration.In a plot of Zr/TiO 2 vs. Nb/Y the Que River rocks define a restricted area dominantly in the fields defined by modern andesite and dacite-rhyodacite. Basalts and rhyolites are apparently absent. In terms of their primary geochemical affinities, the host rocks of the mineralization are probably best described as a high K, calc-alkaline suite, similar in many ways to modern continental margin andesitic rocks. However, the Que River rocks are by comparison abnormally depleted in Ti and enriched in transition metals such as Ni, Cr, and V.The geochemistry of alteration is complex and not readily amenable to quantitative evaluation. Elements such as Fe, Mg, and Mn have been added to the system whereas Ca and Na have been removed. Both Si and K have been partly remobilized during alteration. Among the major and minor elements, Mn shows the greatest relative enrichment over its inferred primary abundance. However, the Mn anomalies are sporadic and do not define a neat halo. Although most of the observed geochemical alteration appears to have accompanied mineralization, the andesites have been modified from their inferred primary compositions, probably during the regional metamorphism. Hydrothermal alteration is not symmetrical about the ore lenses. Although the presently observed displacement between the massive sulfide lenses and the alteration halo is probably due largely to the Devonian deformation, it is possible that there is also a primary component.The apparent immobility of Al implies that the abundant phyllosilicates, whose development is clearly related to mineralization, do not represent simple chemical precipitates or sediments. Rather, their development would appear to be controlled by the local availability of immobile Al.