Scapolitization in the Cloncurry district of north-western Queensland

Abstract

Three extensive areas of intense scapolitization have been found in the Cloncurry district of north-western Queensland, centring on the Dugald River lead prospects, the old Duchess copper mine, and the Trekelano copper mine. At Dugald River the scapolite rocks occur over an area of more than 50 square miles, while at Duchess they outcrop over more than 30 square miles. They lie within the narrow, 150-mile long, north-south belt of Argylla gneisses and schists mapped by Honman, and may be co-extensive throughout this belt. The characteristic rocks are banded scapolite-pyroxene granulites, in which the banding corresponds to the bedding of original fine-grained sediments, thought to have been calcareous shales. The granulites consist essentially of a dipyre, Ma₇₅Me₂₅, and a bright green ferriferous augite, with abundant accessory pink sphene. Some of the granulites contain albite in place of the scapolite, and hornblende or biotite in place of the pyroxene. In places the scapolite granulites are interlayered with the metamorphic equivalents of shales, or grade into scapolite marbles. Elsewhere they appear interbedded with amphibolites. They are intruded locally by gabbroic rocks, amphibolites, granite, syenite, porphyry and aplite, but the volume of acid rocks exposed is small. At Duchess, and at Trekelano, the granite and syenite rocks appear to be responsible for the copper mineralization in those areas. In all three areas the scapolite granulites are associated with “red rocks,” which have resulted from the leaching from the granulites and associated rocks of almost all their magnesia, and much of their iron and lime, with conversion of the residual iron to the ferric state. Portion of the iron oxide has been precipitated as fine haematite “dust” through the residual rocks, rendering them brick red, and almost opaque. At Trekelano this alteration is directly related to the copper mineralization. In its regional character, the scapolitization recalls the scapolitization in the Kiruna district of Sweden.