The volcanogenic sulfide ores at Mount Chalmers, eastern Queensland

Abstract

Copper-gold with minor zinc, lead, and silver mineralization occurs within rhyodactic pyroclastics of Permian age at Mount Chalmers in eastern Queensland. The mineralization is confined to two elongate parallel lodes each composed of an upper massive and partly bedded pyrite-barite lens and a lower stringer sulfide zone. A bedded carbonatepyrite body of limited extent continues downslope from the massive ore and appears to be a facies variant of the latter. The Main lode is situated on the top of a siliceous pyroclastic dome with a present diameter of 300 m and a relative elevation of 120 m. The West lode is situated on a shoulder on the western flank of the dome. The hangingwall sequence consists of rhyodacitic crystal tuffs and cherty siltstones with minor horizons of volcanic breccias and tuffaceous graywackes. Gold is concentrated in the central portions of the lode structure within the massive sulfide ore and the upper levels of the stringer zone. Chalcopyrite occurs with the gold and extends laterally in the massive ore and at depth in the stringer zone. Sphalerite and galena are concentrated in the edges of the massive ore and within the dolomite-pyrite facies downslope from the copper-gold zone. The orebodies and dome structure are interpreted to have developed together, with the zonation of metals in the West lode being strongly affected by the original paleoslope.