Anomalous trace elements in pyrite in the vicinity of mineralized zones at Woodlawn, N.S.W., Australia

Abstract

Pyrite from both chloritized and silicified alteration zones around the Woodlawn, N.S.W., Australia, volcanogenic massive sulphide deposit is characterized by anomalous trace element levels which reflect the presence of base metal inclusions. Ag, As,Bi, Cd, Co, Cu, Mn, Ni, Pb, Sb, Sn and Zn contents of 76 pyrite samples from 48 drill cores have been determined by a semi-quantitative emission spectroscopic method and the samples classified by a cluster analysis technique which defined four groups. Group 1 represents pyrite from the zone of chloritization in the footwall, and members of this group have been recognized at distances greater than 0.5 km from mineralized zones. The group is characterized by high Pb, Ag, Zn and Cu contents and a mean Co/Ni ratio of 0.8. Inclusions, peripheral replacement by base metal sulphides, and rounded crystal edges are commonly observed features. Group 2 includes pyrite generally found as fine-grained aggregates in cross-cutting, chlorite-rich bands in footwall units. Pyrite of this group contains lower base metal values than members of group 1. Group 3 members, from the zone of silicification around the ore deposit, have the highest Zn, Cu, Cd, Sn and Bi contents, high As, Pb, Sb and Ag, and low Ni contents, with a high Co/Ni ratio (mean 21.0). Group 4 pyrite is from rocks equivalent to the hanging wall remote from mineralization and has low base metal, Ag and Ni, but high As and Bi contents, with a high Co/Ni ratio (mean 7.9). Affinity is shown between groups 1 and 2, and between groups 3 and 4 on the basis of Co, Ni, As and Bi contents and Co/Ni ratios, but enrichment of group 1 and 3 members in the ore-forming elements shows them to have been associated with mineralizing solutions. The distribution and mineralogical nature of inclusions in pyrite from alteration zones encountered in volcaniclastic piles has potential to indicate proximity to mineralization and whether the pile itself is mineralized.