Application of pyrite trace element chemistry to exploration for SEDEX style Zn-Pb deposits: McArthur Basin, Northern Territory, Australia

摘要

Sedimentary pyrites in black shales contain abundant trace elements that provide information on the chemistry of the seawater at the time of sedimentation. This study focuses on the Barney Creek Formation (similar to 1640 Ma) in the McArthur Basin in the Northern Territory of Australia, which is host to one of the world's largest SEDEX Zn-Pb-Ag deposits, and several smaller deposits. Fine-grained sedimentary pyrite has been sampled from three drill holes through the Barney Creek Formation at various distances from SEDEX mineralisation. Samples were selected through the stratigraphy of each hole and analysed by LA-ICPMS for a suite of 14 trace elements. The data show that sedimentary pyrite at the base of the Barney Creek Formation, closest (within 1 km) to SEDEX mineralisation, is strongly enriched in Zn and Tl by one to two orders of magnitude compared to the global average for sedimentary pyrite. In contrast sedimentary pyrite from the hole furthest from SEDEX mineralisation (similar to 60 km) contains mean Zn and Tl values equal to, or less than, the global average. Based on the three drill hole pyrite data sets it is concluded that trace elements that are contributed to the basin during hydrothermal exhalation, and adsorbed into contemporaneous sedimentary pyrite, are principally Zn, Tl, Cu, Pb, Ag and As. In contrast, trace elements that are adsorbed into sedimentary pyrite from background seawater are principally Mo, Ni, Co, Se and As. These differences have enabled the development of a SEDEX fertility diagram for sedimentary basins, based on the composition of sedimentary pyrite, that distinguish high Zn, but barren shales, from high zinc SEDEX-related shales. In parallel with the increase in Zn and Tl in sedimentary pyrite approaching mineralisation there is a decrease in Ni, Co and Mo. This means that the ratios Zn/Ni and Tl/Co are particularly good pyrite vectors to SEDEX mineralisation in the McArthur Basin, varying over 4 to 6 orders of magnitude from barren shales to mineralised shales. It is speculated that the reason for the reverse relationship between Ni, Co and Zn, Tl may be caused by hydrothermal exhalations into the water column that effect the ion-exchange pyrite surface complexation processes that alter the uptake of these elements into sedimentary pyrite.