Criteria for determining initial lead isotopic compositions of pyrite in Archaean lode-gold deposits: a case study at Victory, Kambalda, Western Australia

Abstract

The initial Pb isotopic compositions of ore-associated sulphides are important in isotopic tracer studies. Studies of Archaean lode-gold deposits from Western Australia indicate that the Pb isotopic data distribution for ore-associated galenas and pyrites is commonly linear, and determination of the initial Pb ratio relies on selecting the least-radiogenic composition. This detailed Pb isotopic study of sulphides from the 32 Ore Zone, which transects a variety of rocks, in the Victory mine (mineralisation dated at 2627 Ma) has resolved the origin of the Pb isotopic heterogeneities which yield linear arrays. It has led to the development of criteria for sample selection which more efficiently and effectively target pyrite samples which best preserve the initial Pb ratio of the sulphide and hence the ore fluid. The most important controls on the displacement of the measured Pb ratio of pyrite from the initial Pb ratio are, in order of importance: (1) Pb content of pyrite; (2) pyrite abundance; (3) proximity of pyrite to ore-fluid channelway (i.e. vein); and (4) host rock. The host rock is important because the Th, U and Pb contents and presence of minor UTh-bearing phases influence the amount and nature of any post-formation Pb represented in the pyrite data. The first three parameters control the magnitude of that influence. A high Pb content, high pyrite abundance and setting in a vein or at a vein margin produce a Pb isotopic composition which is more likely to be dominated or buffered by fluid Pb (i.e. more accurately reflects the initial Pb isotopic ratio). Conversely, pyrite with a low Pb content, low abundance and siting distal from veining is likely to be more susceptible to post-formation modification by contributions from the host rock, resulting in a more radiogenic Pb isotopic composition.