Rare-earth element distribution in exhalites associated with Broken Hill-type mineralisation at the Pinnacles deposit, New South Wales, Australia

Abstract

Rare-earth element (REE) concentrations in exhalites and pelites associated with strata-bound base-metal (Broken Hill-type) mineralisation, have been measured for the Early Proterozoic Pinnacles deposit in western New South Wales. Garnetites, garnet-rich rocks and sulphide-rich garnet-gahnite-bearing rocks exhibit a wide variety of REE concentrations, particularly LREE and Eu, which cannot be related entirely to changes in mineralogy, but show some correlation with stratigraphic position relative to the main orebody. Exhalites lying in the footwall to the mineralised rocks have negative Eu anomalies and steep, LREE-enriched patterns, whereas exhalites in the hanging wall to the ores and silicate-rich rocks from within the ore lenses have zero or positive Eu anomalies and less LREE-enriched patterns. HREE have more consistent values throughout the deposit, but in places are strongly depleted. Non-mineralised pelites are slightly enriched in the REE relative to shale composite values and have negative Eu anomalies. It is argued in this paper that, because the variations in data can be related to the stratigraphic location of samples relative to the orebodies, that the observed REE distributions are due to fluctuations in REE concentrations in the primary exhalative fluids rather than differential mobilisation of REE during a second, post-depositional episode of hydrothermal alteration, or mobilisation of the REE during prograde granulite-facies metamorphism or retrograde amphibolite-facies metamorphism which has affected the area. The lack of association between metamorphic mineral assemblages and REE distribution and the absence of any homogenisation of the REE concentrations, suggest that the data reflect the primary exhalative fluid composition and that the variations in REE data reflect changes in palaeoenvironmental conditions such as f O 2 and temperature. A corollary to this conclusion is that there was minimal mobilisation of the REE during subsequent metamorphism.