Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia)

Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and Nd and Sr isotopic compositio ns can be used to constrain the path and origin of the mineralising fluids and the age of the hydrothermal activity. Micro-analyses by laser ablation high resolution inductively coupled mass spectroscopy and cathodoluminescen ce imaging reveal a very inhomogeneous distribution of the REE in single sc heelite grains from the Mt. Charlotte and Drysdale Archaean gold deposits i n Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched, with no or minor positive Eu-anomaly, wherea s type II is flat or MREE-depleted with a strong positive Eu-anomaly. The c hemical inhomogeneity of these scheelites is related to oscillatory zoning involving type I and type II patterns, with zone widths varying from below 1 to 200 mu m Intra-sectorial growth discontinuities, syncrystallisation br ittle deformation, and variations in the relative growth velocities of crys tallographically equivalent faces suggest a complex crystallisation history under dynamic hydraulic conditions. The co-existence of MREE-enriched and MREE-depleted patterns within single scheelite crystals can be explained by the precipitation of a mineral which strongly partitions MREE relative to light and heavy REE. Scheelite itself has such characteristics, as does flu orapatite, which is locally abundant and has REE contents similar to that o f scheelite. In this context, the systematic increase of the Eu-anomaly bet ween type I and type II patterns is produced by the difference between the partition coefficients of EU2+ and Eu3+, and not by fluid mixing or redox r eactions. Consequently, the high positive Eu-anomaly typical of scheelite f rom gold ores may not necessarily be inherited from the hydrothermal fluid, but may reflect processes occurring during ore deposition. This case study demonstrates that in hydrothermal systems characterised by low REE concent rations in the fluid, and by the precipitation of a REE-rich mineral which strongly fractionates the REE, the REE patterns of such a mineral will be h ighly sensitive to the dynamics of the hydrothermal system.