Inhomogeneous distribution of REE in scheelite and dynamics of Archaean hydrothermal systems (Mt. Charlotte and Drysdale gold deposits, Western Australia) |
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| Authors: | Joël Brugger Yann Lahaye Sylvie Costa David Lambert Roger Bateman |
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| Institution: | (1) VIEPS, Department of Earth Sciences, VIC-3800 Monash University, Australia Fax: ++61-3-9905-4903; e-mail: joelb@mail.earth.monash.edu.au, AU;(2) Kalgoorlie Consolidated Gold Mines, PMB 27 Kalgoorlie, 6430 Western Australia, AU |
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| Abstract: | Scheelite is a widespread accessory mineral in hydrothermal gold deposits, and its rare earth element (REE) patterns and
Nd and Sr isotopic compositions 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 cathodoluminescence
imaging reveal a very inhomogeneous distribution of the REE in single scheelite grains from the Mt. Charlotte and Drysdale
Archaean gold deposits in Western Australia. Two end-member REE patterns are distinguished: type I is middle REE (MREE)-enriched,
with no or minor positive Eu-anomaly, whereas type II is flat or MREE-depleted with a strong positive Eu-anomaly. The chemical
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 μm. Intra-sectorial growth discontinuities, syn-crystallisation brittle deformation, and variations
in the relative growth velocities of crystallographically 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 fluorapatite, which is locally abundant and has REE contents similar to that of scheelite.
In this context, the systematic increase of the Eu-anomaly between 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 reactions. Consequently, the high positive Eu-anomaly typical of scheelite from 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 concentrations 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 highly sensitive to
the dynamics of the hydrothermal system.
Received: 1 November 1999 / Accepted: 4 February 2000 |
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