New Exposure and new Insights into the role of Granulite Metamorphism at the Challenger Gold Mine, South Australia
Abstract
The Challenger Au mine, hosted by 2440 Ma migmatitic gneisses within the Gawler Craton, South Australia, is the type-example of a granulite-hosted Au deposit. The overall geometry, mineralization, and metamorphic evolution of the deposit were previously described by Tomkins & Mavrogenes (Econ. Geol., 2002) based on drill core recovered during the exploratory phase of mine development. Based on a number of key textural, mineralogical, and experimental constraints, Challenger was interpreted by these authors to have formed by partial melting of a pre-existing deposit in which Au was remobilized in coexisting sulphide and silicate melts. Challenger has since become an open pit, allowing us to map and sample the new exposure with an eye towards refining the relationships between granulite metamorphism, deformation, partial melting, and Au remobilization. Structures exposed in the Challenger pit are dominated by strong plunge-parallel transposition fabrics. High-strain zones containing multiply refolded leucosomes separate lower-strain domains characterized by cross-cutting extensional structures that host coarse-grained (locally pegmatitic) Kfs-rich leucosomes. Boudinaged leucosomes are common throughout the deposit, with many boudin-necks filled with coarser-grained Kfs-rich material. This pattern of strong layer-parallel transposition overprinted by later extensional structures may help to explain the localization of silicate and sulphide melts in low-strain domains. A reassessment of internal zoning and chemistry of high-grade minerals (Al in orthopyroxene, F in biotite, REE in garnet) combined with petrographic evidence for melt-solid reactions (Melt + Opx (or Grt) = Bt + Qtz) suggest initial isobaric cooling rather than decompression following peak-T. This retrograde P-T path may help to explain why silicate and sulphide melts remained in the rock. Replacement of Grt by Bt + Qtz intergrowths also gave rise to tiny (<20 micron) monazite grains that can been used to directly date the timing of melt crystallization. Bt + Qtz intergrowths that formed where melt back-reacted with Opx and Grt are themselves locally overprinted by a second generation of Opx and Grt indicating that a second high-grade event has affected Challenger. If this is the case, care must be taken when interpreting ore textures at Challenger because of the potential for multiple sulphide melting, remobilization, and chemical fractionation events.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V34A..02M
- Keywords:
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- 3660 Metamorphic petrology;
- 3665 Mineral occurrences and deposits;
- 8025 Mesoscopic fabrics;
- 8102 Continental contractional orogenic belts