Stavely Project Report 11 - Illite crystallinity and mica lattice geothermobarometry and XRD/SEM mineralogy

Category: Stavely Project	Product Code: MP-R-158954
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Introduction:
The Stavely Project is a collaborative geoscience research project between Geoscience Australia and the Victorian Department of Economic Development, Jobs, Transport and Resources (represented by the Geological Survey of Victoria). The aim of the Stavely Project is to improve the understanding of the regional geological architecture and mineral prospectivity of western Victoria, in particular the Cambrian-aged Stavely Arc.

Executive Summary:
This report contains metamorphic pressure and temperature (P/T) estimates for the Cambrian basement hosting mineralised porphyries in the Stavely region of western Victoria. Mudstone drill core samples were analysed by X-Ray Diffraction, with the b₀ atomic lattice spacing of white mica providing an estimate of pressure whilst the crystallinity of illite provides an estimate of temperature. Samples from across the length and breadth of the Grampians-Stavely Zone all had similar prehnite-pumpellyite facies metamorphic conditions. Pressure estimates suggested relatively uniform mid-crustal depths of 11-16 km and the temperatures clustered around 220-280⁰C. The limited spread of both the pressure and temperature across the entire region suggests a relatively consistent level of exhumation. This regional consistency of exhumation is encouraging for the preservation of mineralised porphyries which intruded into this basement shortly after the Cambrian metamorphism. Scanning Electron Microscopy of the mineralogy of the basement samples was conducted as part of the analytical procedure. This analysis revealed previously unknown diagnostic differences between the Glenthompson Sandstone and Nargoon Group, with the former being more micaceous whilst the latter was more chloritic and feldspathic. These mineralogical differences reflect the different geological histories of the two rock packages and could be used in future work to better define the mapped boundary between them.

Bibliographic reference:
McKnight, S.W. & Taylor, D.H., 2019. Illite crystallinity and mica lattice geothermobarometry and XRD/SEM mineralogy analysis of sedimentary bedrock samples from the Stavely Project area. Stavely Project Report 11. Geological Survey of Victoria. Department of Jobs, Precincts and Regions.

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The downloadable version of this report is supplied in PDF format (20.4MB) and MS Word text only (50kb).