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Open File Report 159

Regolith architecture and geochemistry of the Byrock Area, Girilambone Region, North-western NSW

R.A. Chan, R.S.B. Greene, M. Hicks, M. Le Gleuher, K.G. McQueen, K.M. Scott and S.E. Tate

Stage 3 of the Girilambone (Cobar-Bourke) Project has involved collaborative work between CRC LEME and the NSW Department of Mineral Resources in the Byrock area. This work has provided 2713 m of drilling for regolith study with 60 holes, generally 1-3 km apart along a major road traverse across the Byrock 1:100 000 sheet area and more widely spaced across parts of the Glenariff 1:100 000 sheet area.

Regolith-landform mapping conducted in association with drill hole logging reveals that colluvial and alluvial sediments cover most of the Byrock area. Islands of in situ regolith are surrounded by sheetwash deposits on rises grading to depositional plains, and large areas of stagnant alluvial plains, especially towards the northwest. Analysis and interpretation of aircore drill samples has allowed identification of two sequences of sediments, weathered bedrock lithologies (altered and unaltered), as well as various types of induration of both transported and in situ regolith. A 16.8 Ma leucitite lava marker flow separates the younger sequence of sediments (Sequence 1) with inferred arid alluvial, colluvial and aeolian sediments from the older sequence of sediments (Sequence 2) with inferred lacustrine sediments. Sequence 1 includes magnetic sediments whose areal extent can be detected on first vertical derivative airborne magnetic imagery.

Major and trace element geochemical data from the drilling program indicate a different geochemical terrain to that previously examined further south. There appears to be a greater abundance of weathered mafic rocks (probably including dykes and volcanics) and fractionated granites. There are a number of areas where gold is elevated in the regolith. These appear to be in a “gold-only” association and some are spatially associated with mafic rocks, including those in the western part of the Byrock sheet near the Mt Dijou – Bald Hills area. In the Lord Carington area, northeast of Byrock, anomalous gold values (>0.04 and up to 0.15 g/t Au) were intersected over a 39 m interval in weathered chloritephengite-magnetite schists. There is potential for vein and lode-style gold mineralisation associated with mafic rocks in regolith concealed areas.

Detailed work on the soils in the Byrock and adjacent areas has clearly established the presence of a significant aeolian component. This is predominantly in the near 70 µm size fraction. To reduce the diluting effect of this extraneous component, the >100 µm fraction is the most appropriate fraction for geochemical analysis.



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