Wednesday, November 7, 2007 - 10:30 AM
321-3

Natural Ultrathin Nanoplates of Supergene Gold.

Robert M. Hough1, Rob D. Hart2, Ryan Noble1, David Vaughn3, Steve M. Reddy2, Greg Hitchen1, Martin Saunders4, Peta Clode4, and Ravi R. Anand1. (1) CSIRO, CSIRO Exploration and Mining/CRC LEME, PO Box 1130, Bentley, 6102, Australia, (2) Curtin University of Technology, PO Box U1897, Bentley, Australia, (3) ., PO Box 187, Subiaco, Australia, (4) University of Western Australia, Perth, Australia

The Golden Virgin pit to the south of Southern Cross in Western Australia is a small gold deposit where primary mineralisation occurs as small high-grade quartz veins blanketed by up to 30 m of completely weathered cover (28 m of saprolite and 2 m of transported sands and gravels). The veins near the base of the pit contain visible primary Au in the quartz and have weathered fracture surfaces lined with several generations of secondary iron oxides, clays, sulphates, and a population of supergene Au crystals. Gold is normally bright in SEM BSE; however, there are supergene Au some grains that are dark grey. Many of these crystals are transparent to the electron beam; underlying materials are visible through them. Monte Carlo modelling combined with imaging at different accelerating voltages indicates the crystals are only 20-50 nm thick. Some direct analysis of plate edges indicate plates <17 nm thick.  SEM observations also reveal complex internal plate structures where contouring of the lattice appears to produce these textures and small holes appear to act as seed points. TEM indicates these structures are probably bend contours related to lattice strain, possibly caused by buckling during settling onto the substrate. These single crystals are defect rich. This gold occurs closely associated with salt crystals and some are intergrown with barite, supporting the premise that native gold (Au/Ag alloy) was dissolved locally into groundwater, the Ag remained in solution, and pure gold was precipitated from the saline and slightly acidic groundwater. The gold crystallography implies rapid deposition of supergene gold in days.


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