A Nanoparticulate Origin for Orogenic Gold
Mark A Pearce1, Michael F Gazley
2, Robert Hough
2, Martin Saunders
3, Louise Fisher
2, Charlie Kong
4 and Guillaume Duclaux
5,6, (1)CSIRO Mineral Resources, Perth, Australia, (2)CSIRO Mineral Resources Flagship, Perth, Australia, (3)The University of Western Australia, Centre for Microscopy, Characterisation and Analysis, Perth, Australia, (4)University of New South Wales, Electron Microscopy Unit, Sydney, Australia, (5)CSIRO Mineral Resources Flagship, Sydney, Australia, (6)University of Bergen, Department of Earth Science, Bergen, Norway
Contact First Author: Mark A Pearce; mark.pearce@csiro.au
English Abstract:
Gold concentrations in gold deposits are several orders of magnitude higher than in most ore forming fluids and up to five orders of magnitude more concentrated than in typical crustal rocks. At such low concentrations, vast quantities of fluids must pass through small rock volumes to deposit the metal found in even a modest deposit. Nanoparticulate suspensions or colloids present an alternative transport medium to the aqueous ionic gold complexes that are typically considered to have formed gold deposits. By combining micro-and nano-scale characterisation of the crystallographic and chemical variation in gold grains from the Plutonic Gold Mine, Western Australia, we present the first direct evidence that large, high economic grade, orogenic gold deposits may form from gold nanoparticles.
Electron backscatter diffraction (EBSD) shows that there micron-scale domains of orientation variation within the grain. At the sub-micron-scale these domains contain grains on the order of 50 – 100 nm that have a strong crystallographic texture. In between the gold nanoparticles are nanoparticles of platinum and platinum-iron alloy that have crystallised with the gold. 10 micron-scale inclusions of Fe-Ca aluminosilicate are also present within the gold grains. The inclusions contain nanoparticulate mixtures of gold, Fe-Ca-Al oxide and SiO2, derived from the same transport medium as the gold. This hypothesis is supported by the strong spatial correlation between calcium-aluminium silicate alteration and gold throughout the deposit. The mixture of oxide and metal/alloy nanoparticles shows that large quantities of gold can be transported as mixed colloids with gold nanoparticles being prevented from coagulating by the oxides at temperatures in excess of 350°C. Post-depositional annealing of the nanoparticulate microstructure has not gone to completion meaning that we have the first evidence that colloids play an important role in orogenic gold deposits.
