Controls to low sulphidation Au-Ag deposits

Low Sulphidation Epithermal Ore shoots, with wider and higher Au grade, are formed by the coincidence of several of:
  • Host rock as competent rocks fracture well
  • Structure as dilatant fracture mediums of fluid flow and sites of mixing
  • Style of mineralisation as higher Au-Ag and variable Ag:Au ratios in different styles
  • Mechanism of Au-Ag deposition as more efficient mechanisms give higher grades
  • Dilution including barren quartz, dykes, calcite
  • Surficial supergene Au enrichment
  • Competent host rocks required for throughgoing fractures which host fissure veins
  • Layered volcanics provide sub-horizontal ore shoots
  • Mostly andesite flows, locally welded tuffs and silicified non-reactive sediments such as sandstone
  • Some domes host only stockwork veins
  • Important exploration opportunities where incompetent clay altered permeable reactive rocks cap mineralised host rocks

Exploration implications – Low sulphidation epithermal Au deposits

Different styles of low sulphidation epithermal Au vary:
  • With arc, intra arc rift and back arc settings
  • Relationship to intrusion source
  • In the paragenetic sequence:
  • Au grades & Au:Ag ratios
  • Quartz-sulphide Au susceptible to supergene Au
  • Carbonate-base metal Au irregular Au grade and association with diatreme breccias
  • Epithermal quartz Au-Ag high fineness and bonanza
  • Chalcedony-ginguro Au-Ag low fineness and bonanza
  • Sediment hosted replacement Au refractory Au

Mechanisms of Au deposition

  • More efficient mechanisms of Au deposition provide higher Au grades
  • Several mechanisms to consider
    • Boiling
    • Cooling
    • Rapid cooling
    • Sulphidation reactions
    • Carbon reactions
    • Mixing with oxygenated groundwaters
    • Mixing with bicarbonate waters
    • Mixing with low pH waters
  • Have a minor effect on Ag:Au ratios