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Carlin Trend Alteration

Gold was deposited from hydrothermal (hot water) fluids. These hot, slightly acidic, metal-bearing solutions changed, or altered, the mineralogy and chemistry of the rocks they passed through. The altered rock, “alteration halo”, surrounding these deposits is much larger in volume than the gold deposit itself, and serves as a guide in the exploration process for these deposits. Important alteration types are listed below.

Decalcification (decarbonitization)

  • The removal of carbonate from calcareous rocks by slightly acidic gold-bearing fluids


  • The addition of silica (quartz) through replacement or veining of a given rock type
  • “jasperoid” – a decalcified and pervasively silicified carbonate rock


  • Introduction of fine-grained, gold-bearing pyrite – often called “sooty sulfides” (e.g. pyrite, arsenical pyrite or marcasite)


  • May occur as a supergene or hypogene process whereby sulfide minerals (pyrite) are exposed to oxygenated fluids to form iron oxide minerals such as hematite, limonite or goethite
  • Vertically-extensive oxidation zones may exceed 300m in some gold deposits
  • Many Carlin Trend gold deposits are vertically-zoned where an oxidized gold zone transitions to a reduced gold zone at depth


  • Black, residual and/or hydrothermal carbon is common in reduced zones and on the margins of gold deposits


  • Dolomite or ferroan dolomite occurs as replacement flooding or as veins

Argillic Alteration

  • Formation of clay minerals such as kaolinite, illite, montmorillonite, dickite

Common Accessory Minerals

  • Barite, orpiment, realgar, stibnite

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