Geological controls for pre-concentration in an intrusive related gold deposit

The mineralogy, texture and hardness of rocks are factors that influence amenability to pre-concentration during breakage. This study uses samples from an intrusive-related Au deposit to understand the geological features that control preferential gold deportment into the fine size fractions after breakage. Deposit host rocks are mainly sandstone interlayered with siltstone. Gold mineralization occurs in quartz-pyrite and monomineralic pyrite veins as well as in pyrite, hematite, or goethite disseminations. Mineralization occurs as fine-grained native Au, typically associated with hematite or goethite in the oxidized area, or in the fresh ore as Au inclusions or fracture fillings associated with pyrite. The head gold grade is not related to the pre-concentration potential, however, hardness, mineralogy, grain size and density of veins are important factors. Sandstones with relatively coarse grain size, composed of quartz with silica or carbonate cement, and a higher presence of veins are more amenable to concentrate gold into the fine size fraction. Fine-grained sandstones intercalated with siltstone, and with fewer veins present, are more likely to break along soft minerals and deport more gangue minerals into the fine size fraction, thus reducing the gold pre-concentration potential.