Cu-Mo-Au ratios in porphyry-style ore deposits: fluid chemical controls

A series of magmatic and hydrothermal processes involved in the transport and deposition of metals seem to control Cu-Mo-Au ratios in porphyry-type ore deposits and commonly involve three different types of fluids: i) a moderate salinity (2 - 15 wt% NaCl(equiv.)) aqueous fluid of intermediate density, often trapped in the single-phase field; ii) a high-salinity (35 - >50 wt% NaCl(equiv.)) aqueous liquid fluid; and iii) a low-salinity (<5 wt% NaCl(equiv.)) vapour. In many cases the earliest fluids in porphyry-type ore deposits are of the single-phase type, which separates into coexisting high-density brine and low-density vapour. Cesium (Cs) concentrations in the fluid exsolving from a magma correlate with the timing of fluid exsolution relative to the degree of fractional crystallisation of the magma. Highly fractionated magmas exsolve fluids rich in Cs and metals such as Sn, W and Mo, whereas more primitive magmas saturate in fluids poor in Cs and rich in metals like Cu and Au. Phase separation is responsible for the decoupling of e.g. Cu and Au from Mo. The separating vapour is enriched in Cu and to a minor extent in Au, Ag, Sn and As. On the other hand Mo, W, Pb and Zn do not preferentially partition. Finally, precipitation of ore minerals is temperature dependant and therefore may be of selective nature. Copper and Au commonly precipitate from vapour fluids, but there is evidence that Mo mineralization forms from brine fluids, which is rather immobile compared to the vapour. In summary, magmatic-hydrothermal processes, such as fluid exsolution, phase separation and mineral precipitation have a strong influence on the metal ratios in porphyry-type ore deposits.