The Exploration Significance of Ag/Au, Au/Cu, Cu/Mo, (Ag×Au)/(Cu×Mo) Ratios, Supra-ore and Sub-ore Halos and Fluid Inclusions in Porphyry Deposits: A Review

This paper documents the exploration significance of Ag/Au, Au/Cu, Cu/Mo and (Ag×Au)/Cu×Mo) ratios of the supra-ore and sub-ore halos versus fluid inclusion evolution for 24 Cu, 6 Cu- Mo and 10 Cu-Au porphyry deposits worldwide. The ratios are based only on the economic and mineralized hypogene alteration zones. The results indicate that (Ag×Au)/(Cu×Mo), Au/Cu and Cu/Mo ratios increase with decreasing depth and could be considered as the possible exploration guides for many as-yet- undiscovered unexposed porphyry deposits. These ratios are well correlated with vertical elemental vector zoning and alteration halos from the center (proximal potassic) outward (distal propylitic-zeolitic) and follow the order of Cu-Mo-Au, Cu-Zn, Zn-Pb-Ag, Ag-Pb- As-Sb-Hg-Au. On the basis of the (Ag×Au)/(Cu×Mo) ratios and fluid inclusion data, the Iranian continental arc porphyry copper deposits show multiphase halite-sylvite saturated fluid inclusions and formed between 2 to 4.75 km depth. The simple and multiple geochemical ratios and fluid inclusion evolution highlight that the Cu, Cu-Mo and Cu-Au porphyry deposits of the world were probably formed at < 8-0.6, < 5-2 and < 4-1 km, respectively. This depth zoning could be used as an exploration target for hypogene as well as supergene mineralized zones, vectoring the position of ore body with respect to the primary halos or even level of erosion for unexposed porphyry systems. The main factors controlling the elemental ratios are related to the depth of mineralization, hydrothermal complexing of metal ligands and the presence or absence of sulfides minerals and gold-silver alloys.