APPLICATION OF METAL ZONING TO GOLD EXPLORATION IN PORPHYRY COPPER SYSTEMS

Recent gold discoveries peripheral to gold-enriched porphyry copper deposits demonstrate the potential for gold mineralization within roughly concentric zones surrounding the copper deposits. In gold-enriched porphyry copper systems the close correlation between gold grade and copper grade in the potassic altered zone is well documented (Dos Pobres, Granisle, Bell, Dizon, and others). More recently discoveries at Fortitude (Copper Canyon), Star Pointer (Ely), and elsewhere point to a gold-rich zone that falls outside of the copper ore body, and inside of the lead-zinc halo. This zone, referred to as the intermediate gold zone, is partly coincident with the pyrite zone peripheral to many porphyry copper systems. Some Carlin-type gold deposits (Barneys Canyon, Bau, Yauricocha), located 3-5 kilometers from the outer margin of gold-enriched porphyry copper deposits, suggest the existence of a distal gold zone. Within an idealized gold-enriched porphyry system, a composite zoning pattern from the center of the system outward may include the following zones: (1) barren (or subeconomic) core; (2) molybdenum; (3) bornite-gold; (4) chalcopyrite; (5) pyrite halo (gold in shear zones, and distal skarns); (6) lead-zinc-silver; and (or) (7) distal epithermal gold. Considerable variability exists from district to district in the details of the zoning relationships. Initial gold distribution can be significantly modified by late-stage remobilization and redeposition. In many gold-enriched porphyry copper districts there has been little systematic exploration for gold mineralization peripheral to the copper deposit. These zoning relations can define multiple gold target areas within gold-bearing porphyry systems, especially those intruding carbonate sequences. Metal ratios, particularly Pb/Cu, Pb + Zn/Cu, Cu/Au, and Au/Ag are useful for clarifying zoning patterns and, in conjunction with structure and stratigraphy, can be used to define and evaluate specific exploration targets.