METAL ENRICHMENT AND ZONING IN PORPHYRY COPPER DEPOSITS (Invited Presentation)

Porphyry Cu±Mo±Au deposit formation requires a magmatic source of fluids, ligands, and metals at ca. 700°C; upward transport of aqueous fluids; and trapping via precipitation reactions associated with fluid reactions with wall-rock. Magma composition, including silica, chlorine and water content, alkalinity, and oxygen fugacity, largely dictate initial fluid metal content and ratios. Magmatic-hydrothermal fluids hydrofracture host-rock and ascend vertically into the subvolcanic environment (<3 km), where mixing with ground water and lateral fluid flow dominate.

Deep deposits (ca. 5-10 km) are formed from single-phase parental intermediate density fluids, and are characterized by sparse quartz veins and chalcopyrite±pyrite or bornite in early halo selvages (bio-Ksp-musc) at ca. 500-600°C. At these relatively high pressures magmatic SO₂ gas disproportion or reacts with wall-rock to form H₂SO₄ and H₂S that produce hydrolytic (acid) wall-rock reactions, precipitate anhydrite, and fix sulfides.

In shallow deposits (ca. 2-5 km), parental fluids separate into brine and vapor, and the latter is dominant and enriched in SO₂ and HF and ascends into the epithermal environment (<2 km) to form barren quartz-alunite-clay lithocaps. Brine transports most metal and produces abundant A-type quartz veins with chalcopyrite-bornite±magnetite in bio-Ksp alteration zones in the porphyry environment.

In both deep and shallow deposits, late intermediate density fluids are extracted from deep magmas, and produce widespread sericitic (muscovite) rock alteration and pyrite (ca. 350°C) that transport metals from great depth to the epithermal environment.

Metal zonation is dominated by decreasing solubility as temperature declines and acid is neutralized in the order Cu-Mo to Sn-W-Te-Se to As-Sb-Bi to Li-Cs. Additionally, hydrolytic wall-rock alteration removes metals (Pb, Ba, Sr from feldspar; Fe, Zn, Mn, Cr, V from mafic minerals; Cu from sulfides) and transports them upward. Fe released from these reactions is largely fixed as sulfide where H₂S is sufficient.

The distribution of base metals and critical minerals is thus both governed both by initial magmatic-hydrothermal alteration, but also by evolving wall-rock reactions and thermal conditions.