Formation of giant high-grade hypogene porphyry copper deposits during phyllic to advanced argillic alteration: textural evidence from automated SEM mapping, Resolution and Hugo Dummett North deposits

Porphyry Cu deposits host the majority of global Cu resources and high-grade hypogene porphyry Cu deposits are of particular interest to industry because of the reduced waste and energy consumption required in exploitation, leading to favorable economics and reduced environmental impact. Detailed core logging, combined with TESCAN TIMA mineral quantification at two globally significant, high hypogene Cu grade, supergiant porphyry deposits – Resolution, USA and Hugo Dummett North, Mongolia, indicate that the majority of the chalcopyrite±bornite-pyrite are intergrown with muscovite that overprints earlier potassic alteration assemblages containing biotite and/or K-feldspar. Copper grades increase with the intensity of muscovite overprinting on primary potassic assemblages supporting the link between high-grade Cu mineralization and phyllic alteration. Another zone of high-grade Cu mineralization occurs in the upper parts of the phyllic alteration zone and/or within later advanced argillic alteration, associated with high-sulfidation bornite±digenite±covellite±chalcocite-pyrite assemblages, that partly replace earlier chalcopyrite. These two high grade domains have comparable features in many other significant HGHP deposits (Chuquicamata, Rosario, MMH, Onto, Butte) – all strongly telescoped systems that host significant amounts of high-grade Cu mineralization in phyllic and/or advanced argillic alteration that overprint potassic alteration.

We suggest there are at least three reasons for the development of high-grade hypogene ore in telescoped porphyry systems: 1) rapid unroofing and exhumation can generate steep thermal gradients, promoting a rapid decrease in Cu solubility and efficient precipitation of sulfides; 2) the most significant permeability creation in porphyry systems often develops late – during rapid, syn-mineralization exhumation and magma doming stages – when the rock mass behaves in an increasingly brittle fashion; 3) telescoping during syn-mineralization exhumation leads to overprinting of early sulfide assemblages by late-stage acidic and oxidized hydrothermal fluids that remobilize and concentrate early Cu, leading to the precipitation of sulfides with high Cu/S ratios. We conclude that the coincidence of rapid exhumation and long-lived hydrothermal activity exerts a first order control on the formation of high-grade hypogene porphyry Cu mineralization, meanwhile some other factors (such as favorable host rocks, high density of veins and breccias) are potential to form an individual high-grade porphyry Cu deposit.