TIMING OF HYPOGENE CU INTRODUCTION DURING THE EVOLUTION OF MAGMATIC-HYDROTHERMAL SYSTEMS FORMING PORPHYRY COPPER DEPOSITS

Hypogene mineralization in porphyry copper deposits comprises large, low-grade stockwork and disseminated sulfide zones that are spatially associated with shallowly emplaced igneous stocks and dike swarms. The stockwork zones in porphyry deposits are composed of a characteristic sequence of crosscutting vein generations that differ in morphology, vein mineralogy, and associated alteration. In many porphyry deposits, including Ann-Mason in Nevada studied by John Dilles and Marco Einaudi (1992), copper occurs within distinct sulfide veins referred to as C veins, which consist of variable amounts of chalcopyrite, bornite, and pyrite. C veins commonly reopen and exploit earlier barren A and B veins. They range from coatings on fracture surfaces to distinct veins that are several millimeters thick. In high-grade ore zones, the sulfide veins are closely spaced and crosscut each other.

Detailed petrographic investigations show that C veins contain little to no quartz. Where present, quartz in C veins shows dissolution textures. This lack of quartz as a gangue mineral can be explained by the formation of these veins under conditions of retrograde quartz solubility. Quartz solubility calculations suggest that retrograde quartz solubility occurs over a narrow temperature range of ~375–450°C in single-phase fluids under hydrostatic pressures. Fluid inclusion studies on C veins from several porphyry deposits confirm that the ore-forming fluids were intermediate-density fluids with critical or near-critical densities and low salinities (<10 wt.% NaCl equiv.). Interaction of these fluids with the host rocks results in chlorite, chlorite-K-feldspar, or chlorite-sericite alteration in most deposits.

Petrographic evidence suggests that hypogene Cu mineralization in many porphyry deposits postdates the earlier high-temperature quartz veins and associated potassic alteration. The research highlights the importance of intermediate-density magmatic-hydrothermal fluids in the formation of this important deposit type. These observations are inconsistent with previous models proposing that Cu is precipitated at high temperatures or remobilized during the late phases of porphyry formation. These findings have implications for the design of mineral exploration programs.