Fluid and Vein Evolution, Timing, and Temperature of Cu-Au-Mo Sulfide Deposition at the Encuentro Porphyry Cu-Au-Mo Deposit, Northern Chile

Abstract

Abstract The enormous economic potential of porphyry systems makes them the most explored and researched ore deposits in the last century. Despite all these efforts, debate remains around the timing and pressure-temperature conditions of metal introduction and precipitation. In this study, we document the abundance and spatial distribution of each vein type in the porphyry environment, followed by cathodoluminescence (CL) imagery, Ti-in-quartz, and fluid inclusion microthermometry to estimate the timing and conditions of metal-bearing sulfide deposition. Below, we provide evidence that most of the Cu-Fe sulfides were deposited early in the evolution of the system, at relatively high temperature, and synchronously with K-silicate alteration. We recognized a sequence of at least five porphyry intrusions that are linked spatially, temporally, and genetically to the bulk of Cu-Au mineralization. Each Encuentro porphyry developed a similar sequence of biotite veinlets, early dark micaceous halos, and A quartz veins with Au-bearing chalcopyrite ± bornite. A veins are the most abundant at Encuentro and constitute 80 vol % of all quartz veins in the deposit. Their distribution and abundance define the shape and geometry of the quartz vein stockwork and the Cu and Au grade shells. The abundance of A veins, Cu-Fe sulfide content, and Cu and Au grades progressively decreased in each cycle of intrusion, consistent with a decline of the magmatic-hydrothermal fluid flux with time. Continuous extraction of Cu-Au–rich fluids impoverished the hidden underlying magma chamber in these metals but generated younger Mo-rich fluid that formed B veins and later quartz-anhydrite-molybdenite (QAM) veins. This process produced Cu-Au and Mo mineralization zones that are decoupled in time and space at the deposit scale. Single-phase intermediate-density fluid inclusions were trapped in A, B, and QAM veins and may include parental fluids modified by postentrapment processes. Depressurization of similar fluids from lithostatic to near hydrostatic pressures along near adiabatic paths caused unmixing to form brine-rich and vapor-rich fluids and furthermore caused the quartz precipitation in these veins and formation of associated K-silicate alteration at >500°C and 0.4- to 1.0-kbar pressures (~3- to 4-km depth). Copper-Fe and Mo sulfides in A, B, and QAM veins were found intimately associated with high-temperature bright- and gray-CL quartz, K-feldspar, and anhydrite, implying that vein formation and sulfide deposition occurred concomitantly during K-silicate alteration. The K-silicate alteration and associated early veins are cut by four vein types stable with sericitic alteration. Three of these vein sets are closely related and zoned upward and outward from deep C-type chalcopyrite-pyrite veinlets, to chlorite-white mica-chalcopyrite-pyrite veinlets, to distal pyrite-rich D veins with well-developed sericitic selvages. The spatial zonation and similar mineral assemblages suggest that these veins were produced by the same fluid, which was more deeply sourced, less voluminous, and cooler than early fluids and, therefore, lower in pH upward as a result of acid dissociation. The youngest fluids at Encuentro are associated with the formation of tourmaline veins along the eastern side of the deposit, which cut and offset all previous veins. Sericitic alteration and associated veins formed at 350° to 460°C and 0.2 to 0.4 kbar (~2.4- to 5-km depth) via depressurization and cooling through the pressure-temperature zone of retrograde quartz solubility, consistent with paucity of quartz in C-type, D, and tourmaline veins. Liquid-rich fluid inclusions trapped in tourmaline veins indicate that the latest fluids remained as single-phase without intercepting the brine-vapor solvus. Veins stable with sericitic alteration are volumetrically minor in the center of the deposit, only contain Cu-Fe sulfides when transgressing high-grade zones, and do not correlate with Cu and Au grade shells, implying that most of the Cu they contained was derived from early-deposited sulfides.