Abstract

The Pebble Cu-Au-Mo deposit in southwest Alaska is one of the world's largest porphyry deposits. The deposit contains over 80 billion pounds (Blbs) Cu, 107 million ounces (Moz) Au, and 5.6 Blbs Mo in all resource categories. Copper and gold grades vary across diverse hydrothermal alteration assemblages with higher grades associated with sericite- and pyrophyllite-rich assemblages in an advanced argillic alteration zone. Moderate grades are associated with the potassic and sodic-potassic alteration assemblages that dominate the deposit and low grades are found in quartz-illite-pyrite assemblages. These variations reflect fluctuations in both the temperature and composition of the magmatic-hydrothermal fluids in time and space. QEMSCAN scanning electron microscope-based mineral mapping and in situ laser ablation inductively coupled plasma-mass spectrometry methods designed to document the spatial and temporal variations in metal deportment across alteration zones show that gold occurs as inclusions of electrum and high-fineness gold in chalcopyrite and pyrite and as free grains hosted by silicate minerals, with the proportion of each related to the principal host alteration assemblage. Gold associated with the earliest high temperature sodic-potassic and potassic assemblages occurs as electrum inclusions in chalcopyrite and to a lesser extent pyrite, and pyrite trace element signatures have high gold, silver, and copper concentrations (pyrite-1). During cooling of the hydrothermal system lower temperature illite and illite-kaolinite alteration overprinted the earlier assemblages with varying degrees of recrystallization and precipitation of more pyrite-rich assemblages with pyrite having a gold-poor trace element composition (pyrite-2). Gold released during recrystallization formed high-fineness gold inclusions in newly formed pyrite and narrow gold-rich pyrite rims (pyrite-3). A second magmatic-hydrothermal event resulted in a sericite- and pyrophyllite-rich advanced argillic overprint, which introduced more copper and gold to the system and recrystallized preexisting sulfides into both high and low sulfidation assemblages with high-fineness gold inclusions in pyrite and chalcopyrite and solid solution gold in bornite. A second generation of pyrite-1 with elevated palladium concentrations is associated with this hydrothermal fluid pulse and also possibly high arsenic pyrite (pyrite-4).

Understanding the controls on gold deportment provides genetic constraints on ore deposit genesis. Alteration and sulfide assemblages and gold compositions provide information on the hydrothermal fluid compositions, pH, and temperature evolution of the magmatic-hydrothermal system. The presence of electrum inclusions in chalcopyrite and pyrite in potassic and sodic-potassic alteration is consistent with these zones forming from early high-temperature magmatic fluids because both gold and silver are transported together in such fluids. High-fineness gold inclusions in younger pyrite related to low-temperature clay alteration are evidence that the gold in early electrum was partially remobilized and reprecipitated. The low-temperature clay alteration most likely formed from a mixture of H2S-rich vapor with meteoric waters, conditions where gold is transported as a bisulfide complex and silver is not, resulting in the separation of the two metals. A second, structurally controlled pulse of magmatic fluids formed a well-mineralized advanced argillic alteration assemblage containing high-fineness gold inclusions in chalcopyrite and pyrite, the only part of the deposit where high-fineness gold is hosted by chalcopyrite. In the pyrophyllite stability field acidic fluid compositions transport gold and silver as different complexes and gold solubility is significantly lower than silver leading to the precipitation of high-fineness gold inclusions. Palladium is transported by the same complex as gold under these conditions, consistent with the elevated palladium content of pyrite in the advanced argillic alteration assemblage.

The temporal and spatial studies of variations in gold deportment across the Pebble deposit provide critical inputs to optimization of mineral processing design. The greatest influence on metallurgical gold recovery at Pebble is the proportion of gold that is hosted by pyrite. Pyrite-hosted gold may require different mineral processing methodologies compared with gold hosted by chalcopyrite. Therefore, defining domains of consistent hydrothermal alteration, and sulfide mineralogy and gold deportment is the key to the geometallurgical characterization of the deposit.

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