Geo- and thermochronology of the Ertsberg-Grasberg Cu-Au mining district, west New Guinea, Indonesia

The prolific Ertsberg-Grasberg Cu-Au mining district, located on the island of New Guinea in Indonesia, is host to the supergiant Grasberg porphyry copper deposit, and multiple giant skarns. The well-studied nature of the district provides geologic context for high resolution geochronology and thermochronology studies. The supergiant Grasberg porphyry copper deposit is hosted in the Grasberg Igneous Complex. Intrusions were dated using the novel zircon U/Pb depth profiling technique, and age results show the magmatic system was active from 3.6-3.1 Ma. Cu-Au mineralization initiated following intrusion of the MGI (3.22 ± 0.04 Ma) and predates the EKI (3.20 ± 0.04 Ma) and LKI (3.09 ± 0.05 Ma). Based on these cross-cutting relationships, the high grade core of the Grasberg deposit formed in less than 100 to 220 kyr. Age results for the Ertsberg pluton (31-2.8 Ma) and other minor intrusions shows that magmatism in the district took less than 1 myr. Zircon and apatite (U-Th)/He ages from a 2.2 km vertical profile in the Grasberg deposit record minimum cooling rates of 25°C/10 kyr near surface and 4°C/10 kyr at depth. These results indicate Grasberg ore formation occurred immediately following maar volcanism and was short-lived. Rapid cooling of surface samples precludes the presence of a 2 km volcanic edifice overlying the orebody. Rapid cooling at 2 km depth necessitates emplacement into cold country rock. As copper sulfide precipitation is temperature dependent, the tightness of isotherms in the ore zone contributes to the localization of copper mineralization into a small volume, resulting in an extraordinarily high ore grades. Garnet samples from the Big Gossan skarn were dated using the newly developed LA-ICP-MS garnet U/Pb chronometer. Age results show the skarn formed between 2.9–2.7 Ma, over a timespan of approximately 200 kyr. High U contents (10-100 ppm) and a consistent common Pb composition improve precision, and garnet ages agree with external age constraints. This study demonstrates that andradite garnet U/Pb chronometry can be a robust dating technique for constraining the timing and duration of skarn-forming hydrothermal systems.Geological Science

Structural geology of the Mount Polley Cu-Au district, south-central British Columbia

The Mount Polley copper-gold deposit is one of a series of porphyry copper deposits that occur in a belt of accreted Mesozoic age island arc terranes in the Canadian Cordillera. The deposit comprises three breccia-hosted Cu-Au ore zones that are associated with a series of monzodiorite to monzonite intrusions emplaced into the Nicola Group in the Late Triassic. The Mount Polley deposit has been deformed in a number of structural events and the relative sequence of folding, faulting, and tilting provides insight into the tectonic history of the Quesnel Terrane. During a period of 8 million years, from 205 Ma to 197 Ma, the Mount Polley deposit was emplaced into the Nicola Group, exhumed to the surface, and buried beneath a sequence of conglomerates, sandstones, and a quartz latite ignimbrite. The oldest faults in the district are the north-northwest-striking reverse faults. These include the Polley Fault and East Cariboo Fault, which displace intrusions, mineralized ore zones, and plagioclase and K-feldspar porphyry dikes. Reverse faults are coeval with a regional folding event that formed a doubly plunging synform that dips to the northwest in the Mount Polley district. The North Springer Fault is a southwest-striking sinistral fault that cuts the Polley Fault and the East Cariboo Fault. The youngest faults in the district are the Green Giant Fault and the Center Fault, which is inferred from drill hole data. The Green Giant Fault cuts the intrusions, mineralized ore zones, and both porphyry dikes and post-mineral dikes. A tilting event of unknown age resulted in a 30-35° NW tilt of the sedimentary beds, ore bodies, and dikes in the Mount Polley district. A limited number of apatite fission track data suggest that the district cooled through the apatite partial annealing zone during the Paleocene to Eocene. This study provides insight into the local structural geology of the Mount Polley Cu-Au deposit, and the history of island arc accretion, hinterland development, and post-convergence extensional modification of allochthonous terranes in the Canadian Cordillera

Potential for Volcanogenic Massive Sulfide Mineralization at the A6 Anomaly, North-West British Columbia, Canada: Stratigraphy, Lithogeochemistry, and Alteration Mineralogy and Chemistry

The Middle Jurassic A6 Anomaly is located 30 km southeast of Eskay Creek, north-central British Columbia and consists of thick, altered felsic igneous rocks overlain by a mafic volcano-sedimentary package. Lithogeochemistry on igneous rocks, X-ray diffraction on altered felsic units, and electron probe microanalysis and secondary ion mass spectrometry on illite and quartz were applied to explore the volcanogenic massive sulfide (VMS) potential, characterize alteration, and determine fluid conditions at the A6 Anomaly. Lithogeochemistry revealed calc-alkaline rhyodacite to trachyte of predominantly FII type, tholeiitic basalts with Nb/Yb < 1.6 (i.e., Group A), and transitional to calc-alkaline basalts and andesites with Nb/Yb > 2.2 (i.e., Group B). The felsic units showed weakly to moderately phyllic alteration (quartz–illite with minor orthoclase and trace chlorite–pyrite–calcite–barite–rutile). Illite ranged in composition from illite/smectite (K = 0.5–0.69 apfu) to almost endmember illite (K = 0.69–0.8 apfu), and formed from feldspar destruction by mildly acidic, relatively low temperature, oxidized hydrothermal fluids. The average δ18O composition was 10.7 ± 3.0‰ and 13.4 ± 1.3‰ relative to Vienna Standard Mean Ocean Water for illite and quartz, respectively. Geothermometry involving illite composition and oxygen isotope composition on illite and quartz yielded average fluid temperatures of predominantly 200–250 °C. Lithogeochemical results showed that the A6 Anomaly occurred in a late-Early to Middle Jurassic evolving back-arc basin, further east then previously recognized and in which transitional to calc-alkaline units formed by crustal assimilation to enriched Mid-Ocean Ridge Basalt (EMORB) (i.e., felsic units, Group B), followed by thinning of the crust resulting in tholeiitic normalized MORB basalts (i.e., Group A) with a minor crustal component. The alteration assemblage is representative of distal footwall alteration, and metal transport in this zone was limited despite favorable temperature, pH, and redox state, indicating a metal depleted source (i.e., felsic units)