The Porgera gold deposit: Fluid characteristics, ore deposition processes, and duration of the ore forming event.

摘要

This study focuses on ore formation at the Porgera gold deposit, Papua New Guinea. The duration of the ore-forming event was determined using the laser probe 40Ar/39Ar technique on magmatic and hydrothermal minerals, where the date on igneous biotite (5.99 +/- 0.11 Ma) was interpreted to be the age of the intrusive event, and the average date obtained from two hydrothermal roscoelite samples (5.99 +/- 0.08 Ma) is the age of ore formation. These results show that ore formation occurred within ≤0.1 my of intrusion.; Gold-bearing veins from a high-grade zone (North Zone) were characterized petrographically, and the fluid from which the vein material precipitated was investigated using fluid inclusion microthermometry, gas and ion chromatography, and stable isotope analysis (O, H, C) on vein quartz. The aim of these analyses was to study the evolution of the hydrothermal fluid along the North Zone and the process(es) by which this zone formed.; The veins consist dominantly of quartz but the main ore mineral, native gold, is intergrown with roscoelite [K(V,Al)2(AlSi3O 10)(OH,F)2], pyrite, and minor quartz. The vein centers are filled with anhydrite and carbonate. The homogenization temperature of primary inclusions hosted by quartz ranges from 127° +/- 12°C (n = 37) to 165° +/- 25°C (n = 27), and average salinities fall between 8 and 9 equiv. wt % NaCl where a minority of samples contains a low salinity group (∼4 equiv. wt % NaCl) in addition to the higher salinity inclusions. Vapor-rich inclusions were observed to coexist with liquid-rich inclusions in a minority of samples. The microthermometric and petrographic data were used together with the results of the chromatographic analyses as input parameters for numerical simulations of ore deposition using the software CHILLER. The modeling results indicate that stage II veins are most likely formed by cooling of a slightly acidic, oxidized fluid that had interacted with the surrounding sedimentary rocks. The observation of vapor-rich fluid inclusions suggesting that phase separation occurred during vein formation, and continuous salinity trends in some samples indicates that fluid mixing occurred locally. In addition, gold was observed in pyrite in the altered wall-rock. Hence, the results indicate that more than one precipitation mechanism was involved in vein formation. This result connects and integrates previous hypotheses of ore formation at Porgera.