FLUID CHEMISTRY AND ORE DEPOSITIONAL MECHANISMS AT THE PORGERA GOLD DEPOSIT, PAPUA NEW GUINEA

 

The Porgera Au deposit is hosted by alkalic intrusions that were emplaced into sedimentary rocks 6 Ma ago. Minor Au occurs in pyrite in base-metal-carbonate veins, but quartz-roscoelite-pyrite-Au veins are economically more significant. The latter veins exhibit at least three different paragenetic and textural styles: (1) quartz-rich breccias; (2) banded veins with quartz-rich centers; (3) quartz veinlets with roscoelite-pyrite-Au wallrock alteration.

Microthermometric data from liquid-rich fluid inclusions in quartz from quartz-roscoelite-pyrite-Au veins show the presence of two fluids of different salinities. The majority of inclusions are characterized by salinities between 7.5 and 8.8 equiv wt % NaCl and average homogenization temperature (Th) values of 152° ± 17° C (n=995). The second fluid exhibits lower salinities, between 4.4 and 6.2 equiv wt % NaCl, and similar average Th values (145° ± 9° C; n=77). In 3 of 27 samples, a continuous salinity trend is observed with similar to slightly lower Th values for the lower salinity group, indicative of fluid mixing. The higher salinity fluid contains up to 2 mole% CO₂, 0.11 mole% CH₄, and 0.065 mole% N₂. Six of 27 samples contain vapor-rich inclusions coexisting with liquid-rich inclusions, indicating boiling conditions.

Fluid inclusion microthermometric data, ion chromatographic data from bulk fluid inclusion analyses, and paragenetic information were used to estimate the composition of the high salinity fluid involved in quartz-roscoelite-pyrite-Au vein formation. These compositions were used to model potential ore deposition mechanisms using the software CHILLER. Boiling, fluid mixing with sedimentary formation water, cooling, and fluid-rock reaction were modeled.

Modeling results support interpretations that boiling, fluid-rock reaction, and fluid mixing, all accompanied by cooling, caused ore deposition under various conditions. Boiling was most likely responsible for ore deposition in large hydrothermal breccia veins, and fluid-rock reaction caused gold deposition where fluid/rock ratios were low. In situ fluid mixing resulted in gold deposition locally. These results paint a picture of a complex 4-dimensional hydrothermal system, and the large size of the resulting ore body may be a consequence of this complexity.