THE BILIMOIA AU-BI-W-TE DEPOSIT, KAINANTU, PAPUA NEW GUINEA: GEOLOGY, MINERALOGY, AND CONDITIONS OF FORMATION

The Bilimoia deposit (2.32 Mt @ 24 g/t Au), Papua New Guinea, is a fault-controlled quartz-gold vein system hosted by 290-221 Ma years old basement rocks that were regionally metamorphosed to the greenschist facies. Mineralization occurred during events S₃-S₄ in a four stage (S₁-S₄) deformation sequence that was related to rapid uplift, crustal thickening and a change in plate motion-type from oblique-transpression to orthogonal convergence. The Bilimoia deposit is spatially and temporally related to I-type, mafic and intermediate to felsic intrusions emplaced during the Late Miocene. Hydrothermal alteration and associated mineralization formed in 10 stages: 1. Chlorite-epidote selvaged quartz-specularite-calcite veins; 2. Quartz-sericite-mariposite-fuchsite-pyrite; 3. Quartz-wolframite; 4. Hematite; 5. Pyrite-pyrrhotite-arsenopyrite; 6. Quartz-adularia-sericite-illite; 7. Quartz-amethyst-sphalerite-galena-chalcopyrite; 8. Quartz-chalcopyrite-bornite-covellite-chalcocite-enargite-tennanite and minerals in the system Au-Ag-Bi-Pb-Sn-Cu-Te-S; 9. FeħMnħMg carbonates and clays; and 10. Supergene overprint. Gold occurs as native gold, electrum, calaverite, petzite, kostovite, and sylvanite; calaverite and kostovite host 90-95% of the total gold. In the supergene zone (<200 m), mustard-textured native gold in oxidized gold-tellurides indicates in-situ disintegration. Fluid inclusions in stage 3 quartz are mixed aqueous-carbonic, low salinity (0.9-5.4 wt% NaCl) fluids that homogenized at 210^o-330^oC. Coexisting liquid- and vapor-rich (including CO₂-bearing) inclusions that homogenized at 285^o-330^oC suggest phase separation during mineral deposition. Wolframite was deposited from low salinity (0.9-1.1 wt% NaCl) mixed aqueous-carbonic fluids at 240^o-260^oC. Gold deposition in stage 8 was associated with declining temperature (350^o to 100^oC) and sulfur activity, fluctuating pressure, and a change in the oxidation and sulfidation states of the fluid. Bilimoia is a Te-rich (up to 1,300 ppm), intrusion-related gold deposit formed in the mesothermal to epithermal regime. Ore-forming fluids were derived from late-stage, crystallizing intrusions associated with oxidized magma that was contaminated by reduced basement and subducted sediments