The Laloki and Federal Flag deposits are two of the many (over 45) polymetallic massive sulfide deposits that occur in the Astrolabe Mineral Field, Papua New Guinea. New data of the mineralogical compositions, mineral textures, and fluid inclusion studies on sphalerite from Laloki and Federal Flag deposits were investigated to clarify physiochemical conditions of the mineralization at both deposits. The two deposits are located about 2 km apart and they are stratigraphically hosted by siliceous to carbonaceous claystone and rare gray chert of Paleocene–Eocene age. Massive sulfide ore and host rock samples were collected from each deposit for mineralogical, geochemical, and fluid inclusion studies. Mineralization at the Laloki deposit consists of early‐stage massive sulfide mineralization (sphalerite‐barite, chalcopyrite, and pyrite–marcasite) and late‐stage brecciation and remobilization of early‐stage massive sulfides that was accompanied by late‐stage sphalerite mineralization. Occurrence of native gold blebs in early‐stage massive pyrite–marcasite‐chalcopyrite ore with the association of pyrrhotite‐hematite and abundant planktonic foraminifera remnants was due to reduction of hydrothermal fluids by the reaction with organic‐rich sediments and seawater mixing. Precipitation of fine‐grained gold blebs in late‐stage Fe‐rich sphalerite resulted from low temperature and higher salinity ore fluids in sulfur reducing conditions. In contrast, the massive sulfide ores from the Federal Flag deposit contain Fe‐rich sphalerite and subordinate sulfarsenides. Native gold blebs occur as inclusions in Fe‐rich sphalerite, along sphalerite grain boundaries, and in the siliceous‐hematitic matrix. Such occurrences of native gold suggest that gold was initially precipitated from high‐temperature, moderate to highly reduced, low‐sulfur ore fluids. Concentrations of Au and Ag from both Laloki and Federal Flag deposits were within the range (<10 ppm Au and <100 ppm Ag) of massive sulfides at a mid‐ocean ridge setting rather than typical arc‐type massive sulfides. The complex relationship between FeS contents in sphalerite and gold grades of both deposits is probably due to the initial deposition of gold on the seafloor that may have been controlled by factors such as Au complexes, pH, and fO2 in combination with temperature and sulfur fugacity.

We present the first sulfur (S) isotope data of sulfides, sulfates, pyrite in host mudstone, and bulk sulfur of gabbroic rocks from the Laloki and Federal Flag massive CuZn-Au-Ag deposits in the Astrolabe mineral district, Papua New Guinea. Early-stage pyrite-marcasite, chalcopyrite, and sphalerite from Laloki display wide range of δ 34 S values from −4.5 to +7.0‰ (n=16). Late-stage pyrite, chalcopyrite, and sphalerite have restricted δ 34 S values of −1.9 to +4.7‰ (n = 16). The mineralizing stage these correspond to had moderately saline (5.9-8.4 NaCl eq. wt%) mineralizing fluids of possible magmatic origin. A single analysis of late-stage barite has a value of δ 34 S +17.9‰, which is likely similar to coexisting seawater sulfate. Pyrite from the foot-wall mudstone at Laloki has very light δ 34 S values of −36.1 to −33.8‰ (n=2), which suggest an organic source for S. Pyrite-marcasite and chalcopyrite from Federal Flag show δ 34 S values of −2.4 to −1.9‰ (n=2), consistent with a magmatic origin, either leached from intrusive magmatic rocks or derived from magmatic-hydrothermal fluids. The very narrow range and near-zero δ 34 S values (−1.0 to +0.6‰) of bulk gabbroic samples is consistent with mantle-derived magmatic S. Sulfur isotope characteristics of sulfides and sulfates are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and sedimented midocean ridges. The most reasonable interpretation is that the range of the sulfide and sulfate δ 34 S values from both Laloki and Federal Flag massive sulfide deposits is indicative of the complex interaction of magmatic fluids, seawater, gabbroic rocks, and mudstone.

We interpreted and modelled aeromagnetic data from helicopter surveys of the Morobe goldfield, Papua New Guinea, by comparing gridded magnetic data with mapped surface geology. In some cases, geologic units may be recognized and distinguished based on contrasts in magnetic intensity and texture, including the Morobe Granodiorite, Kaindi Metamorphics, Bulolo Volcanics, and Langimar Beds. Magnetic anomalies occur due to intrusions of Edie Porphyry and related igneous rocks that are associated with gold mineralization. Numerous faults control the size and shape of the intrusions. Magnetic anomalies of interest were identified in gridded data from high-resolution surveys and modelled using flight line data. Two-dimensional magnetic models included dipping slabs with thicknesses of 100-400 m. Three-dimensional models were prepared, including a dipping tabular body and a dipping cylinder. A 2008 survey of the Papuan peninsula sponsored by the European Union (EU) provided continuous coverage of the Morobe goldfield area and helped resolve regional features that were only partially covered by previous smaller surveys. In particular, the EU survey covers the full length of the Sunshine Fault and greatly extends coverage of outcrops of the Morobe Granodiorite and Langimar Beds. The EU survey contains isolated anomalies south-west of Wau that may correspond to intrusions of Edie Porphyry, Morobe Granodiorite.

Ore mineralization and wall rock alteration of Crater Mountain gold deposit, Papua New Guinea, were investigated using ore and host rock samples from drill holes for ore and alteration mineralogical study. The host rocks of the deposit are quartz-feldspar porphyry, feldspar-hornblende porphyry, andesitic volcanics and pyroclastics, and basaltic-andesitic tuff.The main ore minerals are pyrite, sphalerite, galena, chalcopyrite and moderate amounts of tetrahedrite, tennantite, pyrrhotite, bornite and enargite. Small amounts of enargite, tetradymite, altaite, heyrovskyite, bismuthinite, bornite, idaite, cubanite, native gold, CuPbS 2, an unidentified Bi-Te-S mineral and argentopyrite occur as inclusions mainly in pyrite veins and grains. Native gold occurs significantly in the As-rich pyrite veins in volcanic units, and coexists with Bi-Te-S mineral species and rarely with chalcopyrite and cubanite relics.Four mineralization stages were recognized based on the observations of ore textures. Stage I is characterized by quartz-sericite-calcite alteration with trace pyrite and chalcopyrite in the monomict diatreme breccias; Stage II is defined by the crystallization of pyrite and by weak quartz-chlorite-sericite-calcite alteration; Stage III is a major ore formation episode where sulfides deposited as disseminated grains and veins that host native gold, and is divided into three sub-stages; Stage IV is characterized by predominant carbonitization. Gold mineralization occurred in the sub-stages 2 and 3 in Stage III. The fS 2 is considered to have decreased from~10 -2 to 10 -14 atm with decreasing temperature of fluid.