Abstract

The Porgera Au deposit, located in the highlands of Papua New Guinea, contains proven and probable reserves of 51.5 million metric tons at 0.23 oz Au/t. The deposit is spatially and temporally associated with a late Miocene age (6.0 + or - 0.3 Ma) epizonal intrusive complex, characterized by gabbroic and porphyritic rocks of volatile-rich alkali basaltic, hawaiitic, to mugearitic composition. Magmatism and mineralization shortly preceded early Pliocene collision between the northeast Australasian continental margin and an island arc and were probably related to changes in the tectonic configuration leading up to that event.The stocks and dikes which constitute the Porgera intrusive complex have experienced variable degrees of propylitic alteration. Gold mineralization postdates this alteration and occurs in two main stages: (I) disseminated auriferous pyrite in zones of pervasive phyllic alteration, plus minor native gold (Au degrees ) in associated base metal sulfide veins, and (II) locally abundant Au degrees and Au-Ag tellurides with roscoelite, pyrite, and minor barite in banded, vuggy quartz veins and hydraulic breccias related to late normal faulting.Fluid inclusions from calcite, apatite, and garnet indicate that propylitic alteration mostly involved relatively dilute fluids (< or =8 wt % NaCl equiv) at temperatures of up to approximately 450 degrees C, but more saline fluids (up to 16.8 wt % NaCl equiv) were also present locally. The observation that some inclusions in calcite were trapped under near-critical conditions restricts fluid pressures to approximately 450 bars. Stable isotope compositions of hydrothermal epidote indicate that these fluids were dominantly of ground-water origin, but the data do not enable distinction between sea or meteoric waters. Higher salinity inclusions may record the presence of residual magmatic fluids, and isotopic compositions of disseminated pyrite from propylitically altered rocks (delta 34 S = 1.4-4.5ppm) suggest that S was of magmatic origin.Hypersaline fluid inclusions occur in minor quartz from samples of intense stage I phyllic alteration and indicate trapping of a halite-saturated fluid at 200 degrees to 210 degrees C, with approximately 32 wt percent NaCl equiv. Calculated values of delta 18 O and delta D for fluids associated with phyllic alteration range from 9.1 to 10.4 per mil and -55 to -37 per mil, respectively, and overlap the range of Porgera magmatic waters calculated from igneous hornblende and biotite compositions (delta 18 O (sub H 2 O) = 7.9 to 9.3ppm, delta D (sub H 2 O) = -63 to -49ppm). Isotopic compositions of stage I disseminated pyrite (delta 34 S = 2.4-5.2 ppm) are similar to those from propylitic alteration.Fluid inclusions in sphalerite and quartz from stage I base metal sulfide veins are characterized by moderate salinities (9.5 + or - 1.8 wt % NaCl equiv, n = 142) and relatively high homogenization temperatures (299 degrees + or - 33 degrees C, n -- 274). An average formation temperature of 325 degrees C is estimated for these veins, assuming trapping at approximately 450 bars. The delta 18 O (sub H 2 O) value calculated from a single quartz sample intergrown with sphalerite is 8.6 per mil and falls within the range of Porgera magmatic waters. Isotopic compositions of vein pyrite, sphalerite, and galena (delta 34 S = 2.4-5.4ppm) show a similar range to sulfides from propylitic and phyllic alteration. The delta 13 C and delta 18 O values of late-stage vein Mn-Fe-Ca-Mg-carbonates range from -7.1 to -4.5 per mil and 14.8 to 18.3 per mil, respectively. These values are consistent with a magmatic source for C, but delta 18 O (sub H 2 O) values of 4.8 to 8.3 per mil calculated at 200 degrees C are similar to values derived from late vuggy quartz in the same veins (2.7-6.1ppm) and indicate the involvement of ground waters.Homogenization temperatures of fluid inclusions from stage II quartz-roscoelite-Au veins average 146 degrees + or - 13 degrees C (n = 519). Salinities fall into two populations, averaging 4.3 + or - 0.4 (n = 119) and 7.8 + or - 0.7 wt percent NaCl equiv (n = 439), and inclusions directly associated with roscoelite-Au deposition typically belong to the higher salinity group. Stage II veins are estimated to have formed at approximately 165 degrees C, and textural and physicochemical considerations suggest that flashing may have occurred during the initial stages of vein rupture. Phase separation may have been triggered by fault-related seismic activity and promoted by the presence of minor CO 2 in the ore fluids. Stable isotope compositions of these fluids indicate the involvement of isotopically exchanged meteoric ground waters (delta 18 O (sub H 2 O) = 2.0 to 5.4ppm, delta D (sub H 2 O) = -62 to -34ppm). The delta 34 S values of stage II pyrite range from -14.0 to -11.4 per mil, whereas late vuggy barite and anhydrite yield values of 22.2 and 20.6 per mil, respectively. These data are interpreted to reflect partial oxidation of a fluid with bulk delta 34 S [asymp] 4 per mil.Gold is suggested to have been transported as a chloride complex in early magmatically derived hypersaline fluids, and deposited as disseminated auriferous pyrite during cooling, as a result of sulfidation and sericitization reactions with the mafic igneous wall rocks. In contrast, Au was probably transported as a bisulfide complex in stage I and II veins, and was precipitated after depletion of reduced S in these more dilute fluids through sulfide deposition and phase separation, respectively.Comparisons with other magmatic-related ore-forming systems suggest that a key feature of the Porgera Au deposit is its association with volatile-rich, mafic, alkalic magmas, which were emplaced at shallow crustal levels. Similarities in process are noted with calc-alkalic porphyry Cu deposits, but differences in detail, such as metal inventory and alteration and mineralization styles, reflect a fundamental difference in magmatic affiliation. Stage II mineralization at Porgera shares many characteristics with Au-Ag-Te vein-type, or alkalic-type, deposits. In particular, similarities of magmatic association may reflect a direct link between metallogenesis and large-scale geodynamic processes. Specifically, it is suggested that regions undergoing changes in tectonic configuration (collision, subduction reversal) within a broadly convergent framework, may be suitable for generation of relatively small volumes of anomalous mantle-derived magmas. A relationship to subduction activity is thought to be important in generating melts with relatively high H 2 O contents. Rapid emplacement at shallow crustal levels and exsolution of an aqueous volatile phase distinguish these systems from typical gabbroic intrusions and associated orthomagmatic deposits, whereas the mafic, alkalic nature of the parental magmas appears to correlate with high Au/Cu ratios in the ore, and distinguishes them from calc-alkalic porphyry Cu deposits.

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