University Of Tasmania
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The formation of the Panguna Porphyry copper deposit, Bougainville, Papua New Guinea: with an appendix on the Frieda porphyry copper prospect, New Guinea.

posted on 2023-05-26, 05:25 authored by Eastoe, CJ
Various hydrothermal processes have been suggested as important in the formation of porphyry coppers, e.g. orthomagmatic evolution of salt-rich liquid, condensation of salt-rich liquid from magmatic vapour, convection of groundwater driven by magmatic heat, boiling of groundwater. A fluid inclusion study based on detailed two-dimensional sampling indicates that all of these processes appear to have contributed to the evolution of the Panguna deposit, but suggests that copper was deposited mainly by salt-rich liquid expelled direct from the magma. The deposit formed at the southern contact of the Kaverong Quartz Diorite with the Panguna Andesite. Three smaller porphyritic stocks, the Biotite Granodiorite, the Leucocratic Quartz Diorite and the Biuro Granodiorite, were emplaced in the deposit during mineralisation, which comprised three phases of hydrothermal activity. The·first, phase A, took place when the southern part of the Kaverong Quartz Diorite was at 0 temperatures over 700 c. The Panguna Andesite was pervasively altered to an amphibole-magnetite-plagioclase assemblage, upon which was superimposed copper mineralisation and associated K-silicate alteration. The limit of copper deposition and quartz veining to the southwest coincides closely with a zone in which salt-rich liquid was cooled and diluted. A pyritic halo parallels this zone. The system cooled below 400°C before undergoing renewed mineralisation at temperatures over 400°C in two approximately concurrent but separate phases B and c. These phases were accompanied by the intrusion of porphyritic stocks. Phase B formed a well-defined cell bounded by a pyritic halo and centred on the Leucocratic Quartz Diorite. Phase C was expressed as veining of the Biotite Granodiorite, the Biuro Granodiorite and the area between them. Copper mineralisation took place at a pressure near 300 bars and at temperatures between 350°C and 700°c or higher. Cu,Fe sulphides, quartz, anhydrite and hematite in veins, and potassium silicate alteration were formed from boiling salt-rich liquid, of density 1.2 - 1.5 g/cm3, mostly of magmatic origin. The composition of these liquids (which nucleated both KCl and NaCl in fluid inclusions) in terms of the system NaCl-KCl-H2o varied between 76% salts (60% NaCl, 16% KCl) and 46% salts (30% NaCl, 16% KCl) by weight. Other liquids, apparently more dilute, nucleated only NaCl. The salt-rich liquids also contained Fe, Ca and S, and minor quantities of Mg, Cu, Mn and zn. A Cu concentration tration of 1900 ppm has been estimated in one liquid. The atomic K/Na ratios of salt-rich liquids from three principal phases of vein mineralisation and from quartz phenocrysts conformed to a single trend, increasing from 0.17 to 0.46 as the NaCl content decreased. Groundwater, mainly of less than 5% salinity, inundated the orebody between phase A and phases B and c, and again after phases B and c, at temperatures below 400 c. Groundwater deposited quartz-pyrite and probably pyrite-clay and sphalerite-pyrite veins at temperatures near 300°C and caused local phyllic alteration. Given a hydrostatic pressure regime in the groundwater system, the depth of formation was near 3 km. Fluids of groundwater composition, trapped as inclusions at or above their critical points, seem to bound the. regions in which two fluids coexisted during phases A and B, and possibly c. The evolution of fluid compositions and phase properties across the two-phase region is consistent with the predicted evolution of boiling salt-rich liquid expelled unsaturated from the magma, cooled to saturation and supersaturation by 500°c, then cooled and diluted by mixing with salt-rich liquid formed by the concentration of groundwater (as high as 45% salts) by boiling. The salt-rich liquids were unsaturated near 430°C, and at lower temperatures the liquid and gas compositions converged to the critical composition at the boundary. Pressures fell sharply from lithostatic between the magma and the zone of supersaturated liquids of the ore-zone, and were hydrostatic in the lower-temperature unsaturated fluids. In the zone of supersaturation, pressures may have been lower than in the groundwater. Salt-rich liquid was pumped into the ore-zone by the lithostatic-hydrostatic pressure difference, then descended through the ore-zone because of its density. The transport of Fe and possibly CU in the vapour is insignificant under porphyry copper conditions, but Zn and Mo may undergo signific·ant vapour transport. This may explain the separation of zn and Mo from Fe and Cu in porphyry copper systems. The absence of major sericite alteration (as opposed to the K-feldspar commonly associated with the salt-rich liquid) suggests that boiling removed excess HCl formed during the alteration of plagioclase and amphibole to biotite. The sulphate in anhydrite deposited by saltrich liquid probably originated from the decomposition of SOz. This mechanism does not account for increased sulphide deposition below 500°C because the liquid maintained a constant S0z:H2S ratio but the reduction of SOz by Fe 2+ may have become important at lower temperatures. The high oxidation state of magmatic fluids during copper mineralisation was due to the loss of H2 from the magma in those early-evolved volatiles that formed the amphibole-bearing assemblage. Chalcopyrites have a .834s range of -1.6 to 1.5%., pyrites +0.5 to 3.l%and anhydrites +7.6 to 16.0%. The salt-rich liquid that deposited anhydrite and chalcopyrite had o34s = +1%.The complexity of the hydrothermal processes indicates that there was not a simple relationship between these values and the 834s values of sulphur in the magma.


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