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

摘要

Various hydrothermal processes have been suggested as importantudin the formation of porphyry coppers, e.g. orthomagmatic evolution ofudsalt-rich liquid, condensation of salt-rich liquid from magmaticudvapour, convection of groundwater driven by magmatic heat, boiling ofudgroundwater. A fluid inclusion study based on detailed two-dimensionaludsampling indicates that all of these processes appear to have contributedudto the evolution of the Panguna deposit, but suggests that copperudwas deposited mainly by salt-rich liquid expelled direct from the magma.udThe deposit formed at the southern contact of the Kaverong QuartzudDiorite with the Panguna Andesite. Three smaller porphyritic stocks,udthe Biotite Granodiorite, the Leucocratic Quartz Diorite and the BiuroudGranodiorite, were emplaced in the deposit during mineralisation, whichudcomprised three phases of hydrothermal activity. The·first, phase A,udtook place when the southern part of the Kaverong Quartz Diorite was atud0 temperatures over 700 c. The Panguna Andesite was pervasively alteredudto an amphibole-magnetite-plagioclase assemblage, upon which was superimposedudcopper mineralisation and associated K-silicate alteration. Theudlimit of copper deposition and quartz veining to the southwest coincidesudclosely with a zone in which salt-rich liquid was cooled and diluted.udA pyritic halo parallels this zone. The system cooled below 400°C beforeudundergoing renewed mineralisation at temperatures over 400°C in twoudapproximately concurrent but separate phases B and c. These phases wereudaccompanied by the intrusion of porphyritic stocks. Phase B formed audwell-defined cell bounded by a pyritic halo and centred on theudLeucocratic Quartz Diorite. Phase C was expressed as veining of theudBiotite Granodiorite, the Biuro Granodiorite and the area between them. Copper mineralisation took place at a pressure near 300 bars andudat temperatures between 350°C and 700°c or higher. Cu,Fe sulphides,udquartz, anhydrite and hematite in veins, and potassium silicate alterationudwere formed from boiling salt-rich liquid, of density 1.2 - 1.5udg/cm3, mostly of magmatic origin. The composition of these liquidsud(which nucleated both KCl and NaCl in fluid inclusions) in terms of theudsystem NaCl-KCl-H2o varied between 76% salts (60% NaCl, 16% KCl) andud46% salts (30% NaCl, 16% KCl) by weight. Other liquids, apparentlyudmore dilute, nucleated only NaCl. The salt-rich liquids also containedudFe, Ca and S, and minor quantities of Mg, Cu, Mn and zn. A Cu concentrationudtration of 1900 ppm has been estimated in one liquid. The atomic K/Naudratios of salt-rich liquids from three principal phases of veinudmineralisation and from quartz phenocrysts conformed to a single trend,udincreasing from 0.17 to 0.46 as the NaCl content decreased.udGroundwater, mainly of less than 5% salinity, inundated the orebodyudbetween phase A and phases B and c, and again after phases B and c, atudtemperatures below 400 c. Groundwater deposited quartz-pyrite andudprobably pyrite-clay and sphalerite-pyrite veins at temperatures nearud300°C and caused local phyllic alteration. Given a hydrostatic pressureudregime in the groundwater system, the depth of formation was near 3 km.udFluids of groundwater composition, trapped as inclusions at orudabove their critical points, seem to bound the. regions in which twoudfluids coexisted during phases A and B, and possibly c. The evolutionudof fluid compositions and phase properties across the two-phase region isudconsistent with the predicted evolution of boiling salt-rich liquidudexpelled unsaturated from the magma, cooled to saturation and supersaturationudby 500°c, then cooled and diluted by mixing with salt-richudliquid formed by the concentration of groundwater (as high as 45% salts) by boiling. The salt-rich liquids were unsaturated near 430°C, and atudlower temperatures the liquid and gas compositions converged to theudcritical composition at the boundary. Pressures fell sharply fromudlithostatic between the magma and the zone of supersaturated liquidsudof the ore-zone, and were hydrostatic in the lower-temperatureudunsaturated fluids. In the zone of supersaturation, pressures may haveudbeen lower than in the groundwater. Salt-rich liquid was pumped intoudthe ore-zone by the lithostatic-hydrostatic pressure difference, thenuddescended through the ore-zone because of its density.udThe transport of Fe and possibly CU in the vapour is insignificantudunder porphyry copper conditions, but Zn and Mo may undergo signific·antudvapour transport. This may explain the separation of zn and Mo from Feudand Cu in porphyry copper systems.udThe absence of major sericite alteration (as opposed to theudK-feldspar commonly associated with the salt-rich liquid) suggests thatudboiling removed excess HCl formed during the alteration of plagioclaseudand amphibole to biotite. The sulphate in anhydrite deposited by saltrichudliquid probably originated from the decomposition of SOz. Thisudmechanism does not account for increased sulphide deposition below 500°Cudbecause the liquid maintained a constant S0z:H2S ratio but the reductionudof SOz by Fe 2+ may have become important at lower temperatures. The highudoxidation state of magmatic fluids during copper mineralisation was dueudto the loss of H2 from the magma in those early-evolved volatiles thatudformed the amphibole-bearing assemblage.udChalcopyrites have a .834s range of -1.6 to 1.5%., pyrites +0.5 toud3.l%and anhydrites +7.6 to 16.0%. The salt-rich liquid that depositedudanhydrite and chalcopyrite had o34s = +1%.The complexity of the hydrothermaludprocesses indicates that there was not a simple relationshipudbetween these values and the 834s values of sulphur in the magma.