Abstract

Copper mineralization at Panguna took place at a pressure of 200 to 300 bars and at temperatures between 350 degrees and 700 degrees C or higher. Cu, Fe sulfides, quartz, anhydrite, and hematite were deposited by dense, boiling, salt-rich liquids of magmatic origin. The composition of the salt-rich liquids (those which nucleated both KCl and NaCl in fluid inclusions) in terms of the system NaCl-KCl-H 2 O varied between 76 percent salts (60% NaCl, 16% KCl) and 46 percent salts (30% NaCl, 16% KCl) by weight. Other liquids, apparently more dilute, nucleated only NaCl. The salt-rich liquids also contained Fe, Ca, and SO 4 and minor quantities of Mg, Cu, Mn, and Zn. A Cu concentration of 1,900 ppm has been estimated in one liquid. The atomic K/Na ratios of salt-rich liquids from three principal phases of vein mineralization and from quartz pheno-crysts conformed to a single trend, increasing from 0.17 to 0.46 as the NaCl content decreased. The first phase of mineralization (phase A) took place when the southern part of the Kaverong Quartz Diorite was at temperatures higher than 700 degrees C. The temperature and salinity distributions for this phase were asymmetric; cooling and dilution of the salt-rich liquids took place south and southwest of the orebody but have not been detected to the north or east. The zone of cooling and dilution corresponds closely with the 0.3 percent Cu ore-grade contour and the edge of major quartz deposition and is paralleled by a pyritic halo. The system cooled below 400 degrees C before undergoing renewed mineralization at temperatures over 400 degrees C in the two approximately concurrent 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 centered on the Leucocratic Quartz Diorite. Phase C was expressed as veining of the Biotite Granodiorite, the Biuro Granodiorite, and the area between them. Ground water, generally of less than 10 percent salinity, inundated the orebody between phase A and phases B and C, and again after phases B and C, at temperatures below 400 degrees C. Within the orebody there was no salinity continuum between ground water and the salt-rich liquids. Ground water deposited quartz-pyrite and probably pyrite-clay and sphalerite-pyrite veins at temperatures near 300 degrees C and caused local phyllic alteration. The distribution of fluids in space and time is consistent with the porphyry copper model in which a magmatic-hydrothermal system is surrounded by a cooler meteoric-hydrothermal system, the latter collapsing inward as the former wanes. Given a hydrostatic pressure regime in the meteoric-hydrothermal system, the depth of formation was 2 to 3 km.Extract from Melanesian pidgin abstractLiklik Tok I Autim Bikpela Tok I Kamap AnanitBipo tru, kopa i kam pinis long graun long Panguna taim graun i hat moa moa yet, na taim ston i karamapim main inap tripela kilomita. Wara i bin karkarim kopa i kam. na dispela lain wara i hevi tru bikos em i gat planti sol insait long en. ...TranslationAbstract (Literally, little talk (which) divulges big talk that follows underneath)A long time ago copper arrived in the ground at Panguna when the ground was very very hot and when the mine was covered by 3 kilometers of rock. Water carried the copper in, and this type of water was very dense because it contained much salt. . . .

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