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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.

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Eastoe, CJ (1979) The formation of the Panguna Porphyry copper deposit, Bougainville, Papua New Guinea: with an appendix on the Frieda porphyry copper prospect, New Guinea. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Copyright Holders: The Author
Copyright Information:

Copyright 1979 the Author

Date Deposited: 19 Aug 2011 02:05
Last Modified: 11 Mar 2016 05:56
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