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The Geology and mineralisation of the Waisoi porphyry copper deposits, Namosi Province, Republic of Fiji

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Ellis, PD (1968) The Geology and mineralisation of the Waisoi porphyry copper deposits, Namosi Province, Republic of Fiji. Research Master thesis, University of Tasmania.

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Abstract

The Waisoi porphyry copper/gold deposits at Namosi, in the Republic of Fiji, are
hosted by several units of strongly faulted, fractured and altered volcanic rocks.
The lower host volcanic unit consists of coherent and autobrecciated basaltic
andesite lavas of the Late Oligocene to Late Miocene (32 to 10 Ma), Wainimala
basement (early island arc magmatic affinities), whereas the upper host volcanic
unit is dominated by Late Miocene (8 to 5.5 Ma), calc-alkaline andesitic dacite
lavas and volcaniclastics of the Namosi Andesite (mature island arc magmatic
affinities). These two units are separated by a +5my time break, with a mass
flow conglomerate unit (Basal Namosi Conglomerate) marking the beginning of
the calc-alkaline volcanism. These rocks are intruded (at 5.5 - 6.0 Ma) by dacitic
quartz diorites of similar chemistry to the Namosi Andesite volcanics. Fluids
associated with these intrusives caused extensive alteration in the Namosi area
and led to formation of the 930 million tonne Waisoi porphyry mineralisation,
averaging 0.43% copper and 0.14 ppm gold.
Petrological, mineralogical, alteration and fluid inclusion investigations,
combined with field observations of temporal relationships between geological
features, demonstrated that the paragenetic sequence consists of early,
regional, greenschist facies, burial metamorphism and albitisation followed by
porphyry-related alteration. The latter (porphyry-related alteration) consists of
early sericitisation, followed by silicification, before the main mineralising fluids
were introduced. These resulted in a potassic alteration event, during which
most of the copper and gold was deposited, followed by a chloritisation event,
also with significant copper mineralisation. The variable phase mineralising
event was followed by several carbonate-rich alteration events.
Minor copper mineralisation was deposited as chalcopyrite during the
silicification event, but most of the copper and gold was introduced and
deposited as chalcopyrite and bornite during the subsequent main
mineralisation episode. This involved a series of events, from those with highly
saline fluids at temperatures in excess of 500°C, to high salinity (+45 wt.% NaCI
equivalent) fluids at temperatures above 380°C, to fluids with lower salinities (10 wt.% NaCI equivalent) and temperatures around 300°C. Minor copper was also
deposited as chalcopyrite during the early stages of the carbonate alteration
event.
Mineralisation distribution is controlled by lithology and structure. Regional
structure controlled the location of the mineralising porphyries, the fluid
pathways and the porosity/permeability of the host rocks, whereas the
lithologies controlled the degree of chemical reaction and the style of
mineralisation. The dominant structures controlling the emplacement of the
porphyries are northeast-southwest and northwest-southeast trending corridors,
with lesser influence of east-west zones. East-west and north-south structures
tended to occur later, and disrupt the mineralisation on a series of block faults.
The high erosion rate of the Waisoi area has resulted in the exposure of the
Waisoi mineralisation, but lower erosion at nearby Waivaka has only exposed
the top of a similarly mineralised system.

Item Type: Thesis (Research Master)
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Date Deposited: 19 Aug 2011 02:01
Last Modified: 11 Mar 2016 05:56
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