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Geology, geochemistry and genesis of the Namosi porphyry Cu-Au deposits, Fiji

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Orovan, EA (2016) Geology, geochemistry and genesis of the Namosi porphyry Cu-Au deposits, Fiji. PhD thesis, University of Tasmania.

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Abstract

The porphyry Cu-Au(-Mo) deposits of the Namosi district are located 30 km west-northwest
of Suva, southeastern Vitu Levu, Republic of Fiji. The district comprises three principal porphyry
deposits (Wainaulo, Waisoi West, and Waisoi East) and several other peripheral porphyry, polymetallic
vein, and skarn prospects. The combined Wainaulo, Waisoi West, and Waisoi East resources
are in excess of 7.86 million tonnes of copper and 7.97 million ounces of gold.
The Namosi porphyry district formed after a major perturbation in the Australian-Pacific
plate tectonic regime during the Late Miocene. The collision of the Melanesian Border Plateau
with the Vitiaz Arc led to a localized reversal of arc polarity, wherein the South Fiji Basin began
to subduct northeastward along the Matthew-Hunter Trench. This short-lived subduction system
had a low-angle component beneath the Fiji Platform that may have triggered the generation of
the Namosi district porphyry deposits. A north-dipping and locally flat-lying orphaned slab from
the fossil subduction zone currently abuts the westward subducting Pacific plate ~ 350 km beneath
the Fiji platform.
The Namosi district consists of a gently dipping, andesitic, volcanic and volcaniclastic succession
≥ 1 km thick (known as the Namosi Andesite Formation of the Medrausucu Group), which
was deposited on the submerged flank of a shoaling seamount in the Late Miocene. The Namosi
Andesite Formation unconformably overlies steeply dipping, low-K tholeiitic submarine basaltic
andesite volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Wainimala
Group. The Namosi porphyry deposits are temporally and genetically associated with a sequence
of medium-K calc-alkalic porphyritic rocks that intruded both the Wainimala Group and Namosi
Andesite Formation. Diorite plutons and a dike complex were emplaced at Wainaulo, quartz-diorite
stocks were intruded at Waisoi West, and dacite stocks and dikes were emplaced at Waisoi
East.
The high 143Nd/144Ndi (~ 0.51304) and low 87Sr/86Sri (~ 0.7036) values of the Namosi district
porphyries suggest a primitive mantle source with no crustal input. Neodymium isotopic compositions
infer more primitive sources throughout the evolution of the Namosi district, interpreted
to reflect the onset of rifting in the vicinity of the Fiji Platform in the Early Pliocene.
Two contrasting styles of porphyry deposits have been recognized within the Namosi district.
The first style is a variation on the plutonic-hosted porphyry deposit model (Wainaulo and Wainaulo
West), consisting of dike complexes emplaced into pre-mineralization porphyritic diorite
plutons. The ore bodies are localized around dikes and are vertically attenuated (> 800 m). The
second type are classic volcanic-hosted porphyry deposits (Waisoi West and Waisoi East), consisting
of multiphasic porphyritic stocks that intruded the volcanic host sequence. The ore bodies are
shallow and annular, occurring in and around the cupola of the porphyritic stocks.
Alteration assemblages at Wainaulo are mineralogically complex, but have well-constrained
zonation patterns. A central zone of biotite – albite – magnetite – actinolite ± muscovite ± rutile
and chalcopyrite ± molybdenite bearing veins is spatially and temporally associated with
two early-mineralization diorite phases. Separating the central biotite-bearing assemblage from
the peripheral chlorite – albite – epidote – calcite ± montmorillonite ± magnetite assemblage is a
zone of actinolite – albite – chlorite – magnetite ± epidote ± rutile ± titanite alteration. Overprinting
the central biotite-bearing assemblage are domains of chlorite – albite – quartz – actinolite
– epidote alteration that are spatially and temporally associated with the main-stage quartz-diorites
and abundant quartz – magnetite – bornite and quartz – sulfide veins. The last significant
ore-bearing event at Wainaulo consists of epidote – sulfide – anhydrite – calcite – hematite veins
with conspicuous K-feldspar – epidote – phengite vein halos that overprint the main-stage diorite
and quartz-diorite complex. Structurally focused illite – muscovite – anhydrite – calcite – pyrite
and kaolinite – montmorillonite – ankerite ± hematite, as well as ubiquitous chlorite – illite-smectite
assemblages overprint the earlier alteration domains throughout the deposit.
The alteration assemblages at both Waisoi deposits consist of an inner biotite – albite – magnetite
± actinolite ± K-feldspar core that grades out to a peripheral chlorite – albite – magnetite
± calcite ± epidote domain. The bulk of the copper mineralization is hosted in a sulfide-bearing
quartz stockwork that overprinted the earlier formed alteration assemblages. Paragonite – quartz
– pyrite and chlorite – illite-smectite alteration is ubiquitous, but is most intensely developed
overprinting the intrusions associated with mineralization at each deposit.
High-resolution CA-TIMS U–Pb dating on zircons and Re–Os dating on molybdenite from
each of the principal deposits has revealed that mineralization in the Namosi district occurred
rapidly over a span of ≤ 520,000 years from the Late Miocene to Early Pliocene. Re–Os ages overlap
with U–Pb ages of interpreted causative intrusions at Wainaulo (5.690 ± 0.023 Ma) and Waisoi
West (5.473 ± 0.022 Ma), whereas at Waisoi East molybdenum mineralization post-dates the interpreted
source intrusion by ≥ 2000 years (5.248 ± 0.022 Ma). These strong correlations demonstrate
the genetic link between mineralization and intrusive activity at each deposit.
The sulfur isotopic compositions at Wainaulo indicate sulfides precipitated from a redox neutral
to SO42--dominant exsolved magmatic fluid that had probably mixed with seawater. There is
a zonation of sulfur isotopic compositions at Wainaulo (i.e., δ34Ssulfide values range from > 1 ‰
within and directly above the deposit core to < -3 ‰ at the deposit periphery; δ34Ssulfate values
vary from > 11.5 ‰ in the core of the deposit to < 11.5 ‰ distally), whereas at the progressively
more reduced deposits at Waisoi West (δ34Ssulfide from -2.9 to +0.6 ‰; mean = -1.4 ‰) and Waisoi
East (δ34Spyrite from +1.8 to +3.4 ‰; mean = +2.4 ‰), no sulfur isotopic zonation was discernible.
The calculated δ18O and δD composition of water in isotopic equilibrium with gangue minerals
from Wainaulo, Waisoi West and Waisoi East reflect the involvement of three different fluid
sources during the evolution of the Namosi district porphyry deposits: exsolved magmatic fluid,
Late Miocene seawater and a local meteoric component. Results from a Sr isotopic study on Namosi
district epidote separates confirm the minor involvement of seawater during hydrothermal
alteration.
The Namosi district records a rapid evolution of contrasting porphyry deposit styles (low
tonnage-high grade to large tonnage-low grade) within a time period of ≤ 520,000 years. These
findings have significant implications to porphyry exploration strategies that need to be considered
when working at a district scale. Geology, hydrothermal alteration, geochemistry, structural
trends, mechanisms for ore concentration, deposit footprints and deposit size can vary significantly
over a short period of time and within a confined space. The rapid evolution of the Namosi
district reflects the dynamic tectonic setting during ore-formation. The recognition of tectonic
environments that reflect arc fragmentation and rotation can be valuable targets for identifying
new metallogenic belts worldwide.

Item Type: Thesis (PhD)
Keywords: Fiji, porphyry, geochemistry, geochronology, hydrothermal, isotopes
Copyright Information:

Copyright 2016 the Author

Date Deposited: 31 Oct 2016 22:12
Last Modified: 17 Nov 2017 16:00
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