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Fluid-inclusion and geochemical studies at the Emperor gold mines, Fiji.

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Ahmad, Masood (1979) Fluid-inclusion and geochemical studies at the Emperor gold mines, Fiji. PhD thesis, University of Tasmania.

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Abstract

The gold-silver telluride deposits of the Emperor mine
(Vatukoula, Fiji) are fairly typical of the epithermal deposits found
in association with Tertiary volcanic activity in the circum-Pacific
belt. The mine produces about 60,000 troy ounces of gold per annum,
together with silver and tellurium. It lies on the western margin of
a Pliocene caldera (the Tavua caldera or Tavua basin) and the host
rocks for the mineralization are olivine basalt flows and andesite
dykes. The olivine basalt sequence is about 2000 m thick and is
probably underlain by mid-Tertiary calcareous sedimentary rocks .
approximately 1500 m thick. The sedimentary rocks are probably underlain
by early Tertiary andesitic rocks.
Five phases of volcanic (and associated intrusive) activity
have been recognised in the Tavua cauldron. The K-Ar dating indicates
two major periods of igneous activity, Which commenced at about 10 m.y.
with widespread eruptions of olivine basalt (phase I) and continued to
about 7.5 m.y. A period of quiescence followed. The later phases of
volcanic activity (phases II to V) date between 5 and 4.3 m.y.
Phase II volcanism is represented by minor eruptions of trachybasalt . ,
phase III by augite trachyandesite, phase IV by biotite trachyandesite
and phase V by monzonite and trachyte. The later phases (phases III to
V) are essentially intrusive in nature and are represented by cone
• sheets, dykes and small plugs, and generally occur within.the limits of
the caldera. Te 130-Xe130 dating on a telluride sample gave an age o f
7.2 m.y. indicating that the bulk of the mineralization was probably
associated with the first period of igneous activity (10 to 7.5 m.y.).

Initial hydrothermal activity resulted in widespread
propylitization and carbonatization of basalt, and this was followed by
limited K-silicate alteration and mineralization. The lodes are
essentially fracture fillings with well-developed crustification
textures and vugs. Ore-mineral, sulphide and carbonate precipitation
appears to have been intermittent, whereas quartz precipitation was
essentially uninterrupted, and continued after the cessation of oremineral
precipitation. The ore-minerals are native gold, native
tellurium and gold-silver tellurides. Paragenetic studies suggest
ten stages of ore and gangue mineral precipitation, viz. stage IA:
widespread propylitic alteration with the development of carbonate,
chlorite, actinolite, epidote etc.; stage IB: fringe K-silicate alteration
which includes K-feldspar, sericite, dolomite and sulphides;
stage IIA: barren vein quartz deposition; stage IIB: vein quartz,
dolomite, K-feldspar, sericite, roscoelite, native tellurium and goldrich
tellurides; stage IIIA: barren vein-quartz deposition; stage IIIB:
vein quartz, dolomite, K-feldspar, sericite, roscoelite, native gold
• and silver-rich tellurides; stage IVA: white barren vein quartz;
stage •VB: amber barren vein quartz; stage IVC: white barren quartz;
and stage V: calcite deposition. The bulk of mineralization (stage IIB
tellurides) is restricted to 12 level and above while stage IIIB
tellurides and native gold are distributed throughout.
Fluid-inclusion studies on quartz indicate a temperature range
from 20.5 to 317°C during stage IIA and from 170 to 210°C during
stage IIIA. Temperatures during stage IVA, IVB and IVC ranged from
160 to 2400
C and peaked around 180° C. Fluid inclusions in stage V
calcite are rare and very small, and generally lack a visible vapour
phase; no temperature determinations were possible. Temperatures during
stage IIA show an apparent increase with depth but no such changes are
apparent during other stages. A number of inclusions from near the
top of the mine show evidence that the fluids were boiling and it is
possible to reconstruct dramatic variations in the water table during
the vein formation which apparently overlapped caldera formation.
Freezing temperatures of fluid inclusions indicate about
5.5 wt.% eq. NaC1 and there is no apparent variation in salinity with
time. Leaching studies of fluid inclusions indicate that the fluid
averaged 0.1 molal K, 0.35 molal Na, 0.26 molal Ca and 0.005 molal mg.
The 6D values of fluid-inclusion water from vein quartz
(stages HA to IVC) range from -26 to -58 per mil and there is no
apparent variation in these values with time. 'The 6 180 values Of water
(stages IIA to IVC) calculated from the oxygen isotopic composition of
quartz range from +3 to +9 per mil and it appears that the solutions
were progressively depleted in the lighter isotope with time. The 6 180
values of fresh rocks range from +5.5 to +7 per mil. The 180 values
of propylitized rocks range from +9 to +12.5 per mil and K-silicate
rocks range from +15.5 to +17 per mil. The 6 180 values of vein quartz
(stages IIA to IVC) range from +16 to +21 per mil. it is suggested
that the hydrothermal fluids were largely meteoric waters which underwent
extensive oxygen-isotope exchange with the country rocks. The 6D
values of inclusion water in stage V calcite range from -60 to -80 per
mil suggesting a magmatic source at this stage.
The 6 13C values of carbonates in the propylitic rocks range
from .1.1 to -1.4 per nil. The 6 13C values' of early Vein carbonates
(stages IIB and IIIB) range from +0.3 to -1 per mil. The 6 13 C values
of calcite in the supposedly underlying sedimentary rocks range from
-0.8 to -4 per mil. It is suggested that the hydrothermal carbon
(stages IA to IIIB) was derived from the underlying sedimentary rocks.
The 6 13 C values of stage V calcite range from -4 to -8.6 per mil
suggesting a magmatic parentage. It is suggested that intrusion of
minor plugs and dykes during the second period of igneous activity
(5 to 4.3 m.y.) probably resulted in the formation of shallow
• convection cells and the stage V carbon was either directly derived
from these intrusions or was leached from the surrounding volcanic
rocks.
The 6 34 S values of pyrite (stages IA to IIIB) range from
-5.5 to -15.3 per mil. When considered in conjunction with f02-T-pH
conditions these values are compatible with a magmatic source.
Chemical analyses of fresh and altered rocks indicate that
Si02 was depleted from the propylitic rocks but was added to the
K-silicate rocks. MgO, CaO, Al203, Na20 and TiO2 show progressive
depletion with the intensity of alteration.

Physicochemical conditions of ore deposition have been approximated
from mineral stability and other data. They are: T = 200 to
250°C; pH = 5.5 ± 0.5; mCl = 1; I = 1; mES = 10-3; mEC = 10-2 ; mesr/mESo
= 1 and f02 = 10-35 to 10-40 . Metal solubility calculations
suggest that under these conditions the solutions may carry 10 to 1 ppm
Zn, 100 to 10 ppm Fe, 0.05 to 0.02 ppm Au and 0.1 to 0.01 ppm Pb, Ag
and Cu each.
Stability fields of several tellurides have been constructed
and the existing thermodynamic data indicate that the tellurium concentrations
as low as 10-11 molal will result in stabilising a number of
tellurides. Apparently the thermodynamic data on the aqueous tellurium
species need revision.
A fluid convection model is proposed in which meteoric water
circulated for most of the history of the system through basalt and the
underlying sediments. The probable cauldron geometry requires only one
major discharge zone for the convective system. Though this limits the
xiv potential for further deposits related to the Tavua caldera it
emphasises the ore potential of other similar structures in Fiji. It
is estimated from the settling properties of pyrite grains that the
fluid velocity in the vein was about 4 cm/sec. This with other
assumptions regarding metal solubility and the length and width of the
feeder channel suggest that deposits of the size of Emperor could form
in about 10 3 years.
A model is envisaged in which the circulating meteoric waters
reacted with the underlying calcareous sediments and volcanics and
picked up sedimentary carbon and heavy oxygen. The ore and gangue
mineral constituents were also probably leached from these rocks.
These solutions were channeled upwards through existing fractures and
boiled at the site of deposition. A decrease in temperature together
with a drop in mEs and increase in pH due to boiling were probably the
main causes of ore and gangue mineral precipitation.

Item Type: Thesis (PhD)
Keywords: Gold mines and mining, Tellurides, Emperor Mine, Fiji
Copyright Holders: The Author
Copyright Information:

Copyright 1979 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1979. Bibliography: l. 134-155

Date Deposited: 25 Nov 2014 00:39
Last Modified: 11 Mar 2016 05:56
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