Fluid-inclusion and geochemical studies at the Emperor gold mines, Fiji.
Creators: name: Ahmad, Masood
Publisher: name: University Of Tasmania
Year: 2023

description: 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 ‚Äö 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 ‚Äö and silver-rich tellurides; stage IVA: white barren vein quartz; stage ‚ÄöVB: 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¬¨‚àûC during stage IIA and from 170 to 210¬¨‚àûC during stage IIIA. Temperatures during stage IVA, IVB and IVC ranged from 160 to 2400 C and peaked around 180¬¨‚àû 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 ‚Äö 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¬¨‚àûC; 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.