Prendergast, Kylie (2003) Porphyry-related hydrothermal systems in the Estberg District, Papua, Indonesia. PhD thesis, James Cook University.

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Abstract

The Ertsberg district hosts multiple skarn and porphyry-related deposits comprising one of the largest Cu-Au resources in the world. The geological and geochronological framework suggests economically significant igneous and hydrothermal events in the district occurred as two distinct evolving episodes separated by less than 500,000 years. The igneous complexes (Grasberg Igneous Complex ca <3.33-3Ma and Ertsberg ca 3-2.67Ma) are respectively post-dated by early porphyry-style mineralisation and spatially related skarn Cu-Au mineralisation. Deep-intermediate high sulphidation style mineralisation occurred as a late development at both locations.

The West Grasberg estimated gold resource (ca. 700,000 ounces) overprints limestone breccias and banded clay maar sediments on the western margin of the Grasberg Igneous Complex (GIC). Volcanic-diatreme activity is interpreted to have facilitated banded clay subsidence after extensive porphyry-style potassic, magnetite and quartz alteration occurred in the GIC but prior to cessation of hydrothermal mineralisation. Native gold±pyrite occurrences are spatially associated with smectite-chlorite clay, and Au-As-Bi-Te bearing pyrite. Gold mineralisation may have occurred contemporaneously with high sulphidation Au-As-Bi associations in the Kucing Liar Cu-Au deposit which occurs at depth nearby.

Separate but near identical pyrite-Au-As-Zn-Bi-Te occurrences overprint earlier skarn Cu-Au at Wanagon Gold (ca. 2 million ounces gold) and Big Gossan. The Big Gossan occurrence is younger «2.82±O.04Ma based on a new ⁴⁰Ar/³⁹Ar phlogopite date from the Big Gossan skarn). Adularia deposited in leached sandstone at the Wanagon Gold occurrence has a ⁴⁰Ar/³⁹Ar K-feldspar age of 3.62±O.05Ma. A published K-Ar date (3.81 ±O.06Ma) from the Wanagon Sill constrains formation of the overprinting skarn Cu-Au and late-stage Wanagon Gold occurrence to a period of ca. O.2Ma. δ³⁴ of sulphide (-0.7 to 5.10/00) from skarn Cu-Au and overprinting pyrite-Au-As-Zn-Bi-Te occurrences at both deposits are interpreted as magmatic. Laser ablation analyses show pyrites deposited by copper-depositing hydrothermal fluids contain a maximum of 0.01 ppm Au and 370ppm As whereas pyrite deposited by Au-As-Bi-Te-depositing fluids contains up to 57ppm Au and 20000ppm As. Textural relationships are consistent with late-stage gold being transported as both AuHS- and AuHSo complexes and respectively precipitated by sulphidation reactions and adsorption by pyrite surfaces. Fluid inclusion microthermometry indicates the pyrite-Au-As-Zn-BiTe forming fluids had salinities of ca. 9 wt% NaCI(equiv) and homogenisation temperatures of ca. 248°C. Despite the close spatial and temporal association with skarn Cu-Au, the occurrences are interpreted to represent incursion of a new fluid with a different composition.

78 δ¹⁸O and δ¹³C analyses of hydrothermal carbonates in the district suggest non-magmatic fluids were insignificant contributors except at Kucing Liar where calculated δ¹⁸O fluid values from carbonate associated with leaching, quartz and clay alteration are 2.2-4.8‰. δ¹⁸O(SMOW) values of secondary calcite (11.7‰ to 17.5‰) associated with a carbonate alteration and dissolution event in the Ertsberg East Skarn System were determined in situ by SIMS. Carbonate alteration selveges and vein infill from the Big Gossan skarn and mineralisation have magmatic δ¹⁸O(SMOW) and δ¹³C(PDB) values with δ¹⁸O (H₂O) and δ¹³C of ca. 5.9‰ and ca. -3.2‰ respectively. Analyses of texturally distinct dolomitisation of dolostone and limestone peripheral to the GIC define a mixing line from marine carbonate host rocks (δ¹⁸O(SMOW) = 30.9‰ and δ¹³C(PDB) = 1.2‰) to altered dolostone (δ¹⁸O(SMOW) = 8.7‰ and δ¹³C(PDB) = -1.6‰). Fluids are considered to have been magmatic H₂O+CO₂.

The two key findings from this research include a) recognition of magmatic-dominated δ¹³C and δ¹⁸O compositions from hydrothermal carbonates and b) characterisation of two separate but near identical late stage pyrite-Au-As-Zn-Bi-Te occurrences. The depleted δ¹³C values from skarns and mineralised vein carbonates are interpreted to represent contribution from magmatic carbon. The presence of magmatic CO₂ as a volatile component in intrusion-related deposits has significant genetic consequences as it is considered to promote early phase separation, which can lead to selective partitioning of ore body components (i.e. Cu, Au, As, Zn, S, CI etc) between brine and vapour. The pyrite-Au-As-Zn-Bi-Te occurrences were formed from ca. 9 wt% NaCl(equiv) magmatic fluids that homogenised at temperatures of ca. 248°C. Such fluids were capable of transporting and depositing significant pyrite, Au, As and lesser Zn, Pb, Cu, Bi and Te. These fluids may represent the precursor to fluids documented in epithermal deposits that form at shallower levels with an additional significant meteoric component.

Item ID:	27155
Item Type:	Thesis (PhD)
Keywords:	copper ores; Cu-Au; Ertsberg; geology; gold deposits; hydrothermal deposits; igneous rocks; Indonesia; mineral resources; porphyry; skarn deposits
Date Deposited:	30 May 2013 06:25
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 50% 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 50%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8499 Other Mineral Resources (excl. Energy Resources) > 849999 Mineral Resources (excl. Energy Resources) not elsewhere classified @ 50% 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
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