Textural, geochemical, and fluorescence variations of calcite in porphyry Cu-Au deposits
Creators: name: Thompson, JA
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis evaluates the potential for a combination of calcite mineral and stable isotope geochemistry and ultraviolet fluorescence to be used as an aid to exploration in volcanic-hosted and carbonate-hosted porphyry Cu-Au systems. The Grasberg and Batu Hijau porphyry Cu-Au deposits, Indonesia, are investigated as case study sites. New analytical methods for calcite trace element analyses were developed in order to facilitate this research. Advances in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have allowed for the quantification of elements in calcite that are more precise than the generally accepted 10% error allocated to LA-ICP-MS data. To achieve precise data that can be reliably comparable between analytical laboratories, optimal laser conditions and calibration standards have been defined that produce accurate results for an independently verified calcite sample (P-Cal). NIST-612 as a calibration reference material and BCR-2g, GSD-1g as secondary reference materials for laser fluence values of 3.5 J/cm\\(^2\\) provide the best precision and accuracy. For elements that display a systematic offset between our accepted and LA-ICP-MS values in P-Cal, matrix correction factors are provided. Comparison of several calcite ultraviolet (UV) luminescence imaging methods has highlighted that hyperspectral cathodoluminescence-electron microprobe methods provide the best spatial resolution (micron-level) of UV colour response variation of calcite. However, field-based luminescence investigations across a mineralized porphyry district can readily be conducted using a high-power UV light and controlled photography, which allows mineral luminescence to be identified to the millimetre-scale. Red-pink UV-fluorescence in calcite is an indication of proximity to porphyry mineralisation in the carbonate-hosted Grasberg porphyry system. It is caused by elevated Mn concentrations (up to 3 wt. % Mn) and can be detected up to two kilometres from the deposit centre. The fluorescent response of calcite in the Batu Hijau porphyry deposit is suppressed, possibly due to elevated Fe in calcite (up to 2 wt. %) or obscuring from abundant feldspar blue and red fluorescence colour response. These factors may prevent UV fluorescence from being used as an aid to porphyry exploration in volcanic-hosted environments. Calcite alteration associated with porphyry-skarn style Cu-Au mineralisation in the Grasberg-Ertsberg district, Indonesia has been investigated from the centre of the Grasberg Igneous Complex to distal settings more than five kilometres from the deposit centre. Calcite veins distal to Grasberg record a magmatic-hydrothermal signature defined by characteristic O-C isotopic compositions and trace element contents. Spatial variations in Mn/Mg, Pb, Sr, LREE, and U with respect to distance from the source of magmatichydrothermal fluid define chemical halos up to four kilometres from the deposit centre in the limestone host rocks. Low ˜í¬•13C (6.4 to 16.7 ‚ÄövÑ‚àû) and ˜í¬•\\(^{18}\\)O values (-6.8 to -0.3 ‚ÄövÑ‚àû) in proximal calcite, and distal calcite veins with C-O isotopic values similar to host rock limestones define a trend of increasing wall rock interaction, and chemical exchange as fluids progressively infiltrate the host rock limestones. Positive correlations between proximal red ultraviolet fluorescing colours, calcite mineral chemistry, and isotopic compositions provide an example of multiple geochemical methods that can together provide increased ability for the distal detection of porphyry deposits in carbonate terrains. Four textural types of calcite alteration were identified in volcanic wall rocks of the Batu Hijau district. Calcite veins are common proximal to the deposit, whereas selective calcite alteration of primary megacrystic plagioclase and hornblende is more widely distributed across the district. Calcite flooding of groundmass is restricted to proximal settings (up to 2.5 km from the deposit centre) and vug-filling calcite is restricted to the medial and distal settings (1.6 ‚ÄövÑv¨ 4.2 km distance). Several trace elements in calcite including Mn, Mg, Fe, P, Sr, Ce, Zn, and U, decrease in abundance away from the centre of Batu Hijau. The variations in calcite mineral chemistry with distance from Batu Hijau are attributed to a combination of increasing water-rock interaction, fluid mixing, and decreasing temperature away from the deposit. A subset of proximal calcite-bearing veins precipitated from a latestage, more reduced fluid that experienced vapor condensation. The increased chemical variability of proximal calcite is another indicator of proximity to mineralisation. C-O stable isotope variations are useful in detecting magmatic-hydrothermal fluid alteration in both carbonate-rock hosted and volcanic-hosted porphyry environments. Calcite mineral chemistry is most useful in volcanic-hosted environments where trends in mineral chemistry are not apparent in whole rock analysis. In the Grasberg-Ertsberg district some variations in calcite mineral chemistry were also apparent in the whole rock data, although mineral chemistry provided the opportunity to examine small-scale chemical variations that could not be seen in the whole rock data. Calcite fluorescence as a proxy for mineral chemistry was shown to be useful in carbonate-hosted porphyry environments, but calcite fluorescence in volcanic-hosted alteration assemblages was washed out by strong fluorescing feldspar and had a diminished fluorescence response.