The Mt Carlton high-sulfidation epithermal deposit, NE Australia: geologic character, genesis and implications for exploration

Sahlström, Fredrik (2018) The Mt Carlton high-sulfidation epithermal deposit, NE Australia: geologic character, genesis and implications for exploration. PhD thesis, James Cook University.

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High-sulfidation epithermal (HS) deposits occur in volcanic arc settings around the world. These deposits are sought-after by mining companies due to their endowment in precious and base metals, and due to their genetic connection to porphyry copper deposits. The aim of this thesis is to enhance our understanding of HS deposits and how to explore for them. This has been done via an extensive case study of the Mt Carlton HS deposit in the northern Bowen Basin, NE Australia. The Mt Carlton deposit has an estimated resource of 10.42 Mt with an average grade of 2.92 g/t Au, 30.60 g/t Ag and 0.47 % Cu, and since 2013 it is mined in an open pit operation. Mt Carlton is of particular interest for several reasons: it is one of the world's few known economic, pre-Neogene HS deposits; the shallow-water venting portion of the magmatic-hydrothermal system is preserved; it formed in an active extensional tectonic setting; and it contains high enrichment in several critical elements. As such, Mt Carlton provides an excellent opportunity to study the geologic characteristics, genesis, preservation, and tectonic modification of this deposit type.

The first part of the thesis explores the geologic and genetic framework of the Mt Carlton deposit. This was studied by field mapping combined with a range of mineralogical and geochemical analytical methods. We find that hydrothermal alteration and epithermal mineralization at Mt Carlton formed in a shallow volcano-lacustrine extensional setting, partly contemporaneously with the deposition of volcanic sediments in localized half-graben and graben basins, during the Early Permian backarc rifting stage in the Bowen Basin. Proximal Au-Cu mineralization occurs in networks of steep veins and hydrothermal breccias developed within a massive rhyodacite porphyry. The ore zones extend along ~600 m strike length and exhibit a well-defined metal zonation, from NE (proximal) to SW (distal), of: Au-Cu ➔ Cu-Zn-Pb-Ag ➔ Ag-Pb-(Cu) ➔ Ag. Distal Ag-barite mineralization occurs in a stratabound position within a volcano-lacustrine sedimentary sequence that overlies the rhyodacite porphyry. Locally, it exhibits various types of syn-sedimentary ore textures. Such textures are interpreted to have formed as mineralizing fluids discharged into small lakes developed within localized rift basins, at the same time as the volcano-lacustrine sediments were deposited. At depth, equivalent ore textures were produced within open spaces in the structural roots of the rift basins. The magmatic-hydrothermal system at Mt Carlton was similar to those in subaerial HS deposits, however, it stands out for containing an additional shallow-water venting lacustrine component. As such, the Mt Carlton deposit shows several morphological, textural and geochemical similarities to seafloor massive sulfide deposits formed in shallow-submarine volcanic settings. Prolonged extension in the Bowen Basin preserved the Mt Carlton deposit by rapidly burying it beneath a post-mineralization volcano-sedimentary cover. Continued extension also led to displacement of the mineralized rock pile along a series of shallowly dipping detachment faults and across steeply dipping normal faults. The post-mineralization faults have truncated, segmented, and rotated the ore zones at the Mt Carlton deposit, and on a larger scale, they should have displaced any underlying porphyry mineralization relative to the current location of the open pits.

The second part of the thesis focusses on germanium, gallium and indium; three critical elements in demand due to their increasing usage in various high-tech and green-tech applications. However, the mineralogy and mechanisms of concentration of these elements in different types of hydrothermal ore deposits remain poorly constrained. We investigated the mineralogical distribution of germanium, gallium and indium at Mt Carlton and similar deposits worldwide, using electron probe microanalysis and laser ablation inductively-coupled plasma mass spectrometry. This study revealed three distinct mineral associations in HS deposits that may contain enrichment in critical elements. These include: (1) gallium enrichment in aluminium-bearing hydrothermal sulfates and silicates; (2) germanium ± indium-gallium enrichment in high-sulfidation enargite ores; and (3) indium-gallium ± germanium enrichment in intermediate-sulfidation sphalerite ores. We observe that the deposits from which samples showed high enrichment in critical elements in this study are all hosted in stratigraphic sequences that locally contain carbonaceous sedimentary rocks. We suggest that, in addition to magmatic-hydrothermal processes, such rocks could potentially be important for the collection and precipitation of critical elements in HS deposits. Additionally, a new analytical approach to study trace elements in sphalerite, hyperspectral cathodoluminescence (CL) mapping on the electron microprobe, was used to characterize the composition and CL properties of indium-bearing sphalerite from Mt Carlton. The hyperspectral mapping revealed a high-intensity CL emission directly related to the presence of indium, copper and gallium in the sphalerite structure, centred at wavelengths between ~500 and ~600 nm. The CL peak is demonstrated to be shifted to the higher-wavelength positions when the proportion of indium relative to copper increases. This study shows that hyperspectral CL provides a powerful and efficient tool to study the distribution of indium in sphalerite.

Overall, the study of Mt Carlton highlights that shallow-level HS mineralization can be preserved in Paleozoic settings, under appropriate geodynamic conditions. We suggest that extensional segments of volcanic arcs, such as backarc rifts, are particularly prospective for such mineralization. The study also highlights the importance of understanding the post-mineralization tectonic modification of the porphyry-epithermal system in extensional settings. This concerns exploration in the Mt Carlton district and the northern Bowen Basin, as well as in similar geologic terranes elsewhere. Additionally, this thesis demonstrates the potential for significant enrichment in germanium, gallium and indium in the common ore and alteration mineral assemblages in HS deposits. These findings may assist in the evaluation of HS deposits as potential sources of these critical elements, and for the development of efficient ore processing methods during recovery.

Item ID: 54844
Item Type: Thesis (PhD)
Keywords: cathodoluminescence, copper, critical elements, enargite, epithermal, gallium, germanium, high-sulfidation, high-sulphidation, hyperspectral, indium, LA-ICP-MS, Mt Carlton, sphalerite
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For this thesis, Lars Fredrik Sahlstrom received the Dean's Award for Excellence 2019.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3: Sahlström, Fredrik, Arribas, Antonio, Dirks, Paul, Corral, Isaac, and Chang, Zhaoshan (2017) Mineralogical distribution of germanium, gallium and indium at the Mt Carlton high-sulfidation epithermal deposit, NE Australia, and comparison with similar deposits worldwide. Minerals, 7 (213).

Chapter 4: Sahlström, Fredrik, Blake, Kevin, Corral, Isaac, and Chang, Zhaoshan (2017) Hyperspectral cathodoluminescence study of indium-bearing sphalerite from the Mt Carlton high-sulphidation epithermal deposit, Queensland, Australia. European Journal of Mineralogy, 29 (6). pp. 985-993.

Date Deposited: 31 Jul 2018 00:06
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 75%
04 EARTH SCIENCES > 0403 Geology > 040306 Mineralogy and Crystallography @ 25%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 20%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 80%
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