Behnsen, Helge (2018) Magma fertility related to Au - Cu mineralisation: evaluating the potential for linked porphyry Cu - Au deposits at depths, North Queensland, Australia. Masters (Research) thesis, James Cook University.

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Abstract

Continental magmatic arcs host economically important porphyry ore deposit types for copper, gold, and molybdenum, and, if preserved, epithermal high- to intermediate-sulfidation gold and silver deposits. These deposits are genetically and/or spatially associated with magmatism, so understanding the link between magmatism and mineralisation is seen as a crucial endeavor to assist knowledge of mineralisation processes and strategies for regional-scale mineral exploration.

This thesis examines aspects of the mineralisation potential of continental arc magmatic rocks that are exposed as part of the Permo-Carboniferous Kennedy Igneous Association (KIA) from northeastern Queensland. This belt of upper crustal intermediate to felsic granitoids and associated volcanic rocks is recognised to host many economic ore deposits related to igneous activity. Using well-established analytical techniques to analyze whole-rock and mineral major- and trace element compositions combined with Sm-Nd isotopes, I investigate two different localities related by their occurrence in time and space, with the aim to better understand magma fertility, or more specifically the petrogenetic processes contributing to Permo-Carboniferous Cu-Au mineralisation in northeast Queensland. This work strives to improve the applicability of magma fertility concepts to confidently identify fertile igneous terranes potentially covering high grade Cu-Au deposits at depth.

The first location investigated lies to the south of Townsville at the northern edges of the Bowen Basin, where the early Permian Lizzie Creek Volcanic Group (LCV) hosts the Mount Carlton high-sulfidation epithermal Au-Cu deposit. Established whole-rock geochemical parameters, e.g. Sr/Y, V/Sc, used to distinguish fertile porphyry Cu-Au hosting intrusive rocks from barren intrusions, were tested on the "fertile" LCV succession, hosting the Mt. Carlton deposit and compared to contemporary "barren" volcanic rocks of the same group. The results reveal that the key control on generating the fertile LCV sequence was a high magmatic water content, reflected by early fractionation of amphibole at the source level, whereas the barren sequence was comparably dry, and formed by typical fractionation of plagioclase and clinopyroxene. This outcome has major implications for the applicability of whole rock geochemistry as a magma fertility indicator. The here presented results suggest that for volcanic rocks, which tend to be affected by hydrothermal alteration; a) Sr/Y is not a reliable fertility proxy and; b) Rare earth element ratios (e.g. La/Yb, Dy/Yb), which are relatively resistant to alteration, can be used to reveal differences in magmatic evolution. This supports previous research on magma fertility, and provides strong evidence that fertile magmatic suites can be identified from volcanics sequences that evolved from basalt to rhyolite with a progressive increase in La/Yb, decreasing Dy/Yb, and consistent or slightly increasing V/Sc ratios, despite having experienced some degree of alteration.

The second location investigated in this thesis is the Tuckers Igneous Complex (TIC); a calc-alkaline, I-type igneous complex which intruded the Ravenswood Batholith between ~300-290 Ma and formed within the same convergent margin as the LCV. The TIC is part of the KIA and its relevance for this project lies within its close association with the major early Permian, breccia-hosted Mt. Leyshon Au deposit. The TIC contains a sequence of intrusive rocks from gabbro to felsic granodiorite, and hence offers the opportunity to investigate geochemical evolution, particularly volatile element evolution, of arc magmas at upper crustal levels. Here, I build on previous geochemical and petrographic data for the complex using newly acquired whole-rock geochemistry and in-situ mineral analyses of the major rock forming minerals plagioclase-clinopyroxene-orthopyroxene, and the halogen-bearing minerals apatite and biotite (and amphibole) to monitor and track volatile evolution (Cl and F). As volatile element behavior is recognised to have a fundamental control on magma fertility, this study, offers new insights into the fertility of this Permo-Carboniferous arc, which can be applied to other arc worldwide.

My new results show that the TIC formed through closed-system crystal fractionation from gabbro to mafic granodiorite varieties with only minor mixing and/or assimilation, and likely became an open system during cooling and crystallisation of the more felsic granodiorites. Based on apatite halogen contents, volatile saturation is suggested to have occurred at around ~63-65 wt.% bulk-rock SiO₂, up to which point estimated Cl melt contents steadily rise, and then suddenly drop from ~0.8 to 0.4 wt.%. This abrupt change likely relates to the exsolution of a Cl-rich volatile phase, and also marks important changes within the mineral assemblage from a dry Plag ± Px ± Fe-Ti oxide assemblage, towards a more hydrous and slightly more oxidised Plag ± Hbl ± Bt ± Mag assemblage. Fluorine enriches in the melt with fractionation, even once saturation in a volatile phase is reached, consistent with what is expected from experimental partitioning studies in the system apatite – melt – fluid. Local or temporal changes in the magma's fO₂ is indicated by a measurable increase in apatite S contents in evolved felsic granodiorite, as apatite preferentially incorporates S as its oxidised species S⁶⁺, which also coincides with the general observed changes in the rock forming mineral assemblages as described above. Indicated volatile exsolution, causing loss of significant proportions of Cl together with oxidising conditions at which the bulk of dissolved S may have been present and possibly degassed as S⁴⁺O₂ (and causing the presence of low amounts of available S²⁻ to precipitate base metals) from the TIC magma may have caused mineralization within overlying, but now eroded rocks. A second possibility may be that the TIC was never able to produce mineralising fluids due to early segregation of sulfides, scavenging ore forming metals (e.g. Cu, Au, and Ag) prior to the crystallisation of TIC gabbro, and thereby stripping the TIC of its ore-forming potential early on. However, an exsolved Cl-rich fluid is very capable of transporting ore metals, therefore the role and true nature of such fluid(s) originating from the TIC can be subject of further investigations.

The here presented results help to understand Cu-Au fertility on a regional, magmatic arc terrane (both volcanic and intrusive) from initial lower crustal petrological processes to the surface, but also on a local scale; within confined individual igneous bodies, their respective mineral assemblages, and their potential role towards regionally present Cu-Au mineralisation. In particular the findings on volcanic rocks offer great potential as easily accessible first-order fertility assessment tool for magmatic-hydrothermal Cu-Au exploration worldwide.

Item ID:	56199
Item Type:	Thesis (Masters (Research))
Keywords:	magma fertility, Cu-Au mineralisation, North Queensland, high sulfidation, porphyry copper, exploration, copper, gold, Lizzie Creek, Permian, Tuckers Igneous Complex
Copyright Information:	Copyright © 2018 Helge Behnsen
Date Deposited:	25 Nov 2018 22:46
FoR Codes:	04 EARTH SCIENCES > 0402 Geochemistry > 040201 Exploration Geochemistry @ 50% 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 50%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 50% 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 50%
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