A magmatic copper and fluid source for the sediment-hosted Mount Isa deposit
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Keywords: copper, isotopes, sedimentary deposits, hydrothermal processes, Mount Isa
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Abstract
Figures
Figure 1 Simplified geological map showing the surface projection of the Cu and Zn-Pb ore bodies. The inset shows the location of the Mount Isa deposits in Queensland and within the Mount Isa Inlier. A and B indicate the location of sections shown in Figure 2. KLB-Kalkadoon Leichhardt Belt, WFB-Western Fold Belt, EFB-Eastern Fold Belt. |
Figure 2 Cross sections through (a) the middle of the deposit and (b) across the low grade envelope showing the location of samples collected for isotope measurements and the δ65Cu values for each sample. Red dots – negative values; blue diamonds – positive values. |
Figure 3 Photographs showing typical chalcopyrite (ccp) textures and dolomite (dol) alteration from Mount Isa deposit. |
Figure 4 The δ65Cu values for Mount Isa deposits relative to the distance from (a) major structures and (b) ore textures. (c) Comparison of δ65Cu values from sedimentary copper deposits (Sed Cu), volcanogenic massive sulfide deposits (VMS), porphyry and epithermal deposits (PCD/Epi), Michigan copper deposits and the Mount Isa deposit. |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
The giant Mount Isa copper (with Zn-Pb) deposit, located in NW Queensland (Fig. 1), is a unique deposit type and the second largest copper producer in Australia after Olympic Dam in South Australia (e.g., Mudd, 2021
Mudd, G.M. (2021) Assessing the Availability of Global Metals and Minerals for the Sustainable Century: From Aluminium to Zirconium. Sustainability 13, 10855. https://doi.org/10.3390/su131910855
). Despite intense exploration for similar deposits since its discovery almost 100 years ago, Mount Isa remains the only deposit of its kind in the entire Mount Isa region and the world. Copper mineralisation occurs within the same stratigraphic horizon, the Urquhart Shales, as Zn-Pb mineralisation (Fig. 1), and the deposit is commonly described in the literature as a zoned Zn-Pb-Cu system (e.g., Cave et al., 2020Cave, B., Lilly, R., and Barovich, K. (2020) Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system. Ore Geology Reviews 124, 102647. https://doi.org/10.1016/j.oregeorev.2020.103647
) with copper mineralisation occurring at depth and Zn-Pb mineralisation occurring nearer the surface. Two contrasting models have been put forward to explain the source, transport and deposition of copper. The first model proposes that copper was sourced within the basin from ferruginous red beds by oxidised basinal brines and transported along permeable mud and silt horizons to the pyritic Urquhart Shales where chalcopyrite deposition occurred due to reduction reactions (e.g., McGoldrick and Keays, 1990McGoldrick, P.J., Keays, R.R. (1990) Mount Isa copper and lead-zinc-silver ores; coincidence or cogenesis? Economic Geology 85, 641–650. https://doi.org/10.2113/gsecongeo.85.3.641
; Wilde et al., 2006Wilde, A.R., Jones, P.A., Gessner, K., Ailleres, L., Gregory, M.J., Duncan, R.J. (2006) A geochemical process model for the Mount Isa copper orebodies. Economic Geology 101, 1547–1567. https://doi.org/10.2113/gsecongeo.101.8.1547
). The second model proposes that copper was sourced from basement mafic rocks by oxidised basinal or metamorphic brines, transported along fractures and deposited in the overlying Urquhart Shales either by cooling, pH change or an increase in sulfur fugacity (e.g., Heinrich et al., 1989Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J. (1989) A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia. Economic Geology 84, 529–550. https://doi.org/10.2113/gsecongeo.84.3.529
; Heinrich et al., 1995Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
; Gregory et al., 2008Gregory, M.J., Keays, R.R., and Wilde, A.R. (2008) Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry. Australian Journal of Earth Sciences 55, 1153–1173. https://doi.org/10.1080/08120090802266618
). In this contribution, we present, for the first time, 65Cu/63Cu isotope data (reported as δ65Cu) from chalcopyrite across the Mount Isa deposit and discuss the results in terms of metal source, transport, precipitation and the two most prominent competing genetic models.
Figure 1 Simplified geological map showing the surface projection of the Cu and Zn-Pb ore bodies. The inset shows the location of the Mount Isa deposits in Queensland and within the Mount Isa Inlier. A and B indicate the location of sections shown in Figure 2. KLB-Kalkadoon Leichhardt Belt, WFB-Western Fold Belt, EFB-Eastern Fold Belt.
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Regional and Deposit Geology
The Mount Isa Inlier (Fig. 1) has been subdivided into three subparallel belts that include, from east to west, the Eastern Fold Belt (EFB), the Kalkadoon-Leichardt Belt (KLB), and the Western Fold Belt (WFB). The geology of the inlier is dominated by sedimentary and volcanic sequences that were deposited in three major supra-crustal basins: 1) the 1790−1740 Ma Leichhardt Superbasin, 2) the 1730−1640 Ma Calvert Superbasin, and 3) the 1640−1580 Ma Isa Superbasin (Gibson et al., 2016
Gibson, G.M., Meixner, A.J., Withnall, I.W., Korsch, R.J., Hutton, L.J., Jones, L.E.A., Holzschuh, J., Costelloe, R.D., Henson, P.A., and Saygin, E. (2016) Basin architecture and evolution in the Mount Isa mineral province, northern Australia: Constraints from deep seismic reflection profiling and implications for ore genesis. Ore Geology Reviews 76, 414–441. https://doi.org/10.1016/j.oregeorev.2015.07.013
). These basinal sediments were deposited on top of basement rocks that had been deformed and metamorphosed during the Barramundi Orogeny (∼1890−1850 Ma; Foster and Austin, 2008Foster, D.R.W., and Austin, J.R. (2008) The 1800–1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes. Precambrian Research 163, 7–30. https://doi.org/10.1016/j.precamres.2007.08.010
). Infill of the Leichhardt Superbasin was interrupted by the ∼1750−1710 Ma Wonga Orogeny (Spence et al., 2021Spence, J., Sanislav, I.V., and Dirks, P.H.G.M. (2021) 1750-1710 Ma deformation along the eastern margin of the North Australia Craton. Precambrian Research 353, 106019. https://doi.org/10.1016/j.precamres.2020.106019
; Spence et al., 2022Spence, J., Sanislav, I.V., and Dirks, P.H.G.M. (2022) Evidence for a 1750–1710 Ma orogenic event, the Wonga Orogeny, in the Mount Isa Inlier, Australia: Implications for the tectonic evolution of the North Australian Craton and Nuna Supercontinent. Precambrian Research, 369, 106510. https://doi.org/10.1016/j.precamres.2021.106510
). Infill of the Calvert Superbasin was interrupted by an inversion event between 1690 and 1668 Ma (Southgate et al., 2000Southgate, P.N., Bradshaw, B.E., Domagala, J., Jackson, M.J., Idnurm, M., Krassay, A.A., Page, R.W., Sami, T.T., Scott, D.L., Lindsay, J.F., McConachie, B.A., Tarlowski, C. (2000) Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730–1575 Ma) in northern Australia and implications for base-metal mineralisation. Australian Journal of Earth Sciences 47, 461–483. https://doi.org/10.1046/j.1440-0952.2000.00787.x
), and sediment fill of the Isan Superbasin was interrupted by the ∼1650−1490 Ma Isan Orogeny (Foster and Austin, 2008Foster, D.R.W., and Austin, J.R. (2008) The 1800–1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes. Precambrian Research 163, 7–30. https://doi.org/10.1016/j.precamres.2007.08.010
; Abu Sharib and Sanislav, 2013Abu Sharib, A.S.A.A., and Sanislav, I.V. (2013) Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: Example from the Eastern Fold Belt, Mount Isa Inlier, Australia. Tectonophysics 587, 146–167. https://doi.org/10.1016/j.tecto.2012.06.051
).The Mount Isa deposit is hosted by the Mount Isa Group (Calvert Superbasin) which consists of the Surprise Creek Formation at its base, followed by the Warrina Park Quartzite, Moondara Siltstone, Breakaway Shale, Native Bee Siltstone, Urquhart Shale, Spear Siltstone, Kennedy Siltstone, and the Magazine Shale at its stratigraphic top (Southgate et al., 2000
Southgate, P.N., Bradshaw, B.E., Domagala, J., Jackson, M.J., Idnurm, M., Krassay, A.A., Page, R.W., Sami, T.T., Scott, D.L., Lindsay, J.F., McConachie, B.A., Tarlowski, C. (2000) Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730–1575 Ma) in northern Australia and implications for base-metal mineralisation. Australian Journal of Earth Sciences 47, 461–483. https://doi.org/10.1046/j.1440-0952.2000.00787.x
). The mineralisation (Fig. 2) is hosted by the Urquhart Shales, adjacent to the Paroo Fault, which separates the Mount Isa Group from the basement mafic rocks of the Eastern Creek Volcanics (ECV). The Paroo Fault is crosscut by a series of faults (the Bernborough and J46 Faults; Fig. 2) that have been interpreted as conduits for the fluid that leached copper out of the ECV and deposited it in the Urquhart Shales (e.g., Heinrich et al., 1989Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J. (1989) A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia. Economic Geology 84, 529–550. https://doi.org/10.2113/gsecongeo.84.3.529
; Heinrich et al., 1995Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
; Gregory et al., 2008Gregory, M.J., Keays, R.R., and Wilde, A.R. (2008) Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry. Australian Journal of Earth Sciences 55, 1153–1173. https://doi.org/10.1080/08120090802266618
). The copper orebodies have irregular shapes, with highest grades in zones of most structural complexity (Fig. 2). Chalcopyrite is the dominant copper-bearing mineral; it is texturally late relative to the Zn-Pb-Ag mineralisation (Fig. 3) and postdates peak deformation and metamorphism (e.g., Cave et al., 2020Cave, B., Lilly, R., and Barovich, K. (2020) Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system. Ore Geology Reviews 124, 102647. https://doi.org/10.1016/j.oregeorev.2020.103647
; Mahan et al., 2023Mahan, B., Mathur, R., Sanislav, I., Rea, P., Dirks, P. (2023) Cu isotopes in groundwater for hydrogeochemical mineral exploration: A case study using the world-class Mount Isa Cu–Pb–Zn deposit (Australia). Applied Geochemistry, 148, 105519. https://doi.org/10.1016/j.apgeochem.2022.105519
).
Figure 2 Cross sections through (a) the middle of the deposit and (b) across the low grade envelope showing the location of samples collected for isotope measurements and the δ65Cu values for each sample. Red dots – negative values; blue diamonds – positive values.
Figure 3 Photographs showing typical chalcopyrite (ccp) textures and dolomite (dol) alteration from Mount Isa deposit.
On a deposit scale, chalcopyrite overprints pre-existing silica-dolomite alteration. In hand specimen, the chalcopyrite occurs as infill in fracture networks that crosscut carbonaceous shale (Fig. 3a), along shear fabrics overprinting silica-dolomite alteration (Fig. 3b), replacing silica-dolomite infill of brecciated Urquhart shale (Fig. 3c) and as replacement and infill of massive silica-dolomite breccia (Fig. 3d).
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Chalcopyrite δ65Cu from Mount Isa Deposit
The δ65Cu composition of chalcopyrite grains (Fig. 4a) were measured from ninety samples spread across the entire deposit including the surrounding low grade envelope (Supplementary Information Fig. S-2). The δ65Cu values vary between −0.87 ‰ and +0.88 ‰ with an average value of +0.13 ‰ (Fig. 4a) and a median value of +0.10 ‰ which indicates that at the scale of the deposit the δ65Cu values have a unimodal distribution. The only variation in δ65Cu values is in relation to the major structures. In general, lower δ65Cu values occur closer to major structures whereas higher values are more common further away from these structures (Fig. 2a). Chalcopyrite grains located within 100 metres of a major structure have an average δ65Cu = 0.00, chalcopyrite grains located 100−200 m from a major structure have an average δ65Cu = +0.25 ‰ whereas chalcopyrite grains located 200−300 m have an average δ65Cu = +0.35 ‰ (Fig. 4a). In fact, 86 % of all negative values occur within 100 m of a major structure. The close spatial relationship between δ65Cu and major structures is a general feature at the Mount Isa deposit and occurs within the deposit (Fig. 2a), more pointedly within the low grade alteration halo (Fig. 2b) and at the scale of the entire deposit (Supplementary Information Fig. S-2). This suggests a genetic relationship between fluid pathways (major structures), the fractionation of copper isotopes and the precipitation of chalcopyrite ore. There is no clear relationship between the δ65Cu values and ore textures (Fig. 4b). That is, chalcopyrite grains analysed from disseminated, breccia, fault/shear or vein-hosted ore display a similar variation in the δ65Cu values.
Figure 4 The δ65Cu values for Mount Isa deposits relative to the distance from (a) major structures and (b) ore textures. (c) Comparison of δ65Cu values from sedimentary copper deposits (Sed Cu), volcanogenic massive sulfide deposits (VMS), porphyry and epithermal deposits (PCD/Epi), Michigan copper deposits and the Mount Isa deposit.
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Copper Isotopes as Indicators of Hydrothermal Fluid Pathways
The close spatial relationship (Fig. 2 and Fig. 4) between the low δ65Cu values and major structures suggests that copper isotopes can be used in some cases to identify ancient hydrothermal fluid pathways with important implications for the exploration and discovery of new deposits. Lighter δ65Cu values occur close to the Paroo Fault and to the steep crosscutting structures (the Bernborough and J46 Faults; Fig. 2) suggesting that these structures acted as conduits for the mineralising fluids. However, the Paroo Fault shows signs of reactivation only along the steeper sections and most likely was not an active fluid conduit during mineralisation.
The lighter δ65Cu values close to the Paroo Fault can be explained by fluid ingress along the fault, at intersections with the steeper structures, and the contact with the Urquhart Shale which constitutes a strong chemical contrast. The mafic rocks of the underlying ECV constitute the most likely metal source (e.g., Heinrich et al., 1995
Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
; Gregory et al., 2008Gregory, M.J., Keays, R.R., and Wilde, A.R. (2008) Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry. Australian Journal of Earth Sciences 55, 1153–1173. https://doi.org/10.1080/08120090802266618
) whereas the crosscutting steep structures such as the Bernsborough and J46 Faults represent the fluid pathways. Fluid inclusion studies, alteration and ore mineral chemistry all suggest that copper deposition at Mount Isa occurred at temperatures between 300 and 350 °C (e.g., Heinrich et al., 1989Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J. (1989) A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia. Economic Geology 84, 529–550. https://doi.org/10.2113/gsecongeo.84.3.529
; Cave et al., 2020Cave, B., Lilly, R., and Barovich, K. (2020) Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system. Ore Geology Reviews 124, 102647. https://doi.org/10.1016/j.oregeorev.2020.103647
). Experimental work indicates that under these conditions the δ65Cu equilibrium fractionation between chalcopyrite and dissolved Cu is −0.22 ± 0.16 ‰ (Syverson et al., 2021Syverson, D.D., Borrok, D.M., Niebuhr, S., and Seyfried Jr, W.E. (2021) Chalcopyrite-dissolved Cu isotope exchange at hydrothermal conditions: Experimental constraints at 350°C and 50 MPa. Geochimica et Cosmochimica Acta 298, 191–206. https://doi.org/10.1016/j.gca.2021.02.005
). Assuming that the ECV has an average δ65Cu similar to Proterozoic basalts (δ65Cu = +0.14 ‰; Liu et al., 2015Liu, S.-A., Huang, J., Liu, J., Wörner, G., Yang, W., Tang, Y.-J., Chen, Y., Tang, L., Zheng, J., and Li, S. (2015) Copper isotopic composition of the silicate Earth. Earth and Planetary Science Letters 427, 95–103. https://doi.org/10.1016/j.epsl.2015.06.061
) and that equilibrium fractionation between chalcopyrite and the hydrothermal fluid occurred, the expected δ65Cu in chalcopyrite precipitated close to the fluid path (i.e. steep structures) will be ∼0 ‰ whereas the δ65Cu of the remaining hydrothermal fluid will have values between 0.38−0.58 ‰ as calculated by Syverson et al. (2021)Syverson, D.D., Borrok, D.M., Niebuhr, S., and Seyfried Jr, W.E. (2021) Chalcopyrite-dissolved Cu isotope exchange at hydrothermal conditions: Experimental constraints at 350°C and 50 MPa. Geochimica et Cosmochimica Acta 298, 191–206. https://doi.org/10.1016/j.gca.2021.02.005
. The average δ65Cu = 0 ‰ of chalcopyrite grains collected within 100 metres (Fig. 4a) from the major structures is consistent with such calculations. Since the remaining hydrothermal fluid will be enriched in heavy isotopes, precipitation of chalcopyrite at increasing distance away from the major structures will record ever higher δ65Cu values. The δ65Cuaverage = 0.25 ‰ at 100−200 m and δ65Cuaverage = 0.35 ‰ at 200−300 m from major structures (Figs. 2, 4a) agree with this model and indicate that equilibrium fractionation and the distance to the fluid conduits can qualitatively account for the observed variation in copper isotopes at the Mount Isa deposit.The general increase in copper isotope values away from the fluid pathways mimics the patterns documented for hydrothermal systems associated with porphyry copper deposits (Gregory and Mathur, 2017
Gregory, M.J., and Mathur, R. (2017) Understanding Copper Isotope Behavior in the High Temperature Magmatic-Hydrothermal Porphyry Environment. Geochemistry, Geophysics, Geosystems 18, 4000–4015. https://doi.org/10.1002/2017GC007026
). The central parts of the Pebble deposit in Alaska, the Bingham deposit in Utah, and the Grasberg deposit in Papua, Indonesia, yield copper isotope values in chalcopyrite that are lower than the distal parts of these fossilised hydrothermal systems (Gregory and Mathur, 2017Gregory, M.J., and Mathur, R. (2017) Understanding Copper Isotope Behavior in the High Temperature Magmatic-Hydrothermal Porphyry Environment. Geochemistry, Geophysics, Geosystems 18, 4000–4015. https://doi.org/10.1002/2017GC007026
; Mathur et al., 2013Mathur, R., Munk, L., Nguyen, M., Gregory, M., Annell, H., and Lang, J. (2013) Modern and Paleofluid Pathways Revealed by Cu Isotope Compositions in Surface Waters and Ores of the Pebble Porphyry Cu-Au-Mo Deposit, Alaska. Economic Geology 108, 529–541. https://doi.org/10.2113/econgeo.108.3.529
). Similar patterns have been recognised in shallower epithermal systems in chalcopyrite and other ore minerals (Duan et al., 2016Duan, J., Tang, J., Li, Y., Liu, S.-A., Wang, Q., Yang, C., and Wang, Y. (2016) Copper isotopic signature of the Tiegelongnan high-sulfidation copper deposit, Tibet: implications for its origin and mineral exploration. Mineralium Deposita 51, 591–602. https://doi.org/10.1007/s00126-015-0624-x
; Wu et al., 2017Wu, L.-Y., Hu, R.-Z., Li, X.-F., Liu, S.-A., Tang, Y.-W., and Tang, Y.-Y. (2017) Copper isotopic compositions of the Zijinshan high-sulfidation epithermal Cu–Au deposit, South China: Implications for deposit origin. Ore Geology Reviews, 83, 191–199. https://doi.org/10.1016/j.oregeorev.2016.12.013
). In porphyry systems, the increase in δ65Cu values away from the core of the system is related to the temperature change as the hydrothermal fluid cools down by interacting with the host rocks (Gregory and Mathur, 2017Gregory, M.J., and Mathur, R. (2017) Understanding Copper Isotope Behavior in the High Temperature Magmatic-Hydrothermal Porphyry Environment. Geochemistry, Geophysics, Geosystems 18, 4000–4015. https://doi.org/10.1002/2017GC007026
). Fluid inclusions and alteration studies (Heinrich et al., 1995Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
) at the Mount Isa deposit indicate that there is no temperature gradient away from the fluid pathways and cooling can be discarded as a major cause for isotope fractionation. Thus, the most likely cause for the observed isotope fractionation pattern is fluid reaction with the host rock. Indeed, sulfur isotope studies indicate that the sulfur required to precipitate chalcopyrite was locally derived from diagenetic pyrite within the Urquhart Shales (Heinrich et al., 1995Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
). Despite resulting from different fractionation mechanisms, the common pattern of light copper isotopes close to fluid pathways and heavier isotopes away from fluid pathways observed in porphyry copper systems and the Mount Isa deposit indicate that this could be a characteristic feature of hydrothermal systems and can be used in exploration for mineral deposits.top
The Metal and Fluid Source at the Mount Isa Deposit
The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989
Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J. (1989) A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia. Economic Geology 84, 529–550. https://doi.org/10.2113/gsecongeo.84.3.529
; McGoldrick and Keays, 1990McGoldrick, P.J., Keays, R.R. (1990) Mount Isa copper and lead-zinc-silver ores; coincidence or cogenesis? Economic Geology 85, 641–650. https://doi.org/10.2113/gsecongeo.85.3.641
; Heinrich et al., 1995Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
; Wilde et al., 2006Wilde, A.R., Jones, P.A., Gessner, K., Ailleres, L., Gregory, M.J., Duncan, R.J. (2006) A geochemical process model for the Mount Isa copper orebodies. Economic Geology 101, 1547–1567. https://doi.org/10.2113/gsecongeo.101.8.1547
Gregory et al., 2008Gregory, M.J., Keays, R.R., and Wilde, A.R. (2008) Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry. Australian Journal of Earth Sciences 55, 1153–1173. https://doi.org/10.1080/08120090802266618
; Cave et al., 2020Cave, B., Lilly, R., and Barovich, K. (2020) Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system. Ore Geology Reviews 124, 102647. https://doi.org/10.1016/j.oregeorev.2020.103647
). The unimodal distribution and the low spread around the average δ65Cu values measured in this study have implications for understanding the metal and the fluid source. The mafic volcanics of the ECV are significantly depleted in copper with quantitative estimates indicating at least 70 % depletion in whole rock copper from K-altered zones (e.g., Heinrich et al., 1995Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
; Gregory, 2006Gregory, M.J. (2006) Copper mobility in the Eastern Creek Volcanics, Mount Isa, Australia: evidence from laser ablation ICP-MS of iron-titanium oxides. Mineralium Deposita 41, 691–711. https://doi.org/10.1007/s00126-006-0086-2
). This suggests that copper was almost completely leached out from the sulfide and oxide phases and thus the average δ65Cu most likely represents fractionation about the mean and reflects the average value of the source rock. This assumption is reasonable considering the almost identical δ65Cu average (+0.13 ‰) to the average δ65Cu value (+0.14 ‰) for Proterozoic basalts (Liu et al., 2015Liu, S.-A., Huang, J., Liu, J., Wörner, G., Yang, W., Tang, Y.-J., Chen, Y., Tang, L., Zheng, J., and Li, S. (2015) Copper isotopic composition of the silicate Earth. Earth and Planetary Science Letters 427, 95–103. https://doi.org/10.1016/j.epsl.2015.06.061
) and the strong depletion in copper from the mafic volcanics surrounding the deposit. Moreover, as discussed above, the only fractionation of the copper isotopes at the scale of the deposits appears to be related to the fluid pathways and distance from major structures and is consistent with equilibrium fractionation from an evolving fluid having an average δ65Cu similar to the overall deposit and typical for Proterozoic basalts. Figure 4c shows the ranges and mean δ65Cu values for sedimentary copper deposits (chalcopyrite), volcanogenic massive sulfide deposits (chalcopyrite), porphyry copper (chalcopyrite and bornite) and epithermal deposits (chalcopyrite), Mount Isa (chalcopyrite) and the native copper deposits from Michigan.Two important observations can be made. First, there is a narrow range in δ65Cu for Mount Isa in comparison to all the other deposit types but the Michigan deposits. Second, the average δ65Cu compositions for volcanogenic massive sulfide, porphyry copper/epithermal, Michigan and Mount Isa deposits are almost identical. Although the Mount Isa deposit is sediment-hosted, its δ65Cu is clearly different from sedimentary copper deposits. The sediment copper deposits form at moderate temperature compared to the other deposit types, require an oxidised source rock leached by highly oxidised basinal brines, and precipitation by mixing with a reduced fluid (e.g., Hitzman et al., 2010
Hitzman, M.W., Selley, D., and Bull, S. (2010) Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits through Earth History. Economic Geology 105, 627–639. https://doi.org/10.2113/gsecongeo.105.3.627
).Copper isotopes are fractionated during oxidative alteration of clastic sediments (Mathur et al., 2005
Mathur, R., Ruiz, J., Titley, S., Liermann, L., Buss, H., Brantley, S. (2005) Cu isotopic fractionation in the supergene environment with and without bacteria. Geochim Cosmochim Acta 69, 5233–5246. https://doi.org/10.1016/j.gca.2005.06.022
; Mounyer et al., 2017), and most likely the copper isotope signature of oxidised red beds is variable and departs from the average value of igneous rocks, whereas the negative fractionation seen in sedimentary copper deposits can likely be attributed to the reduction of Cu to form Cu sulfides by mixing with a reduced fluid (e.g., Asael et al., 2007Asael, D., Matthews, A., Bar-Matthews, M., and Halicz, L. (2007) Copper isotope fractionation in sedimentary copper mineralization (Timna Valley, Israel). Chemical Geology 243, 238–254. https://doi.org/10.1016/j.chemgeo.2007.06.007
; Asael et al., 2012Asael, D., Matthews, A., Bar-Matthews, M., Harlavan, Y., and Segal, I. (2012) Tracking redox controls and sources of sedimentary mineralization using copper and lead isotopes. Chemical Geology 310–311, 23–35. https://doi.org/10.1016/j.chemgeo.2012.03.021
). The clear difference in the range and mean δ65Cu signatures between sedimentary copper deposits and the Mount Isa deposit indicate that the ore forming processes (source, mobilisation and deposition) for the Mount Isa deposit are distinct from those responsible for the formation of sedimentary copper deposits, and the syn-sedimentary copper model (e.g., McGoldrick and Keays, 1990McGoldrick, P.J., Keays, R.R. (1990) Mount Isa copper and lead-zinc-silver ores; coincidence or cogenesis? Economic Geology 85, 641–650. https://doi.org/10.2113/gsecongeo.85.3.641
; Wilde et al., 2006Wilde, A.R., Jones, P.A., Gessner, K., Ailleres, L., Gregory, M.J., Duncan, R.J. (2006) A geochemical process model for the Mount Isa copper orebodies. Economic Geology 101, 1547–1567. https://doi.org/10.2113/gsecongeo.101.8.1547
) can be discarded.The nearly identical average δ65Cu for the Mount Isa, VMS, porphyry/epithermal and Michigan deposits overlaps with the average δ65Cu for igneous rocks (BSE = +0.07 ± 0.10 ‰; Moynier et al., 2017
Moynier, F., Vance, D., Fujii, T., and Savage, P. (2017) The isotope geochemistry of zinc and copper. Reviews in Mineralogy and Geochemistry 82, 543–600. https://doi.org/10.2138/rmg.2017.82.13
) suggesting an igneous source for copper either by leaching of igneous rocks (VMS, Michigan and Mount Isa deposits) or by separation from magma (porphyry/epithermal deposits). The wide range of δ65Cu observed in VMS deposits can be explained by mixing of hot hydrothermal fluids and cold seawater during the precipitation of copper sulfides, by hydrothermal re-working of copper sulfides and by late stage low temperature processes (Rouxel et al., 2004Rouxel, O., Fouquet, Y., and Ludden J.N. (2004) Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev Sea-Floor Hydrothermal Fields on the Mid-Atlantic Ridge. Economic Geology 99, 585–600. https://doi.org/10.2113/gsecongeo.99.3.585
). In porphyry/epithermal deposits the large variation in δ65Cu has been explained by two main processes, cooling and boiling with metal transport in a vapour phase causing significant copper and other transition metal isotope fractionation (Ikehata et al., 2011Ikehata, K., Notsu, K., Hirata, T., Navarrete, J.U., Borrok, D.M., Viveros, M., and Ellzey, J.T. (2011) Copper isotope characteristics of copper-rich minerals from Besshi-type volcanogenic massive sulfide deposits, Japan, determined using a femtosecond LA-MC-ICM-MS. Economic Geology 106, 307–316. https://doi.org/10.2113/econgeo.106.2.307
; Maher et al., 2011Maher, K.C., Jackson, S., and Mountain, B. (2011) Experimental evaluation of the fluid–mineral fractionation of Cu isotopes at 250°C and 300°C. Chemical Geology 286, 229–239. https://doi.org/10.1016/j.chemgeo.2011.05.008
).In the Mount Isa deposit there is no evidence that any of the processes responsible for the wide range in δ65Cu values in VMS and porphyry/epithermal deposits occurred. The δ65Cu values from Mount Isa deposits are almost identical to those reported from the Michigan native copper deposits suggesting a similar mineralisation process. For both deposit types, the copper was sourced from underlying mafic rocks by oxidised brines and transported upward along structures to the deposition site (e.g., Bornhorst and Mathur, 2017
Bornhorst, T.J., and Mathur, R. (2017) Copper isotope constraints on the genesis of the Keweenaw Peninsula native copper district, Michigan, USA. Minerals 7, 185. https://doi.org/10.3390/min7100185
). The almost identical average δ65Cu from Mount Isa (δ65Cu = 0.13 ‰) and the δ65Cu measured from Proterozoic basalts (δ65Cu = 0.14 ‰; Liu et al., 2015Liu, S.-A., Huang, J., Liu, J., Wörner, G., Yang, W., Tang, Y.-J., Chen, Y., Tang, L., Zheng, J., and Li, S. (2015) Copper isotopic composition of the silicate Earth. Earth and Planetary Science Letters 427, 95–103. https://doi.org/10.1016/j.epsl.2015.06.061
) indicate that the copper was most likely sourced from the underlying ECV (Gregory et al., 2008Gregory, M.J., Keays, R.R., and Wilde, A.R. (2008) Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry. Australian Journal of Earth Sciences 55, 1153–1173. https://doi.org/10.1080/08120090802266618
). The source of the fluid that mobilised and transported copper at Mount Isa is unlikely to be basinal or metamorphic, since the deposition of copper postdates the basin formation and metamorphism by at least 130 Myr and 60 Myr respectively (e.g., Cave et al., 2020Cave, B., Lilly, R., and Barovich, K. (2020) Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system. Ore Geology Reviews 124, 102647. https://doi.org/10.1016/j.oregeorev.2020.103647
). CO2-rich fluid inclusions are subordinate, pre-date mineralisation and probably reflect peak metamorphism whereas syn-mineralisation fluid inclusions are NaCl-rich aqueous fluids with O and H isotope values overlapping the magmatic field (Heinrich et al., 1989Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J. (1989) A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia. Economic Geology 84, 529–550. https://doi.org/10.2113/gsecongeo.84.3.529
). Although, the evidence for magmatism at the time of mineralisation in the WFB is restricted to pegmatitic intrusions there is synchronous voluminous magmatism in the EFB and a magmatic source for the mineralising fluid at the Mount Isa deposit cannot be completely discarded.top
Acknowledgements
The authors would like to acknowledge funding from Mount Isa Mines (MIM) Limited, Anglo-American Exploration (Australia) Pty Limited and the Geological Survey of Queensland.
Editor: Horst R. Marschall
top
References
Asael, D., Matthews, A., Bar-Matthews, M., and Halicz, L. (2007) Copper isotope fractionation in sedimentary copper mineralization (Timna Valley, Israel). Chemical Geology 243, 238–254. https://doi.org/10.1016/j.chemgeo.2007.06.007
Show in context
Copper isotopes are fractionated during oxidative alteration of clastic sediments (Mathur et al., 2005; Mounyer et al., 2017), and most likely the copper isotope signature of oxidised red beds is variable and departs from the average value of igneous rocks, whereas the negative fractionation seen in sedimentary copper deposits can likely be attributed to the reduction of Cu to form Cu sulfides by mixing with a reduced fluid (e.g., Asael et al., 2007; Asael et al., 2012).
View in article
Asael, D., Matthews, A., Bar-Matthews, M., Harlavan, Y., and Segal, I. (2012) Tracking redox controls and sources of sedimentary mineralization using copper and lead isotopes. Chemical Geology 310–311, 23–35. https://doi.org/10.1016/j.chemgeo.2012.03.021
Show in context
Copper isotopes are fractionated during oxidative alteration of clastic sediments (Mathur et al., 2005; Mounyer et al., 2017), and most likely the copper isotope signature of oxidised red beds is variable and departs from the average value of igneous rocks, whereas the negative fractionation seen in sedimentary copper deposits can likely be attributed to the reduction of Cu to form Cu sulfides by mixing with a reduced fluid (e.g., Asael et al., 2007; Asael et al., 2012).
View in article
Abu Sharib, A.S.A.A., and Sanislav, I.V. (2013) Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: Example from the Eastern Fold Belt, Mount Isa Inlier, Australia. Tectonophysics 587, 146–167. https://doi.org/10.1016/j.tecto.2012.06.051
Show in context
Infill of the Calvert Superbasin was interrupted by an inversion event between 1690 and 1668 Ma (Southgate et al., 2000), and sediment fill of the Isan Superbasin was interrupted by the ∼1650−1490 Ma Isan Orogeny (Foster and Austin, 2008; Abu Sharib and Sanislav, 2013).
View in article
Bornhorst, T.J., and Mathur, R. (2017) Copper isotope constraints on the genesis of the Keweenaw Peninsula native copper district, Michigan, USA. Minerals 7, 185. https://doi.org/10.3390/min7100185
Show in context
For both deposit types, the copper was sourced from underlying mafic rocks by oxidised brines and transported upward along structures to the deposition site (e.g., Bornhorst and Mathur, 2017).
View in article
Cave, B., Lilly, R., and Barovich, K. (2020) Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system. Ore Geology Reviews 124, 102647. https://doi.org/10.1016/j.oregeorev.2020.103647
Show in context
Copper mineralisation occurs within the same stratigraphic horizon, the Urquhart Shales, as Zn-Pb mineralisation (Fig. 1), and the deposit is commonly described in the literature as a zoned Zn-Pb-Cu system (e.g., Cave et al., 2020) with copper mineralisation occurring at depth and Zn-Pb mineralisation occurring nearer the surface.
View in article
Chalcopyrite is the dominant copper-bearing mineral; it is texturally late relative to the Zn-Pb-Ag mineralisation (Fig. 3) and postdates peak deformation and metamorphism (e.g., Cave et al., 2020; Mahan et al., 2023).
View in article
Fluid inclusion studies, alteration and ore mineral chemistry all suggest that copper deposition at Mount Isa occurred at temperatures between 300 and 350 °C (e.g., Heinrich et al., 1989; Cave et al., 2020).
View in article
The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989; McGoldrick and Keays, 1990; Heinrich et al., 1995; Wilde et al., 2006 Gregory et al., 2008; Cave et al., 2020).
View in article
The source of the fluid that mobilised and transported copper at Mount Isa is unlikely to be basinal or metamorphic, since the deposition of copper postdates the basin formation and metamorphism by at least 130 Myr and 60 Myr respectively (e.g., Cave et al., 2020).
View in article
Duan, J., Tang, J., Li, Y., Liu, S.-A., Wang, Q., Yang, C., and Wang, Y. (2016) Copper isotopic signature of the Tiegelongnan high-sulfidation copper deposit, Tibet: implications for its origin and mineral exploration. Mineralium Deposita 51, 591–602. https://doi.org/10.1007/s00126-015-0624-x
Show in context
Similar patterns have been recognised in shallower epithermal systems in chalcopyrite and other ore minerals (Duan et al., 2016; Wu et al., 2017).
View in article
Foster, D.R.W., and Austin, J.R. (2008) The 1800–1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes. Precambrian Research 163, 7–30. https://doi.org/10.1016/j.precamres.2007.08.010
Show in context
These basinal sediments were deposited on top of basement rocks that had been deformed and metamorphosed during the Barramundi Orogeny (∼1890−1850 Ma; Foster and Austin, 2008).
View in article
Infill of the Calvert Superbasin was interrupted by an inversion event between 1690 and 1668 Ma (Southgate et al., 2000), and sediment fill of the Isan Superbasin was interrupted by the ∼1650−1490 Ma Isan Orogeny (Foster and Austin, 2008; Abu Sharib and Sanislav, 2013).
View in article
Gibson, G.M., Meixner, A.J., Withnall, I.W., Korsch, R.J., Hutton, L.J., Jones, L.E.A., Holzschuh, J., Costelloe, R.D., Henson, P.A., and Saygin, E. (2016) Basin architecture and evolution in the Mount Isa mineral province, northern Australia: Constraints from deep seismic reflection profiling and implications for ore genesis. Ore Geology Reviews 76, 414–441. https://doi.org/10.1016/j.oregeorev.2015.07.013
Show in context
The geology of the inlier is dominated by sedimentary and volcanic sequences that were deposited in three major supra-crustal basins: 1) the 1790−1740 Ma Leichhardt Superbasin, 2) the 1730−1640 Ma Calvert Superbasin, and 3) the 1640−1580 Ma Isa Superbasin (Gibson et al., 2016).
View in article
Gregory, M.J. (2006) Copper mobility in the Eastern Creek Volcanics, Mount Isa, Australia: evidence from laser ablation ICP-MS of iron-titanium oxides. Mineralium Deposita 41, 691–711. https://doi.org/10.1007/s00126-006-0086-2
Show in context
The mafic volcanics of the ECV are significantly depleted in copper with quantitative estimates indicating at least 70 % depletion in whole rock copper from K-altered zones (e.g., Heinrich et al., 1995; Gregory, 2006).
View in article
Gregory, M.J., and Mathur, R. (2017) Understanding Copper Isotope Behavior in the High Temperature Magmatic-Hydrothermal Porphyry Environment. Geochemistry, Geophysics, Geosystems 18, 4000–4015. https://doi.org/10.1002/2017GC007026
Show in context
The general increase in copper isotope values away from the fluid pathways mimics the patterns documented for hydrothermal systems associated with porphyry copper deposits (Gregory and Mathur, 2017).
View in article
The central parts of the Pebble deposit in Alaska, the Bingham deposit in Utah, and the Grasberg deposit in Papua, Indonesia, yield copper isotope values in chalcopyrite that are lower than the distal parts of these fossilised hydrothermal systems (Gregory and Mathur, 2017; Mathur et al., 2013).
View in article
In porphyry systems, the increase in δ65Cu values away from the core of the system is related to the temperature change as the hydrothermal fluid cools down by interacting with the host rocks (Gregory and Mathur, 2017).
View in article
Gregory, M.J., Keays, R.R., and Wilde, A.R. (2008) Magmatic history of the Eastern Creek Volcanics, Mt Isa, Australia: insights from trace-element and platinum-group-element geochemistry. Australian Journal of Earth Sciences 55, 1153–1173. https://doi.org/10.1080/08120090802266618
Show in context
The second model proposes that copper was sourced from basement mafic rocks by oxidised basinal or metamorphic brines, transported along fractures and deposited in the overlying Urquhart Shales either by cooling, pH change or an increase in sulfur fugacity (e.g., Heinrich et al., 1989; Heinrich et al., 1995; Gregory et al., 2008).
View in article
The Paroo Fault is crosscut by a series of faults (the Bernborough and J46 Faults; Fig. 2) that have been interpreted as conduits for the fluid that leached copper out of the ECV and deposited it in the Urquhart Shales (e.g., Heinrich et al., 1989; Heinrich et al., 1995; Gregory et al., 2008).
View in article
The mafic rocks of the underlying ECV constitute the most likely metal source (e.g., Heinrich et al., 1995; Gregory et al., 2008) whereas the crosscutting steep structures such as the Bernsborough and J46 Faults represent the fluid pathways.
View in article
The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989; McGoldrick and Keays, 1990; Heinrich et al., 1995; Wilde et al., 2006 Gregory et al., 2008; Cave et al., 2020).
View in article
The almost identical average δ65Cu from Mount Isa (δ65Cu = 0.13 ‰) and the δ65Cu measured from Proterozoic basalts (δ65Cu = 0.14 ‰; Liu et al., 2015) indicate that the copper was most likely sourced from the underlying ECV (Gregory et al., 2008).
View in article
Heinrich, C.A., Andrew, A.S., Wilkins, R.W.T., and Patterson, D.J. (1989) A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia. Economic Geology 84, 529–550. https://doi.org/10.2113/gsecongeo.84.3.529
Show in context
The second model proposes that copper was sourced from basement mafic rocks by oxidised basinal or metamorphic brines, transported along fractures and deposited in the overlying Urquhart Shales either by cooling, pH change or an increase in sulfur fugacity (e.g., Heinrich et al., 1989; Heinrich et al., 1995; Gregory et al., 2008).
View in article
The Paroo Fault is crosscut by a series of faults (the Bernborough and J46 Faults; Fig. 2) that have been interpreted as conduits for the fluid that leached copper out of the ECV and deposited it in the Urquhart Shales (e.g., Heinrich et al., 1989; Heinrich et al., 1995; Gregory et al., 2008).
View in article
Fluid inclusion studies, alteration and ore mineral chemistry all suggest that copper deposition at Mount Isa occurred at temperatures between 300 and 350 °C (e.g., Heinrich et al., 1989; Cave et al., 2020).
View in article
The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989; McGoldrick and Keays, 1990; Heinrich et al., 1995; Wilde et al., 2006 Gregory et al., 2008; Cave et al., 2020).
View in article
CO2-rich fluid inclusions are subordinate, pre-date mineralisation and probably reflect peak metamorphism whereas syn-mineralisation fluid inclusions are NaCl-rich aqueous fluids with O and H isotope values overlapping the magmatic field (Heinrich et al., 1989).
View in article
Heinrich, C., Bain, J.H.C., Mernagh, T.P., Wyborn, L.A.I., Andrew, A.S., and Waring, C.L. (1995) Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia. Economic Geology 90, 705–730. https://doi.org/10.2113/gsecongeo.90.4.705
Show in context
The second model proposes that copper was sourced from basement mafic rocks by oxidised basinal or metamorphic brines, transported along fractures and deposited in the overlying Urquhart Shales either by cooling, pH change or an increase in sulfur fugacity (e.g., Heinrich et al., 1989; Heinrich et al., 1995; Gregory et al., 2008).
View in article
The Paroo Fault is crosscut by a series of faults (the Bernborough and J46 Faults; Fig. 2) that have been interpreted as conduits for the fluid that leached copper out of the ECV and deposited it in the Urquhart Shales (e.g., Heinrich et al., 1989; Heinrich et al., 1995; Gregory et al., 2008).
View in article
The mafic rocks of the underlying ECV constitute the most likely metal source (e.g., Heinrich et al., 1995; Gregory et al., 2008) whereas the crosscutting steep structures such as the Bernsborough and J46 Faults represent the fluid pathways.
View in article
Fluid inclusions and alteration studies (Heinrich et al., 1995) at the Mount Isa deposit indicate that there is no temperature gradient away from the fluid pathways and cooling can be discarded as a major cause for isotope fractionation.
View in article
Indeed, sulfur isotope studies indicate that the sulfur required to precipitate chalcopyrite was locally derived from diagenetic pyrite within the Urquhart Shales (Heinrich et al., 1995).
View in article
The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989; McGoldrick and Keays, 1990; Heinrich et al., 1995; Wilde et al., 2006 Gregory et al., 2008; Cave et al., 2020).
View in article
The mafic volcanics of the ECV are significantly depleted in copper with quantitative estimates indicating at least 70 % depletion in whole rock copper from K-altered zones (e.g., Heinrich et al., 1995; Gregory, 2006).
View in article
Hitzman, M.W., Selley, D., and Bull, S. (2010) Formation of Sedimentary Rock-Hosted Stratiform Copper Deposits through Earth History. Economic Geology 105, 627–639. https://doi.org/10.2113/gsecongeo.105.3.627
Show in context
The sediment copper deposits form at moderate temperature compared to the other deposit types, require an oxidised source rock leached by highly oxidised basinal brines, and precipitation by mixing with a reduced fluid (e.g., Hitzman et al., 2010).
View in article
Ikehata, K., Notsu, K., Hirata, T., Navarrete, J.U., Borrok, D.M., Viveros, M., and Ellzey, J.T. (2011) Copper isotope characteristics of copper-rich minerals from Besshi-type volcanogenic massive sulfide deposits, Japan, determined using a femtosecond LA-MC-ICM-MS. Economic Geology 106, 307–316. https://doi.org/10.2113/econgeo.106.2.307
Show in context
In porphyry/epithermal deposits the large variation in δ65Cu has been explained by two main processes, cooling and boiling with metal transport in a vapour phase causing significant copper and other transition metal isotope fractionation (Ikehata et al., 2011; Maher et al., 2011).
View in article
Liu, S.-A., Huang, J., Liu, J., Wörner, G., Yang, W., Tang, Y.-J., Chen, Y., Tang, L., Zheng, J., and Li, S. (2015) Copper isotopic composition of the silicate Earth. Earth and Planetary Science Letters 427, 95–103. https://doi.org/10.1016/j.epsl.2015.06.061
Show in context
Assuming that the ECV has an average δ65Cu similar to Proterozoic basalts (δ65Cu = +0.14 ‰; Liu et al., 2015) and that equilibrium fractionation between chalcopyrite and the hydrothermal fluid occurred, the expected δ65Cu in chalcopyrite precipitated close to the fluid path (i.e. steep structures) will be ∼0 ‰ whereas the δ65Cu of the remaining hydrothermal fluid will have values between 0.38−0.58 ‰ as calculated by Syverson et al. (2021).
View in article
This assumption is reasonable considering the almost identical δ65Cu average (+0.13 ‰) to the average δ65Cu value (+0.14 ‰) for Proterozoic basalts (Liu et al., 2015) and the strong depletion in copper from the mafic volcanics surrounding the deposit.
View in article
The almost identical average δ65Cu from Mount Isa (δ65Cu = 0.13 ‰) and the δ65Cu measured from Proterozoic basalts (δ65Cu = 0.14 ‰; Liu et al., 2015) indicate that the copper was most likely sourced from the underlying ECV (Gregory et al., 2008).
View in article
Mahan, B., Mathur, R., Sanislav, I., Rea, P., Dirks, P. (2023) Cu isotopes in groundwater for hydrogeochemical mineral exploration: A case study using the world-class Mount Isa Cu–Pb–Zn deposit (Australia). Applied Geochemistry, 148, 105519. https://doi.org/10.1016/j.apgeochem.2022.105519
Show in context
Chalcopyrite is the dominant copper-bearing mineral; it is texturally late relative to the Zn-Pb-Ag mineralisation (Fig. 3) and postdates peak deformation and metamorphism (e.g., Cave et al., 2020; Mahan et al., 2023).
View in article
Maher, K.C., Jackson, S., and Mountain, B. (2011) Experimental evaluation of the fluid–mineral fractionation of Cu isotopes at 250°C and 300°C. Chemical Geology 286, 229–239. https://doi.org/10.1016/j.chemgeo.2011.05.008
Show in context
In porphyry/epithermal deposits the large variation in δ65Cu has been explained by two main processes, cooling and boiling with metal transport in a vapour phase causing significant copper and other transition metal isotope fractionation (Ikehata et al., 2011; Maher et al., 2011).
View in article
Mathur, R., Ruiz, J., Titley, S., Liermann, L., Buss, H., Brantley, S. (2005) Cu isotopic fractionation in the supergene environment with and without bacteria. Geochim Cosmochim Acta 69, 5233–5246. https://doi.org/10.1016/j.gca.2005.06.022
Show in context
Copper isotopes are fractionated during oxidative alteration of clastic sediments (Mathur et al., 2005; Mounyer et al., 2017), and most likely the copper isotope signature of oxidised red beds is variable and departs from the average value of igneous rocks, whereas the negative fractionation seen in sedimentary copper deposits can likely be attributed to the reduction of Cu to form Cu sulfides by mixing with a reduced fluid (e.g., Asael et al., 2007; Asael et al., 2012).
View in article
Mathur, R., Munk, L., Nguyen, M., Gregory, M., Annell, H., and Lang, J. (2013) Modern and Paleofluid Pathways Revealed by Cu Isotope Compositions in Surface Waters and Ores of the Pebble Porphyry Cu-Au-Mo Deposit, Alaska. Economic Geology 108, 529–541. https://doi.org/10.2113/econgeo.108.3.529
Show in context
The central parts of the Pebble deposit in Alaska, the Bingham deposit in Utah, and the Grasberg deposit in Papua, Indonesia, yield copper isotope values in chalcopyrite that are lower than the distal parts of these fossilised hydrothermal systems (Gregory and Mathur, 2017; Mathur et al., 2013).
View in article
McGoldrick, P.J., Keays, R.R. (1990) Mount Isa copper and lead-zinc-silver ores; coincidence or cogenesis? Economic Geology 85, 641–650. https://doi.org/10.2113/gsecongeo.85.3.641
Show in context
The first model proposes that copper was sourced within the basin from ferruginous red beds by oxidised basinal brines and transported along permeable mud and silt horizons to the pyritic Urquhart Shales where chalcopyrite deposition occurred due to reduction reactions (e.g., McGoldrick and Keays, 1990; Wilde et al., 2006).
View in article
The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989; McGoldrick and Keays, 1990; Heinrich et al., 1995; Wilde et al., 2006 Gregory et al., 2008; Cave et al., 2020).
View in article
The clear difference in the range and mean δ65Cu signatures between sedimentary copper deposits and the Mount Isa deposit indicate that the ore forming processes (source, mobilisation and deposition) for the Mount Isa deposit are distinct from those responsible for the formation of sedimentary copper deposits, and the syn-sedimentary copper model (e.g., McGoldrick and Keays, 1990; Wilde et al., 2006) can be discarded.
View in article
Moynier, F., Vance, D., Fujii, T., and Savage, P. (2017) The isotope geochemistry of zinc and copper. Reviews in Mineralogy and Geochemistry 82, 543–600. https://doi.org/10.2138/rmg.2017.82.13
Show in context
The nearly identical average δ65Cu for the Mount Isa, VMS, porphyry/epithermal and Michigan deposits overlaps with the average δ65Cu for igneous rocks (BSE = +0.07 ± 0.10 ‰; Moynier et al., 2017) suggesting an igneous source for copper either by leaching of igneous rocks (VMS, Michigan and Mount Isa deposits) or by separation from magma (porphyry/epithermal deposits).
View in article
Mudd, G.M. (2021) Assessing the Availability of Global Metals and Minerals for the Sustainable Century: From Aluminium to Zirconium. Sustainability 13, 10855. https://doi.org/10.3390/su131910855
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The giant Mount Isa copper (with Zn-Pb) deposit, located in NW Queensland (Fig. 1), is a unique deposit type and the second largest copper producer in Australia after Olympic Dam in South Australia (e.g., Mudd, 2021).
View in article
Rouxel, O., Fouquet, Y., and Ludden J.N. (2004) Copper isotope systematics of the Lucky Strike, Rainbow, and Logatchev Sea-Floor Hydrothermal Fields on the Mid-Atlantic Ridge. Economic Geology 99, 585–600. https://doi.org/10.2113/gsecongeo.99.3.585
Show in context
The wide range of δ65Cu observed in VMS deposits can be explained by mixing of hot hydrothermal fluids and cold seawater during the precipitation of copper sulfides, by hydrothermal re-working of copper sulfides and by late stage low temperature processes (Rouxel et al., 2004).
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Southgate, P.N., Bradshaw, B.E., Domagala, J., Jackson, M.J., Idnurm, M., Krassay, A.A., Page, R.W., Sami, T.T., Scott, D.L., Lindsay, J.F., McConachie, B.A., Tarlowski, C. (2000) Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730–1575 Ma) in northern Australia and implications for base-metal mineralisation. Australian Journal of Earth Sciences 47, 461–483. https://doi.org/10.1046/j.1440-0952.2000.00787.x
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Infill of the Calvert Superbasin was interrupted by an inversion event between 1690 and 1668 Ma (Southgate et al., 2000), and sediment fill of the Isan Superbasin was interrupted by the ∼1650−1490 Ma Isan Orogeny (Foster and Austin, 2008; Abu Sharib and Sanislav, 2013).
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The Mount Isa deposit is hosted by the Mount Isa Group (Calvert Superbasin) which consists of the Surprise Creek Formation at its base, followed by the Warrina Park Quartzite, Moondara Siltstone, Breakaway Shale, Native Bee Siltstone, Urquhart Shale, Spear Siltstone, Kennedy Siltstone, and the Magazine Shale at its stratigraphic top (Southgate et al., 2000).
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Spence, J., Sanislav, I.V., and Dirks, P.H.G.M. (2021) 1750-1710 Ma deformation along the eastern margin of the North Australia Craton. Precambrian Research 353, 106019. https://doi.org/10.1016/j.precamres.2020.106019
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Infill of the Leichhardt Superbasin was interrupted by the ∼1750−1710 Ma Wonga Orogeny (Spence et al., 2021; Spence et al., 2022).
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Spence, J., Sanislav, I.V., and Dirks, P.H.G.M. (2022) Evidence for a 1750–1710 Ma orogenic event, the Wonga Orogeny, in the Mount Isa Inlier, Australia: Implications for the tectonic evolution of the North Australian Craton and Nuna Supercontinent. Precambrian Research, 369, 106510. https://doi.org/10.1016/j.precamres.2021.106510
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Infill of the Leichhardt Superbasin was interrupted by the ∼1750−1710 Ma Wonga Orogeny (Spence et al., 2021; Spence et al., 2022).
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Syverson, D.D., Borrok, D.M., Niebuhr, S., and Seyfried Jr, W.E. (2021) Chalcopyrite-dissolved Cu isotope exchange at hydrothermal conditions: Experimental constraints at 350°C and 50 MPa. Geochimica et Cosmochimica Acta 298, 191–206. https://doi.org/10.1016/j.gca.2021.02.005
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Experimental work indicates that under these conditions the δ65Cu equilibrium fractionation between chalcopyrite and dissolved Cu is −0.22 ± 0.16 ‰ (Syverson et al., 2021).
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Assuming that the ECV has an average δ65Cu similar to Proterozoic basalts (δ65Cu = +0.14 ‰; Liu et al., 2015) and that equilibrium fractionation between chalcopyrite and the hydrothermal fluid occurred, the expected δ65Cu in chalcopyrite precipitated close to the fluid path (i.e. steep structures) will be ∼0 ‰ whereas the δ65Cu of the remaining hydrothermal fluid will have values between 0.38−0.58 ‰ as calculated by Syverson et al. (2021).
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Wilde, A.R., Jones, P.A., Gessner, K., Ailleres, L., Gregory, M.J., Duncan, R.J. (2006) A geochemical process model for the Mount Isa copper orebodies. Economic Geology 101, 1547–1567. https://doi.org/10.2113/gsecongeo.101.8.1547
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The first model proposes that copper was sourced within the basin from ferruginous red beds by oxidised basinal brines and transported along permeable mud and silt horizons to the pyritic Urquhart Shales where chalcopyrite deposition occurred due to reduction reactions (e.g., McGoldrick and Keays, 1990; Wilde et al., 2006).
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The main differences between the genetic models for Mount Isa deposit relate to the metal source (ferruginous red beds vs. basement mafic rocks), timing (syngenetic vs. epigenetic), and fluid source (basinal brines vs. metamorphic brines) (e.g., Heinrich et al., 1989; McGoldrick and Keays, 1990; Heinrich et al., 1995; Wilde et al., 2006 Gregory et al., 2008; Cave et al., 2020).
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The clear difference in the range and mean δ65Cu signatures between sedimentary copper deposits and the Mount Isa deposit indicate that the ore forming processes (source, mobilisation and deposition) for the Mount Isa deposit are distinct from those responsible for the formation of sedimentary copper deposits, and the syn-sedimentary copper model (e.g., McGoldrick and Keays, 1990; Wilde et al., 2006) can be discarded.
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Wu, L.-Y., Hu, R.-Z., Li, X.-F., Liu, S.-A., Tang, Y.-W., and Tang, Y.-Y. (2017) Copper isotopic compositions of the Zijinshan high-sulfidation epithermal Cu–Au deposit, South China: Implications for deposit origin. Ore Geology Reviews, 83, 191–199. https://doi.org/10.1016/j.oregeorev.2016.12.013
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Similar patterns have been recognised in shallower epithermal systems in chalcopyrite and other ore minerals (Duan et al., 2016; Wu et al., 2017).
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Supplementary Information
The Supplementary Information includes:
- Methodology
- Table S-1
- Figures S-1 to S-4
- Supplementary Information References
Download the Supplementary Information (PDF)
Figures
Figure 1 Simplified geological map showing the surface projection of the Cu and Zn-Pb ore bodies. The inset shows the location of the Mount Isa deposits in Queensland and within the Mount Isa Inlier. A and B indicate the location of sections shown in Figure 2. KLB-Kalkadoon Leichhardt Belt, WFB-Western Fold Belt, EFB-Eastern Fold Belt.
Figure 2 Cross sections through (a) the middle of the deposit and (b) across the low grade envelope showing the location of samples collected for isotope measurements and the δ65Cu values for each sample. Red dots – negative values; blue diamonds – positive values.
Figure 3 Photographs showing typical chalcopyrite (ccp) textures and dolomite (dol) alteration from Mount Isa deposit.
Figure 4 The δ65Cu values for Mount Isa deposits relative to the distance from (a) major structures and (b) ore textures. (c) Comparison of δ65Cu values from sedimentary copper deposits (Sed Cu), volcanogenic massive sulfide deposits (VMS), porphyry and epithermal deposits (PCD/Epi), Michigan copper deposits and the Mount Isa deposit.