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Very rapid subsurface hydrothermal ore deposition mechanisms and the origin of breccia-hosted iron-oxide copper–gold deposits
Journal of Geochemical Exploration 101 (2009) 76
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Very rapid subsurface hydrothermal ore deposition mechanisms and the origin of breccia-hosted iron-oxide copper–gold deposits Nicholas H.S. Oliver a,⁎, Michael J. Rubenach a, Jaiby-Ann Jacob a, Brian G. Rusk a, Martina Bertelli a, James S. Cleverley a,b, Thomas G. Blenkinsop a, David R. Cooke c, Timothy Baker a, Damien Jungman d, Bruce W.D. Yardley e, Timothy Laneyrie a a
Economic Geology Research Unit, James Cook University, Townsville, Qld, 4811, Australia CSIRO Exploration and Mining, ARRC PO Box 1130 Bentley Western Australia 6102, Australia c CODES, University of Tasmania, Private Bag 126, Hobart, Tasmania, 7001, Australia d Xstrata Copper Exploration Pty Ltd, Locked Mail Bag 100 Mount Isa QLD 4825, Australia e School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK b
Ore precipitation at seafloor black smokers or during deposition of volcanogenic breccias is widely considered to be a result of quick changes in solubility of metals and sulphur due to phase changes associated with very steep gradients in temperature, pressure and chemistry. The association of breccias with iron-oxide–copper–gold deposits is common, but most of these breccias are not considered to be surface features. However, because some IOCGs are not brecciahosted, the mechanisms proposed for the mid- to upper-crustal precipitation of ores have been regarded as ‘conventional’ in the sense that fluid transport in permeable shear zones or pipes, facilitated by ingress of both magmatic fluids and/or surface brines, is a common theme. No surprises, but little model clarity. In the Proterozoic Cloncurry District of the Mount Isa Block, we propose that a process of explosive fluid release from the top of crystallizing gabbro–granite complexes resulted in formation of IOCG deposits over a very short time period, during the transport and release phase of fluidized, diatreme-like pipes. Voluminous fluid release (“first boiling”) and rapid phase changes (“second boiling”) from supercritical (salt–water–CO2) to liquid + CO2-dominant gas have direct parallels in the shallower, water-dominant porphyry-epithermal environment; the difference here is that these processes initiated
⁎ Corresponding author. Tel.: +61 7 47815047; fax: +61 747814020. E-mail address: [email protected] (N.H.S. Oliver). 0375-6742/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.069
at around 10 km depth. We relate these directly to the genesis of the breccia-hosted Ernest Henry IOCG deposit (~200 Mt at 1% Cu and 0.5 g/t Au), and by comparison, to Olympic Dam and other large to giant breccia-hosted IOCGs. The ore host breccias have physical attributes consistent with fluidization. Discordant cm- to m-scale magnetite-chalcopyrite seams with internal clast grading cut the main orebody, which itself shows milled, corroded and transported clasts of wallrock with ore as infill. CO2 fluid inclusion densities recorded in the regional and ore breccias reveal syn-breccia pressure fluctuations from 400 to b150 MPa. The combination of these observations suggests that ore precipitation was not a long-lived process dependent on the permeability of pre-ore breccias. Rather, we infer that ore deposition occurred over very short geological time scales due to very rapid emplacement of fluidized, buoyant, CO2- and sulfur-bearing breccia pipes into structural traps containing metal-rich fluids and rocks. The energy and buoyancy of these fluids was derived ultimately from the mingling of mafic and felsic magma, liberating CO2 into pooled granite-derived brines, which triggered rapid pressure buildup and consequent explosive breaching of contact metamorphic aureoles and fault-related structural traps. This may be a defining model for this particular type of (large to giant) IOCGs.
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Very rapid subsurface hydrothermal ore deposition mechanisms and the origin of breccia-hosted iron-oxide copper–gold deposits Nicholas H.S. Oliver a,⁎, Michael J. Rubenach a, Jaiby-Ann Jacob a, Brian G. Rusk a, Martina Bertelli a, James S. Cleverley a,b, Thomas G. Blenkinsop a, David R. Cooke c, Timothy Baker a, Damien Jungman d, Bruce W.D. Yardley e, Timothy Laneyrie a a
Economic Geology Research Unit, James Cook University, Townsville, Qld, 4811, Australia CSIRO Exploration and Mining, ARRC PO Box 1130 Bentley Western Australia 6102, Australia c CODES, University of Tasmania, Private Bag 126, Hobart, Tasmania, 7001, Australia d Xstrata Copper Exploration Pty Ltd, Locked Mail Bag 100 Mount Isa QLD 4825, Australia e School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK b
Ore precipitation at seafloor black smokers or during deposition of volcanogenic breccias is widely considered to be a result of quick changes in solubility of metals and sulphur due to phase changes associated with very steep gradients in temperature, pressure and chemistry. The association of breccias with iron-oxide–copper–gold deposits is common, but most of these breccias are not considered to be surface features. However, because some IOCGs are not brecciahosted, the mechanisms proposed for the mid- to upper-crustal precipitation of ores have been regarded as ‘conventional’ in the sense that fluid transport in permeable shear zones or pipes, facilitated by ingress of both magmatic fluids and/or surface brines, is a common theme. No surprises, but little model clarity. In the Proterozoic Cloncurry District of the Mount Isa Block, we propose that a process of explosive fluid release from the top of crystallizing gabbro–granite complexes resulted in formation of IOCG deposits over a very short time period, during the transport and release phase of fluidized, diatreme-like pipes. Voluminous fluid release (“first boiling”) and rapid phase changes (“second boiling”) from supercritical (salt–water–CO2) to liquid + CO2-dominant gas have direct parallels in the shallower, water-dominant porphyry-epithermal environment; the difference here is that these processes initiated
⁎ Corresponding author. Tel.: +61 7 47815047; fax: +61 747814020. E-mail address: [email protected] (N.H.S. Oliver). 0375-6742/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.069
at around 10 km depth. We relate these directly to the genesis of the breccia-hosted Ernest Henry IOCG deposit (~200 Mt at 1% Cu and 0.5 g/t Au), and by comparison, to Olympic Dam and other large to giant breccia-hosted IOCGs. The ore host breccias have physical attributes consistent with fluidization. Discordant cm- to m-scale magnetite-chalcopyrite seams with internal clast grading cut the main orebody, which itself shows milled, corroded and transported clasts of wallrock with ore as infill. CO2 fluid inclusion densities recorded in the regional and ore breccias reveal syn-breccia pressure fluctuations from 400 to b150 MPa. The combination of these observations suggests that ore precipitation was not a long-lived process dependent on the permeability of pre-ore breccias. Rather, we infer that ore deposition occurred over very short geological time scales due to very rapid emplacement of fluidized, buoyant, CO2- and sulfur-bearing breccia pipes into structural traps containing metal-rich fluids and rocks. The energy and buoyancy of these fluids was derived ultimately from the mingling of mafic and felsic magma, liberating CO2 into pooled granite-derived brines, which triggered rapid pressure buildup and consequent explosive breaching of contact metamorphic aureoles and fault-related structural traps. This may be a defining model for this particular type of (large to giant) IOCGs.