Isan Orogeny Research Articles

Genesis of the Mount Colin Cu-Au deposit, NW Queensland: Evidence from geology, multi-mineral U–Pb geochronology, S-, O- and C-isotope constraints

The Mount Colin Cu-Au deposit is located in the Mary Kathleen Domain of the Mount Isa Inlier, NW Queensland. The deposit is hosted in the Corella Formation, which transitions into the Burstall Granite at a depth of ~400 m. The host rocks of the deposit are overprinted by Na-Ca alteration and a Ca-Mg-Fe skarn assemblage. Na-Ca alteration yields a titanite 207Pb/206Pb weighted mean age of 1717 ± 8 Ma (MSWD = 0.32) and is composed of albite, actinolite, zircon, titanite and scapolite with retrograde calcite, prehnite and sericite. The Ca-Mg-Fe skarn yields a titanite 207Pb/206Pb weighted mean age of 1712 ± 7 Ma (MSWD = 1.2) and is composed of hastingsite, K-feldspar, clinopyroxene, albite, titanite, calcite, biotite, and zircon with retrograde quartz and chlorite. Based on temporal relationships, Na-Ca alteration and the Ca-Mg-Fe skarn is interpreted to have formed from a hydrothermal magmatic fluid during late-stage magmatism of the (1740–1710 Ma) Wonga-Burstall plutons. A second generation of titanite (denoted titanite II) is present within Na-Ca alteration and the Ca-Mg-Fe skarn assemblage. Titanite II yields a 207Pb/206Pb weighted mean age of 1576 ± 7 Ma (MSWD = 0.37), and is interpreted to have formed from an external metamorphic hydrothermal fluid during widespread deformation associated with the (1610–1500 Ma) Isan Orogeny. The Mount Colin Cu-Au deposit consists of a predominantly coarse-grained, mineralogically zoned assemblage that infills along the NW-striking Mount Colin Fault. The centre of the deposit comprises a largely unmineralized core composed of calcite and/or quartz ± apatite ± microcline. Cu-Au mineralization is positioned along the outer margins of the deposit and consists of biotite, actinolite, chalcopyrite and pyrrhotite with trace abundances of magnetite, pyrite, galena, melonite and arsenopyrite. Apatite from the core of the deposit yields a U–Pb age of 1523 ± 16 Ma (MSWD = 1.2). The δ34SCDT values of chalcopyrite and pyrrhotite range from 0.1–2.1 ‰ and 0.9–1.5 ‰ respectively. The δ13CVPDB values of calcite range from −5.54 ‰ to −4.54 ‰ with corresponding δ18OVSMOW values ranging from 8.16–11.26 ‰. Based on the temporal and isotopic constraints presented in this study, a hydrothermal magmatic origin for Cu-Au mineralization is favoured. The timing of Cu-Au mineralization is coeval with a regional-scale metamorphic/hydrothermal event throughout the Mary Kathleen Domain and widespread magmatism throughout the Eastern Fold Belt of the Mount Isa Inlier.

The Dugald River-type, shear zone hosted, Zn-Pb-Ag mineralisation, Mount Isa Inlier, Australia

The Dugald River Zn-Pb-Ag mine is situated in the Mount Isa Inlier, a globally significant base metal province. Zn-Pb deposits in the Mount Isa Inlier are stratabound with four main genetic models, including SEDEX-style, remobilised SEDEX, epigenetic and Broken Hill-type mineralisation applied to interpret their formation. We propose that the Zn-Pb-Ag mineralisation at Dugald River represents a unique, shear zone hosted deposit type that formed through a series of successive deformation events during the Paleoproterozoic Isan Orogeny that concentrated the mineralisation within the Dugald River Shear Zone during two main mineralising phases. The first phase of mineralisation occurred during regional D2 shortening, which is associated with the formation of large-scale F2 folds and a regionally penetrative S2 fabric. During this phase, progressive tightening of upright F2 folds resulted in several sets of secondary space accommodating quartz-carbonate veins that were progressively rotated into parallelism with the pervasive, steep, W-dipping S2 cleavage. The quartz-carbonate veins were coevally replaced by sulphides, which migrated to extensional sites (boudin necks and fold hinges) in tight folds. Thereby creating a sulphide-rich horizon within a developing high strain zone, which during D4 developed into the Dugald River Shear Zone. The second phase of mineralisation occurred during the regional D4 transpressional deformation event and resulted in significant metal enrichment and the current geometry of the ore bodies. The significant enrichment of the mineralisation during D4 resulted from further fold tightening within the high strain zone, which resulted in the attenuation and dismembering of folds and produced a transposed fabric (S4). The sulphide veins were transposed into parallelism with S4 forming sulphide-rich planar ore textures. Strain partitioning at the contact between the ductile deforming sulphide horizon and the brittle deforming slates resulted in the development of an anastomosing shear zone, known as the Dugald River Shear Zone. A right-handed releasing bend in the shear zone produced a dilational jog and a thick, high-grade ore body. The mobilisation of sulphides within the dilational jog involved fragmentation of sulphides and wall rock, brecciation, rotation and rolling of fragments, and the formation of durchbewegung texture.

New age constraints for the Tommy Creek Domain of the Mount Isa Inlier, Australia

The Tommy Creek Domain is a complex, yet little studied, terrane in the Eastern Subprovince of the Mount Isa Province, northwest Queensland Australia. In this study, we take advantage of modern low-cost and rapid geochronology techniques to undertake an iterative dating approach integrated with detailed fieldwork to define the ages and extents of numerous lithologies and units of the Tommy Creek Domain. This includes some units not previously identified, lithologies previously grouped together based on field observations but now shown to have multiple distinct ages and dates not commonly represented in Mount Isan time–space plots. We identify an episode of felsic magmatism at ca 1640 Ma, and multimodal intrusions (ca 1615 Ma) immediately preceding the onset of the Isan Orogeny. A major rock package of the Tommy Creek Domain, the Milo beds, are characterised here as the youngest pre-Isan Orogeny sedimentary unit in the Eastern Subprovince (1660–1620 Ma), confirming that sedimentation and possibly rifting continued after deposition of the Soldiers Cap, Mount Albert and Kuridala groups (ca 1690–1650 Ma) before the onset of the Isan Orogeny (ca 1600 Ma). The Milo beds are thus age equivalent to the Mount Isa and McNamara groups of the Western Succession. There is evidence of a compositional shift in sedimentation coincident with the ca 1640 Ma Riversleigh Inversion event, previously only observed in the Western Subprovince in the Lawn Hill Platform. The application of geochronology as part of the mapping workflow can assist with differentiating geological units in terranes where field evidence is ambiguous and can aid in the focusing of objectives for field campaigns to enable the best possible interpretations to be made. KEY POINTS New ages constrain the Milo beds in the Tommy Creek Domain as the youngest stratigraphy in the Eastern Subprovince of the Mount Isa Province. The Milo beds are age equivalents of the McNamara and Mount Isa groups of the Western Subprovince of the inlier. Recognition of felsic and mafic intrusions with ca 1640 Ma and ca 1615 Ma ages. Evidence for Riversleigh Inversion event ca 1640 in the Eastern Subprovince.

A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes

The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa. Although previous workers have noted the existence of a dolerite dyke at the Hilton Zn-Pb-Ag deposit (Valenta, 1994b; Valenta, 1994a), a dolerite dyke at the George Fisher Zn-Pb-Ag deposit has not been previously documented. Progressive underground expansion at the George Fisher mine has resulted in the discovery of a dolerite dyke in the southernmost portion of the deposit. The temporal and petrological relationship between the dolerite dykes and the adjacent Zn-Pb-Ag mineralisation at the George Fisher and Hilton Zn-Pb-Ag deposits has not been considered by previous studies, and has significant implications for the currently accepted syn-diagenetic metallogenic model. In assay data, the dolerite dykes correspond with a large spike in TiO2 and V values. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 ± 21 Ma (MSWD = 0.93) and 1619 ± 22 Ma (MSWD = 0.53) for the dolerite dykes at the George Fisher and Hilton deposits, respectively. This age is consistent with dyke intrusion during the earliest phases of deformation during the (1620–1500 Ma) Isan Orogeny. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining, which have paragenetic equivalents in the adjacent Zn-Pb-Ag orebodies. The hydrothermal stages include: (1) Quartz ± albite ± K-feldspar alteration/veining; (2) Dolomite alteration/veining; (3) Biotite-chlorite-sulphide alteration; (4) Paragenetically late calcite, dolomite and/or sulphide veining. Monazite from a quartz ± albite ± K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 ± 16 Ma (MSWD = 1.1), constraining the maximum age of alteration within the dolerite dykes. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein. Calcite from the pre-mineralisation alteration assemblage produces a Lu-Hf age of 1501 ± 32 Ma (MSWD = 1.04), which is interpreted to constrain the maximum age of stratabound Zn-Pb-Ag mineralisation. Calcite from the late cross-cutting vein produced a Lu-Hf age of 1289 ± 26 Ma (MSWD = 1.00), which is interpreted to constrain the age of post-mineralisation faulting throughout the deposit. The evidence presented in this study indicates that the dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralisation.

Linking uplift and mineralisation at the Mount Novit Zn-Pb-Ag Deposit, Northern Australia: Evidence from geology, U–Pb geochronology and sphalerite geochemistry

The subeconomic Mount Novit Zn-Pb-Ag deposit is located approximately 20 km south of Mount Isa, Queensland. In contrast to the nearby Mount Isa, Hilton and George Fisher Zn-Pb-Ag deposits, mineralisation at Mount Novit is situated to the west of the regional-scale Mount Isa Fault and is hosted in the Moondarra Siltstone as opposed to the Urquhart Shale. Lower-grade (<4 wt.% Zn + Pb) Zn-Pb-Ag mineralisation primarily replaces pre-existing carbonate alteration and veining and consists of pyrrhotite, pyrite and sphalerite with lesser galena. Higher-grade (>10 wt.% Zn + Pb) mineralisation occurs as a matrix supported breccia dominated by sphalerite and pyrrhotite with galena, pyrite, and magnetite. In-situ U–Pb geochronology was completed on apatite and two textural varieties of monazite. Fine-grained (<50 µm) subhedral to anhedral monazite is located within highly foliated biotite alteration directly adjacent Zn-Pb-Ag mineralisation and yields a mean weighted 207Pb/206Pb age of 1527 ± 18 Ma (MSWD = 1.06). This age is consistent with the formation of highly foliated biotite alteration during D3 deformation of the Isan Orogeny. Apatite from the same fabric yields a lower intercept age of 1443 ± 29 Ma (MSWD = 1.30). Consistent with previous studies, this age is interpreted to represent the age of a major thrusting event along the Mount Isa Fault that resulted in the cooling of the Mount Novit area below ∼375 °C. Coarse-grained monazite is coeval with Zn-Pb-Ag mineralisation and yields a mean weighted 207Pb/206Pb age of 1457 ± 11 Ma (MSWD = 0.28). Sphalerite from Mount Novit has low concentrations (<1 ppm) of Ge and Ga and a relatively high concentration of In (5 to >10 ppm), possibly reflecting the leaching of the metals from an underlying basement unit. The GGIMFis geothermometer (Frenzel et al., 2016) produced a mean formation temperature of 345 ± 52 °C. The timing and temperature of Zn-Pb-Ag mineralisation is consistent with the age and cooling temperature of apatite presented in this study. Based on these correlations, we suggest that Zn-Pb-Ag mineralisation at Mount Novit was emplaced during an episode of major thrusting along the Mount Isa Fault, with the precipitation of Zn-Pb-Ag mineralisation driven by the cooling of the Mount Novit area below ∼375 °C. A key implication of this study is a new model for synorogenic Zn-Pb-Ag mineralisation to the south of Mount Isa, which contrasts with the widely accepted regional-scale syngenetic metallogenic model.

Heterogeneous Exhumation of the Mount Isa Orogen in NE Australia After 1.6 Ga Nuna Assembly: New High‐Precision40Ar/39Ar Thermochronological Constraints

AbstractThe circa 1.60 Ga Isan Orogeny in NE Australia has been ascribed to the collision of Australia and Laurentia (North America), marking the final assembly of the Proterozoic supercontinent Nuna. However, details regarding the tectonic evolution of the orogen remain poorly constrained. To investigate the late orogenic to postorogenic thermal evolution and exhumation history,40Ar/39Ar dating on hornblende, muscovite, and biotite was conducted in the Mount Isa Inlier, NE Australia, where intense crustal imbrication occurred during the Proterozoic continental collision. Published thermochronological results were recalculated using the current decay constant. Petrological examination and calculation of sample‐specific40Ar/39Ar closure temperatures and cooling rates were used to reconstruct the pressure‐temperature evolution of individual structural domains. Diachronous cooling histories are revealed between western, central, and eastern belts through ~525–330°C, mainly between 1.53 and 1.48 Ga. Contrasting cooling across postmetamorphic fault zones records the reactivation of inherited normal (i.e., early basinal) and reverse (i.e., orogenic) faults. Estimated exhumation rates are generally low (&lt; ~0.5 mm yr−1), pointing to a modest local relief of &lt; ~1,000 m which is comparable to modern analogs, and suggest a “soft” collision with limited crust thickening. Exhumation shortly following orogenesis was contemporaneous with felsic magmatism (1.55–1.48 Ga) in the eastern belt. Magmatism transitioning from trondhjemitic to A‐type granitoids over this period suggests progressive heating of the orogen base, ascribed to lower crust delamination. Thus, thermochronological data reveal a regionally heterogeneous exhumation history controlled by orogenic collapse‐related extensional faulting following the final assembly of the supercontinent Nuna.

1750–1710 Ma deformation along the eastern margin of the North Australia Craton

In this contribution, we present field-based evidence that in the Mary Kathleen Domain from the Eastern Fold Belt of the Mount Isa Inlier the 1790–1750 Ma metasediments of the Leichhardt Superbasin were sheared and folded between 1750 and 1710 Ma. The Mary Kathleen Domain consists of a series of anastomosing high and low strain domains intruded syn- to late-tectonically by a series of 1740–1710 Ma felsic and mafic plutons and dykes. The timing relationships in the high strain domains are unclear due to mylonitisation and transposition but in the lower strain domains are well-preserved. The earliest deformation, D1, consists of a series of low angle truncation surfaces and shear zones that are folded around large scale N-S trending upright folds during D2. The D2 folds were refolded during D3 along NW to EW trending fold axes. An undeformed granitic pluton from the Mt Godkin area dated at ~1710 Ma that crosscuts the D2-D3 folds and the youngest detrital zircon population of ~1750 Ma constrains the timing of D1-D3 deformation between 1750 and 1710 Ma. Later, N-S (D4) and N-E (D5) trending brittle-ductile shears are interpreted to indicate a late Isan Orogeny overprint. The occurrence of contemporaneous folding in the Western Fold Belt suggests that between 1750 and 1710 Ma the eastern margin of the North Australia Craton was experiencing a significant deformation event that can be correlated with similarly aged deformation events from other parts of the North Australia Craton, from South Australia Craton and from East Antarctica. Following the assembly of most of the Nuna Supercontinent by 1.8 Ga, we postulate that the 1750–1710 Ma deformation and plutonism observed in Mount Isa Inlier is part of a series of worldwide orogenic events that contributed to the final configuration of the Nuna Supercontinent.

Geology, Apatite Geochronology, and Geochemistry of the Ernest Henry Inter-lens: Implications for a Re-Examined Deposit Model

The Ernest Henry Iron-Oxide-Copper-Gold deposit is the largest known Cu-Au deposit in the Eastern Succession of the Proterozoic Mount Isa Inlier, NW Queensland. Cu-Au mineralization is hosted in a K-feldspar altered breccia, bounded by two major pre-mineralization shear zones. Previous research suggests that Cu-Au mineralization and the ore-bearing breccia formed simultaneously through an eruption style explosive/implosive event, facilitated by the mixing of fluids at ~1530 Ma. However, the preservation of a highly deformed, weakly mineralized, pre-mineralization feature (termed the Inter-lens) within the orebody indicates that this model must be re-examined. The paragenesis of the Inter-lens is broadly consistent with previous studies on the deposit, and consists of albitization; an apatite-calcite-quartz-garnet assemblage; biotite-magnetite ± garnet alteration; K-feldspar ± hornblende alteration; Cu-Au mineralization and post-mineralization alteration and veining. Apatite from the paragenetically early apatite-calcite-quartz-garnet assemblage produce U–Pb ages of 1584 ± 22 Ma and 1587 ± 22 Ma, suggesting that the formation of apatite, and the maximum age of the Inter-lens is synchronous with D2 deformation of the Isan Orogeny and regional peak-metamorphic conditions. Apatite rare earth element-depletion trends display: (1) a depletion in rare earth elements evenly, corresponding with an enrichment in arsenic and (2) a selective light rare earth element depletion. Exposure to an acidic NaCl and/or CaCl2-rich sedimentary-derived fluid is responsible for the selective light rare earth element-depletion trend, while the exposure to a neutral to alkaline S, Na-, and/or Ca-rich magmatic fluid resulted in the depletion of rare earth elements in apatite evenly, while producing an enrichment in arsenic. We suggest the deposit experienced at least two hydrothermal events, with the first event related to peak-metamorphism (~1585 Ma) and a subsequent event related to the emplacement of the nearby (~1530 Ma) Williams–Naraku Batholiths. Brecciation resulted from competency contrasts between ductile metasedimentary rocks of the Inter-lens and surrounding shear zones against the brittle metavolcanic rocks that comprise the ore-bearing breccia, providing permeable pathways for the subsequent ore-bearing fluids.

The ca. 1740–1710 Ma Leichhardt Event: Inversion of a continental rift and revision of the tectonic evolution of the North Australian Craton

Interpretation and modelling of high resolution regional geophysical data of the central Leichhardt River Fault Trough in the Mount Isa Inlier are used to determine the timing of a major basin inversion event following the development of the ca.1780–1740Ma Leichhardt Superbasin. Inversion of the Leichhardt Superbasin formed the regional north-south trending Leichhardt Anticline during east-west shortening. The limbs of the anticline are overprinted by several east-west trending wedge-shaped, non-magnetic sub-basins filled with ca.1710Ma Calvert and Isa superbasin successions. These relationships suggest inversion of the Leichhardt Superbasin occurred between ca.1740 and 1710Ma. The event is also known to have affected the northern and eastern North Australian Craton. The scale of the inversion suggests it was a significant event that we have defined as the Leichhardt Event. This event requires a major tectonic driver to the east of the North Australian Craton, possibly the accretion of a micro-continental ribbon to the east of the Mount Isa Inlier. The results of this study have implication for paleogeographic reconstruction of the Australian continent during the formation of Nuna because eastern North Australian Craton faced an ocean at ca.1740–1720Ma. The results also challenge the significance and intensity of crustal shortening associated with the ca.1600–1500Ma Isan Orogeny throughout the western Mount Isa Inlier.

Changes in Relative Plate Motion during the Isan Orogeny (1670–1500 Ma) and Implications for Pre‐Rodinia Reconstructions

AbstractFoliation inflexion/intersection axes (FIAs) preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE–SW, N–S, E–W to NW–SE. This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny. Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined. Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE–SW shortening, 1645±7 Ma for N–S shortening, and 1591±10 Ma for that directed E‐W. Inclusion trail asymmetries indicate shear senses of top to the SW for NW–SE FIAs and dominantly top to the N for E–W FIAs, reflecting thrusting towards the SW and N. No evidence for tectonism related to early NE–SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier. Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE–SW and N–S‐directed thrusting as the ages of tectonism are similar. Overlapping dates, tectonic, metamorphic, and metallogenic similarities between eastern Australia (Mount Isa and Broken Hill terranes) and the southwest part of Laurentia imply a most probable connection between both continental masses. Putting Australia in such position with respect to North America during the Late‐Paleo‐to‐Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent.

Release of uranium from highly radiogenic zircon through metamictization: The source of orogenic uranium ores

Although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals, rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits. Here, we investigate the source of metals for a series of fault zone– and shear zone–hosted uranium occurrences in the Mount Isa inlier, Australia. As well as containing uranium, these deposits are enriched in Zr and rare earth elements (REEs), requiring that unusual fluids were responsible for addition of these typically immobile elements. During the Isan orogeny, highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion, which contains elevated U, Th, F, Zr, and REEs. Synchrotron X-ray fluorescence spectroscopy and high-resolution electron microprobe mineral mapping show that this radioactive element–rich characteristic caused zircon crystals to become highly metamict, allowing the elements therein to become mobile. Thus, when orogenic fluids pervasively infiltrated along shear zones ?100 m.y. after granite intrusion, their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite, providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons.

Structural architecture of the southern Mount Isa terrane in Queensland inferred from magnetic and gravity data

The basement rocks of the southern Mount Isa terrane are concealed under younger sedimentary units and its crustal architecture is understood using constrained regional potential field analysis. Prominent N-S- to NNW-trending geophysical anomalies extend for ∼250km south of the exposed Mount Isa Inlier and are abruptly terminated by the NE-trending Cork Fault. Palaeoproterozoic basinal successions and major Palaeo- to Mesoproterozoic structures recorded in the Mount Isa Inlier are interpreted to continue southward under the Palaeozoic cover. The intensely positive geophysical signature of the region is mostly attributed to shallowing of the Barramundi-aged basement and distribution of metamorphosed sedimentary and volcanic rocks deposited during the formation of the ca. 1790–1730Ma Leichhardt Superbasin. Regional low magnetic and low gravity responses may reflect basinal sequences deposited during the formation of the ca. 1725–1690Ma Calvert Superbasin and the ca. 1675–1595Ma Isa Superbasin. Short wavelength magnetic anomalies and co-located low intensity Bouguer gravity anomalies are interpreted to represent shallow and variously magnetized granitic intrusions.The geophysical signature of the southern Mount Isa terrane reflects the bipartite fault architecture of the region. In the Western Fold Belt, Palaeoproterozoic multiple basin formation was controlled by west-dipping faults in half graben setting, within the NNW-trending Leichhardt River Fault Trough. To the east of the Pilgrim Fault, thick Palaeoproterozoic sedimentary and volcanic successions were controlled by major NNW-trending, east-dipping normal faults.The Mesoproterozoic Isan Orogeny reactivated the existing extensional fault network. Deformation during the ca. 1595–1580Ma Early Isan Orogeny is not apparent in the southern Mount Isa terrane and may have been confined to the northern part of the terrane. The ca. 1570–1550Ma Middle Isan Orogeny produced inversion of major NNW-trending rift faults and resulted in regional scale culminations. The ca. 1550–1540Ma wrench tectonics is represented by the activation of NW- and NE-oriented strike-slip faults overprinting the prominent N-S- to NNW-oriented structures.The emplacement of elongated batholitic bodies may be coeval to the emplacement of the ca. 1540–1520Ma Williams Supersuite recorded in the Mount Isa Inlier and thus may represent its southern extension. The southern Mount Isa terrane also appears to have been affected by post-Isan tectonics that resulted in deposition and magmatic intrusions focused along the northern side of the NE-trending Cork Fault.

Basin architecture and evolution in the Mount Isa mineral province, northern Australia: Constraints from deep seismic reflection profiling and implications for ore genesis

Deep seismic reflection profiling confirms that the Paleo- to Mesoproterozoic Mount Isa mineral province comprises three vertically stacked and partially inverted sedimentary basins preserving a record of intracontinental rifting followed by passive margin formation. Passive margin conditions were established no later than 1655Ma before being interrupted by plate convergence, crustal shortening and basin-wide inversion at 1640Ma in both the 1730–1640Ma Calvert and 1790–1740Ma Leichhardt superbasins. Crustal extension and thinning resumed after 1640 Ma with formation of the 1635–1575Ma Isa Superbasin and continued up to ca. 1615Ma when extensional faulting ceased and a further episode of basin inversion commenced. The 1575Ma Century Pb–Zn ore-body is hosted by syn-inversion sediments deposited during the initial stages of the Isan Orogeny with basin inversion accommodated on east- or northeast-dipping reactivated intrabasinal extensional faults and footwall shortcut thrusts. These structures extend to considerable depths and served as fluid conduits during basin inversion, tapping thick syn-rift sequences of immature siliciclastic sediments floored by bimodal volcanic sequences from which the bulk of metals and mineralising fluids are thought to have been sourced. Basin inversion and fluid expulsion at this stage were entirely submarine consistent with a syn-sedimentary to early diagenetic origin for Pb–Zn mineralisation at, or close to, the seafloor. Farther east, a change from platform carbonates to deeper water continental slope deposits (Kuridala and Soldiers Cap groups) marks the position of the original shelf break along which the north–south-striking Selwyn-Mount Dore structural corridor developed. This corridor served as a locus for strain partitioning, fluid flow and iron oxide–copper–gold mineralisation during and subsequent to the onset of basin inversion and peak metamorphism in the Isan Orogeny at 1585Ma. An episode of post-orogenic strike-slip faulting and hydrothermal alteration associated with the subvertical Cloncurry Fault Zone overprints west- to southwest-dipping shear zones that extend beneath the Cannington Pb–Zn deposit and are antithetic to inverted extensional faults farther west in the same sub-basin. Successive episodes of basin inversion and mineralisation were driven by changes in the external stress field and related plate tectonic environment as evidenced by a corresponding match to bends in the polar wander path for northern Australia. An analogous passive margin setting has been described for Pb–Zn mineralisation in the Paleozoic Selwyn Basin of western Canada.

Assessing the origin of old apparent ages derived by Pb stepwise leaching of vein-hosted epidote from Mount Isa, northwest Queensland, Australia

Epidote metasomatism affected large areas of tholeiitic metabasalts of the ~1,780 Ma Eastern Creek Volcanics in the Western Fold Belt of the Proterozoic Mount Isa inlier. Hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant S2 fabrics which formed during the regional metamorphic peak at ~1,570 Ma associated with the Isan orogeny. Previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin (with an evaporitic component). Application of the Pb stepwise leaching technique to the epidote does not separate radiogenic Pb4+ and common Pb2+, generating little spread in 206Pb/204Pb (between 16.0 and 30.5). The causes for this relatively low range are twofold: There is little radiogenic Pb in the epidotes (the most radiogenic steps account for <1 % of Pb released) and both Pb2+ and uranogenic Pb4+ substitute into the same site in the epidote crystal lattice. Consequently, age regressions using the Pb stepwise leaching data give ages between 150 and 1,500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak. These old ages are attributed to chemical inheritance from the host metabasalts, via radiogenic Pb release by breakdown of phases such as zircon, monazite, titanomagnetite, and ilmenite during metamorphism. This idea is supported by trace element data and chrondrite-normalized rare earth element patterns that are similar to both the metabasalts and epidotes (except for a variable Eu anomaly in the latter). Relatively high fO2 during vein formation (Fe3+ dominates in the epidote crystal lattice) would allow the incorporation of Th4+ and exclusion of U6+ and would explain elevated Th/U ratios (up to 12) in epidote compared with the host metabasalts. Non-incorporation of U would explain the relatively low U/Pb ratios and non-radiogenic character of the epidote. This process may provide a source of metal for the small U deposits around Mount Isa and may also suggest a relationship between U mineralization and regional Cu mobilization during the Isan orogeny. Our work suggests that non-conventional geochronometers should be used only if additional geological information and geochemical data (e.g., mineral chemistry, trace elements) are available to evaluate any resulting age calculations.

Chemical and stable isotopic characteristics of syn-tectonic tourmaline from the Western fold belt, Mount Isa inlier, Queensland, Australia

Understanding hydrothermal fluid flow events related to orogenic events is important because they provide large-scale redistribution of metals which may become concentrated in ore deposits. In this study we determine the origin of tourmaline-bearing quartz veins and pegmatites in the Western fold belt of the Proterozoic Mount Isa inlier, a region which host numerous syn-orogenic Cu deposits. This paper uses tourmaline mineral chemistry and isotopic analyses (in-situ B and bulk O and H isotopes) to fingerprint the fluid reservoirs that contributed to the fluid budget during the Isan orogeny. All the tourmaline samples have schorl–dravite solid solution compositions. Quartz–tourmaline δ18O (11.0–12.6 and 7.7–9.4‰ V-SMOW, respectively) pairs yield equilibrium formation temperatures of 305 to 575°C with the higher temperatures related to pegmatitic tourmaline. Tourmaline δD values have a relatively narrow range from −68 to −56‰ V-SMOW. In most cases calculated δ18OFluid and δDFluid values are compatible with primary magmatic water with a small component of metamorphic/18O-enriched formation waters. However, there are large variations in tourmaline δ11B values which range from +10.7 to −17.0‰, with the majority of values between −10 and −15‰. Variations in δ11BTur in the pegmatites suggest that boron degassing generated relative 11B depletion during crystallization. Similar δ11BTur values in quartz–tourmaline veins spatially associated with the pegmatites, in cordierite- and sillimanite-bearing metamorphic rocks west of the Mount Isa fault, demonstrate a similar origin for B in this tourmaline. In contrast, quartz–tourmaline veins of similar timing hosted by greenschist grade metamorphic rocks east of the Mount Isa fault have much higher δ11BTur values (+6.2 to +10.7‰) which reflect the involvement of a distinct B isotopic reservoir; mostly likely a near-surface derived evaporitic brine. Thus, the B isotopic data reveal important differences in fluid origin that are not reflected in the δ18O values. Therefore, at least two isotopically distinct fluids, one sourced from fractionating anatectic pegmatites and the other from external basinal brines, circulated contemporaneously in separate areas during the Isan orogeny. There is no evidence, based on the tourmaline isotopic data, to suggest that these fluids mixed, which demonstrates that the hydrothermal plumbing system in the Western fold belt during the Isan orogeny was disconnected, probably due to differences in thermal structure/metamorphic grade across the Western fold belt during orogenesis.

Switching bulk horizontal shortening directions and regional-scale partitioning of deformation during the Isan Orogeny in the Eastern Fold Belt, Mount Isa Inlier, Australia

Prolonged deformation for ca 150 Ma along the Eastern Fold Belt, Mount Isa Inlier, differentially partitioned into three distinct Mesoproterozoic tectonic domains. NW–SE-trending structures dominate the northern domain, whereas E–W- and N–S-trending structures dominate the central and southern domains, respectively.Changing the direction of bulk horizontal shortening from NE–SW to N–S to E–W shifted the locus of maximum tectono-metamorphic effect. This accounts for the different generations of structures preserved in these three domains. Overprinting relationships and geochronological data reveal a component of deformation partitioning in time as well as space.Rheological contrasts in the Soldiers Cap Group between a thick interlayered pelitic, psammitic and volcanic units on the one hand, and ca 1686 Ma, competent mafic intrusives and genetically related metasomatic albitite bodies present in its lower part, on the other, enhanced strain localisation during the long-lived Isan Orogeny (ca 1670–1500 Ma).

Structural and Lithological Controls on Iron Oxide Copper-Gold Deposits of the Southern Selwyn-Mount Dore Corridor, Eastern Fold Belt, Queensland, Australia

The Eastern fold belt of the Proterozoic Mount Isa inlier has undergone numerous episodes of deformation, hydrothermal alteration, and iron oxide copper-gold mineralization. The structural and lithological controls on the location and geometry of five iron oxide copper-gold deposits (Starra, Mount Elliott, SWAN, Mount Dore, and Lady Ella) in the Selwyn-Mount Dore corridor were studied through geologic mapping, core logging, structural analysis, and examination of 3-D grade distribution. All the deposits in this area occur in close proximity to the Selwyn shear, a long-lived, regional structure which developed at the contact between carbonate-rich and siliciclastic-dominated rock packages. This fault and a series of subparallel structures have been the focus of multiple reactivation events and have controlled the distribution of alteration and mineralization, which occurred in this area at ~1595, 1570, 1530, 1515, 1500, and 1485 Ma. Ironstone-hosted mineralization at Starra developed in a preexisting zone of sodic and iron oxide alteration along the N-striking Starra shear. Kinematic indicators along the Starra shear reveal that it has been reused numerous times; early D2 thrusting, post-D2 sinistral shearing, and pre-D4 (syn-D3?) dextral brittle faulting were all associated with sodic, sodic-calcic, and iron oxide alteration. Either the post-D2 or pre-D4 strike-slip faulting events can explain the steeply plunging nature of the orebodies at Starra. However, the dextral pre-D4 event produces dilational structures in ironstones and is associated with regionally important sodic and iron oxide alteration along NE- to ENE-trending faults, which intersect the Starra shear close to the main orebodies; we consider this the more likely main mineralizing event at Starra. Both strike-slip faulting events correspond with a change from E-W–oriented compression during the main phase of the Isan orogeny to transpression that may have been driven by crustal overthickening during orogenesis. At Mount Elliott and SWAN, mineralization is associated with post-D4 dextral and reverse movement across a sinistral fault jog along the NNW-striking Mount Elliott fault during transpression. Mineralization, alteration, and veins at these deposits clearly overprint all ductile structural features. Overall, the orebody geometry is controlled by the orientation of the Mount Elliott fault, but high-grade ore shoots are controlled by breccia formation in elongate pods which are parallel to preexisting F2 fold axes. In contrast, gently plunging grade shells at the slightly younger Mount Dore and Lady Ella deposits formed in response to renewed E-W compression and reverse displacement along N- to NE-striking fault segments of the Mount Elliott fault and newly formed subparallel faults. Examination of the structural setting of Mount Dore and Lady Ella also demonstrates that strain partitioning between mechanically heterogeneous rock sequences (metasedimentary rocks, metabasalts, granite, and strongly silicified rock types) exerted strong controls on the location of later mineralized systems. The important episodes of iron oxide copper-gold mineralization in the Eastern fold belt between 1595 and 1485 Ma appear to be associated temporally with compressive or transpressive structural regimes. These events developed in response to far-field tectonic stresses that evolved in response to the collision of Laurentia with the proto-Australian continent during the Isan orogeny. Late in the orogenic history, the emplacement of voluminous A-type batholiths associated with hot-spot plume activity likely generated localized transient shortening events, which are also associated with iron oxide copper-gold mineralization. This unique geodynamic evolution was responsible for the large-scale metal enrichment and remobilization into suitable chemical and structural traps over significant time periods in the Eastern fold belt iron oxide copper-gold district.

Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: Example from the Eastern Fold Belt, Mount Isa Inlier, Australia

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group, the southeastern corner of Mount Isa Inlier, record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (~1650–1500Ma). Low-P/high-T metamorphism (M1) prevailed during a period of NE–SW bulk horizontal shortening as indicated by the early growth of cordierite porphyroblasts, which entrap monazite grains that gave an average age of 1649±12Ma. This was followed by medium-P/high-T (M2) and high-P/high-T (M3) metamorphisms that accompanied periods of N–S and W–E bulk horizontal shortening, respectively. Growth of first generation garnet, andalusite and staurolite porphyroblasts having an average age of 1645±7Ma identify the former, whereas growth of the 1591±10Ma second generation garnet, staurolite and andalusite porphyroblasts together with fewer kyanite porphyroblasts characterizes the latter.The sediments of the Soldiers Cap Group were deposited in a tectonic setting that has the characteristics of an intra-continental rift basin. The upper limit of the age of sedimentation is constrained by detrital zircons at 1654±4Ma suggesting that tectonism and metamorphism were either active during the final stage of the basin filling or immediate after deposition.Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was the major source of heat.

Paleoproterozoic–earliest Mesoproterozoic basin evolution in the Mount Isa region, northern Australia and implications for reconstructions of the Nuna and Rodinia supercontinents

Paleoproterozoic-earliest Mesoproterozoic sequences in the Mount Isa region of northern Australia preserve a 200 Myr record (1800-1600 Ma) of intracontinental rifting, culminating in crustal thinning, elevated heat flow and establishment of a North American Basin and Range-style crustal architecture in which basin evolution was linked at depth to bimodal magmatism, high temperature-low pressure metamorphism and the formation of extensional shear zones. Rifting initiated in crystalline basement ≥1840 Ma old and produced three stacked sedimentary basins (1800-1750 Ma Leichhardt, 1730-1670 Ma Calvert and 1670-1575 Ma Isa superbasins) separated by major unconformities and in which depositional conditions progressively changed from fluviatilelacustrine to fully marine. By 1685 Ma, a deep marine, turbidite-dominated basin existed in the E and basaltic magmas had evolved in composition from continental to oceanic tholeiites as the crust became increasingly thinned and attenuated. Except for an episode of minor deformation and basin inversion at c. 1640 Ma, sedimentation continued across the region until onset of the Isan Orogeny at 1600 Ma. A near-identical record of crustal thinning and basaltic magmatism accompanied basin formation (lower Willyama Supergroup) in the formerly contiguous Broken Hill region from 1730-1670 Ma. This was followed by further extension and a second phase of basin development that lasted until at least 1640 Ma. Modern-day rifted continental margins preserve a comparable record of crustal thinning and near-continuous basin formation over 100-200 Myr timescales, supporting suggestions that the late Paleoproterozoic-early Mesoproterozoic rift basins of Mount Isa and Broken Hill similarly evolved to continental breakup and formed part of a continental margin sequence no later than 1640 Ma and possibly as early as 1670 Ma. This rifted margin predates assembly and breakup of the Neoproterozoic Rodinia supercontinent to which Australia once belonged and best accords with a pre-Rodinia, SWEAT-like super-continent (Nuna) that matches the E-facing late Paleoproterozoic-early Mesoproterozoic rift sequences of eastern Australia against rocks of comparable age in western Canada. Reconstructions of Rodinia (AUSWUS) based on the distribution of Grenville-age orogenic belts that coincidently position the continental rift sequences of Broken Hill along strike from more juvenile 1700-1650 Ma accreted terranes in the SW United States (Yavapai and Mazatzal provinces) are only possible if the proposed alignment of terranes is not original but an artefact of Neoproterozoic supercontinent assembly. The SWEAT hypothesis avoids this complication but, like AUSWUS, presupposes that eastern Australia and western Laurentia remained juxtaposed throughout the Mesoproterozoic until onset of Rodinia breakup after 830 Ma.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.