Soldiers Cap Group Research Articles

The discovery of the Artemis polymetallic deposit

The Artemis Cu-Au-Zn-Ag deposit is located approximately 43 km southeast of Cloncurry and 19 km west of Eloise in northwest Queensland. The deposit, hosted within the Paleoproterozoic Mount Norna Quartzite unit which is part of the Lower Soldiers Cap Group, is a steeply-dipping massive sulphide body. The Artemis deposit was discovered by Minotaur in July 2014, only 9 months after taking possession of the project. The deposit is a new type of mineralisation that has previously not been identified in the Cloncurry district. The discovery was made by meticulously piecing together the historical geophysical and geological data, complimented by new geophysical data sets including airborne EM and ground EM to improve the understanding of prospective targets throughout the area. The dominant sulphide within the deposit is pyrrhotite, hence the deposit is highly conductive. Airborne and ground EM techniques were therefore the main tools in resolving the mineralisation. The pyrrhotite is non-magnetic and therefore the deposit has no discernible magnetic signature and magnetics played no part in the discovery process. The deposit had no associated surface geochemical anomalism and very little alteration, making this deposit geologically difficult to find and essentially reliant on EM techniques in making the discovery.

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).

Depositional systems in the Mt Isa Inlier from 1800 Ma to 1640 Ma: Implications for Zn–Pb–Ag mineralisation

Two facies models are proposed to explain siliciclastic and carbonate depositional systems of 1800 Ma to 1640 Ma age in the Western Fold Belt of the Mt Isa Inlier. Both models record the response of depositional systems to storm-driven processes of sediment transport, dispersal and deposition on a shallow water shelf. The same suite of facies belts can also be identified in sedimentary successions of the Eastern Fold Belt. Slope driven processes of sediment transport and dispersal characterise turbidite and debrite deposits of the Soldiers Cap Group and Kuridala Formation and provide evidence for significantly greater water depths in this part of the basin from ca 1685 Ma. Through the recognition of unconformity surfaces, their correlative conformities, maximum flooding and ravinement surfaces, the facies belts are packaged into seven supersequences for the interval ca 1800–1640 Ma. The new correlations are shown in an event chart that correlates linked depositional systems across the entire Mt Isa Inlier. Thick successions of turbidite and debrite deposits are restricted to the eastern parts of the Mt Isa Inlier and do not occur in the Western Fold Belt. A major phase of extension and rifting commenced at ca 1740 Ma and by ca 1690 Ma led to significant crustal thinning and increased rates of accommodation over an area east of the Mount Dore Fault and Burke River Structural Belt. In the Mitakoodi Anticline and Kuridala-Selwyn Block, the rapid transition from shallow-water shelf depositional systems of the Prize Supersequence to significantly deeper-water slope environments of the Gun Supersequence coincided with the development of a platform margin, the deposition of turbidite and debrite deposits in deep water on the continental slope and the intrusion of mafic sills and dykes. Turbidite and debrite depositional systems of the Soldiers Cap Group and Kuridala Formations are restricted to a lowstand wedge of siliciclastic facies deposited basinward of a platform margin. Basin geometries and sediment architectures associated with this extensional event and recorded in the Gun Supersequence (ca 1685 Ma to 1650 Ma) provide an explanation for the geographic separation and fluid evolution pathways responsible for the Mt Isa Type and Broken Hill Type Zn–Pb–Ag deposits.

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.

Foliation intersection axes (FIAs) preserved within porphyroblasts: resolving plan view orthogonal refolding, Eastern Fold Belt, Mt Isa Inlier

The relative timing of the sequence of events is problematic where deformation partitioning strongly separates zones dominated by folds that formed in different periods of orogeny. This is particularly the case where they form orthogonal to one another in plan view. A succession of foliation inflection/intersection axes (FIAs) preserved within porphyroblasts readily resolves the sequence. In situ monazite age dating of porphyroblasts preserving W–E and N–S-trending FIAs yielded ages of 1645 ± 7 and 1591 ± 10 Ma, respectively. Peaks in the distribution of the two FIA sets coincide with the trends of the axial planes of the regional folds. The succession of FIAs, as well as foliations in the matrix in the Soldiers Cap Group of the eastern Mount Isa Inlier, supported with geochronological data reveal W–E directed bulk horizontal shortening followed N–S bulk horizontal shortening. This is despite the apparent regional lack of curvature of the W–E-trending folds around those trending N–S. Furthermore, N–S and W–E-directed bulk shortening resulted in at least 2 cycles of subvertical and subhorizontal foliation development that define the W–E and N–S-trending FIAs.

Hydrothermal mineral alteration patterns in the Mount Isa Inlier revealed by airborne hyperspectral data

High-resolution mineral maps derived from hyperspectral imaging (4.5 m pixel) enable the recognition of various types of hydrothermal alteration and the identification of fluid pathways. Airborne hyperspectral images from the Eastern Fold Belt of the Mount Isa Inlier were tested as a new tool for the detection of Fe-oxide Cu–Au (IOCG) related alteration. Four different types of hydrothermal alteration were identified with the hyperspectral mineral maps: (1) Metasomatic 1: white mica mineral maps show the spatial distribution of regional sodic–calcic alteration in metasedimentary successions of the Soldiers Cap Group in the Snake Creek Anticline. (2) Metasomatic 2: alteration zonation is evident from albitised granites assigned to the Williams–Naraku Suite along the Cloncurry Fault. These show characteristic absorption features in the shortwave infrared range (SWIR) which are depicted on the white mica mineral maps (white mica composition, white mica content, white mica crystallinity index). Alteration zonation in gabbros of the Cloncurry District was detected by a combination of MgOH and Fe2+ mineral maps (MgOH content, MgOH composition, amphibole/chlorite and Fe2+ and MgOH) combined with white mica mineral maps (white mica composition and white mica content). (3) Fluid channels 1: major fault zones, such as the Mt Dore fault zone in the Selwyn Corridor, are interpreted as important fluid pathways, where gradual changes in the mineral chemistry are highlighted with mineral maps (e.g. white mica content, white mica composition, white mica crystallinity index). (4) Fluid channels 2: MgOH and Fe2+ mineral maps were used to map breccia pipes in the northern Cloncurry District north of the Saxby Granite (Suicide Ridge). The MgOH and Fe2+ mineral maps were also used to distinguish various mafic rocks from amphibolites, which are host rocks for some of the IOCG deposits in the Eastern Fold Belt (e.g. Mount Elliott), and calcsilicate breccias pipes (e.g. Suicide Ridge).

A fluid inclusion study of the Suicide Ridge Breccia Pipe, Cloncurry district, Australia: Implication for Breccia Genesis and IOCG mineralization

The Suicide Ridge Barren Breccia Pipe (∼1527 ± 4 Ma) is one of numerous breccia occurrences that characterize the Cloncurry district, Australia. Interest in breccia systems in the Cloncurry district is related to their possible link with iron oxide copper–gold deposits. The Suicide Ridge Barren Breccia Pipe is temporally and spatially associated with a granite that outcrops at its southwestern boundary and which is ascribed to the Williams and Naraku magmatic event (ca. 1550–1500 Ma). The breccia is characterized by clasts of variable dimensions, degree of rounding and composition that includes dominant calc-silicate clasts of the Corella Formation that underlie rocks of the Soldiers Cap Group that host the pipe, indicating a dominant upward transport of the fragments. Other breccia fragments include granite bodies characterized by albite–quartz mineralogy and textures typical of the magmatic-hydrothermal transition. These rocks are interpreted to have formed in a fractionated granite carapace during the exsolution and accumulation of volatiles associated with the cooling magma. Two dominant types of inclusions have been identified at the Suicide Ridge Barren Breccia Pipe. These include early primary and pseudosecondary carbonic fluid inclusions, observed only in magmatic-hydrothermal transitional textures of granite breccia clasts, and secondary sodic (-calcic) brine inclusions that occur in all the collected samples. Variable homogenization temperatures in the primary and pseudosecondary pure CO 2 fluid inclusions indicate that they were trapped under different pressure conditions. Pressures estimated from the primary CO 2 fluid inclusions suggest that they were trapped at 4.2 kbar, at least 1 kbar above the inferred lithostatic pressure. This likely represents the overpressuring produced by release of CO 2-rich volatiles during the crystallization of magma and which ultimately caused the subsequent brecciation. Estimated pressures for pseudosecondary CO 2 fluid inclusions indicate pressure entrapment conditions around 2.7 kbar, similar to the lithostatic pressure and 1.5 kbar lower than pressures recorded by type Ia fluid inclusions. This significant drop of pressure likely represents the sudden pressure depression occurring in response to brecciation. Evidence from field relationships, breccia characteristics, mineralogy and textures of granite clasts within the pipe and fluid inclusion studies support an origin of the Suicide Ridge Barren Breccia Pipe in response to overpressuring generated by the release, expansion and possibly phase separation of fluids of dominant carbonic composition from a crystallizing magma beneath and adjacent to the pipe. Sodic (-calcic) fluid inclusions represent a later influx of fluids in the breccia system that produced widespread albitization. Significantly, high temperature, ultrasaline multisolid fluid inclusions abundant in IOCG deposits and considered to be samples of the mineralizing fluid were not observed at the Suicide Ridge Breccia Pipe. Their absence in the barren systems suggests that they were critical for the formation of IOCG deposits.

Defining unconformities in Proterozoic sedimentary basins using detrital geochronology and basin analysis—An example from the Mount Isa Inlier, Australia

Detrital zircons from sedimentary rocks below and above a major regional Palaeoproterozoic unconformity (the ∼1690–1670 Ma Gun unconformity of the Mount Isa Inlier) were dated using U–Pb SHRIMP techniques to test whether calculated maximum depositional ages could be used to identify this ∼20 my boundary, and determine which facies provide the best opportunity to identify this surface. We show that samples taken in a landward position directly above the regional unconformity surface from fluvial or incised valley-fill facies contained significant numbers of ∼1675 Ma ages, suggesting that these settings are the most useful for recording maximum depositional ages close to the actual age of deposition. Likewise, the large numbers of ∼1680 Ma ages in basinal turbidite deposits indicate that this facies may also be a good target for determining maximum depositional ages close to depositional ages. Integrating this new detrital zircon data with the magmatic record from the ∼1670 Ma Carters Bore Rhyolite, Sybella Granite and unnamed igneous horizons permits correlation between basin development and magmatism in the Mount Isa Inlier. Magmatic inflation associated with intrusion of the Sybella Granite led to uplift in the Western Succession associated with the Gun unconformity, with erosion and recycling of this hinterland leading to transportation of this material eastward into the deeper water parts of the basin represented by the Soldiers Cap Group.

The 1800–1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes

The chronostratigraphic and magmatic framework for the Proterozoic Mount Isa Inlier, northwest Queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years. This material and existing chronometric constraints is used to construct a new time–space plot for the Inlier and to compare Eastern Succession volcanosedimentary accumulations with temporally equivalent units in the Western Fold Belt. The accumulation of volcanosedimentary sequences in the Eastern Succession occurred in four main periods: namely 1890–1850 Ma, 1800–1750 Ma, 1710–1690 Ma, and 1680–1610 Ma. Temporally equivalent units can be identified in the Western Fold Belt. Periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the Inlier. The locus of bimodal volcanism migrates from west to east with time. Intracontinental rifting is the most likely tectonic setting for the sedimentary basins. The majority of the Soldiers Cap Group was deposited between ca. 1675 and 1650 Ma with the exception of units in the Boomarra Horst now known to belong to older sequences (deposited ca. 1775–1750 Ma), and the Llewellyn Creek Formation at Snake Creek which may be as old as ca. 1690 Ma. Upper amphibolite facies rock units in the southwest (e.g. the Gandry Dam Gneiss) are likely to be higher metamorphic grade, temporal equivalents of the Soldiers Cap Group. Five rock units previously assigned to the Cover Sequence 2 Mary Kathleen Group; namely, the Answer Slate, Kuridala Formation, Staveley Formation, Marimo Slate and Agate Downs Siltstone are now known to have been deposited between ca. 1675 and 1610 Ma. These units (bar the Kuridala Formation) have been removed from the Cover Sequence 2 Mary Kathleen Group and given the informal name ‘Young Australia Group’. The Kuridala Formation has been included in the Soldiers Cap Group. The rock units within the Young Australia Group are likely to be lateral facies equivalents of the younger units within the Soldiers Cap Group. Rock units within the Mount Albert Group (although undated) are considered likely to be temporal equivalents of the Soldiers Cap Group and the Young Australia Group. The Roxmere Quartzite is included here in the Young Australia Group that it tectonically overlays. The Ballara Quartzite and Mitakoodi Quartzite represent a single regionally extensive sequence deposited at ca. 1755 Ma. The Double Crossing Metamorphics (previously mapped as basement) are likely to correlate with the Argylla Formation and Mitakoodi Quartzite. Most of the Eastern Succession to the west of the Pilgrim Fault is likely to have been a topographic high between ca. 1780 and 1755 Ma, prior to the Ballara Quartzite unconformably overlying the ca. 1780 Ma Argylla Formation. Detrital zircons of ‘Argylla’ and ‘Wonga’ ages indicate that the Mary Kathleen Belt is likely to have been a topographic high at least periodically during accumulation of the Soldiers Cap and Young Australia Groups. Soldiers Cap Group units in the southeast contain numerous detrital zircons giving ages between ca. 1730 and 1710 indicating a possible siliciclastic source to the east. The host rocks to the Pb–Zn–Ag deposits are restricted to Cover Sequence 3 units deposited between ca. 1680 and 1590 Ma and, with the exception of Century, young from the southeast towards the northwest. Cu ± Au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation. However, many of the most significant deposits are found within the younger sequences.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Soldiers Cap Group iron‐formations, Mt Isa Inlier, Australia, as windows into the hydrothermal evolution of a base‐metal‐bearing Proterozoic rift basin

The Proterozoic Soldiers Cap Group, a product of two major magmatic rift phases separated by clastic sediment deposition, hosts mineralised (e.g. Pegmont Broken Hill‐type deposit) and barren iron oxide‐rich units at three main stratigraphic levels. Evaluation of detailed geological and geochemical features was carried out for one lens of an apatite–garnet‐rich, laterally extensive (1.9 km) example, the Weatherly Creek iron‐formation, and it was placed in the context of reconnaissance studies of other similar units in the area. Chemical similarities with iron‐formations associated with Broken Hill‐type Pb–Zn deposit iron‐formations are demonstrated here. Concordant contact relationships, mineralogy, geochemical patterns and pre‐deformational alteration all indicate that the Soldiers Cap Group iron‐formations are mainly hydrothermal chemical sediments. Chondrite normalised REE patterns display positive Eu and negative Ce anomalisms, consistent with components of both high‐temperature, reduced, hydrothermal fluid (≥250°C) and cool oxidised seawater. Major element data suggest a largely mafic provenance for montmorillonitic clays and other detritus during chemical sedimentation, consistent with westward erosion of Cover Sequence 2 volcanic rocks, rather than local mafic sources. Ni enrichment is most consistent with hydrogenous uptake by Mn‐oxides or carbonates. Temperatures inferred from REE data indicate that although they are not strongly enriched, base metals such as Pb and Zn are likely to have been transported and deposited prior to or following iron‐formation deposition. Most chemical sedimentation pre‐dated emplacement of the major mafic igneous sill complexes present in the upper part of the basin. Heating of deep basinal brines in a regional‐scale aquifer by deep‐seated mafic magma chambers is inferred to have driven development of hydrothermal fluids. Three major episodes of extension exhausted this aquifer, but were succeeded by a final climactic extensional phase, which produced widespread voluminous mafic volcanism. The lateral extent of the iron‐formations requires a depositional setting such as a sea‐floor metalliferous sediment blanket or series of brine pools, with iron‐formation deposition likely confined to much smaller fault‐fed areas surrounded by Fe–Mn–P‐anomalous sediments. These relationships indicate that in such settings, major sulfide deposits and their associated chemical sediment marker horizons need not overlie major igneous sequences. Rather, the timing of expulsion of hydrothermal fluid reflects the interplay between deep‐seated heating, extension and magmatism.

Soldiers Cap Group iron‐formations, Mt Isa Inlier, Australia, as windows into the hydrothermal evolution of a base‐metal‐bearing Proterozoic rift basin

The Proterozoic Soldiers Cap Group, a product of two major magmatic rift phases separated by clastic sediment deposition, hosts mineralised (e.g. Pegmont Broken Hill‐type deposit) and barren iron oxide‐rich units at three main stratigraphic levels. Evaluation of detailed geological and geochemical features was carried out for one lens of an apatite‐garnet‐rich, laterally extensive (1.9 km) example, the Weatherly Creek iron‐formation, and it was placed in the context of reconnaissance studies of other similar units in the area. Chemical similarities with iron‐formations associated with Broken Hill‐type Pb–Zn deposit iron‐formations are demonstrated here. Concordant contact relationships, mineralogy, geochemical patterns and pre‐deformational alteration all indicate that the Soldiers Cap Group iron‐formations are mainly hydrothermal chemical sediments. Chondrite normalised REE patterns display positive Eu and negative Ce anomalisms, are consistent with components of both high‐temperature, reduced, hydrothermal fluid (≥250°C) and cool oxidised seawater. Major element data suggest a largely mafic provenance for montmorillonitic clays and other detritus during chemical sedimentation, consistent with westward erosion of Cover Sequence 2 volcanic rocks, rather than local mafic sources. Ni enrichment is most consistent with hydrogenous uptake by Mn‐oxides or carbonates. Temperatures inferred from REE data indicate that although they are not strongly enriched, base metals such as Pb and Zn are likely to have been transported and deposited prior to or following iron‐formation deposition. Most chemical sedimentation pre‐dated emplacement of the major mafic igneous sill complexes present in the upper part of the basin. Heating of deep basinal brines in a regional‐scale aquifer by deep‐seated mafic magma chambers is inferred to have driven development of hydrothermal fluids. Three major episodes of extension exhausted this aquifer, but were succeeded by a final climactic extensional phase, which produced widespread voluminous mafic volcanism. The lateral extent of the iron‐formations requires a depositional setting such as a sea‐floor metalliferous sediment blanket or series of brine pools, with iron‐formation deposition likely confined to much smaller fault‐fed areas surrounded by Fe–Mn–P–anomalous sediments. These relationships indicate that in such settings, major sulfide deposits and their associated chemical sediment marker horizons need not overlie major igneous sequences. Rather, the timing of expulsion of hydrothermal fluid reflects the interplay between deep‐seated heating, extension and magmatism.

SHRIMP U–Pb zircon dating of the host rocks of the Cannington Ag–Pb–Zn deposit, southeastern Mt Isa Block, Australia

SHRIMP U–Pb zircon ages are reported from a paragneiss, a pegmatite, a metasomatised metasediment and an amphibolite taken from the upper amphibolite facies host sequence of the Cannington Ag–Pb–Zn deposit at the southeastern margin of the Proterozoic Mt Isa Block. Also reported are ages from a middle amphibolite‐facies metasediment from the Soldiers Cap Group approximately 90 km north of Cannington. The predominantly metasedimentary host rocks of the Cannington deposit were eroded from a terrane containing latest Archaean to earliest Palaeoproterozoic (ca 2600–2300 Ma) and Palaeoproterozoic (ca 1750–1700 Ma) zircon. The ca 1750–1700 Ma group of zircons are consistent with sedimentary provenance from rocks of Cover Sequence 2 age that are now exposed to the north and west of the Cannington deposit. The metasedimentary samples also include a group of zircon grains at ca 1675 Ma, which we interpret as the maximum depositional age of the sedimentary protolith. This is comparable to the maximum depositional age of the metasediment from the Maronan area (ca 1665 Ma) and to previously published data from the Soldiers Cap Group. Metamorphic zircon rims and new zircon grains grew at 1600–1580 Ma during upper amphibolite‐facies metamorphism in metasedimentary and mafic magmatic rocks. Zircon inheritance patterns suggest that sheet‐like pegmatitic intrusions were most likely derived from partial melting of the surrounding metasediments during this period of metamorphism. Some zircon grains from the amphibolite have a morphology consistent with partially recrystallised igneous grains and have apparent ages close to the metamorphic age, although it is not clear whether these represent metamorphic resetting or crystallisation of the magmatic protolith. Pb‐loss during syn‐ to post‐metamorphic metasomatism resulted in partial resetting of zircons from the metasomatised metasediment.

SHRIMP U–Pb monazite dating of 1600–1580 Ma amphibolite facies metamorphism in the southeastern Mt Isa Block, Australia

SHRIMP U–Pb monazite dates of ca 1600–1580 Ma are reported from three samples taken from the southeastern margin of the Proterozoic Mt Isa Block. The samples include an upper amphibolite facies paragneiss and a pegmatite from the host sequence of the Cannington Ag–Pb–Zn deposit and a middle amphibolite facies metasediment from the Soldiers Cap Group near Maronan station. These dates are interpreted to represent the timing of amphibolite facies metamorphism at the southeastern margin of the Mt Isa Block. They are in accordance with the results of earlier SHRIMP U–Pb zircon and 40Ar/39Ar dating, which suggested that metamorphism in the southeastern Mt Isa Block occurred approximately 50 million years earlier than metamorphism in the western Mt Isa Block. This challenges the common perception of orogeny in the Mt Isa Block in which ‘peak metamorphism’, and the deformation events associated with it, can be correlated across the entire terrane.

Alteration, mineralization, and fluid evolution at the Eloise Cu-Au deposit, Cloncurry District, Northwest Queensland, Australia

The Eloise deposit (3.1 million metric tons, Mt,5.5% Cu, 1.4 g/t Au and 16 g/t Ag) is hosted by Proterozoic rocks of the Soldiers Cap Group in the Eastern fold belt of the Mount Isa inlier. The deposit is characterized by very high grade, chalcopyrite-pyrrhotite-rich mineralization hosted by mafic silicate alteration. Alteration and mineralization lie within and adjacent to a major shear zone. The main metasomatism comprised three stages. Early pervasive albite alteration (stage I) was overprinted by stage II hornblende-biotite-quartz veins and alteration; stage III chalcopyrite, pyrrhotite, magnetite, pyrite, calcite, quartz, chlorite, and actinolite replaced and brecciated the earlier mafic silicates. The lodes occupy a narrow portion of a zoned, mineralized system that is at least 2 km in length. Magnetite-pyrite-rich mineralization occurs in the south, with chalcopyrite-pyrrhotite-rich mineralization in the central lode zone, and pyrrhotite-rich mineralization in the north. The system has a distinct chemistry, with enriched Cu, Au, Ag, Co, Ni, and Zn.Fluid inclusions in pre-, syn- and late- to postmineralization veins indicate an evolving fluid history. Primary inclusions and associated hornblende-biotite assemblages in premineralization veins indicate a high-temperature fluid (450 degrees -600 degrees C), and salinity estimates suggest it was an ultrasaline brine (32-68 wt % total salts). Fluid inclusions in stage III quartz in mineralized veins display a wide range of homogenization temperatures ( approximately 100 degrees -500 degrees C), and gangue assemblages (actinolite-chlorite-muscovite) suggest mineralization was a cooler event (200 degrees -450 degrees C). Inclusions associated with the mineralization have a lower salinity (30-47 wt % total salts) than do premineralization inclusions, and variable fluid inclusion types occur within the same clusters, which may reflect fluid mixing. The ultrasaline nature of the fluids and preliminary sulfur isotope results (0.4-2.0ppm) suggest the ore fluid was predominantly magmatic. Metals were carried as chlorocomplexes and H 2 S was the dominant sulfur species. Important factors in ore deposition were a decrease in temperature and salinity, and sulfidation of Fe-silicates. Eloise is interpreted to be a structurally controlled, distal, magmatic, hydrothermal vein deposit.

Aspects of geochronology and crustal evolution in the Eastern Fold Belt, Mt Isa Inlier*

Zircon U‐Pb SHRIMP ages and crustal Sm‐Nd signatures are reported for metasediments, meta‐volcanics, and a variety of pre‐ and post‐tectonic granitoid intrusives in the Eastern Fold Belt of the Mt Isa Inlier. These enable tracing of the broad crustal evolution of the rocks, erection of a chrono‐stratigraphic framework in the Eastern Fold Belt, and correlation of these rocks with those in other parts of the Mt Isa Inlier and in other Proterozoic inliers of northern Australia. The SHRIMP results and Sm‐Nd model ages provide quantitative information related to the provenance history of the metasediments and the sources and crustal interactions of magmatic rocks. Most of the latter have Nd Tdm of 2270–2200 Ma, suggesting similar source(s) to the metasediments and gneisses (2360–2140 Ma). The Nd TDM model ages are younger than those found in the Kalkadoon‐Leichhardt Belt, suggesting that the Eastern Fold Belt crust received less Archaean source input. Rocks of Barramundi age (1870–1840 Ma) are not found in the Eastern Fold Belt. The Eastern Fold Belt includes an older, calc‐silicate‐dominated sequence (broadly equivalent to the Mary Kathleen Group) some of which was deposited in the interval 1750 ± 7 Ma to 1725 ± 3 Ma. Felsic to intermediate volcanism in this period is evident in the host rocks at the Ernest Henry Cu‐Au deposit which have been dated nearby at 1746 ± 9 Ma and 1742 ± 6 Ma. This volcanism may have been coeval with ca 1740 Ma formation of older deformed granitoids in the Eastern Fold Belt and was possibly related to the Wonga extensional event evident elsewhere in the inlier. The SHRIMP geochronology clarifies the stratigraphic position of the Soldiers Cap Group, a sequence of metamorphosed turbiditic sediments and mafic volcanics. Most of the Soldiers Cap Group (including paragneisses that host the Cannington Ag‐Pb‐Zn deposit) is probably similar in age to the ca 1655 Ma Mt Isa Group, host for the Cu and Pb‐Ag‐Zn orebodies at Isa and Hilton Mines in the Western Fold Belt. However, part of the metasedimentary package previously assigned to the Soldiers Cap Group is comparable in age to the much older Argylla Formation, as it includes volcaniclastic rocks with depositional ages of 1775 ± 4 Ma and 1774 ± 4 Ma. High‐uranium metamorphic zircon selvedges in Soldiers Cap Group gneisses define an age of 1584 ± 17 Ma. This provides a direct age for a high‐grade metamorphic event(s) in the Eastern Fold Belt and invites comparison to similarly timed tectonism in the Georgetown region. The majority of granitoids in the Williams and Naraku Batholiths were emplaced either in the interval 1550–1530 Ma or close to 1500 Ma, again suggesting links in Mesoproterozoic time to the Etheridge Province in the Georgetown region. Hydrothermal zircon overgrowths of this age (1500 Ma) are found not only in the 1500 Ma intrusions but also in some older rocks. These provide evidence for the timing of a pervasive sodic alteration event which pre‐dates Cu‐Au mineralisation in the region. The geochronology of the Eastern Fold Belt country rocks and granites supports arguments based on lithostratigraphy and evidence from Pb‐isotope data which imply that stratabound Pb‐Zn mineralisation in the Eastern Fold Belt and Georgetown regions may have been generated in a similar tectonic environment and from similar source rocks at about the same time (ca 1680–1660 Ma). A post‐1700 Ma connection and correlation of these two terrains, and possibly the Broken Hill Inlier, has to be seriously considered.

Structural development of the Soldiers Cap Group in the Eastern Fold Belt of the Mt Isa Inlier: A succession of horizontal and vertical deformation events and large‐scale shearing

Six discrete episodes of sequential, alternating horizontal (D1, D3, D5) and vertical (D2, D4, D6 ) compressive deformation have been recorded in the Proterozoic Soldiers Cap Group of the eastern Mt Isa Inlier. A seventh compressive horizontal deformation (D7) is recognised locally. The deformations range in age from ca 1600 Ma to post‐1480 Ma. Quartz inclusion trails in garnet porphyroblasts preserve the orientations of early foliations that have been correlated with foliations recorded at the outcrop, Both inclusion trails and foliations in outcrop, are consistent with north‐south shortening during D1, east‐west shortening during D3 and D5, and north‐south shortening during D7. Field and microstructural relationships are consistent with an orogenic model involving three episodes of horizontal shortening each of which was followed by gravity collapse of the overthickened orogenic rock mass resulting in widespread extension at the centre of the orogen and thrusting along its sides. Deep within the orogen, shallowly dipping cleavages (S2, S4, S6) imparted by the vertical compression of the gravitationally collapsing rock masses overprinted the steep cleavages formed by the earlier horizontal compression and were in turn overprinted themselves by the steep cleavages of the next horizontal compression. Local unfolding of D5 folds by reactivation of D3 cleavage allowed the formation of several small mineralised enclaves within the Soldiers Cap Group.

Variation in copper‐gold styles through time in the Proterozoic Cloncurry goldfield, Mt Isa Inlier: A reconnaissance view

The Cloncurry copper‐gold field, Mt Isa Inlier, contains a variety of copper‐gold mineralisation styles, in which Cu/Au ratios vary radically from Cu‐ to Au‐only. Several of these mineralisation styles are not recognised elsewhere in the Mt Isa Inlier and are rare in Proterozoic metallogeny, They developed concurrently with intense, high‐temperature, saline metasomatism exposed in the Cloncurry metasomatic zone 13 km to the southwest and may represent mineralisation styles that occur lateral to and above such albite‐actinolite alteration. Mineralisation varied through time in response to changing structural style. Initially small quartz vein‐hosted Au‐only deposits (Gilded Rose‐style) formed during regional D1 thrusting. These are known only from one east‐west oriented antiform in the Soldiers Cap Group, and do not display the high‐temperature saline fluid‐related alteration of subsequent systems. Quartz vein/replacement‐hosted Cu‐Au ± Zn‐As‐Co‐Bi‐Mn deposits (Eloise‐style) were formed in ductile D2/D3‐shears within metasediments. Brittle‐ductile D3 to post‐D3/syn‐granite deformation produced the most common deposit styles, including stratabound iron‐formation‐hosted Cu‐Au‐U‐Co‐Ba‐Mn‐P‐Zn‐rich deposits (e.g. Monakoff, Hot Rocks), albitite‐associated Cu‐Au‐Co vein networks (e.g. Evening Star), and, most abundantly, Cu‐Au‐Co vein/replacement deposits in amphibolite (Mt Freda‐style). The sources of ore components for D1‐ and D2‐related deposits are uncertain, but deposits that formed during D3 have a close spatial relationship to inferred isolated Williams Batholith intrusions, which supports some magmatic contribution for these. The albitite‐related deposits have some features in common with porphyry copper systems with histories of external saline fluid interaction. For Mt Freda‐style deposits, which mainly contain reduced pyrrhotite‐pyrite assemblages, pH change that was induced by wall‐rock reaction, with concurrent fluid unmixing in dilatant zones, is favoured to account for ore deposition. Oxidation by magnetite‐bearing wall rocks, occurring concurrently with pH change, can account for unusual magnetite‐bearing Mt Freda‐style examples such as Great Australia, but this difference produces a fluid path which greatly favours Cu deposition over Au, producing Cu‐only orebodies.

Metamorphic and metasomatic evolution of the Snake Creek Anticline, Eastern Succession, Mt Isa Inlier

The Snake Creek Anticline is a regional D2 structure, with the overturning of the western limb and the curvature of the axial trace from north to east interpreted as an effect of D3, which produced folds and crenulations with shallowly dipping axial surfaces. Low‐pressure/high‐temperature metamorphism which affected the Mesoproterozoic Soldiers Cap Group reached its peak with the growth of sillimanite during intense foliation development during D2. However, microstructural studies have revealed multiple porphyroblast growth stages, namely: (i) pre‐ to early D1, (ii) late D1 to early D2, (iii) late to post‐D2, (iv) syn‐D3 and (v) syn‐D4. The kyanite/andalusite, sillimanite, and sillimanite/K‐feldspar isograds are D2‐related, whereas the garnet and andalusite/staurolite are composite isograds representing multiple growth events. To explain the occurrence of kyanite partially replaced by andalusite, and the replacement of early growth andalusite by staurolite + muscovite, it is suggested that the pressure‐temperature‐deformation path included a fall in temperature after D1, followed by a rise in both temperature and pressure by the end of D2. Quartz veining occurs throughout the evolution of the rocks, but the most notable events are: (i) an early (pre‐/syn‐D2) phase of albitite veining, (ii) syn‐D2 quartz (+ staurolite + andalusite + kyanite) veining, and (iii) late (syn‐/post‐D5) albitite veining. Preliminary fluid‐inclusion studies show that hypersaline fluids were dominant during at least some of this history. The Snake Creek Anticline and the Cloncurry Fault provided structural controls for the infiltration of the metasomatic fluids. Inferred to be initially of igneous origin. The fluids may have been important in the transfer of heat during D2.

Eloise Cu‐Au deposit, East Mt Isa Block: Structural environment and structural controls on ore

The blind Eloise Cu‐Au deposit is hosted by the subsurface eastward extension of the Proterozoic East Mt Isa Block. The host rocks comprise highly deformed, amphibolite‐grade metasediments and amphibolite of the Soldiers Cap Group. The deposit formed during the waning phase of the Isan Orogeny (D3, 1540–1500 Ma) in a regional Cu‐Au metallogenic event synchronous with emplacement of the Williams Batholith (1540–1490 Ma). The deposit lies within and adjacent to a series of anastomosing shear zones at a local jog within the regional‐scale Levuka Shear Zone, This was a major fluid channel similar to the parallel Mt Dore and Cloncurry Fault Zones to the west. Alteration and mineralisation in the jog were synchronous with ductile‐brittle deformation, and were emplaced in ductile shear zones and cleavage, and brittle stockworks and faults. Brittle mineralisation styles were developed in competent enclaves in more competent primary host rock or silicic/feldspathic alteration. Three principal paragenetic stages are recognised. Stage I albitisation post‐dated peak (D2) metamorphism, and was overprinted by high temperature (>450°C) stage II hornblende + biotite alteration. Stage III sulfide mineralisation fractured and replaced the mafic silicates during a subsequent cooler event (200–450°C). Deposition of stage III ore, in refracture‐infill of stage II veins and selective replacement of stage II alteration, was controlled by sulfidisation of the mafic stage II assemblages. The changing distribution of alteration during the mineralising event (silicic/feldspathic versus hornblende/ phyllosilicate) controlled the competency architecture and in consequence the local mineralisation style. Each mineralisation stage has ductile and brittle styles, each locally overprinting the other. Ductile‐brittle deformation fluctuated in time and space at all scales, producing mineralisation characterised by locally ambiguous textures with respect to the timing of deformation. Massive sulfide veins show durchbewegung texture and breccias are ductilely deformed, yet identical mineralisation assemblages elsewhere occur in veins cross‐cutting cleavage. Ambiguity is compounded by a strong component of replacement in the mineralisation, which produced selvages along cleavage‐parallel veins which mimic cleavage, The paragenesis is determined from unambiguous overprinting relations in the brittle styles, and is then extrapolated to the more equivocal ductile styles.