Thrust-controlled, gold quartz-vein mineralisation at the Tom's Gully mine, Northern Territory, Australia

Metasedimentary and minor metavolcanic rocks of the Early Proterozoic Pine Creek Inlier rest unconformably on Late Archaean granitic basement. Three basin-wide, regional deformation events at ca.1885–1870 Ma are recognised: I) W- to NW-verging thrusts and recumbent folds (D₂), II) upright, open to tight, doubly-plunging, NNE- to NNW-trending folds (D₃), and III) open, upright, E-trending folds (D₄). In the centre of the Pine Creek Inlier, post-tectonic granites (1835–1820 Ma) are spatially, temporally and probably genetically associated with mesothermal gold-quartz vein deposits. The Tom's Gully deposit consists of a shallowly S-dipping quartz reef in graphitic shale and siltstone within the thermal aureole of the post-tectonic (1831 ± 6 Ma) Mt Bundey pluton. Gold mineralisation comprises two(?) SSW-plunging sulphidic ore-shoots which are intimately associated with brecciation and recrystallisation of early barren quartz. Where early quartz is absent from the thrust, gold mineralisation is not developed, indicating that this secondary brittle fracturing was essential to sulphide and gold deposition. The ore-shoots plunge parallel to the trend of D₃ fold axes. The reef is hosted by a D₂ thrust fault with transport to the NW. D₃ folds in the hangingwall and footwall decrease in amplitude toward the reef indicating that, during continued E-W compression, the thrust acted as a décollement zone. Field relationships and microstructural studies suggest that quartz and sulphide were deposited in a reactivated thrust during wrench shear along several NNE-trending faults associated with emplacement of the Mt Bundey pluton.     

Proterozoic deformation in the east Pilbara Craton and tectonic setting of fault-hosted manganese at the Woodie Woodie mine

Neoarchean and Mesoproterozoic sequences in the Oakover Basin provide a record of deformation and sedimentation along the eastern edge of the Archean Pilbara Craton. The early extensional history of the Oakover Basin is overprinted by subsequent compressional events. Five distinct deformation events are recognised in the Woodie Woodie region; the Archean D₁ event, comprising west-northwest–east-southeast extension associated with formation of the Neoarchean Hamersley Basin; the Mesoproterozoic D_2a event, with northwest–southeast extension and basin formation associated with manganese mineralisation; the D_2b event, with renewed extension associated with intrusion of Davis Dolerite during the ca 1090–1050 Ma Warakurna event; the D₃ event, comprising northeast–southwest-directed compression attributed to the ca 900 Ma Edmundian Orogeny; the Neoproterozoic D₄ event, with east-northeast–west-southwest extension producing large D₄ grabens associated with the opening of the Officer Basin; and, the Neoproterozoic D₅ event comprising north–south-directed compression attributed to the ca 550 Ma Paterson Orogeny. Abundant manganese deposits are hosted by the Neoarchean and Mesoproterozoic sequences in the Oakover Basin, including the large high-grade manganese deposits at Woodie Woodie. The orebodies are predominantly hydrothermal in origin, with a late supergene overprint, and deposition of primary manganese mineralisation was synchronous with northwest–southeast Mesoproterozoic D_2a extension and basin formation. The manganese is associated with normal faults, and many of these represent growth faults related to basin formation. Stratabound manganese is found above or adjacent to fault-hosted manganese. An initial structural framework established during Archean rifting was reactivated in the D_2a event and provided a major structural control on manganese distribution. High-grade manganese deposits at Woodie Woodie mine appear to be located in a zone of oblique dextral extension on major north-northwest- to north-trending faults that mark the eastern ‘active’ or faulted margin of an early rift basin. These large north-northwest-trending normal faults are linked to a major northwest-trending transform fault zone (Jewel-Southwest Fault Zone) that separates the Oakover Basin into a northern and southern basin. The transform fault represents a major deep fluid conduit for hydrothermal fluids and most likely accounts for the concentration of significant manganese occurrences immediately to the north and south of this structure.     

Geological observations in high‐grade mid‐Proterozoic rocks from Else Platform,northern Prince Charles mountains region,east Antarctica

Granulite facies rocks on Else Platform in the northern Prince Charles Mountains, east Antarctica, consist of metasedimentary gneiss extensively intruded by granitic rocks. The dominant rock type is a layered garnetbiotite‐bearing gneiss intercalated with minor garnet‐cordierite‐sillimanite gneiss and calc‐silicate. Voluminous megacrystic granite intruded early during a mid‐Proterozoic (ca 1000 Ma) granulite event, M₁, widely recognized in east Antarctica. Peak metamorphic conditions for M₁ are in the range of 650–750 MPa at ～800°C and were associated with the development of a gneissic foliation, S₁ and steep east‐plunging lineation, L₁. Strain partitioning during progressive non‐coaxial deformation formed large D₂ granulite facies south‐dipping thrusts, with a steep, east‐plunging lineation. In areas of lower D₂ strain, large‐scale upright, steep east‐plunging fold structures formed synchronously with the D₂ high‐strain zones. Voluminous garnet‐bearing leucogneiss intruded at 940 ±20 Ma and was deformed in the D₂ high‐strain zones. Textural relationships in pelitic rocks show that peak‐M2 assemblages formed during increasing temperatures via reactions such as biotite + sillimanite + quartz ± plagioclase = spinel + cordierite + ilmenite + K‐feldspar + melt. In biotite‐absent rocks, re‐equilibration of deformed M₁ garnet‐sillimanite‐ilmenite assemblages occurred through decompressive reactions of the form, garnet + sillimanite + ilmenite = cordierite + spinel + quartz. Pressure/temperature estimates indicate that peak‐M₂ conditions were 500–600 MPa and 700±50°C. At about 500 Ma, north‐trending granitic dykes intruded and were deformed during D₃‐M₃ at probable upper amphibolite facies conditions. Cooling from peak D₃‐M₃ conditions was associated with the formation of narrow greenschist facies shear zones, and the intrusion of pegmatite. Cross‐cutting all features are abundant north‐south trending alkaline mafic dykes that were emplaced over the interval ca 310–145 Ma, reflecting prolonged intrusive activity. Some of the dykes are associated with steeply dipping faults that may be related to basin formation during Permian times and later extension, synchronous with the formation of the Lambert Graben in the Cretaceous.     

Chronology and evolution of the Middle Proterozoic Albany‐Fraser Orogen,Western Australia

Geochemical and Sm‐Nd isotopic data, and 19 ion‐microprobe U‐Pb zircon dates are reported for gneiss and granite from the eastern part of the Albany‐Fraser Orogen. The orogen is dominated by granitic rocks derived from sources containing both Late Archaean and mantle‐derived components. Four major plutonic episodes have been identified at ca 2630 Ma, 1700–1600 Ma, ca 1300 Ma and ca 1160 Ma. Orthogneiss, largely derived from ca 2630 Ma and 1700–1600 Ma granitic precursors, forms a belt along the southeastern margin of the Yilgarn Craton. These rocks, together with gabbro of the Fraser Complex, were intruded by granitic magmas and metamorphosed in the granulite facies at ca 1300 Ma. They were then rapidly uplifted and transported westward along low‐angle thrust faults over the southeastern margin of the Yilgarn Craton. Between ca 1190 and 1130 Ma, granitic magmas were intruded throughout the eastern part of the orogen. These new data are integrated into a review of the geological evolution of the Albany‐Fraser Orogen and adjacent margin of eastern Antarctica, and possibly related rocks in the Musgrave Complex and Gawler Craton.     

Evidence for 1808–1770 Ma bimodal magmatism,sedimentation, high-temperature deformation and metamorphism in the Aileron Province,central Australia

Field relationships and LA-ICP-MS U–Pb geochronology from the Yundurbungu Hills (Aileron Province, central Australia) reveal a record of 1808–1770 Ma bimodal magmatism, sedimentation, high-temperature deformation and metamorphism. Specifically, the data presented here provide the first unequivocal evidence for ca 1774 Ma high-temperature deformation and metamorphism during the 1790–1770 Ma Yambah Event in the southern part of the North Australian Craton. Granitic lithologies were synkinematically emplaced between 1808 and 1770 Ma, with early phases recording D₁ deformation and the youngest phase postdating D₁ deformation. The protolith to a D₁ deformed metasedimentary unit was deposited between 1792 and 1774 Ma, followed by the intrusion and deformation of a composite mafic–felsic magmatic association at ca 1774 Ma. An S₁ migmatitic fabric in the composite mafic–felsic gneiss is truncated by the youngest (ca 1770 Ma) phase of granitic magmatism, constraining the timing of S₁ deformation. A second period of sedimentation appears to post-date D₁ deformation, with deposition occurring sometime after ca 1774 Ma. Subsequent overprinting during the 1590–1550 Ma Chewings Event is recorded by the growth of metamorphic monazite and zircon. This event deformed the ca 1774 Ma S₁ gneissic fabric, producing a composite S₁/S₂ gneissic fabric in early metasedimentary and magmatic lithologies and a simple S₂-only fabric in lithologies that were intruded or deposited after ca 1774 Ma. Consistent with previous work, we suggest that localised high-temperature deformation and bimodal magmatism at ca 1774 Ma in the Yundurbungu Hills is consistent with a back-arc setting linked to prolonged north-directed subduction.     

Provenance,tectonic setting and source of Archean metasedimentary rocks of the Browns Range Metamorphics,Tanami Region,Western Australia

This study combines U–Pb age and Lu–Hf isotope data for magmatic and detrital zircons, with whole-rock geochemistry of the Browns Range Metamorphics (BRM), Western Australia. The BRM are medium- to coarse-grained metasandstones that consist of angular to sub-rounded detrital quartz and feldspars with minor granitic lithic fragments. The sequence has undergone partial to extensive quartz–muscovite alteration and rare-earth-element mineralisation and has been intruded by mafic/ultramafic, syenitic and pegmatitic intrusive rock units. Uranium–Pb and Lu–Hf isotopic data on detrital zircons from the metasandstones and intruding granitic rocks yield a well-defined age of ca 3.2 to ca 3.0 Ga for all samples, with relatively radiogenic ?_Hf values (?_Hf = –1.7 to 5.1) indicating derivation from Mesoarchean granite basement of juvenile origin. This is consistent with geochemical and petrological data that support deposition from a granitic source in a continental rift basin setting. The timing of sediment deposition is constrained between the ca 3.0 Ga age of the source rocks and ca 2.5 Ga age of the granitic intrusive bodies that cross-cut the metasedimentary rocks. The ca 2.5 Ga zircons from the intrusive rocks have ?_Hf model ages of ca 3.4 to ca 3.1 Ga, which is consistent with formation via partial melting of the BRM, or the Mesoarchean granite basement. Zircons of the Gardiner Sandstone that unconformably overlies the BRM return detrital ages of ca 2.6 to ca 1.8 Ga with no trace of ca 3.1 Ga zircons, which discounts a significant contribution from the underlying BRM. The Mesoarchean age and isotopic signatures of the BRM zircons are shared by some zircon records from the Pine Creek Orogen, and the Pilbara, Yilgarn and Gawler cratons. Collectively, these records indicate that juvenile Mesoarchean crust is a more significant component of Australian cratons than is currently recognised. This work also further demonstrates that detrital minerals in Paleoproterozoic/Archean sedimentary rocks are archives to study the early crustal record of Earth.     

Thermal evolution of the central Halls Creek Orogen,northern Australia

The Halls Creek Orogen in northern Australia records the Palaeoproterozoic collision of the Kimberley Craton with the North Australian Craton. Integrated structural, metamorphic and geochronological studies of the Tickalara Metamorphics show that this involved a protracted episode of high‐temperature, low‐pressure metamorphism associated with intense and prolonged mafic and felsic intrusive activity in the interval ca 1850–1820 Ma. Tectonothermal development of the region commenced with an inferred mantle perturbation event, probably at ca 1880 Ma. This resulted in the generation of mafic magmas in the upper mantle or lower crust, while upper crustal extension preceded the rapid deposition of the Tickalara sedimentary protoliths. An older age limit for these rocks is provided by a psammopelitic gneiss from the Tickalara Metamorphics, which yield a ²⁰⁷Pb/²⁰⁶Pb SHRIMP age of 1867 ± 4 Ma for the youngest detrital zircon suite. Voluminous layered mafic intrusives were emplaced in the middle crust at ca 1860–1855 Ma, prior to the attainment of lower granulite facies peak metamorphic conditions in the middle crust. Locally preserved layer‐parallel D₁ foliations that were developed during prograde metamorphism were pervasively overprinted by the dominant regional S₂ gneissosity coincident with peak metamorphism. Overgrowths on zircons record a metamorphic ²⁰⁷Pb/²⁰⁶Pb age of 1845 ± 4 Ma. The S₂ fabric is folded around tight folds and cut by ductile shear zones associated with D₃ (ca 1830 Ma), and all pre‐existing structures are folded around large‐scale, open F₄ folds (ca 1820 Ma). Construction of a temperature‐time path for the mid‐crustal section exposed in the central Halls Creek Orogen, based on detailed SHRIMP zircon data, key field relationships and petrological evidence, suggests the existence of one protracted thermal event (>400–500°C for 25–30 million years) encompassing two deformation phases. Protoliths to the Tickalara Metamorphics were relatively cold (～350°C) when intruded by the Fletcher Creek Granite at ca 1850 Ma, but were subsequently heated rapidly to 700–800°C during peak metamorphism at ca 1845 Ma. Repeated injection of mafic magmas caused multiple remelting of the metasedimentary wall rocks, with mappable increases in leucosome volume that show a strong spatial relationship to these intrusives. This mafic igneous activity prolonged the elevated geotherm and ensured that the rocks remained very hot (≥650°C) for at least 10 million years. The Mabel Downs Tonalite was emplaced during amphibolite facies metamorphism, with intrusion commencing at ca 1835 Ma. Its compositional heterogeneity, and the presence of mutual cross‐cutting relations between ductile shear zones and multiple injections of mingled magma suggest that it was emplaced syn‐D₃. Broad‐scale folding attributable to F₄ was accompanied by widespread intrusion of granitoids, and F₄ fold limbs are truncated by large, mostly brittle retrograde S₄ shear zones.     

Petrogenesis of granitoids in the Wulan area: Magmatic activity and tectonic evolution in the North Qaidam,NW China

Numerous granitic intrusions crop out in the eastern segment of the North Qaidam block (NQ), NW China. To evaluate their ages, petrogenesis and genetic relationships to other granitoids in the NQ, we present geochemical and geochronologic data for six intrusive bodies and review regional data. Zircon U-Pb (SHRIMP) dating yielded ages of 413 ± 3 Ma for the Hadesengou granite; 254 ± 3 Ma for the Xugeigou granite; 251 ± 1 Ma for the Qiluoshan granite; 249 ± 1 and 248 ± 2 Ma for the Chahannuo hornblende diorite and granite, respectively; 240 ± 2 Ma for the Chahanhe granite; and 250 ± 1 and 244 ± 3 Ma for the Shailekegoulei granodiorite and granite, respectively. Consequently, the Wulan plutons can be divided into two petrologic groups: Early Devonian (D₁) quartz monzonite and syenogranite, and Late Permian to Early Triassic (P₃-T₁) hornblende diorite, granodiorite, and granite. The D₁ granitic intrusions have geochemical affinities with A-type granites (A₂-type) characterized by low Ca, Sr, Ba and Nb, and high Fe, Ga, Y and Rb, consistent with derivation by partial melting of metapelitic source rocks containing a small amount of metagraywacke. The P₃-T₁ I-type granitic intrusions are geochemically typical of active continental margin rocks, consistent with derivation by partial melting of metabasalt and clay-poor metagraywacke. Combined with previous studies, we recognize five periods of granitic magmatism in the NQ: (1) 465–473 Ma; (2) 423–446 Ma; (3) 391–413 Ma; (4) 372–383 Ma; and (5) 240–271 Ma. Based on the temporal-spatial distribution of granitic intrusions in the NQ and the regional tectonic evolution, we interpret the first and second periods of granitic magmatism as related to normal plate subduction, and the third period to slab break-off and exhumation of the subducted plate. The fourth stage of granitic magmatism is attributed to large-scale lithospheric mantle delamination, involving the differential movement of orogenic blocks. The fifth period of granitic plutonism probably reflects northward subduction of the East Kunlun Paleotethys oceanic crust and southward subduction of Zongwulong oceanic crust beneath the Oulongbuluke continental block.     

Detrital zircon geochronology of the Speewah Group,Kimberley region,Western Australia: evidence for intracratonic development of the Paleoproterozoic Speewah Basin

Field observations integrated with new petrographic and sensitive high-resolution ion microprobe (SHRIMP) U–Pb age data for detrital zircons from the Paleoproterozoic Speewah Group of northern Western Australia provide evidence of depositional conditions, source of detritus, timing and evolution of the sedimentary rocks in the Speewah Basin. The Speewah Group is a 1.5 km-thick succession of poorly outcropping, predominantly siliciclastic rocks that preserve a fluviatile to marine, transgressive and regressive event. The Speewah Group unconformably overlies crystalline rocks of the Lamboo Province that were stabilised by the 1870–1850 Ma Hooper Orogeny, then accreted as the Kimberley region onto the North Australian Craton during the 1835–1810 Ma Halls Creek Orogeny. Unconformably overlying the Speewah Group is about 4 km of predominantly siliciclastic marine sedimentary rocks of the Kimberley Group in the Kimberley Basin. This study has detected a detrital zircon component within the Speewah Basin at 1814 ± 10 Ma, with a youngest zircon at 1803 ± 12 Ma (1σ) in fluviatile sandstones located beneath a volcaniclastic rock with magmatic zircons that have been dated at ca 1835 Ma. Previous studies proposed that the Speewah Basin developed as a retro-arc foreland basin during accretion of the North Australian Craton. We interpret the ca 1835 Ma zircons in the volcaniclastic rocks to be xenocrystic in origin. This new 20 million years younger maximum depositional age indicates that the Speewah Group in the Speewah Basin, similarly to the overlying Kimberley Group in the Kimberley Basin, developed in a post-orogenic setting on the North Australian Craton rather than in a syn-orogenic setting associated with the 1835–1810 Ma Halls Creek Orogeny.     

Deformation history and timing of granite emplacement in the Butchers Hill — Helenvale region,northern Hodgkinson Province,Queensland

Granite plutons of the Whypalla Supersuite in the Butchers Hill — Helenvale region of north Queensland were intruded into the upper crust of the Hodgkinson Formation during contractional deformation associated with the Permian‐Triassic Hunter‐Bowen Orogeny. A four‐stage structural history has been resolved for the area, with fabric overprinting relationships, porphyroblast‐matrix microstructural geometries and isotopic ages being consistent with granite emplacement during D₄ shortening at ca 274 Ma. Microstructural relationships suggest the possibility of a minor syn‐D₃ phase of granite emplacement. The deformation‐emplacement history of the Butchers Hill — Helenvale area is consistent with that recognised regionally for the Hodgkinson Province, indicating province‐wide synchronous syntectonic granite intrusion during a major phase of contractional deformation. Intense syn‐emplacement deformation partitioning was ongoing in the country rocks during progressive D₄ and was associated with upward translation of country rock from the microscale to the macroscale along D₄ cleavages and shears. Kinematic indicators show that this progressive uplift, at the scale of the area examined, was east‐side‐up.     

Palaeoproterozoic subduction‐related and passive margin basalts in the Halls Creek Orogen,northwest Australia

Palaeoproterozoic basaltic rocks in the Halls Creek Orogen form part of two stratigraphic sequences on either side of a major structure, the Angelo‐Halls Creek‐Osmond Fault System. The two sequences have contrasting geological histories and probably formed in different tectonostratigraphic terranes. To the east, basalts of the Biscay Formation, which are part of the Halls Creek Group, were erupted at ca 1880 Ma and deformed and metamorphosed first at low grade between ca 1845 and ca 1820 Ma. To the west, basalts of the Tickalara Metamorphics were deposited after 1865 Ma, and were metamorphosed at medium to high grade and intruded by tonalite and leucogranite sheets at 1850 to 1845 Ma. Two groups of metabasalts are identified in the Biscay Formation. Group 1 samples have compositions similar to enriched (E‐)MORB. Group 2 samples have lower TiO₂, P₂O₅, Cr, Y, Nb and Zr contents, and trace‐element ratios (e.g. Ti/V and Zr/Nb), similar to low‐TiO₂ continental flood basalts. Metabasalts from the Tickalara Metamorphics consist of depleted and enriched types. The depleted samples have high field strength element (HFSE) and rare‐earth element (REE) abundances similar to oceanic island arc/backarc basin tholeiites. The enriched samples have compositions similar to E‐MORB, and are similarto group 1 samples from the Biscay Formation. Basalts of the Biscay Formation were erupted on a passive continental margin, whereas those of the Tickalara Metamorphics formed in an oceanic island arc/backarc basin or ensialic marginal basin, the two terranes being brought together by ca 1820 Ma. This is consistent with the evolution of the Halls Creek Orogen during the Palaeoproterozoic by plate‐tectonic processes similar to those operating in the Phanerozoic.     

Proterozoic igneous activity in the Tennant Creek region,Northern Territory,Australia, and its relationship to Cu-Au-Bi mineralisation

Proterozoic silicic magmatic activity in the Tennant Creek area includes a suite of pre- to syn-orogenic granitoids (i.e. the Tennant Creek Granite and compositionally similar porphyries) predominantly intruded during the period 1870–1830 Ma, a group of silicic volcanics and volcaniclastics (Flynn Subgroup) erupted shortly after the main D₁ deformation, and apparently anorogenic silicic intrusives represented by the Warrego Granite. Chemical and petrographic data indicate that the majority of the pre- to syn-orogenic granitoids and porphyries are I-type or infracrustal-derived, whereas the peraluminous muscovite-rich Warrego Granite has many characteristics of supracrustal-derived (S-type) granitoids. The Warrego Granite also appears to be moderately fractionated with relatively low Ba, Sr, Zr, K/Rb, and high Th, Nb, Be, Bi, Rb/Sr compared with the associated I-type granitoids, porphyries and volcanics. Fractionation of feldspar and minor zircon has also resulted in substantial relative enrichment in LREE together with a pronounced negative Eu anomaly in the Warrego Granite. Previous studies suggest that the Cu-Au-Bi mineralisation at Tennant Creek post-dates formation of the host ironstones, but the absolute timing is imprecisely known. Mass-balance calculations utilising background Au concentrations for potential source rocks suggest it is feasible for the Au in some of the deposits to have been leached by relatively high-temperature (250 to 300 °C) deep basinal brines, and deposited by reaction with the ironstones. However, the inferred hydrothermal leaching cells would need to have been very large unless the leaching process was very effective. An alternative preferred model is that the mineralising fluids were exsolved from incompatible element-enriched, fractionated granitic magma, mixed with ground water, and reacted with the ironstones to deposit the base and precious metals. On the basis of chemical similarity to other granitoids closely associated with mineralisation, the Warrego Granite seems the most likely source of the fluids. However, problematic constraints imposed by the apparent age of the mineralisation (1810 Ma), and an emplacement age (Rb-Sr) of 1670 Ma for the Warrego Granite indicate a need for additional geochronological studies.     

Depositional age and provenance of the Cobar Supergroup

Abstract

The turbidite-filled, Lower Devonian Cobar Basin is characterised through a detrital zircon study. Uranium–Pb age data for six samples were combined with published data to show the basin has a unique age spectrum characterised by a subordinate Middle Ordovician (ca 470?Ma) peak superimposed on a dominant ca 500?Ma peak. Maximum depositional ages for 3 samples were ca 425?Ma, close to the published Lower Devonian (Lochkovian 419–411?Ma) biostratigraphic ages. A minor ca 1000?Ma zircon population was also identified. The major source of the 500?Ma zircons was probably the local Ordovician metasedimentary basement, which was folded, thickened and presumably exposed during the ca 440?Ma Benambran Orogeny. The ca 470?Ma age peak reflects derivation from Middle Ordovician (Phase 2) rocks of the Macquarie Arc to the east. The I-type Florida Volcanics, located ～50?km eastward from the Cobar Basin, contains distinctive Middle and Late Ordovician zircon populations, considered to be derived from deeply underthrust Macquarie Arc crust. Protracted silicic magmatism occurred before, during and after Cobar Basin deposition, indicating that the basin formed by subduction-related processes in a back-arc setting, rather than as a continental rift.     

Age,petrogenesis, and tectonic setting of granitic gneiss and amphibolite from the Weizigou gold deposit,Heilongjiang Province,NE China: Constraints from geochronology and geochemistry

The Weizigou gold deposit is located in the western Jiamusi Massif, Northeast China. Gold mineralization is hosted in the amphibolite, which intruded the granitic gneiss. Although the deposit shows similarities to iron-oxide–copper–gold deposits, the detailed ore-forming process remains uncertain. To determine the formation age, petrogenesis, and tectonic setting of the granitic gneiss and amphibolite, LA–ICP–MS zircon, titanite, and monazite UPb dating, whole-rock major- and trace-element analyses, and LA–ICP–MS in situ zircon Hf isotope analyses were conducted on samples from these rocks. The granitic gneiss yielded two age populations of 951–882 Ma, and ca. 500 Ma, with a monazite UPb concordia age of 501.5 ± 5.1 Ma. The amphibolite yielded a crystallization age of 292 Ma, consistent with the results for magmatic titanite UPb dating, and a metamorphic age of 272–258 Ma. The granitic gneiss contains typical aluminum-rich minerals, such as garnet and muscovite, mean SiO₂ = 73.31 wt%, and molar ratio Al₂O₃/(CaO + K₂O + Na₂O) values of 1.02–1.07, indicating an S-type granite protolith. The amphibolite belongs to the tholeiitic basalt series and has low SiO₂ and high MnO contents. These results, together with ε_Hf(t) values and two-stage model ages ranging from ?9.5 to 2.3 and ? 0.3 to 5.7, and from 2010 to 1659 Ma and from 1331 to 947 Ma, respectively, allow us to infer that the parental magmas of the granitic gneiss and amphibolite were derived from the partial melting of Paleoproterozoic lower crust and the partial melting of metasomatized depleted mantle, respectively. The granitic gneiss is characterized by positive Th and Hf anomalies, and negative Nb, Ta, Sr and Ti anomalies, whereas the amphibolite is enriched in K, Rb, and depleted in Ba, Nb, Ti, and Zr. These geochemical features suggest that the S-type granite was formed in an active continental margin during the Neoproterozoic and underwent granulite-facies metamorphism during the early Paleozoic. The protolith of the amphibolite was gabbro that formed in an extensional setting (e.g., a backarc basin) associated with westward subduction of the Paleo-Pacific oceanic plate beneath the eastern Jiamusi Massif during the early Permian. The gold mineralization can most likely be attributed to contact metasomatic metamorphism of gabbro during the middle–late Permian.     

Relationships between gold concentration and structure in quartz veins from the Hodgkinson Province, northeastern Australia

The Hodgkinson Province is a tract of␣multiply deformed Silurian-Devonian rocks in north␣Queensland, Australia. Gold-bearing quartz veins from the West Normanby Goldfield in the northern Hodgkinson Province were emplaced during the Permian D₄ event, broadly coeval with regional granite emplacement. Taylors Fault, a major structure that formed during D₂, hosts the veins which infill dilatational jogs opened during sinistral-normal reactivation of the fault in D₄. Veins contain graphitic laminations that formed when fault planes segmented wallrocks adjacent to the veins, producing tabular clasts that were tectonically sliced into the reefs. Laminations are the result of progressive shear strain, associated with continued movement on the faults, which caused strain-enhanced dissolution of silicate minerals and residual graphite enrichment in the clasts. This process produced graphite-coated shear planes that delimit zones of grain size reduction in the veins. Laminations commonly contain stylolites, which nucleated on pronounced sinuosities of the shear planes due to progressive shortening during D₄. Gold particles have preferentially nucleated in zones of relatively coarser-grained quartz adjacent to the shear planes, where shortening strain caused microfracturing and allowed fluid access. Gold may have been introduced with the quartz, but was redistributed within the reefs and localized along the laminations by the effects of synchronous, progressive deformation. Regionally, gold deposits show close spatial relationships with granite plutons of the Permian Whypalla Supersuite. Relationships in the West Normanby Gold Field support a regional model of reef emplacement and gold mineralization during the Permian D₄ event. Received: 24 August 1997 / Accepted: 14 October 1997     

Constraining sequence stratigraphy in north Australian basins: SHRIMP U–Pb zircon geochronology between Mt Isa and McArthur River

Fifty‐five new SHRIMP U–Pb zircon ages from samples of northern Australian ‘basement’ and its overlying Proterozoic successions are used to refine and, in places, significantly change previous lithostratigraphic correlations. In conjunction with sequence‐stratigraphic studies, the 1800–1580 Ma rock record between Mt Isa and the Roper River is now classified into three superbasin phases—the Leichhardt, Calvert and Isa. These three major depositional episodes are separated by ～20 million years gaps. The Isa Superbasin can be further subdivided into seven supersequences each 10–15 million years in duration. Gaps in the geological record between these supersequences are variable; they approach several million years in basin‐margin positions, but are much smaller in the depocentres. Arguments based on field setting, petrography, zircon morphology, and U–Pb systematics are used to interpret these U–Pb zircon ages and in most cases to demonstrate that the ages obtained are depositional. In some instances, zircon crystals are reworked and give maximum depositional ages. These give useful provenance information as they fingerprint the source(s) of basin fill. Six new ‘Barramundi’ basement ages (around 1850 Ma) were obtained from crystalline units in the Murphy Inlier (Nicholson Granite and Cliffdale Volcanics), the Urapunga Tectonic Ridge (‘Mt Reid Volcanics’ and ‘Urapunga Granite’), and the central McArthur Basin (Scrutton Volcanics). New ages were also obtained from units assigned to the Calvert Superbasin (ca 1740–1690 Ma). SHRIMP results show that the Wollogorang Formation is not one continuous unit, but two different sequences, one deposited around 1730 Ma and a younger unit deposited around 1722 Ma. Further documentation is given of a regional 1725 Ma felsic event adjacent to the Murphy Inlier (Peters Creek Volcanics and Packsaddle Microgranite) and in the Carrara Range. A younger ca 1710 Ma felsic event is indicated in the southwestern McArthur Basin (Tanumbirini Rhyolite and overlying Nyanantu Formation). Four of the seven supersequences in the Isa Superbasin (ca 1670–1580 Ma) are reasonably well‐constrained by the new SHRIMP results: the Gun Supersequence (ca 1670–1655 Ma) by Paradise Creek Formation, Moondarra Siltstone, Breakaway Shale and Urquhart Shale ages grouped between 1668 and 1652 Ma; the Loretta Supersequence (ca 1655–1645 Ma) by results from the Lady Loretta Formation, Walford Dolomite, the upper part of the Mallapunyah Formation and the Tatoola Sandstone between ca 1653 and 1647 Ma; the River Supersequence (ca 1645–1630 Ma) by ages from the Teena Dolomite, Mt Les and Riversleigh Siltstones, and Barney Creek, Lynott, St Vidgeon and Nagi Formations clustering around 1640 Ma; and the Term Supersequence (ca 1630–1615 Ma) by ages from the Stretton Sandstone, lower Doomadgee Formation and lower part of the Lawn Hill Formation, mostly around 1630–1620 Ma. The next two younger supersequences are less well‐constrained geochronologically, but comprise the Lawn Supersequence (ca 1615–1600 Ma) with ages from the lower Balbirini Dolomite, and lower Doomadgee, Amos and middle Lawn Hill Formations, clustered around 1615–1610 Ma; and the Wide Supersequence (ca 1600–1585 Ma) with only two ages around 1590 Ma, one from the upper Balbirini Dolomite and the other from the upper Lawn Hill Formation. The Doom Supersequence (<1585 Ma) at the top of the Isa Superbasin is essentially unconstrained. The integration of high‐precision SHRIMP dating from continuously analysed stratigraphic sections, within a sequence stratigraphic context, provides an enhanced chronostratigraphic framework leading to more reliable interpretations of basin architecture and evolution.     

Hunter–Bowen deformation in South Percy Island,northeastern Australia

South Percy Island is located approximately 50 km off the central Queensland coast and comprises a disrupted ophiolite mass alongside a diverse array of metamorphosed felsic and mafic rocks that record several episodes of magmatism, volcanism and deformation from the Permian to Early Cretaceous. This paper aims to constrain the age, affinity and deformation history of these units, as well as to establish the tectonic significance of the terrane. The trace-element compositions of mafic and felsic meta-igneous rocks record a change from MORB-like prior to ca 277 Ma to subduction-related by ca 258 Ma. Overprinting relationships between intrusive phases and deformation features reveal a relative chronology for the tectonothermal evolution of the area, while U–Pb and ⁴⁰Ar/³⁹Ar geochronology provides absolute age constraints. Deformation is localised around a NNE-striking tectonic contact that separates serpentinised ultramafic rocks from metamorphosed pillow lavas. Early formed ductile fabrics associated with the main episode of deformation (D₁) preserve bulk flattening strains at greenschist-facies conditions. Emplacement and post-kinematic cooling ages of a pre-D₁ quartz-monzonite dyke constrain the age of D₁/M₁ deformation and metamorphism to the period between ca 258 and ca 248 Ma. Minor brittle deformation (D₂) occurred at ca 230 Ma, based on U–Pb dating of a syn-D₂ diorite dyke (ca 231 ± 10 Ma) and several ca 230 Ma ⁴⁰Ar/³⁹Ar cooling ages. The deformation, metamorphism, and supra-subduction zone magmatism preserved on South Percy Island is correlated with the nearby Marlborough Terrane and more broadly with the second pulse of the Hunter–Bowen Orogeny, which affected much of the central and northern parts of eastern Australia in the late Permian and Early Triassic. Our results support previous suggestions that the second pulse of the Hunter–Bowen Orogeny involved coeval thrust systems in both the inboard and outboard parts of the orogen.     

Paleoproterozoic gold deposits in the Bald Hill and Coyote areas, Western Tanami, Western Australia

Significant gold deposits in the western Tanami region of Western Australia include deposits in the Bald Hill and Coyote areas. The ca. 1,864 Ma Bald Hill sequence of turbiditic and mafic volcanic rocks hosts the Kookaburra and Sandpiper deposits and a number of smaller prospects. The ca. 1,835 Ma turbiditic Killi Killi Formation hosts the Coyote deposit and several nearby prospects. The Kookaburra deposit forms as a saddle reef within a syncline, and the Sandpiper deposit is localized within graphitic metasedimentary rocks along a limb of an anticline. Gold in these deposits is hosted by anastomosing quartz–(–pyrite–arsenopyrite) veins within quartz–sericite schist with disseminated arsenopyrite, pyrite, and marcasite (after pyrrhotite). Based on relative timing relationships with structural elements, the auriferous veins are interpreted to have been emplaced before or during the ca. 1,835–1,825 Ma Tanami Orogeny (regional D₁). Gold deposition is thought to have been caused by pressure drops associated with saddle reef formation (Kookaburra) and chemical reactions with graphitic rocks (Sandpiper). The Coyote deposit, the largest in the western Tanami region, consists of a number of ore lenses localized along the limbs of the Coyote Anticline, which formed during the Tanami Orogeny. The largest lenses are associated with the Gonzalez Fault, which is located along the steeply dipping southern limb of this fold. Gold was introduced at ca. 1,790 Ma into dilatant zones that formed in local perturbations along this fault during later reactivation (regional D₅) towards the end of a period of granite emplacement. Gold is associated with quartz–chlorite–pyrite–(arsenopyrite–galena–sphalerite) veins with narrow (<?5 mm) chloritic selvages. A quartz–muscovite–biotite–K–feldspar–(tourmaline–actinolite–arsenopyrite) assemblage, which is interpreted to relate to granite emplacement, overprints the regional greenschist facies metamorphic assemblage. The mineralogical similarity between this overprinting assemblage and the vein assemblage suggests that the auriferous veins at the Coyote deposit are associated with the granite-related metamorphic–metasomatic assemblage. Gold deposition is thought to have been caused by pressure drops within dilatant zones.     

The late orogenic timing of mineralisation in some slate belt gold deposits,Victoria, Australia

Gold mineralisation in classic Australian slate belt gold deposits at Ballarat, Bendigo, St. Arnaud and Inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main slaty cleavage. This has been revealed through the examination of microstructural relationships in gold-bearing quartz veins and their host rocks from these deposits, which has established a D₁ to D₄ deformation-stage history and consistent timing for gold mineralisation over a wide area. The gold was deposited synorogenically but during the fourth deformation stage (D₄) of the orogeny, a relatively weak event occurring two deformations after the main slaty cleavage producing event, D₂. Previously, D₂ had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event. However, most gold is hosted in breccia veins that formed during D₄. The wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits. This alteration both crosscuts and preferentially mineralises wallrock S₄ allowing the timing of the breccia veins, alteration and gold deposition to be defined as syn-D₄ in age.     

Three Palaeoproterozoic basins—Yerrida,Bryah and Padbury—Capricorn Orogen,Western Australia

The Palaeoproterozoic Yerrida, Bryah and Padbury Basins record periods of sedimentation and magmatism along the northern margin of the Archaean Yilgarn Craton. Each basin is characterised by distinct stratigraphy, igneous activity, structural and metamorphic history and mineral deposit types. The oldest of these basins, the Yerrida Basin (ca 2200 Ma) is floored by rocks of the Archaean Yilgarn Craton. Important features of this basin are the presence of evaporites and continental flood basalts. The ca 2000 Ma Bryah Basin developed on the northern margin of the Yilgarn Craton during backarc sea‐floor spreading and rifting, the result of which was the emplacement of voluminous mafic and ultramafic volcanic rocks. During the waning stages of the Bryah Basin this mafic to ultramafic volcanism gave way to deposition of clastic and chemical sedimentary rocks. At a later stage, the Padbury Basin developed as a retroarc foreland basin on top of the Bryah Basin in a fold‐and‐thrust belt. This resulted from either the collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny) or the ca 2000 Ma westward collision of the southern part of the Gascoyne Complex and the Yilgarn Craton (Glenburgh Orogeny). During the Capricorn Orogeny the Bryah Group was thrust to the southeast, over the Yerrida Group. Important mineral deposits are contained in the Yerrida, Bryah and Padbury Basins. In the Yerrida Basin a large Pb–carbonate deposit (Magellan) and black shale‐hosted gossans containing anomalous abundances of Ba, Cu, Zn and Pd are present. The Pb–carbonate deposit is hosted by the upper units of the Juderina Formation, and the lower unit of the unconformably overlying Earaheedy Group. The Bryah and Padbury Basins contain orogenic gold, copper‐gold volcanogenic massive sulfides, manganese and iron ore. The origin of the gold mineralisation is probably related to tectonothermal activity during the Capricorn Orogeny at ca 1800 Ma.