Capricorn Orogeny Research Articles

Gold metallogeny of the northern Capricorn Orogen: The relationship between crustal architecture, fault reactivation and hydrothermal fluid flow

The timing and distribution of gold mineralization in Proterozoic orogens is influenced by crustal architecture which is often established long before gold mineralization occurs. Gold occurrences in such settings are commonly associated with crustal-scale faults formed at cratonic margins. Once established, these faults provide critical pathways for hydrothermal and mineralizing fluids which during repeated fault reactivations can result in remobilization or introduction of new auriferous fluids resulting in overprinting gold events. Recently published geochronological data for the northern part of the Proterozoic Capricorn Orogen in Western Australia show it has experienced at least three episodes of gold mineralization occurring at c. 2400, 1770 and 1680 Ma. This information, combined with new data directly linking the timing of hydrothermal activity, gold mineralization and fault reactivation show that the crustal-scale Nanjilgardy and Baring Downs Faults, and their subsidiary structures, were (re)activated during discrete orogenic events. Many of the gold deposits are associated with intracratonic reworking during the 1820–1770 Ma Capricorn Orogeny and 1680–1620 Ma Mangaroon Orogeny. Intracratonic settings are not normally considered prospective for gold mineralization due to a lack of input of juvenile material. However, it appears that repeated hydrothermal fluid flow during intracratonic events, has the potential to upgrade gold mineralization or increase gold endowment throughout the orogen, either through gold remobilization or through introduction of new gold, increasing the potential for economic gold deposits.

Imaging sedimentary basins from high-resolution aeromagnetics and texture analysis

Image texture is an important property of gridded aeromagnetic data that together with the availability of closer survey line-spacings and more sensitive instrumentation, has become more relevant for qualitative interpretations. Problems arise though when it is used as a basis for geological interpretation, since visual textures are difficult to define. This can introduce bias when interpretations are not validated against repeatable and testable quantitative measures. We propose the use of well-established unsupervised texture segmentation techniques that use multi-scale, orientation-selective Gabor filters to automatically domain contrasting textures in aeromagnetic maps. These methods are applied to tilt-derivative reduced-to-pole magnetics over the Palaeoproterozoic Earaheedy Basin, that was deposited over the northeastern Yilgarn Craton in a marginal setting prior to the ca. 1800 Ma Capricorn Orogeny Western Australia. These results are then validated against a geophysically constrained interpretation of the area and a review of the magnetic characteristics of the region. Our results demonstrate how automatic texture domaining from gridded magnetic-field data can produce geologically feasible results that can be used to constrain the architecture of basins that contain magnetic sedimentary rocks.

SHRIMP U–Pb phosphate dating shows metamorphism was synchronous with magmatism during the Paleoproterozoic Capricorn Orogeny

Unlike many Phanerozoic orogens, where the primary effects of orogenic events can be easily determined, Precambrian orogens are commonly characterised by repeated tectonothermal events making it challenging to decipher the geological history. The Capricorn Orogen is a complex Precambrian intraplate orogen located within the West Australian Craton that has been subjected to four separate reworking tectonic events between 1820 and 900 Ma. Although direct U–Pb ages for metamorphism have been obtained for the younger events, there is only limited geochronological data for the oldest event, the 1820–1770 Ma Capricorn Orogeny. This is primarily because of multiple episodes of deformation and metamorphism overprinting and obscuring the original tectonic fabrics and destroying metamorphic chronometers. In this study, we use in situ U–Pb monazite and xenotime geochronology, from a feldspathic metasandstone, a quartz–muscovite–chlorite–garnet pelitic schist, a quartz–muscovite–tourmaline schist and a garnet–biotite–plagioclase pelitic gneiss, to obtain the first direct age constraints for metamorphism during the Capricorn Orogeny in the northern Gascoyne Province. Metamorphism was synchronous with the 1820–1775 Ma magmatism in the northern part, and possibly in the southern part, of the Gascoyne Province. Furthermore, our results hint at a late stage hydrothermal fluid event at ca 1750–1730 Ma, post-dating the magmatism in the northern Gascoyne Province.

Unravelling complex geologic histories using U–Pb and trace element systematics of titanite

Unravelling the spatio-temporal evolution of orogenic terranes requires a comprehensive understanding of the duration and extent of metamorphic events and hydrothermal alteration. Commonly used minerals such as zircon and monazite may not fully record geological histories in complex tectonic settings because their elemental constituents do not react under many metamorphic and metasomatic conditions. Here, we complement the current geochronological record of the Capricorn Orogen, Western Australia, with titanite U–Pb geochronology and geochemistry of felsic intrusive rocks to draw conclusions about the use of titanite in understanding the evolution of orogenic terranes. Because titanite usually incorporates common-Pb and may be variably reset by multiple metamorphic and hydrothermal events, a workflow is provided here for the systematic and robust interpretation of titanite U-Pb data. The addition of trace element data in titanite is particularly effective for differentiating whether a grain is igneous, recrystallized or metamorphic. We have developed several petrogenetic indices to differentiate these three types of titanite using Zr-in-titanite temperature, Th/U, Th/Pb, Al/(Al + Fe), light to heavy rare earth element ratio, and Eu anomalies. The addition of trace element geochemistry can also highlight anomalously radiogenic 207Pb/206Pb reservoirs. Utilization of our workflow in the Capricorn Orogen reveals that titanite ages from the same samples as published zircon U–Pb data range from coeval to several hundreds of Myr of age difference between the two minerals. Titanite geochronology and trace element geochemistry indicates ~20 Myr of previously unrecognized prolonged cooling for the Capricorn Orogeny to ca. 1750 Ma. The spatial extent of the ca. 1210–1170 Ma part of the Mutherbukin Tectonic Event is also broadened significantly farther north and south than previously recognized. Incorporating titanite geochronology and trace geochemistry with more commonly used techniques (e.g., zircon and monazite petrochronology) extends our ability to resolve the complete history of large-scale orogenic terranes.

Linking gold mineralization to regional-scale drivers of mineral systems using in situ U–Pb geochronology and pyrite LA-ICP-MS element mapping

Proterozoic orogens commonly host a range of hydrothermal ores that form in diverse tectonic settings at different times. However, the link between mineralization and the regional-scale tectonothermal evolution of orogens is usually not well understood, especially in areas subject to multiple hydrothermal events. Regional-scale drivers for mineral systems vary between the different classes of hydrothermal ore, but all involve an energy source and a fluid pathway to focus mineralizing fluids into the upper crust. The Mount Olympus gold deposit in the Proterozoic Capricorn Orogen of Western Australia, was regarded as an orogenic gold deposit that formed at ca. 1738 Ma during the assembly of Proterozoic Australia. However, the trace element chemistry of the pyrite crystals closely resembles those of the Carlin deposits of Nevada, with rims that display solid solution gold accompanied by elevated As, Cu, Sb, Hg, and Tl, surrounding gold-poor cores. New SHRIMP U–Pb dating of xenotime intergrown with auriferous pyrite and ore-stage alteration minerals provided a weighted mean 207Pb*/206Pb* date of 1769 ± 5 Ma, interpreted as the age of gold mineralization. This was followed by two discrete episodes of hydrothermal alteration at 1727 ± 7 Ma and 1673 ± 8 Ma. The three ages are linked to multiple reactivation of the crustal-scale Nanjilgardy Fault during repeated episodes of intracratonic reworking. The regional-scale drivers for Carlin-like gold mineralization at Mount Olympus are related to a change in tectonic regime during the final stages of the intracratonic 1820–1770 Ma Capricorn Orogeny. Our results suggest that substantial sized Carlin-like gold deposits can form in an intracratonic setting during regional-scale crustal reworking.

Genesis of the Archean–Paleoproterozoic Tabletop Domain, Rudall Province, and its endemic relationship to the West Australian Craton

ABSTRACTThe Tabletop Domain of the Rudall Province has been long thought an exotic entity to the West Australian Craton. Recent re-evaluation of this interpretation suggests otherwise, but is founded on limited data. This study presents the first comprehensive, integrated U–Pb geochronology and Hf-isotope analysis of igneous and metasedimentary rocks from the Tabletop Domain of the eastern Rudall Province. Field observations, geochronology and isotope results confirm an endemic relationship between the Tabletop Domain and the West Australian Craton (WAC), and show that the Tabletop Domain underwent a similar Archean–Paleoproterozoic history to the western Rudall Province. The central Tabletop Domain comprises Archean–Paleoproterozoic gneissic rocks with three main age components. Paleo–Neoarchean (ca 3400–2800 Ma) detritus is observed in metasedimentary rocks and was likely sourced from the East Pilbara Craton. Protoliths to mafic gneiss and metasedimentary rocks are interpreted to have been emplaced and deposited during the early Paleoproterozoic (ca 2400–2300 Ma), and exhibit age and isotopic affinities to the Capricorn Orogen basement (Glenburgh Terrane). Mid–late Paleoproterozoic mafic and felsic magmatism (ca 1880–1750 Ma) is assigned to the Kalkan Supersuite, which is exposed in the western Rudall Province. The Kalkan Supersuite provided the main source of detritus for mid–late Paleoproterozoic metasedimentary rocks in the Tabletop Domain. Similarities in the age and Hf-isotope compositions of detrital zircon from these metasedimentary rocks and Capricorn Orogeny basin sediments suggests that a regionally extensive, linked basin system may have spanned the northern WAC at this time. The Tabletop Domain records evidence for two metamorphic events. Mid–late Paleoproterozoic deformation (ca 1770–1750 Ma) was high-grade, regional and involved the development of gneissic fabrics. In contrast, early Mesoproterozoic (ca 1580 Ma) high-grade deformation was localised and associated with more widespread, late-stage, greenschist facies alteration. These new findings highlight that the Tabletop Domain experienced a much higher grade of deformation than previously assumed, with a Paleoproterozoic metamorphic history similar to that of the western Rudall Province.

A new Paleoproterozoic tectonic history of the eastern Capricorn Orogen, Western Australia, revealed by U–Pb zircon dating of micro-tuffs

The Earaheedy Basin is one of several Paleoproterozoic siliciclastic basins in the eastern part of the Capricorn Orogen in Western Australia. In the absence of an adequate chronostratigraphic framework, multiple conflicting models have been proposed for the tectonic history of the basin. We present in situ SHRIMP U–Pb zircon dates of 1971±11Ma, 1954±5Ma and 1949±10Ma from three thin tuff horizons (“micro-tuffs”) in the Yelma Formation at the base of the basin succession. These micro-tuffs are only slightly younger than the 1990±6Ma Imbin porphyry, which has been interpreted as a basement inlier. Our new dates, combined with field mapping, suggest that the Imbin porphyry is a volcanic–subvolcanic complex within the basin. Micro-tuffs in the Yelma Formation are more than 50m.y. older than c. 1890Ma micro-tuffs within the overlying Frere Formation. Deposition of the Yelma Formation was coeval with continent–continent collision farther west in the Orogen marked by the 2005–1950Ma Glenburgh Orogeny, and the formation is possibly correlative with peripheral foreland deposits of the Padbury Group. The Frere Formation was deposited in shallow marine conditions within an intracratonic basin about 60m.y. after the Glenburgh Orogeny. The Frere Formation is unconformably or disconformably overlain by the Miningarra Group, the youngest unit of which is the Mulgarra Sandstone. This sandstone was deposited after c. 1865Ma, and derived either from exposed Frere Formation or from a now-buried source within the Capricorn Orogen that was uplifted by inception of the 1820–1770Ma Capricorn Orogeny. Therefore, the Earaheedy Basin comprises at least three distinct, unconformity-bound successions deposited in contrasting tectonic settings.

The Paroo Station Mine supergene lead deposits, Western Australia: Geological and geochemical constraints

The Paroo Station Mine lead project is situated near Wiluna in the central part of Western Australia. It is located in an outlier of the Paleoproterozoic Earaheedy Group overlying the southeast Yerrida Basin. The total Measured and Indicated Mineral Resource is currently (as at 31st of December 2014) estimated at 31.8Mt of ore at 4.4% Pb. The deposits are hosted by deeply weathered carbonate and clastic sediments of the Paleoproterozoic Yelma Formation. The lead-only mineralisation contains only traces of sulfides and is composed of few supergene minerals, predominantly cerussite and anglesite with minor pyromorphite and plumbogummite. The Paroo Station Mine demonstrates a high degree of supergene Pb mobility which has resulted in almost complete destruction of primary Pb distribution pattern and development of characteristic for the oxidation zones coupled depletion zone and subhorizontal enrichment blankets below.The deposits are interpreted to have formed from Mississippi Valley-type (MVT), stratabound relatively low-grade sulfide PbZn mineralisation located within dolomites of the Sweetwaters Well Member of the Yelma Formation. The primary mineralisation based on modelling the Pb distribution was controlled by a network of northeast, northwest and meridional oriented fractures and small faults. Lead isotope study yielded from 1.80 to1.88Ga ages of the mineralisation based on the Stacey and Kramers (1975) two-stage model; the mineralising event is related to the Capricorn Orogeny. This data is well in line with previous UPb dating of monazites interpreted to be related to the mineralisation (1.82Ga; Muhling et al., 2012). High μ (238U/204Pb) values (10.26–10.58) are indicative of the Archean basement contribution and/or more likely with sediments originally derived from that basement. The Paroo Station Mine mineralisation is thought to be similar in style and formed synchronously with the MVT prospects in the Teague area in the Earaheedy Basin approximately 120km to the northeast.The mineralisation at the Paroo Station Mine was significantly upgraded through a multi-stage supergene remobilisation and reprecipitation of lead in the course of long weathering history of the region. Lead was almost completely depleted from the upper zone and accumulated below as subhorizontal enrichment blankets at three levels (from top to bottom): (1) the base of the collapse quartz-clay breccia unit marking the dolomite weathering front; (2) the contact of siltstone and sandstone packages coincident with the feldspar/kaolinite weathering front and; (3) the mid- to lower parts of the sandstone unit close to the base of weathering. The lowest enrichment zone extends as a continuous blanket at a significant, up to 500m distance (at a 1% Pb cut-off) beyond the upper mineralisation projection. This enrichment is interpreted to be formed by lateral Pb transport in waters close to the groundwater table through the most permeable layers of weathered sandstones. This active lateral lead dispersion is of high exploration significance for the region providing a much larger exploration target and vector to the primary mineralisation.

New age constraints on metamorphism, metasomatism and gold mineralisation at Plutonic Gold Mine, Marymia Inlier, Western Australia

ABSTRACTThe Plutonic Well Greenstone Belt (PWGB) is located in the Marymia Inlier between the Yilgarn and Pilbara cratons in Western Australia, and hosts a series of major Au deposits. The main episode of Au mineralisation in the PWGB was previously interpreted to have either accompanied, or shortly followed, peak metamorphism in the late Archean at ca 2650 Ma with a later, minor, event associated with the Capricorn Orogeny. Here we present new Pb isotope model ages for sulfides and Rb–Sr ages for mica, as well as a new 207Pb–206Pb age for titanite for samples from the Plutonic Gold Mine (Plutonic) at the southern end of the PWGB. The majority of the sulfides record Proterozoic Pb isotope model ages (2300–2100 Ma), constraining a significant Au mineralising event at Plutonic that occurred >300 Myr later than previously thought. A Rb–Sr age of 2296 ± 99 Ma from muscovite in an Au-bearing sample records resetting or closure of the Rb–Sr system in muscovite at about the same time. A younger Rb–Sr age of 1779 ± 46 Ma from biotite from the same sample may record further cooling, or resetting during a late-stage episode of metasomatism in the PWGB. This could have been associated with the 1820–1770 Ma Capricorn Orogeny, or a late-stage hydrothermal event potentially constrained by a new 207Pb–206Pb age of 1725 ± 26 Ma for titanite in a chlorite–carbonate vein. This titanite age correlates with a pre-existing age for a metasomatic event dated at 1719 ± 14 Ma by U–Pb ages of zircon overgrowths in a sample from the Marymia Deposit. Based on the Pb-isotope data presented here, Au mineralising events in the PWGB are inferred to have occurred at ca 2630, 2300–2100 Ma, during the Glenburgh and Capricorn orogenies, and 1730–1660 Ma. The 2300–2100 Ma event, which appears to have been significant based on the amount of sulfide of this age, correlates with the inferred age for rifting of the Marymia Inlier from the northern margin of the Yilgarn Craton. The texturally-later visible Au may have been deposited during the Glenburgh and Capricorn orogenies.

A Major Geophysical Experiment in the Capricorn Orogeny, Western Australia

A major geophysical experiment has begun in the Capricorn Orogen in Western Australia. Orogen-scale passive seismic and magnetotelluric surveys are on-going and preliminary results suggest have successfully delineated the base of the crust and major structures and tectonic boundaries. Airborne electromagnetic data have successfully mapped features in the near-surface such as palaeovalleys.The integration of the different geophysical datasets with each other and with parallel geological studies are intended to lead to a better understanding the Capricorn Orogen and develop exploration approaches and appropriate toolkits that significantly improve our ability to prospect under cover.

U–Pb geochronology and paleomagnetism of the Westerberg Sill Suite, Kaapvaal Craton – Support for a coherent Kaapvaal–Pilbara Block (Vaalbara) into the Paleoproterozoic?

Precise geochronology, combined with paleomagnetism on mafic intrusions, provides first-order information for paleoreconstruction of crustal blocks, revealing the history of supercontinental formation and break-up. These techniques are used here to further constrain the apparent polar wander path of the Kaapvaal Craton across the Neoarchean–Paleoproterozoic boundary. U–Pb baddeleyite ages of 2441±6 Ma and 2426±1 Ma for a suite of mafic sills located on the western Kaapvaal Craton in South Africa (herein named the Westerberg Sill Suite), manifests a new event of magmatism within the Kaapvaal Craton of southern Africa. These ages fall within a ca. 450 Myr temporal gap in the paleomagnetic record between 2.66 and 2.22Ga on the craton. Our older Westerberg Suite age is broadly coeval with the Woongarra magmatic event on the Pilbara Craton in Western Australia. In addition, the Westerberg Suite on the Kaapvaal Craton intrudes a remarkably similar Archean-Proterozoic sedimentary succession to that on the Pilbara Craton, supporting a stratigraphic correlation between Kaapvaal and Pilbara (i.e., Vaalbara). The broadly coeval Westerberg–Woongarra igneous event may represent a Large Igneous Province. The paleomagnetic results are more ambiguous, with several different possibilities existing. A Virtual Geomagnetic Pole obtained from four sites on the Westerberg sills is 18.9°N, 285.0°E, A95=14.1°, K=43.4 (Sample based VGP, n=34: 16.8°N, 2879.9°E, dp=4.4°, dm=7.7°). If primary (i.e., 2441–2426 Ma), it would provide a further magmatic event within a large temporal gap in the Kaapvaal Craton's Paleoproterozoic apparent polar wander path. It would suggest a relatively stationary Kaapvaal Craton between 2.44 Ga and 2.22 Ga, and ca. 35° of latitudinal drift of the craton between ca. 2.66 Ga and 2.44 Ga. This is not observed for the Pilbara Craton, suggesting breakup of Vaalbara before ca. 2.44 Ga. However, it is likely that the Woongarra paleopole represents a magnetic overprint acquired during the Ophtalmian or Capricorn Orogeny, invalidating a paleomagnetic comparison with the Westerberg Sill Suite. Alternatively, our Westerberg Virtual Geographic Pole manifests a 2.22 Ga magnetic overprint related to Ongeluk volcanism. The similarity between Ongeluk and Westerberg paleopoles however may also infer magmatic connections if both are primary directions, despite the apparent 200 million year age this difference.

Hydrothermal origin of the Paleoproterozoic xenotime from the King Leopold Sandstone of the Kimberley Group, Kimberley, NW Australia: Implications for a ca 1.7 Ga far-field hydrothermal event

A mineral chemical study by electron microprobe microanalyser was carried out to decipher the origin of xenotime overgrowths from the King Leopold Sandstone, the Kimberley Group, NW Australia. Results show that the King Leopold Sandstone xenotime overgrowths are characterised by LREE depletion and MREE–HREE enrichment. The King Leopold Sandstone xenotime overgrowths also show higher Zr content and lower U, Fe, and Lu contents, suggestive of hydrothermal origin rather than diagenetic origin, as previously claimed. The intrusive contact relationships between the Hart Dolerite and King Leopold, Warton and Pentecost sandstones suggest that xenotime overgrowths from the latter two units should also be hydrothermal in origin. The differing U and Th concentrations suggest a different chemical pore fluid would have been responsible for the formation of xenotime overgrowths from the King Leopold, Warton and Pentecost sandstones as the depth of siliciclastic sediments reached the zone of xenotime precipitation. The ca 1.7 Ga ages of xenotime overgrowths from the King Leopold Sandstone can be equated with the ca 1.7 Ga xenotime overgrowth ages obtained from the Warton and Pentecost sandstones of the same group and the ca 1.7 Ga zircon U–Pb ages from the Dingo Granite in northern Australia, and together they are consistent with the age of Capricorn orogeny. As such, the ca 1.7 Ga ages point to a common hydrothermal event possibly associated with the low temperature far-field Dingo Granite intrusion during Capricorn orogeny in the Paleoproterozoic Australia. The Paleozoic Alice Springs Orogeny resulted in radiogenic lead loss and thus is likely responsible for the 1.6–1.3 Ga discordant ages from the King Leopold, Warton and Pentecost sandstones.

Dating fluid flow and Mississippi Valley type base-metal mineralization in the Paleoproterozoic Earaheedy Basin, Western Australia

The ages of deposition and metamorphism of low-grade Precambrian metasedimentary sequences can be difficult to define in the absence of interlayered volcanogenic rocks. Monazite and xenotime can grow at temperatures below 400°C and can give vital evidence for the timing of diagenesis, hydrothermal fluid flow, and low-grade deformation and metamorphism in Precambrian basins. The histories of sedimentary basins in the eastern Capricorn Orogen of the West Australian Craton are generally poorly constrained. However, sandstones and siltstones of the basal Yelma Formation in the Earaheedy Basin contain authigenic monazite and xenotime associated with sulphide minerals related to Mississippi Valley Type base-metal mineralization hosted within carbonate of the overlying Sweetwaters Well Member. In the Teague area, within the Earaheedy Basin, siliciclastic rocks overlying granites of the Yilgarn Craton contain authigenic monazite that gives an in situ SHRIMP 207Pb/206Pb age of 1811±13Ma. Monazite in weathered sandstones and siltstones of the Cano secondary Pb deposit on Magellan Hill, overlying the Yerrida Basin, gives an indistinguishable age of 1815±13Ma. The Cano monazite is interpreted to have been intergrown with primary sulphide minerals and therefore to record the age of mineralization. Xenotime outgrowths on detrital zircons from Cano have high common Pb but define an isochron with an age of 1832±36Ma. The isochron is co-linear with Pb isotope ratios from Pb ore at Magellan, suggesting that xenotime growth is also related to mineralization. The timing of fluid flow is synchronous with the 1820–1770Ma Capricorn Orogeny, and is the first evidence for activity of this age in the eastern part of the Capricorn Orogen. The age of mineralization provides a firm minimum age for the Yelma Formation, and constrains its deposition to between ∼2.0Ga, the youngest age of detrital zircons, and the mineralization age.

Spatio-temporal evolution of the Satpura Mountain Belt of India: A comparison with the Capricorn Orogen of Western Australia and implication for evolution of the supercontinent Columbia

Reconstruction of the Neoproterozoic supercontinent Rodinia shows near neighbour positions of the South Indian Cratons and Western Australian Cratons. These cratonic areas are characterized by extensive Paleoproterozoic tectonism. Detailed analysis of the spatio-temporal data of the Satpura Mountains of India indicates presence of at least three episodes of Proterozoic orogeny at ∼2100–1900Ma, ∼1850Ma and ∼1650Ma, and associated basin development and closing. A subdued imprint of the Grenville orogeny (∼950Ma) is also found in rock records of this Mountain Belt. The Capricorn Orogen of Western Australia also shows three episodes of orogeny: Opthalmian–Glenburgh Orogeny (2100–1950Ma), Capricorn Orogeny (∼1800Ma) and Mangaroon Orogeny (∼1650Ma), and basin opening and closing related to these tectonic movements. These broad similarities suggest their joint evolution possibly in a near neighbour position during Paleoproterozoic Era. In view of juxtaposition of the Western Australia along the east coast of India, at the position of the Eastern Ghats, during Archean, it is suggested that the breaking of this Archean megacraton at ∼2400Ma led to northward movement of the broken components and formation of the Satpura–Capricorn Orogen (at ∼2100 and ∼1800Ma) due to the collision of cratonic blocks with the pre-existing northern cratonic nuclei of India and Western Australia. This is also the time of formation of the supercontinent Columbia. A phase of basin opening followed the ∼1800Ma event, followed by another phase of collisional event at ∼1600Ma at the site of the Satpura–Capricorn Orogen. Subsequent evolutions of the Satpura and the Capricorn Orogens differ slightly, indicating separate evolutional history.

Tectonic evolution of the Satpura Mountain Belt: A critical evaluation and implication on supercontinent assembly

The Satpura Mountain Belt (also referred as Central Indian Tectonic Zone in recent literature) forms an important morphotectonic unit in the central part of India. Some of the recent workers have reported an orogenic event at ∼1000–900 Ma (termed “Sausar orogeny”) which led to amalgamation of the North Indian Block and the South Indian Block and formation of the Satpura Mountain Belt. In this model the stratigraphic relations of two important lithostratigraphic units on either side of the Satpura Mountain Belt (the Sausar Group in the south and the Vindhyan Supergroup on the north) are suggested to be revised from previously held ideas. Critical analyses of available published work in the region to assess the status of the Sausar Group vis a vis the Vindhyan Supergroup was carried out. It is found that the ideas proposed by the recent workers stem from an earlier interpretation that the Sausar Group has monocyclic evolution and the earliest fabric in the Sausar Group is marked by a schistosity with EW strike. Re-mapping of the gneissic rocks and adjacent matasedimentary rocks of Khawasa, Deolapar, and Kandri–Mansar areas revealed presence of gneissic rocks and granulites of two generations, and of four phases of superposed deformations in the metasediments and gneisses. The older gneisses and granulites constitute the basement over which the rocks of the Sausar Group were deposited; and the younger gneisses developed by metamorphism and migmatisation of the rocks of the Sausar Group. The latter types are found in the Khawasa–Ramakona areas. Contrary to the belief of the recent workers that no volcanic activity is present in the Sausar Group, volcanic rocks marked by amygdular basic flows and tuffs have been mapped from different parts of the Sausar Group. Migmatisation and metamorphism of these volcanic rocks (of the Sausar Group) have given rise to amphibolites and granulites in Khawasa and Ramakona areas. Therefore, the use of fabric patterns in these areas to suggest that the granulite facies metamorphism in the Ramakona–Katangi granulite domain was pre-Sausar in age is debatable. Available geochronological data of the Satpura Mountain Belt and its eastward continuation into the Chhotanagpur Gneiss terrain indicate that the basement and cover rocks of these areas were subjected to multiple deformation and metamorphic episodes of similar style and nature. The earliest deformation and metamorphism of the rocks of the Sausar Group and its equivalent rocks to the east took place at ∼2100–1900 Ma. The regional EW trend of the belt developed during the second deformation at ∼1800–1700 Ma and again at ∼1600–1500 Ma. This deformation was accompanied by migmatisation and granulite facies metamorphism in the northern domain of the Sausar Belt and in the Chhotanagpur Gneiss region. Late phase low intensity deformations in the region were associated with thermal events at ∼1100–1000 Ma and ∼900–800 Ma. The ∼EW trending fabric, referred as “Satpura orogenic trend” in Indian literature marks a major compressional tectonic event, developed during the second deformation of the Sausar Group. This has its counter part in Western Australia as the Capricorn orogeny (∼1780–1830 Ma). The development of the Satpura Mountain Belt during the Grenvillian orogeny is ruled out from the synthesis of event stratigraphic data of the region and from its comparison with the Western Australian Craton.

In situ U–Th–Pb geochronology of monazite and xenotime from the Jack Hills belt: Implications for the age of deposition and metamorphism of Hadean zircons

Quartz-pebble conglomerate and quartzite at Eranondoo Hill in the Jack Hills belt in the northwestern Yilgarn Craton, Australia, contain monazite and xenotime grains in heavy mineral layers. In situ SHRIMP U–Th–Pb geochronology of detrital monazite and xenotime, as well as monazite and xenotime inclusions in detrital quartz, gives ages between 3.3 and 3.1 Ga. The conglomerates, which host abundant Hadean zircon grains, contain metamorphic monazite in the muscovite matrix that yields an age of 2653 ± 5 Ma. This date is interpreted to represent the time of low-grade metamorphism and therefore is a direct minimum age for sediment deposition. The conglomerates also contain authigenic xenotime that gives an age of 800 ± 25 Ma, which suggests that part of the succession was affected by a previously unrecognised Neoproterozoic event. Authigenic xenotime in conglomerate immediately to the east of Eranondoo Hill gives an age of 2660 ± 10 Ma. Metamorphic monazite in garnet-bearing quartz-mica schist from the middle of the belt records an age of 1858 ± 6 Ma, which likely reflects an episode of deformation and low-grade metamorphism associated with major shearing. Several monazite grains from the schist indicate that the centre of the belt also experienced metamorphism at ∼2.65 Ga. Massive magnetite–hematite ore from banded iron formation in the northeastern part of the belt contains authigenic xenotime that formed at 3080 ± 20 Ma. Co-existing monazite gives a similar age (3080 ± 35 Ma), indicating that at least part of the belt was deposited before ∼3.08 Ga. A second population of xenotime was identified, indicating a later episode of heating and fluid flow at 1820 ± 25 Ma, possibly related to reactivation during the 1.83–1.78 Ga Capricorn orogeny. The sedimentary rocks dated in this study were deposited before ∼2.65 Ga, and in some cases, before ∼3.08 Ga, indicating that much of the Jack Hills belt is Neoarchean or older. The metasedimentary rocks, and the Hadean zircons they preserve, experienced a protracted history of deformation and heated fluid flow spanning more than two billion years (3080-800 Ma).

Proterozoic deformation in the northwest of the Archean Yilgarn Craton, Western Australia

The Narryer Terrane within the northwestern Yilgarn Craton contains the oldest crust in Australia. The Jack Hills greenstone belt is located within the southern part of the Narryer Terrane, and structures cutting it and surrounding rocks have been dated using the 40Ar/ 39Ar technique. The results show that east-trending, dextral, transpressive shearing was related to the 1830–1780 Ma Capricorn Orogeny, followed by further deformation and/or cooling between c. 1760 and 1740 Ma. These results confirm that major deformation has affected the northwestern part of the Yilgarn Craton in an intracratonic setting during the Proterozoic. Proterozoic structures have been interpreted to extend south beyond the Narryer Terrane into the northern part of the Youanmi Terrane (Murchison Domain), and include the Yalgar Fault, previously interpreted as the boundary between the Narryer and Youanmi Terranes. Terrane amalgamation pre-dated the emplacement of c. 2660 Ma granites in both terranes, and the current expression of the Yalgar Fault must represent a younger, reworked, post-amalgamation structure, possibly controlled by the tectonic boundary. However, new aeromagnetic and gravity imagery does not show the eastern part of the Yalgar Fault as a major structure. Its signature is more akin to a series of east- to east-northeast trending faults that are interpreted to be Proterozoic in age. This suggests that this part of the Yalgar Fault may not be a terrane boundary, and is possibly no older than Proterozoic. The 40Ar/ 39Ar dating also shows a younger, less intense deformation and/or cooling event at c. 1172 Ma.

The Jack Hills greenstone belt, Western Australia: Part 2: Lithological relationships and implications for the deposition of ≥4.0 Ga detrital zircons

The Jack Hills greenstone belt is situated along the southern margin of the Narryer Terrane in the northwest of the Yilgarn Craton, in fault contact with Archean granitic gneiss and granitic rocks. The belt has endured a long deformation history and the effects of recrystallisation, alteration, and the occurrence of a substantial proportion of lithologically similar metasedimentary rocks makes it difficult to correlate units and assign a stratigraphy. We have therefore divided the lithological units into four associations. These are: (1) an association of banded iron formation, chert, quartzite, mafic and ultramafic rocks; (2) an association of pelitic and semi-pelitic schist, quartzite, and mafic schist; (3) an association of mature clastic rocks including pebble metaconglomerate; (4) an association of Proterozoic metasedimentary rocks. Some of the second association was probably part of the same succession as association 1. The first two associations are of Archean age, were probably deposited at c. 3000 Ma, and have been intruded by Neoarchean granitic rocks. The mature clastic rocks of association 3 host ≥4.0 Ga detrital zircons that have been the focus of most previous work on the belt because they provide insight into early Earth processes. The mature clastic association may also have been deposited at c. 3000 Ma, but there is no direct evidence of intrusion by the Neoarchean granites, and it has a different, simpler, structural history than the first two associations. The maximum depositional age and deformation history of the Proterozoic metasedimentary rocks of association 4 suggest it may have been deposited during the late stages of the Capricorn Orogeny. This could have occurred in small transpressional pull-aparts within the Cargarah Shear Zone, which is a major, dextral, transpressional east-trending shear zone that was active during the Capricorn Orogeny between c. 1830 and c. 1780 Ma. Reactivation of the shear zone and overprinting by semi-brittle faulting has produced a complex network of fault slivers, and further sedimentation may have occurred after the Capricorn Orogeny.

The Jack Hills greenstone belt, Western Australia: Part 1: Structural and tectonic evolution over >1.5 Ga

The Jack Hills greenstone belt is situated within the Narryer Terrane, Western Australia, and is an approximately 70 km long greenstone and metasedimentary belt surrounded by granitic gneisses and various granitic rocks. Detailed fieldwork, supported by previous geochronological data shows that the Jack Hills greenstone belt has undergone a long-lived, complex deformation history, spanning at least 1.5 billion years. This includes, from oldest to youngest: (1) an early stage of recumbent and chevron folding probably associated with thrust faulting; (2) east-trending, major transpressional shearing including jog formation; (3) followed by kink and conjugate-style folding, brittle faulting, and fault reactivation. Transpressional shearing is interpreted to have taken place during the Paleoproterozoic Capricorn Orogeny. Strong overprinting and reworking with coaxial geometry gives the belt an apparent simplistic structural style that in places appears to be dominated by a single foliation. Pronounced static recrystallisation and fluid flow also mask structural complexity. Satellite and aeromagnetic imagery illustrate the large-scale geometry and show that the belt has a pronounced sigmoidal curvature, suggestive of dextral kinematics associated with transpression along major, east-trending structures. The majority of kinematic indicators in the field support this interpretation. The various structures are not developed uniformly across all lithological units within and outside the belt. Structural analysis, and an increased understanding of the lithological relationships between the various units, have allowed distinctions to be made between successive tectonic events. This has provided key criteria towards understanding the depositional timing and character of the units, and understanding and relating formation of structural elements such as mineral lineations and dominant foliations to specific events in time and space. On a regional scale, the Jack Hills greenstone belt shares some structural similarities with the nearby Errabiddy Shear Zone, which marks the northern margin of the Yilgarn Craton. This, and relationships in the Jack Hills greenstone belt shows that the Paleoproterozoic Capricorn Orogeny has affected a greater portion of the Yilgarn Craton than previously thought. Unravelling these complex structural relationships is a fundamental step towards understanding granite-greenstone formation, and the early evolution of one of Earth's oldest crustal fragments.

Structural controls of bedded iron ore in the Hamersley Province, Western Australia – an example from the Paraburdoo Ranges

Structure is the most important control on the location of hematite deposits in the Hamersley Province in the Pilbara region of Western Australia. Unravelling the structural history of mineralised areas, however, is complex because the Province underwent three orogenic events, and three regional extension events followed the main episodes of compression. Steeply E to ENE dipping normal faults immediately predated deposition of the Beasley Quartzite of the lower Wyloo Group after the Ophthalmian Orogeny. Steeply NE and SW–S dipping normal faults predated deposition of the McGrath formation of the middle Wyloo Group, following the Panhandle Orogeny. The resulting interpreted rift was subsequently reactivated and tilted during the Capricorn Orogeny. Late NE trending normal faults are confined to the eastern half of the Province. High-grade hematite deposits occur in rocks that range from intensely folded (Mt Whaleback, Giles Mini) to rocks with less, or only little deformation (Paraburdoo, Channar), suggesting that folding had at best limited influence on ore genesis. In contrast, extensional normal faults are always associated with high-grade hematite deposits, and in particular faults that link the underlying dolomites of the Wittenoom Formation with the overlying iron formation, thus providing a pathway for mineralising alkaline fluids.