Magmatic ore deposits in mafic–ultramafic intrusions of the Giles Event, Western Australia

More than 20 layered intrusions were emplaced at c. 1075 Ma across > 100 000 km2 in the Mesoproterozoic Musgrave Province of central Australia as part of the c. 1090–1040 Ma Giles Event of the Warakurna Large Igneous Province (LIP). Some of the intrusions, including Wingellina Hills, Pirntirri Mulari, The Wart, Ewarara, Kalka, Claude Hills, and Gosse Pile contain thick ultramafic segments comprising wehrlite, harzburgite, and websterite. Other intrusions, notably Hinckley Range, Michael Hills, and Murray Range, are essentially of olivine-gabbronoritic composition. Intrusions with substantial troctolitic portions comprise Morgan Range and Cavenagh Range, as well as the Bell Rock, Blackstone, and Jameson–Finlayson ranges which are tectonically dismembered blocks of an originally single intrusion, here named Mantamaru, with a strike length of > 170 km and a width of > 20 km, constituting one of the world's largest layered intrusions. Over a time span of N200 my, the Musgrave Province was affected by near continuous high-temperature reworking under a primarily extensional regime. This began with the 1220–1150 Ma intracratonic Musgrave Orogeny, characterized by ponding of basalt at the base of the lithosphere, melting of lower crust, voluminous granite magmatism, and widespread and near-continuous, mid-crustal ultra-high-temperature (UHT) metamorphism. Direct ascent of basic magmas into the upper crust was inhibited by the ductile nature of the lower crust and the development of substantial crystal-rich magma storage chambers. In the period between c. 1150 and 1090 Ma magmatism ceased, possibly because the lower crust had become too refractory, but mid-crustal reworking was continuously recorded in the crystallization of zircon in anatectic melts. Renewed magmatism in the form of the Giles Event of the Warakurna LIP began at around 1090 Ma and was characterized by voluminous basic and felsic volcanic and intrusive rocks grouped into the Warakurna Supersuite. Of particular interest in the context of the present study are the Giles layered intrusions which were emplaced into localized extensional zones. Rifting, emplacement of the layered intrusions, and significant uplift all occurred between 1078 and 1075 Ma, but mantle-derived magmatism lasted for N50 m.y., with no time progressive geographical trend, suggesting that magmatism was unrelated to a deep mantle plume, but instead controlled by plate architecture. The Giles layered intrusions and their immediate host rocks are considered to be prospective for (i) platinum-group element (PGE) reefs in the ultramafic–mafic transition zones of the intrusions, and in magnetite layers of their upper portions, (ii) Cu–Ni sulfide deposits hosted within magma feeder conduits of late basaltic pulses, (iii) vanadium in the lowermost magnetite layers of the most fractionated intrusions, (iv) apatite in unexposed magnetite layers towards the evolved top of some layered intrusions, (v) ilmenite as granular disseminated grains within the upper portions of the intrusions, (vi) iron in tectonically thickened magnetite layers or magnetite pipes of the upper portions of intrusions, (vii) gold and copper in the roof rocks and contact aureoles of the large intrusions, and (viii) lateritic nickel in weathered portions of olivine-rich ultramafic intrusions

Is the rate of supercontinent assembly changing with time?

To address the question of secular changes in the speed of the supercontinent cycle, we use two major databases for the last 2.5 Gyr: the timing and locations of collisional and accretionary orogens, and average plate velocities as deduced from paleomagnetic and paleogeographic data. Peaks in craton collision occur at 1850 and 600 Ma with smaller peaks at 1100 and 350 Ma. Distinct minima occur at 1700–1200, 900–700, and 300–200 Ma. There is no simple relationship in craton collision frequency or average plate velocity between supercontinent assemblies and breakups. Assembly of Nuna at 1700–1500 Ma correlates with very low collision rates, whereas assemblies of Rodinia and Gondwana at 1000–850 and 650–350 Ma, respectively correspond to moderate to high rates. Very low collision rates occur at times of supercontinent breakup at 2200–2100, 1300–1100, 800–650, and 150–0 Ma. A peak in plate velocity at 450–350 Ma correlates with early stages of growth of Pangea and another at 1100 Ma with initial stages of Rodinia assembly following breakup of Nuna. A major drop in craton numbers after 1850 Ma corresponds with the collision and suturing of numerous Archean blocks. Orogens and passive margins show the same two cycles of ocean basin closing: an early cycle from Neoarchean to 1900 Ma and a later cycle, which corresponds to the supercontinent cycle, from 1900 Ma to the present. The cause of these cycles is not understood, but may be related to increasing plate speeds during supercontinent assembly and whether or not long-lived accretionary orogens accompany supercontinent assembly. LIP (large igneous province) age peaks at 2200, 2100, 1380 (and 1450?), 800, 300, 200 and 100 Ma correlate with supercontinent breakup and minima at 2600, 1700–1500, 1100–900, and 600–400 Ma with supercontinent assembly. Other major LIP age peaks do not correlate with the supercontinent cycle. A thermochemical instability model for mantle plume generation can explain all major LIP events by one process and implies that LIP events that correspond to the supercontinent cycle are independent of this cycle. The period of the supercontinent cycle is highly variable, ranging from 500 to 1000 Myr if the late Archean supercratons are included. Nuna has a duration of about 300 Myr (1500–1200 Ma), Rodinia 100 Myr (850–750 Ma), and Gondwana–Pangea 200 Myr (350–150 Ma). Breakup durations are short, generally 100–200 Myr. The history of angular plate velocities, craton collision frequency, passive margin histories, and periodicity of the supercontinent cycle all suggest a gradual speed up of plate tectonics with time

Mafic-ultramafic intrusions of the Giles event, Western Australia : petrogenesis and prospectivity for magmatic ore deposits

More than a dozen mafic–ultramafic layered intrusions were emplaced across >100 000 km2 in the Musgrave region of central Australia at c. 1075 Ma as part of the c. 1090–1040 Ma Giles Event. The intrusions crystallized from tholeiitic magma of variable composition (170 km and a width of at least 20 km, making it one of the world’s largest layered intrusions. The Giles Event intrusions were emplaced into the Musgrave Province, a complex Proterozoic terrane at the intersection between the West Australian, North Australian, and South Australian Cratons. The region underwent several episodes of orogeny and rifting over a time span of >200 Ma. The oldest event that clearly affected the entire province was the 1220–1150 Ma Musgrave Orogeny. It arose either in an intracratonic setting or as a distal back-arc and featured early, rapid, and substantial lithospheric thinning. These events allowed convecting mantle to be channelled upward along the contacts with surrounding craton keels to the newly exposed base of the Musgrave lithosphere. The result was large-degree mantle melting and subsequent ponding of basalt at (and intrusion into) the base of the lithosphere, lower-crustal melting, voluminous granite magmatism, and widespread mid-crustal ultra-high-temperature (UHT) metamorphism. The ductile (UHT) nature of the lower crust, and the development of substantial crystal-rich magma storage chambers — or melt, assimilation, storage, and homogenization (MASH) zones — prevented ascent of basic magmas into the upper crust. This resulted in the predominantly felsic character of magmatism during the Musgrave Orogeny. The c. 100 Ma (1220–1120 Ma) duration of UHT mid-crustal conditions suggests that re-establishment of lithospheric mantle was significantly retarded. Magmatism largely ceased between c. 1150 and 1090 Ma, possibly because the lower crust became too refractory, or because a buoyant lithospheric mantle began to form. Therefore, the MASH zone may have solidified; however, mid-crustal temperatures remained anomalously high, as suggested by the continued growth of migmatite-related zircon for more than another c. 80 Ma. Renewed mantle melting from c. 1090 Ma onwards led to the magmatism-dominated Giles Event (c. 1090 to 1040 Ma), comprising voluminous basic and felsic volcanic and intrusive rocks grouped into the Warakurna Supersuite. One particularly notable component of the Giles Event was the Warakurna Large Igneous Province, represented by doleritic intrusions that outcrop across ~1.5 million km2 of central and western Australia (Wingate et al., 2004). The source to the Giles basic magmas was largely asthenospheric, reflected by their relatively minor crustal component (low large ion lithophile elements [LILE], ƐNd up to +2), and low Pt/Pd ratios. The long-lasting magmatism and UHT metamorphism in the Musgrave Province suggests that magmatism was plate driven rather than plume driven. In many regards, the Giles Event can be viewed as an extension of the anomalous thermal regime established during the Musgrave Orogeny. Although initial extension and rifting, emplacement of the layered G1 Giles intrusions, and then significant uplift all happened between 1078 and 1075 Ma, mantle-derived magmatism lasted for >50 Ma and is unrelated to a deep mantle plume. Periods of deformation (both extension and compression) during both the Musgrave Orogeny and the Giles Event may be related to far-field compressive influences that allowed the formation of thick sill complexes, ultimately resulting in some of the world’s largest layered intrusions. A comparison of current models of ore formation with the geology generated by the Giles Event indicates that the region has potential prospectivity for the following types of mineral occurrences: - platinum group element (PGE) reefs in the ultramafic–mafic transition zones of layered intrusions, and in magnetite layers in the differentiated portions of the intrusions. Potential PGE reefs are more likely in the early (G1) intrusions, whose parental magmas failed to interact with abundant juvenile sulfur of relatively late-stage felsic volcanic rocks - Cu–Ni sulfide deposits within magma feeder conduits of late basaltic pulses that could assimilate sulfur-rich felsic volcanic rocks - vanadium in the lowermost magnetite layers within the most fractionated intrusions - apatite in the unexposed uppermost magnetite layers of the fractionated intrusions - ilmenite as granular disseminated grains in magnetite layers within the upper portions of the intrusions - iron, particularly in tectonically thickened magnetite layers or magnetite pipes of the upper portions of intrusions - gold and copper in the roof rocks and contact aureoles of the large intrusions, and in associated granites and felsic volcanic rocks - lateritic nickel in weathered portions of the olivine-rich ultramafic portions of intrusions