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

摘要

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.\udOver 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\udgranite magmatism, and widespread and near-continuous, mid-crustal ultra-high-temperature (UHT) metamorphism.\udDirect ascent of basic magmas into the upper crust was inhibited by the ductile nature of the lower crust\udand the development of substantial crystal-rich magma storage chambers. In the period between c. 1150 and\ud1090 Ma magmatism ceased, possibly because the lower crust had become too refractory, but mid-crustal\udreworking 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\udinterest 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\udbetween 1078 and 1075 Ma, but mantle-derived magmatism lasted for N50 m.y., with no time progressive\udgeographical trend, suggesting that magmatism was unrelated to a deep mantle plume, but instead controlled\udby plate architecture.\udThe 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,\ud(iv) apatite in unexposed magnetite layers towards the evolved top of some layered intrusions, (v) ilmenite\udas granular disseminated grains within the upper portions of the intrusions, (vi) iron in tectonically thickened\udmagnetite 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.