Abstract

Proterozoic Fe oxide Cu-Au-Co mineralization in the Cloncuriy district is spatially and temporally related to intrusion of voluminous postpeak metamorphic granites that form most of the Williams and Naraku batholiths. Detailed mapping and geochemical study of these granites in the southern part of the district have identified two groups of intrusions that are referred to here as the Cloncurry Supersuite and Eureka Supersuite.The Cloncurry Supersuite is composed mainly of monzogranite and syenogranite which consist of quartz, plagioclase, alkali feldspar, hornblende, biotite with accessory magnetite, titanite, apatite, and zircon. The timing of intrusion of the Wiley Igneous Complex is interpreted to be pre- or syn-D 3 , based on overprinting relationships, while hornblende geobarometry suggests an emplacement depth of approximately 14 km. The granites are subalkaline, high-K, metaluminous rocks and range from 59 to 74 wt percent SiO 2 . Increasing SiO 2 is accompanied by increasing K 2 O, Rb, and Th, and decreasing MgO, Fe 2 O 3 , FeO, TiO 2 , CaO, P 2 O 5 , Sr, Eu, Zr, Cu, Zn, Ni, V, and Sc. This is consistent with fractionation of observed phases including plagioclase, amphibole, biotite, magnetite, titanite, and apatite.The Eureka Supersuite ranges from diorite to syenogranite in composition. More mafic varieties contain pyroxene and hornblende, while more felsic varieties contain biotite as the only ferromagnesian silicate. Sodiccalcic alteration events are closely linked in space and time to intrusion of Eureka Supersuite magmas. The timing of intrusion is interpreted to be pre- and post-D 3 , based on overprinting relationships between different plutons and mylonitic fabrics. Hornblende geobarometry suggests an emplacement depth of approximately 10 km for the Mount Angelay Igneous Complex. Eureka Supersuite granites in the Mount Angelay Igneous Complex range from 51 to 76 wt percent SiO 2 . The granites are alkaline and K 2 O-rich, leading to their classification as a shoshonite series. With increasing SiO 2 content, TiO 2 , Al 2 O 3 , Fe 2 O 3 (T), MnO, MgO, CaO, P 2 O 5 , Sr, Eu, Cu, Zn, Ni, V, and Sc decrease, indicating fractionation of observed minerals including apatite, magnetite, pyroxene, hornblende, plagioclase, and biotite. Some samples are characterized by very high abundances of K 2 O, P 2 O 5 , Sr, Ba, and light rare earth elements (LREE) compared to other Eureka Supersuite samples. This may reflect their derivation from a source that was variably enriched in incompatible elements. Compared with many other shoshonitic rocks, high-K Eureka Supersuite samples are strongly depleted in Sr and to a lesser extent Eu, and enriched in Y and heavy rare earth elements (HREE). This suggests that garnet was not a stable phase in the source region, that the source contained plagioclase as a stable phase, and/or the granites evolved by extensive plagioclase fractionation.Partial melting in the lower crust to generate post-1540 Ma granites was most likely caused by intrusion of basic magma into the crust. Eureka Supersuite magmas were probably derived from partial melting of variably incompatible element-enriched gabbroic rocks, while Cloncurry Supersuite magmas were most likely derived from partial melting of more felsic material of dioritic to tonalitic composition.

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