Abstract

New Re-Os in molybdenite and U-Pb in titanite and zircon age data have been used to discern several discrete alteration, mineralization, and igneous events that occurred in iron oxide-copper-gold (IOCG) deposits in the Selwyn-Mount Dore corridor of the Proterozoic Mount Isa inlier. Two distinct sodic-calcic alteration events that occurred prior to mineralization have been recognized. This suggests that sodic-calcic alteration may be a fundamental precursor to IOCG mineralization as it mobilizes large amounts of metals, including Fe, K, and Cu. Sodic-calcic alteration at Starra was early synmetamorphic (Isan orogeny) at 1594 ± 8 Ma. Some ironstone-hosted mineralization at Starra may have formed during this event, but an Re-Os molybdenite age of 1568 ± 7 Ma suggests that mineralization also occurred during a (late) metamorphic tectonic event. These ages demonstrate that the Starra system is unrelated to igneous activity; a metamorphic fluid source is proposed. Titanite from a preore alteration assemblage at Mount Elliott yields a U-Pb age of 1530 ± 11 Ma which is within error of the ~1515 Ma Re-Os molybdenite ages from both Mount Elliott and the adjacent SWAN deposit. A magmatic-related origin for the Mount Elliott and SWAN deposits is favored given that the titanite and main-stage molybdenite ages are similar to the emplacement ages for the nearby Squirrel Hills Granite. Molybdenite-bearing calcite veins that crosscut main-stage IOCG mineralization at SWAN have been dated at ~1355 Ma, indicating that this paragenetic stage is unrelated to the Squirrel Hills Granite. U-Pb SHRIMP zircon analyses of volumetrically minor trachyandesitic dikes from Mount Elliott and SWAN reveal relatively young crystallization ages of 1119 ± 15 and 1096 ± 10 Ma, therefore they are not a suitable fluid or metal source for these deposits. Two Re-Os molybdenite analyses from Mount Dore (1503 ± 5 and 1508 ± 5 Ma) are similar to the ~1515 Ma ages from Mount Elliott and SWAN emplacement age of and the Mount Dore Granite. However, two other Re-Os dates (1497 ± 6 and 1501 ± 5 Ma) demonstrate that parts of the Mount Dore system may be slightly younger than Mount Elliott and SWAN. This younger timing is consistent with relative timing criteria that demonstrate mineralization postdates the crystallization of the Mount Dore Granite. Molybdenite from Lady Ella gives an age of 1487 ± 5 Ma. Mineralization at Mount Dore and Lady Ella is probably related to a late-stage, evolved magmatic fluid that generated more potassic alteration. The revised geochronological framework for alteration and mineralization in the Cloncurry district partially explains the fundamental differences in geologic characteristics between IOCG deposits in the district and clearly demonstrates that not all of the IOCG occurrences are related to the intrusion of the voluminous Williams-Naraku batholith. Instead metal-rich reservoirs were generated under the Mount Isa inlier by subduction along the southern margin of the North Australia craton; these reservoirs have been periodically tapped during tectonothermal events, including metamorphism during the Isan orogeny and hot spot activity that generated extensive A-type magmatism that have contributed to the significant metal enrichment across the Mount Isa inlier.

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