IN SITU U-Pb MONAZITE GEOCHRONOLOGY RECORDS MULTIPLE EVENTS AT THE MOUNT ISA Cu (± Zn-Pb-Ag) DEPOSIT, NORTHERN AUSTRALIA

Abstract

The Mount Isa Cu (± Zn-Pb-Ag) deposit is the largest Cu deposit in the Western fold belt of the Mount Isa inlier. Previous geochronological studies on the deposit have produced a large range (>150 m.y.) in ages for Cu mineralization and associated hydrothermal alteration. This study combines detailed petrology with in situ monazite U-Pb geochronology on four monazite-bearing samples in order to constrain the age of hydrothermal and tectonic events experienced by the Mount Isa Cu (± Zn-Pb-Ag) deposit and enclosing host shale. Samples EY108402 and EX102476 contain singular subangular monazite grains included in dolomite and siderite, which are associated with premineralization silica-dolomite alteration. Monazite from these samples yields mean weighted 207Pb/206Pb ages of 1587 ± 43 (mean square of weighted deviates [MSWD] = 0.57) and 1623 ± 25 Ma (MSWD = 0.61), respectively. These ages constrain the maximum age of silica-dolomite alteration and Cu mineralization, reflecting monazite growth during periods of peak metamorphism and early basin inversion, respectively. A sample from the 1100 Cu orebody (DDR012-2) contains two clusters of fine-grained monazite that replace siderite associated with silica-dolomite alteration, envelop chalcopyrite, and are crosscut by chlorite-quartz-orthoclase microveins. Monazite from these clusters produces 207Pb/206Pb ages ranging from ca. 1620 to ca. 1360 Ma. The large variation in ages is attributed to variable radiogenic Pb loss from a precursor monazite due to (1) continuous coupled dissolution-reprecipitation reactions over ca. 260 m.y. or (2) partial recrystallization by a ca. 1360 Ma fluid event. As monazite from this sample envelops chalcopyrite, the ca. 1360 Ma age can be used to infer the minimum age of Cu mineralization. Sample 1758-1 is from a highly silicified and fractured section of the Eastern Creek Volcanics located adjacent the deposit. The sampled fracture plane bears a chlorite-illite-rutile infill assemblage with fine-grained irregular-shaped monazite. Monazite from this sample produces a lower intercept age of 1376 ± 32 Ma (MSWD = 1.3) and is interpreted to represent the age of a major fluid flow event coeval with uplift along the Mount Isa fault. The monazite U-Pb geochronology presented in this study brackets the age of Cu mineralization and records the presence of multiple tectonic/hydrothermal events over the history of the deposit and enclosing host rocks.