SOURCES AND TIMING OF THE MINIMUM-MELT MOLE GRANITE, NEW ENGLAND OROGEN, NSW AUSTRALIA

The Mole Granite is a highly mineralized leucogranite that was emplaced in the Lower to Middle Triassic, late in the development of the New England Batholith. It has a minimum-melt composition at ~1-2 kb and is highly enriched in incompatible and fluid-mobile elements such as F, Rb, U, Th, Y, and the REE. Over 1200 largely sub-economic Sn-W and base metal deposits occur within and around the granite. We present a geochronological and geochemical study of textural variants and mineralization in the Mole Granite to better understand the timing of granite emplacement, the nature of its source, and the magmatic to hydrothermal transition.

Zircons were extracted from the coarse-grained main phase and cross-cutting, fracture-controlled microgranites. Zircons from these two lithologies have radically different morphologies. Those from the main phase are euhedral with concentric zoning whereas those from the microgranite have thick metamict rims. However, U-Pb SHRIMP ages in both lithologies are identical at 247 ± 2 Ma. This suggests that the two lithologies were emplaced within a brief interval although the high common Pb in the metamict rims complicates the dating. Microgranites are enriched in fluid-mobile elements such as Cu, Pb, and U, linking them with the mineralization either as fluid conduits or as sources of the mineralizing fluids. Oxygen (+8.0) and Hf (+6.0) isotopic compositions of zircons also show no significant differences between the main phase and microgranite, and indicate a relatively immature source for the Mole Granite despite its highly evolved composition.

Mineralization has been dated directly by Rb-Sr and Sm-Nd isochrons based on tourmaline and U-Pb TIMS dating of cassiterite using novel techniques developed for this study. These data allow a maximum time between zircon and cassiterite precipitation of 2 ± 1 Ma. Initial Sr (0.7065) and Nd (-4) isotopic compositions are consistent with a significant juvenile component in the source.

The Mole Granite was emplaced by multiple injections of highly evolved magma over a relatively short period of time. Mineralization was closely linked with emplacement of the granite at shallow depths. The minimum melt composition probably reflects fractional crystallization of a much larger volume of parental granitic magma at depth.