Abstract

Sm-Nd isotope and rare earth element geochemical data for U-rare earth element ore of the Mary Kathleen mine and surrounding country rocks reveal a complex history of metasomatic alteration and multistage U-rare mineralization in metasedimentary rocks of the host Corella Formation. Widespread high-temperature contact metasomatism and skarn formation related to the emplacement of the U-rich, 1,730-m.y. A-type Burstall Granite produced large volumes of banded calc-silicate skarn. An Sm-Nd age of 1,766 + or - 80 m.y. (2Sigma ) was obtained on skarn produced at the time of granite emplacement. Sm-Nd data on other skarn types, including massive garnet-diopside skarn which hosts the Mary Kathleen mineralization, suggest that most of the skarn formed during this contact alteration event.The combined Sm-Nd isotope data for whole rocks and minerals of two high-grade ores yield a significantly younger age of 1,472 + or - 40 m.y. (2Sigma ) which is similar to an earlier reported 1,550 + or - 15 m.y. U-Pb age for uraninite. The age discrepancy is considered the result of minor postcrystallization disturbance of the Sm-Nd isotope systems of ore minerals. These ages indicate that the Mary Kathleen mineralization formed long after emplacement of the 1,730-m.y. U-rich Burstall Granite and its related metasomatism. Differences between initial 143 Nd/ 144 Nd ratios of the ore (epsilon Nd = -9.5) and the contemporaneous country rocks including the Burstall Granite and the host skarn (epsilon Nd = -5 to -7.5) at 1,470 m.y. preclude a simple genetic relationship between them.The data suggest a regional metamorphic origin for the present mineralization, probably related to structurally controlled hydrothermal fluid flow around and within massive skarn bodies. The operation of lithological-tectonic control mechanisms on metamorphic fluid circulation has recently been demonstrated for large parts of the Mary Kathleen fold belt, in particular the area affected by massive premetamorphic skarn formation and garnetization. This is supported by a 1,557 + or - 40-m.y. Sm-Nd age for complex garnet-pyroxene-feldspar skarn associated with metamorphic-hydrothermal alteration around prominent D 2 /D 3 deformation and minor rare earth element-U mineralization.The significantly lower epsilon Nd of the ore relative to the country rocks at the time of ore deposition is consistent with an origin from remobilized older, highly light rare earth element-enriched sources. Although other possibilities cannot be ruled out completely, we consider a 1,730-m.y.-old U-rare earth element skarn protore the most likely source for both light rare earth elements and U. Isotopic and mass balance constraints require this protore to be very similar in element contents and ratios to the present mineralization, i.e., it was highly enriched in light rare earth elements and had a long history of a very low Sm/Nd ratio. The preservation of this protore isotopic signature during formation of the present mineralization involved essentially in situ hydrothermal reworking of the protore without significant additions of externally derived rare earth elements.The results of this study demonstrate that combined rare earth element geochemical and Sm-Nd isotope studies provide a powerful tool to date U mineralizing events and to trace the origin of rare earth elements and U in the ore solutions.

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