CASSITERITE U-Pb GEOCHRONOLOGY OF THE SANTA BARBARA TIN DISTRICT, RONDONIA TIN PROVINCE, BRAZIL

The Mesoproterozoic Rondonia Tin Province of the Amazonian craton records a protracted history of about 600 m.y. of successive rare-metal granite intrusions and hosts the youngest known event of tin-granite emplacement of the craton-a rare-metal granite suite known as the Younger Granites of Rondonia intrusive suite. The similar to 1 Ga suite is currently interpreted as intracratonic magmatism resulting from a Grenvillian-age orogeny during the assembly of Rodinia. The Santa Barbara massif is a tin-granite system of the Younger Granites of Rondonia intrusive suite that hosts Sn-Nb-Ta-W-bearing endogreisen and stockwork, as well as important placer deposits. The Santa Barbara mine produces about 800 to 1,000 t Sn/year from placers and weathered greisen and represents about 20% of the tin mine output of the Rondonia Tin Province. Here, we report laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) cassiterite U-Pb ages of 989 +/- 3 and 987 +/- 6 Ma for the Santa Barbara greisen and the cassiterite-quartz vein system, respectively. Alluvial cassiterite from placer mining has a U-Pb age of 995 +/- 4 Ma, which is, within uncertainty, indistinguishable from those of primary cassiterite. These ages agree well with the previously published zircon and monazite U-Pb ages for the Santa Barbara granite (978 +/- 13 and 989 +/- 13 Ma), which indicate a coeval relationship between hydrothermal tin mineralization and granite magmatism. The previously suggested 20-to 30-m.y. time span between granite magmatism and hydrothermal tin mineralization, which was based on mica K-Ar and Ar-Ar age data, is likely due to younger thermal disturbance of the isotopic systems.