LA-ICP-MS U–Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt, SE Brazil.

Abstract

The presence of hydrothermal rutile enables the dating of mineralization related to hydrothermal processes. Refinement of the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique is key to enabling the quick analysis of individual rutile crystals in high spatial resolution and dating of crystals with low U contents, as a prerequisite for the reliable characterization of hydrothermal deposits. For this study rutile crystals were selected from quartz and topaz veins that extend along the southeastern portion of the Araçuaí Belt, part of the long-lived (630 - 490 Ma) polyphase Brasiliano Orogen that formed during West Gondwana amalgamation. The purpose was to allow a systematic evaluation of in situ dating of hydrothermal rutile by LA-ICPMS. U–Pb ages from different rutiles range between 527 ± 6 Ma and 487 ± 5 Ma, indicating that different parts of the orogen were affected by episodic fluid flow under oxidizing conditions over a similar time interval. Our U–Pb dating results of rutiles associated with the Imperial Topaz mineralization in the Quadrilátero Ferrífero (southern portion of the São Francisco Craton, SE Brazil) constrains the age of those deposits to 500- 498 Ma, which is within the range of U–Pb ages of hydrothermal systems and mineralized zones previously reported for the region. The chemical composition of the studied hydrothermal rutiles, as determined by electron microprobe and LA-ICP-MS analysis, together with their U–Pb ages, suggests that the extensive fluid production required to form the hydrothermal deposits was generated from multiple sources. Fluid production was related to extensional collapse of the Araçuaí Orogen. The findings of this research clearly shows the utility of U–Pb rutile dating via LA-ICP-MS for geochronological study of a range of hydrothermal deposits.