Mica geochemistry, and paired in situ [superscript87]Rb/[superscript87]Sr and in situ [superscript40]Ar/[superscript39]Ar geochronology of the Brazil Lake Li-Cs-Ta pegmatite, Nova Scotia, Canada

Abstract

The timing of critical mineral deposit formation is challenging to determine, particularly in areas with a complex tectono-thermal history. We apply in situ 87Rb/87Sr geochronology to investigate the emplacement timing of the Brazil Lake pegmatite Lithium-Cesium-Tantalum deposit, Nova Scotia, Canada. The novel in situ method for 87Rb/87Sr geochronology is compared with the more established 40Ar/39Ar method. Six mica types were identified in and around the pegmatite. Geochronology of the pegmatite is dictated by the initial mineralization age, determined from the oldest 87Rb/87Sr date of 390.9±1 Ma, and the maximum resetting/re-equilibration age ca. 330-320 Ma. Quartz-hosted fluid inclusions indicate post-solidus temperatures were sufficient to reset 40Ar/39Ar, but not 87Rb/87Sr systematics. Re-equilibration was driven by a combined thermal-fluid event recognized in the BLP and southern Nova Scotia.