Rare Earth Minerals as Byproducts of Metamorphosed Volcanogenic Massive Sulfide (VMS) Deposits

Abstract We report on a potential economic source of rare earth elements (REE) within a metamorphosed Archean VMS deposit in Ontario, Canada. High concentrations of monazite, xenotime, and allanite occur in mafic tuffs near the contact with the 2721 ± 0.9 Ma high-silica rhyolites of the Winston Lake VMS deposit in the Wawa Subprovince of the Superior Province. The rhyolites are locally depleted in REE, whereas some of the biotite-chlorite altered mafic tuffs are significantly enriched. A suite of eight samples analyzed from the tuffs returned an average concentration of 0.57 wt.% total rare earth oxides (TREO), with a range of 0.36–0.98 wt.% TREO, and a suite of seven samples returned an average concentration of 0.21 wt.% TREO. Field relationships suggest that the original source of REE was the high silica rhyolites. The rare earths and some of the base metals were mobilized from the rhyolite by high temperature, low pH fluids during metamorphic recrystallization and re-precipitated in the mafic tuffs as rare earth minerals and minor sulfides. The previously established 2667 ± 5 Ma U-Pb age of monazite, xenotime, and allanite in the mafic tuffs defines the time of REE mobility in these rocks (i.e., enrichment in the tuffs and depletion in the rhyolites). The above age coincides with the time of peak metamorphism in the area, which took place >50 m.y. after felsic volcanism and massive sulfide deposition. It is suggested that late metamorphic fluids circulating through reactivated synvolcanic faults and fractures in high silica rhyolites have the potential of removing REE from the rhyolite and re-concentrating them (as rare earth minerals) to economic grade within the adjacent mafic tuffs or porous volcaniclastic rocks at metamorphosed VMS deposits.