Rb-Sr provenance ages from weathered and stream transported quartz grains from the Harney Peak Granite, Black Hills, South Dakota

Abstract

Weathered quartz grus and stream transported quartz of the Harney Peak Granite, Black Hills, South Dakota, contain low concentrations of Rb (generally 0.3-6.8 ppm) and Sr (0.2-2.0 ppm) and variable Sr isotopic ratios (0.759-1.070). Six of seven single grains of large composite quartz grus which recently entered the weathering environment define an apparent isochron age (about 1800 Myr) and initial 87Sr86Sr ratio (0.7066) that approximate the whole-rock isochron age (1707 Myr) and initial ratio (0.7143) of the Harney Peak Granite. Apparently the Rb-Sr systematics of these grains were not significantly altered during initial weathering. Leached fluid inclusion material from a ca. 2 g aggregate of composite quartz grains contains very little Rb or Sr (0.019 and 0.17 μg, respectively) and has a very low 87Sr86Sr ratio (0.739). The Rb and Sr content of the quartz grains appears to be concentrated in minute, heterogeneously-distributed mineral inclusions. Five aggregates of more completely weathered, small non-composite quartz grains produce a widely scattered pattern on an isochron diagram with all samples plotting below the 1707 Myr isochron. Examination by SEM of these grains shows solution and precipitation features on their relatively large effective surface areas. The differential precipitation of Rb is believed to have been the major perturbating chemical process during weathering. Three aggregates of stream quartz grains define an apparent isochron age of 1777 Myr and an initial 87Sr86Sr ratio of 0.720 that suggest the initial 'igneous' Rb-Sr characteristics of the stream quartz were re-attained during their transportation, probably as a result of removal of the outer weathered surface by abrasion. The apparent resistance to chemical weathering of stream quartz and quartz which has just entered the weathering environment suggests that this mineral may be extremely useful in studies of provenance and the geochronology of strongly weathered terranes. © 1979.