GEOSTATISTICAL MODELING OF FIELD SPECTRA TO MAP VARIATION IN WHITE MICA COMPOSITION AND METAMORPHIC INTENSITY IN THE BLACK HILLS, SOUTH DAKOTA

Visible/near infrared spectroscopy of the Al-OH absorption feature of white mica offers a field-based technique to quickly and economically map regional trends in metamorphic intensity. Wavelength variations of the Al-OH feature is a monitor of the aluminoceladonite exchange [(Mg,Fe)²⁺_Oct + Si⁴⁺_Tet = Al³⁺_Oct + Al³⁺_Tet] in white mica. Previous studies in the Lower Proterozoic metamorphic rocks of Black Hills, South Dakota suggested a strong relationship between Al-OH wavelength and metamorphic grade. However, those studies included a limited range in rock types, raising the question of whether bulk composition or mineral assemblage might complicate the relationship between Al-OH wavelength and metamorphic grade. Furthermore, in previous studies, Al-OH wavelength spatial variations suggested the possibility of unmapped anomalies in metamorphic intensity.

Field spectra of 1952 rock samples were acquired in greenschist and lower amphibolite facies metasedimentary rocks in the Black Hills. The wavelength of the Al-OH band shifts from 2218 nm in the biotite zone to 2195 nm in the staurolite zone, representing a decrease in Al-OH wavelength with increasing metamorphic temperatures. Semiquantitative white mica compositions from 45 representative thin sections demonstrated a systematic increase in Al and decrease of Mg, Fe, and Si in samples with decreasing Al-OH wavelength, representing the aluminoceladonite exchange.

Spatial variations in Al-OH wavelength follow previously established metamorphic patterns; however, regions of anomalous Al-OH values were identified. These include low wavelength values along the Thompson Draw synform to Deerfield Lake and low wavelength values southeast of Nemo. Samples with anomalously low wavelengths contain white micas with higher interlayer Na and lower Al than predicted by regional trends. Additionally, investigation of one low wavelength anomaly led to the identification of previously-unknown andalusite-bearing outcrops in the garnet zone.

Geostatistical analysis of the distribution of Al-OH wavelengths of the study area is presented as a series of maps illustrating key geospatial relationships and the resulting geostatistical interpolation. Exploratory data analysis characterized potential relationships between Al-OH wavelength and geospatial variables beyond metamorphic grade. A final kriging interpolation of Al-OH wavelength distribution is presented as a potential model to the variation in metamorphic intensity in the Black Hills.