GEOCHEMISTRY AND TECTONIC SETTING OF THE YATES UNIT OF THE POORMAN FORMATION (DUSEL BEDROCK) AND OTHER NORTHERN BLACK HILLS AMPHIBOLITES

The planned Deep Underground Science and Engineering Laboratory (DUSEL) site is the Homestake Mine in Lead, SD. Facilities will be constructed in Early Proterozoic (~2012 Ma) amphibolites of the Yates unit of the Poorman Formation. In anticipation of underground studies, a suite of samples was collected in the two areas of surface exposure of the unit, Blacktail Gulch and Cleopatra Creek (formerly Squaw Creek). Additional amphibolites from the northern Black Hills were collected for comparative study.

11 of 18 Yates unit samples are amphibolites with compositions reflecting hydration of tholeiitic basalt protoliths. Two other samples have high MgO, CaO, Ni, and Cr suggesting cumulate protoliths. Remaining samples are meta-sedimentary, including one pelitic phyllite. The meta-basalts have LOI of 1.5-3.8 wt%, with normalized wt% of SiO₂=47.3-52.3, MgO=6.0-12.3, Al₂O₃=11.5-15.5, FeO*=10.9-13.5. Of relevance to DUSEL low-background physics projects, K₂O=<0.01-0.9 wt%, U=0.06-2.11 ppm, and Th=0.2-9.5 ppm (excl. phyllite). Ni declines steadily with MgO and fluid-immobile incompatible elements are nearly flat, suggesting that MgO variation reflects low degrees of olivine-dominated fractionation.

Yates meta-basalts are slightly LREE-depleted and plot in MORB fields on many tectonic discrimination diagrams. Van Boening & Nabelek (2008: Precamb. Res. 167) studied amphibolites south of the study area. They recognized several E-MORB packages, at least one of which (Pactola-Rushmore) is thought to correlate with the Yates unit, and interpreted these to reflect plume-influenced rift volcanism. They also recognized rocks with OIB affinity, interpreted as reflecting a mantle plume, in the Rochford area, and analysis of other units in that area confirms this signature. However, others interpret the Rochford rocks to be up to 130 m.y. younger than the Yates unit (Redden & DeWitt, 2008: USGS SI Map 2777). In Th/Yb-Nb/Yb and TiO₂/Yb-Nb/Yb plots the Yates is closer to N-MORB than other Black Hills units. Samples collected between the Yates unit and Pactola area create an array between N-MORB and OIB on a TiO₂/Yb-Nb/Yb plot. This suggests that the Yates was more distal from the postulated plume, perhaps beyond its influence. Alternatively, the separation could be in time. The strong N-MORB signature suggests a mature rift.