Characteristics of the Yates Unit Amphibolite

Introduction The lowest known portion of the Poorman Formation as determined from diamond drilling and mapping is the Yates Unit. It largely comprises amphibolite, but also contains interbedded volcaniclastics, calc-silicate beds, metachert, and carbonate-rich semipelitic rocks. This unit is exposed at the surface in the Blacktail Gulch area north of Lead and continues down-plunge to the southeast through the core of the Lead Anticline. Extending from surface outcrops, amphibolite of the Lower Unit has been found in mine exposures to the 6800 Level and in drill core down to about the 8600 equivalent level; a known plunge length of eleven kilometers. In plan view, the amphibolite unit consists of multiple, lobate masses that are elongated NNW-SSE and comprise most of the core of the Lead Anticline. These large, nearly strain-resistant masses have served as structural “buffers” for the eastern ledges in the Homestake mine, compared to the highly attenuated structures found in the western ledges.