WIDESPREAD, LATE-STAGE FLUID INDUCED METAMORPHISM IN A BUCHAN-TYPE SETTING, SOUTH DAKOTA BLACK HILLS

Precambrian terranes, in the amphibolite facies of the Black Hills core, typically exhibit two stages of metamorphism. The earlier, moderately low-pressure event accompanied the development of pervasive structures, including two phases of folding. The later metamorphism produced a mostly thermal overprint of static porphyroblasts concentrated along lithologic boundaries, veins and shear zones. Latest tectonic and metamorphic features have been attributed to the final emplacement of the Harney Peak Granite (HPG) and its pegmatites at about 1715Ma (D₃). In a pattern around the HPG, the later metamorphism approximates a contact aureole and structures define a tectonic doming in the surrounding metasedimentary and metavolcanic units.

Recent study north of the HPG exposures and into regions of lower-grade regional metamorphism indicates that the latest metamorphism persists in hydrothermal character, associated primarily with steeply-dipping shear zones. In the vicinity of Pactola Dam in Pennington County, the shears developed at the brittle-ductile transition, and cut biotite-grade supracrustals. Selective growth of euhedral, randomly-oriented phases (biotite, amphibole, pyrite and more rarely, K-feldspar) took place along the shears and occur with or without accompanying quartz, carbonate and chlorite in veins. At Pactola, the dominant rock types are mafic metavolcanic flows (?), and volcanogenic metasedimentary units (greywacke and carbonaceous, sulfidic shales, originally). A great abundance of carbonate phases plus chlorite in all units may indicate the high activity of CO₂ as well as H₂O during the latest metamorphism.

Just north of HPG outcrops in the Keystone area, garnet-grade units compositionally equivalent to Pactola Dam, are also hosts to late, steeply-dipping mineralized veins. Even though no sign of granitic influence is obvious, static-growth tourmaline exists along chlorite-rich shears.

The HPG may only crop out in the southern Black Hills, but suspicions are that its regional influence may be more ubiquitous. Related metamorphism/metasomatism far beyond the dome, possibly occurs in the Lead area and even northwest to Tinton. Undated, granitic pegmatite similar to the HPG has been mined in Tinton.