Relations of zoned pegmatites to other pegmatites, granite, and metamorphic rocks in the southern Black Hills, South Dakota Available to Purchase

The pegmatite field and the Harney Peak Granite of the southern Black Hills, South Dakota, form an igneous system that progresses from slightly biotitic muscovite granite through layered pegmatitic granite, with alternating sodic and potassic rocks, to simple plagioclase-quartz-perthite pegmatites, and on to zoned pegmatites. The zoned pegmatites range from rather simple ones, which have been mined for sheet muscovite or potassium feldspar, to complex ones with minerals of Li, Be, Nb, Ta, Sn, and Cs.

Most of the country rocks are Lower Proterozoic mica schists that were isoclinally folded and metamorphosed to biotite and almandine grades before granitic activity began. At 1700 Ga, intrusion of the Harney Peak Granite created a large dome in these rocks, a thermal aureole with a staurolite, a first sillimanite isograd, and a small area of metamorphism above the second sillimanite isograd. The estimated pressure is 3.7 kbar.

The granite has an area of 104 km², mostly in a single pluton consisting of thousands of sills and dikes. The pegmatite field is exposed over an area of 711 km². An isogram map of the abundance of pegmatites has many local irregularities. Calculations from the isograms show a total of 24000 pegmatites. About 2% of these are zoned pegmatites.

The zoned pegmatites have a strong tendency to occur in clusters, and the types of pegmatites are different in different clusters. A less obvious tendency is a regional zonation in which rare-mineral pegmatites become more abundant and muscovite pegmatites less abundant toward the outskirts of the region. This trend is parallel to a drop in crystallization temperatures from more than 660 °C in parts of the granite, some of which have primary sillimanite instead of muscovite, to less than 600 °C in inner zones of a few rare-mineral pegmatites, many of which are outside the first sillimanite isograd.

The composition of the granite indicates that its magma originated by partial melting of metasedimentary mica schists similar to those at the present surface. The pegmatitic nature of most of the granite probably reflects exsolution of an aqueous phase. The resulting processes continued into the simple pegmatites and zoned pegmatites, which are 4% and 0.5% of the exposed granitic rocks. For rare-element pegmatites, residual concentration in granitic magma can account for their modest contents of Be, Sn, Ta, Nb, and Cs, but Li presents complications because a large share of the Li in the exposed parts of the system is in Li-rich zoned pegmatites. The clustering of zoned pegmatites shows them to be products of localized subsystems that behaved differently.