Layered pegmatites, southern Wind River mountains, Fremont County, Wyoming

Abstract

A swarm of more than 1,000 Precambrian granite pegmatites, mainly layered and including both concordant and discordant types, intrudes tightly folded, low- to medium-grade schists. Well exposed granitic stocks, a granitic sill, and a probable batholith are spatially associated with the pegmatite masses. Superposed folding affected the schists, pegmatite and granite bodies. Non-folded Precambrian basic dikes of north-easterly trend cut these rocks and east trending faults displace the dikes and older rocks. Tertiary sediments partially cover the Precambrian rocks, which have been re-exhumed through later Tertiary uplift and subsequent erosion.Geometric classification of the pegmatite bodies by size (length X width) and shape (length: width ratio) shows small ( 20,000 ft 2 ) and nearly equidimensional (1:1 to 2:1) to very elongate (10:1) bodies. Texture ranges from fine to very coarse-grained. Magnetite-, tourmaline- and tourmaline-beryl-, banded garnet-, biotite- and graphic granite pegmatites are the common mineralogical varieties of the granite pegmatites. The more exotic minerals are uncommon. Distribution of the pegmatite types suggests a regional zoning spatially related to large exposed intrusive bodies.Texture and/or mineralogy determine internal layering of the bodies. Successive layers from less than an inch to several feet thick and grain size from 0.04 inch or less to as much as three feet may occur within the same body. Passive intrusion into the host rock, assimilation and mobilization of host rock and later crystallization, essentially in situ, by pegmatitic fluids genetically related to nearby granitic intrusions is suggested as the most likely origin for these pegmatite bodies.