TESTING MODELS OF LI-PEGMATITE PETROGENESIS USING DIRECT DATING METHODS AND IMPLICATIONS FOR MINERAL RESOURCE ASSESSMENTS

Models of Li-pegmatite (LiPeg) formation include derivation from highly evolved parental magmas (residual melts of S-, A- and I-type granites-RMG) and direct products of anatexis (DPA). Ages of LiPegs, proximal intrusions, and regional tectonothermal events are essential to understanding the petrogenetic processes involved in LiPeg formation. LiPegs that are directly related to an RMG should be approximately the same age as the parental intrusion, whereas DPA LiPegs are a result of discrete tectonothermal events, and may or may not be similar in age to intrusions in the associated terrane.

We investigate LiPeg occurrences in the Black Hills of South Dakota and in the northern U.S. Appalachians as probable examples of RMG and DPA pegmatite end members, respectively. Direct dating of LiPegs from these occurrences was accomplished by using U-Pb Laser Ablation ICPMS geochronology studies of cassiterite (SnO₂), a mineral commonly associated with LiPegs and a proven reliable geochronometer in these settings.

The Black Hills LiPegs are intimately associated with the ~1715 Ma Harney Peak Granite. Mineral and chemical zoning patterns of the pegmatites are correlated with distance from the Harney Peak Granite, with barren proximal pegmatites and rare-metal/Li-enriched distal pegmatites. Preliminary geochronology studies of cassiterite sampled from the Black Hills LiPegs suggest they formed at approximately the same time as the parental granite. In the northern Appalachians, LiPegs appear to be associated with Appalachian orogenesis and generally constrained to peri-Gondwanan and Gondwanan affinity terranes. Most igneous rocks in this realm are associated with the Paleozoic Acadian-Neoacadian-Alleghanian orogenies. Pegmatites in the region are often hosted in metasedimentary rock units deformed during Appalachian orogenic events. At least eight generations of LiPegs have been identified in the Appalachians (up to six in New England alone) with ages between ca. 395 Ma and 250 Ma. Most, if not all LiPegs, are apparently younger than identified and proximal plutonic rocks in the region by 10’s of millions of years.

Understanding the processes of formation is critical to exploration and mineral resource assessments of LiPegs. Direct dating of minerals associated with LiPegs such as cassiterite allow one to test models of their formation in prospective regions.