SAMARSKITE-(YB): A NEW MINERAL INDICATING EXTREME HREE ENRICHMENT IN THE SOUTH PLATTE NYF PEGMATITE DISTRICT, COLORADO

The South Platte Pegmatite district, located in Jefferson County, Colorado is hosted within the 1.0 Ga Pikes Peak batholith. These pegmatites are noted for their strong enrichment in rare earth elements (REE), Nb, Y, and F. Within the district, a geographical separation exists between pegmatites enriched in light REE (LREE) versus those enriched in heavy REE (HREE). Composite core pegmatites in the southern part of the district contain large masses of allanite-(Ce) and are LREE enriched. Quartz core pegmatites, located in the northern portion of the district, have only minor allanite-(Ce), but contain large radiating nodules of samarskite “group” minerals up to 30 cm in diameter. The samarskite "group" species occur in albite-rich, core-margin replacement units associated with HREE-rich zircon (cyrtolite), ferrocolumbite, monazite-(Ce) and yttrian fluorite. This "samarskite" has elevated HREE content compared to "samarskite" from other pegmatites (e.g. Kunar, Afghanistan, Spinelli, CT, Spruce Pine, NC, and Oxford, ME). Its Y content, however, is equivalent. Such strong HREE enrichment of nearly an order of magnitude is sufficiently high that numerous samples have Yb>Y. These Yb-dominant samples are the new mineral, samarskite-(Yb), recently approved by the CNMMN (# 2004-001).

The separation of LREE and HREE enriched minerals into separate pegmatite groups is related to late-stage enrichment of F and other volatiles into the upper portion of the pluton and to selective complexing of HREE with F. Thus, HREE, which more readily form complexes with F, were preferentially partitioned into the F-rich upper portion of the pluton. The pegmatitic melts evolved from subsequent fractionation of this volatile-rich upper portion. The HREE in the pegmatitic melts remain complexed with F until F begins to be removed by the formation of F-bearing minerals such as fluorite. Samarskite-(Yb) and other HREE minerals then begin to form in the very final stages of crystallization, typically in replacement units.