MINERALOGY AND PETROLOGY OF A NEWLY DISCOVERED REE PEGMATITE NEAR WELLINGTON LAKE, SOUTH PLATTE PEGMATITE DISTRICT, COLORADO (USA)

The pegmatites in the South Platte district, Colorado, in the 1.09 Ga ‘A-Type’ Pikes Peak Batholith, represent globally significant examples of the Nb-Y-F (NYF) type. Here we present a petrographic and mineralogical analysis of a yet undescribed REE pegmatite near Wellington Lake. The pegmatite is a steeply dipping, lenticular, concentrically zoned body (5 x 8 x 3m) with a narrow graphic granite transition into a coarse zone of microcline-perthite, ‘cleavelandite’ albite, biotite, and Fe-oxides. Accessory minerals include fluocerite, bastnäsite, columbite, ‘cyrtolite’ zircon, and minor secondary U/Th species. Notable is the absence of fluorite or phosphate minerals (monazite) in the pegmatite body.

The granite host with HREE rich pattern is characteristic for coarse-grained Pikes Peak granite with LREE/HREE (normalized) ratio of 2:1. Towards the pegmatite core the REE content decreases, with similar REE pattern compared to the host granite, but with a less significant Eu anomaly. Inside the pegmatite a pronounced relative HREE enrichment is observed for microcline and albite and most notably for a Fe-rich breccia developed in certain areas.

Of the accessory minerals, most notable for the pegmatite are well-developed tabular crystals of fluocerite [(Ce,La)F₃] which are epitaxially overgrown by bastnäsite [(Ce,La)[CO₃]F]. Electron microprobe analyses shows a chemical quite homogeneous composition for the fluocerite core, in contrast to the bastnäsite overgrowth that is zoned with respect to REE and generally depleted in light REE compared to the fluocerite.

Fluid inclusions in pegmatite core-zone quartz analyzed by micro-thermometry suggest the presence of at least two types of fluids in the form of a H₂O-NaCl and a H₂O-CO₂ fluid, with no evidence of boiling. Few higher temperature fluid inclusions might represent a distinctly earlier fluid of higher temperature. The H₂O-CO₂ fluid observed in inclusions may have also affected the growth and REE zonation of the fluocerite/bastnäsite crystals, which appear to be paragentically late, through selective complexation of certain REE with CO₃^2-.

ε_{Nd1.09Ga =} -1 for this late stage fluocerite suggest that the pegmatite is cogenetic and Nd and other REE are derived from the granite magma having been at isotopic equilibrium at 1.09 Ga.