GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 3-6
Presentation Time: 9:30 AM

ROLE OF SILICATE-FLUORIDE MELT IMMISCIBILITY IN THE FORMATION OF PROTEROZOIC F- AND REE-RICH SEGREGATIONS WITHIN APLITE VEINS IN THE LONGS PEAK-ST. VRAIN BATHOLITH NEAR JAMESTOWN, COLORADO, USA


STERN, Charles R.1, ROSS, Jeremy1, ALLAZ, Julien M.1, RASCHKE, Markus B.2, FARMER, G. Lang1 and SKEWES, M. Alexandra1, (1)Department of Geological Sciences, University of Colorado Boulder, 2200 Colorado Avenue, UCB 399, Boulder, CO 80309-0399, (2)Department of Physics, Department of Chemistry, and Jila, University of Colorado, Boulder, CO 80309-0390, Charles.Stern@Colorado.EDU

F- and REE-rich segregations, with 40-46 wt % REE and 5-7 % F, occur contained within aplite veins in the Proterozoic 1.42(±3) Ga Longs Peak-St. Vrain Silver Plume-type granitic intrusion near Jamestown, Colorado (EN Goddard & JJ Glass 1940, AmMin 25: 381-404; J Allaz et al. 2015, AmMin 100: 2123-2140). The segregations are zoned with gray/purple-colored cores containing dominantly fluorbritholite-(Ce), monazite-(Ce), fluorite, and minor quartz, uraninite, and sulfides, surrounded by black, typically millimeter-thick allanite-(Ce) rims, with minor monazite-(Ce) in the inner part of that rim. Monazite-(Ce) and uraninite U-Th-Pb microprobe ages yield 1.420(±25) and 1.442(±8) Ga, respectively, for the F- and REE-rich segregations, suggesting a co-genetic relationship with the host granite. However, various petrochemical characteristic of the aplites and the F- and REE-rich segregations indicate that these were derived from an independent source from the granite. These features include 1) different εNd1.42Ga for the granites and associated pegmatites, which range from -3.3 to -4.7 and average of -3.9 for 6 samples, compared to the aplites and REE-rich segregations, which range from -1.0 to -2.2 and average of -1.6 for 8 samples, the latter two phases being similar to each other; 2) different mineral compositions for the granites (plagioclase Ab>90; biotite FeO/MgO = 2.5-10, and F <1.5 wt%) and aplites (plagioclase Ab76-82; biotite FeO/MgO <2 and F = 2-3 wt%); 3) granite (La/Yb)N <50, and with a significant negative Eu anomaly, in contrast to both the apalites and segregations with (La/Yb)N >80 and no negative Eu anomalies; and 4) the distinctive granular texture of the aplite. Based on the textures of the F- and REE-rich segregations, which are in many cases preserved as rounded blebs in the aplite veins, we suggest that the aplites and REE-rich segregations initially intruded as a single hypersolvus melt, which then separated by immiscibility into a silicate-rich aplite and a F-, P2O5- and REE-rich phase. Small but ubiquitous concentration of fluorite and monazite-(Ce), surrounded by Fe-sulfides, within the fluorbritholite-(Ce)-rich cores of the REE-rich segregations, suggest a second stage of immiscible separation of a relatively F-, S- and P-rich phase from a less F-, S- and P-rich, but more REE-rich one.