1 Manuscript 5253 – REVISION 1 2 Age, petrochemistry, and origin of a REE-rich 3 mineralization in the Longs Peak - St. Vrain pluton near 4 Jamestown, Colorado (USA) 5 Julien Allaz1*, Markus B. Raschke2, Philip M. Persson3, and Charles R. Stern1 6 7 1 Department of Geological Sciences, University of Colorado Boulder, 399 UCB, Boulder, CO 8 80309 9 * Corresponding author:
[email protected] 10 2 Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, 11 CO 80309, USA 12 3 Department of Geology & Geological Engineering, Colorado School of Mines, 1516 Illinois 13 Street, Golden CO 80401 14 15 Abstract 16 An unusual rare earth element (REE) mineralization occurs at a locality known as the “Rusty 17 Gold” within the anorogenic 1.4 Ga Longs Peak - St. Vrain monzo- to syenogranite Silver 18 Plume-type intrusion near Jamestown, Colorado (USA). Irregular shaped centimeter- to 19 decimeter-sized mineralized pods and veins consist of zoned mineral assemblages dominated by 20 fluorbritholite-(Ce) in a grey-colored core up to 10 cm thick, with monazite-(Ce), fluorite, and 1 21 minor quartz, uraninite, and sulfides. The core zone is surrounded by a black, typically 22 millimeter-thick allanite-(Ce) rim, with minor monazite-(Ce) in the inner part of that rim. 23 Bastnäsite-(Ce), törnebohmite-(Ce) and cerite-(Ce) appear in a thin intermediate zone between 24 core and rim, often just a few hundreds of μm wide. Electron microprobe analyses show that the 25 overall REE content increases from rim to core with a disproportionate increase of heavy REE 26 (ΣHREE increases 10-fold from 0.2 to 2.1%) compared to light REE (ΣLREE increases 2-fold from 27 21.3 to 44.3%).