CIM Montreal 2011
Malcolm Bucholtz,
Abstract Deposits containing rare earth elements (REE) are found throughout New Mexico, USA. With the projected increase in demand of REE, domestically and globally, areas such as the Gallinas Mountains in New Mexico are being re-examined for additional REE potential. Minimal past production of REE in the 1950s, as bastnaesite, came from the Gallinas Mountains. Since then, several companies and the U.S. Bureau of Mines (USBM) have conducted various exploration programs to identify and delineate REE resource potential. The igneous rocks in the Gallinas Mountains are metaluminous to peraluminous, alkaline volcanic rocks, and have chemical compositions similar to A-type granitoids. Trachyte/syenite and latite are possibly related, but the rhyolite could be a separate magmatic event. The geochemical data suggest a crustal source for the igneous rocks. Four types of deposits are found in the Gallinas Mountains: epithermal REE-F veins, Cu-REE-F veins, REE-F breccia pipes and iron skarn deposits; all are associated with Tertiary alkaline to alkalic-calcic igneous rocks. District zonation is defined by Cu-REE-F (±Pb, Zn, Ag) veins that form center of the district, surrounded by REE-F veins. The magmatic-hydrothermal breccia pipe deposits form a belt partially surrounding the veins. Iron skarns formed at the top and edge of the trachyte/syenite body and are likely the earliest stage of mineralization. The iron skarns are probably related to the REE-F veins and breccias because they typically contain bastnaesite and fluorite and are similar in trace element geochemistry. The paragenesis is defined by four stages of brecciation and faulting with three stages of fluorite deposition. REE minerals were deposited during the 1st and 2nd stage of fluorite deposition. A genetic model is summarized by intrusion/extrusion of crustal-derived igneous source rock in an extensional terrain possibly related to an alkaline-carbonatite complex with mineralization related to mixing of magmatic-hydrothermal and formation fluids. In 1991-1992, USBM calculated an inferred resource of 537,000 short tons with a grade of 2.95% total REE (not NI-43-101 compliant).
Keywords: Fluorite, iron skarn, breccia pipes, REE, Alkaline rocks
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