RARE EARTH MINERALIZATION OF SOUTHERN CLARK COUNTY, NEVADA

Rare earth mineralization was first reported in the New York Mountains of southern Nevada and eastern California (Volborth, 1962), 30 kilometers to the east of the rare earth carbonatite complex at Mountain Pass, California. Our research examines the northern portion of this mineralized trend lying within southern Clark County, Nevada. The New York Mountains are comprised of orthogneissic granitoids emplaced in a north-south-trending zone at about 1.65 to 1.80 Ga (Miller and Wooden, 1994). Greenschist to amphibolite grade metamorphism overlapped batholitic emplacement. XRF whole rock analyses indicate the host rocks are predominantly silica-oversaturated granite to quartz monzonite. However, field observations suggest a period of silica flooding coincident with metamorphism or hydrothermal alteration with the original protolith composition most likely monzonite to syenite.

Rare earth mineralization occurs in pods, dikes and veins along a 2.5 kilometer trend striking N20°E, informally termed the “Lopez Trend”. Controls for mineralization are enigmatic. Numerous northeast –striking faults have been mapped throughout the northern New York Mountains, but mineralized outcrops lack slickensides or any evidence of lithologic discontinuity. In addition, most dikes closely parallel regional foliation. XRF, XRD and thin section analyses reveal that mineralization occurs predominantly as rare-earth fluorapatite with lesser monazite. A few grains of epidote (var. allanite) were observed in hand sample. Locally some dikes report high concentrations of Na₂O (>10%). These were examined and found to be comprised almost exclusively of albite, suggesting the mineralizing event was related to emplacement of albitite dikes (Na-metasomatism). A strikingly similar series of rare earth-bearing dikes have been described on the island of Sardinia (Palomba, 2001). There, mineralization is ascribed to a metasomatic event occurring during the latter stages of rifting. As the nearby Mountain Pass carbonatite complex is thought to be related to a 1.3-1.4 Ga rifting event, it seems plausible that the rare earth mineralization of the northern New York Mountains may represent a distal phase emplaced along permeable zones in the gneissic complex by circulating metasomatic fluids.