PETROCHEMISTRY OF A RARE EARTH OCCURRENCE WITHIN THE NORTHERN NEW YORK MOUNTAINS OF SOUTHERN NEVADA

The northern New York Mountains, southern Clark County, Nevada contain anomalous concentrations of rare earth elements within a Proterozoic (1.6-1.8 Ga) orthogneiss. The host for the mineralization is a weakly alkaline granite, however, the highest rare earth concentrations occur in close proximity to the contact between granite and a more mafic intrusive phase varying in composition from granodiorite to diorite.

Four rare earth-bearing minerals have been identified. Small amounts of rare earths occur within the abundant fluorapatite of large aggregates that frequently enclose smaller grains of the other phosphates. Much higher concentrations of rare earth are present within monazite-Ce and xenotime. The monazite-Ce also contains significant amounts of lanthanum, neodymium, and thorium. Xenotime, in contrast, is enriched in gadolinium, dysprosium, and erbium. A thorium mineral species also contains small amounts of rare earth. The mineralization generally occurs as pods and disseminations within the host granite, however, some veining particularly of monazite was noted in thin section. Zoning of REEs and thorium within individual grains was common.

Alteration consists of widespread Na-metasomatism, in which k-spar has altered to albite and sodium clays, and more localized chloritization. The latter appears most commonly within breccia zones that likely were created by faulting. The relationship of the alteration to the rare earth mineralization remains enigmatic. Although some mineralization is present in areas of intense alteration other pods of rare earth mineralization have no associated alteration. The relationship is complicated by a Mesozoic copper-bearing intrusive near the north end of the New York Mountains. Fluid circulation associated with that event could account for some or all of the alteration associated with the rare earth mineralization.