MINERALOGY OF RARE EARTH BEARING APATITE VEINS WITHIN THE NORTHERN NEW YORK MOUNTAINS OF SOUTHERN NEVADA

An occurrence of anomalous concentrations of rare earth element (REE) bearing apatite veins occur within a Proterozoic (1.6-1.8Ga) orthogneiss of the northern New York Mountains of southern Nevada. The primary host rock is a weakly alkaline granite with the highest concentrations of the rare earths occurring in a series of apatite veins in close proximity to a more mafic phase varying in composition from granodiorite to diorite.

Individual apatite veins are typically about 1 cm thick, and in cores are numerous throughout a thickness of about 3 m. Primary minerals in the apatite veins are allanite, monazite, and zircon. Most of the REEs occur in the allanite and monazite, with low concentrations in the apatite itself. Rare and small grains of xenotime are present in the veins. Considerable thorium (Th) is present in the monazite. In SEM and microprobe backscattered electron imaging all of the vein minerals show zoning. In the monazite the zoning is of the patchy type that is often attributed to partial dissolution and regrowth during formation. This zoning largely reflects differences in Th content. In allanite the zoning largely reflects differences in REE content.

Considerable secondary alteration is also observed in allanite and monazite. Much of the monazite close to the apatite veins is also altered. This monazite altered largely to a mixture of thorogummite and secondary apatite, but occasional alteration products included thorianite and a REE fluoride. Comparatively, unaltered monazites contain a higher concentration of REE’s than altered monazites, possibly due to the removal of REEs by alteration fluids. In another section secondary allanite is observed coating apatite crystals. Our study indicates that apatite veins may contain significant amounts of REEs for future economic potential.