CATHODOLUMINESCENCE IDENTIFICATION OF LATE STAGE FLUID REPLACEMENT IN THE BLACK MOUNTAIN PEGMATITE

The Black Mountain pegmatite, located in Oxford County, Maine, is an internally zoned and chemically evolved, rare-element granitic pegmatite that exhibits extensive, yet sometimes, subtle replacement features caused by late-stage fluids. Reaction of Na-rich, residual fluids with earlier formed potassium feldspar, quartz, muscovite, lepidolite, and spodumene, resulted in the replacement of these minerals by fine-grained (saccharoidal) and bladed (cleavelandite) albite. The replacement of pre-existing minerals by albite may not be readily obvious with the unaided eye, but can be easily identified using cathodoluminescence (CL).

The principle CL feature that is observed in albite from Black Mountain is the reduction of luminescence, which varies from a bright green to blue in non-replacing albite to dark green luminescence in albite that replaces other minerals. The mechanism by which this occurs is still not known, but a few hypotheses have been proposed: 1) the incorporation of a quenching agent in the albite during the replacement process 2) the reduction of activators (Ti⁴⁺, Mn²⁺, Fe²⁺, Fe³⁺) in albite during the breakdown of replaced minerals, and 3) an increase in the amount of activating elements leading to concentration quenching in the albite.