COMPOSITIONAL HETEROGENEITY IN FLUORITE AND THE GENESIS OF FLUORITE DEPOSITS: INSIGHTS FROM LA ICP MS ANALYSIS

Abstract

Fluorite from four fluorite– REE mineral deposits has been analyzed by LA–ICP–MS to determine the viability of this method for fluorite and to assess spatial and paragenetic compositional heterogeneity in the fluorite. Two of the deposits studied hav e an affiliation to an alkaline suite (Gallinas Mountains, New Mexico and Rock Canyon Creek, British Columbia), and two are associated with granites (South Platte, Colorado and St. Lawrence, Newfoundland). We show that LA–ICP–MS is a valid technique for the analysis of fluorite and that the nature of, and controls on, the trace-element content of fluorite are more complex th an previously indicated from bulk analyses. Significant, small-scale (less than 2 mm) variation occurs in the trace-element composition of fluorite within individual deposits. Patterns of trace-element enrichment and depletion differ between, but are gener ally consistent within, individual deposits. Fluorite associated with alkaline magmatism is characterized by a flat to LREE-enriched chondrite-normalized REE pattern that lacks a negative Eu anomaly and has a positive Y anomaly. Fluorite associated with granitic magmatism is characterized by a flat to LREE-depleted chondrite-normalized REE pattern that has a negative Eu anomaly and a positive Y anomaly. With the exception of the Eu and Y anomalies, the chondrite-normalized REE patterns of the fluorite are similar to those of the associated intrusive rocks. The compositional heterogeneity of fluorite indicates that its use as a guide in classifying mineralization or in mineral exploration should be limited to instances where results of small-scale analyses (L A– ICP–MS) are available.