Geochemistry of biotites from mineralized and barren intrusive systems

Abstract

Ninety-two biotite separates from 35 different igneous systems in western North America and the Caribbean region have been analyzed for Cl, F, H 2 O, Cu, Zn, Fe, and Mg. Biotites in fresh magmatic samples from igneous systems genetically related to significant mineralization (dominantly porphyry copper type) form a population with average chlorine and fluorine contents greater than and average copper content less than the average of the remaining barren intrusive systems (at the 90 percent confidence level). However, the magnitude of this halogen enrichment is too small to permit its use as a routine exploration tool. There are no systematic differences in Cl:F, F:H 2 O or other multicomponent ratios of biotite compositions that can be used to distinguish between mineralized and barren host intrusions with a high degree of confidence. Although the average halogen content of biotites from both mineralized and barren potassium-rich intrusions is higher than that for potassium-poor intrusions, biotites from mineralized potassium-rich intrusions are slightly enriched in halogens with respect to their barren counterparts. Magmatic biotites from the Boulder batholith do not exhibit systematic spatial variations in halogen abundance related to known areas of mineralization, although biotites from altered rocks around Butte exhibit chlorine depletion. The copper content of biotites from the same samples indicates the location of mineralization at Butte more reliably, however.