CATHODOLUMINESCENCE EVIDENCE OF MULTIPLE GENERATIONS OF QUARTZ IN K-METASOMATIZED VOLCANIC ROCKS

Pervasive K-metasomatism is seen in ore deposits and unmineralized volcanic terranes, but is often of cryptic origin.  To better understand this alteration style, we are studying Tertiary volcanic rocks at Creede, CO (an environment of epithermal Ag-Pb-Zn mineralization), the Harcuvar Mountains, AZ (the upper plate of a detachment fault), and Socorro, NM (an arid playa extensional environment), with petrographic and geochemical methods that include CL.  These terrains underwent extensive low-T (<250^o C) metasomatism that produced similar chemical and mineralogical effects.  Rocks are enriched in K and Rb, and depleted in Na, Sr, Ca, via replacement of yellow- and blue-luminescent feldspar phenocrysts by brown to non-luminescent, Or_>95 K-feldspar, clay minerals, and quartz, and oxidation of magnetite and mafic minerals to hematite.  CL documents multiple generations of quartz growth that helps to delineate complex metasomatic histories.

Spectroscopy of quartz from these rocks is essential, because observed color is an unreliable provenance indicator.  Relict igneous phenocrysts emit zoned CL in both the red and blue regions of the spectrum, yielding “purple” CL that may appear blue or red to the eye. Much fine-grained groundmass quartz in altered tuffs is non-luminescent.  Bright red-luminescent quartz occurs as overgrowths on phenocrysts, in former melt inclusion structures and in the groundmass of some tuffs. The latter may reflect vapor phase crystallization, because it is seen primarily in lithophysae. Recrystallized groundmass quartz from the welded Willow Creek tuff (Creede) displays coarse-grained quartz with red CL.

Hydrothermal vein quartz exhibits several CL colors, whereas quartz related to K-metasomatism is typically non-luminescent.  For example, mineralization-related vein quartz at Creede shows oscillatory and sector zoned yellow, blue, and red CL; in contrast, vein quartz from K-metasomatized samples isolated from mineralization are non-luminescent.  This may reflect lower-T growth of the latter.  Trace element compositions of adularia from veins that also contain quartz with yellow CL differ from those of adularia that has replaced phenocrysts, suggesting distinct fluid-rock events.  CL could prove useful as a prospecting tool in regions that display multiple alteration events.