COARSE MUSCOVITE ALTERATION IN PORPHYRY COPPER SYSTEMS OF ARIZONA: CHARACTERISTICS AND IMPLICATIONS

Hydrothermal alteration styles and distribution are important tools for effective exploration for porphyry copper deposits. Many porphyry systems along the Laramide arc in Arizona have been dismembered and tilted by Basin and Range extension, exposing some porphyry systems to paleodepths of > 12 km. These exposures have allowed documentation of varying forms of deep hydrothermal alteration, largely beneath copper ore bodies in porphyry copper systems, including coarse muscovite alteration. Recent documentation of coarse muscovite alteration allows for a more detailed understanding of the roots of porphyry systems.

Coarse muscovite veins and alteration have been documented in multiple porphyry copper systems across Arizona. These veins and alteration show spatial, temporal, and chemical patterns between districts. Coarse muscovite veins (muscovite-quartz-K-feldspar-albite-pyrite ± chalcopyrite) are spatially extensive and coarse muscovite wall-rock alteration is common but spatially restricted. Mineral assemblages indicate that coarse muscovite-forming fluids contain cation loads dominated by K and Na. The magmatic composition of associated Laramide stocks and plutons appears to relate to the mineral assemblages observed in coarse muscovite veins; veins associated with more mafic (quartz monzonitic to granodioritic) magmas contain a less diverse mineral assemblage whereas coarse muscovite alteration associated with more felsic (quartz monzonitic and granitic) magmas may also contain molybdenite ± fluorite ± scheelite ± wolframite ± sphalerite. Coarse muscovite veins are interpreted to have formed either in subhorizontal and/or subvertical orientations. Where timing relationships were observed, coarse muscovite veins tend to cut and offset potassic veins. Coarse muscovite veins are distinct from earlier sugary quartz veins in texture and mineral assemblage. Hydrogen isotope compositions of coarse hydrothermal muscovite and calculated hydrogen isotopic compositions of fluids in equilibrium with coarse muscovite suggest a magmatic origin for associated aqueous fluids. These data suggest that coarse muscovite-forming fluids are magmatic in origin, with mineral assemblages and compositions that vary with associated magmatic composition, and formed after the formation of porphyry copper ores.