Aluminosilicate mineral assemblages in the Cargo Muchacho Mountains, southern California: metasomatism and gold concentration associated with magmatism and deformation in mesozonal environments

Abstract

Aluminous, kyanite-bearing mineral assemblages with more than 30% Al2O3 and less than 2% CaO + Na2O + K2O, like aluminous occurrences elsewhere, are spatially associated with gold deposits in the Cargo Muchacho Mountains, southeastern California. They are part of a zoned sequence of mineral assemblages that grade from regional amphibolite-facies quartzofeldspathic gneiss into locally occurring quartz–oligoclase–biotite–epidote–magnetite (feldspar zone), quartz–muscovite–biotite–magnetite–apatite–tourmaline (muscovite zone), and quartz–kyanite–magnetite–rutile–apatite–tourmaline–lazulite (kyanite zone). Field relations show that the mineral assemblages developed during emplacement of a Jurassic metaluminous–peraluminous plutonic suite, and that magmatism and aluminosilicate formation were partly contemporaneous with crosscutting low-angle shear zones.Mineral assemblage variation and bulk rock compositions point to aluminosilicate formation by pervasive H -ion metasomatism. Replacement of biotite by magnetite and variation in biotite Fe/(Fe+Mg) from 0.51 in the feldspar zone to 0.38 in the kyanite zone indicate oxidation was important during metasomatism. Calculated pH is 6.5 or less, and oxygen fugacity is 10−27 to 10−18 bars (1 kbar = 100 MPa) for fluids in equilibrium with the kyanite zone. Staurolite-bearing kyanite zone limits pressures to more than 400 MPa, and garnet–biotite geothermometry gives temperatures of 500–550 °C for metasomatism. The data are consistent with an origin by oxidizing, near-neutral magmatic fluids arising from crystallization of Jurassic mesozonal metaluminous–peraluminous magma. The high oxidation state suggests gold would have been transported to more reduced chlorite- and pyrite-bearing assemblages distal to the aluminosilicates.