AMARGOSA PROSPECT, BAJA CALIFORNIA NORTE, MEXICO: INTENSE REGIONAL METASOMATISM AS A RESULT OF IOCG-STYLE ALTERATION

The Amargosa prospect, located south of El Rosario, Baja California Norte, Mexico, contains intensely altered volcanic rocks of the Cretaceous Alisitos Formation. Mapping and petrographic studies indicate a complex alteration and mineralization paragenetic sequence which is compatible with an iron oxide Cu-Au (IOCG) hydrothermal system. Hypogene alteration and mineralization occurred in six stages. The initial alteration event (sodic) resulted in massive albite replacement and disseminated specular hematite. Sodic alteration was followed by sodic-calcic (I) alteration which precipitated an assemblage of epidote, magnetite, albite, and musketovite (magnetite replacement of hematite). Both sodic and sodic-calcic (I) alteration depleted LREE and U in the host rocks. While sodic alteration primarily affected fine-grained rocks and the matrix of volcaniclastic rocks, sodic-calcic (I) alteration affected clasts within the volcaniclastics. A weak potassic event characterized by biotite growth, followed sodic-calcic (I) alteration. Potassic alteration was followed by silica-boron (I) alteration which precipitated irregular zones of fine-grained quartz, radiating dravitic tourmaline, pyrope-almandine garnet, and minor pyrite and chalcopyrite replacing magnetite. Silica-boron (I) assemblages are cut by a sodic-calcic (II) alteration assemblage of epidote, ferro-actinolite, chlorite, albite, magnetite, and musketovite. The final hypogene event, silica-boron (II), resulted in the formation of a quartz, blocky schorl, pyrite assemblage. This alteration paragenesis indicates that repetitive pulses of relatively similar hydrothermal fluids with varying oxidation states affected the area. Although strongly altered, rocks at the Amargosa prospect are only weakly mineralized and lack the well-developed potassic alteration event associated with copper mineralization at the nearby San Fernando prospect (see Lopez et al., this volume).