StructureSAN JUAN-UNCOMPAHGRE CALDERAS, LAKE CITY CALDERA, SAN JUAN VOLCANIC FILED, SAN JUAN DEPRESSION
AlterationSILIFICATION, SERICITIZATION, AND PYRITIZATION OF VEIN WALLROCKS; SUPERGENE ALTERATION OF ARGENTIFEROUS CU SULFIDE TO PYRARGYRITE, NATIVE AG, COVELLITE, BORNITE, CHALCOCITE; NEAR-SURFACE OXIDATION OF FE-BEARING SULFIDE TO OXIDES. SOLFATERIC ALTERATION SEQUENCE OF RED MOUNTAIN REPLACEMENT ALUNITE DEPOSIT. HALL'S (1978, 1980) GENERALIZED ZONATION: (1) SILICEOUS CORE OR CAP, IN WHICH VOLCANIC ROCKS HAVE BEEN THOROUGHLY LEACHED OF ALKALINE COMPONENTS AND ALUMINA, POSSIBLY CONTAINING SOME NATIVE S, AND REPRESENTING PRINCIPAL VENT OR CHANNEL FOR ACIDIC HYDROTHERMAL FLUIDS; (2) QUARTZ-ALUNITE AND CONTAINING SMALL AMOUNTS OF KAOLINITE OR DICKITE OR OTHER ALTERATION MINERALS; (3) INNER ARGILLIC ZONE, CHARACTERIZED BY DOMINANT MICROQUARTZ PHASE WITH SUBEQUAL AMOUNTS OF ALUNITE AND KAOLIN MINERALS, SUBORDINATE AMOUNTS OF SERICITE, PYROPHYLLITE, AND DIASPORE, AND FE STAINING; (4) OUTER ARGILLIC ZONE, SIMILAR TO INNER ZONE EXCEPT THAT ROCK IS LESS ALTERED, LESS BLEACHED, AND PRESERVES MORE OF PRIMARY ROCK TEXTURE, AND CONTAINS SPARSE ALUNITE, PREDOMINANT KAOLINITE, OR SUBEQUAL KAOLINITE AND SERICITE OR ILLITE OR HYDROMICA; (5) PROPYLITIC ZONE, MILD ALTERATION, GRAYISH-GREEN COLOR, PRESERVED ROCK TEXTURES, SOME SERICITIZED FELDSPAR, AND CHARACTERISTIC EPIDOTE, CHLORITE, ZEOLITES, CALCITE, AND PYRITE.