StructureSAN JUAN-UNCOMPAHGRE CALDERAS, LAKE CITY CALDERA, SILVERTON CALDERA, EUREKA GRABEN, SAN JUAN VOLCANIC FIELD, SAN JUAN DEPRESSION
AlterationALTHOUGH OVERALL ALTERATION EFFECTS IN DISTRICT ARE VARIABLE DUE TO SUPERPOSITION OF SURFICIAL ALTERATION ON HYDROTHERMAL AND TO VARIABILITY OF ROCK TYPES, KRASOWSKI (1976) SUMMARIZED GENERAL ALTERATION SEQUENCE AROUND QUARTZ VEIN CUTTING PRECAMBRIAN GRANITE: (1) VEIN, CONSISTING OF MASSIVE QUARTZ AND SOME APORADIC DISSEMINATED OYRITE, SURROUNDED BY SILICIC ZONE (WITH SULFIDES) EXTENDING 1 TO 5 FT INTO NINBRECCIATED ROCK, ALTHOUGH QUARTZ-SERICITE MAY OCCUR ADJACENT TO QUARTZ VEIN IN PLACE OF SILICIFICATION; (2) IN SOME DEPOSITS, SILICIC ZONE MAY GRADE DIRECTLY INTO SERICITE ZONE AVERAGING 5 FT WIDE, GRADING INTO (3) ARGILLIC ZONE CHARACTERIZED BY INTENSE ALTERATION OF PLAGIOCLASE AND K FELDSPAR TO CLAYS, OCCASIONALLY VERY NONUNIFORM IN BRECCIA ZONES, GRADING INTO (4) PROPYLITIC ZONE OF MORE REGIONAL EXTENT AND ASSOCIATED MOSTLY WITH EARLIER PERIODS OF MINERALIZATION, CHARACTERIZED BY QUARTZ-CARBONATE-CHLORITE ASSEMBLAGE CHANGING AT DEPTH TO QUARTZ-CARBONATE-ALBITE-EPIDOTE-CHLORITE, AND BY ALTERATION OF MAFIC MINERALS TO CHLORITE, EPIDOTE, SERICITE, AND OPAQUES (MAGNETITE, PYRITE, RARE CHALCOPYRITE, ALTERATION OF PLAGIOCLASE TO EPIDOTE CLAYS, AND CARBONATE (CALCITE?), AND ALTERATION OF K FELDSPAR TO CLAY AND EPIDOTE.ALTHOUGH OVERALL ALTERATION EFFECTS IN DISTRICT ARE VARIABLE DUE TO SUPERPOSITION OF SURFICIAL ALTERATION ON HYDROTHERMAL AND TO VARIABILITY OF ROCK TYPES, KRASOWSKI (1976) SUMMARIZED GENERAL ALTERATION SEQUENCE AROUND QUARTZ VEIN CUTTING PRECAMBRIAN GRANITE: (1) VEIN, CONSISTING OF MASSIVE QUARTZ AND SOME APORADIC DISSEMINATED OYRITE, SURROUNDED BY SILICIC ZONE (WITH SULFIDES) EXTENDING 1 TO 5 FT INTO NINBRECCIATED ROCK, ALTHOUGH QUARTZ-SERICITE MAY OCCUR ADJACENT TO QUARTZ VEIN IN PLACE OF SILICIFICATION; (2) IN SOME DEPOSITS, SILICIC ZONE MAY GRADE DIRECTLY INTO SERICITE ZONE AVERAGING 5 FT WIDE, GRADING INTO (3) ARGILLIC ZONE CHARACTERIZED BY INTENSE ALTERATION OF PLAGIOCLASE AND K FELDSPAR TO CLAYS, OCCASIONALLY VERY NONUNIFORM IN BRECCIA ZONES, GRADING INTO (4) PROPYLITIC ZONE OF MORE REGIONAL EXTENT AND ASSOCIATED MOSTLY WITH EARLIER PERIODS OF MINERALIZATION, CHARACTERIZED BY QUARTZ-CARBONATE-CHLORITE ASSEMBLAGE CHANGING AT DEPTH TO QUARTZ-CARBONATE-ALBITE-EPIDOTE-CHLORITE, AND BY ALTERATION OF MAFIC MINERALS TO CHLORITE, EPIDOTE, SERICITE, AND OPAQUES (MAGNETITE, PYRITE, RARE CHALCOPYRITE, ALTERATION OF PLAGIOCLASE TO EPIDOTE CLAYS, AND CARBONATE (CALCITE?), AND ALTERATION OF K FELDSPAR TO CLAY AND EPIDOTE.