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88 GOLD '90 strikes east-west and dips 15' to 25° to the south (Pig. 2). The fault changes to a more northerly strike and dip 15° to 25° to the west dip in the northwest part of the Weat Ore Zone. The presence of brecciated, folded, and non-diatturbed quartz veins in single outcrops indicates that faulting was at least in part syn-mineralization. Movement on the fault ie inferred to be down-dip to the south and southwest. This ie based upon interpretations of slickeneide features, strikes and dips 'of hanginguall foliations, and the apparent northeastward rotation of beds and flows in the Santa Fe Formation. The clay zone which defines the actual fault slip surface hea been er»ded from the Baet Ore Zone, but caps the West Ore Zone. The deposit area ie cross-cut by numerous north- eouth striking, steeply-dipping faults. Despite this, the Eeat and West Ore Zones do not appear to have been eienificantly offset by poat- mineralization fault movement. Mineralization in the West Ore Zone is partially cutoff on the north by a steeply-dipping, east-nest striking fault. Mineralization in the East Ore Zone may be partially cutoff on the south by a steeply-dipping east-northeast striking fault. Inferred north-south striking normal faults neat of the deposit probably downdrop basement rocks and sediments to the west hundreds to thousands of meters. Rydrothermal Alteration Four alteration assemblages are identified at San Iuis: (1) silica replacementsfveins, (2) quartz-sericite-pyrite replacements/veins, (3) chlorite-carbonate replacements/veins, and (4) quartz-specularite veins. The moat intense alteration is silica replacement/veining which ie commonly associated with high-grade gold mineralization immediately beneath the clay zone. Silicification typically grades into quartz- sericite-pyrite alteration. The least intense hydrothermal alteration is chlorite-carbonate replacement/ veining. Though discussed here separately, the paragenetic and spatial distributions of these assemblages are not cell established at this time. p„ r.._a Cular;+e si„= Quartz-specularite veins and aeaociated specularite fracture coatings formed early along the low-angle fault zone. Specularite ie abundant peripheral to the deposit but appears to have been overprinted by more intense silica- serlcite events Sn the core of the deposit. Gold is not sasociated with quartz-specularite veins. Silica r ola m nra/vim; fSili iFi -+ on) Silica replacement and veining occur ae pervasive matrix replacement of biotite gneiss breccis. Silicification also occurs ae a more localized replacement of biotite gneiss cataclasite. Silica ranges in color from dark Lo light gray. Dark gray silica contains <_3X fine-grained disseminated pyrite. Very high gold grades are sasociated with dark gray silicification which is most intense immediately below the fault clay in the Weat Ore Zone. Dark gray silicification grades laterally and downward into light gray sllcificatioa and eericitic alteration. Replacement silicification is commonly cross-cut by quarter veinlets. The relationship of these quartz veinlets to mineralization is not understood at this time. ~+art_-a ri 7. -tvri .. r_ala em n.4 /ve'na Quart2- aericite-pyrite alteration occurs as an intense gray-green matrix replacement of biotite gneiss breccis. Quartz veinlets commonly cross-out the altered matrix. Pyrite associated with quartz- eericite-pyrite alteration is generally localized in clots and veinlets and is less commonly disseminated. Quartz-sericite-pyrite alteration grades upward into dark gray silicification and downward into chlorite-caroonate alteration. Gold mineralization is associated with quartz-sericite- pyrlte alteration but is not ae intense as in the dark-gray silicification. Quartz-sericite-pyrite alteration crops out in the Bast Ore Zone. Ch)o +_ -. *bo a. v_ina Chlorite-carbonate alteration is relatively widespread and includes an overall lees intense alteration assemblage of chlorite ± carbonate ± magnetite ± clay t pyrite. Chlorite commonly occurs a: a foliation replacement in biotite gneiss catacleaite and biotite amiss. In thin section, primary quartz, microcline, and plagioclase are partially replaced by fine-grained clay, sericite, and carbonate, and biotite is replaced by chlorite ± magnetite. Mineralization Mineralization is present in hydrothermally altered zones in the biotite gneiss breccfa and to a leaser extent in biotite aneisa cataclasite. Gold mineralization correlates slightly with silicification and sulfide ~:ontent. The dominant sulfide present ie pyrite, in three polvmorphs, with minor occurrences of chalcopyrite, molybdenite, galena. and possible acanthite (Suthard, 1968). The average gold to silver ratio ie 1: 1. Specularite and chlorite are usually found in more weakly mineralized wrens and are aeaociated with less intense hydrothermal alteration. Fluorite commonly is present in narrow quartz- fluorite veinlets and in open-apace fillings. Three forms of pyrite are present in mineralized zones. Two appear to have no consistent association with ore-grade gold mineralization. Euhedral. untarnished. relatively coarse-grained pyrite has little correlation with gold mineralization. Untarnished, poorly-twinned pYritohedrone also have little or no correlation with gold mineralization. Irregular fine-grained clots of tarnished pyrite appear to have the moat consistent correlation with gold mineralization. Ore microscopy studies of sulfide conrentratea indicate the preaenca, in order of decreasing abundance, of pyrite. chalcopyr•ite, hematite, geothite, anataee, galena, m<,lybdenite, covellite, gold, and pyrrhotite (Scharer and Fuchs. 199A). Gold ie present along fractures in F'vrite grains, as small blebs within pvrite grains. and as discrete particles. Gold grain size ranges from 2 to 160 microns, averaging 20 microns.