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GEOLOGY OF THE SAN LUIS GOLD DEPOSIT. COLORADO A-mode factor analysis was applied to multi- element whole rock data from 85 drill hole intercepts in the West Ore Zone (Junes, 1985). Cold mineralization factors suggest a correlation of gold With silver, lead, copper, molybdenum, and fluorine. These correlations indicate that gold deposition at San Luis Nos part of a polymetallic mineralizing event. Gold also correlates With both iron-rich and iron-poor factors reflecting an inconsistent association with pyrite. GEOLOGY OF THE SAN LUIS DEFCISIT ORE ZONES The San Luis Deposit contains 11,021,500 tonnes (12,199,000 tons) of ore at 1.4 g/t (0.040 oz/st) of gold in two mineable areas named the East Ore Zone and the West Ore Zone (Fig. 2). The East Ore Zone contains 1,277,300 tonnes (1,908,000 tons) with an average grade of 1.68 g/t (.049 oz/st) of gold and lies in the extreme east portion of the mine property (Fig. 2). The West Ore Zone contains 9,744,000 tonnes (10,741,000 tons) with an average grade of 1.34 g/t (.039 oz/st) of gold and lies in the central and western portion of the mine property (Johnson, 1989). East Ore Zone The East Ore Zone strikes east-west and dips 15° to 25° to the south. Ore is mostly confined to biotite gneiss breccia with minor mineralization occuring below in biotite gneiss cataclasite. The dominant alteration Se intense quartz-aericite- pyrite alteration of biotite gneiss breccia. Silicification is intense but less so than in the West Ore Zone. Sulfides occur in clots and pods more than in disseminations. Breccia cleat size generally decreases up section from the lower parts of the biotite gneiss breccia towards the clay zone. Biotite gneiss breccia zones range in thickness from 0 to 30m, averaging 15m. In the more intensely quartz-sericite altered breccias, claets constitute less than lOX of the rock. The clay zone is not present in the Esat Ore Zone but ie inferred to have capped the deposit prior to erosion. Asymmetrical folding suggests normal movement along the law-angle fault. West Ore Zone Tha strike and dip of the West Ore Zone ie roughly conformable to that of the host fault breccia. As the strike of the fault changes from nearly east-west in the southern portion of the West Ore Zone to north-south in the northern portion of the zone (Fig. 2) so dose the strike of the ore zone. The Weat Ore Zone dips 15° to 25° degrees to the south in the southern portion of the zone and 15° to 25° degrees to the west in the northern portion of the zone. Breccia cleats are typically <2mm and make up upwards of 15X of the rock volume. Breccia thickness in the ranges from 0 to 45 meters, averaging 20 meters. The hangingwall of the West Ora Zone is fault clay and gneissic granite (Fig. 4); the thickness of the gneissic granite section decreases from north to south. 89 The West Ure Zone is confined to intensely silicified and sericitizer! biotite gneiss breccia and cataclasite. Pyrite content varies from XY t Y%. The highest gold grades of the 9eposit are within the West Ore .:one in dark gray, pyritic 9i11Ca directly below the clay zone. The most intense silicification of the deposit is within t West Ore Zone where primary breccia textures are overprinted and destroyed by silica replacement a veining. )ISCUSSION The San Luis deposit developed ae part of a eilicificationiquartz-sericite-pyrite alteration event hosted by a low-angle fault zone. Mineralization is polvmetallic with Bold depositic closely associated with silver, lead. copper, molybdenum. and fluorine. Textural evidence clearly indicate that the major episode of faultir predated alteration and mineralization. A simple genetic model of the deposit can be derived from discussion of the origin of the fault zone and th< origin of the subsequent hydrothermal system. Origin of the fault zcme In Laramide time t}ie area of the present day Sangre de Cristo range underwent strong ' compressional deformation which resulted in thrust faulting and attendant folding in Precambrian •,nd Paleozoic stratigraphies (Tweto, 1979). Hors recent Neogene extensional normal faulting has dissected this Iaramide uplift. On the basis ~,f this geologic history, the fault zone at San Luie has most commonly been interpreted as a thrust ' fault. We depart from this interpretation and suggest an extensional "detachment"-style origin for the fault. zone. Arguments for detachment-style faulting at the San Luis deposit are based upon local and regional observations. The moat important of these argument are: (1) the preservation of the fault clay zone a its contact with the Santa Fe Formation: (2) the location of the fault :one at the edge of Neogene depositional basins; (3) the tilting of the Santa Fe Formation above the fault Zone: and, i4) the evidence that faulting was in part synchronous wit emplacement of felsite intrusions. First, the fault clay zone always separates hangingwall gneissic granite from footwall biotite gneiss. Ia several core ho lea Precambrian hangingwall rocks are not present and the fault clay ie in direct contact with Santa Fe Formation. If the Santa Fe were deposited on an eroded Laramide throat fault surface. the clay zone would have been rapidly removed long before sedimentation. Clay zone preservation could only occur if it were immediately buried by unconsolidated Santa Fe sediments upon removal of the overlying Precambrian plate. This constrains major movement of the upper plate ae synchronous with development of Santa Fe depositional basins, Neogene event.