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GEOLOGY OF THE SAN LUIS GOLD DEPOSIT', COLORADO <br />~ Clbola. The first recorded mining activity started <br />in 1890 near what is now known as the East Ore <br />Zone. Lead and silver were the principal metals <br />mined and the presence of galena accounts for the <br />historical name of "E1 Plome" for the San Luis <br />deposit. A gold mill with an amalgamation circuit <br />~ and later a cyanide circuit operated intermittently <br />from 1897 through 1934. The mill feed material <br />' mostly came from an open cut immediately south of <br />' the East Ore Zone (Fig. 2). Production records from <br />this time are limited (Deakin and Lehman, 1988). <br />San Luie was leased by ASARCO in 1931 and 1959, <br />and by Inspiration Copper Co. in 1968. Earth <br />Sciences. Inc. (ESI) leased the property in 1972 <br />and delineated portions of the East and West Ore <br />Zones. Other companies subsequently examined the <br />I property but lost interest. Battle Mountain Gold <br />Company (BMGC) optioned San Cuis from ESI in May. <br />1987. After initial test drilling, BMCC exercised <br />its option in August, 1987, and renamed the <br />deposit "San Cuis", after the Costilla County seat. <br />~ Development and feasibility work continued <br />throughout 1987 and 1988. BMGC has drilled 482 <br />exploration and development holes on the property <br />' to date. Of these drill holes, 89 were cared and <br />399 were drilled with reverse-circulation and <br />conventional roatary methods. A permit to mine was <br />issued by the State of Colorado Mined Land <br />Reclamation Division in March, 1988. An process <br />amendment to change to a complete milling process <br />i was granted bye the Mined Card Reclammation in <br />January, 1990. Pro,iected startup of production is <br />scheduled for the last quarter of 1990. <br />8: <br />Quaternary time (Tweto, 1979). <br />Smchronous with Neogene uplift of the Sangrc <br />Cristo range uas the development of the San Lui; <br />basin and deposition into it of poorly-sorted. <br />high-energy sediments of the Miocene to Pliocene <br />Santa Fe Formation and the Pliocene to Quaternar <br />Alamosa Formation. The northerly trending San C <br />basin extends over x:40 km from Taos N.H. to Ponr. <br />Pass, Colo., and is an integral part of the Rio <br />Cranda rift system (Fig. 1). Estimated depths o <br />basin fill in the San Luis area are on the order <br />several thousand meters (Keller and others, 1964 <br />The San Luis deposit lies ad.7 scent to a porti <br />of the basin known as the Culebra reentrant (Figg,, <br />1., Upson, 1939). Within the reentrant the Santl <br />Fe Formation ie present and deeply eroded at <br />elevations up to 10,)00 ft. Elsewhere in the ba. <br />Santa Fe sediments 1Le at less than 8,000 ft <br />elevation. It appea^s that the San Luis deposit <br />lies within a "failed" portion of the Rio Grande <br />rift where Tertiary :sedimentary rocks are preserc <br />at a higher atructur+sl level. <br />No large Tertiary intrusions are exposed in t! <br />vicinity of the San Luis deposit. The nearest <br />volumetrically signii'.icant intrusive centers are <br />the 26-22 Ma Quests uagmatic system to the south <br />(Lipman and others, )986), the 25-19 Ma Spanish <br />Peaks system to the east ('lweto, 1979), and aural) <br />sill-like intrusions near Ia Veta pass to the <br />north. Small dikes cf rhyolite and andeaite are <br />present at San Luis tut nowhere do these exceed <br />10's of meters in thickness. <br />Mineable Reserves <br />! The San Lula Deposit contains 11,021,500 tonnes <br />i (12. 199,000 tons) of ore at 1.4 g/t (0.040 oz/st) <br />of Hold in two mineable areas named the East Ore <br />i Zone and the West Ore Zone. The East Ore Zone <br />contains 1,277,300 Lonnes (1,408,000 tone) with an <br />' average grade of 1.68 g/t (.049 opt) of gold and <br />lies in the extreme east portion of the mine <br />property (Fig. 2). The Weat Ore Zone contains <br />9,744,000 tonnes (10,741, 000 tone) with an average <br />grade of 1.39 g/t (.039 opt) of Bold and lies in <br />the central and western portion of the mine <br />property (Fig. 2; Johnson, 1989). <br />GEO[,OGIC SEITING <br />The Sangre de Cristo Hountaine are a northerly <br />trending chain of 12.000- to 14,000-foot peaks <br />which extend over 340 km from Santa Fe, New Mexico <br />to Salida, Colorado. The San Luia deposit lies <br />between 8200 to 9000 ft. of elevation along the <br />west slope of the range at the seat edge of the San <br />Luis basin (Fie- 1). The bulk of the Sangre de <br />~ Cristo range in the deposit area consists of 1,800 <br />Ha Proterozoic grseieaea which have been intruded by <br />1,700 and 1,400 Ha granitic rocks (Tweto, 1979). <br />The eastern half of the range consists dominantly <br />~ of Paleozoic sedimentary rocks which were steeply <br />upturned and folded during Laramide orogenic <br />events. The present relative uplift of the range <br />is inferred to have occurred during Neogene and <br />GEOLOGY OF THE SAN LUIS DEPOSIT <br />Gold mineralization at San Luis occurs as <br />tabular bodies within and below a low-angle foul <br />zone in catacleatically deformed Precambrian <br />metamorphic rocks (Figs. 3, 4). The ma,)or <br />dislocation surface of the fault zone is preeerv <br />as unmineralized clay fault gouge which marks the <br />upper extent of economic mineralization. <br />Hangingwall to the fault zone are cataclasticall <br />deformed Precambrian natamorphic rocks and <br />Tertiary-Quaternary sediments with minor <br />interbedded volcanic :'lows. Higher gold grades 1 <br />the deposit are cloee:ty associated with <br />silicification and quartz-eericite-pyrite <br />alteration that have destroyed the primary <br />textures. Peripheral areas are dominated by <br />chlorite-carbonate anti quartz-epecularite <br />alteration which carry low-grade to non-detectabl <br />gold values. <br />Primary litholoeies <br />Precambrian rocks The dominant litholoeies at Say <br />Luis are Proterozoic tiotite gneiss and gneissic <br />granite. Biotite gneiss is s gran 1800 Ha (Tweto <br />1979) quartz-feldspar-biotite gneiss which is the <br />dominant rock type below the Low-angle fault zone <br />(Figs. 3, 4). Structurally deformed and <br />hydrothermally altered biotite gneiss is the host <br />to gold mineralization. Atypical modal <br />