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
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