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<br />III IIIIIIIIIIIII III
<br />Chapter 9
<br />GEOIAGY OF THE SAN LUIS GOLD DEPOSIT,
<br />COSTILCA COUNTY. COLORADO
<br />Robert G. Benson ~
<br />Battle Hountain Gold Company
<br />San Luis, Colorado t~ \`
<br />~ ,~K'Qh e
<br />David M. Jones ~~d~
<br />Battle Mountain Exploration Company
<br />Tucson, Arizona
<br />ABSTRACT
<br />The San Luis deposit of Costilla County,
<br />Colorado contains 11,021.000 tonnes (12.149,000
<br />tone) of ore at 1.9 g/t (0.040 oz/st) gold in two
<br />mineable zones. Gold mineralization ie associated
<br />with silicification and quartz-sericite-pyrite
<br />alteration localized in breccias along slow-angle
<br />"detachment" fault zone in Precambrian gneiss.
<br />Gangue minerals include chlorite, specularite,
<br />chalcopyrite, molybdenite, and fluorite.
<br />Mineralization ie mid-Tertiary in age and may have
<br />an indirect genetic relation to the emplacement of
<br />felsic intrusions. Portions of the ore body are
<br />unconformably overlain by sedimentary rocks of the
<br />Tertiary Santa Fe Formation. The deposit formed
<br />during the early stages of extensional tectonism
<br />along the eastern edge of the present day Rio
<br />Grande rift.
<br />INTRODUCTION
<br />Location
<br />The San Luis deposit ie located about 5 miles
<br />northeast of the town of San Luis in Costilla
<br />County, south-central Colorado (Fig 1. ). The
<br />deposit topography conaiets of gentle southern-
<br />dipping slopes at elevations from 8200 ft. to over
<br />9000 ft. The dominant vegetation ie sagebrush,
<br />suniper, and pinon pine. Average rainfall at the
<br />mine site is 12" annually. The Rito Seco flows
<br />westward across the southern part of the deposit
<br />area.
<br />Historical activity and production
<br />The San Cuie deposit lies within the historical
<br />Sangre de Cristo land grant which was deeded by the
<br />Mexican government in 1844 to persons of purported
<br />Spanish ancestry (Simmons, 1979). After the 1848
<br />Treaty of Guadalupe Hidalgo, the U.S. Congress
<br />formally recognized the grant ae a private
<br />inholding in the newly established New Mexico
<br />Territory.
<br />First knowledge of Sold mineralization at San
<br />Luis probably dates back to the late 16th or early
<br />17th century when Spanish expeditions from Mexico
<br />went in search of the mythical golden cities of
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<br />EXPLANATION
<br />a SWlmanfary rata n e oe9oe.n. and ae°r
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<br />Figure 1. Locntion map of the San Luis Deposit and
<br />the San Luis Basin (modified from
<br />Keller, et al, 1984)
<br />83
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