New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/35 Geology of the Cochiti mining district, Sandoval County, New Mexico David J. Wronkiewicz, David I. Norman, Gary A. Parkison, and Karl M. Emanuel, 1984, pp. 219-222 in: Rio Grande Rift (Northern New Mexico), Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J.; [eds.], New Mexico Geological Society 35th Annual Fall Field Conference Guidebook, 379 p. This is one of many related papers that were included in the 1984 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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EMANUEL' 'Department of Geology, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801; 2NICOR Mineral Ventures, Inc., 2659-G Pan American Freeway, NE, Albuquerque, New Mexico 87107 INTRODUCTION the area of the Bland townsite with perhaps an additional 250 at the The Cochiti or Bland mining district is located in Sandoval County Albermarle townsite during this peak of activity. Prior to 1903, esti- in north-central New Mexico. The district lies about 80 km (50 mi) mated production from the district was $695,000 in gold and $345,000 due north of Albuquerque and 50 km (30 mi) due west of Santa Fe on in silver. The best description of the geology is by L. C. Gration in the southeast flank of the Jemez Mountains (Fig. 1). Elevations in the Lindgren and others (1910). Other historical accounts of the district district range from about 2,100 to 2,500 m. The topography in the area were published by Wynkoop (1900), Barbour (1908), and Statz (1912). is very rough, with steep northwest—southeast-trending canyons and More recent descriptions of the geology are by Bundy (1958), Elston intervening ridges. Two main drainages occur in the area, Bland Canyon (1967), and Stein (1983). The largest mine in the district is the Alber- to the east and Colle Canyon to the west. The old townsite of Bland marle, where total past production may have been 105,000 tons (95,250 has only a few buildings still standing and about three permanent res- mt) at an average grade of about 0.22 oz/ton (6.8 g/t) gold and 4.0 oz/ idents. ton (124 g/t) silver. Total recovered value is estimated to have been $667,500. The mine was developed by an inclined shaft some 220 m HISTORY deep and a total of about 1,500 m of drifts. One of the first cyanide Earliest prospecting and mining activity in the Cochiti district dates mills in New Mexico was installed at the mine site, with a capacity of back to 1893 when the first claims were located. Work at the two largest approximately 250 TPD. mines in the district, the Albermarle and the Lone Star, was concurrent Total past production from the Lone Star mine is estimated to have and lasted from about 1897 until 1903. Some smaller mines in the area, been 65,000 tons (59,100 mt) grading 0.20 oz/ton (6.2 g/t) gold and notably the Iron King, Washington, and Crown Point, were also active 4.0 oz/ton (124 g/t) silver. About 30,000 tons (27,250 mt) of this ore at this time. It has been estimated that nearly 1,000 people inhabited were produced between 1897 and 1903. The production from this period was either direct-shipped or processed at the Woodbury mill at the mouth of Bland Canyon. From 1914 to 1916 about 33,000 tons (30,000 mt) were mined and milled at the recently constructed Cossak mill. This 20-stamp cyanide leach facility was constructed adjacent to the Lone Star mine. Lesser activity from 1932 to 1936 resulted in about 2,000 tons (1,800 mt) of ore being direct-shipped to the El Paso smelter. Total production from the Washington, Iron King, and Crown Point mines was typically 5,000 tons (4,500 mt) or less each. This ore was generally hand-sorted and direct-shipped. A small flotation mill was built near the Iron King mine in 1947. After processing a total of 12 tons of ore, the mill was sold for back taxes in 1951. REGIONALGEOLOGY The Cochiti district is located on the southeast flank of the Jemez Mountains. This late Cenozoic volcanic landform owes its present top- ographic relief to the evolution of the Toledo and Valles calderas, as well as development of the Rio Grande rift and Nacimiento fault sys- tems. Much of the volcanic stratigraphy underlying the Cochiti district was formerly interpreted to be Eocene and Oligocene in age (Bailey and others, 1969; Smith and others, 1970). Stein (1983) reports a single K—Ar date which suggests that the volcanic stratigraphy of the Cochiti district might actually be of Miocene age and thus significantly younger than previously estimated. The informal Bland group (Stein, 1983) is composed of a predom- inantly hypabyssal and extrusive suite of gabbro, basalt, basaltic an- desite, dacite, and volcanic breccias. This sequence has been intruded by a composite sequence of quartz monzodiorites consisting of an older granular intrusion which has been injected by a younger porphyritic dike swarm (Fig. 2). These rocks appear to reflect the emplacement history of a single intrusive event. Outcrop exposures of the Bland group are dominated by quartz-monzodiorite porphyries followed by intimately mixed basaltic andesite and dacite collectively referred to as andesite. Outcrops of arkosic sandstone have been interpreted to be older than any rocks of the Bland group and may represent xenoliths or roof pendants within the younger intrusive rocks. Overlying and intruding the rocks of the Bland group are those of the Keres Group which has been subdivided into the Paliza Canyon Formation and the younger Bearhead Rhyolite (Smith and others, 1970). The Keres Group is petrogenetically similar to the Bland group, al- 220 WRONKIEWICZ et al. though stratigraphic continuity between the two groups has not been established as yet. The Paliza Canyon Formation generally consists of flow-banded rhyodacite, volcanic breccia, and massive andesite in about equal amounts. The Bearhead Rhyolite is generally seen to have intru- sive contacts with older rocks. This unit is white to gray, flow-banded, and is generally restricted to the western part of the district. The Tewa Group represents the latest episode of volcanism in the Jemez Mountains and is represented in the area by the Bandelier Tuff (Smith and others, 1970). The Bandelier has been subdivided into the basal air-fall and non-welded ash-flow deposits of the Otowi Member and the ovelrying welded ash-flow deposits of the Tshirege Member. The Tewa Group in the Cochiti district appears to occupy a paleovalley coincident with the present Bland Canyon. Quaternary surficial deposits cover large areas underlain by older units. Geochronology of the rocks of the Cochiti district (Fig. 2) is based on correlations and radiometric-age data, and has recently been sum- marized by Stein (1983). From an extensive review of existing and new K—Ar dates, the Keres Group appears to range from 13 to 6 m.y. (Gardner and Goff, this guidebook). A single K—Ar date from quartz- monzodiorite porphyry (Bland group) of 1L2 m.y. suggests that the Bland group and Keres Group may, in part, be contemporaneous and could be locally equivalent (Stein, 1983; Gardner and Goff, this guide- MINERALIZATION book). K—Ar dates from 7.1 to 6.2 m.y. have been obtained from the Bearhead Rhyolite, establishing it as the youngest intrusive unit in the Mineralization at the Cochiti district is fairly typical of epithermal area (Bailey and Smith, 1978; Gardner and Goff, this guidebook). quartz-vein-hosted gold—silver deposits (Buchanan, 1981). The min- eralization is similar to better known epithermal districts such as Oatman The structure of the district is characterized by numerous generally (Arizona), Mogollon (New Mexico), and Creed (Colorado).