New Mexico Geological Society Annual Spring Meeting — Abstracts

Dakota Sandstone crops out east of Ancho in central New Mexico,forming the north rim of a southeast-draining canyon. The Dakota deposits consist of clay shale and sandy clay interbedded with sandstones. Resistant sandstones form an escarpment around the perimeter of a synclinal basin and topographic high which is 3 mi across at its short axis. Clay-rich beds are exposed in the southwest and south face of the escarpment, locally striking N25°W and dipping 30° NE.

The shales contain 7 to 12 wt.% sand and larger-size, 66 to 68 wt.% silt-size, and 20 to 27 wt.% clay-size particles. Clay minerals are dominantly kaolinite, with lesser amounts of mixed-layer illite-smectite (I/S) and minor illite. Refractory clay was mined chiefly from four stratigraphic horizons, by underground methods in the escarpment face and by open-pit methods in the escarpment crest. Most of the underground-mine openings have collapsed and there is no record of past production, however, the uniform thickness of clay and shale beds in outcrop indicates significant reserves.

The refractory clay was mined from 1902 until 1922. Phelps Dodge Corporation purchased the plant, located just east of the town on Ancho, and the mines in 1917 from an unknown Iowa company. Phelps Dodge enlarged the plant to 16 round-downdraft kilns and other facilities to produce structural tile, face brick, and firebrick. The structural tile and face brick were produced from reddish argillaceous siltstone of the Artesia Group adjacent to the plant in Ancho. Ancho No. 1 refractory brick from the Dakota Formation was sold as far north as Kansas.

Van Sandt reported, in an unpublished 1964 U.S. Bureau of Mines report, reported the results of four refractory-clay analyses of the clay beds. The lowest sample, near the base of the middle shale member of the Dakota Formation, was of a 6-ft brownish-gray basal shale bed that expanded at 1315°C (2400°F) and had a PCE value of 16. The second sample was taken from a 6-ft-thick dark-gray to grayish-black hard shale some 50 ft stratigraphically above the basal clay bed. It burned white at 1150°C (2100°F) and had a fired linear shrinkage of a constant 10% from 1205°C through 1315°C (2200°F through 2400°F). Its PCE rating was 30-31, indicating it is a intermediate refractory clay. The third sample was of a 5-ft-thick gray shale bed an unstated distance above the second sampled bed, but stratigraphically 10 ft below the base of the open pit at the top of the escarpment. It had a PCE rating of 20-23, making it possibly acceptable as a low-duty refractory clay. The uppermost and fourth sample represented a composite of 20 ft of clay-rich beds in the upper part of the workings and was exposed along the crest of the escarpment. Refractory tests indicated it has a PCE rating of 23 and could be used as a low-duty refractory clay.

In summary, the Ancho refractory-clay resources are kaolinitic shales with lesser amounts of liS, illite, and non-clay minerals, in particular quartz. Although considerable reserves are present, it is doubtful that these deposits will be mined again. The units are too thin and with dips 30° steep, making highly expensive underground mining the only practical method of extracting the clay. In addition, the clay is only low-duty to immediate-duty refractory grade, too low to command the premium price paid for highgrade refractory clays.