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Bulletin 48—Geology of the Cerrillos area, Santa Fe County, New Mexico

By A. E. Disbrow and W. C. Stoll, 1957, reprinted 1980, 73 pp., 2 tables, 8 figs., 5 plates, 1 index.

The center of the Cerillos area lies 15 mi southwest of the city of Santa Fe in north-central NM. Strata ranging in age from the Upper Triassic Chinle formation to recent alluvium are exposed in the area. Marine and continental sediments, chiefly sand and clay, were deposited during repeated advances and withdrawals of the Mesozoic sea. Throughout the Cenozoic era the Cerillos area has been above sea level and subjected to erosion and the deposition of continental material.

Laramide uplift of the Sangre de Cristo Range resulted in a thinning by erosion of Upper Cretaceous at a rate of 130 ft per mile in a northeasterly direction. Subsequent deposition of sand, clay, and gravel of the Galisteo Formation overlapped this surface toward the Sangre de Cristo Range. In Oligocene time uplift and complex folding and faulting occurred during the emplacement of a series of monzonitic stocks, plugs, laccoliths, sills, and dikes. The first intrusion was hornblende monzonite porphyry, the second and third were hornblende-augite monzonite porphyry, and the fourth was augite-biotite monzonite. With the possible exception of the first intrusive, each intrusive type has an extrusive equivalent.

After consolidation of the youngest monzonitic intrusive, a system of fracturing developed. Subsequent mineralizing solutions formed deposits along many of these fractures. These fractures also provided planes of weakness along which Miocene limburgite magma was intruded to form many dikes throughout the area. Associated volcanic rocks of the Cieneguilla limburgite rest unconformably on an erosion surface that cuts into monzonitic intrusive and volcanic rocks along the northern edge of the area. The Cieneguilla limburgite is conformably overlain by lacustrine limestone, clay, and tuffaceous sandstone assigned to the lowermost unit of the Santa Fe Group.

In the Cerillos area, deformation during the late Pliocene Sandia uplift resulted in northward trending normal faults along the western edge of the area. The western block was dropped about 4,500 ft to form a part of the Rio Grande depression. In the eastern block, strata were tilted about 12 degrees to the east. During the Pleistocene time the Ortiz pediment was graded to the Rio Grande from the highlands of the Cerillos, the Ortiz Mountains, and the Sangre de Cristo Range. Sand and gravel of the Ancha Formation were deposited on the pediment. Upon this gravel, in the northwestern sector of the area, Pleistocene Cuerbio basalt flows were erupted.

After extrusion of the Cuerbio basalt, recurrence of normal faulting along the western edge of the area again dropped the western block, deepening the Rio Grande depression. A period of dissection of the lava- and gravel-covered pediment followed this faulting. The Cerillos mining district has produced zinc, lead, copper, silver, and gold from one group of deposits and turquoise from another. Most of the turquoise in this area was produced from the mines at Turquoise Hill and Mt. Chalchihuitl from prehistoric time to the present. Turquoise occurs in nodules in fracture zones and in narrow veinlets which cut intrusive and volcanic rocks that have been strongly bleached by hydrothermal alteration.

The earliest metal mining in the district was by the Spaniards in the 17th century. Modern metal mining started with a boom period in 1879 but subsided after a few years. From 1909 to 1952 a production of 25,843 tons of metalliferous ore was obtained from 12 mines. This ore contained 920.68 ounces of gold, 22,187 ounces of silver, 174,894 pounds of copper, 1,379,040 pounds of lead, and 1,671,527 pounds of zinc. The Cash Entry and Tom Payne mines were the chief producers. The primary metalliferous ores contain sphalerite, galena, chalcopyrite, and pyrite, as well as small quantities of silver and gold. Quartz, ankerite, calcite, siderite, barite, opal, and chalcedony are the most prominent gangue minerals.

Ore mineralization occurred in shear zones from a few inches to 16 ft in width, which are chiefly within the main intrusive masses. Most of the mineralized zones are traceable for a few hundred feet; a few are continuos for distances of 1,500–2,500 ft. Most of the ore bodies occur as shoots in the shear zones. Within these shoots the sulfides form aggregates and crustified bands or are disseminated in vein quartz or sheared wall rock.

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