Precambrian Rocks of South-Central New Mexico

NEW MEXICO GEOLOGICAL SOCIETY • SIXTH FIELD CONFERENCE • SOUTH-CENTRAL NEW MEXICO

PRECAMBRIAN ROCKS OF SOUTH-CENTRAL NEW MEXICO

This paper, which describes briefly the Pre- quartz-mica schist, and quartzite. In some places an cambrian rocks of the region in which the Sixth older granite intruded into this series has been con- Field Conference of the New Mexico Geological Soci- verted to gneiss. Locally unmetamorphosed" basic ety is to be held, is essentially a review of published igneous rocks are intruded into the older rocks. The literature, and is intended to provide basic informa- youngest and one of the most widespread of the Pre- tion for more extensive and detailed investigations. cambrian rocks is a coarse-grained white or pink The literature describing the Precambrian rocks of granite with associated pegmatite, aplite, and lam- New Mexico is not voluminous and, but for a few prophyric dikes." exceptions, is limited to the location and petrology Stark and Dapples (1946) examined the Precam- of the Precambrian rocks and their associated min- brian rocks of the Los Pinos Mountains of north- eral deposits. More extensive studies have been made eastern Socorro County and summarized their de- by Kelley and Silver (1952) , Kuellme • (1954) , Mont- scription as follows : gomery (1953) , and Stark and Dapples (1946) , and "The pre-Cambrian core, the area of greatest from their works some conception of Precambrian relief, is composed of the Sais quartzite (oldest) , problems in New Mexico can be gained. Toward this Blue Springs muscovite schist (originally siltstones) , end comment upon their work is made. White Ridge quartzite, and Sevilleta rhyolite, all intruded by the Los Pinos (pre-Cambrian) granite. GENERAL SURVEY OF THE The pre-Cambrian bedded rocks exceed 12,000 feet in PRECAMBRIAN OF NEW MEXICO thickness, of which over 4,500 feet is rhyolite flows. Exposures of Precambrian rocks (Darton 19- All dip westward (average 50 r ), and schistosity 28a) , as compared with those of the Paleozoic and parallels the dip. Schistosity of the Los Pinos granite younger sedimentary rocks, are relatively not exten- (N. 20 e E.) parallels that in other pre-Cambrian sive. They are limited to uplifted areas where they rocks. Granitization is pronounced along its borders, have been laid bare by erosion. They occur in the particularly along the contacts with the White Ridge Sangre de Cristo and other mountains of north- quartzite." central New Mexico, in the Zuni, Manzano, Los Montgomery (1953) made a detailed investi- Pinos, Pedernal, Ladrones, and Magdalena Moun- gation of the Precambrian rocks of the Picuris tains, and as smaller areas elsewhere. In southern Range of north-central New Mexico. He recognized New Mexico they are found along the eastern base two formations, the older Ortega formation and the of the San Andres Mountains, along the west front younger Vadito formation. An unconformity sepa- of the Oscura, Fra Cristobal, and Caballo Mountains, rates the two formations, and both are intruded in the Black Range, and in the Burro Mountains. by the Embudo granite, also of Precambrian age, Other exposures of Precambrian rock are present and diabase dikes of probable late Precambrian age. Thus a start toward more comprehensive inves- in the Silver City district, Cooks Range, and Florida Mountains and as small areas elsewhere. The Pre- tigation of the Precambrian rocks of New Mexico has cambrian of the subsurface of southeastern New been made but much remains to be done before these Mexico has been studied by Flawn (1954a, b). rocks can be divided into units, their petrographic In general, knowledge of the petrologic and character and relative ages determined, the units petrographic character, and the relative ages of the correlated from one region to another, and the struc- Precambrian rock units is limited. Granites, gneisses, tural history deciphered. Detailed mapping and pet- schists, and associated rocks are known and, accord- rographic identification of Precambrian rock types ing to Kelley (1949, p. 21) ; "Both Archean and should prove to be a fruitful field of research. Algonkian rocks may be present in New Mexico, but their definite differentiation has not been made. SAN ANDRES MOUNTAINS In many places the oldest rocks of the Precambrian The Precambrian rocks of the San Andres consist of a thick series of isoclinally folded and Mountains have not been described in detail. We are dynamically metamorphosed igneous and sedimen- indebted to Darton (1928a, b) , Kelley (1949) , and tary rocks. These have been converted to rhyolitic Lasky (1932) for most of our information concern- gneiss and schist, amphibolite, talc schist, greenstone, ing these rocks and the associated mineral deposits.

62 NEW MEXICO GEOLOGICAL SOCIETY • SIXTH FIELD CONFERENCE • SOUTH-CENTRAL NEW MEXICO

The Precambrian rocks are shown on the geologic numerous acidic and basic dikes, and intersected by map of New Mexico (Darton, 1928a) as a narrow quartz veins having chiefly a northwest-southeast strip along the eastern slope of the mountains, ex- course. The granite is of reddish or brownish color tending from the Organ Mountains at the south and much jointed. almost to the Oscura Mountains at the north, a dis- Darton (1928b, p. 320) mentioned the Precam- tance of approximately seventy-five miles. They brian rocks of the Caballo Mountains briefly and have been uplifted by faulting along the west margin stated that massive granite predominates with some of the Tularosa Basin and exposed to erosion. gneiss or schist, cut locally by darker rocks that Darton (1928b, p. 183) reported that the Pre- appear to be diorites. cambrian rocks of the San Andres Mountains are in Kelley and Silver (1952, p. 32-33) described the general similar to the Precambrian rocks in many Precambrian rocks in greater detail : other uplifts in New Mexico. The most abundant "In general the Precambrian terrain is irregular rock of this age is massive, coarse-grained granite. and knobby. It commonly has a reddish or brownish Schist was observed in Cottonwood, Deadman, Sul- aspect, owing largely to the weathering of the pink phur, Membrillo, and San Andres Canyons. Diorite ferroan feldspars of the granite and granite gneiss. containing much hornblende was reported by him in The dominant rock of the Precambrian is a pinkish Lostman Canyon, and granite in Goodfortune and coarse-grained granite. In many outcrops the rock Membrillo Canyons, near Rhodes Canyon, and from is gneissoid. J. J. Glass (Rothrock, et al, 1946, p. 143) Lava Gap northward. found that the dominant feldspar of the granite is Kelley (1949, p. 228) briefly mentioned the Pre- microcline and that the chief dark minerals are bio- cambrian rocks and stated that they are dominantly tite and magnetite. Small dikes of simple granite pink granite, but that considerable areas of schist, pegmatite and silexite are fairly common. Small lam- silicic and basic phyllites, talc schist, quartzite, and prophyric dikes are occasionally present. Here and dioritic rocks exist. The intrusive pink granite has there, small patches of mica schist, metadiorite, much disturbed the structure of the metamorphic greenstone, and other rocks occur." rocks but uniformity of trend over great distances They reported between Granite Wash and Bur- is apparent. bank Canyon several square miles of metamorphic Lasky (1932, p. 73-85) , in reporting on the ore rocks, the steep foliation of which strikes mainly deposits of Socorro County, mentioned the Precam- almost due east. These rocks are chiefly mica schist, brian rocks of the San Andres Mountains and the but granite gneiss, metadiorite, and greenstone are associated mineral deposits. He stated that granite is common. Very little quartzite is present. The meta- the chief rock of the Precambrian, but schist is very morphic rocks are intruded by pink granite. Age prominent locally. Practically all of the ore deposits units of the Precambrian rocks were not determined, on the eastern slope of the range are within or close but the authors express their belief that the metamor- to the Precambrian rocks, and are associated wi th phic rocks may be of Archeozoic age, and that the the faulting and related fracturing that elevated the granite may be of either late Archeozoic or early rocks of this age. The bismuth deposits of Grand- Proterozoic age. view Canyon occur at the contact of Precambrian Harley (1934, p. 197, 213), in describing the schist and granite, and the copper deposits in Sulphur mineral deposits of the Caballo Mountains, mentioned Canyon is in schist. Other occurrences of minor min- the rock types which occur in the Precambrian and eral deposits within or related to the Precambrian reports that the gold mined in the Shandon placers rocks are described. in and near Apache Canyon has been derived from the quartz veins in the Precambrian granites and CABALLO MOUNTAINS schists. Rothrock, Johnson, and Hahn (1946, p. 146- 166) and Jahns (1942) , reported occurrences of The Precambrian rocks of the Caballo Moun- fluorite in veins in Precambrian rocks. Kelley and tains are exposed with local interruptions along the Silver (1952, p. 33) stated that the mineral deposits western base of these mountains from a point east in the Precambrian rocks include fissure veins con- of Truth or Consequences southward approximately taining fluorspar, lead, and copper, probably of Ter- twenty miles (Kelley and Silver, 1952). They have tiary age, and gold in small veins probably of Pre- not been examined in great detail. cambrian age. Lindgren, Graton, and Gordon (1910, p. 225) reported granite in the vicinity of Shandon associat- MUD SPRINGS MOUNTAINS ed with hornblende schist and granitic gneiss, cut by Precambrian rocks occupy a small area of about

63 NEW MEXICO GEOLOGICAL SOCIETY • SIXTH FIELD CONFERENCE • SOUTH-CENTRAL NEW MEXICO

one square mile or less in the Mud Springs Mountains brian rocks were reported. about three miles northwest of Truth or Consequen- SIERRA CUCHILLO NEGRO ces. Harley (1934, p. 193) reported granite of Pre- cambrian age in this area and Kelley and Silver Precambrian rocks have been reported in the (1952, p. 253) described the Precambrian rocks Sierra Cuchillo Negro of northwestern Sierra County briefly as pink granite with small pegmatite dikes but apparently have not been described in detail. and inclusions of biotite schist. Kelley (1949, p. 202) stated that Precambrian meta- andesite lies about 350 feet below and west of a saddle FRA CRISTOBAL RANGE in the range in the vicinity of the iron deposits. The The Precambrian rocks of the Fra Cristobal New Mexico Geological Society Guidebook (1952, p. Range are represented on the geological map of New 79) states that the Cuchillo Negro Mountains are a Mexico (Darton, 1928a) by an area about ten miles small, eastward tilted fault block consisting of rocks long and a mile or slightly more wide. They are shown which range from Precambrian to Tertiary in age. as separated from the Paleozoic rocks by a north- south fault. REFERENCES Harley (1934, p. 196) described the Precam- The appended bibliography includes only the papers of brian rocks as predominantly granite. Schist is lo- those authors cited in the text. It does not list the works cally prominent, and at the north end of the range of earlier geologists who have mentioned or described occurrences of Precambrian rocks in New Mexico, and it is there- diorite containing abundant hornblende is exposed. fore incomplete. References to the observations of these Pegmatite dikes occur locally. No important mineral geologists appear for the most part in the papers listed. Darton, N. H. (1928a) Geologic Map of New Mexico: U. S. deposits were reported. Geol. Survey. (1928b) "Red beds" and associated formations THE BLACK RANGE in New Mexico: U. S. Geol Survey Bull. 794. Flawn, Peter T. (1954a) Texas basement rocks; a progress The geologic map of New Mexico (Darton, 1928 report: Amer. Assoc. Petrol. Geol. Bull., v. 38, p. 900-912. a) shows an area of Precambrian rock in the Black (1954b) Summary of southeast New Mexico basement rocks: N. M. Geol. Soc., Fifth Field Conference, Range of western Sierra County. Darton mentioned p. 114-116. the occurrence of granite (1928b, p. 326) in these Harley, G. T. (1934) The geology and ore deposits of Sierra County, New Mexico. New Mexico Bur. Mines and Mineral mountains, and Harley (1934, p. 98) reported Pre- Res. Bull. 12. cambrian granite, gneiss, and schist in the Kingston Jahns, R. H. (1942) Fluorspar deposits near Hot Springs, New Mexico: U. S. Geol. Survey unpublished report, p. 9. district. Kuellmer (1954, p. 6-10) described these Kelley, V. C. (1949) Geology and economics of New Mexico rocks in some detail and noted that the Precambrian iron-ore deposits: Univ. of New Mexico Pub. in Geol., No. 2. rocks at Kingston consist of graywacke, granite, and Silver, C. (1952) Geology of the Caballo and metadiabase. He stated that detailed mapping Mountains: Univ. of New Mexico Pub. in Geol., No. 4. Kuellmer, F. J. (1954) Geologic section of the Black Range has shown that the exposures of Precambrian rocks at Kingston, New Mexico: New Mexico Bur. Mines and are small and discontinuous in the Kingston region, Mineral Res. Bull. 33. Lasky, S. G. (1932) The ore deposits of Socorro County, New in contrast with Dartons map (1928a). The gray- Mexico: New Mexico Bur. Mines and Mineral Res. Bull. 8. wacke is fine to coarse-grained, massive to thin-bed- Lindgren, W., Graton, L. C., and Gordon, C. H. (1910) The ore deposits of New Mexico: U. S. Geol. Survey Prof. ded, gray to dark green or black. It weathers green Paper 68. to brown, and it is penetrated by thin quartz string- Montgomery, Arthur (1953) Pre-Cambrian geology of the Picuris Range, north-central New Mexico: New Mexico ers. The granite is brown to pink and medium-grain- Bur. Mines and Mineral Res. Bull. 30. ed. It weathers to a reddish or yellowish brown. N. M. Geol. Soc. (1952) Third Field Conference. Rothrock, H. E., Johnson, C. H., and Hahn, A. D. (1946) Metadiabase dikes transect both the graywacke and Flourspar Resources of New Mexico: New Mexico Bur. granite. It is a darker green to black rock, medium to Mines and Mineral Res. Bull. 21. Stark, J. T. and Dapples, E. C. (1946) Geology of the Los fine grained, and it contains abundant laths of a Pinos Mountains, New Mexico: Geol. Soc. America Bull., chloritic material. No mineral deposits in Precam- v. 57, p. 1121-1172.