Trigo Mts, La Paz Co., Arizona, USA

‡Ref.: Wilson, E.D. (1933) Geology and Mineral Deposits of Southern Yuma County, Arizona. Arizona Bureau of Mines Bulletin 134: 50-51.

Wilson, E.D., et al (1951), Arizona zinc and lead deposits, part II, Arizona Bureau of Mines Bull. 158: 83-87.

Farnham, L.L. & L.A. Stewart (1958), Manganese deposits of western Arizona, U.S. Bureau of Mines Information Circular 7843.

Galbraith, F.W. & D.J. Brennan (1959), Minerals of Arizona: 49.

Parker, F.Z. (1966) The Geology and Mineral Deposits of the Silver District Trigo Mountains, Yuma County, Arizona. Masters Thesis, San Diego State College: 131-132.

Garner, W.E., Frost, E.G., Tanges, S.E., and Germinario, M.P. (1982) Mid-Tertiary detachment faulting and mineralization in the Trigo Mountains, Yuma county, Arizona, in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada - Anderson-Hamilton Volume: San Diego, Cordilleran Publishers, p. 158-171.

Weaver, B.F. (1982) Reconnaissance geology and K-Ar geochronology of the Trigo Mountains detachment terrane, Yuma County, Arizona: San Diego, San Diego State University, M.S. thesis, 97 p.

Tosdal, R.M. (1986) Mesozoic ductile deformations in the southern Dome Rock Mountains, northern Trigo Mountains, Trigo Peaks and Livingston Hills, southwestern Arizona, and Mule Mountains, southeastern California, in Beatty, B., and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest: 16: 62-71.

Barth, A.P., Tosdal, R.M., and Wooden, J.L. (1988) Characteristics and implications of Triassic and Jurassic granitoids in the San Gabriel, Mule, and Trigo Mountains, southern California and southwestern Arizona [abs.]: Geological Society of America Abstracts with Programs: 20(3): 141.

Bullock, J.H., Jr., Vaughn, R.B., Smith, D.B., Welsch, E., and Fey, D.L. (1989) Analytical results and sample locality map of stream-sediment, heavy-mineral-concentrate, and rock samples from the Trigo Mountain Wilderness Study Area (AZ-050-023B), La Paz County, Arizona: U.S. Geological Survey Open-File Report 89-0472, 24 p., 1 sheet, scale 1:50,000.

Sherrod, D.R., Tosdal, R.M., Vaughn, R.B., Smith, D.B., Kleinkopf, M.D., and Wood, R.H., II (1989) Mineral resources of the Trigo Mountains Wilderness Study Area, La Paz County, Arizona, Chapter J, in Mineral resources of Wilderness Study Areas: Southwestern and south-central Arizona: U.S. Geological Survey Bulletin 1702-J, p. J1-J16.

Sherrod, D.R., and Hughes, K.M. (1993) Tertiary stratigraphy of the southern Trigo Mountains, Ariz., and eastern Chocolate Mountains, Calif.: Picacho State Park area, in Sherrod, D.R., and Nielson, J.E., eds., Tertiary stratigraphy of highly extended terranes, California, Arizona, and Nevada: U.S. Geological Survey Bulletin 2053, p. 189-191.

Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 172, 235, 308, 342, 374, 417.

A Ag-Pb-Zn-Au-Mo-V-Cu-Bi mining area.

The terrane as a whole is remarkably rough, although its differences in altitude are not great. Irregular steeply-sided peaks and serrated ridges alternate with canyons or valleys that are several hundred feet deep and drain southward or westward to the Colorado River.

The oldest rocks of this range are of the metamorphic type and are mapped as schist. Locally, this unit includes some areaqs of gneiss. Much od the schist is moderately fissile and consists of fine-grained quartz, sericitized feldspar, and bands of partly chloritized biotite.

Intruding the schist and gneiss are irregular masses of granitic rocks which weather into steep slopes. These "granites" include two or more varieties of which one is light gray and another dark gray. The lighter is sodic granite and the darker is classifed as a granodiorite.

The schist and granite are intruded by dikes of aplite, pegmatite, and various dark-colored porphyries of intermediate to basic composition. The schist and granite are presumed to be Precambrian.

Unconformably overlying the schist and granite is a thick, extensive series of volcanic flows, breccias, and tuffs, locally intruded by dikes of rhyolitic, intermediate, and basic composition.

The flows of the immediate area consist mainly of andesite, trachyte, and rhyolite,. Basalt caps prominent mesas northeast of the Clip Mine. The flow-breccias are mainly andesite and trachytic. he tuffs are white, pink, buff, or locally banded.

This region has undergone intense crustal disturbance during several geologic periods. The older metamorphic rocks reflect ancient folding and faulting upon which later structural deformation has been superimposed. Their foliation commonly strikes either northwest, northeast, or northward but is subject to local variations.

The granite has been broken by several systems of fractures of which the most prominent trend parallel to the ridges. In places, the schist and granite are separated from the volcanic rocks by faults, also sub-parallel to the ridges. As the main ridges of schist and granite were initiated prior to eruption of the volcanics, it is presumed that these faults represent renewed movement upon ancient, pre-volcanic breaks.
The volcanic rocks prevailingly strike northwesterly and dip northeastward at medium to low angles. Southeast of the Red Cloud Mine; however, they appear to lie in a broad, low anticline of which the axis trends approximately S.70ºE.

Faults are conspicuous in many places where they separate volcanic rocks from granite and schist, but elsewhere they may be obscure at the surface. Much of the faulting was earlier than the Pb-Zn-Ag mineralization, and some was probably later.

The principal faults strike irregularly NNW-ward and dip from 35º to vertically. Branches from them strioke N.20º to 30ºE. Less conspicuous breaks strike S.60º to 70ºE. and commonly offset the N-NW faults. In addition, there are northeast and northwest fissures along which little or no movement has occurred.

Displacements on the faults probably range up to several hundred feet.