Trigo Mountains, La Paz County, Arizona, USAi

Regional Level Types
Trigo Mountains	Mountain Range
La Paz County	County
Arizona	State
USA	Country

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.

Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Trigo_Mountains
Wikidata ID:	Q7841762
GeoNames ID:	5318137

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References