MINING IN MEXICO — The ore deposits of the Sierra Madre

The Mexican epithermal precious metal province coincides with the Sierra Madre Occidental that covers western Mexico from the state of Sonora, just below the border with the U.S., south to the Neovolcanic axis in the state of Jalisco.

This province is 1,200 km long by 200-300 km wide with elevations up to 3,000 meters above sea level. (Wisser, 1966)

The Sierra Madre Occidental is a great pile of tertiary volcanics up to 3,000 meters thick deposited on a basement of Mesozoic, Paleozoic and Precambrian rocks. Basement rocks are occasionally exposed in the higher elevations of the Sierra, where the volcanics are thick and erosion has carved deep, steep sided canyons (barrancas). A good example of that is the Barranca del Cobre in the state of Chihuahua that is more than 2,100 metres deep. In the winter, tropical fruit grows in the lower elevations while snow covers the plateau above.

The only Precambrian rocks exposed in the area cover a small section in the Altar region of northwestern Sonora. Overlying the Precambrian rocks is a thick series of Paleozoic rocks, mainly limestones, ranging in age from Cambrian to Permian. Overlying the Paleozoic rocks are Mesozoic limestones intercalated in places with lavas. These pre-tertiary rocks form the basement of the Sierra Madre Occidental province.

Volcanics, mainly andesites, up to 1,500 meters thick unconformably overlie the basement rocks. These tertiary rocks make up the Lower Volcanic Series that is the host formation for most of the epithermal deposits of the Sierra Madre Occidental province.

Covering the andesites of the Lower Volcanic Series is the Upper Volcanic Series consisting mainly of rhyolitic tuffs and ignimbrites with thicknesses up to 1,500 meters. This formation forms impressive cliffs on the flanks of erosional valleys in the Barrancas sub-province.

The Sierra is divided from west to east into six physiographic divisions or sub-provinces that, with minor exceptions, apply to the entire Sierra Madre Occidental province. On the west coast, the Coastal Plain forms the western sub-province in the states of Nayarit and Sinaloa. Going north along the Sonoran coast with the Sea of Cortz, the Coastal Plain section changes into the Sonoran Desert sub-province. On the western flank of the Sierra Madre is the Parallel Range and Valley sub-province of the state of Sonora. The Barranca sub-province (King, 1939) forms the western part of the Plateau sub-province of the Sierra Madre in the states of Chihuahua, Durango and Nayarit. The eastern part of the Sierra is characterized by northwesterly trending small valleys and ranges called the Altas Llanuras sub-province that covers eastern Chihuahua, eastern Durango and western Zacatecas. The Sierra Madre Occidental is a metallogenic as well as a structural province and is considered by some authorities to be the greatest epithermal precious metal area in the world. The epithermal fissure veins containing gold, silver and base metals are mainly found in the andesites of the Lower Volcanic Series, but are also found in the basement intrusive and sedimentary rocks as well as in the lowest part of the rhyolites and ignimbrites of the Upper Volcanic Series.

The most common features of the epithermal precious metal deposits of the Sierra Madre Occidental are (Wisser, 1966):

* the ore deposits are associated with andesite-rhyolite volcanics * they are middle-to-late-tertiary in age

* they occur mainly as veins in fractures

* hydrothermal fluids ascended these fractures and propylitized the andesite wall rock; after the propylitization, sericitization occurred and, finally, silicification

* ore was deposited in these fractures at depths ranging from 100 metres or so to perhaps 600 meters below the surface that existed then * at such relatively shallow depths, the rock was brecciated into complex fracture patterns

* vein formation commonly occurred along with faulting normal or parallel to the veins so that earlier vein material was added to by later vein minerals * the principal gangue is quartz; minor gangue minerals are calcite, adularia, barite, dolomite, fluorite and manganese

* silver occurs as argentite or complex sulfosalts with arsenic or antimony; pyrite, sphalerite, and galena may be sparse or abundant and, if abundant, their distribution is apt to be sporadic rather than uniform. * the vein texture exhibits crustified or colloform banding, drusy cavities, or a breccia of altered rock fragments cemented by vein material * the typical orebody is longer on strike than on dip; the ore zone has a definite bottom; below the precious metal zone, gangue and base metals may persist, but sometimes vein matter gives out, completely leaving a barren fracture

* epithermal precious metal deposits are often associated with local anticlines or domes produced by vertically acting forces.

An additional feature of all Mexican epithermal precious metal deposits is that they are derived from nearby intrusive silicic dikes or plugs, mainly quartz monzonite and granodiorite. (Ing. G. P. Salas)

Explorationists should look for porphyry copper and gold in the Sonoran Desert, Parallel Range and Valley, and Coastal Plain sub-provinces of the states of Sonora and Sinaloa.

Searching for epithermal deposits of silver and gold is best done in the Barranca sub-province just to the east of the Coastal Plain sub-province in the states of Chihuahua, Durango, Sinaloa and Nayarit.

Epithermal silver, minor gold and base metals deposits mainly occur in the Altas Llanuras sub-province in the states of Chihuahua and Durango. — The writer is a partner with Encinas y Gerling, a mining consulting firm with offices in Durango, Mexico.