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This province (Fig. 1) is named after the mountain chains with which it coincides physiographically. Though these are not continuous, their Mesozoic sedimentary formations, folded and faulted during the Eocene, follow the trend of the Pacific coast. The older (Precambrian and Paleozoic) units strike north-south, whereas the post-Eocene mineralizing intrusions generally follow northwesterly trends (Fig. 1).

The mountain chains developed in thick sequences of effusive and pyroclastic rocks unconformably overlying considerable sections of northwest-trending, folded and faulted Mesozoic limestones, which in turn overlie Paleozoic formations near theTomellin Canyon in northeastern Oaxaca. Late Precambrian (750 to 850 Ma) foliated schists and gneisses crop out both at Oaxaca and in southern Chiapas.

Extensive schists and gneisses unconformably underlying the Cretaceous sedimentary section in northern Oaxaca and southwestern Guerrero are assigned to the Mesozoic on the basis of isotope geochronology; the schists are host rocks for part of the Taxco (Guerrero) ore deposits.

The mineralization in Michoacan, Guerrero, Oaxaca, and the few deposits known in Chiapas are of Oligocene-upper Miocene age.

Mineralization appears associated with felsitic intrusive stocks and dikes; it occurs at the periphery of the stocks or above them when they are not exposed.

The abundant southeastern Guerrero and southwestern Oaxaca metasomatic iron ore deposits are low-volume, which makes them uncommercial for the time being (1984). The Cemento Cruz Azul Company is presently developing a small ironore deposit (El Carmen) for clinker.

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