Dos Marias Mine, Manuel Benavides, Manuel Benavides Municipality, Chihuahua, Mexicoi

Regional Level Types
Dos Marias Mine	Mine
Manuel Benavides	Town
Manuel Benavides Municipality	Municipality
Chihuahua	State
Mexico	Country

For completely unknown reasons at various times this small mine has been known as either Tres Marias or Dos Marias. It lies just south of the Rio Grande, just west of Big Bend National Park in the mountains above Nuevo Lajitas, Chihuahua; essentially just across the border from the Terlingua-Lajitas, Texas stretch of the Rio Grande. There is some recent ore deposits literature under the name Tres Marias stemming from recent interest in the high Germanium content of the ores (cf Saini-Eidukat et al., 2009, Mineralium Deposita v. 44, p. 363-370) Unfortunately not enough has been found to justify production and no additional orebodies have been found.
The mine was operated intermittently between the late 40s and 1992 for zinc oxide mineralization with a high Germanium content. Eagle Picher was the principal purchaser of the ores, which they blended with moderate Ge lead-zinc ores from San Pedro Corralitos. Perhaps 125,000 tonnes were mined. A number of exploration companies have evaluated the property since the 1990s with extensive work and limited success.
The orebody is an inverted carrot-shaped breccia chimney in lower Cretaceous Santa Elena (Georgetown) Limestone, probably a result of hydrothermal karsting by acidic ore fluids ascending along intersecting structures. Mineralization preferentially occurred around the margins of the breccia pipe as both open-space fillings and partial replacement of limestone fragments. Oxide ores were the principal mining interest, with zinc grades to 40%. Primary mineralization is galena and sphalerite, with lesser marcasite, pyrite, wurtzite, and cinnabar. The sphalerite is low-iron, honey colored and commonly occurs as stalactitic masses coated with a thick crust of pyrobitumen (called gilsonite by some workers). Silver grades are very low, but Germanium ranges from 300-960 ppm. Secondary minerals include cerussite, anglesite, wulfenite, mimetite, descloizite, plumbojarosite, minium (?), greenockite, hemimorphite, smithsonite and willemite in a matrix of mixed iron-manganese oxides. Calcite, fluorite, dolomite, fine-grained quartz and barite are the principal gangue species.
The metals content, mineralogy and style of mineralization indicate a Mississippi Valley Type brine-origin. The presence of cinnabar suggests a genetic link to (or overprinting by) the nearby mercury mineralization of the Terlingua District across the river.

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References