Platosa Mine, Bermejillo, Mun. de Mapimí, Durango, Mexico
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Name(s) in local language(s): Mina la Platosa, Bermejillo, Municipio Mapimi, Durango, Mexico
The historic workings of the Platosa Mine are a series of flat-lying stopes, connected by tabular open zones developed along NW-SE faults and fractures. The deposit was found in outcrop, probably in the mid-1600s when Ojuela was just getting started and early mining focused on very high-grade oxidized silver-lead-zinc-copper ores. Mineralogically, these ores consisted dominantly of argentiferous cerussite and anglesite, with some remnant galena) and probably at least trace chlorargyrite. The oxides also contain small amounts of schulenbergite, orthoserpierite, malachite, linarite and serpierite.
Post-mineral ground-water movement along the NW-SE faults dissolved large open caverns up to 8 m wide, 30 m high and 250 m long. These contain very large gypsum crystals...to 2.5 m long, often characterized by dark brown to black phantoms of organic material. Many also show pale yellow-white phantoms under SWUV, probably also reflecting included organic materials. A fair number of these gypsum crystals are partially encrusted with hydrozincite and a small number are studded with light blue rice-grain smithsonites to 4 mm. Many gypsum specimens from Platosa have been misattributed to Naica...the fluorescent phantoms are diagnostic however.
Sulfur isotope analyses indicate that the gypsum in the crystals is derived from gypsum-rich evaporite deposits that lie deep in the local stratigraphic section. Field relations and extensive collapse breccias indicate that the deep gypsums were dissolved to form large caverns that ultimately collapsed creating breccia chimneys that subsequent fluids...themselves dissolving gypsum...followed and partially to completely filled with gypsum.
In 1997, exploration drilling by Excellon Resources found the unoxidized faulted continuations of the Platosa mantos essentially just below cover at the base of the mountain. These mantos have been proven to contain over 1 million tonnes of very high-grade silver-lead-zinc mineralization in the form of massive galena, sphalerite, acanthite and minor pyrite with trace copper sulfosalts. In the 5 Manto, silver grades locally exceeded 2% silver in areas rich in proustite. Proustite crystals to 1 cm in diameter and 3 cm long were found, many encaased in limpid crystallinee gypsum. Barite and celestine are locally abundant with clear evidence of galena replacing celestine in several places.
There are a multitude of celestine and barite showings throughout the Sierra Bermejillo, the majority forming as infillings of collapsed caverns. Most have been prospected and produced, although little specimen-quality material has been found. The cross-cutting relationships indicate the celesto-barite mineralization was early, formed from brines expelled from the nearby Central Mexico Basin. The silver-lead-zinc mineralization is clearly younger, probably forming in the mid-Tertiary.
Other oxidized metallic deposits occur in the area, some with adamite, mimetite, wulfenite and copper oxide species.
23 entries listed. 22 valid minerals.
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Megaw and ramirez, 2010: Geologia Economica de Mexico: Clark, K.F., and others, editors. Servicio Geologico Mexicano Publication.