La Platosa Mine, Bermejillo, Mapimí Municipality, Durango, Mexicoi

Regional Level Types
La Platosa Mine	Mine
Bermejillo	Town
Mapimí Municipality	Municipality
Durango	State
Mexico	Country

Platosa is a typical Mexican Carbonate Replacement Deposit lying approximately 5 km northwest of the town of Bermejillo, Durango. Platosa lies on the northeastern flank of the Sierra Bermejillo, less than 2 km off the Pan American Highway. Although it is in the Municipality of Mapimí it has no apparent relationship to the Ojuela Mine area of the Mapimí District.

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 (up to 2.5 m long), often characterized by dark brown to black phantoms of organic material. Many also show pale yellow-white phantoms under SW-UV, 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 encased in limpid crystalline 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.

The 2015 Excellon PEA for Platosa says: Gypsum occurs as fairly late stage fracture-fillings and veins throughout the district. Collector quality
gypsum crystals have been extracted from open fractures in the historic Platosa Mine, and are what originally led Excellon to the property. The gypsum commonly cuts across mantos and cements large areas of sulphide breccia. Crystalline gypsum, containing extremely fine-grained inclusions of galena dust, is often found tens of metres from these breccias and is a reliable indicator of proximity to coherent sulphides. Cross-cutting relationships and sulphur isotopic analyses indicate that the gypsum is probably not genetically related to the mineralizing event but is most likely derived from the solution and reprecipitation of gypsum derived from the underlying Acatita Formation evaporates.

Other Regions, Features and Areas containing this locality

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