Cortes and 5 co-authors reported in 2010 that the mineralogy of the two uranium deposits (Victorino and San Marcos I) of Sierra San Marcos, located 30 km northwest of Chihuahua City, Mexico, was studied by optical microscopy, powder X-ray diffraction with Rietveld analysis, scanning electron microscopy with energy dispersive X-ray analysis, inductively coupled plasma spectrometry, and gamma spectrometry.
At the San Marcos I deposit, uranophane Ca(UO2)2Si2O7·6(H2O) (the dominant mineral at both deposits) and metatyuyamunite Ca(UO2)(V2O8)·3(H2O) were observed. Uranophane, uraninite (UO2+x), masuyite Pb(UO2)3O3(OH)·3(H2O), and becquerelite Ca(UO2)6O4(OH)6 ·(8H2O) are present at the
Victorino deposit.
Field observations, coupled with analytical data, suggest the following sequence of mineralization:
1) deposition of uraninite,
2) alteration of uraninite to masuyite,
3) deposition of uranophane,
4) microfracturing,
5) calcite deposition in the micro-fractures, and
6) formation of becquerelite.
The investigated deposits were formed by high-to low-temperature hydrothermal activity during post-orogenic evolution of Sierra San Marcos. The secondary mineralization occurred through a combination of hydrothermal and supergene alteration events. Becquerelite was formed in situ by reaction of
uraninite with geothermal carbonated solutions, which led to almost complete dissolution of the precursor uraninite.
The Victorino deposit represents the second known occurrence of becquerelite in Mexico, the other being the uranium deposits at Peña Blanca in Chihuahua State.
The authors concluded that the following uranium minerals were identified in the deposits of the San Marcos area: uraninite, uranophane, metatyuyamunite, becquerelite and a uranium–lead oxy hydroxide (probably masuyite).
The occurrence of becquerelite is the second one reported in a Mexican uranium deposit. In contrast to the previously-made brief description of this mineral from Peña Blanca (Pearcy et al. 1994), the identification of becquerelite in the present work is based on convincing analytical evidence.
The uranium deposits of the San Marcos area in Chihuahua, Mexico, are similar to other uranium deposits associated with rhyolitic rocks (NEA 1998). Particularly, the main characteristics of the Victorino deposit are similar to those of the Nopal I deposit at Peña Blanca.
At the Victorino deposit, the U-enriched rock occurs in an area of approximately 3,000 m2 in the southwestern part of the San Marcos area. The U mineralization occurs in breccia in ash flow tuff near its contact with a rhyolitic dome. At San Marcos I, U ore occurs in veins up to 2 cm in width cross cutting altered tuff of the USA. The extent of the secondary mineralization decreases with distance from the deposit. Similarities in mineralogy and alteration patterns in both deposits suggest that they are different manifestations of the same mineralizing system.
The secondary mineralization is interpreted to have occurred through a combination of hydrothermal and supergene alteration events. Based on mineralogical and textural evidence, the development and subsequent evolution of the U mineralization involved the following events:
- 1) uraninite deposition,
- 2) alteration of uraninite to masuyite (in zones of nomalously high Pb concentrations),
- 3) micro-cracking,
- 4) calcite deposition in the micro-cracks, and
- 5) formation of metatyuyamunite, becquerelite and uranophane.
Becquerelite was formed in situ by geothermal alteration of the uraninite which had been almost totally leached away.
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