STRATIGRAPHY AND MINERALOGY OF THE MIOCENE BOLEO FORMATION GYPSUM IN THE SANTA ROSALIA BASIN OF BAJA CALIFORNIA SUR, MEXICO

Gypsum outcrops within the Santa Rosalia rift basin of east central Baja California Sur were defined by Wilson (1955) as the basal deposits of the Boleo Formation, predating the Cu-Zn-Mn-Co mineralization. We studied the late Miocene Boleo Fm gypsum and interbedded clastic units exposed as broad folds along Arroyo del Boleo to determine the temporal relationship of these rocks to the ore mineralization.

We measured a 110.5 m-thick stratigraphic section and divided it into five gypsum/anhydrite sections separated by four clastic layers. The basal gypsum/anhydrite unit (34.5 m thick) is massive or flat/wavy laminated sections with thinly bedded gypsum/anhydrite. This is cut by channelized and graded clastic beds (14 m thick). The second section of gypsum (11.5 m thick) contains black massive gypsum/anhydrite and wavy to flat laminated gypsum/anhydrite. Seismites are present within the laminated sequence attesting to active syndepositional tectonism. Copper staining is evident on the face of this outcrop. A second clastic unit (3.5-0.5 m thick) contains graded fluvial beds, a debris flow, and a clastic dike. The third (33 m thick) gypsum/anhydrite section contains massive to laminated or thinly bedded gypsum/anhydrite with some intercalated thin siltstone beds. The third clastic unit (1 m thick) is a sequence of interbedded laminated to thin beds of sandstone and mudstone. Above is (8.5 m thick) massive to laminated or thinly bedded gypsum/anhydrite interbedded with mud/siltstone. The fourth clastic layer (1 m thick) contains graded thin beds of mudstone and silty sandstone marked at its base by a notable angular unconformity. The uppermost unit (6.5 m thick) has poorly laminated gypsum/anhydrite interbedded with silt and massive to poorly laminated gypsum/anhydrite.

The trace mineralogy of the evaporites include strontianite, tenorite and Fe, Mn and Cu-Co oxides, and fine-grained sulfide grains, including a Cu-Fe sulfide, villamaninite ((Cu,Fe,Co,Zn)S2), and an unidentified Co-S phase with minor Cu and Fe. The sulfides occur along bedding planes, suggesting an influx of metal-bearing fluids into the evaporitic environment with periodically reduced conditions. Such mineralization suggests that the gypsum deposition is coeval with ore formation. This study is part of the NSF-funded Baja Basins REU.