Abstract The E1 Sauzal gold-silver deposit is situated in the western Sierra Madre Occidental of Mexico, near the border between the states of Chihuahua and Sinaloa (Fig. 1). The deposit was discovered in 1995 by Francisco Gold during grass-roots exploration centered on the historic silver district of Batopilas, about 15 km to the NE (Fig. 1), (Charest et al., 2004). Colonial-era and other significant workings were completely absent at El Sauzal. Geologists travelling by mule and on foot in the deep, roadless canyon of the Rio Urique made the discovery by direct rock-chip sampling of resistant, highly altered outcrops. Between 1996 and 1999, a camp was established and a total of 188 diamond-drill holes were completed. Drilling was focused on the potential for shallow, oxidized ores and was largely guided by gold in rock-chip and channel samples.

Abstract The Minas Viejas Formation consists of carbonates and sulfates that are the first evidence of marine incursion into northeastern Mexico during the Late Jurassic (Oxfordian). In the area southwest of Galeana, Nuevo Leon, this evaporite sequence is intensively deformed, but a consistent stratigraphic succession and separation of two members is recognizable. In addition to the Las Minas Member that was defined by Götte (1988), we introduce the La Primavera Member. Our data suggest that only one largely evaporitic succession exists in the region and that the terms Minas Viejas and Olvido are synonyms for the same stratigraphic unit. Lateral and vertical changes of facies in the Minas Viejas Formation are the result of syndepositional normal faulting and relate to the onset of sea-floor spreading in the Gulf of Mexico. Alkaline volcanic rocks occur in the La Primavera Member of the Minas Viejas Formation. This Oxfordian volcanism is hitherto undescribed in the area and links the tectonostratigraphic evolution of northeastern Mexico to early sea-floor spreading in the Gulf of Mexico. In addition, barite deposits in the Galeana area likely are related to this Late Jurassic volcanism. Barite mineralization is restricted mainly to stratigraphic levels older than the alkaline volcanism in the Minas Viejas Formation and is not the result of magmatism of Tertiary age. Apparently, carbonatite magmatism that provided the source for barium by hydrothermal activity was associated with Late Jurassic volcanism.

Abstract The Jurassic sequence of northeastern Mexico includes Oxfordian through Tithonian-aged marine sediments that were deposited in eight environments: (1) sabkha, (2) nearshore, (3) lagoonal, (4) shoal, (5) inner-shelf, (6) restricted-shelf, (7) open-shelf, and (8) pelagic. Lithologic correlations relate the sections studied with the type sections of coeval rocks described from the Sierra de Parras. Based on their lithic characteristics they are assigned to the Minas Viejas, Zuloaga, La Gloria, La Caja, La Casita, and San Angel Formations. Three biohorizons are established: (1) the first stratigraphic occurrence of the ammonite Idoceras in the early Kimmeridgian, (2) the first appearance of calpionellids in the middle Tithonian, and (3) the first occurrence of Calpionella elliptica Cadish at the Jurassic/Cretaceous boundary. Two lowerings of sea level are identified in the sections studied, the first in early Kimmeridgian and the second in the early Tithonian. These two drops of sea level are separated by a sea-level rise in the middle Tithonian. Eustatic changes of sea level are associated with tectonism in the area. Active transform faulting in the Gulf of Mexico region resulted in the juxtaposition of Oxfordian-Kimmeridgian shallow- and deep-water facies. The translations along transcurrent faults result in a SE-NW displacement of about 300 kilometers, which caused the formation of the Sabinas Basin of northeastern Mexico. Late Jurassic sea-floor spreading in the central Gulf of Mexico is regarded as the driving mechanism. The Oxfordian-Kimmeridgian and late Tithonian paleogeographies of northeastern Mexico are reconstructed, and parallel lineal facies belts are envisioned which account for the sporadic distribution of the Jurassic shallow-water facies in northeastern Mexico.

Abstract Strata exposed in the La Popa Basin of northern Mexico range in age from Late Jurassic through middle Eocene and record the evolution of the region currently lying in the foreland of the Sierra Madre orogen from extensional to contractional tectonics. The facies distribution and geometry of the strata demonstrate that salt diapirism was active at least from late Aptian–middle Eocene. Jurassic rocks include evaporites of the Oxfordian-Kimmeridgian Minas Viejas Formation, which is exposed only in diapiric bodies in the basin. The evaporites contain three types of blocks transported to their present levels during diapirism: (1) mafic to intermediate metaigneous rocks with latest Jurassic 40 Ar/ 39 Ar cooling ages (146 Ma); (2) laminated and nodular gypsum; and (3) Kimmeridgian limestone (Zuloaga Limestone). This Upper Jurassic lithic assemblage represents different parts of a formerly thick (at least 1000 m) stratigraphic section deposited in an extensional, or pull-apart, basin. Post-Jurassic rocks of the basin range from Early Cretaceous to middle Eocene and record both carbonate and siliciclastic deposition in dominant deltaic, shallow-marine, and tidal settings, as well as subordinate basinal and coastal-plain environments. This exposed section is at least 6400 m thick. The Lower Cretaceous section is thin (∼100 m) and locally consists of carbonate biostromes deposited on bathymetric highs adjacent to a diapiric salt wall. These carbonates are late Aptian–late Albian in age and much thinner than their regional correlatives. The lower part of the Upper Cretaceous is represented by the Indidura and Parras Formations, the former a basinal carbonate, the latter a prodeltaic or basinal shale that underlies the Difunta Group, a constructional continental-margin clastic wedge or embankment that filled the La Popa Basin. The Difunta Group in the La Popa Basin spans the Maestrichtian–middle Eocene. Lenticular carbonate beds as much as 350 m thick are interbedded with mud-rock intervals of the Difunta Group and represent deposition on bathymetric highs created by rising salt bodies.

Abstract The Taxco Mining District is located in the Neovolcanic Axis Metallogenetic Province in northern Guerrero State, at an elevation of 1,700 m (Fig. 1). This district includes the San Antonio, Guerrero, Remedios, Guadalupe, and Golondrina Mines. In 1534, Conquistadors Juan de La Cabra, Juan Salcedo, Muriel and Hernán Cortés mined and processed silver from the Socavón del Rey (now known as Mina del Pedregal), which was worked sporadically until 1747. In 1802, José de la Borda and José Vicente de Anza discovered a large silver bonanza in Tehuilotepec; José de la Estaca found another in the Juliántra area; these mines were almost inactive from 1802 to 1880. A flotation plant was installed in 1920 by the American Smelting and Refining Company. Ores were processed by amalgamation. Industrial Minera México, S.A. (IMMSA), owner of all the properties in the district, which employs 1,298 workers at the IMMSA mines and Taxco District plant, was producing 3,300 t/d in 1984.

Abstract The Real de Angeles deposit is the result of geologic events causing a mass of marine sedimentary rocks approximately 400 m long by 400 m wide and 400 m of known depth to become more or less uniformly mineralized in silver, lead, zinc, and other elements. Exploration and quantification of the deposit′s potential, and subsequent planning, financing, and construction of the necessary installations have resulted in its successful development. In 1985, opencast mining produced 11,400,000 troy ounces of silver, 37,800 tons of lead, and 30,600 tons of zinc, approximately equivalent to 19 percent of the silver, 21 percent of the lead, and 16 percent of the zinc produced in Mexico. Minera Real de Angeles has countered the drastic price reductions by increasing treated ore volumes from the original 10,000 tons/day capacity to 14,500 tons/day, reducing costs and increasing productivity indexes; 85 percent of the financial commitments will have been covered by the second half of 1986. Mina Real de Angeles is located in Noria de Angeles Municipality, eastern Zacatecas State in central Mexico, at geographic coordinates 22°25′N and 101°54′W (Fig. 1), at the center of a triangle formed by the towns of San Luis Potosí, Zacatecas, and Aguascalientes, to which it is connected by roads; the Aguascalientes- San Luis Potosí railroad stops at Estación La Honda, 10 km east of the mine.

Abstract Exposed near Galeana, Nuevo Leon, are sedimentary deposits and contemporaneous structures that record the rifting and opening of the Gulf of Mexico, passive-margin development, and Laramide compression in the region. A mapping, sedimentologic, and structural study utilizing thin sections, measured sections, aerial photos, and kinematic models has produced a detailed stratigraphic section that records the transition from terrestrial to open-marine deposition and the orientation and timing of deformational events. Triassic to Early Jurassic red beds in the Huizachal Group are composed of fluvial and marine sands. The La Boca and La Joya formations are separated by a polymictic cobble conglomerate. The Callovian Minas Viejas represents the initial stages of marine transgression and the overlying interbedded carbonates and evaporites of the Zuloaga and Olvido Formations were deposited in response to cyclic eustatic sea-level fluctuations. The increase of biodiversity through the upper Jurassic strata reflects a change from restricted to open-marine conditions. Rift-related tectonics and gravity driven brittle extensional features within the carbonates have been overprinted by structures that formed during the Laramide orogeny. The red beds have been folded and faulted into a broad Laramide anticlinorium, in which smaller intrusion cored folds have amplitudes on the order of tens of meters. The decollement for Laramide thin-skinned deformation occurs within an evaporate unit. There is a high degree of structural complexity in the overlying carbonates as indicated by tight folding patterns. By integrating concepts of depositional systems with structural deformation the hypothesis, “There is an exhumed preraft block in the Zuloaga Limestone around San Pablo de Tranquitas.” will be tested.

This study describes the geology of a well-exposed but previously unmapped section of Paleozoic–early Cenomanian metamorphic, sedimentary, and igneous rocks in the Frey Pedro study area of the Agalta Range of east-central Honduras. The objective of the study is to use these new structural, stratigraphic, biostratigraphic, and geochemical data to better constrain the geologic and tectonic history of this part of the Chortis block during the period of time from Aptian to early Cenomanian. The study revealed that the topographic Agalta Range exposes a thick stratigraphic section (3.5 km) deposited in an Albian-Aptian intra-arc rift and on the rift shoulders. This rift feature, named here the Agua Blanca rift, presently trends northwest and is parallel to three other belts of deformed Cretaceous rocks in Honduras (the Comayagua, Minas de Oro, and Montaña de la Flor belts) that also may correspond to Cretaceous intra-arc rifts produced during the same phase of intra-arc extension. These other three deformed belts are west of the Agalta Range and also form topographically elevated mountain ranges. Albian-Aptian calc-alkaline volcanic rocks and pyroclastic flows of the Manto Formation record arc affinity, while the volcaniclastic wedges of the Tayaco Formation record syn-rift deposition. These rift and arc-related units occupy the stratigraphic position between two major, extensive, shallow-water carbonate units, the lower and upper Atima Formations, also of middle Cretaceous age. Thickening of volcaniclastic rocks of the Tayaco Formation strata in the Agua Blanca rift accompanied erosion of the adjacent rift shoulders and eruption of Manto calc-alkaline volcanic rocks both within and adjacent to the Agua Blanca rift. The Agua Blanca intra-arc rift was inverted by a regional shortening event presumably in the Late Cretaceous. Rocks within the rift were intensely shortened, while rocks on the rift shoulders were shortened less because they are underlain by more competent metamorphic basement rocks. In order to better understand the Aptian–early Cenomanian tectonic setting for intra-arc rifting and subsequent rift inversion on the Chortis block, we reconstructed the Chortis block relative to the terranes studied by previous workers in southern Mexico. Five geologic and tectonic features were selected for realigning the two now widely separated areas: (1) areas of Precambrian basement outcrops, (2) areas of similar Mesozoic stratigraphy, (3) aligned trends of Mesozoic volcanic arc rocks exhibiting a similar arc geochemistry, (4) aligned trends of Late Cretaceous folds and thrusts, and (5) alignment of magnetic boundary.