Abstract Middle to Late Triassic turbidite sequences are exposed in the states of Zacatecas and San Luís Potosí in central Mexico. These strata, assigned mostly to the Zacatecas Formation, accumulated in continental slope, toe-of-slope, and basin-plain environments along the passive continental margin of western Pangea. Strata of the Zacatecas Formation are age equivalent to rocks of the Antimonio Formation and Barranca Group in Sonora, the La Boca Formation in Tamaulipas and Nuevo León, and unnamed strata in Baja California. Based on their age, the Zacatecas turbidites correlate with a drop in sea level during the Permian-Triassic assembly of Pangea. The Triassic paleogeographic setting of Mexico is complex and poorly understood, because only dispersed Triassic outcrops exist across Mexico. However, the biogeographic affinities of the faunas from the Zacatecas Formation in central Mexico with those from equivalent strata in Baja California and Sonora suggest that these three regions were connected through the eastern Pacific, and that the Atlantic Ocean did not exist during the Ladinian-Carnian. The Zacatecas sequences underwent three periods of compressive deformation: one during their obduction onto the continental margin at some time during the latest Triassic-earliest Jurassic (?); a second during the Middle to Late Jurassic (Oxfordian) (?), apparently related to transpression; and a third during the Late Cretaceous to Tertiary Laramide orogeny.

Abstract The first part of the trip travels northwest from San Luis Potosi to Zacatecas. An extensive collection of Tertiary volcanic rocks covers the base sediments west of San Luis Potosi. Rocks are mainly rhyolite tuffs, latites, and layered volcanic limolites. Another important volcanic center is the Los Pinos, which is a complex formed by clastic rocks, red beds, trachytes, and rhyolite tuffs. Historically, mining in the area has been active and was first reported in epithermal Au-Ag vein mineralization related to the mid-Tertiary sequence formed by domes, associated lava flows, and ash-flow tuffs. Peñon Blanco (San José) is an emergent mining district that has been explored recently. Rocks reported in the area include the basement, formed by Cuesta del Cura limestone and rocks of the Chilitos Formation. This sequence of rocks is intruded by the Peñon Blanco batholith, which has created a halo of marble rock in the contact with the sedimentary and volcanosedimentary rocks. Mineralization occurs in quartz veins and skarn, and main ores are silver, lead, zinc, and little copper. The city of Zacatecas is situated within a horst formed by NE-trending normal faults. The Mesozoic Chilitos Formation, siltstones, phyllites, and rhyolites are the main host rocks for mineralization in the region. Two of the major producer mines are located nearby: The Francisco I. Madero and the Cozamin deposits. The Madero deposit is hosted in thin-layered siliceouscarbonaceous units, with chlorite, calcite, and pyrite alteration that can be seen only underground. The Cozamin deposit occurs in veins hosted by the Chilitos Formation, andesites, phyllites, and siltstones. General vein trending is N 40° to 65° W dipping 65° to 75° NE, and the entire group of veins is known as the Mala Noche vein system. Good morning! Take the side road, San Luis Potosi-Mexico, to the east. 0.4 Turn left on Colorines street and take the San Luis-Mexico Highway to the west. 0.9 Jump into the Distribuidor Vial Benito Juarez and take the road San Luis Potosi-Matehuala to the north. 2.1 Turn left and follow the Boulevard Rio Santiago to the west. 2.6 Turn right on Fray Diego de la Magdalena Street. 1.1 To the right you'll see the Tangamanga local park, a Formula 3 racing course. Continue straight to Zacatecas by way of the Federal Highway no. 49. 4.1 Pass over the bridge of the Anillo Periferico Norte. 14.6 Route passes by cultivated areas and a series of small

Abstract The La Colorada district in northwest Zacatecas state in Mexico is located near the transition between the Mesa Central and Sierra Madre Occidental physiographic provinces. The district is underlain by a sequence of Cretaceous, deep-water, marine rocks comprised of the Fresnillo, Cuesta del Cura, and Indidura Formations. A dacite flow-dome complex of lower Tertiary age unconformably overlies the sediments and a middle Tertiary sequence of rhyolite tuffs overlies the dacites. Four phases of deformation are seen in the district including tight folding and local thrust faulting of the sedimentary sequence which occurred during the Laramide orogeny, lower Tertiary east-west- and north-northeast-trending normal faulting, a district-scale doming, and postmineral normal faulting with reactivation of earlier faults. Silver-lead-zinc mineralization in the district consists of several sets of predominantly east-west-trending epithermal veins, small vein-related mantos, a deep zone of replacement, breccia, stockwork, and skarn mineralization, and numerous intrusive breccia pipes. Vein-related alteration consists of argilliza-tion above ore and pyrite-illite-sericite-adularia at deeper levels. In sediment-hosted replacement, breccia and stockwork ores within the deep zone, alteration consists of weak silicification and local marble development with abundant calcite veinlets. Skarn exhibits calc-silicate alteration that has been retrograded to a calcite-quartz-chlorite assemblage and the breccia pipes have argillic alteration in the rock-flour matrix. Economic minerals in the various ore types include galena, sphalerite, silver sulfosalts, the tetra-hedrite-tennantite series, native silver, and chalcopyrite. Average ore grades in the minable portions of the veins are 600 g/t Ag with 3.5 percent combined Pb-Zn. The deep zone is characterized by variable but lower grade silver with 6 percent Pb-Zn. The breccia pipes average 90 g/t Ag with 3.0 percent Pb-Zn, and the skarn bodies discovered to date have subore-grade mineralization. All mineralization types are considered to be part of the same lower Tertiary episode of hydrothermal activity and an intrusive body responsible for driving the system is postulated to exist at depth. The vertical transition between the epithermal vein environment and the deeper replacement, stockwork, and skarn environment occurs about 350 m below the current erosion surface and it coincides roughly with the dacite-sediment unconformity in the hanging wall of the No Conocida group of veins. The center of the hydrothermal system is probably located under the eastern part of the district as indicated by the distribution of deep skarn mineralization and the higher temperature breccia pipes. Current resources in the vein and replacement environments stand at 4.33 Mt. The geologic resource in the breccia pipes is 14.3 Mt. Exploration continues at depth along the No Conocida, Recompensa, and Amolillo-Palomas structures, and in the breccia pipes. The development schedule calls for the construction of a minimum 850 tpd underground mine and flotation mill with production beginning in early 2000.