Road Log From Fresnillo to Zacatecas
Most of the day will be spent underground and on the surface in the Fresnillo district. This consists of replacement chimney, manto and fissure-vein deposits, and disseminated sulfide ores, hosted mainly by Cretaceous marine sedimentary rocks. The base metal - silver deposits are zoned to the southeast, away from a quartz monzonite stock, with increasing silver, but decreasing base-metal content, distal to the stock. Wall rock alteration adjacent to the stock consists of silicification and calc-silicate formation, partially replaced by sulfide mineral. Wall rock alteration around veins exhibits potassic, phyllic, argiJlic and propyllitic alteration with increasing distance from the mineralized fractures. The ores contain recoverable amounts of lead, zinc, copper, silver and gold.
The latter part of the day consists of a short journey from Fresnillo to Zacatecas, passing over a wide, cultivated valley, but climbing in elevation to about 2,250 m, where the colonial city of Zacatecas is situated, flanked by sierra in which the fissure-vein mineral deposits are located.
Figures & Tables
Mexican Silver Deposits
Much of the first day's route is spent in travelling in a southerly direction through desert basins to Chihuahua City (Fig. 1). Numerous ranges are visible from the roadside and some stops are scheduled for description of lithologies and structures, particularly where mapping has been accomplished. Of particular interest is the lower Cretaceous in Sierra del Presidio and Banco Lucero, Los Medanos (dune field), Sierra La Candelaria and Sierra El Kilo intrusions, and the Tertiary volcanic stratigraphy of Sierra Gallego (near El Sueco), Sierra El Nido, Sierra de La Campana, and Sierra Sacramento. The Sierra de Samalayuca, near the community of the same name, has been considered by some to be Early Jurassic or Paleozoic in age, and it consists of weakly-metamorphosed sediments in an anticlinal structure.
The underground visit to the manto and breccia-pipe deposits at the Potosí mine, West Camp, Santa Eulalia completes the day.
0.0 Buenos dias. Proceed north on Santa Fe Street.
0.1 Turn right on frontage road (Wyoming) and proceed eastward through 4 traffic lights.
0.9 Join I-10, travelJing east, passing Southern Pacific railroad yards.
2.2 Pass Southern Pacific railroad yards on right. Continue east on I-10; keep in right lane. At 9:00 is southern flank of Franklin Mountains, where Bliss Sandstone is overlain by Ordovician strata (Fig. 2), and the El Paso Group overlies a Precambrian (Red Bluff) crystalline complex. The tectonic style of the Franklin Mountains comprises a north-trending, west-dipping, tilted fault block typical of Basin and Range structure (see Lovejoy, 1980).
The Rio Grande Valley is aligned from northwest to southeast, but curves prominently between Cerro Cristo Rey and the Southern Franklin Mountains, forming El Paso del Norte. Between the Franklin Mountains and the Hueco Mountains (to the east) lies the Hueco Bolson; a relatively flat valley which is filled with sand and silts and is the major source of El Paso's water supply.
The Franklin Mountains range is linear, extending from its southern limit near central El Paso, north and slightly west for approximately 24 km, to a point about 6 km north of the Texas New Mexico boundary. The range is roughly 5 km wide. The highest point, North Franklin, or Cabeza de Vaca Peak, is approximlately 2,390 m above sea level. Precambrian igneous rocks comprise North Franklin Peak, making it the area of greatest structural relief in the state of Texas.
The oldest rocks exposed in