Preliminary investigation of the nature and origin of the Sierra Mojada Non-sulfide Zn deposits, Coahuila, Mexico
Abstract
The Sierra Mojada district consists of multiple types of mineral concentrations ranging from polymetallic sulfide deposits, 'non-sulfide Zn' deposits (separate smithsonite and hemimorphite zones), and a Pb carbonate manto hosted by Upper Jurassic to Lower Cretaceous carbonates. The district is typically grouped with other polymetallic carbonate replacement deposits of southwestern North America, but the intrusive rocks that commonly are associated with these types of deposits are not known at Sierra Mojada. The Sierra Mojada district located near the boundary of the Coahuila Platform and the Sabinas Basin that formed during Late Jurassic and Cretaceous tectonic extension. The east-trending San Marcos fault runs through the Sierra Mojada district and is thought to have acted as the major conduit for basinal fluids, responsible for local dolomitization, sulfide mineralization, and petroleum in the region. The polymetallic sulfides are the stratigraphically and structurally highest ore zone and occur north of the San Marcos fault. Non-sulfide Zn (hemimorphite, smithsonite and sauconite) and lead (cerussite) concentrations occur south of the fault in the Aurora and La Pena Formations. The Iron Oxide Manto consists of stratabound zones of dominantly of hemimorphite pore-filling in Fe-oxide rich dolostones giving the ore a distinct red to orange color. The Smithsonite Manto has distinct karst features, including internal sediments interbanded with smithsonite in the lower part of the orebody. The non-sulfide Zn ores are being studied to document their morphologies, growth patterns, and paragenetic relationships using conventional petrography, SEM, and CL-based ESEM. In the Smithsonite Manto, banded and colloform smithsonite consists of aggregates of rhombohedral crystals that grew into open space and also occurs within internal sediment bands with hemimorphite or Zn clays and Zn oxides. Black dendrites of Mn oxides with Fe oxides are encased in banded smithsonite. Euhedral hemimorphite is present in both mantos, although it is most abundant in the Iron Oxide Manto. Hemimorphite occurs in the Smithsonite Manto as layers with smithsonite suggesting that they may have precipitated together, although locally hemimorphite cross-cuts smithsonite bands. Locally, hemimorphite and smithsonite have been altered to sauconite and hydrozincite. Associated minerals include barite and calcite that seem to have formed later than the major Zn mineral formation. Preliminary isotope studies reveal that smithsonites from Sierra Mojada have δ18OVSMOW values ranging from 19.1 to 22.1 ‰ and δ13CVPDB values of -7.0 to +1.0 ‰. While the range of carbon isotope values is typical for supergene smithsonites, the oxygen isotope values are much lower than those recorded in most supergene deposits. The unusually low oxygen isotope values in smithsonite require either highly 18O- depleted waters of less than -12 ‰ if oxidation occurred at temperatures of less than 20C° or elevated temperatures (>40C°) during oxidation if waters had isotope compositions similar to present- day ground waters in the area. Studies are in progress to further constrain the paragenesis, mineralizing fluid character, and timing of mineralization in the Sierra Mojada district.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMMA11A..05A
- Keywords:
-
- 1034 Hydrothermal systems (0450;
- 3017;
- 3616;
- 4832;
- 8135;
- 8424);
- 1039 Alteration and weathering processes (3617);
- 1041 Stable isotope geochemistry (0454;
- 4870);
- 1832 Groundwater transport;
- 3665 Mineral occurrences and deposits