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2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 9
Presentation Time: 3:45 PM

THE CARBONATE HOSTED TRES MARIAS ZN-GE DEPOSIT, CHIHUAHUA, MEXICO: CHARACTERIZATION OF GE-HOSTING PHASES AND OF ASSOCIATED HYDROCARBONS


SAINI-EIDUKAT, Bernhardt1, MELCHER, Frank2, OSTERTAG-HENNING, Christian2 and LODZIAK, Jerzy2, (1)Department of Geosciences, North Dakota State Univ, Fargo, ND 58108-6050, (2)Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, 30655, Germany, bernhardt.saini-eidukat@ndsu.edu

The Tres Marias mine is a Ge-rich Zn (+Pb) deposit in Chihuahua, Mexico. A carbonate hosted replacement deposit, it occurs in Cretaceous sedimentary rocks of the Chihuahua Trough. The ore body is a 70 m diameter collapse breccia extending 100 m below surface.

Sphalerite occurs in contact with host rocks, or as breccia clasts. It consists of spheroidal aggregates with cores of fine-grained gray sphalerite with a distinctive micron-scale bladed to dendritic texture. These contain (ave.) 900 ppm Ge, 9.0 wt. % Fe, 540 ppm As and 2900 ppm Cd. Sulfur isotope ratios of bulk sulfide ore range from 5.0 to 8.3 permil δ34S(CDT). A galena-sphalerite mineral pair shows values of 4.8 and 6.1 permil δ34S(CDT), respectively, which we interpret as non-equilibrium.

A significant zone of “oxidized ore” consisting of Zn-silicates, carbonates and oxide minerals is an alteration product of the sulfide ore and associated limestone and breccia host. Microprobe analyses show that Pb, As, and Ge in willemite range from below detection limits to approximately 2 wt.%, 6000 ppm, and 4000 ppm, respectively. Across single zoned crystals, Pb and Ge are positively correlated, while Pb vs. Zn, and Zn vs. Ge are negatively correlated.

Sphalerite is often intimately associated with abundant hydrocarbon. Common modes of hydrocarbon occurrence include vein fillings in the host rock or sulfide ore, masses within vugs associated with sulfides, and as fluid and solid inclusions in sphalerite. Hydrocarbons impregnating sulfide ore were analyzed by Rock-Eval pyrolysis. Tmax values ranging from 421 to 430 °C and HI values ranging from 299 to 444 indicate the material was not heated to temperatures over 100 °C. GC-MS analyses of the aliphatic fraction and especially selected biomarkers reveal different degrees of oxidation of the remaining bitumen.

War Eagle Mining and the German-American Fulbright Commission are gratefully acknowledged.

Session No. 149

Economic Geology
Tuesday, 30 October 2007: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 39, No. 6, p.413
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