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Zinc–germanium ores of the Tres Marias Mine, Chihuahua, Mexico


The Tres Marias carbonate-hosted Zn–Ge deposit in Chihuahua, Mexico contains sphalerite with the highest average Ge (960 ppm) and willemite with the highest reported Ge contents of Mississippi-Valley-type (MVT) deposits worldwide. This has prompted current exploration efforts to focus on the deposit as a high-grade source of germanium. The sulfide-rich ore type (>125,000 t at 20% Zn and 250 g/t Ge) contains Fe-rich botryoidal sphalerite (type I) associated with solid hydrocarbons. This type exhibits distinctive intimately intergrown lamellar texture of high-Fe sphalerite (average 9.9 wt.% Fe and 800 ppm Ge) and a somewhat less Fe-rich sphalerite phase (average 5.5 wt.% Fe and 470 ppm Ge). Reddish-brown banded sphalerite (type II, average 5.7 wt.% Fe and 1,320 ppm Ge) is subordinately followed by galena and pyrite. The sulfide-poor “oxidized” zinc ore (up to 50 wt.% Zn; 250 to 300 ppm Ge) is a fine-grained, often friable, alteration product of the sulfide ore and associated limestone and breccia host. While some areas are dominated by carbonates and sulfates, others are enriched in silicates such as hemimorphite and willemite. The gangue assemblage includes goethite, hematite, and amorphous silica or quartz. Minor wulfenite, greenockite, cinnabar, and descloizite also occur. Willemite occurs as interstitial replacement of sphalerite and fracture fillings in the oxidized ore and can be unusually rich in Pb (up to 2.0 wt.%) and Ge (up to 4,000 ppm). Oscillatory zonation reflects trace element incorporation into willemite from the oxidation of primary Ge-bearing sphalerite and galena by siliceous aqueous fluids. The Tres Marias deposit has hybrid characteristics consisting of a primary low-temperature MVT Ge-rich Zn–Pb sulfide ore body, overprinted by Ge-rich hemimorphite, willemite, and Fe oxide mineralization.

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This research was undertaken while BSE was a Senior Researcher at the Federal Institute for Geosciences and Natural Resources (BGR) funded by the German–American Fulbright Commission. North Dakota State University is gratefully acknowledged for providing developmental leave to carry out this research. We thank Dan Reetz for assistance with artwork, and Dave Hackman, Roberto del Rio, and Alistair Logan of War Eagle Mining for their support in the field and with artwork. Ulf Hemmerling (Technical University of Clausthal) provided excellent polished sections. Support by Thomas Oberthür (BGR) is greatly appreciated.

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Correspondence to Bernhardt Saini-Eidukat.

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Saini-Eidukat, B., Melcher, F. & Lodziak, J. Zinc–germanium ores of the Tres Marias Mine, Chihuahua, Mexico. Miner Deposita 44, 363 (2009).

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  • Tres Marias
  • Mexico
  • Willemite
  • Sphalerite
  • Germanium
  • Mississippi Valley type