Zinc–germanium ores of the Tres Marias Mine,Chihuahua, Mexico |
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Authors: | Bernhardt Saini-Eidukat Frank Melcher Jerzy Lodziak |
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Institution: | (1) Department of Geosciences, North Dakota State University, Fargo, ND 58105, USA;(2) Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany |
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Abstract: | 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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Keywords: | Tres Marias Mexico Willemite Sphalerite Germanium Mississippi Valley type |
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