Abstract

Lead-zinc-(silver) mineralization is widespread in carbonate rocks of the Permian Chochal and Cretaceous Coban-Ixcoy formations in a 250 X 50 km zone elongated east-west across central Guatemala. Massive galena-sphalerite-pyrite-quartz mineralization is concentrated along bedding planes and fault surfaces in areas up to 1,500 X 100 m. Minor barite, chalcopyrite, and pyrargyrite are present locally and the silver content of galena increases southward across part of the mineralized zone. Most mineralization is near the base of the lowest carbonate unit in the local stratigraphic section and does not appear to be related, in surface outcrop, to the depositional and collapse breccias that are widespread throughout the Chochal and Coban-Ixcoy carbonate rocks. Mineralization appears to be late Cretaceous in age.delta S 34 values for sulfides in the mineralized area exhibit a spread of 25 per mil, do not center on zero, and are similar to those of many Mississippi Valley deposits. Lead isotope values are only slightly radiogenic, however, and show greater similarities to magmatic-hydrothermal lead-zinc deposits. Limited sulfur isotope and fluid inclusion data suggest that mineralization took place below 250 degrees C. Sulfides with the observed variation in delta S 34 values could have been deposited below 250 degrees C under a small range of f (sub O 2 ) and pH if the bulk isotopic composition of the ore-forming sulfur was +18 to +26 per mil. The lower part of this range coincides with estimates of delta S 34 for Jurassic-Cretaceous sea water, which is widespread in the mineralized zone as the gypsum facies of a large evaporite basin that occupied northern Central America during Jurassic-Cretaceous time.

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