Zinc and germanium in the sedimentary rocks of Gale Crater on Mars indicate hydrothermal enrichment followed by diagenetic fractionation

Abstract Zinc and germanium enrichments have been discovered in sedimentary rocks in Gale Crater, Mars, by the Alpha Particle X‐ray Spectrometer on the rover Curiosity. Concentrations of Zn (910 ± 840 ppm) and Ge (65 ± 58 ppm) are tens to hundreds of times greater than in Martian meteorites and estimates for average silicate Mars. Enrichments occur in diverse rocks including minimally to extensively altered basaltic and alkalic sedimentary rocks. The magnitude of the enrichments indicates hydrothermal fluids, but Curiosity has not discovered unambiguous hydrothermal mineral assemblages. We propose that Zn‐ and Ge‐rich hydrothermal deposits in the source region were dispersed in siliciclastic sediments during transport into the crater. Subsequent diagenetic mobilization and fractionation of Zn and Ge is evident in a Zn‐rich sandstone (Windjana; Zn ~4000 ppm, Ge ~85 ppm) and associated Cl‐rich vein (Stephen; Zn ~8000 ppm, Ge ~60 ppm), in Ge‐rich veins (Garden City; Zn ~2200 ppm, Ge ~650 ppm), and in silica‐rich alteration haloes leached of Zn (30–200 ppm). In moderately to highly altered silica‐rich rocks, Ge remained immobile relative to leached elements (Fe, Mn, Mg, and Ca), consistent with fluid interaction at pH ≪ 7. In contrast, crosscutting Ge‐rich veins at Garden City suggest aqueous mobilization as Ge‐F complexes at pH < 2.5. Multiple jarosite detections by the CheMin X‐ray diffractometer and variable Zn concentrations indicate diagenesis of lower Mount Sharp bedrock under acidic conditions. The enrichment and fractionation of Zn and Ge constrains fluid events affecting Gale sediments and can aid in unraveling fluid histories as Curiosity's traverse continues.