FERRICRETES, MANGANOCRETES, AND EXOTIC CU DEPOSITS FORMED IN WEATHERING AND DISPERSION FROM PORPHYRY CU DEPOSITS IN ARIZONA

Exotic Cu deposits form when a headward-eroding stream cuts into the vadose zone of an active supergene enrichment system. Acidic supergene mineralizing water then discharges laterally to the alluvial aquifer and evolves downgradient, depositing a zoned system of, first, hematite, then, copper wad, and then, chrysocolla, cementing conglomerates (Münchmeyer, 1996). New work in Arizona confirms this model and adds to it.

Mineral zonation of these systems is a reliable exploration tool, pointing upgradient to an enriching bedrock porphyry Cu deposit, though unzoned small deposits also occur. Exotic Cu deposits develop best in channels incised into bedrock, and important tonnages are found in fractured bedrock rarely extending to 100 m below the alluvium. And, just as supergene enrichment of bedrock deposits is ongoing in Arizona, so, also, are exotic Cu, ferricrete, and manganocrete mineralization ongoing, at several places. Exotic mineralizing water is O₂-poor, because it earlier weathered bedrock sulfides, so it deposits metals as oxides where it again meets atmospheric O₂. The climate must be semi-arid, because too much background alluvial ground water will dilute the mineralizing water, and too little water stops the downgradient transport. Metals deposited due to evaporation are evident from special textures, further emphasizing the seasonal aridity and limits on water volumes. Where the bedrock is merely pyritic, goethitic ferricrete forms downgradient and nearby, and manganocrete lacking Cu forms farther below. Uniquely, though, the ferricrete of Cu-rich systems has red hematite, comparable with red hematite in leached cappings above Cu-rich supergene blankets. Chrysocolla of exotic deposits is commonly black due to substituted Mn.

Study of these deposits is critical to understanding environmental geochemistry in the Southwest, because, in nature, acid rock drainage was “remediated” by depositing ferricretes, manganocretes, and exotic Cu deposits.

Münchmeyer, C., 1996, Exotic deposits – products of lateral migration of supergene solutions from porphyry copper deposits, in Camus et al., eds., Andean copper deposits: new discoveries, mineralization styles and metallogeny: SEG Spec. Pub. 5, p. 43-58.