A native copper deposit near Jefferson City, Montana

Abstract

Native copper occurs as a cement placer material within a restricted area near Jefferson City, Montana. The deposit is situated in a shallow canyon near the headwaters of the major Prickly Pear Creek drainage system where an intimate association of peat muck, bog-iron oxide, and alluvial gravels combine to establish a unique set of natural reducing conditions that attend the resulting deposition of secondary copper from percolating sub-surface waters. The copper is derived by oxidation of narrow epigenetic veins that occur up-stream in rocks of the Boulder batholith. It is carried downward as copper sulphate in solution until it reaches the reducing environment.Although both carbonaceous peat muck and iron oxide may be reducing agents, it appears that, in this instance, the iron oxide deposit plays the dominant role in reduction. The peat muck, in fact, seems to have even kept the copper in solution in most cases, and thus has permitted its localization on or near the limonitic bog material.