Abstract

The Silver Reef mining district had a recorded production of more than 7 million ounces of silver prior to 1900. It is noted for rather unique occurrences of ore-grade silver halides (reportedly silver chloride, i.e., cerargyrite or horn silver) in sandstones of the Triassic Moenave Formation, unaccompanied by obvious alteration or substantial base metal ores. The beds crop out at the noses and flanks of anticlines. Our work, conducted during underground development and surface mapping, as well as that of others, indicate the mineral deposits have minimal similarity to most known significant silver deposits. The ore grades declined substantially with depth in the major mines, where the rock becomes more impervious and the dip of the beds flattens. The district lies in a silver-rich metallogenic province noted for silver in a variety of environments.

Silver halide is the primary mineral in the Silver Reef district; some native silver occurs, but silver sulfides have not been documented with certainty. Old reports state that silver sulfide was locally encountered at depth. Carbonaceous matter (fossil wood, sedimentary trash) locally contains much silver. Significant selenium and possibly minor silver selenides (Ag₂Se) are present. Mercury, sometimes reported as abundant, was at least partly introduced by old amalgamation plants, which used it in large quantities. Anomalous, not ore grade, gold has been reported. Locally significant copper and uranium concentrations correlate poorly with silver occurrences.

Evidence for oil migration through the rocks also suggests a chloride brine was present. Such a brine, in this regionally-anomalous silver metallogenic province, may have complexed silver as a chloride. Passing upward into anticlinal traps, the low-temperature brine may have encountered reducing conditions and/or low-salinity ground water. Dilution of the brine would break the complex and precipitate cerargyrite. Reduction would accomplish a similar effect. The system had a low sulfide content. Oxidation potential is unknown. No modeling of Ag concentrations or flow rates has been done. Subsequent supergene enrichment likely created some high-grade silver areas, as well as uranium concentrations. Although more work on chemistry is needed, this model best explains the observed mineral deposits.