The crustal heritage of silver and gold ratios in Arizona ores

The ratios of produced grades of silver and gold in ore deposits of Arizona, representing nearly a century of mining, correlate with their geologic setting. The deposits occur south and west of the Colorado Plateaus where two geological domains are recognizable on the basis of type and age of rocks and geological history. The eastern or “Naco” domain has a basement of Proterozoic Pinal Schist, formed from a protolith of mostly clastic sedimentary rocks, and widely scattered younger Proterozoic granites, overlain by Paleozoic platform strata and Mesozoic volcanic and clastic rocks. The “Western Deserts” domain is characterized by an older Proterozoic section of largely submarine volcanic and volcaniclastic strata, together with Proterozoic granites; it is largely devoid of Paleozoic strata, but Mesozoic clastic and volcanic strata are widely exposed. In both domains, middle and late Tertiary volcanic rocks are extensive.

Ore deposits of many genetic types, of Proterozoic, Nevadan, Laramide, and younger Tertiary ages, are present; deposits of Laramide and younger Tertiary ages occur in both domains, but deposits of Proterozoic age are confined mostly to the Western Deserts, and those of Nevadan age are restricted to the Naco domain. The ore deposits evolved at different times under different conditions of tectonic stress.

The produced grades of silver and gold, and the silver to gold ratios, reveal a bimodal distribution. Most deposits of all epochs and genetic type in the Naco domain show Ag enrichment relative to the ratio of the Ag:Au crustal abundance (clarke ratio) (17.5:1). Except for two Laramide porphyry copper deposits and the Proterozoic massive Cu-Pb-Zn ores, whose ratios are close to the clarke ratio, most other districts of the Western Deserts domain are enriched in gold with respect to the ratio.

The regional zoning transgresses ore-deposit type, time, tectonic style, and reported igneous rock types and metal associations. It does not appear to be related to the hypothetical results of subduction processes wherein metals and magmas are directly derived from partial melting of a descending slab of oceanic rocks. Rather, the precious-metal characteristics may represent inheritance from specific kinds of crust.