Interpretation of the regional geochemistry of the Tonopah 1 degree by 2 degrees Quadrangle, Nevada, based on analytical results from stream-sediment and nonmagnetic heavy-mineral-concentrate samples

Abstract

Results of more than 2,400 analyses of <0.25-mm fraction of stream sediments and nonmagnetic heavy-mineral concentrates from stream sediments from 1,217 sites in the Tonopah quadrangle demonstrate the existence of anomalous concentrations of many metals of economic interest in many areas. Single-element anomalies of As, Pb, or Sb in stream sediment provide general guides to possible mineralization, but multi-element suites are more reliable to characterize types of mineralization. Various combinations of Ag, As, Bi, Cu, Mo, Pb, Sb, and W in stream sediments and concentrates provide guides to five general types of deposits and their geologic environments. Multi-element suites that recur in stream-sediment media resemble those observed in mineralized rock samples, and the distribution of the suites corresponds to the distribution of known types of deposits and to the general geology of the region. Multi-element geochemical anomalies are prominent in the Pilot Mountains, Monte Cristo Range, Paradise Range, Lone Mountain, northeastern Toiyabe Range, Cedar Mountains, and Toquima Range. Strongest multi-element anomalies are associated with plutonic rocks in the western part of the quadrangle. Two base-metal suites are enriched in and around plutons: (1) Bi + W + Pb + Mo ± As, Cu, Sb, a suite that resembles metals enriched in rocks samples from skarn deposits; and (2) Pb + Mo ± As,Cu,Sb, which resembles base-metal enrichments in sulfide vein and replacement deposits, generally associated with intrusions. A third variety of base-metal enrichment that contains Ag is distinguished for its possible economic importance. This Ag-Pb-rich suite occurs both near plutons and in volcanic terrane and generally is near areas of known Ag-rich polymetallic deposits. Many sites are characterized by modest enrichments of two to five elements in the suite As-Mo-Sb-Zn-Ag that resemble enrichments in epithermal ore deposits. Many anomalies of this type are from drainages underlain by Tertiary welded tuff, commonly lacking alteration and mineral prospects. Some of these anomalous sites correlate with areas of known epithermal deposits or siliceous alteration of calcareous rocks, thus this suite may be a guide to precious-metal deposits if there is supporting geologic evidence. The results from regional sampling of stream sediments, utilizing the <0.25-mm fraction and nonmagnetic heavy-mineral-concentrate media, effectively outline most areas of known deposits or alteration in the Tonopah quadrangle. Some clusters of anomalies are larger than the area of known mineral prospects, suggesting that other deposits remain to be discovered. This regional geochemical information is most effective for exploration and resource analysis when integrated with other geologic information, especially rock alteration.