Title: Anomalous chlorine in iron-rich strata; Yellowjacket formation, Lemhi County, Idaho: Analytical data and discussion
Abstract: Uncommonly high concentrations of chlorine, in the range of 0.1 to 1.10 wt percent, occur in samples collected from biotite-rich strata of the Middle Proterozoic Yellowjacket Formation.New analytical results for Cl and F in 539 samples are reported and briefly described.Although many of the biotiterich samples having very high Cl are from Co-Cu-Au deposits of the Blackbird mining district, similar lithologies more than 10 km from ore deposits also are Cl-rich.Chlorine correlates with biotite content, and locally with scapolite, with Fe and K, but not with F. Microprobe analyses have shown that Cl resides in the lattice of Fe-rich biotite.Chlorine appears to have been enriched early in the history of these rocks, but the mechanisms are not established.Unlike some metasedimentary rocks containing scapolite and high Cl, there is no evidence for evaporitic rocks in the Yellowjacket Formation that could have been a source of the Cl.More likely, the Cl reflects a stratified submarine brine that also carried Fe and K. Lithostratigraphic zones rich in Fe-K-Cl appear to be lateral equivalents of those hosting stratiform base-metal deposits.Rocks having anomalous Fe-K-Cl are considered to be a guide to subbasins favorable for stratiform base-metal deposits.