Studies conducted on two Alaskan shale-hosted stratiform Pb-Zn-Ag deposits showed that the NH4 content of illites increased toward the ore bodies and might be used as an exploration tool. This study compares the NH4 chemistry of five shale-hosted stratiform deposits in Selwyn Basin and verifies the existence of a halo near some.
The five are: Howards Pass, Driftpile (Gataga District), Clear Lake, Tom, and Jason (Macmillan Pass). NH4 was found in feldspars, but not in illites. This is probably due to a transferral of NH4 from illites to authigenic feldspars during regional metamorphism. NH4 in feldspars was found to increase in the hangingwall over the Howards Pass and Clear Lake deposits. Concentrations reached as high as 2000 ppm. Evidence suggests that NH4 halos around the other deposits were disturbed by detrital input.
The NH4 anomalies near deposits in Selwyn Basin resemble those near active hydrothermal systems in the Guaymas Basin, Gulf of California - a modern analogous rift basin. A model is proposed for the formation of an NH4 halo.
