EFFECT OF ULTRAMAFIC INTRUSIONS AND ASSOCIATED MINERALIZATION ON THE AQUEOUS GEOCHEMISTRY OF THE TANGLE LAKES AREA, ALASKA

Stream water was collected at 30 sites within the Tangle Lakes area of the Delta River Mining District. Sampling focused on streams near the ultramafic rocks of the Fish Lake Intrusive Complex south of Eureka Creek and the Tangle Complex area east of Fourteen Mile Lake, as well as streams within the deformed metasedimentary, metavolcanic, and intrusive rocks of the Specimen Creek drainage and east of Eureka Glacier. Major, minor, and trace elements were analyzed in aqueous samples for this reconnaissance aqueous geochemistry effort. The lithologic differences within the study area are reflected in the major-ion chemistry of the water. The dominant major cation in streams draining mafic and ultramafic rocks is Mg²⁺; abundant Mg and low Ca in these streams reflects the abundance of Mg-rich minerals in these intrusions. Nickel and Cu are detected in 84% and 87% of the filtered samples, respectively. Nickel concentrations range from Ni < 0.4 to 10.1mg/L, with a median of 4.2 mg/L and Cu < 0.5 to 27 mg/L, with a median of 1.2 mg/L. Trace-element concentrations in water are generally low relative to freshwater aquatic life criteria; however, Cu concentrations exceed the hardness-based critera for both chronic and acute exposure at some sites. Complete rare earth element (REE) suites are found in samples from the Specimen Creek sites MH5, MH4, and MH6 and, with the expectation of Tm, at site MH14. The North American shale composite (NASC) normalized REE profiles from the sites along Specimen Creek indicate that the REE are in the dissolved phase in Specimen Creek, while those of site MH14 indicate that the REEs are associated with very fine colloidal matter at this site.