GEOCHEMISTRY AND PRELIMINARY CONSTRAINTS ON PROVENANCE OF PLACER GRAINS OF THE FORTYMILE GOLD DISTRICT, EAST CENTRAL ALASKA

The Fortymile district has produced approximately 500,000 troy ounces of placer gold since its original discovery in 1886. Although a few poorly defined lode deposits (i.e. Napoleon and Purdy) are present around Chicken, AK, no major deposits have been discovered to date. Since no definitive bedrock gold source has been found, the provenance of placer deposits in the Fortymile district of Alaska is a long-standing enigma for geologists and prospectors alike. This study focuses on creeks and terraces around Chicken in the southwest part of the district. The principle objectives of this study are to 1) determine the minor trace element concentrations (Ag, Sn, Sb, Te, Hg, Pb, Bi) of the gold grains at each location, 2) characterize mineral inclusions within gold grains, and 3) infer the bedrock deposit type (e.g. orogenic or intrusion-related). Similar studies within the Klondike district have indicated the usefulness of these techniques in determining the provenance of placer gold and we intend to employ and expand on these methods. EMPA for Au-Ag-Hg show a slightly higher fineness of gold in the Uhler Creek and Lost Chicken samples and a relatively consistent average Hg content in all the samples ranging from 1.33-2.66 wt%. Tellurium was detected in the grains from Stone House and Gold Coin and Sn in the samples from Uhler Creek and Gold Coin. SEM analysis of mineral inclusions of Bi, Ag, and Te sulfides and tellurides occur in both Gold Coin and Stone House, while Lost Chicken and Uhler Creek have little to none. The higher fineness, presence of gold telluride, and lack of Bi in gold or mineral inclusions from Lost Chicken and Uhler Creek suggests that they are derived from orogenic gold deposits. The lower fineness and presence of Bi in gold, bismuth telluride and sulfide inclusions from Gold Coin and Stone House suggests that they are derived from intrusion-related gold deposits. The presence of Sb at Stone House and Sn at Uhler Creek complicates this interpretation, as Sb is often associated with orogenic deposits and Sn with intrusion-related deposits.