ARSENIC SPECIATION IN SURFACE WATER DRAINING THE GOLDEN ZONE BRECCIA DEPOSIT IN CENTRAL ALASKA

The Golden Zone breccia deposit is an active copper/silver/gold exploration prospect adjacent to the southern boundary of Denali National Park in Central Alaska. The deposit was mined briefly from 1941 to 1942 via an underground adit that intersected a highly mineralized breccia pipe, containing arsenopyrite, chalcopyrite, pyrrhotite and pyrite associated with a Cretaceous-age porphyry stock.

To investigate the geochemical signal of Bryn Mawr creek which drains the mineralized area, water samples were collected in July and August 2011 and arsenic species separated in-situ using a field technique for later laboratory analysis using ICP-MS. Duplicate samples were collected for a suite of major and trace elements. Arsenopyrite mineralization at Golden Zone imparts an extensive arsenic signature to the surrounding watershed and it can be used as a pathfinder element. Arsenic concentrations in excess of 250 µg/L in were measured in Bryn Mawr creek adjacent to the mine adit accompanied by elevated levels of antimony.

Arsenic speciation in Bryn Mawr was dominated by As(V) at the majority of sampling stations. Only a few stations were predominantly As(III) species and correlated with water discharge from the adit or groundwater interaction, suggesting that As(III) species are rapidly oxidized over a distance of 100 meters or less in the well-oxygenated waters of the Bryn Mawr. Arsenic species ratios may be a complimentary tool in interpreting arsenic signatures at other similar deposits.