Geologic and environmental characteristics of porphyry copper deposits with emphasis on potential future development in the Bristol Bay Watershed, Alaska (Appendix H)

This report is prepared in cooperation with the Bristol Bay Watershed Assessment being conducted by the U.S.
Environmental Protection Agency. The goal of the assessment is to help understand how future large-scale
development in this watershed may affect water quality and the salmon fishery. Mining has been identified as a
potential source of future large scale development in the region, especially because of the advanced stage of
activity at the Pebble prospect. The goal of this report is to summarize the geologic and environmental
characteristics of porphyry copper deposits in general, largely on the basis of literature review. Data reported in the
Pebble Project Environmental Baseline Document, released by the Pebble Limited Partnership in 2011, are used to
enhance the relevance of this report to the Bristol Bay watershed.
The geologic characteristics of mineral deposits are paramount to determining their geochemical signatures in
the environment. The geologic characteristics of mineral deposits are reflected in the mineralogy of the
mineralization and alteration assemblages; geochemical associations of elements, including the commodities being
sought; the grade and tonnage of the deposit; the likely mining and ore-processing methods used; the
environmental attributes of the deposit, such as acid-generating and acid-neutralizing potentials of geologic
materials; and the susceptibility of the surrounding ecosystem to various stressors related to the deposit and its
mining, among other features (Seal and Hammarstrom, 2003). Within the Bristol Bay watershed, or more
specifically the Nushagak and Kvichak watersheds, the geologic setting is permissive for the occurrence of several
mineral deposit types that are amenable for large-scale development. Of these deposit types, porphyry copper
deposits (e.g., Pebble) and intrusion-related gold deposits (e.g., Shotgun) are the most important on the basis of
the current maturity of exploration activities by the mining industry. The Pebble deposit sits astride the drainage
divide between the Nushagak and Kvichak watersheds, whereas the Humble, Big Chunk, and Shotgun deposits
are within the Nushagak watershed. The Humble and Big Chunk prospects are geophysical anomalies that exhibit
some characteristics similar to those found at Pebble. Humble was drilled previously in 1958 and 1959 as an iron
prospect on the basis of an airborne magnetic anomaly. Humble is approximately 85 miles (137 km) west of
Pebble; Big Chunk is approximately 30 miles (48 km) north-northwest of Pebble; and Shotgun is approximately 110
miles (177 km) northwest of Pebble. The H and D Block prospects, west of Pebble, represent additional porphyry
copper exploration targets in the watershed.