Mineral Resources of the Lower John Day and Thirtymile Wilderness Study Areas, Sherman and Gilliam Counties, Oregon

At the request of the U.S. Bureau of Land Management, approximately 25,163 acres of the Lower John Day (OR-005-006) and Thirtymile (OR-005-001) Wilderness Study Areas (17,831 and 7,332 acres, respectively) in northcentral Oregon were evaluated for identified mineral resources (known) and mineral resource potential (undiscovered). In this report, "wilderness study areas" and "study areas" refer to the 25,163 acres for which mineral surveys were requested. Fieldwork for this report was carried out in 1985 and 1986. No mines or mining districts are located inside the study areas; however, both areas are under lease for oil and gas. The Thirtymile Wilderness Study Area has 790,000 cubic yards of subeconomic gold resources averaging 0.003 troy ounces per cubic yard in two river bars along the John Day River. The Lower John Day Wilderness Study Area has five river bars that have gold occurrences. The Lower John Day and the Thirtymile Wilderness Study Areas both have a low mineral resource potential for placer gold in the canyon of the John Day River. In addition, part of the Thirtymile Wilderness Study Area has a low mineral resource potential for gold and (or) mercury in hydrothermal deposits. Both study areas have a low resource potential for oil and gas and no potential for geothermal energy resources. Character and Setting The Lower John Day and Thirtymile Wildcrnesss Study Areas are located in north-central Oregon about 40 mi southeast of The Dalles and about 15 mi west of Condon (fig. 1). Within the study areas, the gently rolling hills Manuscript approved for publication June 22, 1988. of the region are deeply incised by the steep, 1,800-ft-deep canyon of the John Day River. Basalt flows of the Columbia River Basalt Group cover the entire area studied. A drill hole to the east indicates that the basalt is at least 2,000 ft thick and that it overlies rocks of the John Day Formation. Entrenched meanders and perched alluvium were left along canyon walls by the John Day River as it cut into the surrounding terrain. Identified Resources Deposits of alluvium along the John Day River in the study areas contain placer gold. The Thirtymile Wilderness Study Area has 790,000 yd3 of inferred subeconomic gold resources averaging 0.003 troy ounces per cubic yard (oz/ yd3) in two river bar deposits. No mineral resources were identified in the Lower John Day Wilderness Study Area; however, five river bars in the study area have gold occurrences totaling 1.0 million yd3 averaging 0.0004 troy oz/yd3 gold. Stone, sand, and gravel from the study areas could be quarried and used for construction purposes; however, suitable material is common elsewhere in the region and more accessible to possible markets. Mineral Resource Potential of the Lower John Day Wilderness Study Area The Lower John Day Wilderness Study Area has low mineral resource potential for placer gold in alluvium occurring in scattered small river bars in the canyon of the John Day River. This potential is indicated by anomalously high gold concentrations in stream-sediment samples. Mineral Resources of the Lower John Day and Thirtymite Wilderness Study Areas, Oregon A1 120°45' 120°30' 120° 15' APPROXIMATE BOUNDARY OF LOWER JOHN DAY WILDERNESS STUDY AREA (OR-005-006) Standard Kirkpatrick