MINERALOGY AND GEOCHRONOLOGY OF AU-TE MINERALIZATION AT THE JEWETT AND BENTON MINES, JOSEPHINE COUNTY, OREGON

The Jewett and Benton mines in the Klamath Mountains of Josephine County, Oregon exploited high-grade Au-Te bearing veins, similar to that seen throughout the Klamath Geologic Province. The mines are separated by ~66 km (41 miles), yet, despite the distance, they share several common traits, suggesting a similar origin.

The Jewett mine is located SE of the Grants Pass pluton (139 ± 2 Ma (Harper et. al., 1994), 147.9 ± 1.1 Ma (Barnes, 2014)) and 2.4 km (1.5 miles) W-SW of the Greyback pluton (157.3 ± 1.4 Ma (Hacker et al., 1995)). Quartz veins within the most recently worked Tanner-Michael pit are thin, irregular and pinch and swell along stacked SE dipping faults. Visible gold, tetradymite (Bi₂Te₂S) and tellurobismuthite (Bi₂Te₃) are irregularly distributed within the quartz veins. New radiometric dating utilizing the ⁴⁰Ar/³⁹Ar method on hydrothermal mica within the Au-Te quartz veins demonstrates an age of mineralization to be 153.67 ± 0.46 Ma. This indicates that mineralization is in fact older than the Grants Pass pluton, but much closer in age to the Greyback pluton.

Porphyritic hornblende diorite-quartz diorite dikes are also found throughout the Jewett mine. The majority are subvertical and strike NNW to SSE and SW to NE. In the pit, small hornblende dikes are cut by mineralized faults, however, a similar porphyry dike NE of the pit gave an ⁴⁰Ar/³⁹Ar age of 147.11 ± 0.56 Ma, younger than mineralization and similar to the age of the Grants Pass pluton.

The Benton mine Au-Te quartz vein system consists of highly irregular, structurally controlled veins lying in parallel sheets that pinch and swell from 1-20cm and dip approximately 45° N-NW. The veins are hosted in an undated dioritic body that intrudes the Late Jurassic Rogue Volcanics. Quartz veins contain 0.1-2cm thick pyrite veinlets which host gold in calaverite (AuTe₂), petzite (Ag₃AuTe₂) and electrum. Ages of 149.35 ± 0.37 Ma and 151.17 ± 1.14 Ma were obtained using the ⁴⁰Ar/³⁹Ar method on hydrothermal mica within the veins.

The source of fluids and metals of Au-Te veins in the Klamath Mountains is presently ambiguous and requires more research to address. However, geochronology indicates that mineralization at the Jewett and Benton mines formed at a similar time that coincides with both regional metamorphism and magmatism, making either a possible source.