STRUCTURAL PRESERVATION OF GOLD DEPOSITS: NORTHWEST-TRENDING FAULTS IN THE BOISE BASIN

The Boise Basin is Idaho’s largest gold district with documented production of nearly 3 million ounces from placer and lode mines. Preliminary mapping of newly recognized, northwest-trending faults in the Basin suggests the faults may have controlled formation and location of placers at changes in stream gradient and later preserved portions of the Pleistocene-age high gravels in downdropped blocks and grabens. The N40W to N60W structural grain is visible on digital elevation maps of the basin, orthophotos, and panoramic photographs of the Boise Basin. Anderson (1947) briefly mentioned northwest-trending faults in the Gambrinus District, a portion of the Basin. Additional evidence for the faults includes: northwest-trending, slickensided surfaces exposed along Thorn Creek; topographic breaks; alignment of major and minor stream drainages; and northwest-trending Eocene dikes, one fault and straight lithologic contacts mapped by Kiilsgaard (1997). In addition, sheared and iron-stained outcrops along Bogus Basin road were observed along one of the inferred structures.

Lode mineralization occurs principally along northeast-trending structures and Eocene porphyry dikes related to the Trans-Challis Fault System. Mineralization of northwest-trending structures in the Gambrinus district requires initial movement prior to or during Eocene time. If the northwest-trending faults offset Eocene intrusions and bound sub-basins which localized and preserved the placer deposits, then much movement is post-Eocene and probably Miocene in age, extending down to 10-15 Ma. Kiilsgaard (1997) shows one short northwesterly fault cutting Oligocene rhyolite, and similar faults may displace Payette Formation sediments, recently dated at 11.8 Ma (Forester, et al., 2002). The Boise Basin faults generally parallel the nearby Western Snake River Plain and Olympic-Wallowa lineament. Major activity on the Boise Basin faults was probably coincident in time with development of the Western Snake River Plain graben, which Wood and Clemens (2002, in press) suggest formed from 11 to 9.5 Ma. Recognition of these northwest-trending Basin faults provides a tool for the discovery of blind ore bodies offset from the rich veins mined in the late 1800s through the 1930s.