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A Permian rhenium–osmium radiometric age for bornite at the Rock Creek deposit, Spar Lake copper–silver district (Montana, USA)—a link to the Sonoma Orogeny and the copper–silver–vanadium Midcontinent Belt?
Element partitioning in magnetite under low-grade metamorphic conditions – a case study from the Proterozoic Belt Supergroup, USA
A rapid lake-shallowing event terminated preservation of the Miocene Clarkia Fossil Konservat-Lagerstätte (Idaho, USA)
New regional mapping documents that a thick quartzite sequence in the Lemhi subbasin of the Belt-Purcell basin lies near the top of the Mesoproterozoic stratigraphic column, and that two finer-grained units have been miscorrelated. This observation requires reassessment of the subbasin's stratigraphy, which we present here. Determination of the relationships between the stratigraphic units of the Lemhi Range and Salmon River and Beaverhead Mountains and better-known Belt Supergroup units to the north has been hampered by miscorrelation of this upper quartzite sequence with older strata, and by miscorrelation of the type Apple Creek Formation with a similar but stratigraphically lower unit. The base of the upper quartzite sequence includes the Swauger and Lawson Creek Formations, which are the highest units previously identified in the Lemhi subbasin. This sequence continues upward through quartzite units described here that underlie or comprise lateral equivalents of the type Apple Creek Formation in the Lemhi Range. The spatial distribution of these quartzite units extends the Lemhi subbasin farther east and north in Montana and northwest in Idaho. The complete stratigraphy reflects the stratigraphic separation of the two “Apple Creeks” and expands the type Apple Creek Formation to accommodate the quartzite units into the regional Mesoproterozoic stratigraphy. Our proposed correlation of the thick upper quartzite sequence with the Bonner Formation and higher units of the Missoula Group in the main part of the Belt basin requires that subsidence of the Lemhi subbasin was significantly faster than that of the main part of the Belt basin during deposition of the upper Missoula Group. Therefore, the two parts of the Belt basin were distinct tectonically, although they shared common sediment sources.
A recent 1:24,000 scale mapping project within the northern Beaverhead Mountains along the Idaho-Montana border has resulted in a reinterpretation of both the Mesoproterozoic stratigraphy and the regional structural framework. A 15-km-thick stratigraphic section of the Mesoproterozoic Lemhi subbasin was initially deformed by northeast-southwest shortening into giant northwest-striking, northeast-verging folds, probably during Cretaceous Sevier orogenesis. These initial folds were then dissected by a system of subparallel and anastomosing, oblique-slip reverse, thrust, and normal faults that generally strike northwest, but that exhibit east-west–oriented lineations, suggesting components of strike-slip displacement. Contractional faulting appears to have been followed by Eocene to Miocene extensional faulting, with many normal faults following the preexisting fabrics. Extension opened Tertiary basins along some of these faults, including the Salmon Basin along the southwestern side of the Beaverhead Range. Subparallel faults in the surrounding region appear to have a similar complex history, and all appear to be part of a major northwest-striking Cretaceous fold-and-thrust belt that was later dissected by Tertiary extension. Although the faults of the Beaverhead Mountains are significant and long-lived, they are not terrane-bounding structures separating the Belt and Lemhi sedimentary sequences. Instead, Lemhi strata extend across the range and northward to Missoula, where they grade into correlative Missoula Group strata.
We investigated the crustal structure of the central Mesoproterozoic Belt Basin in northwestern Montana and northern Idaho using a crustal resistivity section derived from a transect of new short- and long-period magnetotelluric (MT) stations. Two- and three-dimensional resistivity models were generated from these data in combination with data collected previously along three parallel short-period MT profiles and from EarthScope MT stations. The models were interpreted together with coincident deep seismic-reflection data collected during the Consortium for Continental Reflection Profiling (COCORP) program. The upper-crustal portion of the resistivity model correlates well with the mapped surface geology and reveals a three-layer resistivity stratigraphy, best expressed beneath the axis of the Libby syncline. Prominent features in the resistivity models are thick conductive horizons that serve as markers in reconstructing the disrupted basin stratigraphy. The uppermost unit (up to 5 km thick), consisting of all of the Belt Supergroup above the Prichard Formation, is highly resistive (1000–10,000 Ω·m) and has relatively low seismic layer velocities. The intermediate unit (up to 7 km thick) consists of the exposed Prichard Formation and 3+ km of stratigraphy below the deepest stratigraphic exposures of the unit. The intermediate unit has low to moderate resistivity (30–200 Ω·m), relatively high seismic velocities, and high seismic reflectivity, with the latter two characteristics resulting from an abundance of thick syndepositional mafic sills. The lowest unit (5–10 km thick) is nowhere exposed but underlies the intermediate unit and has very high conductivity (4–8 Ω·m) and intermediate seismic velocities. This 17–22-km-thick three-layer stratigraphy is repeated below the Libby syncline, with a base at ~37 km depth. Seismic layer velocities indicate high mantle-like velocities below 37 km beneath the Libby syncline. The continuous high-conductivity layer in the lower repeated section is apparently displaced ~26 km to the east above a low-angle normal fault inferred to be the downdip continuation of the Eocene, east-dipping Purcell Trench detachment fault. Reversal of that and other Eocene displacements reveals a >50-km-thick crustal section at late Paleocene time. Further reversal of apparent thrust displacements of the three-layer stratigraphy along the Lewis, Pinkham, Libby, and Moyie thrusts allows construction of a restored cross section prior to the onset of Cordilleran thrusting in the Jurassic. A total of ~220 km of Jurassic–Paleocene shortening along these faults is indicated. The enhanced conductivity within the lowest (unexposed) Belt stratigraphic unit is primarily attributed to one or more horizons of laminated metallic sulfides; graphite, though not described within the Belt Supergroup, may also contribute to the enhanced conductivity of the lowest stratigraphic unit. A narrow conductive horizon observed within the Prichard Formation in the eastern part of the transect correlates with the stratigraphic position of the world-class Sullivan sedimentary exhalative massive sulfide deposit in southern British Columbia, and it may represent a distal sulfide blanket deposit broadly dispersed across the Belt Basin. By analogy, the thick conductive sub–Prichard Formation unit may represent repeated sulfide depositional events within the early rift history of the basin, potentially driven by hydrothermal fluids released during basaltic underplating of attenuated continental crust.
SHRIMP U–Pb and REE data pertaining to the origins of xenotime in Belt Supergroup rocks: evidence for ages of deposition, hydrothermal alteration, and metamorphism
Early Mississippian Orbital-Scale Glacio-Eustasy Detected From High-Resolution Oxygen Isotopes of Marine Apatite (Conodonts)
The Spar Lake Strata-Bound Cu-Ag Deposit Formed Across a Mixing Zone Between Trapped Natural Gas and Metals-Bearing Brine
SHRIMP U-Pb Ages of Xenotime and Monazite from the Spar Lake Red Bed-Associated Cu-Ag Deposit, Western Montana: Implications for Ore Genesis
Geochemistry of Magnetite from Hydrothermal Ore Deposits and Host Rocks of the Mesoproterozoic Belt Supergroup, United States
AGE AND ORIGIN OF QUARTZ-CARBONATE VEINS ASSOCIATED WITH THE COEUR D’ALENE MINING DISTRICT, IDAHO AND WESTERN MONTANA: INSIGHTS FROM ISOTOPES AND RARE-EARTH ELEMENTS
On-site repository construction and restoration of the abandoned Silver Crescent lead and zinc mill site, Shoshone County, Idaho
Abstract From the early 1900s through the 1950s the Silver Crescent mine and mill processed lead, zinc, and silver from ore found in the Precambrian metasedimentary rocks of the Belt Supergroup. Approximately 150,000 cubic yards of tailings and waste rock were deposited in the floodplain of Moon Creek less than 2 miles upstream of what is now a residential area. The actively eroding tailings impoundments were a source of heavy metal contamination to the surface and groundwater flowing through the site. The U.S. Forest Service began a CERCLA non-time-critical removal action at the Silver Crescent mine in 1998. Removal action goals included reduction of particulate and dissolved metal loading into Moon Creek and local groundwater. These goals were successfully achieved in part by incorporating the tailings and waste rock dumps into an on-site capped repository. The nearly $2 million Silver Crescent removal action construction phase was completed in late 2000 with the final habitat restoration phase scheduled for completion in 2007.