Geologic Map of the Northern Half of the Pintler Lake 7.5' Quadrangle and the Southern Half of the Warren Peak 7.5' Quadrangle, Southwestern Montana

Howlett, Caden J.; Reynolds, Aislin; Laskowski, Andrew K.

doi:10.1130/abs/2019am-331565

Cited by 2 publications

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“…Zircon fission-track ages of ca. 27 Ma (Foster et al, 2010) and mapped field relationships (Howlett et al, 2019) suggest that slip could have occurred for much longer than previously thought, into the Oligocene. This raises the possibility that the Pioneer District gold placers, if initially concentrated in Oligocene-Miocene Flint Creek Basin conglomerates, were shed from the AMCC footwall during active extension.…”

Section: Geologic Settingmentioning

confidence: 78%

“…The absence of strata of the Beaverhead Group in the hypothesized source area itself (Fig. 2) is probably due to its erosion during the Paleocene (e.g., Houston and Dilles, 2013;Schwartz and Schwartz, 2013) and contemporaneous with exhumation of the AMCC in the Eocene and Oligocene (Foster et al, 2010;Howlett et al, 2019;Reynolds et al, 2019). Composite cumulative distribution and KDE plots produced by the placer samples and the Beaverhead Group reveal obvious similarities in their DZ signatures (Fig.…”

Section: Detrital Zircon Geochronology Unmixing Modelingmentioning

confidence: 97%

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Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

Howlett

Laskowski

2020

Geosphere

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Despite the widespread occurrence and economic significance of gold placer deposits, modern provenance studies of placer sediments remain largely qualitative. This study applies detrital zircon (DZ) geochronology to determine the source of zircon in placer deposits. We then evaluate the provenance of the zircon to assess whether the gold might have been derived from the same sources, thereby providing a case study of the use of DZ geochronology applied to placers. We present a new set of DZ U-Pb ages (n = 1058) and Lu-Hf (n = 61) isotopic data from four placer deposit samples collected from the Pioneer District of western Montana (USA). Each of the four samples yielded similar age spectra, with a range of U-Pb ages between 3000 and 25 Ma. We interpret that ≥250 Ma zircons were recycled from the Mesoproterozoic Belt Supergroup, Paleozoic–Mesozoic sedimentary rocks, and the Upper Cretaceous–Paleocene Beaverhead Group. Our 237 DZ U-Pb ages ≤250 Ma reveal two prominent age-probability peaks centered at ca. 69 Ma and ca. 26 Ma, which we interpret to record first-cycle derivation from the Royal stock and nearby Dillon Volcanics, respectively. We evaluate these data using an inverse Monte Carlo DZ unmixing model that calculates relative contributions from plausible source units, determining a 12% contribution from the Royal stock and a 43% contribution from the Beaverhead Group. A current absence of the Beaverhead Group in the hypothesized source region suggests complete erosion of the unit into the placer-bearing basin. Detrital zircon geochronology, Hf isotopic data, and the unmixing modeling results offer the first zircon-based support for previous interpretations that the Late Cretaceous Royal stock precipitated gold along its contact with overlying Proterozoic–Mesozoic sedimentary strata. Subsequent exhumation and erosion of the lode source led to gold deposition in the Anaconda metamorphic core complex supradetachment basin during the late Oligocene–late Miocene. The worldwide occurrence of gold placer deposits with unknown source areas provides abundant opportunity to apply these techniques elsewhere.

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Section: Geologic Settingmentioning

confidence: 78%

Section: Detrital Zircon Geochronology Unmixing Modelingmentioning

confidence: 97%

Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

Howlett

Laskowski

2020

Geosphere

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“…In the northeastern Anaconda Range, the ADF is expressed as a 300-500 m thick lower-to middle-greenschist-facies mylonitic shear zone (Foster et al, 2007(Foster et al, , 2010. Exposures of the detachment become more isolated and lower-grade in the south-central and southern Anaconda Range (Elliott, 2017;Wallace et al, 1992;this study). In the south-central Anaconda Range, the detachment dips shallowly southeast (∼12-15°), has been recognized as a complex zone of anastomosing brittle-plastic strain, and is expressed as multiple detachments in certain stretches along strike (Elliott, 2015;Howlett et al, 2020).…”

Section: The Amccmentioning

confidence: 99%

Magmatism and Extension in the Anaconda Metamorphic Core Complex of Western Montana and Relation to Regional Tectonics

2021

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Metamorphic core complexes (MCCs) are domal geological structures that result from the exhumation of the mid-to-lower crust (e.g., Crittenden et al., 1980). These structures are significant because they represent locations of extreme crustal extension and provide illuminating windows into the thermomechanical properties of Earth's lithosphere (e.g., Platt et al., 2015;Whitney et al., 2013). First described in the western United States, MCCs consist of a ductilely deformed metamorphic-plutonic footwall separated from a brittlely deformed hanging wall by a low-angle normal fault (detachment fault) (Armstrong, 1982;Coney, 1980;Wernicke, 1981). Despite their widespread occurrence and tectonic significance, the origin of core complexes-specifically the regional tectonic processes that facilitate footwall exhumation from mid-crustal depths-remains controversial (e.g., Konstantinou et al., 2013). Several orogen-scale dynamic models have been proposed for MCC formation in the North American Cordillera, including: (a) a change in plate motions, (b) dynamic processes of the downgoing slab (e.g., slab rollback), and (c) late or post-orogenic collapse due to overthickened continental crust (e.g., Whitney et al., 2013). In the North American Cordillera, MCCs form an N-S trending belt that traces a pre-extensional lithospheric welt, where crustal thicknesses and

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Geologic Map of the Northern Half of the Pintler Lake 7.5' Quadrangle and the Southern Half of the Warren Peak 7.5' Quadrangle, Southwestern Montana

Cited by 2 publications

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Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

Magmatism and Extension in the Anaconda Metamorphic Core Complex of Western Montana and Relation to Regional Tectonics

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