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

Analytical techniques, data reduction methods, ICP-MS zircon U-Pb datasets, comparison of age-probability curves generated using PDPs and various KDE bandwidths, sample information, and NMF details and results.

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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

Geosphere ◽

10.1130/ges02226.1 ◽

2020 ◽

Author(s):

Caden J. Howlett ◽

Andrew K. Laskowski

Keyword(s):

Detrital Zircon ◽

Source Region ◽

Metamorphic Core Complex ◽

Western Montana ◽

Gold Deposition ◽

Isotopic Data ◽

Core Complex ◽

Gold Placer ◽

Placer Deposits ◽

Metamorphic Core

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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Supplemental Material: Mesoproterozoic–Early Cretaceous provenance and paleogeographic evolution of the Northern Rocky Mountains: Insights from the detrital zircon record of the Bridger Range, Montana, USA

10.1130/gsab.s.12670904.v1 ◽

2020 ◽

Author(s):

Chance B. Ronemus ◽

et al.

Keyword(s):

Early Cretaceous ◽

Data Reduction ◽

Rocky Mountains ◽

Detrital Zircon ◽

Analytical Techniques ◽

Northern Rocky Mountains ◽

Sample Information ◽

Icp Ms ◽

Reduction Methods

Analytical techniques, data reduction methods, ICP-MS zircon U-Pb datasets, comparison of age-probability curves generated using PDPs and various KDE bandwidths, sample information, and NMF details and results.

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Accelerating exhumation in the Eocene North American Cordilleran hinterland: Implications from detrital zircon (U-Th)/(He-Pb) double dating

Geological Society of America Bulletin ◽

10.1130/b35160.1 ◽

2019 ◽

Vol 132 (1-2) ◽

pp. 198-214 ◽

Cited By ~ 1

Author(s):

Andrew S. Canada ◽

Elizabeth J. Cassel ◽

Daniel F. Stockli ◽

M. Elliot Smith ◽

Brian R. Jicha ◽

...

Keyword(s):

North American ◽

Detrital Zircon ◽

Sedimentary Basins ◽

Metamorphic Core Complex ◽

Crustal Thickening ◽

Sediment Provenance ◽

Core Complex ◽

Extensional Faulting ◽

Lag Times ◽

Metamorphic Core

AbstractBasins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve records of deformation that are unavailable in footwall rocks. Following prolonged late Mesozoic–early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth’s best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermochronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105–740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (<4–6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to <100 m.y. in 45–43 Ma strata and to <10 m.y. in 43–41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1–20 m.y. lag times. These data reflect acceleration of basement exhumation rates by >1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere.

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