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Ford, R.C., 1993

Geology, geochemistry, and age of gold lodes at Bluff and Mt. Distin, Seward Peninsula, Alaska

Bibliographic Reference

Ford, R.C., 1993, Geology, geochemistry, and age of gold lodes at Bluff and Mt. Distin, Seward Peninsula, Alaska: Golden, Colorado, Colorado School of Mines, Ph.D. dissertation, 302 p., illust., 5 maps.


Gold lodes at Bluff and Mt. Distin, 76 km east-southeast and 30 km north of Nome, Alaska, respectively, are hosted by deformed Paleozoic(?) metasedimentary and metavolcanic rocks of the Seward terrane. The Seward terrane includes low-grade metasedimentary and metavolcanic rocks that surround gneissic and plutonic rocks of the Kigluaik, Bendeleben, and Darby mountains; these underwent deformation and high pressure-low temperature (HP-LT) metamorphism at > 160 Ma, resulting in the formation of three fold sets (Drm 1a, 1b, 1c). Relict lithologic layering (S0) is locally observed, although it has often been obliterated by subsequent deformational events. Drm 1a folds are locally recognized as rootless isoclines of S0 that are refolded. Rare Drm 1b folds are upright, tight to isoclinal, with no associated axial planar fabric. Drm 1c folds deform both Drm 1a and Drm 1b and are tight to isoclinal, recumbent intrafolial with an axial planar, transpositional crenulation cleavage (Srm 1c) that is the dominant planar element in both areas. A prominent stretching lineation (Lrm 1c) is parallel to Drm 1c hinges. Significant tectonic transport has occurred along Drm 1c-related low-angle faults in thick marble sequences and at schist-marble boundaries. The change in vergence of low-angle faults between Bluff and Mt. Distin may reflect unroofing of the Darby and Kigluaik Mountains, respectively. Upright and open folds (D2) formed late in the kinematic history of the terrane, along with extensional joint sets. Gold-bearing veins at Bluff are hosted by extensional joints, and show a complex mineral assemblage. Marginal alteration is locally pervasive. Gold is also found in silicified, mylonitic marble at schist contacts. The primary focus of exploration at Mt. Distin is a quartz + stibnite +/- gold vein hosted by chlorite + feldspar schist. 40Ar/39Ar geochronology on metamorphic and vein-related muscovite show that disturbed metamorphic spectra cooled below muscovite closure at ~127 Ma (Bluff) and 157.1 - 169.6 (Mt. Distin). Three vein muscovite samples from Bluff show plateaus of ~109 Ma, demonstrating that crystallization of vein muscovite coincides with Barrovian metamorphism and plutonism in the Kigluaik, Bendeleben, and Darby mountains. Gold-bearing fluids were generated during this thermal event.

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