301 Geochronology (Posters)

Wednesday, 22 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

40Ar/39Ar Constraints on the Timing of the Manh Choh Au-Cu-Ag Skarn, Alaska

Paper number: 301-0
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

The Manh Choh Au-Cu-Ag skarn deposit is located in the Tintina Gold Belt in the eastern part of Interior Alaska. The main ore phases in the amphibole-rich skarn are: pyrrhotite, chalcopyrite, native bismuth, arsenopyrite, galena, sphalerite, marcasite, jonassonite, gold-electrum, and pyrargyrite; classic deposit-scale skarn calc-silicate mineral zoning is still being mapped. Key petrogenetic components of the deposit model are poorly understood, such as skarn formation relative to regional metamorphic and magmatic events. We present preliminary 40Ar/39Ar data of quartz-mica schist, calc-silicate hornfels, magnetite skarn, amphibole skarn, amphibolite, andesite, and quartz monzonite. 

Preliminary observations made with transmitted light and scanning electron microscopy revealed that samples chosen for 40Ar/39Ar analyses contain target phases such as feldspar, hornblende, and mica. Results of step-heated white mica 40Ar/39Ar from quartz-mica schist yielded plateau ages of 120 Ma, which match reported ages of quartz-mica schist mapped in the Tanacross Quadrangle in the biotite gneiss and schist unit.  This represents the age of the most recent regional metamorphic event, demonstrating that emplacement-related heating did not exceed the muscovite closure temperature of ~350 °C. Preliminary results for other step-heated minerals yield 40Ar/39Ar plateau ages of 73 Ma for groundmass in andesite, 66 Ma for hornblende in amphibolite,  67 Ma for feldspar in quartz monzonite, and 68 Ma for hornblende in magnetite skarn. Ongoing work will continue to extend 40Ar/39Ar data to minerals from other lithologies in the region and a thermal model using HeFTy. These data will constrain the timing and thermal evolution of skarn formation at Manh Choh and provide insight into deposit formation relative to regional metamorphic and magmatic events.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414361

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