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UBC Theses and Dissertations

Geological setting, nature and evolution of reduced intrusions and gold bearing quartz veins of the 4021 prospect, Goodpaster district, east-central Alaska Dilworth, Katherine M.

Abstract

Gold deposits associated with Cretaceous reduced granitic rocks are common in Yukon and Alaska. The Liese zone and spatially related prospects in the Goodpaster district, eastcentral Alaska, are associated with an arcuate, easterly trending belt of Cretaceous reduced granite to tonalite that compose the older of two Early Cretaceous intrusive suites. A younger suite of calc-alkaline diorite rocks (-94 Ma) are slightly more oxidized and are unrelated to the gold mineralization. Both calc-alkalic suites have low magnetic susceptibilities and oxidation states. The older suite, which includes a phase of the Goodpaster Batholith, is weakly peraluminous and contains monazite and zircon. Biotite is common and hornblende is subordinate to absent. Older granitoids of the suite are deformed and syn- to- late kinematic to regional metamorphism at about 109-107 Ma (U-Pb; zircon and monazite). Post-kinematic intrusions of the suite are as young as 104 Ma (U-Pb SHRTMP-RG). Sulphide deposition at -104.3 Ma in the Liese Zone, inferred by a Re-Os date on molybdenite, indicates a temporal overlap. Gold veins in the region have a reduced sulphide assemblage (lollingite and pyrrhotite stable) and are characterized by an Au-Bi-Te geochemical signature. Fluid inclusions from gold veins and pegmatite are similar and contain ubiquitous aqueous-carbonic fluid inclusions ± CH4. Locally, complex saline fluids coexist with C02-rich fluids. Sulphide minerals from the Liese zone and gold prospects have Pb isotopic values (²⁰⁶Pb/²⁰⁴Pb =19.4-19.5) that generally overlap with thase of the reduced granite to tonalite (206Pb/204Pb= 19.4-19.6). The sulphide Pb isotopic values extend to slightly higher ²⁰⁶Pb/²⁰⁴Pb values, likely due to fluid rock interaction with the isotopically heterogeneous host gneiss of the Yukon-Tanana terrane. The diorite suite has lower values (²⁰⁶Pb/²⁰⁴Pb = 19.1-19.2) than most of the sulphides, and thus is precluded from contributing Pb to the bulk of the sulphides. Gold in the Pogo area is related to a suite of reduced granite to tonalite intrusions that were emplaced late- to post-kinematic to regional deformation in the late Early Cretaceous. Gold-bearing quartz veins formed from magmatic-hydrothermal fluids exsolved from the reduced granites and were focused along shallowly dipping regional folation.

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