| Summary: | 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. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
|
|---|
