The Dolphin deposit is an intrusion-related gold deposit (IRGD) located approximately 30 km north of Fairbanks, Alaska. The deposit is in--and adjacent to--a composite mid-Cretaceous pluton intruding amphibolite facies metamorphic rocks. NI43-101 compliant gold resource estimations for the deposit utilizing a 0.3 g/t cut-off, is 61.5 Million tonnes (Mt) at 0.69 g/t indicated (1.36 million oz = Moz) and 71.5 Mt at 0.69 g/t inferred (1.58 Moz).
Due to extensive hydrothermal alteration of the intrusion, identifying lithology in hand sample and thin section, as well as by standard compositional techniques (SiO2 vs. Na2O + K2O), has proven problematic. By plotting wt% TiO2 vs. P2O5 obtained from XRF analysis and four-acid digest ICP-MS data, two distinct population clusters appear. By comparison with least-altered intrusive rock analyses from the Fairbanks district, the igneous units can be identified as granite and tonalite. Because there is no gradational transition through an intermediate granodiorite unit, they were most likely derived from two separate magmatic bodies rather than in-situ fractionation from a single parent. Tonalite is concentrated along the northern and eastern margins of the stock with granite composing the rest of the body. Tonalite xenoliths in granite and granite dikes intruding tonalite prove that tonalite is the older unit.
Investigations of hydrothermal alteration (based on chemical analyses, X-ray diffraction, and thin section examination) show albitic and advanced argillic (kaolinite-quartz) alteration are the dominant styles with sericite common throughout. Advanced argillic is a low temperature (<300°C) low pH alteration style that has not been previously identified in intrusion related gold deposits (IRGDs) in interior Alaska. Albitic alteration resulted from higher temperature, more neutral pH fluids.
Gold investigations show that gold occurs as coarse-grained Au°, aurostibite, and maldonite in quartz + sulfide veins; fine-grained Au° in the oxide zone; and in many forms in disseminated sulfide. These forms include Au° inclusions in pyrite and arsenopyrite, solid-solution Au within compositionally zoned arsenopyrite, and as Au° nanoparticles in pyrite and arsenopyrite. Using UAF’s JEOL JXA-8530F microprobe, I found that solid-solution gold occurs only in arsenopyrite with strong compositional zoning. These have low As cores; gold-bearing mantles with moderate % As; and high As rims. In contrast, compositionally homogenous arsenopyrite does not contain solid-solution gold. Pyrite is commonly arsenian and carries dissolved gold (if any) near detection limits. Gold mineralization has not been tied to any lithology or alteration style; however, gold does seem to correlate with abrupt changes in alteration, especially between sericite + albite and kaolinite + sericite alteration and gold-bearing, zoned arsenopyrites are associated with advanced argillic alteration.
|Advisor:||Newberry, Rainer J.|
|Commitee:||Keskinen, Mary, Larsen, Jessica|
|School:||University of Alaska Fairbanks|
|School Location:||United States -- Alaska|
|Source:||MAI 58/01M(E), Masters Abstracts International|
|Keywords:||Arsenopyrite, Dolphin, Fairbanks mining district, Gold, Intrusion-related gold deposit|
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