Journal article
The Lake Cretaceous Donlin Creek gold deposit, Southwestern Alaska; controls on epizonal ore formation
Economic geology and the bulletin of the Society of Economic Geologists, Vol.99(4), pp.643-671
07/2004
DOI: 10.2113/gsecongeo.99.4.643
Abstract
The Donlin Creek gold deposit, southwestern Alaska, has an indicated and inferred resource of approximately 25 million ounces (Moz) Au at a cutoff grade of 1.5 g/t. The ca. 70 Ma deposit is hosted in the Late Cretaceous Kuskokwim flysch basin, which developed in the back part of the arc region of an active continental margin, on previously accreted oceanic terranes and continental fragments. A hypabyssal, mainly rhyolitic to rhyodacitic, and commonly porphyritic, 8- X 3-km dike complex, part of a regional ca. 77 to 58 Ma magmatic arc, formed a structurally competent host for the mineralization. This deposit is subdivided into about one dozen distinct prospects, most of which consist of dense quartz+ or -carbonate veinlet networks that fill north-northeast-striking extensional fractures in the northeast-trending igneous rocks. The sulfide mineral assemblage is dominated by arsenopyrite, pyrite, and, typically younger, stibnite; gold is refractory within the arsenopyrite. Sericitization, carbonatization, and sulfidation were the main alteration processes. Fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent CO (sub 2) + or -CH (sub 4) and a few tenths to a few mole percent NaCl+KCl. The gold-bearing fluids were mainly homogeneously trapped at approximately 275 degrees to 300 degrees C and at depths of 1 to 2 km. Some of the younger stibnite may have been deposited by late-stage aqueous fluids at lower temperature. Measured delta (super 18) O values for the gold-bearing quartz range between 11 and 25 per mil; the estimated delta (super 18) O fluid values range from 7 to 12 per mil, suggesting a mainly crustally derived fluid. A broad range of measured delta D values for hydrothermal micas, between -150 and -80 per mil, is suggestive of a contribution from devolatilization of organic matter and/or minor amounts of mixing with meteoric fluids. Gold-associated hydrothermal sulfide minerals are characterized by delta (super 34) S values mainly between -16 and -10 per mil, with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin. A smaller group of delta (super 34) S measurements, which shows values as depleted as -27 per mil, suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite. Initial epsilon (sub Nd) of -8.7 to -3.1 and (super 87) Sr/ (super 86) Sr measurements of 0.706 to 0.709 for the ore-hosting dikes also indicate a crustal reservoir for some of the Late Cretaceous magmatism. Overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both. Copper-and gold-bearing stockwork veinlets in hornfels occur at Dome, a prospect located at the northern end of the Donlin Creek deposit. These stockworks are cut by the younger auriferous gold veins that define the main Donlin Creek gold mineralization. Highly saline, gas-rich, heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100 degrees C hotter than those of the main gold-forming event at Donlin Creek. The genetic relationship of the Dome prospect to the main Donlin Creek gold resource is equivocal. The epizonal Donlin Creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related; it shows important differences from typical epithermal and Carlin-like deposits. The ore-forming fluids were derived by either broad-scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component.
Details
- Title: Subtitle
- The Lake Cretaceous Donlin Creek gold deposit, Southwestern Alaska; controls on epizonal ore formation
- Creators
- Richard J Goldfarb - United States Geological SurveyRobert A Ayuso - United States Geological SurveyMarti L Miller - United States Geological SurveyShane Ebert - University of British ColumbiaErin E Marsh - United States Geological SurveyScott A Petsel - NovaGold Resources Inc., 4420 Taku Blvd., Juneau, Alaska 99801Lance D Miller - Juneau Economic Development Council, 612 W. Willoughby, Juneau, Alaska 99801Dwight C Bradley - United States Geological SurveyCraig A JohnsonWilliam C McClelland - University of Idaho
- Resource Type
- Journal article
- Publication Details
- Economic geology and the bulletin of the Society of Economic Geologists, Vol.99(4), pp.643-671
- Publisher
- Economic Geology Publishing Company
- DOI
- 10.2113/gsecongeo.99.4.643
- ISSN
- 0361-0128
- eISSN
- 1554-0774
- Language
- English
- Date published
- 07/2004
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984240894902771
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