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Juneau Gold Belt - Berners Bay, Eagle River, Snettisham, Sumdum, Juneau |
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Alaska, USA |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The mesothermal Juneau gold belt quartz lodes stretch for 200 km along the Alaskan panhandle coast from south of Skagway in the north, through Juneau and 100 km further south. They follow the Coast Range Megalineament and are approximately 125 km inboard from the Gulf of Alaska and 20 km west of the Coast Batholith.
Following the discovery of placer gold in the area in 1869, some 210 tonnes of lode gold were recovered between 1880 and 1944. The more significant deposits and districts include Berners Bay district (including the Ophir, Bear, Kensington, Horrible, Ivanhoe, Northern Belle, Comet, Johnson and Jualin mines), Eagle River, Juneau, Snettisham and Sumdum.
The gold bearing quartz lodes of this belt were developed in an accretionary terrane of Permian to Cretaceous meta-sediments ± volcanic rocks which have undergone greenschist grade metamorphism. These include phyllite and greywacke, with lesser quartzite, schist and marble, and interbedded thick meta-mafic volcanics (now greenstones). Many of the intrusives in the belt are Mesozoic in age, although younger belts of 67 to 64 and 50 Ma granitoids are found a few kilometres to the east of the gold belt.
Most of the lodes occur as stockworks and massive veins within plutonic rocks and as stringers along cleavage in argillaceous rocks. Quartz dominates, with lesser carbonates (mainly ankerite). Sulphides (pyrite or pyrrhotite, with lesser sphalerite and arsenopyrite) commonly comprise <5%, and there is usually a direct relationship between gold occurrence and sulphide abundance.
Wall rock alteration is extensive and intense in the larger deposits, with secondary carbonate, sulphide and white mica (±biotite). At some deposits such as at Juneau, this zone may be up to 3 km long by 300 m in widh. Lodes have been dated at 55 to 60 Ma.
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1990.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Goldfarb R J, Newberry R J, Pickthorn W J, Gent C A 1991 - Oxygen, Hydrogen, and Sulfur isotope studies in the Juneau Gold Belt, southeastern Alaska: constraints on the origin of hydrothermal fluids: in Econ. Geol. v86 pp 66-80
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Groves, D.I., Santosh, M., Goldfarb, R.J. and Zhang, L., 2018 - Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits: in Geoscience Frontiers v.9, pp. 1163-1177.
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Miller L D, Goldfarb R J, Snee L W, Gent C A, Kirkham R A 1995 - Structural geology, age and mechanisms of Gold vein formation at the Kensington and Jualin deposits, Berners Bay district, southeast Alaska: in Econ. Geol. v90 pp 343-368
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Sibson R H and Scott J 1998 - Stress/fault controls on the containment and release of overpressured fluids: Examples from gold-quartz vein systems in Juneau, Alaska; Victoria, Australia and Otago, New Zealand: in Ore Geology Reviews v13 pp 293-306
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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