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Crandon |
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Wisconson, USA |
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Zn Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Crandon volcanic-hosted massive sulphide (VHMS) deposit is located 10 km south of the town of Crandon, 120 km NNW of Green Bay in Wisconsin, USA (#Location: 45° 28' 55"N, 88° 55' 29"W).
It lies within the eastern-most of three districts of Palaeoproterozoic base metal and gold bearing massive sulphide deposits in northern Wisconsin. These districts are located within the 1860 to 1820 Ma Penokean Mobile Belt which defines the southern margin of the Archaean Superior Province. All are located, within felsic centres on the flanks of the core of a major 85 km wide and 280 km long, east-west trending Palaeoproterozoic volcanic complex/greenstone belt. This belt is composed of felsic to mafic volcanic and sedimentary rocks and younger granitic intrusions.
Crandon is contained within a sequence of andesitic to dacitic pyroclastics, flows and chemical sediments of the Hemlock Creek Assemblage that dip sub-vertically. The lowermost Nashville unit in this assemblage comprises porphyritic basaltic flows with local thin tuff beds. These are overlain by the Sand Lake Unit, composed of an undertermined thickness of fine-grained andesitic, sericitised, silicified and weakly pyritised tuff with minor debris flows, cherty tuff and flows, which grades with depth into coarser reworked tuff and fine lapilli tuff, all of which are succeeded by up to 100 m of volcanic breccia of the Township subunit. This subunit is composed of three volcanic breccias, namely: (i) monolithic, subangular, chloritic, generally weakly silicified fragments in a siliceous matrix, and grades into fractured and then to unbrecciated Sand Lake unit tuffs; the breccia has been interpreted to be related to explosive fumarolic activity caused by decreasing lithostatic pressure; (ii) heterolithic, 70% of which contains fragments of local lithologies in the Crandon pile, ranging from a few mm to 20 cm in a matrix of quartz, sericite, chlorite and pyrite, and 30% containing lithic clasts and matrix which has been strongly silicified, sericitised and pyritised; this breccia overlies the first type and occurs in the central to western part of the deposit area and has undergone a multistage development and quartz veining before its final assembly; it is interpreted ot have been formed by phreatic and phreomagmatic activity; (iii) a thin (up to 10 m) discontiuous breccia composed of angular, 1 to 100 mm fragments of tuff, argillite, massive sulphides, chert and occassional exotic volcanic fragments set in a sand- to silt-sized matrix, occurring in the central part of the deposit, between the second breccia type and the immediate host to the ore deposit. These breccias are interpreted to have served as permeable conduits that directed metalliferous fluids to be vented into a depositional graben (Lambe and Rowe, 1987).
The ore deposit is hosted by the Crandon unit, which overlies the Sand Lake unit, and consists of discontinuous, thin-bedded sulphide, argillite and tuff with minor chert and dolomite. It has a maximum thickness of 100 m, strike length of ~1700 m trending at 275° and a down-dip extent of 950 m, with dips that range from 70°N in the east, to overturned 85°S in the west. Alteratin varies through the unit laterally and vertically from chloritic to siliceous. Tuff beds increase to the west, although there are no coarse pyroclastics. Massive (i.e., >50%) sulphides occur with chloritic and siliceous, an uncommonly sericitic alteration, with 60 to 90% sulphides. The chloritic facies is associated with, and comonly grades into pyritic argillite (10 to 50% pyrite) and argillite, while siliceous massive sulphide is associated with chert, cherty tuff and cherty argillite with a quartz-sericite matrix. The argillite is typically thin, finely bedded and black in sets that are up to 20 cm thick, containing slumped and contorted laminae of pyrite, chert and dolomite.
The massive sulphide is composed of pyrite (average 54%) and sphalerite (average 15%) with lesser chalcopyrite (<2%) and galena, and a gangue of quartz, chlorite, and minor sericite and dolomite.
The hanging wall to mineralisation is represented by the Mole Lake unit, a dominantly fine-grained chloritic tuff that contains a lower, 10 to 20 m thick, locally up to 50 m, siliceous lapilli tuff (Prospect lapilli tuff) composed of siliceous debris and relatively undeformed and unaltered pumiceous debris in a chloritic matrix, containing andesitic to basaltic flow fragments. This sub-unit is overlain by the 0 to 50 m thick Eagle volcanic sandstone, a fine-grained sandy tuff of quartz grains in a chloritic and sericitic matrix.
The remainder of the Hemlock Creek Assemblage comprises the up to 280 m thick Oak Lake unit basinal pyritic, cherty and sericitic tuffs; the Rice Lake unit of coarse, chloritic to siliceous tuff to tuff breccia debris flows which are up to several hundred metres in thickness; and the 210 to 240 m thick Skunk Lake unit of cherty tuff with lesser interbedded chert, argillite and tuff, represnting a sequence of chemical sediments and ash filling a basin that thickened to the east.
The Hemlock Creek Assemblage is overlain by the Swamp Creek Assemblage that commences with porphyritic rhyolite flows with interbeded tuff, chert and argillite, overlain by basinal cherty tuff, chert and silicified tuff, and a capping sequence of epiclastic argillaceous conglomerate and sandstone with volcanic andgranitic clasts.
Reserves are 61 Mt @ 5.6% Zn, 1.1% Cu, 0.5% Pb, 37 g/t Ag, 1 g/t Au.
The property, which was discovered in 1975, has had serious environmental objections to its development.
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Crandon
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DeMatties T A 1996 - A geologic framework for early Proterozoic volcanogenic massive sulfide deposits in Wisconsin: an exploration model: in LaBerge G L (Ed.) 1996 Volcanogenic massive sulfide deposits of northern Wisconsin: a commemorative volume Institute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI v42, part 2 pp 31-65
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DeMatties T A 1994 - Early Proterozoic volcanogenic massive Sulfide deposits in Wisconsin: an overview: in Econ. Geol. v 89 pp 1122-1151
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Erikson A J, Cote R 1996 - Geological summary - Crandon deposit: in LaBerge G L (Ed.) 1996 Volcanogenic massive sulfide deposits of northern Wisconsin: a commemorative volume Institute on Lake Superior Geology Proceedings, 42nd Annual Meeting, Cable, WI v42, part 2 pp 129-141
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Lambe R N, Rowe R G 1987 - Volcanic history, mineralisation and alteration of the Crandon massive sulfide deposit, Wisconsin: in Econ. Geol. v82 pp 1204-1238
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May E R, Schmidt P G 1991 - The discovery, geology and mineralogy of the Crandon Precambrian massive sulfide deposit, Wisconsin (Abstract and References): in Hollister V F (Ed.), Case Histories of Mineral Discoveries, Porphyry Copper, Molybdenum and Gold Deposits, Volcanogenic Deposits (Massive Sulfides) and Deposits in Layered Rock Society for Mining, Metallurgy and Exploration Inc., Littleton, Colorado v3 p 102
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McCarthy J H, Lambe R N, Dietrich J A 1986 - A case study of soil gases as an exploration guide in glaciated terrain - Crandon massive Sulfide deposit, Wisconsin: in Econ. Geol. v81 pp 408-420
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Schmidt P G 1991 - Discovery case history of the Crandon massive sulfide deposit, Forest County, Wisconsin: in Hollister V F (Ed.), Case Histories of Mineral Discoveries, Porphyry Copper, Molybdenum and Gold Deposits, Volcanogenic Deposits (Massive Sulfides) and Deposits in Layered Rock Society for Mining, Metallurgy and Exploration Inc., Littleton, Colorado v3 pp 99-102
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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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