DESCRIPTIONS of ORE DEPOSITS
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Image: Red Dog Mine Complex, Alsaka.
This tour, which was developed, organised and managed by Mike Porter of Porter GeoConsultancy Pty Ltd (PGC), on behalf of the Australian Mineral Foundation Inc. (AMF), included:
Module 2 - North American Sediment-Hosted Deposits, Mon 27 July to Wed 5 Aug
For information on the remainder of the tour, see the Deposit Descriptions for Module 1 and Module 3
- Sullivan, British Columbia, Canada.
- Coeur d'Alene, Idaho, USA.
- Red Dog, Alaska, USA.
- Faro / Anvil, Yukon, Canada.
- Howard's Pass, Yukon & NWT, Canada.
MODULE 2 - NORTH AMERICAN SEDIMENT HOSTED DEPOSITS
This module commenced with a one day workshop in Vancouver, British Columbia. Its aim was to provide a context to the deposits visited, and an overview of the tectonics, geology and metallogeny of the North American cordillera where all of the deposits to be visited are located. The deposits visited included:
is a stratabound, massive iron-lead-zinc sulphide orebody that lies conformably near the top of the Middle Proterozoic Lower Aldridge Formation. This unit is dominantly composed of fine grained siliciclastic rocks, mainly a rhythmic succession of thin to medium bedded, typically graded, very fine wackes and quartzitic wackes with associated tourmalinites. The northern two-thirds of the orebody is underlain by up to 125 m of intraformational conglomerate. Cross-cutting zones of chaotic breccia disrupt well bedded sedimentary rock and conglomerates under the western half of the orebody. Ore occurs as a convex, gently east dipping tabular lens with lateral dimensions of approximately 2000 x 1600 m. The western part, which is up to 100 m thick, can be divided into three approximately equal intervals. The lower is composed of predominantly pyrrhotite massive sulphides, while the middle layer is also massive sulphides, but mainly galena and sphalerite. The upper part is delicately banded galena, pyrrhotite and sphalerite, with intercalated siliciclastic beds near the top. The eastern part of the orebody consists of five distinct bedded sulphide intervals totalling up to 27 m in thickness, that are intercalated with four siliciclastic bands which are together up to 10 m thick. A funnel shaped tourmaline zone some 1400 x 950 m in plan area at the orebody footwall extends downwards for more than 450 m below the western part of the sulphide deposit. Production + reserves totalled 170 Mt @ 6.1% Zn, 4.9% Pb, 50 g/t Ag. Sullivan was owned and operated by Cominco Ltd.
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Coeur-Galena Complex - Coeur d'Alene
The Coeur d'Alene district has produced several tens of millions of tonnes of ore from a large number of mines spread along the length of a WNW-ESE trending corridor. The hosts are Middle Proterozoic clastics of the Belt Super Group, the US continuation of the Purcell Super Group, that includes the Aldridge Formation, host to Sullivan. The individual deposits are rich, narrow, transgressive veins, or vein swarms with extensive lateral dimensions of hundreds to thousands of metres, but thicknesses of <1 to a few metres. Grades are of the order of 400 to 1000 g/t Ag with associated <1 to 10% Pb, <0.1 to 0.6% Cu, <0.1 to 5% Zn and traces of Au, eg. the Bunker Hill mine which had reserves of 10 Mt @ 4.6% Zn, 2.3% Pb, 41 g/t Ag in 1991. In 1996 the proven reserves at Coeur-Galena totalled 1.63 Mt @ 450 g/t Ag. Total production from the field to 1984 was 31 100 t Ag, 7.60 Mt Pb, 2.9 Mt Zn, 0.15 Mt Cu, 15.9 t Au. The Coeur-Galena Complex was operated by Silver Valley Resources, a joint venture between Coeur d'Alene Mines Corp and ASARCO Inc.
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The Red Dog orebody is hosted by a Carboniferous sequence of black siliceous shale and chert. The ore is located in the second lowest of a stack of eight imbricated and folded allochthonous thrust slices. The main deposit lies within two major and one minor mineralised plates within this slice. The orebody overlies a tectonic mélange zone which separates it from the underlying Cretaceous rocks of the lower allochthon. The orebody is a stratabound accumulation of silica rock, barite and sulphides. The silica rock consists dominantly of growth zoned mosaic aggregates of quartz with accessory sulphides, within and peripheral to the main mass of sulphides. The barite facies comprises coarse and massive barite, contains accessory sulphides and silica, and is found towards the top and periphery of the deposit. The sulphide textures vary from massive, chaotic or fragmental to poorly banded. The major sulphides in decreasing order of abundance are sphalerite, pyrite, marcasite and galena. Rare disseminated chalcopyrite and pyrrhotite are found in sphalerite, while boulangerite occurs in galena. The dominant gangue constituents are quartz, barite and minor shale. The original reserve was 77 Mt @ 17.1% Zn, 5% Pb, 82 g/t Ag, with an additional 76 Mt @ 13.7% Zn, 3.6% Pb in the new Aqqaluk deposit. The mine was a Cominco US operation.
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Faro / Anvil
The Anvil deposits occur within a thick prism of complexly deformed and metamorphosed, late Proterozoic to Silurian metasediments and lesser mafic metavolcanics, intruded by a mid Cretaceous granitic plutonic suite. The district is on the western edge of the Selwyn Basin. The host sequence comprises the lower monotonous, 5000 m thick Upper Proterozoic to Cambrian Mt Mye Formation, and the overlying, 1000 m thick Cambrian to Ordovician Vangorda Formation. The Mt Mye Formation is composed of non-calcareous amphibolite and greenschist facies schists, with lesser, but important carbonaceous phyllite, limestone, marble, thinly banded calc-silicates and amphibolites. The Vangorda Formation comprises a sequence of recessive, generally calcareous, phyllitic, probable meta-turbidites, intercalated with mafic meta-igneous rocks, which are represented by calc-silicates around the orebodies. The Anvil Range district contains a string of 7 deposits distributed over a strike interval of 35 km, roughly parallel to, and 3 to 6 km to the north-east of the major Vangorda fault zone. The massive sulphide deposits are stratabound, distributed over a 150 m stratigraphic interval straddling the Mt Mye and Vangorda Formation boundary, and are associated with a regionally distributed, but laterally discontinuous graphitic phyllite unit. The galena and sphalerite bearing massive sulphide ore includes pyritic, barytic, carbonatic and pyrrhotitic variants, with common post depositional breccia textures. The massive sulphides are fringed laterally and below by quartzose and graphitic disseminated sulphide mineralisation, which may be banded and/or spectacularly brecciated. The ore lenses are typically elongated. The principal members of the Faro Group of deposits are:
Faro - 63 Mt @ 5.5% Zn, 5.5% Pb, 41 g/t Ag.
Vangorda - 6 Mt @ 4.6% Zn, 3.5% Pb, 50 g/t Ag.
Grum - 30 Mt @ 5% Zn, 3.5% Pb, 50 g/t Ag.
DY - 21 Mt @ 6.7% Zn, 5.5% Pb, 48 g/t Ag.
Swim - 4.5 Mt @ 5.5% Zn, 4% Pb, 51 g/t Ag, 0.27% Cu, 0.7 g/t Au;
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Howards Pass is an extensive, sub-economic, stratabound deposit on the eastern margin of the Selwyn Basin in north-western Canada, some 200 km east of Faro. The resource has been quoted at >270 Mt @ 6.4% Zn, 2.6% Pb. It occurs within the lower sections of the lower Palaeozoic Road River Formation. This unit has a basal 1000 m thick middle Cambrian succession of dark grey to black shale and dark grey platy limestone, followed by 800 m of dark grey platy limestone with abundant slump breccias. This passes upwards into lower Ordovician black graptolite bearing shale and chert, followed in turn by lower to middle Devonian sediments, which include brownish-grey basinal shales, argillaceous lime mudstones, dolomitic platy siltstone and sandstone, and dark grey and black shales with some chert, possibly together totalling more than 1000m in thickness. The uppermost unit is composed of calcareous dark grey shale and limestone and dark siliceous shale. The mineralisation occurs as rhythmically laminated, saucer shaped bodies distributed along a 130 km long linear belt coinciding with the interpreted basal basinal facies to the west of the adjacent carbonate platform which was deposited over the Canadian Shield. There are two larger deposits, XY and Anniv, and three smaller accumulations, OP, Hug and Pab. All are deposited in the Howard's Pass Member of the Road River Formation. Individual cycles in the unit that hosts the ore exhibit an ascending trend from limestone, through carbonaceous mudstone, to cherty mudstone, to chert. Sulphide mineralogy consists of fine grained sphalerite, lesser galena and minor pyrite. The mineralisation is only weakly zoned, has a high degree of sedimentary intercalation, and has negligible silver, and low barium (typically <2000 ppm) and copper contents. The property was, at the time, a joint venture between Placer Dome and US Steel.
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For more information contact:
T M (Mike) Porter, of Porter GeoConsultancy
This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.
Porter GeoConsultancy Pty Ltd|
6 Beatty Street
LINDEN PARK, 5065
Telephone: +61 8 8379 7397
Mobile: +61 422 791 776