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 August, 1998
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
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 is 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 is operated by Silver Valley Resources,
a joint venture between Coeur d'Alene Mines Corp and ASARCO Inc.
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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 is a Cominco US operation.
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Faro / 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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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 is currently a joint venture between Placer Dome
and US Steel.
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (email@example.com)
This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.
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Telephone: +61 8 8379 7397
Facsimile: +61 8 8379 7397.