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Kemess South, Kemess North |
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British Columbia, Canada |
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Au Cu Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Kemess South and Kemess North porphyry copper-gold deposits are located in the Toodoggone mining district, 430 km NW of Prince George in the mountains of north-central British Columbia, Canada.
The Kemess deposits are on the eastern margin of the Intermontane Belt within a northwesterly trending belt of Palaeozoic to Tertiary sediments, volcanics and intrusions covering an area of 90x25 km. The basement comprises Neoproterozoic metasedimentary rocks that are unconformably overlain by volcanic and sedimentary units of the Upper Carboniferous to Permian Asitka Group, which are in turn are overlain by Upper Triassic basaltic to andesitic flows, volcaniclastics and minor limestone of the Takla Group. Volcaniclastics of the Lower Jurassic Hazelton Group and rhyolitic to dacitic flows, intrusions and volcaniclastics of the Lower Jurassic Toodoggone Formation (of the Hazelton Group) overlie the Takla Group. Further west, nonmarine Cretaceous Sustut Group sediments overlie the volcanic strata and define the western margin of the district.
The older strata in the central and eastern parts of the region have been intruded by quartz monzonitic to granodioritic suites of the Early Jurassic Black Lake Suite along the eastern margin of the Toodoggone district. Syenomonzonitic and quartz feldspar porphyritic dykes are thought to be feeders to the Toodoggone Formation.
Kemess South
The geology of the Kemess South deposit comprises Asitka Group rhyolites and cherts, Takla Group volcanics, mostly basaltic to andesitic flows with lesser crystal and lapilli tuffs, and intercalated sediments, which are predominantly chert, cherty tuff, argillite and graphitic argillite. The Takla Group has been intruded by Lower-Middle Jurassic quartz monzonite with lesser granodiorite, quartz diorite and some syenite. Dacific ash flow tuffs, lithic tuffs and epiclastic rocks of the Toodoggone Formation overlie all of the above. All of these lithologies have been subjected to brittle fracturing to produce and intense pattern of fractures, faults and breccia zones. A thin layer of overburden varying from 3 to 24 m thick, averaging 10 metres, covers the quartz monzonite.
Mineralisation is hosted by a relatively flat lying quartz monzonite body and extends for a short distance into Takla volcanics and sediments in the footwall. Pyrite and chalcopyrite are disseminated in both quartz veins and the altered host quartz monzonite, with minor molybdenite and magnetite. Mineralisation is hosted in part in the rhyolite of the Asitka Group where it comprises pyrite, magnetite, chalcopyrite and bornite with minor molybdenite and traces of gold. Sulphides occur on fractures and in veins and interstitial to feldspars. The supergene assemblage includes chalcocite, native copper, cuprite and malachite.
The mineralised zone is a gently dipping, blanket shaped mass of approximately 1700x650 m metres, elongated in an east-west direction and ranging from 100 to 290 m in vertical thickness. It occurs very close to the surface in the eastern half of the deposit, dipping to the west to a depth of up to 182 m below the surface.
The most important alteration is pervasive sericitisation which may occupy up to 25% of the rock, while restricted pink potash feldspar selvages are developed on quartz and/or sulphide veins and stockworks. Below these types of alteration, near the lower edge of the pluton, hematite, clay, carbonate and silica are more prevalent. The superposition of alteration styles produced a complex paragenesis of overprinting alteration assemblages that has affected all lithologies, with the possible exception of the cherts and argillites, as follows:
i). Sericitization of plagioclase - the most significant alteration style which has replaced almost all of the plagioclase in the quartz monzonite to form an aggregate of fine-grained sericite and clays;
ii). Quartz - filling fractures to form a stockwork of veinlets and veins from a few mm to a few cm thick that make up from 5 to 30% of the rock over widths of tens of metres and as much as 70% over intervals of a few metres;
iii). Chlorite - mainly in the form of the fault gouge;
iv). Calcite veining - which is frequently accompanied by chlorite and hematite, is widespread throughout, cutting many of the other alteration assemblages, and commonly occurs as quartz-calcite veins where the calcite has re-opened and invaded pre-existing quartz veining;
v). Carbonate veining - which refers to veins containing carbonate minerals other than calcite, most commonly iron carbonate, and like calcite veins, are usually late;
vi). Gypsum - which is invariably a late, but not a volumetrically important phase, although present throughout the lithologic assemblage;
vii). Zeolites - late veinlets contain probable zeolite minerals, which like gypsum, are widespread but not volumetrically important.
The supergene zone, occurs above and overprints the hypogene alteration and is characterised by the increasing development as small flakes of native copper that become leaf like and eventually coalesce to form sheets of native copper at the top of the enriched zone in a red oxidised clay rich rock. The supergene enriched blanket is overlain by a "leached cap" which is enriched in gold but contains very little copper.
Kemess North
Intercalated andesitic flows and pyroclastics of the Takla Group are found in the southern part of the deposit area, with augite andesite to the west and feldspar porphyritic andesite flows and breccias in the east. Local interbedded limestones within the massive volcanics indicate that the Takla Group strikes SE and dips 60° SW, and are cut by NNW to N striking basaltic dykes. An infaulted, gently south dipping sequence of intermediate pyroclastic lithic and crystal tuffs, and tuffaceous breccia in the centre of the deposit area belong to the Toodoggone Formation. The far north-western and southern sections of the deposit area are underlain by granodioritic and quartz monzonitic stocks of the Black Lake Suite which intrude both Takla Group and Toodoggone Formation rocks. In these areas the Takla Group is composed of augite porphyry, basaltic flows and tuffs, and bladed feldspar porphyry, bladed feldspar porphyry tuff breccia and basaltic dykes. These volcanics are also cut by porphyritic stocks and dykes, predominantly syenitic in composition that are comagmatic with the underlying granitic pluton.
The Black Lake Suite is composed of quartz monzonite, granodiorite and by feldspar-quartz-biotite-, feldspar-hornblende-, crowded feldspar-hornblende-, quartz-plagioclase- and leucocratic feldspar-hornblende-porphyries.
The intruded volcanics have been intensely deformed, mainly by multi-generation brittle fracture. The early fractures are healed by quartz, carbonates, zeolites or gypsum while some of the younger structures remain open. Faulting has had a greater disrupted influence on the sequence more than the intrusions, with major normal and transcurrent faults trending at 70°.
Four alteration styles have been recognised, as follows:
i). Phyllic - quartz-sericite-pyrite alteration dominates the centre of the deposit area and envelopes surrounding veinlets of pyrite and micro-fractures. Plagioclase is altered to quartz and sericite and may form up to 15% of the rock, while chlorite and kaolinite constitute up to a further approximately 30%. Only phyllic alteration is known to be directly associated with the mineralisation.
ii). Propylitic - alteration is characterised by local albitisation with variable epidote, chlorite and calcite, and is present in an elongate east-west zone of green coloured rocks parallel to, and south of, the central phyllic zone.
iii). Zeolitic - alteration is most common to the north of the phyllic alteration zone, although it is found locally throughout the deposit area and is common in the Takla Group. Laumontite often occurs as fracture fillings up to 3 mm thick in local shear zones.
iv). Hornfels - alteration occupies an irregular zone of variable intensity parallel to the quartz monzonite and granodiorite intrusive contacts, and principally within crystal tuffs. Strongly hornfelsed rocks are massive, fine grained and pale grey to brown in colour, composed of quartz, andalusite, epidote, sericite and chlorite. Pyrite occurs locally as micro-veinlets and fine-grained disseminations within this alteration facies.
Sulphides within the ore zone include, in decreasing order of abundance are pyrite, chalcopyrite, magnetite, hematite, molybdenite and digenite. Micro-veinlets and disseminations of pyrite constitute from 0.5 to 10% of the rock and is directly proportional to the intensity of the fracturing and alteration. A well developed, east-west trending, elongate, 3300x800 m gossan caps the zone of stronger disseminated pyrite mineralisation. Chalcopyrite occurs in northerly trending veinlets, micro-veinlets and more frequently, as disseminations, and is accompanied by pyrite, magnetite-hematite and gangue minerals of quartz and orthoclase. Molybdenite is spatially associated with the quartz-sericite-pyrite alteration zone as fracture fillings.
The hypogene mineralisation is overlain by a 10 to 20 m thick leached cap and a further up to 30 m thick supergene enrichment blanket. Within the supergene blanket digenite rims hypogene chalcopyrite grains.
Reserves and Resources
In 1995, the Mineable "reserves" at Kemess South were (Royal Oak Mines):
Supergene ore - 45.5 Mt @ 0.20% Cu, 0.75 g/t Au
Hypogene ore - 155 Mt @ 0.23% Cu, 0.59 g/t Au
TOTAL - 200.4 Mt @ 0.22% Cu, 0.63 g/t Au, 0.008% Mo.
Reserves in Dec 31 2005 were (Northgate Minerals website, 2006):
Kemess South
Proven - 68.03 Mt @ 0.65 g/t Au, 0.21% Cu
Probable - 0.20 Mt @ 0.09 g/t Au, 0.90% Cu
TOTAL - 68.23 Mt
Kemess North
Proven - 299.3 Mt @ 0.30 g/t Au, 0.16% Cu
Probable - 124.6 Mt @ 0.29 g/t Au, 0.15% Cu
TOTAL - 423.9 Mt
TOTAL Proven & Probable Reserves for Kemess South and North - 492.13 Mt
Resources (in addition to reserves) in Dec 31 2005 were (Northgate Minerals website, 2006):
Kemess South
Indicated - 11.0 Mt @ 0.40 g/t Au, 0.18% Cu
Kemess North
Measured - 148.5 Mt @ 0.31 g/t Au, 0.16% Cu
Indicated - 137.3 Mt @ 0.28 g/t Au, 0.12% Cu
Nugget Zone (West Cirque)
Measured - 3.3 Mt @ 0.38 g/t Au, 0.07% Cu
Indicated - 6.1 Mt @ 0.36 g/t Au, 0.07% Cu
TOTAL Measured & Indicated Resources South, North and Nugget - 306.29 Mt
TOTAL Reserves + Resources for Kemess Deposits (2005) - 798.42 Mt
The most recent source geological information used to prepare this decription was dated: 2006.
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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Duuring P, Rowins S M, McKinley B S M, Dickinson J M, Diakow L J, Kim Y-S and Creaser R A, 2009 - Magmatic and structural controls on porphyry-style Cu–Au–Mo mineralization at Kemess South, Toodoggone District of British Columbia, Canada: in Mineralium Deposita v.44 435-462
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Duuring P, Rowins S M, McKinley B S M, Dickinson J M, Diakow L J, Kim Y-S and Creaser R A, 2009 - Examining potential genetic links between Jurassic porphyry Cu–Au ± Mo and epithermal Au ± Ag mineralization in the Toodoggone district of North-Central British Columbia, Canada: in Mineralium Deposita v.44 pp. 463-496
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Logan, J.M. and Mihalynuk, M.G., 2014 - Tectonic Controls on Early Mesozoic Paired Alkaline Porphyry Deposit Belts (Cu-Au ± Ag-Pt-Pd-Mo) Within the Canadian Cordillera : in Econ. Geol. v.109, pp. 827-858.
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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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