British Columbia, Canada
Super Porphyry Cu and Au|
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The Mt Milligan alkalic porphyry copper-gold deposit is located approximately 70 km north of Fort St James in central British Columbia, Canada, in the central part of the Quesnel belt.
(#Location: 55° 07' 26" N 124° 01' 39" W).
Published resource and reserve figures include:
1153 Mt @ 0.254 g/t Au, 0.126% Cu, (Geological resource, MBX and Southern Star zones,
Placer Dome, 1991),
298 Mt @ 0.45 g/t Au, 0.22% Cu, (Prefeasibility est. mineable reserve, Placer Dome, 1991),
445 Mt @ 0.415 g/t Au, 0.215% Cu, (Measured + indicated resource, Placer Dome, 1997).
257 Mt @ 0.510 g/t Au, 0.240% Cu, (Reserve, included in resource, Placer Dome, 1997).
Remaining Measured + Indicated resource in 2009 (Terrane Minerals Corp., 2009):
706.7 Mt @ 0.182% Cu, 0.33 g/t Au.
The central Quesnel belt is occupied by rocks that are equivalents of the Middle Triassic to Lower Jurassic Takla, Nicola and Stuhini groups. The belt is bounded to the west by the Pinchi fault, which separates it from the deformed and uplifted Carboniferous to Jurassic rocks of the Cache Creek Complex. To the east, the Manson fault zone juxtaposes the belt with the high-grade metamorphics of the Proterozoic Wolverine Complex and the Carboniferous to Permian Slide Mountain Group. The southern tip of the NW trending, Late Triassic to Early Cretaceous Hogem Intrusive Complex (main phase of 176 to 212 Ma), which is considered to be an intrusive equivalent of the Takla Group, is located 25 Km WNW of Mt Milligan.
In the Mount Milligan district, the Takla Group comprises the:
i). Upper Triassic Rainbow Creek Formation - a basinal slate layer with lesser siltstone and epiclastic interbeds,
ii). Upper Triassic Inzana Lake Formation - an overlying sequence of epiclastic sediments,
iii). Upper Triassic Witch Lake Formation - augite and other porphyritic volcanics and pyroclastics,
iv). Lower Jurassic Chuchi Lake Formation - polymictic lahars and sub-aerial flows.
The Early Jurassic Mount Milligan Intrusive Complex, centred on Mt Milligan, 9 Km to the north of the deposit, comprises at least two separate intrusive phases, including: i). an early, equigranular, massive to foliated quartz-deficient, sphene-bearing monzonite, with gabbro and hornblende granite end members; and ii). a younger, porphyritic, medium-grained granite with peripheral pegmatite and aplite stringers.
Regionally metamorphosed amphibolites and granulites, and contact hornfels form the wall rocks and numerous pendants within the complex. The complex has been uplifted as a horst, accompanied by Late Cretaceous to Early Tertiary felsic intrusive activity.
In the vicinity of the Mt. Milligan deposits, the country rock is mainly composed of NNW striking Witch Lake Formation volcanics and lesser sedimentary rocks of the Rainbow Creek Formation, overlain by early Tertiary volcanic and sedimentary rocks. The Witch Lake rocks are intruded by a number of small coeval Takla and post-Takla Group stocks and dykes of porphyritic monzonite and lesser syenite related to the Mount Milligan Intrusive Complex. These are in general aligned with that complex forming a NW-trending belt, suggesting structurally control. The monzonitic MBX, Southern Star, Goldmark and North Slope stocks host mineralisation at the deposit. Rafts of volcanic rocks are common in the stocks, as are xenoliths of volcanic rock and/or lesser earlier monzonites. The MBX and Southern Star stocks contain up to 30% plagioclase phenocrysts which are 1 to 10 mm in length, set in a fine-grained greyish-pink groundmass composed mostly of K-feldspar with lesser plagioclase and minor quartz, hornblende, biotite and accessory magnetite. Three types of post-mineral dyke cut the deposits; trachytic, monzonitic and dioritic varieties. All are characterised by lack of sulphide mineralisation, with only the monzonitic and dioritic types showing alteration as a weak propylitic and carbonate style respectively.
Mineralisation at Mt. Milligan occurs in two deposits, as follows:
i). Main - comprises the MBX, WBX, 66 and DWBX zones and is hosted within the MBX stock and adjacent latitic and trachytic rocks of the Witch Lake Formation. The latitic country rocks, which occupy most of the area around the MBX stock, are potassic altered andesites. They may be distinguished from the original andesitic volcanics by their darker colour, general absence of visible hornblende, presence of biotite and >30%K-feldspar. The trachytic volcanic rocks are interbedded with latitic volcanic rocks in the eastern portion of the Main deposit. They are characterised by a high K-feldspar content, deficiency of mafic minerals and minor fine-grained plagioclase, and are porous and intensely potassic-altered. Pyrite and chlorite occur as curvilinear partings, along bedding planes and as bedding controlled disseminations. The MBX stock is a moderately west-dipping, approximately 400 m diameter, body of monzonite. In the south-eastern section of the Main deposit, the up to 50 m wide Rainbow, extends from the footwall of the MBX stock to form an elongate bowl-shaped body with gently dipping sides open to the south-east.
ii). Southern Star - occurs in the Southern Star stock, and adjacent andesitic rocks of the Witch Lake Formation. The Southern Star stock is predominantly intruded into andesitic rocks, largely composed of monolithic fragmental varieties characterised by actinolite-altered augite porphyritic lapilli tuff with minor augite crystal and lithic tuffs. Minor augite porphyritic flows and heterolithic debris flows are interbedded with the fragmental rocks. Plagioclase and/or hornblende phenocrysts are locally found within flows and within lapilli or crystal tuffs. The Southern Star stock is a tabular body of monzonite which strikes NNW, dips moderately to the west and forks at its northern end. It is exposed over an area of approximately 800x300 m in area.
Hydrothermal breccias are extensively developed throughout the Southern Star stock and to a lesser extent in adjacent volcanic rocks and along the margins of the MBX stock. These breccias characterised by K-feldspar veinlets and flooding that vary in amount and size.
A pattern of potassic and propylitic alteration is evident throughout both deposits, overprinted in part by post-mineral carbonate alteration. Gold and copper mineralisation is largely confined to the potassic assemblage, which comprises secondary K-feldspar and, where more intense, fine-grained secondary biotite (up to 40%), accompanied by chalcopyrite, lesser magnetite and minor bornite. Pyroxene phenocrysts were replaced by actinolite.
The propylitic alteration is widespread and generally pervasive, and is best developed beyond the zone of potassic alteration, persisting outward from the stocks for as much as 2500 m. It is characterised by an assemblage of epidote, varying calcite, chlorite, albite and pyrite, and locally overprints the potassic assemblage. Less commonly potassic alteration overprints the propylitic assemblage.
Potassic alteration is best developed around the outer contacts of the Southern Star and MBX stocks and the Rainbow dyke, decreasing in intensity towards their cores, and passing into the propylitic halo several hundred metres into the fractured country rocks
Four main episodes of post-mineral faulting are evident in the deposit area:
i). The oldest is represented by the north striking, shallow east-dipping Great Eastern and Rainbow faults. The Great Eastern strikes north to NW and is a regional structure that truncates the south-eastern portion of the Main deposit and separates it from sedimentary rocks of the Rainbow Creek Formation and early Tertiary volcanic and sedimentary rocks. The Rainbow fault follows the Rainbow dyke, and may be a splay off the Great Eastern fault.
ii). NW striking, steeply east-dipping faults cut the Southern Star deposit and the western portion of the Main deposit. The most important if these is the Divide fault which separates the Main deposit and Southern Star deposits.
iii). North-striking structures, manifested by the steeply east-dipping Harris fault, which separates the WBX and DWBX zones, and are possibly related to Tertiary block faulting.
iv). Prominent ENE trending cross faults are the youngest phase and include the Oliver, Southern Star and the Caira faults, although the latter may belong to an earlier episode that predates the Rainbow fault.
Mineralisation is distributed as follows:
i). The Main deposit - hypogene mineralisation occurs as a contiguous, blanket shaped body with dimensions of over 1300x950x240 m forming the MBX, 66, WBX and DWBX zones. The MBX zone contains Au and Cu and is the central portion of the deposit, developed along the footwall of the MBX stock, and surrounding the Rainbow dyke where it protrudes from the stock. It grades into the gold-rich copper-poor 66 zone to the south-east which surrounds the Rainbow dyke. The WBX zone and its down-faulted western extension, the DWBX zone both contain Au and Cu and occur along the hangingwall of the MBX stock to form the northwest portion of the deposit.
i). The Southern Star deposit - occurs in both the hangingwall and footwall of the Southern Star stock, and contains Au and Cu.
Mineralisation is present in a number of associations, including:
i). Predominantly chalcopyrite with lesser magnetite and minor bornite in areas of potassic alteration, best developed in monzonitic and volcanic rocks adjacent the footwall and, to a lesser extent, the hangingwall contact of the stocks. It is also present in and around trachytic volcanic rocks and the Rainbow dyke. The chalcopyrite mostly occurs as fine-grained disseminations and fracture fillings, and less commonly in veinlets and selvages of veinlets. Where found in veins, the chalcopyrite is accompanied by magnetite and pyrite in a gangue of K-feldspar, quartz and calcite. Magnetite occurs as disseminations (common in biotite-rich rock), patches and in veinlets, laminae and breccia matrix. Bornite is found as blebs and disseminations in lensoidal zones within volcanic rocks close to the footwall contacts of the MBX and Southern Star stocks where potassium feldspar veinlets are common. Bornite also occurs within the southern portion of the Southern Star stock.
ii). Pyrite, usually accompanied by propylitic alteration, decreasing in intensity away from the stock. It occurs as disseminations, veinlets, large clots, patches and as replacements in mafic minerals. Several generations of pyrite veinlets are indicated.
iii). Gold occurs as grains ranging from 5 to 100 µm across that adhere to imperfections on the outside of pyrite grains, fill micro-fractures and occur as inclusions in pyrite, chalcopyrite and magnetite grains. Visible Au is rare. Au-Ag bearing veins are present in propylitic altered volcanic rocks adjacent to the MBX and Southern Star stocks, apparently radiating outward from the MBX stock and occurring mainly within 500 m of the stocks. They comprise sulphide-rich and carbonate-quartz rich types. The best gold bearing sulphide veins contain mostly pyrite with lesser chalcopyrite, sphalerite, galena, molybdenite, arsenopyrite and tetrahedrite-tennantite, with minor quartz, K-feldspar and carbonate. The gold bearing carbonate-quartz rich veins, which contain sphalerite, galena and pyrite, occur in propylitic altered latitic volcanics northwest and northeast of the MBX stock.
iv). Sporadic supergene alteration has been recognised in the MBX, WBX and Southern Star zones and is most deeply and extensively developed in the MBX and WBX zones where it is about 20 m thick over most of the area, with localised zones of up to 60 m. Secondary copper minerals include: a). the sulphides covellite, chalcocite and djurleite rimming hypogene chalcopyrite; b). the oxides, cuprite and tenorite occurring as surface coatings on native copper, particularly cuprite; c). the carbonates, malachite and azurite; and d). native copper. Secondary copper minerals are commonly accompanied by goethite, magnetite, hematite and siderite. Limonite, which includes goethite, frequently replaces sulphides or occurs as coatings on fracture surfaces and hairline cracks.
In general, copper and gold mineralisation form a core zone around which peripheral gold-only mineralisation occurs in the upper portion of the hydrothermal system. Higher Au grades and Cu grades is broadly coincident, but with several important variations. Higher Cu values are found as a halo surrounding the MBX stock and along the footwall of the Southern Star stock. Where concentrations of chalcopyrite are greater near the margins of the MBX and Southern Star stocks, gold values are higher, while the chalcopyrite poor and pyrite rich 66 zone has higher gold grades, occurring in zones of pyrite clotting, carbonate and chlorite development. Generally then, gold grades are lower around the Southern Star stock which has a lower gold:copper ratio, but have a much broader distribution around the MBX stock, with higher gold:copper ratios away from that stock, particularly in the 66 zone where the highest grades occur.
The information in this summary is derived from the British Columbia Geological Survey online MINFILE record for this deposit.
The most recent source geological information used to prepare this summary 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.
Jago C P, Tosdal R M, Cooke D R and Harris A C, 2014 - Vertical and Lateral Variation of Mineralogy and Chemistry in the Early Jurassic Mt. Milligan Alkalic Porphyry Au-Cu Deposit, British Columbia, Canada: in Econ. Geol. v.109 pp. 1005-1033|
LeFort D, Hanley J and Guillong M, 2011 - Subepithermal Au-Pd Mineralization Associated with an Alkalic Porphyry Cu-Au Deposit, Mount Milligan, Quesnel Terrane, British Columbia, Canada : in Econ. Geol. v.106 pp. 781-808|
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.|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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