The Mt. Milligan property is situated in the central part of the Quesnel belt which comprises equivalent rocks of the Middle Triassic to Lower Jurassic Takla, Nicola and Stuhini groups. Bounded to the west by the Pinchi fault, the belt is separated from deformed uplifted Carboniferous to Jurassic Cache Creek Complex (Group) rocks. To the east, the Manson fault zone separates the belt from the high-grade metamorphic Proterozoic Wolverine Complex and the Mississippian to Permian Slide Mountain Group. About 25 kilometres west-northwest of the Mt. Milligan deposit is the southern tip of the northwest-elongate Late Triassic to Early Cretaceous Hogem Intrusive Complex. The main phase of the complex is dated at between 176 to 212 Ma, and is considered to be an intrusive equivalent of the Takla Group.
In the Mount Milligan area, the Takla Group has recently been divided into four informal units - the Upper Triassic Rainbow Creek, Inzana Lake and Witch Lake formations and the Lower Jurassic Chuchi Lake Formation (Fieldwork 1990, page 93). The Rainbow Creek Formation is a basinal layer of slate, and lesser siltstone and epiclastic interbeds. Overlying epiclastic sediments of the Inzana Lake Formation are in turn overlain by augite and other porphyritic volcanics and pyroclastics of the Witch Lake Formation. These pass upward into polymictic lahars and subaerial flows of the Chuchi Lake Formation.
The Mount Milligan Intrusive Complex, located about Mount Milligan peak, approximately 9 kilometres northwest of the Mt. Milligan deposit, consists of at least two separate Early Jurassic intrusive phases. It includes sphene-bearing monzonite, with gabbro and hornblende granite end members; and porphyritic granite. Its wallrocks and numerous pendants include regionally metamorphosed amphibolites and granulites as well as contact hornfelses. The earlier of the two plutonic bodies on Mount Milligan is an equigranular, massive to foliated quartz-deficient monzonite. The later plutonic body is a porphyritic, medium-grained granite with peripheral pegmatite and aplite stringers. The complex was later uplifted as a horst, accompanied by Late Cretaceous to Early Tertiary felsic intrusive activity.
The area of the Mt. Milligan porphyry copper deposits are underlain mainly by north-northwest trending volcanic rocks of the Witch Lake Formation and minor sedimentary rocks of the Rainbow Creek Formation; early Tertiary volcanic and sedimentary rocks are also present. The Witch Lake rocks are intruded by coeval Takla Group and post-Takla Group intrusions related to the Mount Milligan Intrusive Complex. The monzonitic MBX, Southern Star, Goldmark and North Slope stocks host mineralization on the property.
The Mt. Milligan deposits are made up of the Main and Southern Star (093N 191) deposits. The Main deposit comprises the MBX zone, the WBX zone, the 66 zone and the DWBX zone. The Main deposit occurs within the MBX stock and adjacent latitic and trachytic rocks of the Witch Lake Formation. The Southern Star deposit occurs in the Southern Star stock, and adjacent andesitic rocks of the Witch Lake Formation. Refer to the Southern Star occurrence for details of that deposit.
Andesitic rocks underlie most of the area around the Southern Star stock and areas away from the MBX stock. Monolithic fragmental varieties, which form most of the unit, are characterized by actinolite-altered augite porphyritic lapilli tuff with minor augite crystal and lithic tuff. Minor augite porphyritic flows and heterolithic debris flows are interbedded with the fragmental rocks. Plagioclase and/or hornblende phenocrysts occur locally within flows and within lapilli or crystal tuffs.
Latitic volcanic rocks are texturally similar to andesitic volcanic rocks since they are potassically altered andesites. They underlie most of the area around the MBX stock and, less commonly, areas adjacent to the Southern Star stock. These rocks can be distinguished from andesitic volcanic rocks through their darker colour; general absence of visible hornblende; presence of biotite; and greater than one-third potassic feldspar (visible when stained).
Trachytic volcanic rocks are characterized by high potassium feldspar content and a lack of mafic minerals; minor fine-grained plagioclase is also present. Pyrite and chlorite occur in the rock as curvilinear partings in a massive rock type, and along bedding planes and disseminated throughout in a bedded type. The trachytic rocks are porous and intensely potassically-altered. They are interbedded with latitic volcanic rocks in the eastern portion of the Main deposit which is the only indication of stratigraphy in the area of the deposits.
The above sequence has been intruded by a number of small stocks and dikes of porphyritic monzonite and lesser syenite related to the Mount Milligan Intrusive Complex and aligned with that complex along a northwest-trending belt, indicating that their emplacement was structurally controlled. The MBX stock is a moderately west-dipping monzonite body approximately 400 metres in diameter. In the southeastern portion of the Main deposit, the Rainbow dike, up to 50 metres wide, extends from the footwall of the MBX stock forming an elongate bowl-like body with gently dipping sides open to the southeast. The Southern Star stock is a moderately west-dipping, north-northwest striking tabular body of monzonite which forks at its northern end. This stock is approximately 800 by 300 metres in area. These stocks contain up to 30 per cent plagioclase feldspar phenocrysts, 1 to 10 millimetres in length, occurring within a fine-grained grayish pink groundmass composed mostly of potassium feldspar with lesser plagioclase feldspar, and minor quartz, hornblende, biotite and accessory magnetite. Rafts of volcanic rocks are common in both MBX and Southern Star stocks, and xenoliths of volcanic rock and/or lesser earlier monzonite occur locally.
Hydrothermal breccia occurs extensively throughout the Southern Star stock and less commonly in adjacent volcanic rocks and along the margins of the MBX stock. It is characterized by potassium feldspar veinlets and flooding that vary in amount and size.
Three types of postmineral dikes cut the deposits; trachytic, monzonitic and dioritic varieties. These dikes are characterized by lack of sulphide mineralization, with only the monzonitic and dioritic types showing alteration of a weak propylitic and carbonate nature respectively.
Potassic and propylitic alteration assemblages are present throughout the deposits with gold and copper mineralization mainly associated with the potassic assemblage. Minor postmineral carbonate alteration is also present. The potassic alteration is best developed around the contacts of the MBX stock, Rainbow dike and Southern Star stock, decreasing in intensity towards the core of the stocks and away from the contacts for several hundred metres into fractured country rocks. The alteration is characterized by secondary potassium feldspar and, in areas of intense potassic alteration, fine-grained secondary biotite (up to 40 per cent), chalcopyrite, lesser magnetite and minor bornite. Pyroxene phenocrysts, where present, are replaced by actinolite. The propylitic alteration is widespread and generally pervasive. It is best developed outside the zone of potassic alteration, outward from the stocks for up to 2500 metres. The assemblage consists of epidote with varying amounts of calcite, chlorite, albite and pyrite. The propylitic alteration locally overprints the potassic assemblage and less commonly potassic alteration overprints the propylitic assemblage; this relationship is possible because of the contemporaneous nature of the two types of alteration.
At least four episodes of postmineral faulting affected the area containing the deposits. The earliest is manifested by the northerly striking, shallow east-dipping Great Eastern and Rainbow faults. The Great Eastern fault is a moderate east dipping, north to northwest striking regional structure that truncates the southeastern 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 dike, and may be a splay off the Great Eastern fault.
Northwest striking, steeply east-dipping faults occur in the Southern Star deposit and in the western portion of the Main deposit. The most important fault of this type is the Divide fault which separates the Main deposit from the Southern Star deposit.
North-striking faults, possibly related to Tertiary block faulting, are manifested by the steeply east-dipping Harris fault, which separates the WBX and DWBX zones.
Prominent east-northeast striking crossfaults are the latest episode of faulting in the area. These include the Oliver fault, Southern Star crossfaults and the Caira faults, although the latter may belong to an earlier episode that predates the Rainbow fault.
Hypogene mineralization within the Main deposit occurs in the MBX, 66, WBX and DWBX zones, apparently forming one contiguous, blanket-shaped mineralized body over 1300 metres long, up to 950 metres wide and up to 244 metres thick. The MBX zone is in the central portion of the deposit along the footwall of the MBX stock, and surrounds the Rainbow dike where it protrudes from the stock. This zone contains gold and copper, and grades into the 66 zone to the southeast. The gold-rich copper-poor 66 zone surrounds the Rainbow dike. The WBX zone and its downfaulted western extension, the DWBX zone, form the northwest portion of the deposit. Both occur along the hangingwall of the MBX stock, and contains gold and copper.
The Southern Star deposit occurs in the hangingwall and footwall of the Southern Star stock, and contains gold and copper.
Mineralization consists mostly of chalcopyrite, lesser magnetite and minor bornite in areas of potassic alteration, and pyrite in areas of propylitic alteration. In areas of potassic alteration, mineralization is developed best 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 dike. In areas of propylitic alteration, mineralization generally decreases away from the stock.
Chalcopyrite occurs mostly as fine-grained disseminations and fracture fillings, and less commonly as veinlets and selvages of veinlets. In veins, chalcopyrite occurs with magnetite and pyrite in a gangue of potassium feldspar, quartz and calcite. Magnetite occurs as disseminations (common in biotite-rich rock), patches and in veinlets, laminae and breccia matrix. A unique occurrence of magnetite breccia occurs as a small zone consisting of 50 per cent massive magnetite veins along the contact of the MBX stock. Bornite occurs 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. Pyrite occurs as disseminations, veinlets, large clots, patches and as replacements in mafic minerals. Several generations of pyrite veinlets are indicated. Gold occurs as grains ranging from 5 to 100 micrometres that fill microfractures, adhere to imperfections on the outside of pyrite grains, and also as inclusions in pyrite, chalcopyrite and magnetite grains. Visible gold is rare.
A diamond-drill hole intersection from the WBX zone across 4.99 metres assayed 12.58 grams per tonne gold and 0.26 per cent copper (George Cross News Letter No.111, 1989). From the gold-enriched 66 zone, a diamond-drill hole intersection across 6.0 metres assayed 4.45 grams per tonne gold and 0.26 per cent copper (George Cross Newsletter No.111, 1989).
Supergene alteration in the deposit is recognized in the MBX, WBX and Southern Star zones. The alteration is deepest and most extensive in the MBX and WBX zones where it is about 20 metres thick over most of the area, with localized areas up to 60 metres thick. However, supergene enrichment is only sporadic and does not form well-defined zones. Secondary copper minerals identified in these areas consist of the sulphides, covellite, chalcocite and djurleite; the oxides, cuprite and tenorite; the carbonates, malachite and azurite; and native copper. The sulphides occur as rims around chalcopyrite; and the oxides, in particular cuprite, occur as surface coatings on native copper. Secondary copper minerals commonly occur with goethite, magnetite, hematite and siderite. Limonite, which includes goethite, commonly replaces sulphide minerals or occurs as coatings on fracture surfaces and hairline cracks.
Gold-silver bearing veins are present in propylitically altered volcanic rocks adjacent to the MBX and Southern Star stocks, apparently radiating outward from the MBX stock and occurring mainly within 500 metres of the stock. They comprise sulphide-rich and carbonate-quartz rich types. The best developed sulphide-rich veins occur in the Creek and Esker zones where 3 to 5 subparallel veins strike northeast and dip steeply northwest through andesitic rocks. The Creek zone is about 400 metres south-southwest of the MBX stock; the Esker zone is about 600 metres southwest of the stock. The veins contain mostly pyrite with lesser chalcopyrite, sphalerite, galena, molybdenite, arsenopyrite and tetrahedrite-tennantite, and minor quartz, potassium feldspar and carbonate. At the Creek zone, a diamond-drill hole intersection across 1 metre assayed 4.4 grams per tonne gold and 82.0 grams per tonne silver (Assessment Report 16966). The carbonate-quartz rich veins, which contain sphalerite, galena and pyrite, occur in propylitically altered latitic volcanics northwest and northeast of the MBX stock.
In general, higher copper values occur along the footwall of the Southern Star stock and as a halo around the MBX stock. The distribution of higher gold grades is similar to copper, but with several important differences. Where concentrations of chalcopyrite are greater near the margins of the MBX stock and the Southern Star stocks, gold values are higher. However, within the 66 zone, where chalcopyrite is sparse and pyrite is abundant, higher gold grades are associated with zones where clots of pyrite, carbonate and chlorite are present. Generally, gold grades are lower around the Southern Star stock and have a much broader distribution around the MBX stock particularly in the 66 zone where the highest grades occur. Lower gold:copper ratios occur in the Southern Star deposit, whereas higher ratios, indicating higher gold grade, occur away from the MBX stock. In general, copper and gold mineralization form a core zone around which peripheral gold-only mineralization occurs in the upper portion of the hydrothermal system.
The area was prospected as early as 1937, but no serious work was done until 1972 when Pechiney Development Ltd. conducted induced polarization, geochemistry and a 5-hole drill program. The property was dormant until 1983 when Selco Inc. did extensive geochemical work which identified an Au-As anomaly east of Heidi Lake. Selco amalgamated with BP Resources Canada Limited in 1984. In that same year, R. Haslinger staked claims on adjacent ground, which BP then optioned. Extensive geological, geochemical, lithogeochemical, magnetic, induced polarization and trenching work was done by BP until 1986, when Lincoln Resources Inc. optioned the property. Lincoln’s twelfth drill hole was the first to intersect the MBX Zone in 1987. Lincoln later reorganized to become United Lincoln Resources Inc., and then amalgamated with Continental Gold Corp in 1989. Drill hole 199 was the first to intersect the Southern Star Zone later that year.
Placer Dome Inc. purchased the property in 1990, did extensive infill and step-out drilling, and completed a Pre-Feasibility Study in 1991.
A prefeasibility study in 1991 estimated the mineable reserve at 298,400,000 tonnes of ore grading 0.45 gram per tonne gold and 0.22 per cent copper (Information Circular 1995-1, page 14).
A geological resource (no economic considerations) for the MBX and Southern Star zones is reported to total 1,153,135,000 tonnes grading 0.254 gram per tonne gold and 0.126 per cent copper (Mt. Milligan Project, Stage 1 Report, Volume 1 - Development Plan (Mine Development Review Process), April 1991, Placer Dome Inc., Table 3.4, page 3-12).
The Mount Milligan deposit has a measured and indicated resource of 445 million tonnes of ore grading 0.415 gram per tonne gold and 0.215 per cent copper. More than 70 per cent of the measured and indicated resource is considered as a mineable reserve which is economically viable to extract. Mount Milligan's reserve is 257 million tonnes of ore grading 0.510 gram per tonne gold and 0.240 per cent copper (Property File - Mt. Milligan Project, Economic Growth for B.C. & Shareholder Value for Placer Dome, 12/15/97).
There has been little work done on the property since 1991. A 1997 geological work program by Placer consisted of examination of 25 intervals of varying lengths in 11 separate drill holes in the MBX Zone. Twenty core specimens were collected for polished thin section examination and 26 samples were analyzed. Placer Dome Inc. re-visited the property in 2004, drilling a dozen HQ bore holes totaling about 2200 metres, in order to provide core for metallurgical testing. Placer Dome plans to complete a full feasibility study on the project in 2005.
In 2006, after Placer Dome was acquired by Barrick Gold Corporation, Terrane Metals Corp. purchased the property. Between 2006 and 2007 Terrane completed 18,507 metres of drilling in 60 holes with the purpose of upgrading the mineral resources. In 2007 the company reported an estimated 213.8 million tonnes grading 0.228 per cent Cu and 0.472 grams per tonne Au Proven mineral reserves and 119.9 million tonnes grading 0.198 per cent Cu and 0.351 grams per tonne Au Probable mineral reserves. The company also reported a combined Measured and Indicated mineral resource of 590.8 million tonnes grading 0.193 per cent Cu and 0.352 grams per tonne Au (Feasibility Study - Technical Report Mt. Milligan Property - Northern BC - Prepared for Terrane Metals Corp. April 2008 www.terranemetals.com). In early 2008 the company conducted a feasibility study for the property which projects production could start in 2012. During 2008, exploration on the Mt. Milligan property was limited to geochemistry and a HeliGEOTEM magnetic-electromagnetic survey.
Terrane plans an updated feasibility study report for late 2009, to guide project implementation in the context of the operating and capital costs pertaining at that time. Meanwhile, the Environmental Assessment review process, permitting, and pre-construction activities will continue.
As of October 2009, the Mt. Milligan copper-gold porphyry deposits contain a Measured and Indicated Mineral Resource of 706.7 million tonnes at 0.18 per cent copper and 0.33 gram per tonne gold, containing 2.84 billion pounds copper and 7.50 million ounces gold (Press Release, Terrane Metals , October 13, 2009). The Mineral Resource was tabulated within a conceptual open pit at a US$4.10/tonne NSR cut-off value using US$2.00/pound copper and US$800/ounce gold.
Mineral Resource Estimate1 (Inclusive of Mineral Reserve)
(Source - Press Release, Terrane Metals , October 13, 2009)
Category Tonnes Cu Au Cu In-Situ Au (oz)(Million)
---------- (millions) (%) (g/t) (x million lb) (x millon oz)
----------------- --------- ----- ------- --------------- ----------------
Measured 334.6 0.197 0.398 1,453 4.28
Indicated 372.1 0.169 0.269 1,386 3.22
+ Indicated) 706.7 0.182 0.330 2,840 7.50
Inferred 20.5 0.154 0.205 70 0.14
Mineral Reserve Estimate
(Source - Press Release, Terrane Metals , October 13, 2009)
Category Tonnes Cu Au Cu In-Situ Au (oz)(Million)
---------- (millions) (%) (g/t) (x million lb) (x millon oz)
----------------- --------- ----- ------- --------------- --------------
Proven 274.6 0.210 0.438 1,273 3.87
Probable 207.8 0.187 0.322 851 2.16
Total 482.4 0.200 0.388 2,124 6.02
In 2012, Thompson Creek Metals Company Inc. released a sustainability and social responsibility report on Mount Milligan. This report states that Mount Milligan is set to begin production in the third quarter of 2013, and is projected to produce an average of 36,700,000 kilograms of copper and 5,500,000 grams of gold annually over the projected 22 year mine life.
On September 24, 2013 Thompson Creek Metals announced the start of commercial production at Mount Milligan. Through September 30, 2013 the mill produced approximately 1600 tonnes of saleable concentrate at 24 per cent Cu and 31 grams per tonne Au. The full production design throughput rate for the mill is 60,000 tonnes per day (Press Release, Thompson Creek Metals Company Inc. October 7, 2013).
As of December 31, 2014 the mine life is estimated to be 24 years (Thompson Creek Metals Company Ltd. Annual Report 2014).
Updated reserves and resources as of December 31, 2014 are:
(Mount Milligan Mine Technical Report, January 21, 2015. Calculated at 0.176 per cent Cu cut-off)
Category Amount (tonnes) Cu (%) Au (g/t)
Measured 43,200,000 0.122 0.465
Indicated 79,100,000 0.172 0.243
Indicated 122,300,000 0.155 0.321
Inferred 10,100,000 0.146 0.337
Proven 300,100,000 0.206 0.424
Probable 242,000,000 0.195 0.269
Probable 542,100,000 0.201 0.355