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Minto - Minto Main, Minto South, Area 2, Area 118, Copper Keel, Wildfire, Ridgetop, Minto East, Minto North

Yukon Territory, Canada

Main commodities: Cu Au Ag
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The Minto copper-gold-silver cluster of deposit is located in central Yukon Territory, 75 km NW of the the town of Carmacks and 240 km NNW of Whitehorse, northwestern Canada. The mine is accessed via the Klondike Highway to a seasonal barge across the Yukon River and then via a private 27 km road to the mine site (#Location: 62° 36' 51"N, 137° 14' 32"W).

  Mineralisation was discovered in the immediate deposit area in 1971 when the current claim package was pegged, based on anomalous results of a regional stream sediment geochemical program shortly after the discovery of the Williams Creek, now Carmacks Copper, deposit. Following the exploration program that ensued, the results of a joint feasibility study were released in 1977, citing a Main zone pre-NI 43-101 reserves of 6.55 Mt @ 1.86% Cu, 0.51 g/t Au, 6.86 g/t Ag. No mining was initiated. Beginning in 1994, engineering and geotechnical studies were commenced to support a feasibility study on already known resources. An initial in situ resource was re-calculated, also pre-NI 43-101, at a cut-off grade of 0.5% Cu for 8.818 Mt @ 1.72% Cu, 0.48 g/t Au, 7.5 g/t Ag. Declining copper prices prevented a mining operation being approved. In early 2005, a new owner took on the project and within five months released the first NI 43-101 compliant Measured + Indicated Mineral Resource of 8.340 Mt @ 1.83% Cu, 0.55 g/t Au, 7.95 g/t Ag, with an Inferred resource of 0.7 Mt @ 1.41% Cu, 0.45 g/t Au, 6.0 g/t Ag (Giroux, 2005). All major permits were granted by June 2006, with pre-stripping of the Main Zone and mill construction commencing in the latter half of the year to be completed in early 2007. The first copper-gold concentrates were produced on May 1, 2007 and the first load of concentrates were delivered to the port of Skagway, Alaska on July 16, 2007. The discovery of mineralisation in Area 2 in early 2006, and between 2006 and 2011, a further nine zones of Cu-Au-Ag mineralisation were discovered, including the Minto South (made up of earlier Area 2, Area 118, Wildfire and Copper Keel discoveries), Ridgetop, Minto East and Minto North (2 deposits). This cluster is distributed over a NNW-SSE trending interval of ~3 km and a width of up to 1 km. Significant exploration potential, particularly for deeper resources, remained on the property in 2016 (Sack et al., 2017). The mine was in continuous production from 2007, first as an open pit until early 2018, then underground until late 2018, when it was placed in temporary care and maintenance. The operation changed ownership in June 2019 and recommenced production in October 2019.

Regional Setting

  See the 'Regional Setting' section of the Carmacks record. Minto lies at the northern end of the NW-SE trending, 75 km long, Carmacks-Minto copper belt defined by Carmacks, Stu, Minto and other similar prospects that are distributed over that interval.
  The Minto orebodies are hosted within foliated rocks that are enclosed within the overall unfoliated granodiorite of the 170 km2 Minto pluton. This pluton is a member of the Minto suite that is, in turn, part of the composite 120 x 15 to 25 km Lower Jurassic Granite Mountain Batholith. The Minto pluton is separated from the section of the batholith that host the Carmacks deposit and Stu prospect by the Upper Cretaceous Carmacks Group basalts and sedimentary rocks. The Minto pluton is predominantly composed of granodiorite, the composition and texture of which varies from crowded K feldspar megacrystic syenite to equigranular tonalite and quartz diorite. In general, its composition comprises 30 to 50% plagioclase, 10 to 50% K feldspar, 20 to 25% quartz and 10 to 15% biotite ±hornblende, with accessory magnetite, epidote, titanite, apatite and zircon, all of which have sharp euhedral crystal faces. K feldspar most commonly occurs as 1 to 3 cm phenocrysts with inclusions of biotite, plagioclase, hornblende, epidote and zircon which are frequently distributed along growth zones (Hood, 2012). Those phases with tonalitic or dioritic compositions, are generally slightly coarser grained and typically contain anhedral K feldspar, as well as up to 1 cm long euhedral plagioclase crystals and anhedral equigranular crystals that are evenly distributed (Hood, 2012). Quartz forms anhedral masses interstitial to feldspar, but locally occurs as glomeroporphyritic masses up to 1 cm in diameter. The most common mafic minerals are biotite and hornblende that are typically <1 cm, and comprise up to 15% of the granodiorite (Hood, 2012). Both magmatic epidote, which has sharp euhedral boundaries with mafic phases and growth zoning, and secondary epidote are present (Zen and Hammarstrom, 1984; Sack et al., 2017).
  The Minto pluton is dated at between 197.6 ±1.6 and 200.1 ±1.1 Ma (U-Pb zircon; Hood 2012). The estimated depth of emplacement for several plutons of the Minto suite ranges from 30 to 20 km (7.2±1.0 to 6.4±0.8 kbar, respectively) have been calculated using the aluminum-in-hornblende geothermobarometer (McCausland et al., 2002; Tafti, 2005; Topham et al., 2016).
  The Minto pluton is crosscut by four sets of relatively late, volumetrically insignificant dykes, that are composed of, from oldest to youngest (Hood, 2012): i). granitoid pegmatite; ii). aplite dykes that have mutually crosscutting relationships with the pegmatoids, suggesting a similar age for both - a pegmatite dyke dated at 195.5 0.7 Ma (Tafti, 2005) indicates they are essentially coeval with the crystallisation of the Minto pluton; iii). andesite dykes that are undeformed, but can be locally altered, and are likely related to the volcanic rocks of the Upper Cretaceous Carmacks Group; iv). hornblende diorite dykes that are unaltered, undeformed and likely related to the Pliocene and younger Selkirk Volcanic Group basalts found to the north of the deposit (Sack et al., 2017).

Geology

  The foliated rocks that host mineralisation at Minto have been variously interpreted to represent various stages of digestion of rafts of older intruded rocks, to various stages of deformation of the Minto pluton granodiorite (Hood, 2012). These foliated rocks are composed of alternating mafic and felsic layers. The mafic layers have millimetre to centimetre thicknesses, comprising moderately aligned biotite, hornblende, epidote, magnetite and titanite, separated by thicker centimetre to tens of centimetre felsic layers composed of medium to coarse-grained quartz and plagioclase with lesser biotite, hornblende, epidote, magnetite and titanite (Hood, 2012). They host most of the sulphides at the Minto deposits, with individual bodies that are up to tens of metres in thickness. Higher grades tends to be with associated with thicker layered, coarser grained and more siliceous lithologies, while lower grades often accompany thin, discontinuous layering and more mafic rocks (Mercer and Sagman, 2012). Contacts with unfoliated rocks are marked by rapid grain-size change and are typically sharp. Locally, relationships highlighed by feldspar phenocrysts in unfoliated granodiorite at the contact with foliated rocks suggest the contacts are, at least locally, intrusive in nature (Hood, 2012). Similarly, individual mafic-felsic bands or foliations vary from being parallel, to a high angle to contacts with unfoliated rocks (Mercer and Sagman, 2012), again suggesting an intrusive relationship. Never the less, the zones of foliated rock form consistently subhorizontal rafts that can be traced laterally for >1 lm in drill data interpretations. The zones are also often stacked in parallel to subparallel sequences (Mercer and Sagman, 2012).

Structure

  Late brittle fracturing and faulting occurs throughout the area (Mercer and Sagman, 2012). The central Main deposit is bisected by the steeply dipping, ENE trending, sinistral, reverse, Minto Creek fault, which also divided the cluster of deposits into north and south areas (Mercer and Sagman, 2012). Both vertical and horizontal displacements are evident by offsets of mineralised horizons, but appear to be minimal. The DEF fault, which strikes east-west and dips NNW, defines the northern end of, and truncates mineralisation in, the Main deposit. Although it may share a similar sense of movement with the Minto Creek fault, in contrast a significant amount of displacement is inferred (Mercer and Sagman, 2012). The two faults are interpreted to be late block faults with a rotational component, which is common to the area (Tempelman-Kluit, 1984). To the south of the Minto deposit, another example of a rotational fault, rotates the Cretaceous Carmacks Group sedimentary rocks by up to 60° from horizontal (Tafti and Mortensen, 2004). Other, but poorly defined faults include northeast-dipping structures with significant displacement, that displace or truncate mineralisation at the Ridgetop deposit and Wildfire resource and others (Mercer and Sagman, 2012).

Mineralisation and Alteration

  Hypogene mineralisation consists of chalcopyrite, bornite, euhedral chalcocite and minor pyrite, with some covellite. Silver telluride (hessite) is observed, whilst native gold and electrum both occur as inclusions within bornite. Coarse free gold is also present locally, associated with chloritic or epidote-lined fractures that crosscut the sulphide mineralisation. This gold is attributed to supergene enrichment of the main copper sulphide mineralisation (Mercer and Sagman, 2012).
  Increased magnetite and biotite, possibly representing potassic alteration, almost always accompanies sulphide mineralisation (Mercer and Sagman, 2012). Sulphide mineralisation predominantly occurs as disseminated and foliation parallel stringers within the deformed rocks, although the sulphide content tends to increase where foliation is intensely disrupted and can result in semi-massive sulphide accumulations up to several metres thick or, locally, massive sulphide accumulations up to 0.5 m in thickness (Mercer and Sagman, 2012). In high sulphide zones, sulphides can occur interstitial to the silicate minerals of the host, and resemble net textured sulphides typical of magmatic sulphide accumulations. The strongest bornite grades are associated with local up to 20 vol.% accumulations of coarse-grained disseminated and stringer-style magnetite mineralisation. Such magnetite stringers are often folded or boudinaged, indicating at least some of the magnetite predates peak ductile deformation and may reflect rheologic differences compared to magnetite (Mercer and Sagman, 2012).
  Re-Os molybdenite dates (Hood, 2012) constraint the age of mineralisation to the range from 197.4 ±0.8 to 201.8 ±0.8 Ma. Molybdenite flakes are observed to occur in two different morphologies, leading to the suggestion that more than one generation of mineralisation may be represented in this age range (Hood, 2012).
  The Minto Main, Minto North and Minto East deposits each exhibit a crude zoning, at the orebody scale, from lower grade chalcopyrite-dominant mineralisation in the east, to higher grade bornite-dominant ore in the west. The latter has a mineral assemblage of bornite-chalcopyrite-magnetite, with locally >10% bornite occurring as strong disseminations and foliation following stringers, and locally as semi-massive to massive lenses up to 2 m thick. In contrast, the chalcopyrite rich zone is characterised by chalcopyrite-pyrite ± very minor bornite and magnetite, typically containing 1 to 2%, but locally reaching >10% sulphides.
  Significant supergene mineralisation at Minto is only known near surface at the Ridgetop orebody and Wildfire resource of the Minto South deposit cluster, where near surface sulphides have been affected by supergene oxidation (Mercer and Sagman, 2012). The deeper part of these are similar to the other Minto South orebodies where oxidation has not been preserved, or did not occur. At Areas 2, 118 and Copper Keel, also of the Minto South deposit cluster, mineralisation is predominantly disseminated, with localised foliation following stringers, but lacks semimassive to massive sulphide mineralisation typical of the Minto Main, North and East deposits. The primary mineral assemblage in this resource sub-domains includes chalcopyrite-bornite-magnetite with minor pyrite. The northern half of the Minto South deposit has enhanced bornite concentrations, up to 8% locally, and a corresponding higher grade (Mercer and Sagman, 2012).

Resources and Reserves

Sack et al. (2017) quote the following global resources including Minto Main (after SRK, 2008) and Minto South, Ridgetop, Minto North and Minto East (Mercer and Sagman, 2012) at a 0.5% Cu cut-off:
  Measured + Indicated Mineral Resource - 54.513 Mt @ 1.21% Cu, 0.45 g/t Au, 4.4 g/t Ag;
  Inferred Mineral Resource - 8.491 Mt @ 0.81% Cu, 0.24 g/t Au, 2.9 g/t Ag;
  TOTAL Mineral Resource - 63.004 Mt @ 1.15% Cu, 0.42 g/t Au, 4.2 g/t Ag.

The published NI 43-101 compliant Mineral Resource at Minto Main pre-mining (Giroux, 2005) was:
  Measured + Indicated Mineral Resource - 8.340 Mt @ 1.83% Cu, 1.55 g/t Au, 7.95 g/t Ag;
  Inferred Mineral Resource - 0.700 Mt @ 1.41% Cu, 0.45 g/t Au, 6.0 g/t Ag;
  TOTAL Mineral Resource - 9.040 Mt @ 1.80% Cu, 1.46 g/t Au, 7.8 g/t Ag.

The most recent source geological information used to prepare this summary was dated: 2017.    
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.


  References & Additional Information
   Selected References:
Sack, P.J., Kerr, R. and McIlveen, D.,  2017 - Update on the Minto deposit: in MacFarlane, K.E., (Ed.), 2017 Yukon Exploration and Geology Overview 2016 Yukon Geological Survey,   Yukon MINFILE 115I 021, 022 pp. 75-87.


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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