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Lemarchant |
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Labrador & Newfoundland, Canada |
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Zn Pb Cu Ag Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Lemarchant volcanic hosted massive sulphide (VHMS) Zn-Pb-Cu-Ag-Au deposit is located 21 km SW of the similar Duck Pond deposit and ~245 km WNW of St John in central western Newfoundland Island (#Location: 48° 31' 43"N, 56° 42' 59"W).
Regional Setting
The Lemarchant deposit lies to the SE of the Red Indian Line within the Tally Pond Belt/Group of the Victoria Lake Supergroup, which in turn falls within the Exploits Sub-zone, in the Dunnage Zone, of the Newfoundland Appalachians. It was deposited in a nascent to mature volcanic arc, arc-rift and back arc basin setting (Swinden et al., 1988; Rogers et al., 2006; McNicoll et al., 2010; Piercey and Hinchey, 2012). The Red Indian Line separates the Dunnage Zone into the peri-Gondwanan Exploits Subzone to the east and the peri-Laurentian Notre Dame Subzone to the east. These two subzones represent the opposite sides of the Iapetus suture zone (Williams et al., 1988; Evans and Kean, 2002; Rogers et al., 2006; McNicoll et al., 2010). The Exploits Subzone volcanic and sedimentary rocks were first accreted to the Ganderian microcontinent during the Mid Ordovician Penobscot Orogeny, then to the composite Laurentia along the Red Indian Line during the Late Ordovician (Rogers and van Staal, 2002; Rogers et al., 2006; van Staal and Barr, 2012; Piercey et al., 2014). The Victoria Lake Supergroup, which hosts the Lemarchant deposit within the of the Exploits subzone, comprises six discrete volcanic assemblages (e.g., Rogers and van Staal, 2002; Rogers et al., 2006; Zagorevski et al., 2007). These overlie Neoproterozoic basement metavolcanic rocks of the Sandy Brook Group and the meta-plutonic Crippleback/Valentine Lake intrusive suites (Rogers et al., 2006; McNicoll et al., 2010).
The ∼513 to 509 Ma Cambrian Tally Pond Group (Pollock, 2004; McNicoll et al., 2010), is the lowermost volcanic assemblage in the Victoria Lake Supergroup, and forms part of the eastern margin of the Exploits subzone. This group is informally divided into the Bindons Pond and Lake Ambrose formations (Rogers et al., 2006; Dunning et al., 1991). The former is composed of transitional to calc-alkaline island arc volcanic rocks, dominated by rhyolite to dacite flows, volcaniclastic rocks and carbonaceous shales (Evans and Kean, 2002; Rogers et al., 2006; Copeland et al., 2008; Piercey and Hinchey, 2012; Piercey et al., 2014). The Lake Ambrose formation is intercalated with the Bindons Pond formation, and predominantly comprises mafic and sub-alkalic to depleted tholeiitic island arc volcanic rocks (Evans and Kean, 2002; Rogers et al., 2006; Copeland et al., 2008; Piercey and Hinchey, 2012). The Bindons Pond formation hosts the bulk of the massive sulphide mineralisation of the Tally Pond Group, including the Duck Pond and Boundary VMS deposits, and the Lemarchant (McNicoll et al., 2010; Piercey and Hinchey, 2012). The latter occurs at a contact between the Bindons Pond felsic volcanic rocks and overlying Lake Ambrose mafic volcanic rocks, and was deposited during a hiatus in eruptive volcanism in an extensional arc-rift setting (Evans and Kean, 2002; Rogers and van Staal, 2002; Rogers et al., 2006; van Staal and Barr, 2012; Piercey et al., 2014).
Geology
The Lemarchant deposit is hosted by a felsic volcanic assemblage within a bimodal felsic-mafic magmatic suite. It has been broadly subdivided into:
• a barite-rich, semi-conformable/stratabound massive to semi-massive sulphide zone that has a strike length of ~350 m northwest, is generally ≤20 m thick, and is located at the contact between the felsic footwall and the mafic hanging wall rocks. A thin, pyritic and variably graphitic mudstone forms the footwall-hanging wall contact immediately above the stratabound sulphides, and is also intercalated with the overlying mafic hanging wall proximal to mineralisation. This metalliferous mudstone is discontinuous with a lateral extent of 1 to 4 km. The contact between the underlying massive sulphides and the mudstones is occupied by a 5 to 40 cm thick zone of Fe-rich red to purple sphalerite that grades down-dip into white- to honey brown-colored sphalerite-dominated massive sulphides. Electrum ± Hg ± Sb also occurs in the mudstones. Massive and granular interstitial barite, which are found overlying the footwall, are variably replaced by sulphide mineralisation.
• a more discordant stringer sulphide zone that is situated below the stratabound sulphides within the footwall felsic rocks. This stringer sulphide mineralisation does not persist far below the stratabound zone, being abruptly truncated, possibly as a result of Silurian-Devonian thrust faulting (Dunning et al., 1991; Squires and Moore, 2004; Rogers et al., 2006).
Another portion of the deposit, the 'Northwest Zone' which has a strike length of ~100 m and is interpreted to have been displaced 200 m to the NW and >100 m below the main mineralised zone, although it is mineralogically and geochemically comparable to the main mineralised zone (Gill et al., 2016).
The footwall rocks comprise aphyric rhyolite flows/domes, breccia, lapilli tuffs and tuff breccia, polylithic lapilli tuffs and minor tuffs (Squires and Moore, 2004; Copeland et al., 2008). Hanging wall rocks are composed of massive basalt to basaltic andesite, vesicular pillow basalt and hyaloclastite breccia, and are variably magnetic (Squires and Moore, 2004; Copeland et al., 2008; Piercey and Hinchey, 2012).
The mineralised lithologies at Lemarchant are crosscut by three types of intrusions, namely i). light brown to green, pyroxene phyric synvolcanic mafic dykes which commonly have peperitic and vesicular margins (Pollock, 2004; Squires and Moore, 2004; Copeland et al., 2008); ii). grey-green medium-grained dolerite to gabbroic dykes with sharp contacts; and iii). pink to white, aphyric to lesser quartz phyric felsic dykes.
Proximal to the overlying stratabound barite band, the felsic volcanic rocks of the footwall has been altered to quartz and sericite with lesser albite and by chlorite. Sections of chlorite-dominated alteration are also found locally within the stringer zone. Devitrified glass fragments, which occur within brecciated rhyolite are also strongly chlorite altered (Copeland et al., 2008; Gill and Piercey, 2014; Gill et al., 2015).
This zone of hydrothermal quartz, sericite, chlorite alteration and Ba enrichment covers an area of ~4000 x 700 m zone, accompanied by anomalous disseminated and stringer-type pyrite, base metal sulphides with lesser amounts of pyrrhotite (Fraser et al., 2012).
The hanging wall mafic rocks are only weakly altered to quartz, chlorite and minor epidote, accompanied by pyrite and rare discordant pyrrhotite, arsenopyrite and chalcopyrite mineralisation (Copeland et al., 2008; Fraser et al., 2012). Fuchsite is found in synvolcanic dykes, whilst late carbonate alteration is abundant, occurring as quartz-carbonate stringers crosscutting all of the lithologies within the deposit; carbonate bleaching of mafic dyke contacts; and trace ankerite disseminated throughout the mafic suite.
Repetition of the bimodal volcanic stratigraphy in the deposit area has been attributed to thrust imbrication, particularly along the shallowly dipping Lemarchant thrust fault which passes below the flat-lying Main Zone. The deposit has been disrupted by a set of WNW-ESE trending possibly reactivated upright normal faults which has down-faulted the Main Zone in a graben and displaced the northern and southern margins, e.g., the 'Northwest Zone' (Squires and Moore, 2004; Copeland et al., 2008; Fraser et al., 2012). Deformation during Silurian-Devonian tectonic activity resulted in the development of low-grade greenschist metamorphism (Dunning et al., 1991; Squires and Moore, 2004; Rogers et al., 2006). However, whilst the deposit has been variably deformed, there is only evidence of minor local remobilisation of sulphide mineralisation, with good preservation of sulphide mineral textures in both the stratbound and stringer zones.
Mineralisation
The Main ore zone at Lemarchant is 1.7 to 30.4 m thick and comprises an idealised zonation involving a barite-rich periphery that grades into a Pb-Zn sulphide-rich annulus, and an interior core of Zn-Cu sulphides, which, in turn, grades stratigraphically downward into stringer mineralisation (Lode et al., 2015). The Main Zone is flat lying overall, and covers an area of ~250 to 300 m in diameter.
Mineralisation at Lemarchant is predominantly composed of sphalerite and pyrite, with lesser galena and chalcopyrite. It also contains abundant sulphosalts, including tetrahedrite-tennantite, and bornite, with minor pyrrhotite, arsenopyrite, marcasite and visible electrum. These are accompanied by bladed barite and calcite as well as trace colusite-germanocolusite, stromeyerite, covellite, polybasite, miargyrite and bournonite, with unidentified silver tellurides, nickel sulphides, Cu-Sb-Ag- and Cu- V-bearing sulphosalts. This assemblage includes minerals that contain enrichments in epithermal suite elements (e.g., Au, Ag, As, Hg, Sb, Bi; Gill and Piercey, 2014). Gold primarily occurs as electrum; although laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) reveals it is also present within pyrite, either as lattice substitutions or as micro-inclusions (Gill et al., 2016). Silver occurs within the tetrahedrite-tennantite series of minerals such as Ag-tetrahedrite and tetrahedrite, in electrum, and in other Ag-bearing sulphosalts. Trace Ag is also present in galena (Gill et al., 2016). Gold contents in electrum are higher in the centre of the deposit, whilst Ag-rich electrum and other Ag-bearing minerals are more abundant at the peripheries of the deposit (Gill et al., 2016).
Five mineral assemblage types have been differentiated on the basis of sulphide mineral associations and textures (Gill et al., 2015; Gill et al., 2016):
• Type 1 - semi-massive granular barite-pale sphalerite-colloform pyrite-galena (±chalcopyrite-tetrahedrite-tennantite);
• Type 2A stringers of bornite-galena-chalcopyrite (±stromeyerite-covellite-Ni-sulphide);
• Type 2B disseminated tetrahedrite-tennantite-galena-bladed barite-white sphalerite-recrystallised pyrite (±electrum-colusite-germanocolusite-polybasite-miargyrite-bournonite-Ag tellurides);
• Type 3 massive dark sphalerite-subhedral to euhedral pyrite-galena-chalcopyrite(±pyrrhotite-arsenopyrite); and
• Type 4 stringers of chalcopyrite-euhedral pyrite (±orange sphalerite-galena).
The type 1 assemblage is intergrown with massive barite, and is partially replaced by type 2A stringers and infilled by the type 2B assemblage in the stratabound zone. These are overlain and partially replaced by type 3 mineralisation in the upper portion of the stratabound zone. Stratigraphically below the stratabound zone, the stringer zone lies is composed of type 4 stringers, and is devoid of barite.
These five mineral assemblages are interpreted to have been deposited during three paragenetic stages, each with distinct pyrite textures (Gill et al., 2016):
• The first stage of paragenesis is inferred to have comprised deposition of barite and low-Fe pale sphalerite type 1 mineralisation, with fine-grained colloform pyrite, from a low temperature (275 to 150°C), likely weakly oxidised (SO4 ≥ H2S) and mildly acidic hydrothermal fluid (Gill et al., 2016).
• The second stage included partial replacement and infill of type 1 mineralisation by intermediate sulphidation, sulphosalt-rich type 2A and type 2B mineral assemblages, resulting in recrystallisation of type 1 pyrite to form rounded to subhedral, fine- to medium-sized grains of pyrite.
• The third stage comprised replacement and minor changes to the stratabound zone by barite-poor, high-Fe dark sphalerite type 3 mineralisation, and formation of the basal stringer zone with chalcopyrite dominated type 4 sulphides. The assemblages deposited during this third stage are consistent with formation from a higher temperature (>300°C), intermediate sulphidation state (SO4 ≈ H2S) hydrothermal fluid (Gill et al., 2016).
The Lemarchant deposit was discovered in 1983, and contains (Fraser et al., 2012):
Indicated Mineral Resource of 1.24 Mt @ 5.38 wt.% Zn, 0.58 wt.% Cu, 1.19 wt.% Pb, 1.01 g/t Au, 59.17 g/t Ag;
Inferred Mineral Resource of 1.34 Mt @ 3.70 wt.% Zn, 0.41 wt.% Cu, 0.86 wt.% Pb, 1.00 g/t Au, 50.41 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2019.
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.
Lemarchant
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Gill, S.B., Piercey, S.J., Layne, G.D. and Piercey, G., 2019 - Sulphur and lead isotope geochemistry of sulphide minerals from the Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive sulphide (VMS) deposit, Newfoundland, Canada: in Ore Geology Reviews v.104, pp. 422-435
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Gill, S.B., Piercey, S.J., Layton-Matthews, D., Layne, G.D. and Piercey, G., 2015 - Mineralogical, sulphur, and lead isotopic study of the Lemarchant Zn-Pb-Cu-Ag-Au-VMS deposit: Implications for precious-metal enrichment processes in the VMS environment: in Peter, J.M. and Mercier-Langevin, P., 2015 Targeted Geoscience Initiative 4: Contributions to the Understanding of Volcanogenic Massive Sulphide Deposit Genesis and Exploration Methods Development, ( Geological Survey of Canada, Open File 7853 pp. 183-195.
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Lode, S., Piercey, S.J. and Devine, C.A., 2015 - Geology, Mineralogy, and Lithogeochemistry of Metalliferous Mudstones Associated with the Lemarchant Volcanogenic Massive Sulfide Deposit, Tally Pond Belt, Central Newfoundland: in Econ. Geol. v.110, pp. 1835-1859.
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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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