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Mt Pleasant - Fire Tower, Saddle and North Zones |
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New Brunswick, Canada |
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W Mo Bi Sn
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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 Mount Pleasant porphyry tungsten-molybdenum-bismuth deposit is located 45 km north of St George in southern New Brunswick, near the northern end of the Appalachian Belt of the eastern US and Canada (#Location: 45° 25' 27"N, 66° 49' 5"W).
It is by far the most significant of a number of known tin and tungsten deposits which are largely associated with pegmatites, and are spread over an interval of some 2500 km within this belt. These occurrences are almost exclusively restricted to a zone of more intense deformation which is accompanied by the development of granitic intrusives, running the length of the Appalachian Belt. The geology of the belt is dominated by Palaeozoic quartzites, shales, sandstones and carbonates, and their metamorphic equivalents, overlying similar Proterozoic sediments and older high grade metamorphics.
The deposit is hosted by the Late Devonian Mount Pleasant Caldera Complex (McCutcheon et al., 1997) along the northern margin of the Late Silurian to Late Devonian Saint George Batholith. This caldera complex has an elliptical shape and underlies an area of ~13 x 34 km, as interpreted from regional gravity and magnetic maps. It overlies pre-caldera (Ordovician and Silurian) sedimentary rocks of the Cookson and Kingsclear groups to the east and west, and is overlain by Upper Carboniferous conglomerate and sandstone to the north. It lies above the older Gander Terrane.
Rocks within the Mount Pleasant Caldera Complex are divided into the Exocaldera, Intracaldera and Late Caldera-Fill sequences (McCutcheon et al., 1997). These sequences are members of the Late Devonian Piskahegan Group and associated subvolcanic intrusions of the Late Devonian to Permian Maritimes Basin.
The Exocaldera Sequence consists, in ascending stratigraphic order, of the Hoyt Station Basalt flows; Rothea Formation lapilli and crystal tuff; South Oromocto Andesite flow units; Carrow Formation, a fining-upward redbed sequence; and Bailey Rock Rhyolite, a porphyritic lava.
The Intracaldera Sequence comprises, in ascending stratigraphic order, the Scoullar Mountain Formation sedimentary breccia and interbedded
andesitic lavas; Little Mount Pleasant Formation crystal tuff and massive porphyritic rhyolite; Seelys Formation, lithic and pumiceous lithic lapilli and banded and pumiceous crystal tuff, and densely welded crystal tuff; and McDougall Brook Granitic Suite. In addition it includes felsic dykes and one mafic dyke that intrude the Scoullar Mountain and Little Mount Pleasant formations, respectively.
The Late Caldera-Fill Sequence includes the Big Scott Mountain Formation porphyritic to nearly aphyric rhyolite, lithic to lithic lapilli and crystal tuff; the Kleef Formation redbeds, porphyritic to glomeroporphyritic basalt, and pumiceous lithic to lithic lapilli tuff; and the Mount Pleasant Granitic Suite with its associated felsic/granitic dykes and small plug-like bodies associated with magmatic to hydrothermal breccias. Although the ages of the Late Caldera-Fill rocks are not firmly established, they are most likely Late Devonian but could range into the Lower Carboniferous.
The Mount Pleasant deposits are developed within two separate small brecciated 500 x 200 m sub-volcanic rhyolitic porphyry plugs, which cut the Intracaldera Sequence and contain the North and Fire Tower mineralised zones. These plugs were intruded on the margin of a 15 km diameter lower Carboniferous (Mississippian) caldera comprising an intrusive-subvolcanic-eruptive complex of granitic, porphyritic and felsic pyroclastic rocks. The separate North and Fire Tower mineralised zones are centred in volcanic necks defined by hydrothermal breccias within this complex. The oldest of three successive granites associated with both brecciated zones, a fine grained phase, hosts porphyry-style W-Mo (stage 1) mineralisation, while the two younger granites in the North Zone contain various stanniferous, polymetallic vein and replacement (stage 2) deposits. All three granites appear to be 300 to 340 Ma in age.
McCutcheon et al., in Fyffe and Thorne (2010) describe these three granites that collectively comprise the Mount Pleasant Granitic Suite, and are interpreted to represent successive cooling stages of one magma body, as follows:
• Granite I occurs as irregular bodies closely associated with the hydrothermal breccias. The granite-breccia contacts are commonly gradational, with fragments of that granite stage locally abundant within the breccias. In relatively unaltered specimens, Granite I is typically fine grained and equigranular, although the textural features of the granite are obscured in most areas by pervasive silicic and chloritic alteration.
• Granite II gradationally underlies Granite I at depth, and dyke-like bodies of Granite II intrude Granite I and the overlying breccias. Outcropping banded porphyry dykes are probably derived from Granite II. Granite II ranges from aplitic to porphyritic and locally contains abundant miarolitic cavities and comb quartz layers. The latter consists of parallel to subparallel layers in which quartz crystals are oriented approximately perpendicular to the planes of layering. This is one in a family of unidirectional solidification textures (USTs) associated with fluid-saturated and/or undercooled magmas.
• Granite III occurs as a large body that gradationally underlies Granite II and locally intrudes both Granites I and II. The contacts are commonly sharp and in many places are marked by thin (0.5 to 2 cm thick) layers of USTs, mainly K feldspar, in Granite III. Granite III ranges from fine to medium grained and can be equigranular porphyritic and pegmatitic. Miarolitic cavities filled with very fine-grained sericite are locally abundant.
The Mount Pleasant Granitic Suite is younger than that of the Exocaldera Sequence of the Piskahegan Group, which
is constrained by a U-Pb radiometric age of 363.4 ±1.8 Ma for the Bailey Rock Rhyolite. K-Ar and Rb-Sr determinations indicate an age of 340 Ma to 330 Ma. However, a K-Ar date of 361 ±9 Ma from biotite hornfels in sedimentary breccia underlain by Granite III appears to confirm a Late Devonian to Carboniferous age.
In both the Fire Tower Zone and North Zone, breccias and associated intrusive rocks form irregular, roughly vertical, pipe-like complexes that were centres of subvolcanic intrusive and related hydrothermal activity. The breccias range from groundmass-supported with rounded fragments to clast-supported with mainly angular fragments. Both the fragments and groundmass material have been altered extensively, and in many places the protoliths of fragment are difficult to identify (McCutcheon et al., 2010).
The tungsten-molybdenum-bismuth mineralisation is directly related to Granite I and is mainly hosted by that granite and related breccia and, to a lesser extent, the surrounding country rocks. It occurs in fractures, quartz veinlets and disseminations in the breccia matrix. Wolframite and molybdenite are the principal ore minerals, accompanied by minor bismuth and bismuthinite. Quartz, topaz, fluorite, arsenopyrite and loellingite are the primary gangue minerals. Several types of alteration are associated with the tungsten-molybdenum-bismuth mineralisation. Intense and pervasive silicic or greisen-type alteration occurs within and above the high-grade tungsten-molybdenum orebodies and is characterised by the complete or near complete replacement of host rocks by quartz, topaz, sericite and fluorite. Alteration passes outward into a less intense silicic that is mainly limited to narrow selvages on mineralised fractures and quartz veinlets. The quartz, biotite, chlorite and minor topaz alteration stage extends laterally up to 100 m beyond the high grade tungsten-molybdenum orebodies. Propylitic alteration, which comprises chlorite and sericite surrounds the silicic alteration, and extends outward for >1000 m before passing into relatively unaltered rock (Pouliotet al., 1978; Kooiman et al., 1986). Tungsten-molybdenum-bismuth mineralisation is mainly found in the Fire Tower Zone, but one subzone of tungsten-molybdenum has also been intersected in the North Zone (McCutcheon et al., 2010).
Physically, the stage 1 wolframite/ferberite and molybdenite mineralisation occurs in quartz ±topaz and/or fluorite veinlets and breccia matrices and as disseminations in the altered wallrock. The disseminated mineralisation comprises around 100 µm ferberite and 1 mm molybdenite flakes. Quartz and topaz with lesser green biotite, chlorite and K feldspar are the principal alteration minerals. Sericite occurs as a late alteration product in the main ore zone. Stage 2 comprises mineralogically complex Sn-In bearing sulphide vein and replacement bodies containing abundant fluorite and chlorite and has been superimposed on stage 1.
That Stage 2 tin-indium mineralisation was directly related by Granite II and occurs as sulphide-rich polymetallic veins and replacement bodies. Sphalerite, chalcopyrite, arsenopyrite and cassiterite are the dominant ore minerals, associated with chlorite, fluorite, topaz and a complex assemblage of sulphides and sulpharsenides that include löellingite, galena, pyrite, marcasite, molybdenite, tennantite, bornite, bismuthinite, wittichenite and roquesite.
Most of the economically significant tin bodies occur in the North Zone from depths of 200 to 400 m. They include the Upper Deep Tin, Deep Tin, Contact Crest, Contact Flank and Endogranitic subzones. The Deep Tin Subzone is a relatively large, irregular body that comprises fracture-controlled and disseminated cassiterite hosted within silicified and chloritised breccia and Granite I. Other minerals associated with cassiterite include arsenopyrite, sphalerite, chalcopyrite and galena. The Contact Crest and Contact Flank subzones mainly occur in breccia or other associated host rocks at the upper contact or along the sides of Granite II, whilst the Endogranitic subzone is mainly found within Granite II. Cassiterite occurs as finely disseminated grains andas fine- to medium-sized grains in veins or veinlets and along fractures within these sub-zones. Associated minerals include arsenopyrite, sphalerite, chalcopyrite, pyrite, and pyrrhotite. Chlorite, fluorite and topaz are the principal alteration minerals. Cross-cutting relationships suggest several stages of alteration and mineralisation may have taken place. Tin-indium mineralisation is primarily found within the North Zone but also has been intersected in the Fire Tower Zone, where indium grades range from 50 to 300 g/t. The indium occurs mainly as a solid solution in sphalerite and, to a lesser extent, in chalcopyrite and stannite (McCutcheon et al., 2010).
Stage 1 porphyry-style W-Mo, and associated bismuth-rich mineralisation occur in the Fire Tower Zone and the North Zone, and were mined during the early 1980s in the former where they are more extensive. The two main orebodies in the Fire Tower zone in 1978 totalled some 7 Mt of 0.41% WO3, 0.27% MoS2, 0.18% Bi and 6% F. They comprised a 150 x 60 x 120 m and a 350 x 150 x 15 m body, within a larger 30 Mt low grade zone averaging 0.1% Mo, 0.25% WO3 and 0.1% Bi. The deposit that was brought into production in 1983 had a reserve of 9.4 Mt @ 0.39% WO3 and 0.2% MoS2.
Small, more or less flat, cassiterite-rich stage 2 'greisen zones' are hosted in granite porphyry to porphyritic granite in the Fire Tower Zone, although the North Zone also contains additional large mineralised bodies, all 120 to 430 m below surface.
According to Inverno and Hutchinson (2004) the Fire Tower Zone porphyry-style W-Mo bodies contain a total of 25 Mt @ 0.21% W, 0.10% Mo and 0.08% Bi. Equivalent bodies in the North Zone amount to 11.5 Mt @ 0.19% W, 0.11% Sn, 0.06% Mo and 0.08% Bi. A third mineralised zone between the Fire Tower and North Zones, the Saddle Zone contains mineralised zones consisting predominantly of tin and some base metals with a total of 2 Mt @ 0.92% Sn and 49.8 g/t In.
The main tin deposits in the North Zone contain a total of 5.1 Mt @ 0.79% Sn and 78.7 g/t In. Shallow tin lodes in the North Zone and Fire Tower Zone amount to 0.53 and 0.15 Mt @ 0.6 and 0.4% Sn, 2.3 and 3.7% Zn, 0.3 and 0.4% Cu and 0.4 and 1.2% Pb, respectively. Two late replacement 'zones' in the Fire Tower Zone contain 1.7 and 4.5 Mt @ 0.32% of both Cu and Zn and 1.0% Zn, respectively.
The total NI 43-101 compliant resource estimate for the Fire Tower Zone in 2014 was (Adex Mining Inc.):
Indicated resource - 13.489 Mt @ 0.33% WO3 and 0.21% MoS2,
Inferred resource - 0.8417 Mt @ 0.26% WO3 and 0.20% MoS2.
At the same date, the North Zone contained an updated NI 43-101 resource estimate of:
Indicated resource - 12.40 Mt @ 0.38% Sn, 0.86% Zn, 64 g/t In,
Inferred resource - 2.80 Mt @ 0.30% Sn, 1.13% Zn, 70 g/t In.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2010.
Record last updated: 15/5/2021
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.
Mt Pleasant
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Charnley, B.E. and Lentz, D.R., 2016 - Petrogeochemical Assessment of the Various Felsic Volcanic and Subvolcanic Igneous Rocks Associated with Sn-Cu-Zn and W-Mo-Bi Mineralization in the North Zone, Mount Pleasant, New Brunswick: A pXRF Study: in New Brunswick Department of Energy and Mines, Geological Surveys Branch, Geoscience Report 2015-5 (Online), 31p.
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Inverno C M C and Hutchinson R W, 2006 - Petrochemical discrimination of evolved granitic intrusions associated with Mount Pleasant deposits, New Brunswick, Canada: in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v115 pp 23-39
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Inverno C M C and Hutchinson R W, 2004 - The endogranitic tin zone, Mount Pleasant, New Brunswick, Canada and its metallogenesis: in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v113 pp B261-288
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Kooiman G J A, McLeod M J 1986 - Porphyry Tungsten-Molybdenum orebodies, polymetallic veins and replacement bodies, and Tin-bearing Greisen zones in the Fire Tower zone, Mount Pleasant, New Brunswick: in Econ. Geol. v81 pp 1356-1373
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McClenaghan, M.B., Parkhill, M.A., Chapman, J.B. and Sinclair, W.D., 2015 - Indicator mineral content of bedrock from the Mount Pleasant Sn-W-Mo-Bi-In deposit, New Brunswick: in Geological Survey of Canada, Open File 7721, 16p. doi:10.4095/295613.
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McCutcheon, S.R., Sinclair, W.D., McLeod, M.J., Boyd, T. and Kooiman, G.J.A 2010 - Mount Pleasant Sn-W-Bi-In Deposit: in Fyffe, L.R., and Thorne, K.G. (compilers), 2010 Polymetallic deposits of Sisson Brook and Mount Pleasant, New Brunswick, Canada - Field Guide No.3, 68p. New Brunswick Department of Natural Resources; Lands, Minerals and Petroleum Division, Part 2: pp 37-68.
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Samson I M 1990 - Fluid evolution and mineralization in a subvolcanic granite stock: The Mount Pleasant W-Mo-Sn deposits, New Brunswick, Canada: in Econ. Geol. v85 pp 145-163
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