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Britannia - Jane, Fairview Zinc, Beta, Bluff, Empress, Victoria, Daisy |
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British Columbia, Canada |
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Cu Zn Ag Au Pb
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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 Britannia volcanic hosted massive sulphide deposits are located immediately to the south of the town of Squamish on the west coast of British Columbia in Canada, approximately 60 km Noth of Vancouver.
The deposits occur within a roof pendant of Cretaceous volcanic and sedimentary rocks, the Britannia Pendant, surrounded by the Cenozoic-Mesozoic Coast Plutonic Complex which comprises older, commonly foliated bodies ranging from diorite to granodiorite and younger quartz diorite to quartz monzonite intrusions (the Squamish Pluton). The pendant is one of many NW trending bodies within and metamorphosed by the Coast Plutonic Complex. The volcanics and sediments of the Britannia Pendant belongs to the Lower Cretaceous Gambier Group andd are fresh to weakly metamorphosed rocks with sharp contacts against plutonic rocks. A contact metamorphic aureoles up to a hundred metres wide extends outwards from the plutonic complex into the rocks of the pendant.
The Britannia Pendant is cut by the NW trending Britannia shear zone, a broad, steeply south dipping zone of complex shearing and metamorphism. All of the known orebodies were localised within this shear zone. A narrow, generally sub-parallel zone of less sheared foliated rocks, known as the Indian River shear zone, dislocates transects the northeast part same pendant. Both shear zones, and the host volcano-sedimentary sequence are cut by dacite and andesite dykes and several major sets of faults.
The Britannia deposits, within the Britannia shear zone, are hosted by a sequence of strongly foliated pyroclastic rocks of dacitic to andesitic volcanism intercalated near the top with, and overlain by, dark marine shales and siltstones. During hiatuses in dacitic volcanism, extensive units of fine-grained andesitic rocks were formed. One such hiatus is coincident with the formation of massive sulphides and related mineralisation following the extrusion of a dacite tuff breccia.
The lower pyroclastic sequence is composed of pyroclastic dacite tuff breccia which contains prominent dark, wispy fragments, minor intercalations of black or green argillite or volcanic sandstone, commonly grades to dacitic crystal and lithic tuffs and passes upwards into distinctive beds of intercalated black argillite and plagioclase crystal tuffs. The dacite tuff breccias are overlain by a sequence of andesitic tuffaceous sediments, andesitic tuffs and cherty andesitic sedimentary rocks. These are in turn overlain by the black argillite and siltstone which are relatively featureless, poorly bedded, but are commonly cleaved, although intercalations of greywacke may show graded bedding, shale sharpstones and minor slump structures.
Two major dyke sequences and a group of small mafic dykes intrude the volcano-sedimentary sequence. These comprise an early phase of dark grey-green andesites that have a fragmental nature and may also contain abundant quartz and chlorite amygdules. These dykes are virtually contemporaneous with the pyroclastic flow rocks and are locally highly deformed and mineralised. The next pulse of dykes are massive grey-green porphyritic dacites, which are largely undeformed except along their margins and appear to closely postdate major mineralisation. Abundant late dykes include lamprophyre, basalt and andesite, but are volumetrically insignificant.
The sulphide deposits and related accumulations of anhydrite, quartz, zones of silicification, cherty andesitic sediments, bedded chert and minor barite are all interpreted to have formed from volcanogenic hydrothermal solutions following the emplacement of the dacite tuff breccia and during deposition of the overlying andesitic sedimentary and tuffaceous rocks.
Sulphides are present as massive and stringer accumulations, as well as disseminations and bedding plane concentrations. Both massive and stringer sulphide style ore deposit and their alteration halos have been modified by shear deformation and segmented by faults. The massive sulphide deposits are mostly localised along and slightly above the upper contact of the dacite tuff breccia and are commonly hosted by or near cherty andesitic rocks. They include the Jane, Fairview Zinc; No. 8 (top), Beta, 040, Bluff; and No. 8, No. 10, Empress, Victoria, West Victoria. Stringer sulphide deposits, representing around 80% of the mined ore and are mainly in silicified dacite tuff breccia below the massive sulphide deposits. Stringer deposits, include the Bluff, East Bluff, Jane, No. 4 (Bluff), No. 5, No. 10 and Fairview Veins. Other zones within and near the mine area include the Daisy, Homestake, Robinson, Furry Creek, Fairwest and 074.
The sulphide orebodies are very heterogeneous mixtures of sulphides, remnant altered host rocks, and discrete veins and have a simple and fairly constant mineralogy. Pyrite is the dominant mineral, with lesser chalcopyrite and sphalerite and minor erratically distributed galena, tennantite, tetrahedrite and pyrrhotite. Pyrite occurs as beds and nodules in andesitic sedimentary rocks and locally shows slumping features characteristic of soft sediment deformation. The ores contain thin bands of sphalerite, pyrite and barite developed parallel to the bedding planes, while galena occurs as irregular intergrowths in sphalerite and is abundant in a few thin layers in zinc and zinc-copper ore. Gold is locally abundant in scattered narrow veins, in high-grade quartz veinlets and throughout some of the stringer orebodies. Locally pyrrhotite and argentite inclusions are found. Anhydrite is abundant in pyritic andesitic sedimentary rocks and less abundant in the dacite tuff breccia in a broad elongate tabular halo around ore centres. The principal gangue minerals include quartz and muscovite/chlorite, anhydrite, barite and siderite. Locally anhydrite forms massive deposits in tuffaceous sedimentary rocks, flanking and above orebodies, and is also found as distinct crosscutting veins in tensional zones. Barite is disseminated and/or well bedded in zinc ore and nearby zinc-rich sedimentary rocks. Cherty andesitic sedimentary rocks and tuffs, locally with abundant pyrite, occur in and near massive sulphide bodies.
The main massive orebodies exhibit a marked zonal structure in which they have one or more high-grade chalcopyrite core enveloped successively by a lower-grade zone and overlapping pyrite and siliceous zones. Zinc-rich ore tends to be localised in the upper central parts of massive bodies and as almost sheet-like masses. In cross section, these orebodies have an overall lens-shape oriented parallel to the schistosity and are frequently connected to a steeply plunging root which may or may not be of ore grade. The stringer lodes and veins comprise thin sheet-like masses of chalcopyrite and pyrite with some quartz that are generally sub-parallel the schistosity but in detail cross it at an acute angle. Trace realgar, orpiment, scheelite, fluorite and pyrolusite occur in post-dacite, northeast trending gash quartz-carbonate veins in the No. 10 orebody.
A broad halo of pervasively silicified rock surrounds all but one of the stringer orebodies in the dacite tuff breccia and are characterised by quartz and quartz-pyrite veins which occur throughout, but increase in abundance and sulphide content toward an orebody.
Structure at the Britannia mine is complex. The earliest deformation (D0) produced widespread, open, concentric, flexural-slip folds. The first episode of shear deformation (D1) was the most intense. The second episode of compressional shear deformation (D2), recrystallised earlier shear fabrics and was accompanied by the introduction of dacitic magma into dilated zones to form a major dyke swarm. A third metamorphic foliation (S3) was formed locally, possibly following the dacite intrusion. Observations suggest that sulphide ores were deformed during D1 and that volcanism, hydrothermal activity, shear deformation, faulting and metamorphism were all dynamic forces centred along a long-lived active axis now reflected by the Britannia shear zone. Rocks are interpreted to have been altered by volcanogenic hydrothermal solutions during sulphide deposition and by metasomatic hydrothermal solutions during shear deformation. Near orebodies, alteration during deformation was superimposed on ore-stage alteration such that the two are indistinguishable. Alteration is more pronounced in andesitic than in dacitic rocks, with andesitic rocks having been altered to an assemblage of quartz-chlorite-sericite (epidote-albite-potassium feldspar-calcite). Late faults cut the dacite dykes and late andesite dykes and commonly contain vuggy quartz-carbonate veins. They have siderite-kaolinite alteration halos that are most intensely developed in rocks with abundant chlorite. A fourth metamorphic foliation (S4) is a widespread strain-slip cleavage and may have formed from a release of compression perpendicular to the shear zone. A major set of post-dacite dyke faults cuts the Britannia shear zone subparallel to its margins and to S1. These faults cut all the orebodies and are characterised by a few centimetres to metres of gouge and/or strongly sheared rock. Many are braided and coalesce.
As many of the orebodies have contacts which are defined by or are near major east striking faults and because most appear to be parts of a typical volcanogenic sulphide deposit, it has been suggested that the present orebodies may represent faulted segments of two pre-deformation major sulphide deposits.
Between 1905 and 1977, the Britannia orebodies produced approximately 47.8 Mt of ore @ 1.1% Cu, 0.65% Zn, 6.8 g/t Ag and 0.6 g/t Au.
Remaining reserves at the closure of mining in 1977 totalled 1.424 Mt @ 1.9% Cu.
This summary is based on the more detailed online British Columbia Geological Survey MINFILE record summarising this deposit.
#Location - 49° 36' 40" N, 123° 08' 28" W
The most recent source geological information used to prepare this decription 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.
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Payne J G, Bratt J A and Stone B G, 1980 - Deformed Mesozoic volcanogenic Cu-Zn sulfide deposits in the Britannia district, British Columbia: in Econ. Geol. v75 pp 700-721
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