The Britannia district is underlain by a roof pendant of mid- Mesozoic volcanic and sedimentary rocks, within the Cenozoic-Mesozoic Coast Plutonic Complex. A broad, steeply south dipping zone of complex shear deformation and metamorphism, the Britannia shear zone, crosses the pendant in a northwest direction; all orebodies are in the shear zone. A narrow zone of foliated rocks, the Indian River shear zone, is subparallel to the Britannia shear zone and transects the northeast part of the Britannia pendant. The deformed rocks are cut by dacite dykes and several major sets of faults. The Britannia roof pendant is one of many northwest trending bodies within, and in part metamorphosed by, the Coast Plutonic Complex. The pendant is comprised of fresh to weakly metamorphosed rocks with sharp contacts against plutonic rocks, and belongs to the Lower Cretaceous Gambier Group. The Coast plutonic rocks consist of older, commonly foliated bodies ranging from diorite to granodiorite and younger quartz diorite to quartz monzonite intrusions (Squamish pluton). The plutonic rocks have produced contact metamorphic aureoles up to a hundred metres wide in the Britannia pendant.

The Britannia mine area within the Britannia shear zone is dominated by strongly foliated pyroclastic rocks of dacitic to andesitic volcanism intercalated near the top and overlain by dark marine shales and siltstones. Extensive units of fine-grained andesitic rocks were formed in the mine area during hiatuses in dacitic volcanism; one hiatus occurred during the period of formation of massive sulphides and related deposits after extrusion of a dacite tuff breccia. The lower pyroclastic sequence and the upper shale-siltstone sequence are cut by many dacitic and andesitic dykes. The lower sequence is composed of pyroclastic dacite tuff breccia (locally called the Bluff tuff breccia) that commonly grades to dacitic crystal and lithic tuffs. This unit contains prominent dark, wispy fragments and grades at the top into distinctive beds which consist of intercalated black argillite and plagioclase crystal tuffs. These may be regularly interbedded, convoluted or disaggregated by soft rock deformation. Within the pyroclastic sequence there are also minor intercalations of black or green argillite or volcanic sandstone; fragments of argillite also form a normal component of the pyroclastic flow rocks. Overlying the dacite tuff breccias are a sequence of andesitic tuffaceous sediments, andesitic tuffs and cherty andesitic sedimentary rocks. The overlying black argillite and siltstone are relatively featureless, poorly bedded, but commonly displays cleavage. Intercalations of greywacke may show graded bedding, shale sharpstones and minor slump structures. Although gross stratigraphic units can be defined over much of the area, numerous lateral lithologic variations, the scarcity of marker units in the mine area, and complex deformation hampers detailed stratigraphic and structural interpretation.

Intruding this package are two major dyke sequences and a group of small mafic dykes. The early dyke intrusions are composed of dark grey-green andesites that commonly have a slightly mottled texture that reflects a fragmental nature; they may also contain abundant quartz and chlorite amygdules. They are clearly almost contempora- neous with the pyroclastic flow rocks and may be highly deformed and mineralized. The second group are massive grey-green porphyritic dacites, which show no deformation or slight deformation on their margins. Their emplacement postdates major mineralization but they have a close spatial and structural relationship to orebodies. Late dykes are common but volumetrically insignificant and include lamprophyre, basalt and andesite.

Sulphide and genetically related deposits of anhydrite, quartz, silicified rock, cherty andesitic sedimentary rocks, bedded chert, and minor barite formed from volcanogenic hydrothermal solutions after formation of the dacite tuff breccia and during deposition of the overlying andesitic sedimentary and tuffaceous rocks. Sulphides occur as massive and stringer deposits and as disseminations and bedding plane concentrations. Massive deposits are mainly along and slightly above the upper contact of the dacite tuff breccia and commonly in or near cherty andesitic rocks. Stringer deposits are mainly in silicified dacite tuff breccia below the massive sulphide deposits. The ratio of stringer (80 per cent of ore) to massive deposits is much greater at Britannia than in most volcanogenic sulphide deposits. Original deposits and alteration halos are modified by shear deformation and segmented by faults. The massive sulphide-type orebodies mined were: Jane, Fairview Zinc (1.5 per cent of total ore mined); No. 8 (top), Beta, 040, Bluff (4.5 per cent of total ore mined); and No. 8 (bottom), No. 10, Empress, Victoria, West Victoria (15 per cent of total ore mined). Stringer-type orebodies mined were the Bluff, East Bluff, Jane, No. 4 (Bluff), No. 5, No. 10 and Fairview Veins (79 per cent of total ore mined). Other zones within and near the mine area include the Daisy, Homestake, Robinson, Furry Creek, Fairwest and 074.

The sulphide orebodies of Britannia are highly heterogeneous mixtures of sulphides, remnant altered host rocks, and discrete veins. The main mineralogy of orebodies is simple and fairly constant. Pyrite is by far the most abundant mineral, with less chalcopyrite and sphalerite and minor erratically distributed galena, tennanite, tetrahedrite and pyrrhotite. The main nonmetallic minerals include quartz and muscovite (chlorite), anhydrite and siderite. The main massive orebodies, the Bluff, East Bluff, No. 5, No. 8 and 040 all show a marked zonal structure in which they have one or more high-grade chalcopyrite cores enveloped successively by a lower-grade zone and overlapping pyrite and siliceous zones. Zinc-rich ore tends to occur in the upper central parts of massive bodies and as almost sheet-like masses, like the Fairview Zinc vein. In section, the main orebodies have a crude lens-like shape oriented within the schistosity and are commonly connected to a steeply plunging root which may or may not be of ore grade. The other orebodies such as the Fairview Veins are stringer lodes and veins composed of thin sheet-like masses of chalcopyrite and pyrite with some quartz that appear generally parallel to the schistosity but actually cut across schistosity in plan at a small angle. Trace realgar, orpiment, scheelite, fluorite and pyrolusite occur in post-dacite, northeast trending gash quartz-carbonate veins in the No. 10 orebody.

The ore contains thin layers of sphalerite, pyrite and barite parallel to the bedding planes (So). Galena forms irregular intergrowths in sphalerite and is abundant in a few thin layers in zinc and zinc-copper ore. Gold is abundant in scattered narrow veins in the Homestake showing, in high-grade quartz veinlets in the No. 8 orebody and throughout the No. 5 and East Bluff orebodies. Massive ore in the No. 10 mine contains pyrrhotite and argentite inclusions within the chalcopyrite-rich massive orebody. Many of the orebodies contain several types of sulphide concentrations; the No. 8 massive orebodies grade from zinc-copper to copper. The No. 8 and No. 8A ore zones contain more zinc than the No. 8B. In the Bluff deposit, sphalerite is abundant only above the 1800 level; locally in this region siliceous copper-zinc stringer ore grades into massive zinc-copper ore toward the structural footwall (stratigraphic top).

A broad zone of pervasively silicified rock surrounds all stringer orebodies in the dacite tuff breccia except the Fairview veins. Quartz and quartz-pyrite veins occur throughout the silicified halos and increase in abundance and sulphide content toward an orebody. Pyrite is abundant as beds and nodules in andesitic sedimentary rocks above the Fairview Zinc orebody and locally pyritic layers show slumping features characteristic of soft sediment deformation. 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. Locally anhydrite forms massive deposits in tuffaceous sedimentary rocks, flanking and above orebodies, and is also found as distinct crosscutting veins in tensional zones. Locally the anhydrite has been converted to gypsum, especially near permeable zones where the gypsum occurs as narrow replacement veinlets. Within 60 to 90 metres of surface the conversion of anhydrite to gypsum is complete. James (1929) reports the presence of native sulphur in the mine. While the native sulphur may have gypsum or anhydrite associated with it none is present in the large gypsum masses (Open File 1991-15, page 35). 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 and host most of the No. 8 ore lenses.

Structure at the Britannia mine is complex; the earliest deformation (Do) produced widespread, open, concentric, flexural-slip folds (Fo) with subhorizontal to gently plunging, west-northwest trending axes. A major anticline was formed in the dacitic pyroclastic rocks and a major syncline was formed in argillite to the north. Further flexural-slip deformation was localized along the Britannia anticline, which became overturned to the north. Under continued stress, deformation consisting of several episodes of inhomogeneous strain produced the Britannia and other shear zones. Rocks were crystallized to S-tectonites with phase assemblages the same as those of lower greenschist facies regional metamorphism. East of the Jane basin, the axis of the Britannia shear zone follows the axis of the Britannia anticline; from the Jane basin to the west, the shear zone cuts across the south limb of the Britannia anticline. On the surface, the shear zone narrows to a single fault west of the Jane basin, whereas at depth and to the east it widens.

The first episode of shear deformation (D1) was the most intense. Parallel orientation of recrystallized chlorite and sericite plates and flattened lithic fragments define a foliation (S1). Numerous isoclinal folds (F1) were formed with S1 as an axial plane cleavage. In the second episode of shear deformation (D2), some sericite which had formed parallel to S1 during D1 was recrystallized to define S2 into steeply dipping west plunging mesoscopic and microscopic folds (F2). A critical factor regarding the origin of the Britannia sulphide deposits is whether they are pre- or post- D1 (and D2). Recent observations support the hypothesis that sulphide and related deposits at Britannia were deformed during D1 (see Economic Geology, Payne, et. al. 1980, for extensive discussion). The existence of stratabound ore lenses within a felsic volcanic sequence, including pyroclastic breccias, suggests that the Britannia area was a structural locus for all initial and subsequent geological processes. Volcanism, hydrothermal activity, shear deformation, faulting, and metamorphism were all dynamic forces centred along the axis presently known as the Britannia shear zone.

Rocks were 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. Andesitic rocks were altered to an assemblage of quartz-chlorite-sericite (epidote-albite-potassium feldspar-calcite). Some strongly altered andesitic rocks are distinguished from strongly altered dacitic rocks by the andesite's much higher TiO2 content. Studies of rocks near several of the orebodies show that much of the variation in chemical composition in all rock types is produced by ore-stage introduction of quartz, sulphides and sulphates.

A major compressional event (ending with D2) was followed by a period of relaxation of stress during which dacitic magma was intruded into dilated zones within the shear zone and surrounding rocks. In the shear zone, dacite formed dykes subparallel to S1 mainly in or near the dacite tuff breccia. Near the axis of the Britannia anticline, dykes coalesce upward and to the west and appear to cap some of the orebodies. Thin continuous andesite dykes are subparallel to S1 and cut the dacite dykes. Outside the shear zones, sills, dykes and irregular bodies of several varieties of dacite cut the Gambier Group rocks. The evidence suggests that most of the dykes at Britannia were intruded in the late stages of D2 deformation.

A third metamorphic foliation (S3) was formed locally, possibly following the dacite intrusion. It is parallel to northeast trending gash fractures in and near the dacites and to a set of northeast trending faults. The 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. The faults converge upward and to the west to form one major fault. To the east, successive faults branch off a major footwall zone and cut diagonally across the shear zone subparallel to S1. These faults are characterized by a few centimetres to metres of gouge and/or strongly sheared rock. Many are braided and coalesce. In the major fault blocks, minor faults of a similar nature are abundant. Some show more than one age of movement. All the orebodies are cut by the minor faults and many are bounded by, or are near, one or more major faults.

Because many orebodies have contacts at or near major east striking faults and because most appear to be parts of a typical volcanogenic sulphide deposit, the present orebodies may represent faulted segments of a few original major sulphide deposits. A predeformation reconstruction suggests that the orebodies are segments of two original massive sulphide deposits; this requires a near vertical displacement along one fault zone followed by sub- horizontal offset with a cumulative right-lateral displacement of a couple of thousand of metres (Economic Geology, Payne et. al., 1980).

In summary, the Britannia ore deposits were formed from hydrothermal solutions genetically related to dacitic volcanism. Massive zinc, zinc-copper and copper deposits were formed near the contact of dacite tuff breccia and overlying fine andesitic tuff and sedimentary rocks. Siliceous stringer zones were formed in the dacitic tuff breccia and grade upward into massive deposits. Massive to disseminated bodies of anhydrite, pyrite, and minor barite were formed near the orebodies from exhalite solutions. Cherty andesitic sedimentary rocks are common near the orebodies. A northeast trending compressive stress couple produced the following events: a) Broad concentric folds, under continued stress, became tighter and slightly overturned at Britannia. The early part of deformation overlapped the late stages of dacitic volcanism and hydrothermal activity, and produced a series of subparallel fractures which acted as channelways for hydrothermal solutions. b) With continuing stress, several episodes of inhomogeneous strain produced the schistose rocks which define the Britannia shear zone. Rocks were recrystallized into S-tectonites and sulphide deposits were deformed i part by fracture and in part by plastic flow, and were segmented into a series of en echelon stringers parallel to S1. Sulphides and quartz in the orebodies show typical deformation textures similar to those of the enclosing rock. c) Ore-stage hydrothermal solutions and deformation stage solutions caused chemical alteration. Andesitic rocks were effected more than dacitic rocks and show increases in Al2O3, K2O, SiO2 and H2O and decreases in CaO, FeO and MnO. TiO2 remains relatively constant and its content can be used to distinguish some strongly altered andesitic rocks from similarly altered dacitic rocks. d) Orebodies were deformed during several periods of faulting. Following an early period of right-lateral movement, dacite dyke swarms were intruded into the shear zone generally parallel to S1 and concentrated in the dacitic tuff breccia. Dykes were cut by northeast trending quartz-carbonate gash fractures, which near orebodies contain sulphides, mainly chalco- pyrite and pyrrhotite, remobilized from the orebodies. e) A major set of late east faults displaces the rock and orebodies with a cumulative right-lateral horizontal component of motion to a maximum of 2438 metres (Economic Geology, Payne, J.G. et. al., 1980).

Measured and drill indicated reserves in the No. 10 mine at the time of closure were 1,424,147 tonnes grading 1.9 per cent copper (Property File - Northcote, K.).

Past work consisted of extensive underground and surface development. Between 1905 and 1977, the Britannia orebodies yielded approximately 47.8 million tonnes of ore grading 1.1 per cent copper, 0.65 per cent zinc, 6.8 grams per tonne silver and 0.6 grams per tonne gold.

The mine site became the B.C. Museum of Mining, a National Historic Site in 1975.