Stratigraphy, lithogeochemistry, alteration and mineralization at the Tulsequah Chief massive sulphide deposit, northwestern British Columbia
Creators: name: Sebert, C.
Publisher: name: University of British Columbia
Year: 2009

description: Tulsequah Chief is a polymetallic, massive sulpliide deposit of Carboniferous age hosted in a bimodal felsic volcanic sequence in the western part of the Stikine terrane, in Northwest British Columbia. Present ore reserves are estimated at 8.7 Mt of 1.27% Cu, 1.18% Pb, 6.35% Zn, 2.43 g/tonne Au and 99.44 g/tonne Ag. Basaltic to basaltic andesite flows and breccias form the footwall of the deposit, and are strongly altered by sericite and quartz-pyrite stringers to &gt;200 m below the sulphide lenses. A distinctive cordierite-biotite mineral assemblage tends to occur peripherally and within the sericitized zone in the footwall. The deposit consists of stacked sulphide-rich lenses hosted in sericitized and variably silicified felsic flows and felsic volcaniclastics. These rocks include variably pumiceous lapillistone, ash and other sandy to blocky felsic-rich volcaniclastics. Relatively unaltered massive rhyolite flows and breccias form the hanging wall of the deposit. A second package of basaltic rocks and volcanic sediments caps the felsic rocks in the mine area. A massive, semiconcordant gabbro sill, of similar mineralogy and chemistry as the overlying basalts, intrudes and splits the felsic sequence above the sulphide lenses. The stratigraphy and sulphide lenses are structurally deformed into a set of open, steeply northward plunging folds. Two post-ore faults, the 4400E and 5300E, with up to 40 m dextral displacement, divide the mine stratigraphy. These structures occur on each side of the sericite-altered zone in the footwall rocks, and on each side of the of the largest massive sulphide lenses, the AB2- and H-Zones, which follow the plunge of a synclinal axis. The upper ore zones are pipe-like to lenticular in shape and of composite architecture. They consist of varying proportions of banded to massive sulphides and gangue. Sulphides include pyrite, sphalerite, galena and chalcopyrite. The gangue is largely quartz and sericite with lesser barite and minor epidote, chlorite and carbonate. Layers of sulphide-bearing debris, constitute a semimassive ore-facies, and are interstratified or cap the sulphide lenses. The debris includes disseminated sulphides and fragments of sulphide and barite. Other mineralization includes crosscutting chalcopyrite, tetrahedrite, sphalerite and galena-bearing veinlets; some veinlets contain visible gold. A zone of anhydrite mineralization occurs stratigraphically below the western edge of the AB2 sulpliide lens east of the 4400E fault and is associated with massive to disseminated pyrite. Other less extensive and less complex pyrite and sphalerite-rich sulpliide lenses are hosted in the felsic stratigraphy below the H-Zone sulphide lens. The elongated morphology of the main ore body along the axis of a fault bounded syncline and the presence of mass flow debris covering a portion of the sulphide lenses imply that sulphide deposition took place in a graben. Preexisting faults, parallel to the 4400E and 5300E structures, may have focused hydrothermal fluids responsible for the sericite alteration and intense quartz-sulphide stringer mineralization in the footwall of the deposit. Lithogeochemical data indicates the presence of three groups of felsic rocks and at least two groups of mafic footwall rocks despite the severe alteration near the deposit. Mass changes calculated for the altered lower felsic rocks indicate large additions of K20 and loss of Na20 and CaO. Samples of least-altered felsic and mafic volcanic rocks at Tulsequah Chief display light rare-earth element enrichments and other chemical features that suggest that they are of tholeiitic to transitional affinity. The volcanic sequence is interpreted to have formed in a rifted arc setting, above a metasomatized mantle wedge. Several rifting events may have occurred in the area of the deposit, with sulphide deposition taking place just after the transition from mafic to felsic volcanism. [Scientific formulae used in this abstract could not be reproducted.]