Stratigraphy, alteration, and mineralization at the Tulsequah Chief massive sulfide deposit, northwestern British Columbia.

Sebert, C. and Barrett, T.J., 1996. Exploration and Mining Geology, v. 5, p. 281-308.

Tulsequah Chief is a polymetallic, massive sulfide deposit of Carboniferous age hosted in a bimodal felsic volcanic sequence in the western part of the Stikine terrane, in northwestern 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 >200 m below the sulphide lenses. A distinctive cordierite-biotite mineral assemblage tends to occur peripherally to the sericite altered zone in the footwall. The mineralization is in the form of stacked sulfide-rich lenses hosted in variably sericitized and silicified felsic flows and felsic volcaniclastic rocks, which include pumiceous lapillistone, ash and sandy to blocky felsic-rich debris. Relatively unaltered massive rhyolite flows and breccias form the hanging wall of the deposit. A second package of basaltic rocks 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 sulfide lenses.

The stratigraphy is structurally deformed into a set of open, steeply northward plunging folds. Two post-ore faults, 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 lens, the AB2-H zone, which follows the plunge of a synclinal axis. The ore lenses consist of varying proportions of disseminated to banded sulfides, which include pyrite, sphalerite, galena and chalcopyrite, interstratified and mixed with altered volcanic debris, which includes sulfide and barite fragments. Other mineralization includes crosscutting chalcopyrite, tetrahedrite, sphalerite and galena bearing veinlets, which contain significant precious metal values. A zone of anhydrite mineralization occurs just below the western edge of the AB2-H sulfide lens and is associated with massive to disseminated pyrite. The two post-ore faults may have followed pre-existing faults that focused hydrothermal fluids responsible for the sericite alteration and intense quartz-sulfide stringer mineralization in the footwall of the deposit. 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 sulfide lenses imply that sulphide deposition took place in an elongated paleotopographic low.

Lithogeochemical data allow the recognition of three groups of felsic volcanic rocks and at least two groups of mafic footwall rocks despite the severe alteration near the deposit. Mass changes calculated for the different groups indicate large additions of K2O in that part of the mafic footwall and the felsic rocks which occur under the sulfide lenses. Additions of MgO, FeO and SiO2 are also present locally in both the mafic footwall and felsic unit.

Samples of least-altered felsic and mafic footwall volcanic rocks at Tulsequah Chief display light rare-earth element enrichments and other chemical features which suggest that they are of transitional to mild calc-alkaline affinity. The volcanic sequence is interpreted to have formed in a mature island-arc setting, above a metasomatized mantle wedge. Several rifting events may have occurredin the area of the deposit, with sulfide deposition taking place just after the transition from mafic to felsic volcanism.

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