Volcanic Stratigraphy, lithogeochemistry, and seafloor setting of the H-W massive sulfide deposit, Myra Falls, Vancouver Island, British Columbia.

Barrett, T.J. and Sherlock, R.L., 1996b. Exploration and Mining Geology, v. 5, p. 421-458.
The Myra Falls massive sulfide deposits occur in Paleozoic Sicker Group rocks of the Wrangellia allochthonous terrane. The largest orebody, the H-W deposit, has produced 7.5 Mt of ore grading 1.9 g/t Au, 30 g/t Ag, 2.1% Cu and 4.2% Zn, with 1996 reserves of 5.9 Mt at 1.8 g/t Au, 24.4 g/t Ag, 1.5% Cu and 3.2% Zn. It lies stratigraphically on the Price formation, which consists of at least 300 m of massive to pillowed flows and breccias of mainly basaltic andesite composition, and is overlain by the Myra Formation, which begins with 50-100 m of felsic subaqueous volcaniclastic and pyroclastic beds, lesser interbedded black mudstones, and, at the northern margin of the deposit, a lens of quartz-feldspar-porphyritic rhyolite up to 60 m thick. U-Pb zircon dating of rhyolitic material just above the orebody yields a weighted mean age of 365±4 Ma, similar to a previously reported data of 366 Ma. A massive mafic sill up to 60 m thick with extensive peperitic margins intrudes the felsic volcaniclastic-mudstone sequence. The felsic volcaniclastic rocks pass upwards over 20-30 m into a thick sequence (>100 m) of mafic volcaniclastic sandstone-siltstone beds which record an overall transition to quiet basin sedimentation (Thelwood Formation).

The orebody is underlain by a strongly sericite-pyrite±quartz altered feeder zone that extends up to 60 m (or more) into the mafic footwall. The top of the feeder zone is overlain by several metres of massive pyrite which grades up into a central zone of massive pyrite with several percent disseminated chalcopyrite which forms the bulk of the orebody. The Cu-bearing massive sulfides are typically flanked to the south, and also overlain by, semi-massive to disseminated sphalerite-barite-galena-tennantite mineralization hosted by sericite-altered pyritic felsic debris. Fluid inclusion homogenization data from primary inclusions in quartz and sphalerite in the ore and feeder zone indicate moderate temperatures of 146 to 247ĄC (average 184ĄC), with salinities ranging from 3.2 to 8.1 equivalent weight % NaCl (assuming no gas phase). Whole-rock oxygen isotope data for mafic and felsic rocks are consistent with temperatures of alteration in the 150-250ĄC range.

Least altered samples indicate that the mafic footwall and the mafic hangingwall sill have low TiO2 contents (0.7-0.9%), with moderate enrichment in light rare-earth elements (LREE), moderate depletion of high-field strength elements, and low Zr/Y values of 3-4. These features are typical of island-arc volcanic rocks derived from metasomatized mantle overlying a subduction zone. The felsic hangingwall rocks are characterized by distinct LREE enrichment and Zr/Y values of mainly 5-7, i.e. a mildly calc-alkaline affinity. Two main alteration 'fans' are apparent in immobile element binary plots of Al2O3, TiO2 and Zr. These 'fans' result from alteration of felsic and mafic precursors with small initial compositional ranges in their immobile element ratios. Calculated mass changes for the mafic footwall reveal very large additions of K2O, and near-total loss of CaO and Na2O for 30 to 60 m below the orebody. Silica was added to some parts of the alteration pipe, but depleted from others. The lowest 15-30 m of the felsic volcanic hangingwall has experienced moderate additions of K2O, and losses of CaO, Na2O and SiO2.

Following formation of a basaltic andesite basement of broad lateral extent, massive sulfides were deposited in a local basin which formed part of a semi-continuous east-wend trending graben system that extended several kilometres. The hydrothermal system was apparently enriched in K relative to Fe+Mg, producing extensive sericite rather than chlorite alteration, even in mafic rocks. A massive rhyolite complex was emplaced on the northern margin of the graben hosting the deposit, whereas the deposit itself was overlain by black pelagic mudstones and/or felsic volcaniclastic debris which was shed southwards from the rhyolite complex. A thick mafic sill was intruded from south towards the north within these hangingwall sediments while they were wet and unconsolidated. The emplacement of the rhyolite complex and the hangingwall mafic sill may reflect important rifting events in the area. The composition of the mafic rocks suggests that they were derived by melting of metasomatized (LREE-enriched) mantle. Comparison with modern occurrences of mafic and felsic volcanism from a variety of tectonic settings suggests that the H-W deposit formed in a back-arc basin generated by rifting of a mature island arc.

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