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Geology of the West Block Area of the Late Devonian Myra Falls VHMS District, B.C., Canada


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McNulty, BA ORCID: 0000-0003-4568-1842 2019 , 'Geology of the West Block Area of the Late Devonian Myra Falls VHMS District, B.C., Canada', PhD thesis, University of Tasmania.

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Myra Falls is a series of polymetallic, volcanic-hosted massive sulfide (VHMS) deposits located in central Vancouver Island, 90 km southwest of Campbell River, British Columbia, Canada. As of April 2013, production has exceeded 30 Mt of ore at average grades of 5.5% Zn, 1.6% Cu, 0.6% Pb, 2.0 g/t Au and 54.0 g/t Ag. There are 10 VHMS deposits at Myra Falls, which are spatially and temporally related to episodic felsic volcanism, in the Devonian Sicker Group, which define two district-scale stratigraphic members. The near surface L-M-P member is host to the Lynx, Myra, and Price orebodies. Whereas the HW, Trumpeter Zone, Battle, Extension Zone, Ridge Zone West, Ridge Zone North, and Marshall Zone orebodies are in the lower H-W member.
While previous studies have focused on the HW, Battle, and Ridge Zone West orebodies, questions regarding the lithostratigraphic architecture and the depositional age of the H-W member remain. The West Block Area provides a geological-link between the Battle and Ridge Zone West orebodies. This research presents: (1) a detailed description of the lithology and nature of VHMS mineralisation in the West Block Area; (2) a proposed geological framework linking the Battle and Ridge Zone West orebodies; (3) mineralogical and geochemical proximity indicators to ore; (4) immobile element discrimination of altered volcanic rocks from systematic pXRF analysis; (5) age constraints from U-Pb zircon geochronology of felsic volcanic rocks in the H-W and L-M-P members; and (6) develop useful criteria for exploration in the Myra Falls VHMS district and elsewhere on Vancouver Island.
Detailed and simplified district-scale cross-section interpretations of the West Block Area were generated from underground drilling, in conjunction with company data, to assess the distribution of identified lithological units, alteration, and sulfide mineralisation. Stratigraphy in the West Block Area comprises coherent, volcaniclastic and marine sedimentary rocks of the Price and lower Myra Formations. The Price Formation, at least 80 metres-thick, is characterised by a submarine, extrusive sequence of coherent, andesitic lavas and related breccias. The 150 to 175 metres-thick H-W member (lower Myra Formation) consists of syn- and post-eruptive volcanic and marine-sedimentary rocks. Polylithic siltstone, sandstone and conglomerate of the Basal Volcaniclastic Unit extends west over 500 metres to the Ridge Zone West orebody, where it marks the base of the lower Myra Formation. The Basal Volcaniclastic Unit is overlain by intercalated argillite and chert of the Caprocks Unit. The overlying HW Rhyolite comprises felsic volcaniclastic rocks, which are overlain by coherent, quartz and feldspar-phyric rhyolite, 10 to 40 metres in thickness. Coherent rhyolite domes of the West Block Area extend east over 500 metres to the Battle orebody and north over 250 metres to the Ridge Zone North orebody. A 10 to 50 metre-thick, plagioclase and pyroxene-phyric coherent andesite lava is the youngest extrusive unit of the H-W member, and overlies the HW Rhyolite, extending >500 metres to the west, above the Ridge Zone West orebody. The Hanging Wall Andesite member unconformably overlies the H-W member, and consists of polylithic, volcaniclastic sandstone and conglomerate, locally with clasts of massive sulfide and chert.
Polymetallic sulfide mineralisation in the West Block Area occurs in altered andesite of the Price Formation (footwall) and a rhyolite-dominant, volcano-sedimentary sequence of the lower Myra Formation. Styles of sulfide mineralisation are disseminated, stringer, semi-massive and massive. Disseminated sulfide is pyrite-rich, and occurs in altered, Price Formation andesite and all rock types of the H-W member. Sphalerite-rich and pyrite-dominant stringer-style mineralisation is present in altered coherent and volcaniclastic rhyolite of the HW Rhyolite. Less common in the West Block Area are semi-massive and massive zinc-rich, polymetallic, mineralisation styles, which are hosted in altered coherent rhyolite and autobreccias of the HW Rhyolite, and polylithic conglomerate of the Basal Volcaniclastic Unit.
Volcanic-hosted massive sulfide mineralisation is located at two stratigraphic positions — upper zone and contact zone. The “upper zone” is stringer-style, semi-massive and massive sulfide mineralisation, 30 to 75 metres above the Price Formation contact. The “contact zone” comprises semi-massive and massive sulfide mineralisation at or near the Price Formation contact. Mineralisation consists of sphalerite, pyrite, chalcopyrite, and galena with accessory tennantite-tetrahedrite, stromeyerite and pearceite-polybasite. Precious metal minerals are electrum and hessite. Four paragenetic stages of mineralisation are proposed. Stage-1 includes early, inclusion-rich pyrite. Stage-2 consists of inclusion-free pyrite rimming Stage-1 pyrite accompanied by the precipitation of sphalerite, with minor galena-chalcopyrite-barite ± tennantite and tetrahedrite. Stage-3 is characterised by the modification of Stage-2 mineral phases in the form of grain coarsening, homogenisation of sphalerite(?), and local remobilisation of sulfide and sulfosalt phases. Stage-4 encompasses metamorphic remobilisation with pearceite-polybasite veinlets crosscutting Stage-2 and -3 mineral assemblages. The timing of electrum and hessite precipitation is unresolved, and could have occurred during Stage-3 and/or Stage-4.
Determining the intensity, zonation and extent of hydrothermal alteration associated with VHMS deposits can provide important implications for mineral exploration. A laterally continuous footwall alteration zone extends beneath the West Block Area and the Battle, Ridge Zone North and Ridge Zone West orebodies. Alteration of the Price Formation is mineralogically and texturally diverse. Weak alteration, defined from the Thelwood Valley locality, consists of a pervasive, texturally non-destructive, chlorite-calcite ± epidote alteration. Moderate alteration, defined from the West Block Area, consists of pervasive, texturally nondestructive, chlorite-calcite-pyrite and chlorite-sericite alteration. Intense alteration occurs in the footwall below the Ridge Zone North and Battle orebodies, and consists of feldspardestructive, sericite-quartz-pyrite, chlorite-rich, and chlorite-sericite-pyrite alteration.
Footwall alteration associated with the West Block Area, Ridge Zone North and Battle orebodies has several mineralogical and geochemical characteristics that show systematic changes with increasing proximity to ore. The Fe/Fe+Mg of chlorite decreases from 0.49-0.27 in least to moderately altered andesite to <0.20 in intensely altered andesite immediately below ore. In general, K-mica compositions shift from phengitic in least-altered andesite to muscovitic in intensely altered andesite, proximal to ore. Least-altered footwall alteration whole-rock lithogeochemical trends, include elevated Na\(_2\)O, Rb/Sr <0.1, and moderate alteration index (AI) and Chlorite-Carbonate-Pyrite Index (CCPI) values that plot in the diagenetic field of the alteration box plot. Proximal, within 100 metres to ore, lithogeochemical trends include depleted Na\(_2\)O (≤0.75 wt. %), variably elevated S (up to 16.0 wt. %), Rb/Sr >1.0, AI values >80, and CCPI values from 35-95. Distal, up to 500 metres away from ore, lithogeochemical signatures are characterised by variable Na\(_2\)O and S values, and Rb/Sr from 0.1-1.0. Alteration Index and CCPI values, that plot in the hydrothermal alteration field on the alteration box plot, form an array from the least-altered andesite field to the chlorite-pyrite and white mica fields. As these mineralogical and lithogeochemical alteration trends change systematically relative to the position of ore they can be used in the exploration for other deposits in the district.
A new method for lithological discrimination of altered volcanic rocks based on systematic portable X-ray fluorescence (pXRF) analysis of drill core has been developed. A compilation of published conventional XRF lithogeochemical data from the Myra Falls VHMS district shows robust and discrete Ti/Zr trends for coherent volcanic rocks. Single-spot pXRF analysis of pressed powder dill core samples and three-spot pXRF analyses measured from the flat, cut and clean surface of drill core samples were compared with conventional XRF results from the same sample sets. Both pXRF sampling methods reproduce the laboratory XRF results for Ti and Zr, and there was no significant improvement in accuracy or precision between drill core powders and unprepared drill core samples. Calibration, estimation of total measurement uncertainty, and data reduction procedures for systematic three-spot pXRF analysis of drill core samples were developed to improve lithological logging of altered volcanic rocks. Portable- XRF analysis, combined with detailed logging of volcanic lithofacies can improve geologic and stratigraphic interpretations, which are vital for developing mineral exploration models for VHMS deposits and other economic mineral systems.
Previous attempts to date the volcanic rocks at Myra Falls have resulted in relatively imprecise crystallisation ages that do not resolve the temporal relationship between the L-M-P and H-W members. New U-Pb zircon CA-ID-TIMS results from the HW Rhyolite and LA-ICPMS results from the LMP Rhyolite, combined with published age constraints and stratigraphic relationships, confirm at least two phases of felsic volcanism at Myra Falls that were episodic over a period of ~7 million years. High precision CA-ID-TIMS results indicate that VHMS mineralisation in the Ridge Zone North orebody is hosted in 362.4 ± 0.4 Ma coherent rhyolite. For the first time, the felsic volcaniclastic stratigraphy of the L-M-P member is constrained by U-Pb zircon geochronology, with LA-ICPMS results providing a maximum deposition age of 355.5 ± 2.8 Ma for the LMP Rhyolite of the Price orebody. These age constraints at Myra Falls correlate with previously dated VHMS deposit stratigraphy in the Cowichan Lake uplift on Vancouver Island. Future VHMS mineral exploration in the Sicker Group on Vancouver Island should consider ~362 Ma felsic stratigraphy located within 200 metres of the upper contact of the Price and Nitinat Formations, as well as ~355 Ma felsic stratigraphy located in the top 100 metres of the Myra and McLaughlin Ridge Formations.
This research advances the overall understanding of the H-W member stratigraphy, the nature of hydrothermal alteration, and the temporal relationship of felsic host rocks at Myra Falls. These advancements have implications for the advancement of mineral exploration in the Myra Falls district and throughout Vancouver Island. In short, VHMS exploration targeting in the belt should focus on felsic volcanic stratigraphy (Ti/Zr ratios between 9-12) with U-Pb zircon crystallisation ages of approximately 362 Ma and 355 Ma. The siliceous Caprocks unit is a distinguishing lithological feature for the H-W member VHMS orebodies, therefore mineral exploration should also target Late Devonian marine basin depositional environments intercalated with felsic volcanic deposits and fine-grained marine sedimentation.

Item Type: Thesis - PhD
Authors/Creators:McNulty, BA
Keywords: geochronology, lithogeochemistry, pXRF, VHMS, Myra Falls
DOI / ID Number: 10.25959/100.00034062
Copyright Information:

Copyright 2019 the author

Additional Information:

Chapter 6 appears to be the equivalent of a post-print version of an article published as: McNulty, B. A., Fox, N., Berry, R. F., Gemmell, J. B., 2018. Lithological discrimination of altered volcanic rocks based on systematic portable X-ray fluorescence analysis of drill core at the Myra Falls VHMS deposit, Canada, Journal of geochemical exploration, 193, 1-21

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