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Volcanic stratigraphy and lithogeochemistry of the Seneca Zn-Cu-Pb Prospect, Southwestern British Columbia McKinley, Sean D.


The Seneca Prospect is a volcanic-hosted Zn-Cu-Pb deposit 120 km east of Vancouver in southwestern British Columbia. Volcanic strata at Seneca form part of the Weaver Lake Member of the Lower to Middle Jurassic Harrison Lake Formation of the Harrison Terrane. The rocks at Seneca comprise four principal facies: 1) vent to vent-proximal facies consisting of basaltic to rhyolitic lavas and associated breccias; 2) vent-proximal to distal facies consisting of volcaniclastic debris flows and siltstones; 3) coeval intrusions consisting of basaltic andesitic to rhyolitic sills and dikes, and 4) distal marine facies consisting of a pumice-bearing argillaceous unit. The volcanic strata can be subdivided into three intervals from bottom to top as follows: 1) the Footwall Interval below the mineralized horizon which comprises subaqueously deposited basaltic lavas and felsic debris flows; 2) the Seneca Horizon which hosts the mineralized zones, and 3) the Hangingwall Interval which is consists largely of felsic flows, intrusions and volcaniclastic rocks. Volcaniclastic rocks in the Footwall Interval are dominated by coarse, poorly-sorted debris flows whereas the volcaniclastic rocks of the Hangingwall Interval are mostly massive to well-bedded ashes and volcanically-derived turbidites. The Seneca volcanic sequence is a bimodal suite of rocks of calc-alkaline to transitional calcalkaline- tholeiitic affinity (Zr/Y ratios >3.5, LaN / YbN ratios > 2.0). Pearce element ratio analysis of the mafic rocks shows that the chemical variation in a least-altered subset can be explained by the fractionation of plagioclase, olivine and clinopyroxene although variation within basaltic and basaltic andesitic subgroups can be explained by plagioclase fractionation alone. Major element variations in the least altered felsic rocks can be explained by fractionation involving the crystallization o f feldspar, quartz ± pyroxene and/or hornblende. Trace element trends can be accounted for by 30 to 40 % fractional crystallization o f the assemblage feldspar-hornblende-magnetite±apatite. Mass change calculations revealed both a vertical zonation and spatial differences in the hydrothermally altered stockwork zones. In general, the stockworks can be subdivided into upper and lower alteration zones. The upper quartz-sericite zone has experienced net mass gain with mass gains of Si02 and K20 and mass losses of Na2O and CaO resulting from silicification and the destruction of feldspar. The lower sericite-chlorite zone has had small net mass gains or losses as a result of mass gains of K20 and MgO and losses of Na2O, CaO and in places Si02 . The MgO gains throughout the Vent Zone are much smaller or absent in the Fleetwood Zone, perhaps indicating that the larger Vent Zone hydrothermal system was more capable of incorporating seawater magnesium than the Fleetwood Zone stockwork systems, which may have been sealed by overlying flows or volcaniclastic sediments. Mineralization in the Pit Area consists of zones of disseminated to conformable massive sphalerite, pyrite, chalcopyrite, galena and barire hosted by the strongly altered ore zone conglomerate (OZC). Stratigraphic relationships indicate that these zones may have formed contemporaneously with the stockwork sphalerite-pyrite-chalcopyrite mineralization in the Fleetwood and Vent Zones. The stockwork zones possibly were vertical conduits for hydrothermal fluids which then migrated laterally through the permeable OZC where they interacted with seawater and formed the Pit Area sulphide mineralization. The volcanic rocks which host the Seneca deposit have geological and geochemical similarities to younger rocks of the Lau Basin and Tofua Arc in the southwest Pacific and the Hokuroku Basin in Japan. These similarities suggest that the Seneca volcanic sequence and sulphide mineralization may be in rifted sub-basins within a calc-alkaline volcanic arc formed at a destructive plate margin involving two oceanic plates with little or no continental crustal influence.

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