Lead- and strontium-isotope geochemistry of Paleozoic Sicker Group and Jurassic Bonanza Group volcanic rocks and Island Intrusions, Vancouver Island, British Columbia

Abstract

Whole-rock and galena lead-isotope analyses have been obtained from the Sicker Group Paleozoic island-arc volcanic package and from a Jurassic island-arc represented by the Bonanza Group volcanics and Island Intrusions. Galena lead-isotope analyses from the volcanogenic ore deposits at the Buttle Lake mining camp in the Sicker Group provide estimates of the initial lead ratios for the Sicker Group. Lead-isotope signatures are uniform within each of the major orebodies, but the Myra orebody is less radiogenic than the older H–W orebody. This has major significance in terms of ore genesis for these important deposits.There are significant differences in isotopic composition between the Sicker Group and Devonian island-arc type rocks in the Shasta district, California, which rules out direct correlations between the rock units of these two areas. Relatively high initial values of 207Pb/204Pb (> 15.56) and 208Pb/204Pb (> 38.00) suggest that large quantities of crustal lead must have been involved in the formation of the Sicker Group volcanic rocks. Thus it is proposed that the trench related to the Paleozoic island arc had a substantial input of continental detritus and may have lain near a continent.The Jurassic island arc is characterized by low 207Pb/204Pb ratios (< 15.59), suggesting a more primitive arc environment than for the Paleozoic arc. Bonanza Group volcanic rocks contain lead that is less radiogenic than lead in the Island Intrusions. Present and initial lead-isotope ratios of both the Bonanza Group volcanics and Island intrusions follow the same trend, supporting the hypothesis that they are comagmatic. Lead isotopes from a galena vein within the Island Copper porphyry deposit plot with the initial ratios for Bonanza Group volcanics and Island Intrusions. This confirms the hypothesis that this mineralization is related to the Jurassic island-arc volcanic event.Initial lead-isotope ratios for the Jurassic rock suite form a linear array on both 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb plots. If interpreted as due to isotopic mixing, the more radiogenic end member has a composition that is lower in 207Pb/204Pb and higher in 206Pb/204Pb than typical upper continental crust. Assimilation of Sicker Group material during the emplacement of the Jurassic arc can explain the mixing trend.

Geologic Map of Baranof Island, southeastern Alaska

“…There is a similar range of K-Ar and 40 Ar/ 39 Ar ages in Glacier Bay and the Chilkat Mountains, which suggests a regional source for the Eocene-Oligocene igneous activity. These ages also correspond in age with the Catface intrusions dated at 41 Ma on Vancouver Island (Isachsen, 1987;Andrew and Godwin, 1989;Cowan, 2003). The tectonic setting of these intrusions is not clear.…”

“…Unconformably beneath the Late Triassic Karmutsen Formation on Vancouver Island, the Sicker Group consists of pre-Middle Devonian calc-alkaline volcanic and volcaniclastic rocks of the Duck Lake, Nitinat, and Price Formations and Devonian to mid-Permian chert, tuff, rhyolite, and basalt breccia of the Myra, Thelwood, Flower Ridge, and McLaughlin Ridge Formations, which are interpreted to represent island arc magmatism (Andrew and Godwin, 1989;Muller, 1980;Jones and others, 2006;Ruks and others, 2010). The siliceous metasedimentary and metavolcanic rocks in this sequence are thinly banded and resemble the thinly banded siliceous lithologies in unit sv on Baranof Island.…”

Lead isotope systematics of sulfide minerals in the Middle Valley hydrothermal system, northern Juan de Fuca Ridge

“…Comparison of the Pb isotopic composition of massive sulfides from Middle Valley with those of major Besshi‐type mining districts [ Andrew et al , 1991; Andrew and Godwin , 1989a; Andrew and Godwin , 1989b; Andrew and Godwin , 1989c; LeHuray , 1984; McCutcheon et al , 1993; Peter and Scott , 1997; Sato and Sasaki , 1976; Swinden and Thorpe , 1984] and other modern seafloor hydrothermal vent deposits [J. Peter, unpublished data, 1998; Hegner and Tatsumoto , 1987; LeHuray et al , 1988]. Stacey‐Kramers average continental crust evolution curve is shown with crosses every 0.5 Ga. NHRL, Northern Hemisphere Reference Line [ Hart , 1984].…”

“…Recent U-Pb studies of the Monashee Complex, southern British Columbia, show that some of these metamorphic rocks include zircon grains that are as old as 2.3 Ga (J. Crowley, personal Figure 6. Comparison of the Pb isotopic composition of massive sulfides from Middle Valley with those of major Besshi-type mining districts [Andrew et al, 1991;Andrew and Godwin, 1989a;Andrew and Godwin, 1989b;Andrew and Godwin, 1989c;LeHuray, 1984;McCutcheon et al, 1993;Peter and Scott, 1997;Sato and Sasaki, 1976;Swinden and Thorpe, 1984] and other modern seafloor hydrothermal vent deposits [J. Peter, unpublished data, 1998;Hegner and Tatsumoto, 1987;LeHuray et al, 1988].…”

The Middle Valley Sulfide Deposits, Northern Juan de FucaRidge: Radiogenic Isotope Systematics

“…Sulfide deposits investigated from Vancouver Island, from which most of the sediment is derived, show that the source rocks for those deposits are much too radiogenic to form this old component (Andrew and Godwin, 1989). Sulfide deposits investigated from Vancouver Island, from which most of the sediment is derived, show that the source rocks for those deposits are much too radiogenic to form this old component (Andrew and Godwin, 1989).…”