U-Pb geochronology and Nd and Pb isotope characteristics of the Au-Ag-rich Eskay Creek volcanogenic massive sulfide deposit, British Columbia

Abstract

The Eskay Creek deposit is an Au-Ag-rich volcanogenic massive sulfide deposit located within the Iskut River area of northwestern British Columbia. At Eskay Creek beds of Au-Ag-rich clastic sulfides and sulfosalts hosted within the contact argillite are underlain by the variably altered Eskay rhyolite, which hosts stockwork mineralization, and overlain by the barren hanging-wall basalt. The Eskay rhyolite has yielded a Middle Jurassic U-Pb zircon age of 175 + or - 2 Ma. Interaction textures between the Eskay rhyolite and contact argillite, which imply emplacement of the rhyolite prior to lithification of the argillite, along with probable local derivation of bedded sulfide clasts indicate a Middle Jurassic age for mineralization, some 7 to 16 m.y. younger than intrusion-related Lower Jurassic mineralization within the region. The data presented in this study demonstrate that there is a second and significant period of Jurassic mineralization within the Iskut River area.Initial epsilon Nd values of 5.5 to 6.9 for the Eskay rhyolite and hanging-wall basalt lie within the field for uncontaminated island-arc volcanic rocks and are consistent with formation in a subduction-related environment. Ratios of immobile elements and REE patterns for the Eskay rhyolite suggest a tholeiitic magmatic affinity, whereas unmineralized rhyolite of the same age (174 (super +2) (sub -1) Ma; U-Pb zircon) is characterized by a slightly less primitive Nd isotope signature (4.3) and a transitional to calc-alkaline affinity.Sulfides and sulfosalts from clastic and rhyolite-hosted mineralization in and along strike from the deposit exhibit a homogeneous Pb isotope signature. The Pb isotope composition of Eskay Creek sulfides and sulfosalts is inconsistent with leaching of lead from Paleozoic to Mesozoic arc assemblages which may occur at depth below the Eskay Creek deposit, but is consistent with derivation from a Lower or Middle Jurassic source.