Geology and lithogeochemistry of the Fyre lake copper-cobalt-gold sulphide-magnetite deposit, southeastern Yukon

The Fyre Lake sulphide-magnetite deposit is located in the Finlayson Lake massive sulphide district in the Yukon-Tanana Terrane, southeastern Yukon Territory, Canada. It is hosted by quartz-chlorite-actinolite schist derived from (probable) Devono-Mississippian-aged mafic volcanic rocks. Overlying the deposit are intercalated metasedimentary and metavolcanic rocks. This mixed sequence is, in turn, overlain by a thick sequence of graphitic phyllite. The deposit consists of three northwest-trending stratiform lenses comprised of massive and semimassive sulphide and magnetite iron formation. Pyrite is the dominant sulphide mineral in the deposit, with lesser amounts of pyrrhotite, chalcopyrite, and locally, sphalerite. The sulphide mineralization is copper- and cobalt-rich and locally contains significant concentrations of zinc and gold. It has low to trace amounts of lead, barium, arsenic, antimony, tin and selenium. Lithogeochemical results indicate that the host mafic metavolcanic rocks are strongly depleted of light rare-earth elements (LREE) and high field strength elements (HFSE), and contain elevated levels of MgO, Ni and Cr. SiO2 in the host rocks ranges from 53 to 58%. Chemically, the mafic metavolcanic rocks are similar to boninitic rocks found in some suprasubduction zone ophiolites such as those at Cyprus. The primitive boninitic chemistry of the host metavolcanic rocks implies that they originated as melts from a depleted mantle in a rifted setting. The presence of felsicmetasedimentary rocks below, within and overlying the host metavolcanic rocks suggests that the Fyre Lake deposit was formed in or near a mature tectonic setting, possibly a continental arc or an evolved island arc. The host mafic metavolcanic rocks are chemically distinct from other mafic and intermediate metavolcanic rocks that outcrop on the Fyre Lake property. A clastic metavolcanic rock unit lies stratigraphically below the deposit. It consists of fragmental LREE- and HFSE-enriched transitional subalkaline basalts. Other discontinuous bodies of metavolcaniclastic and metaflow rocks occur stratigraphically above and peripheral to the Fyre Lake deposit. These are enriched in LREE and Th, and relatively depleted in Nb, Ta and Ti, and generally display the chemical features of transitional basalts and andesites erupted in a continental arc or evolved island arc setting.