Geology and District-Scale Setting of Tilted Alkalic Porphyry Cu-Au Mineralization at the Lorraine Deposit, British Columbia

Abstract

Lorraine is a copper-gold porphyry deposit (6.4 Mt indicated resource at 0.61% copper and 0.23 g/t gold, and 28.8 Mt inferred resource at 0.45% copper and 0.19 g/t gold) in north-central British Columbia—one of the youngest known alkalic porphyry deposits within the Triassic-Jurassic Quesnel arc. The deposit is hosted by the Early Jurassic Duckling Creek Syenite Complex, an alkalic multiphase intrusive dike complex of pyroxenite, syenite, and monzonite composition. The southern part of the complex hosts numerous copper-gold porphyry prospects in addition to Lorraine; together, they constitute the Duckling Creek district. Main-stage Cu-Au porphyry mineralization at Lorraine is hosted by phase 1 of the Duckling Creek Syenite Complex, an SW-dipping, early intrusive suite of silica-undersaturated pyroxenites and syenites. Mineralized zones are up to 2.5 km long by 500 m wide, in the form of northwesterly elongate, SW-dipping panels that generally parallel the phase 1 intrusive contacts. Mineralization is attributed to “blind” porphyries within the phase 1 sequence, which was tilted after main-stage mineralization. Copper sulfides define deposit-scale zonation patterns of bornite-chalcocite cores rimmed by chalcopyrite-rich and outer, weakly developed pyrite-dominated domains. Alteration patterns and intensity are consistent across mineralized zones. Strong, pervasive K-feldspar-biotite (potassic) core zones are rimmed by albite-diopside-magnetite (calc-potassic) alteration that is pervasive near the center of the zones, but varies laterally outward to vein-style alteration. Pervasive albite alteration (sodic) is locally present and is interpreted to be an early alteration event. Mineralization at Lorraine has been cut by later phases of the Duckling Creek Syenite Complex. A second cycle of pyroxenite-syenite-monzonite intrusion (phase 2) is mineralogically similar to phase 1, but is unmineralized and has vertical intrusive contacts that strike northwest. A change to late, silica-saturated intrusions is marked by subvertical phase 3 silica-saturated leucosyenite and pegmatite dikes. U-Pb zircon and Ar-Ar geochronology constrain the timing of main-stage mineralization at Lorraine to an approximately 1.5 m.y. interval between 180 Ma and 178.5 Ma. Phase 1 intrusions are as old as 181.68 ± 0.95 Ma based on an Ar-Ar age on biotite from pyroxenite, but may range to as young as 180.2 ± 0.28 Ma based on a U-Pb zircon age from a mineralized phase 1 syenite. The upper age limit for mineralization is 178.75 ± 0.21 Ma, based on U-Pb data for a postmineral phase 2 syenite. An alternate interpretation considers the presence of zircon antecrysts in an analyzed phase 1 rock, which leads to a more narrow time window of 0.31 m.y. that includes phase 1 intrusion, main-stage porphyry mineralization, and structural tilting, which was followed by phase 2 intrusion. Magmatic and hydrothermal activity in the Lorraine area persisted until at least 176 Ma, based on the U-Pb zircon age of 176.77 ± 0.36 Ma from a phase 3 leucosyenite dike, and a late biotite-albite-apatite vein with an Ar-Ar biotite age of 175.21 ± 0.91 Ma. Several intrusive-hosted porphyry prospects within the Duckling Creek district share mineralogical and alteration characteristics, as well as relative igneous paragenesis, with Lorraine. The model for magmatic and hydrothermal development at Lorraine may broadly be applied to the district as a whole, providing a geologic framework upon which to base future exploration in the district.