Palaeomagnetism of the Howards Pass Zn-Pb deposits, Yukon, Canada

Abstract

SUMMARY The Howards Pass district is located in the Selwyn Basin, Yukon. The district consists of >15 laminated clastic‐dominated (CD) sphalerite–galena (Zn–Pb) deposits in the ‘zinc corridor’ that trends northwest–southeast and extends for ∼35 km. The stratiform mineralized zone, the Active Member, is hosted in carbonaceous cherts and black shales of the Early Silurian Road River Group. Using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization (ChRM) by the core‐magnetization‐angle method from 18 sites (339 specimens) in 18 variably oriented exploration drill hole cores from 6 Zn–Pb mineralized panels. Rock magnetic analyses show that the main remanence carriers are single‐ or pseudosingle‐domain pyrrhotite and titanomagnetite. The deposits’ mean ChRM direction yields a pole position of either ∼170 Ma on the North American apparent polar wander path or ∼162 Ma on a corrected path for the Intermontane Belt (IMB) terranes. A negative palaeomagnetic fold test indicates that the mineralization's ChRM is post‐folding, setting a minimum age for regional metamorphic deformation of ∼170 ± 20 Ma and supporting an Early Jurassic arrival for the IMB's collision and accretion to North America. An autochthonous or para‐autochthonous North American tectonic model is favoured for the ChRM of the metamorphosed Selwyn Basin strata rather than an allochthonous IMB model. Further, the palaeomagnetic age indicates that the coarse‐grained Zn–Pb mineralization in fine fractures that cut the laminated fine‐grained Zn–Pb mineralization at Howards Pass at least was formed by remobilization during Middle Jurassic metamorphism.

Re-Os systematics and age of pyrite associated with stratiform Zn-Pb mineralization in the Howards Pass district, Yukon and Northwest Territories, Canada

“…In contrast, a Middle Jurassic paleomagnetic age was obtained on coarsegrained sulphide-rich rock (Kawasaki and Symons (2012). These authors suggest that the laminated sulphides also possibly, but less likely, formed during the Mesozoic.…”

“…9) and suggests that mineralization occurred during the early Silurian, concurrent with sedimentation or early diagenesis. Formation of the stratiform Zn-Pb deposits during the Jurassic-Cretaceous (Kawasaki and Symons 2012) is ruled out by this isochron age, excluding the remobilization of coarse sulphides into late fractures that likely occurred in the Mesozoic.…”

“…The northeast dip of the cleavage planes and their alignment with northwest-trending regional folds (Gordey and Anderson 1993;Martel 2015) suggest that the coarse-grained sulphides formed during tectonism, the onset of which regionally occurred in the Jurassic to Cretaceous (Dusel-Bacon et al 2002;Mair et al 2006). Furthermore, paleomagnetic dating of coarse-grained sulphide veins in the Howards Pass district indicates that they formed during the Middle Jurassic, leading to the further suggestion that the fine-grained sulphides may also have formed at this time (Kawasaki and Symons 2012). Establishing with certainty the age of the laminated sulphide minerals is therefore critical for developing a robust genetic model for the Howards Pass deposits.…”

Post-Triassic para-autochthoneity of the Yukon-Tanana Terrane: paleomagnetism of the Early Cretaceous Quiet Lake batholith

“…Together the stock and ore give a concordant mid-Cretaceous paleopole for North America, to further indicate that the Selwyn Basin has been an autochthonous part of North America for the past 98 Ma. Finally Kawasaki & Symons (2012) concluded from a paleomagnetic study of the Zn-Pb SEDEX mineralization in six separate tectonic zones of the Howards Pass deposits about 40 km north of Cantung that the basin's Paleozoic strata were accreted as a prism to North America shortly after Early to Middle Jurassic metamorphism. Thereafter the strata were subjected to ∼300 ± 300 km of northward transport with ∼28 • of clockwise rotation on docking.…”