Zonation of chalcophile elements about the howard's pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon

Abstract

ABSTRACT Goodfellow, W.D., Jonasson, I.R. and Morganti, J.M., 1983. Zonation of chalcophile elements about the Howard's Pass (XY) Zn-Pb deposit, Selwyn Basin, Yukon. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 503–542. The Selwyn Basin comprises those rocks of Paleozoic age that were deposited in a fault-controlled epicratonic basin or aulacogen bounded by the Mackenzie Platform on the east and north, and the Cassiar Platform to the west. Boundaries between sedimentary facies within the Selwyn Basin during Ordovician to Middle Silurian time were parallel to and symetrically disposed about the long axis of that basin, consisting of chert in the central and deeper zone and shales in the flanking zones. Conodont dates indicate that the Howard's Pass (XY) Zn-Pb deposit formed during Llandoverian time in a local third-order basin within the shale facies. Various geological features such as abrupt facies changes, local thickening and slumping of sedimentary units suggest that growth faults played an important role in basin development as well as serving as conduits for the discharge of ore-forming fluids. The only sulphides recognized to date at the XY deposit are sphalerite, galena, chalcopyrite and pyrite. Sphalerite and galena can occur as concordant, fine laminations, or remobilized into sedimentary dewatering structures, or transposed and remobilized into a tectonic (slaty) cleavage. Pyrite most commonly forms diagenetic microframboids which postdate the laminated sulphides but predate the sphalerite and galena emplaced in dewatering structures. Within the main ore-bearing unit, i.e. the Active Member, the Pb/(Pb+Zn) ratio decreases stratigraphically upwards and laterally towards the margins of the deposit. This zonation accompanies a change in host-rock composition from cherty limestone at the base to carbonaceous chert towards the top of the Active Member. In addition to Zn and Pb, the other chalcophile elements concentrated in the Active Member are Cd and Hg, both of which are bound isomorphously in sphalerite. Both ore-forming and ore-associated elements are zoned stratigraphically and laterally in the host rocks of the XY deposit. The footwall carbonaceous mudstones are enriched in Ni, Cu, Co, Mo, As, Sb, Zn, Cd, Hg and Pb relative to rocks of similar age and composition located remote from mineralization. Ni, Cu, Co, Mo, As and Sb now reside in diagenetic pyrite but in whole rock chemical analyses show a strong positive correlation with organic C. On the other hand, Zn, Cd, Hg and Pb are bound in sphalerite or galena. By comparison, the overlying phosphatic chert is enriched in Ni and Cu only, both of which correlate positively with organic C. The zonation of these elements reflects a decrease in pyrite content in the rocks and also a decrease in trace-element contents of pyrite itself away from the XY deposit. On the basis of extremely low S/C ratios, the low pyrite content outside the XY deposit is thought to be due to a lack of sufficient Fe2+ to fix S2- generated by bacterial reduction during diagenesis. The higher content of Fe near the XY deposit probably reflects Fe debouched onto the seafloor during hydrothermal activity. The increased trace-element content of pyrite in rocks hosting the XY deposit is due most likely to elevated levels of these elements accumulated by dead organic matter from ore-forming fluids during sedimentation. A period of Devonian hydrothermal activity in the Howard's Pass area can be recognized by elevated levels of Zn, Hg, Sb, Ag, As, Mo and Ni in highly carbonaceous and calcareous rocks. The spatial association of these Devonian metalliferous mudstones with the underlying XY deposit suggests that structures which served as conduits for the discharge of ore-forming fluids during the Lower Silurian were reactivated in Devonian time.