Non-hydrothermal origin of apatite in SEDEX mineralization and host rocks of the Howard's Pass district, Yukon, Canada

Abstract

The Howard's Pass district (HPD) comprises 14 Zn-Pb sedimentary exhalative (SEDEX) deposits and is located within the Selwyn basin, Yukon, Canada. Although the HPD is renowned for its large accumulation of base-metal sulfides, in places the Late Ordovician to Early Silurian host rocks also contain abundant carbonate-bearing fluorapatite (CBFA). This mineral is present stratigraphically below, within, and above the SEDEX deposits and occurs as fine-grained layers that are interbedded with cherty carbonaceous mudstone. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometric analysis reveal that mineral compositions and rare earth element-yttrium (REE-Y) systematics, respectively, are remarkably similar throughout the stratigraphic succession. North American Shale Composite (NASC)-normalized La/Sm and La/Yb ratios indicate that the original REE compositions in CBFA have undergone only minor compositional modification subsequent to deposition. Uniformly negative Ce anomalies indicate that the mineral formed in analogous manner to modern and ancient sedimentary phosphorites under suboxic bottom-water conditions. Europium anomalies are mostly absent, indicating that reduced, slightly acidic high-temperature hydrothermal fluids were not a major source of REE-Y to CBFA. The chemical homogeneity of the mineral irrespective of its stratigraphic position indicates that a common process was responsible for its deposition within the sedimentary rocks of the HPD. On the basis of the similarity of the REE patterns to modern and ancient phosphorites, and the absence of positive Eu anomalies, we conclude that the CBFA is of hydrogenous origin, and not hydrothermal as suggested by previous workers. As such, phosphorite formation in the HPD is casually related to SEDEX Zn-Pb deposit formation.