| Abstract | Polymetallic metalliferous shales are an important global resource for Zn, Ni, Cu, Mo, Se, U, V, ± Cr, Co, Ag, Au, platinum group elements (PGE) and rare earth elements (REE). The deposits are typically
thin (<10 cm) and laterally widespread, covering 1000s of km2. Yukon hosts some of the best examples in the World and the Nick Ni-Zn-Mo-PGE deposit is the best-known example in Canada. However, many basic questions remain unresolved regarding the
formation of these deposits, including the host mineral(s) for the PGE. Our research aims to determine the residence and mineralogical associations of PGE and other elements in the Nick deposit and several other localities with similar
mineralization.
Mineralization is constrained to a thin (<10 cm) layer that is located at the contact between the Cambrian to Middle Devonian Road River Group and the Middle to Upper Devonian Canol Formation. Within this layer, the maximum
metal abundances are as follows: Ni (7.0 wt.%), Mo (0.33 wt.%), Zn (2.3 wt.%), Pt (511 ppb), Pd (202 ppb), Ir (10.9 ppb), Ru (12 ppb), Re (25.8 ppm), Se (0.57 wt.%) and As (1.1 wt.%). The semi-massive sulfides consist predominantly of vaesite (NiS2),
pyrite and sphalerite, with lesser millerite (NiS) and gersdorffite (NiAsS). Gangue minerals are primarily quartz, barite, apatite and trace to minor K-(Ba) feldspar and pyrobitumen.
Vaesite shares mutual boundaries with pyrite, and typically
forms 0.1 to 0.2 mm-thick encrustations on pyrite or occurs as minute grains within pyrite nodules. Less commonly, vaesite occurs as discrete, anhedral grains. Preliminary laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace
element maps indicate that vaesite is the principle PGE host mineral; these data show the highest abundances of Ru (up to 1 ppm), Pd (up to 10 ppm), Os (up to 2 ppm) and Pt (up to 5 ppm) in vaesite, which suggest that PGE- and Ni-bearing sulphides
were deposited coevally. Although Ir is present in relatively high abundances in the lithogeochemical data, our preliminary LA-ICP-MS data do not identify its host mineral, and the residence site(s) for Ir currently remain unresolved. Ongoing
research will continue to investigate the nature and residence of PGE at the Nick deposit and other, similar types of deposits in Yukon. Our salient goal is to develop an internally consistent model that describes the striking metal enrichment within
this regionally significant layer. |