Update on the SEDEX model for deposits of the Canadian Cordillera, Yukon and British Columbia

Abstract

Listen

Translate article

SEDEX (SEDimentary EXhalative) deposits are important resources of Zn and Pb. In addition to Zn and Pb, other potentially economic commodities are: Ag, Au, Cu, Cd, Sb, Sn, and barite. Major metallogenic districts in the Canadian Cordillera that host SEDEX deposits are: - Mesoproterozoic Sullivan district in southeastern British Columbia, which hosts the world-class Sullivan deposit and other smaller deposits such as North Star and Kootenay King. - Late Cambrian Anvil district in the Selwyn Basin of central Yukon, which hosts the Faro, Grum, Vangorda, DY, and Swim deposits. - Early Silurian Howard's Pass district in the Selwyn Basin of the northeastern Yukon, which hosts the world-class Howard's Pass deposits (XY, Brodel, HC, Don, Anniv, OP, Pelly North). - Late Devonian Gataga district of the Kechika Trough (southern extension of Selwyn Basin) in northeastern British Columbia, which hosts the Cirque, Driftpile, and Akie deposits. - Late Devonian MacMillan's Pass district in the Selwyn Basin of northeastern Yukon, which hosts the Tom and Jason deposits. SEDEX deposits are defined as being predominantly composed of Zn and Pb hosted in sphalerite and galena that were deposited at or near the seafloor from basinal metalliferous fluids discharged into riftcontrolled anoxic sedimentary basins. They consist of vent-distal and vent-proximal facies. The former is composed of interbedded sphalerite, galena, iron sulphides and clastic sediments, and the latter of variably veined, infilled and replaced bedded sulphides. Cordilleran SEDEX deposits formed in settings with dynamic redox fronts controlling sulphide and sulphate precipitation within carbonaceous sediments. Zinc, Pb, Cu and other metals are leached from deeply buried clastic sediments during metamorphic mineralogical transformations driven by increasing temperature and pressure during burial. At the Howard's Pass (HP) deposits (Yukon), sulphides precipitated from dense bottom-hugging metalliferous brines that accumulated in a bathymetric low, distal to vent complex(es), and percolated into porous unconsolidated sulphidic carbonaceous muds. At the MacMillan Pass (MP) deposits (Yukon), hydrothermal sulphides precipitated sub-seafloor, proximal to vent complex(es), due to interaction of hot (>250°C), acidic (pH ? 4.5) metal-bearing hydrothermal fluids with H S generated during a number of processes (bacterial 2 and thermochemical sulphate reduction, barite dissolution, and sulphate reduction coupled with anaerobic methane oxidation) in the carbonaceous mudstones. At HP and MP, most of the Zn-Pb mineralisation was precipitated below the seafloor as replacement of early barite (at MP) and finegrained sediments during early diagenesis. Close proximity to a carbonate platform may have been an important factor for the mineralising systems, enhancing access to sources for saline, metal complexing brines. Factors that have potential application in the search for SEDEX include the presence of: 1. Deep-seated synsedimentary faults expressed as abrupt changes in facies and isopachs, intraformational breccias, slumps, debris flows, and fault scarp talus. 2. Sedimentary basins hosting organic-rich sediments with >1% C ; adjacent to carbonate platform. org 3. Ore-stage diagenetic pyrite that are texturally sooty (i.e. inclusion-rich) and anomaleous in Tl, As, Sb and possibly Mn. 4. Anomalous concentrations of redox-sensitive trace elements (e.g. V, Tl, Cd, U, V/Mo, Re/Mo) Mo, Re/Mo, and Ce/Ce* in the host rocks. 5. Widespread hydrothermal alteration (muscovite, carbonates, and silicates). 6. Laterally and vertically extensive distal sediments that are mineralogically and chemically zoned around seafloor vents. 7. Regional euxinic condition is not a prerequisite for the formation of SEDEX deposits. 8. Basin has high biological productivity; should include outer shelf and slope settings and not focus exclusively on anoxic or sulphidic basins.