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.
The Sullivan deposit and two deposits of the Anvil district (i.e., Faro and Grum) are
past-producers. Other deposits have seen or are undergoing intensive exploration and development work (e.g., Howard's Pass, MacMillan's Pass, Drifpile, Akie, and Cirque).
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 rift-controlled 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 occur in intracratonic and epicratonic rifts within reduced marine basins.
The architecture of sedimentary basins is characterized by rapid syn-rift subsidence and clastic sedimentation that is overlain by fine-grained clastic and carbonate sediments that accumulated during a post-rift sedimentation phase. Marine turbidites
with interlayered mafic sills of the syn-rift phase host the Sullivan deposit, whereas organic-rich carbonaceous shales/mudstones of the post-rift phase host deposits of the Selwyn Basin. The deposits most likely formed at or just below the sea floor
from warm to hot (~100° up to 300°C), saline (10 to 20% NaCl equiv.) basinal brines that ascended along basin-controlling synsedimentary faults. Deposition and sequestration of metals occurred by precipitation of sulphide minerals as a result of
mixing of metal-transporting brine with locally derived H2S produced by bacterial (and perhaps thermochemical) reduction of local seawater sulphate.
Important exploration vectors include the presence of:
Deep-seated synsedimentary faults
expressed as abrupt changes in facies and isopachs, intraformational breccias, slumps, debris flows, and fault scarp talus.
Recognition of local fault-controlled paleoenvironments that represent restricted and stratified basins with anoxic
H2S-rich bottom waters.
Organic-rich sediments with >1% Corg.
Anomalous concentrations of redox-sensitive trace elements (e.g., V, Tl, Cd, U, V/Mo, and Re/Mo).
Widespread hydrothermal alteration (muscovite, carbonates, and silicates).
Laterally and vertically extensive distal sediments that are mineralogically and chemically zoned around seafloor vents.