Sedex deposits typically occur in marine sediments of the rift sag phase of epicontinental and intracontinental rifts. However, the Sullivan deposit of Canada and the Broken Hill and
Cannington deposits of Australia occur in sediment-sill complexes of the rift fill, and are major exceptions to the general rule. Although there are no unequivocal examples of Sedex deposits occurring in the rift fill and rift sag phases of the same
rift, current genetic models for Sedex deposits do not preclude this possibility. The Geological Survey of Canada’s TGI-3 Programme has supported several studies aimed at assessing whether the rift sag phase of the Belt-Purcell rift shows evidence of
synsedimentary tectonism and/or hydrothermal activity that would enhance the potential for other Sedex deposits in the district. Two such studies, based on radiometric dating, are:
1. Dating of hydrothermal events within the Sullivan
The sequential mud volcano activity; tourmalinization; sulphide precipitation; pyrite-carbonate replacement in the "Iron Core" and hanging wall albitization indicate that the hydrothermal conduits for the Sullivan deposit were
repetitively re-utilized by successive hydrothermal events (Hamilton et al, 1982) during the 1470-1468 Ma time interval (e.g. Schandl and Davis, 2000). More recent investigations involving isotopic dating have revealed hydrothermal “events” that span
the Mesoproterozoic (e.g. Beaudoin, 2000), Jiang et al, 2000, Slack et al., 2008). Refinement of the ages of these late hydrothermal events evident in the Sullivan deposit should therefore indicate the timing (and hence stratigraphic level) of
hydrothermal movement within the Purcell basin during deposition of the rift sag phase of the Belt-Purcell Basin.
Sm-Nd dating of a suite of 19 sulphide-rich samples from the Sullivan deposit, showing replacement or epigenetic textural features,
was undertaken to test this hypothesis. Samples locations include the core, Transition Zone, and eastern margins of the deposit, and the Kimberley fault zone. The non-calcitic fraction of all samples give poor Mesoproterozoic Sm/Nd isochrons that do
not reliably distinguish their different ages. However, the analyses show that the Sullivan deposit is zoned with respect to REE in much the same way that it is zoned with respect to the ore metals. This demonstrates that REE are mobile under
hydrothermal conditions. The results also demonstrate the important role of subsurface zone refinement for the characteristic chemical zoning in Sedex deposits, because the bulk of the REE was originally contained in the siliciclastic detrital
component of the ores,.
The calcitic fraction of the samples gives an age of 470± 20 Ma. At 470 Ma, the calcite had the same 133Nd/144Nd ratio as the non-calcitic fraction. The preliminary interpretation is that the calcite reequilibrated with a
fluid with high Sm/Nd ratio at 470 Ma. One of the few sources of fluids with high Sm/Nd ratios are pore fluids of the deep part of sedimentary basins below the smectitite-illlite transition (e.g. Smack & Awwiller, 1990). More than 6 km thickness of
Lower Paleozoic sediments are preserved below the pre-Upper Devonian unconformity to the northeast of Sullivan (Price and Sears, 2000), and it is suggested that fluids expelled by compaction of such a sedimentary basin infiltrated the Sullivan
deposit. The Early Paleozoic age for migration of these fluids has regional importance to understanding Zn-Pb
metallogeny in southeastern British Columbia.
2. Dating of host rocks to the Legion (Wild's Creek) Zn-Pb-Ba deposit.
deposits with characteristics of Sedex deposits occur in carbonate rocks (mapped as being part of the Dutch Creek Formation) along the western limb of the Purcell anticlinorium. However, their metallogenic significance is enigmatic. For example, lead
and sulphur isotope compositions of the Mineral King deposit are similar to Paleozoic Sedex deposits of Selwyn Basin, and its carbonate host rocks for the Mineral King may be a thrust slice of Devonian rocks (Lydon and Graf, 2000). Guided by the
mapping of Brown and Klewchuck (1995), clastic sedimentary rocks that overly and underly the host carbonate at the Legion Zn-Pb-Ba deposit were sampled to determine if the deposit is hosted by Mesoproterozoic sediments.
A quartz-sericite schist
from the eastern side of the host carbonate contains abundant zircons in the 1490-1610 Ma age range, typical of rocks in the lower Purcell (Ross and Villeneuve, 2003) and upper Purcell (Gardner, 2008) of the western (but not the eastern) part of the
Purcell anticlinorium. Hence, the results are not diagnostic of the stratigraphic level of the sampled horizon.
A quartzite from the western side of the host carbonate contains a predominant 1675-1750 Ma detrital zircon population, similar to the
Mount Nelson Formation sampled on the eastern limb (Gardner, 2008) and northern nose (Ross and Villeneuve, 2003) of the Purcell anticlinorium, and a subordinate 1350-1425 Ma population also present in the sample of Gardner (2008) but not in the
sample of Ross and Villeneuve (2003). The data suggests that the differences in provenance and lithostratigraphy between the eastern and western parts of the Purcell basin that persisted throughout most of the depositional history of the Purcell
Supergroup, had been eradicated by time the Mount Nelson Formation was deposited.
Zircons were not seen in thin section of a mafic volcanic breccia nor a mafic volcanic arenite mapped as being along strike from the host carbonate, and so zircon
separation was not attempted.
The conclusion from this investigation is that the Legion deposit is probably hosted by a Mesoproterozoic carbonate unit that occurs stratigraphically below the Mount Nelson Formation. Collectively, the detrital
zircon data gathered by the TGI-3 Programme emphasizes the difference in sedimentary history between the western and eastern parts of the Purcell basin. In turn, this difference may reflect an original distance between the two that is much greater
than their present distance, and consequently metallogentic events that are related to basin evolution (i.e. Sedex and MVT deposits) may have different timing and expression in the western part of the basin than those in the eastern part.