The Geology and ore deposits of the Howards Pass Area, Yukon and Northwest Territories : the origin of basinal sedimentary stratiform sulphides deposits
Creators: name: Morganti, John Michael
Publisher: name: University of British Columbia
Year: 2010

description: Economically important sedimentary-type stratiform Zn-Pb deposits exist in the Lower Silurian basinal facies of the Selwyn Basin along the Yukon-Northwest Territories border. Three major similar deposits have been defined to date, and are referred to collectively as the Howards Pass deposits. The Pre-Mesozoic history of the Howards Pass area was dominated by three major basin systems. During the Late Hadrynian to Early Cambrian the 'Grit Unit' was deposited as a clastic wedge with a western source area. Initially deep water turbidites were deposited but continued filling in of the basin led to shallow water deposition. During the Late Cambrian to Early Devonian the Rabbitkettle and Road River groups were deposited in the Selwyn Basin. The Selwyn Basin is defined by shallow water carbonates on the east and a general deep water environment to the west. The carbonates were succeeded by hemipelagic and pelagic organic-rich sediments with occasional terrigenous material deposited by geostrophic currents. Within the Selwyn Basin the Ordovician to the Early Silurian Howards Pass formation contains three major facies from east to west; a slope facies, a base of slope facies and a chert basin facies. The Howards Pass deposits occur in sub-basins in the base of slope facies. The third major basin system in the region was associated with uplift to the west and is represented by the Earn Group. In this later basin turbidity currents associated with submarine fans deposited clastic material derived from the uplifted centre of the Selwyn Basin and underlying rocks. Major barite deposits occur in the Earn Group. The individual Howards Pass deposits consist of complex saucer shaped bodies containing laminated to massive sulphides occurring in the Lower Silurian active member of the Howards Pass formation. The deposits are characterized by simple sulphide mineralogy, predominantly sphalerite, galena and pyrite. The sulphides in the deposits may be divided into six textural types which aid in characterizing the sulphides geologically and metallurgically. Types I, II, and III consist of laminated sulphides, types IV and V consist of laminated to massive sulphides and type VI consists of late diagentic concretionary sulphides. Types I thru V are associated with specific lithofacies in the active member. The Howards Pass deposits show characteristics common to stratiform-sedimentary deposits such as conformity with bedding, no obvious association with volcanic rocks, similar age of the three major associated deposits, single-stage Pb isotope systematics and association with organic-rich sedimentary rocks. In contrast differences between the Howards Pass deposits and other stratiform-sedimentary deposits, include deposition in a starved basin sedimentary environment, a lack of any associated feeder zone within 10 km, a lack of massive pyrite associated with the deposits, relatively low Ag and Cu associated with the deposits and a lack of bedded barite near the deposits. This datum suggests that the Howards Pass deposits are unique, and therefore a model is proposed which is relevant to the geologic setting. The most important part of the model is the synsedimentary deposition of Zn and Pb sulphides within sub-basins occurring at the base of slope of the eastern edge of the Selwyn Basin. The formation of biogenetic sulphide from sea water sulphate is suggested by the sulphur isotope data. This data also supports the existence of an ideal cycle of the active member and suggest a possible sub-basin evolution during an individual cycle. The origin of the metals in the deposit is not clear, but the association of volcanic tuffs near the shale-out at the eastern margin of the Selwyn basin suggests that warm fluids may have been expelled onto the sea floor, migrated down slope and collected in the topographically low sub-basins, or possibly compaction fluids may have been expelled directly into the sub-basins and concentrated during brine evolution. The abundant evidence for slumping and later folding and the possibility of both ordinary and more radiogenic Pb leads to the conclusion that post-depositional mobilization of both sediment and sulphide was important in the final location of Zn and Pb. Exploration for Howards Pass-type deposits, based on the above model, should emphasize the sedimentary nature of the deposits. Regional stratigraphic exploration should be aimed at defining major, platform-marginal, starved basins. The importance of defining the paleogeography, such as the base of slope and specific sub-basins, appears to be critical. This method emphasizes the petroleum methodology of looking for traps, although in the present case these are paleogeographic sulphide traps.