Marg Deposit: Geologic Setting OverviewMarg is a massive sulphide deposit hosted primarily by metavolcanic and metasedimentary rocks of the Devono-Mississippian Earn Group of the Selwyn Basin in central Yukon (Holbek et al., 2001; Hunt, 2002) (Fig. 1). The Marg property is located within map sheets 105 M/15, 105 M16, 106 D/01 and 106 D/02 (Fig. 2).
Marg is one of more than 800 mineral occurrences known in the Selwyn Basin (Heon, 2003), a Neoproterozoic to Paleozoic elongate rift-controlled epicratonic sedimentary basin located between the Mackenzie foreland belt (platform) and the accreted terranes & displaced elements of the ancient North American continental margin (Mair at al., 2006; Goodfellow, 2006). The Selwyn basin was formed in a divergent setting about 760-730 ma and represents structurally complex asymmetrical rift system (Colpron et al., 2003). Gritty quartz sandstone, shale and carbonate rocks of the Hyland Group (older Yusezyu and younger Narchilla formations, Fig. 2) underlie the northwestern Selwyn Basin and are believed to be late Neoproterozoic to Early Cambrian age (based on primitive trace fossils) and are interpreted as slope to basin facies that correlate with the upper part of Windemere Group on the Mackenzie platform (Goodfellow, 2006). Marg is located within the northwest-southeast trending belt of younger Paleozoic rocks cutting through the Hyland Group that can be traced from the Dawson map area (NTS 116 B/C) to the Lansing area (NTS 105 N) and farther southeast towards Macmillan Pass, with its present configuration at least partly defined by the younger Jurassic-Cretaceous Robert Service and Tombstone thrust faults (Fig. 2). At Marg, rocks of the Hyland Group may be exposed within the Robert Service thrust sheet (Fig. 3) and are composed of greywackes, quartz-feldspar grits with minor carbonate (marble) and mafic volcanic rocks (Holbek et al., 2001). The presence of Hyland Group north of Marg (Fig. 3) is debated (Abbott, 1990 a, b).
From Cambrian to Early Devonian relatively thin successions of carbonates, shale and chert of Middle Cambrian-Silurian Road River Group (Gull Lake, Rabbitkettle, Duo Lake and Steel Formations) and Late Silurian-Early Devonian Nogold Unit (not exposed at Marg) were deposited, indicative of shallow marine, periodically anoxic conditions that suggest an isolation of the basin from an open ocean that were punctuated by pulses of rifting. Episodes of anoxia correlate with major SEDEX-forming events in the Selwyn basin and globally during Late Cambrian, Early Silurian and Late Devonian (Mair at al., 2006; Goodfellow, 2006; Goodfellow & Lydon, 2005). Pulses of rifting were reflected in unconformities and accompanied by volcanism during Cambrian, Early-Middle Ordovician and Devonian (Mair et al., 2006). The final and most widespread rifting events occurred during Early-Devonian to Early Mississippian and resulted in subsidence of the entire miogeocline, local uplift and formation of secondary extensional basins. Slide Mountain Ocean was opened to the west in a backarc setting inboard to a Devono- Mississippian arc (Yukon-Tanana terrane) (Godrey et al., 1987; Heon, 2003; Piercey et al., 2004; Mair et al., 2006). Devono-Mississippian rocks host mineralization at Marg and other VMS as well as SEDEX and MVT deposits along the western margin of North America (Fig. 4) (Holbek & Wilson, 1997; Nelson et al., 2000; Goodfellow, 2006; Goodfellow & Lydon, 2005). In the Selwyn Basin, these rocks comprise the Earn Group (Abbott & Turner, 1990; Roots, 1997a; Heon, 2003; Mair et al., 2006).
The Earn Group in the Mayo area (NTS 105 M, Fig. 2) is subdivided into three units (Roots, 1997a): DMv- felsic metavolcanic unit, DMp- metasedimentary unit of siliceous slate, carbonaceous schist, metachert and metaconglomerate (Roots, 1997a) and unit DMe composed of siltstone, shale, chert, sandstone, conglomerate and grit. The DMe unit is only found in the southern part of the Mayo area (Roots, 1997a). In the Lansing map area (105 N) to the east this unit is much more extensive (Roots, 1997 b). Sedimentary rocks of the Earn Group in the Mayo area are interpreted as deep-marine deposits of a turbidite basin (Godrey et al., 1987) with coarse clastic input such as a submarine canyon. Felsic metavolcanic rocks range in thickness from several metres to several hundred metres and extend over 42 km from Tiny Island Lake (NE corner of NTS map 105M) through the Patterson range including the Marg area. The presence of abundant broken phenocrysts suggests regional scale pyroclastic flow from a felsic dome although laterally extensive submarine pyroclastic flow cannot be discounted (Roots, 1997a). 206Pb/238U dating of felsic metavolcanic rocks from the Mayo area produced Upper Devonian ages of 373-380 ma (Roots, 1997a). Fossil evidence from Mayo, Glenlyon (105 L), Lansing (105 N) and Tay River (105 K) areas range from Early Devonian to Early Carboniferous (Roots, 1997a). Elsewhere in the Selwyn Basin the Earn Group is subdivided into the Early to Late Devonian Portrait Formation and the Late Devonian to Middle Mississippian Prevost Formation (Abbott and Turner, 1990; Heon, 2003). In the Macmillan Pass area the Portrait Formation contains alkalic mafic volcanic rocks (Abbott & Turner, 1990; Goodfellow, 2006). At Marg, the Earn Group has been subdivided informally into three units (Fig. 3): the upper unit DMvs of metasedimentary graphitic and argillaceous rocks with intercalations of tuffaceous rocks and lenses of limestone; the middle unit DMps of black argillaceous rocks (graphitic schist) interpreted as approximately 90% black shale or mudstone with minor amounts of volcanic ash and no quartzite; and the lower unit DMv of felsic metavolcanic rocks (interpreted as pyroclastic flows) that is a primary host of the Marg VMS deposit (Holbek at al., 2000). The isotopic composition of lead at Marg is consistent with the Mississippian age of mineralization. Metavolcanic rocks that host mineralization at Marg are presently correlated with the Prevost Formation in other parts of the Selwyn Basin (Heon, 2003). Many questions about the local geology at Marg still remain, for instance, about the structural and stratigraphic relationship between the Earn Group units and the Keno Hill Quartzite or the extent of the Earn Group metavolcanic rocks between Marg and the Robert Service thrust fault (Fig. 3) (Turner & Abbott, 1990; Abbott et al., 1990; Wilson, 2000; Holbek et al., 2001). Early Carboniferous Keno Hill Quartzite overlies the Earn Group (Roots, 1997a) and at Marg, it is believed to be at least partly stratigraphically or structurally interlayered with it (Holbek et al., 2001). Early Carboniferous conodont fossils were recovered from the Keno Hill Quartzite in the Dawson area (NTS 116 B/C). Zircons from the Marg deposit provide a broader Late Devonian-Early Carboniferous age (Roots, 1997a; Murphy, 1997). The Keno Hill Quartzite is composed of massive to foliated quartzite with lesser phyllitic quartzite, chloritic and carbonaceous phyllite and minor limestone. The unit has major economic significance because its fractures host silver veins and polymetallic mineralization in the Elsa-Keno Hill mining camp and gold-rich veins in the Mount Hinton prospect. This unit is exposed extensively at Marg and adjacent areas, especially at the Mayo Lake antiform (Fig. 2) within the footwall of the Robert Service Thrust fault. The Keno Hill Quartzite is interpreted as deposited in a shallow coastal well-aerated environment based on the discovery of plant remains and gastropods (Roots, 1997a). Near the top of the Keno Hill Quartzite lies a unit of light grey to green siliceous muscovite-chlorite phyllite up to 200 metres thick locally including grey marble that was interpreted to be of volcanic origin based on the occurrence of rare quartz ‘eyes’. This unit extends along the same trend as strata hosting the Marg VMS but it is not clear whether it represents the upfolds of the Earn Group or younger metavolcanic rocks deposited along with the Keno Hill Quartzite (Murphy, 1997). 207Pb/206Pb age of a felsic metavolcanic sample collected from the Patterson Range (105 M/15) west of Marg deposit is 378 ma (Upper Devonian) and overlaps with previously discussed Earn Group metavolcanic rocks. Triassic tabular bodies of diorite and gabbro intrude rocks of the Earn Group and Keno Hill Quartzite at Marg and adjacent areas and are locally called the “Greenstone” (Roots, 1997a; Holbek et al., 2001) (Fig. 3). The U/Pb isotopic date of 232 Ma was determined for zircons and baddelyite from southern Ogilvie Mountains in Dawson map area (NTS 116 B/7, B/8) (Roots, 1997a; Holbek et al., 2001). The youngest strata in the Mayo and adjacent areas (do not occur at Marg) is the Triassic-Jurassic Jones Lake Formation composed of slate, shale, sandstone and limestone indicative of shallow-marine environment (Roots, 1997a). A major change in tectonic evolution of the Selwyn Basin occurred when an orogeny commenced in the Jurassic with the exotic elements of the composite Yukon-Tanana terrane colliding with the ancient continental margin. Convergence initiated folding, metamorphism and the development of thin-skinned, N-directed thrust sheets: Dawson, Tombstone and Robert Service (Fig. 2). Timing of fold-and thrust-related deformation is constrained to be Late Jurassic to late Early Cretaceous (Mair et al., 2006). Main thrust faults are more than 200 km long and are rooted in a single detachment in the Yusezyu Formation with faults cutting progressively deeper from north to south. The Tombstone and Robert Service thrusts are nearly parallel to each other and the earlier Robert Service Thrust gets deformed within a highly-strained ductile shear zone in the hanging wall of the Tombstone Thrust called the Tombstone High-Strain Zone or THSZ (Murphy, 1997). Both the Robert Service and Tombstone faults are folded over the regionally extensive McQuesten and Mayo Lake antiforms (Fig. 2) in the Mayo map area southwest of Marg (Mair et al., 2006). Regional metamorphic grade reaches lower to middle greenschist facies in the lower part of the Robert Service and the underlying Tombstone thrust sheets. Marg is located between the Robert Service and Tombstone thrust faults. Rocks of the Hyland Group are thrusted over Devono-Mississippian rocks along the Robert Service thrust fault (Fig. 2, 3). Rocks within the deposit area record a complex history of Jurassic to Cretaceous deformation that is reflected in several generations of folds and the patterns of foliation (Holbek et al., 2001; Mair et al., 2006). It has been suggested that this deformation could have substantially modified the geometry of the initial massive sulphide mineralization and its host rocks (Holbek et al., 2001; Hunt, 2002). The Dawson Thrust fault is a linear Mesozoic fault that juxtaposes rocks of the Selwyn Basin on its south side to a shallow-water sequence of the MacKenzie platform on its north side. The Dawson Fault represents a moderate amount of displacement compared to other faults with an offset that could be as little as 2-4 km. The Dawson Fault is interpreted to be an ancient underlying basement structure, which repeatedly influenced patterns of sedimentation, igneous activity and faulting since later Proterozoic time (Heon, 2003). Both middle Cretaceous and late Cretaceous predominantly felsic to intermediate intrusions cross cut all deformation features and faults in central Yukon (Mair et al., 2006). The intrusions of the 93-92 ma Tombstone Plutonic Suite, are responsible for gold and silver mineralization within the Tintina Gold Belt in Alaska and Yukon including the world famous Keno Hill Silver Camp and the Yukon Gold Mount Hinton vein gold property, both located southwest of Marg (25 km and 16 km, respectively). Collision-related deformation ceased by 105-100 ma and was followed by about 425 km of late Cretaceous-Tertiary dextral displacement of newly-assembled continental margin along the Tintina fault (Fig. 1, 2). The onset of the dextral transcurrent regime resulted from the initiation of the Kula plate at ca. 85 Ma, and its subsequent northward movement relative to North America (Colpron et al., 2003; Mair at al., 2006; Goodfellow, 2006). |
