Yukon Territory Mining

[echarter]

The Wernecke Breccia is a curvilinear Mesoproterozoic breccia province of nearly 50,000 km2 that extends in a west-to-east direction from the Wernecke to the Ogilvie mountains across the Coal Creek and Wernecke inliers of the Cordillera as well as extending northward and reaching the Hart River Inlier (Fig. 2; cf. Bell, 1986; Thorkelson, 2000; Laznicka, 2002; Hunt, 2004). The breccias vary in size from outcrop to mountain scale (0.1 to 10 km2), in colour from grey (sodic) to mottled red and pink (potassic), and in fragment size from <5 cm up to hundreds of metres, making the breccias particularly spectacular (Thorkelson, 2000, p. 17, 31-40). All known occurrences formed within the Paleoproterozoic Wernecke Supergroup sedimentary rocks (Fairchild Lake, Quartet and Gillespie Lake groups) and locally in Bonnet Plume River intrusions within the Wernecke Supergroup (Delaney, 1981; Thorkelson, 2000). Deposition of the Wernecke sediments is inferred to postdate the 1.84 Ga Fort Simpson magmatic arc to the east and must predate emplacement of the 1.71 Ga Bonnet Plume River intrusions, but the exact age of deposition is still uncertain (Thorkelson, 2000, p. 7; Thorkelson et al., 2001a). With this age bracket, the Wernecke Supergroup may correlate with the onset of the Hornby Bay Assemblage sedimentation in the Northwest Territories to the east (MacLean and Cook, 2004; Laughton et al., in press). The Wernecke Supergroup and the Bonnet Plume River intrusions were deformed and metamorphosed at greenschist facies during the post-1.70 Ga and pre-1.60 Ga Racklan orogeny (Brideau et al., 2002), then crosscut by the breccia at 1.60 Ga based on a 1595±5 Ma U-Pb age obtained from an hydrothermal titanite within the Slab Mountain breccia matrix (Thorkelson et al., 2001b). An unconformably overlying volcanic sequence must have been present at the time of brecciation as fragments of non-deformed Slab volcanics recording a volcanic sequence of at least 160 m in thickness are found in the breccias even though such volcanics are not preserved in the exposed tectonostratigraphic record. These observations and the fact that breccia fragments of Wernecke Supergroup and Bonnet Plume River intrusions are deformed and metamorphosed and fragments of Slabs volcanics are not has led Laughton et al. (in press) to advocate for a period of uplift and erosion of Wernecke Supergroup after the Racklan orogeny, a subsequent restricted extrusion of Slab volcanics along valleys and finally a regional-scale surges of hydrothermal fluids to form oxide-rich breccias. A ca. 1.38 Ga sedimentary pile unconformably overlies the breccias and testifies that most of the hydrothermal activity took place prior to 1.38 Ga. However evidence for subsequent minor brecciation and hydrothermal alteration exists and is associated with 1.38 and 1.27 Ga intrusive events (Thorkelson et al., 2001b). The Racklan orogeny may be contemporaneous with the 1.66 Ga Forward orogeny in the Northwest Territories (Thorkelson et al., 2001b, 2003). If so, then the non-deformed Slab volcanics must have been deposited between 1.59 and 1.66 Ga (Laughton et al., in press).

The Wernecke Breccia encompasses at least 65 breccia bodies and includes a number of iron oxide Cu (Au, U, Co) prospects and showings. These are described in the Yukon Geological Survey Minfile database Mineralization occurs as disseminations and veins in Wernecke Supergroup metasediments and in Wernecke Breccia, as clasts and within matrix of heterolithic breccias as well as in carbonate veins across breccias (Hunt, 2004). Current mineral exploration is particularly active at the Monster, Olympic (Lala), Yukon Olympic (Hem), Pike and Hart River showings and prospects (Burke, 2002). Pertinent Yukon minfiles include the following: 116B (84, 99, 102, 103), 106C (6, 7, 13, 15, 16, 17, 44, 71, 86), 106D (49, 52, 68, 75, 76, 77, 79, 87, 96), 106E (2, 3, 5, 9, 11, 22, 25, 30, 31, 40), 106L (61) (Fig. 3; Deklerk, 2003).

Brecciation, pre to post-brecciation hydrothermal sodic-potassic-calcic alteration and associated Cu, Co, Au, Ag, U, and locally Mo mineralization took place with precipitation of hematite, magnetite, calcite, albite, microcline, muscovite/sericite, chalcopyrite and pyrite at regional scale (Thorkelson, 2000). Hematite is the main iron oxide but magnetite can be locally abundant. Brecciation and hydrothermal influx of iron oxide were polyphase resulting in sub-angular to sub-rounded clasts of Wernecke Supergroup sediments, Slab volcanic rocks (up to 0.2 x 0.4 km) and Bonnet Plume River diorite as well as clasts of earlier breccia and mineralization. Brecciation occurred either close to surface leading to near-surface vent with volcanic fragments and sodic, low temperature alteration facies or more commonly well below surface leading to potassic alteration facies with locally derived fragments (Laughton et al., in press). Alteration extends from the breccia into the host sedimentary rocks for meters to hundred meters and crackle breccia, where host rocks display incipient brecciation, is also observed. As such, contacts tend to be mostly gradational. Sharp intrusion of breccia material into non-altered country rocks also occurs. Contacts are also sharp where fault bounded recording fault activity after brecciation. At a regional scale, the breccia are spatially associated with deep-seated fault zones such as the Richardson Fault array while at the local scale, the surges of fluids used pre-existing discontinuities such as core of fold structures, intrusive contacts of Bonnet Plume River Intrusions, sedimentary layering, faults, and shear zones (Hunt, 2004). Normal faults are very commonly associated with the breccia zones but a cogenetic relationship is uncertain as in many cases faults can be shown to postdate brecciation and breccia are found outside of fault zones (Fig. 3; Thorkelson, 2000, p. 45). At the scale of mineral properties (e.g., Yukon Olympic property; Copper Ridge Exploration web site), km-scale Bouguer gravity anomalies can be associated with slightly off set magnetic anomalies. This geophysical response is typical of the Olympic Dam deposit and of other iron oxide (Cu-Au) settings and is a key guide to iron oxide breccia where abundant magnetite occurs in parts of the breccia system and hematite prevails in other areas (Nisbet et al., 2000; Smith, 2002). Unfortunately, such anomalies may exist whether the iron oxide breccias are strongly mineralized or not. Breccias can be exposed or overlain by Paleozoic sedimentary rocks creating blind targets that can only be detected by geophysical methods.

The physical and mineralogical characteristics of the Wernecke Breccia share many similarities with those of the breccias at the Olympic Dam deposit (Hitzman et al., 1992; Thorkelson, 2000). Their ages are coeval, their emplacement is closely related in time with orogenic activities, and their breccias include down-dropped volcanic fragments, which must have been formed relatively close to surface (Oreskes and Einaudi, 1990; Reeve et al., 1990; Laughton et al., in press). One point that has particularly captured the interest of the exploration industry is that both settings could have been contiguous during brecciation based on the SWEAT continental reconstruction (Thorkelson et al., 2001a, b). However, many alternatives exist to such reconstructions and the exact nature of the paleo-environment of the Wernecke Breccia and the underlying crustal architecture is still uncertain (e.g., Snyder et al., 2003 vs. Thorkelson et al., 2003). In contrast to the Olympic Dam where there is a direct link between arc magmatism, orogenic activity and timing of mineralization, the Racklan orogeny and the Wernecke Breccia are not coeval and they are not associated with exposed proximal arc magmatism. The Racklan orogeny is a far-field response to a distant collisional orogen, with the attenuated crust of the Wernecke basin representing a zone of crustal weakness (Thorkelson et al., 2001a). The 1.66 Ga age for the Racklan orogeny would best coincide with the late Paleoproterozoic 1.7-1.6 Ga Labradorian and Mazatzal orogenies that took place along the southern Laurentian margin (Karlstrom et al., 2001; Gower and Krogh, 2002, 2003).

The Olympic Dam deposit is now known to be associated with intra-continental arc magmatism, not with intracratonic, anorogenic activity and to occur within a doubly-vergent orogen with a fold and thrust belt above a reflective crustal-scale ramp that extends to the Moho and transect a non reflective Moho and lower crust as well as above strong horizontal reflectors in mid crust typical of mafic sills (Ferris and Schwarz, 2003; Lyons et al., 2004). These new data puts an end to the anorogenic myth that Creaser (1996), Partington and Williams (2000) and Gandhi (2003) among others have disparaged putting into perspective that an A-type granitic fingerprint does not imply formation in an anorogenic setting and that in fact orogenic activities were taking place at the time of IOCG-related intrusions in the Gawler craton. How the new paradigm is going to influence future exploration models for IOCG deposits and in particular for the Wernecke Breccia is uncertain. However, as pointed out by Thorkelson et al. (2003) the Wernecke Breccia, even if not deformed, is part of a mature Proterozoic orogen, like Olympic Dam. This key area for exploration is currently the site of active geoscientific research by academia and the Yukon Geological Survey (e.g., Burke, 2002; Hunt et al., 2002). With the spectacular exposures that the mountain relief and cliffs provide across the Wernecke Breccia province, the study of these iron oxide Cu (Au, U, Co) breccias are bound to advance knowledge on IOCG-type breccia formation, evolution and settings as well as providing 3D knowledge to be applied to breccia systems such as those of the Central Mineral Belt of the Makkovik Province in Labrador (Marshall et al., 2003) and those within the Lemieux Dome in the Appalachian orogen of Québec (e.g., Beaudoin, 2004). Knowledge of their exact endowment in terms of mineral deposits awaits results of exploration efforts.