Summary:
The Macpass Project covers three distinct deposits of sedimentary rock-hosted, stratiform zinc-lead-silver (Zn-Pb-Ag) mineralization including Tom, Jason, and End Zone, and one deposit containing brecciated, stratiform, and vein-hosted leadzinc-silver mineralization at Boundary Zone. These deposits represent structurally and stratigraphically controlled feeder-fault systems that occur on splays of the Macmillan-Hess fault system.
The sediment-hosted Zn-Pb-Ag (+/- Ga-Ge) deposits occur predominantly within the Portrait Lake Formation at or near the contact between the Fuller Lake Member and the Macmillan Pass Member, while mineralization at Boundary occurs throughout a wide stratigraphic interval spanning sections of the Road River Group, the Portrait Lake Formation of the Earn Group and intercalated Macmillan Pass Volcanics. The Fuller Lake Member consists of massive to thinly laminated, carbonaceous to siliceous, pyrite-rich mudstone, while the Macmillan Pass Member generally consists of interbedded black mudstone with grey siltstone and sandstone (“pinstripe mudstone”) with coarse sandstone and conglomerate intervals. Carbonate-altered mafic volcanic and volcaniclastic rocks occur at several stratigraphic intervals, including within the Macmillan Pass Member, below the Niddery Lake member, and deeper, within the Road River Group.
The Tom, Jason, End Zone, and Boundary Zone deposits are examples of stratiform, stratabound sediment hosted zinc-lead-silver-barite deposits. Historically the term SEDEX was first used in a report describing the Zn-Pb-Ag deposits of the Selwyn Basin by Carne and Cathro (1982) and subsequently for a period of time, the term was used to describe these deposits worldwide. The term SEDEX has been replaced, however, in favour of more descriptive and less genetic terminology.
Tom Deposit
The Tom deposit consists of several stratiform Zn-Pb-Ag-Ba bodies of mineralization which crop out at surface and are located around an open, north-south trending, doubly-plunging anticline. Tom West sits on the western limb of the fold, and Tom East sits on the eastern limb. Tom South and Tom Southeast are located around the southern nose of the fold. Mineralization transitions from well-laminated and thinly bedded to zones of massive sulphide and semi-massive sulphide brecciation proximal to the feeder fault. The following section focuses on descriptions of mineralization styles, extent, and geology.
Mineralization at Tom has been segregated into distinct facies along vertical and lateral transitions from darker to lighter sphalerite colours and progressively lower lead to zinc ratios, interpreted as increasing distance to the feeder zone (Goodfellow 1991; 2007). These facies consist of:
• Massive Sulphide Feeder Facies: Massive pyrite, pyrrhotite, galena, sphalerite, with minor chalcopyrite, arsenopyrite and tetrahedrite with ferroan carbonates, quartz and barite, typically grading 15% to 30% Pb+Zn and a high Pb:Zn Ratio.
• Pink Facies: Interlaminated cream-coloured to pink sphalerite, galena, barite, chert, pyrite, and barium carbonates (witherite). Locally high grades range from 10% to 30% combined Pb and Zn, including greater than 1% Pb.
• Grey Facies: Interlaminated cream to white coloured sphalerite, pyrite, minor galena, white to pale grey barite, pale grey chert and grey to white barium carbonate (witherite) and dark grey barium feldspar (hyalophane and celsian). Typically, with grades in the range of 4% to 5% Pb+Zn and Pb less than 1%.
• Black Facies: Black mudstone and chert interbedded with sections of interlaminated barite, witherite, and fine-grained white sphalerite, galena and pyrite. Typically, with grades in the 4% to 10% Pb+Zn range and a low Pb:Zn ratio.
Jason Deposit
The Jason deposit is hosted by a Devonian sequence of sediments disrupted by synsedimentary faulting and fault scarp material. Bounded to the south by the regional Hess fault, mineralization consists of two stratiform Ba-Zn-Pb-Ag bodies on opposite limbs of the Jason syncline. The Jason syncline is a steeply dipping, upright, west-trending syncline that plunges east, with the Jason Main zone located on the northern limb and Jason South zone occurring on the southern limb. Hosted within the lower Portrait Lake Formation of the Earn Group at the contact with the Macmillan Pass member, the carbonaceous sediments commonly contain mud-hosted diamictite breccias related to fault scarps that thicken towards the Hess Fault.
Mineralization is spatially related with proximity to the feeder fault where diamictite facies, barite lenses, and metal content increase; These horizons can be divided into several distinct mineralization zones or facies, including (after Turner 1991):
• Pb-Zn-Fe Sulphide Facies: Massive, banded sphalerite-galena and galena-pyrite overlain by debris flow deposits.
• Barite-sulphide Facies: Interbedded fine-grained sphalerite, galena, barite, chert and ferroan carbonate forming the bulk of the mineralization at Jason.
• Quartz-sulphide Facies: Interbedded sphalerite, pyrite, quartz and carbonaceous chert with quartz-celsian (barium feldspar) bands in the lower lens.
• Massive Pyrite Facies: Massive pyrite beds interbedded with sphalerite, galena, chalcopyrite, pyrrhotite, and quartz located near the Jason Fault.
• Ferroan Carbonate Facies: Massive beds of siderite and ankerite up to several metres across with irregularly distributed galena, sphalerite, pyrrhotite, pyrite, quartz, muscovite, and pyrobitumen; spatially associated with a breccia pipe.
End Zone Deposit
End Zone is a small, fault bound block of MacMillan Pass Member (and Fuller Lake Member) in fault contact with older Road River Group mudstones and carbonates. This fault block contains high-grade, massive sulphide mineralization (dominantly galena, pyrite, pyrrhotite, and sphalerite), interpreted to be feeder-proximal mineralization similar to the feeder proximal mineralization at Tom. Like the feeder-proximal mineralization at Tom, End Zone also has a high lead to zinc ratio compared to other mineralization styles.
Boundary Zone Deposit
The Boundary Zone area is part of a distinct sub-basin that contains significant volumes of strongly siderite altered basaltic volcaniclastics within and below the Earn Group. Boundary Zone is located adjacent to a major syn-sedimentary fault and also contains large volumes of boulder diamictites indicating that the area underwent active tectonic extension during the formation of the basin, a similar setting to the Tom and Jason areas. The presence of synsedimentary faulting, a distinct sub-basin, volcaniclastic inputs rocks, abundant zinc mineralization, and strong alteration indicate the area is host to a robust mineralized system.
Extensive vein and associated breccia mineralization at Boundary Zone occurs both stratigraphically above and below the stratiform laminated massive sulphide zones described above. This mineralization forms a halo approximately 100 m to 150 m wide on both sides of the stratiform laminated zones and is interpreted as a stockwork of randomly oriented veins and breccia zones that are contained within broadly stratiform bodies. It is not restricted to one stratigraphic horizon or lithology; Boundary Zone Vein Mineralization occurs within mudstones, siltstones, diamictites, conglomerates, and volcaniclastics. This vein mineralization accounts for a significant volume of mineralization at Boundary Zone.