Summary:
The Snow Lake deposits including Lalor are all located within the Trans-Hudson Orogen of the Flin Flon Greenstone Belt. The volcanic assemblages consist of mafic to felsic volcanic rocks with intercalated volcanogenic sedimentary rocks.
The volcanogenic massive sulphide (VMS) deposits located near the town of Snow Lake have been subdivided into two different groups: Cu-Zn-rich (Cu-Zn, Cu-Zn-Au) and Zn-Cu-rich (Zn-Pb-Cu-Ag) types. The Cu-Zn-rich deposits mainly occur in the Anderson sequence and the Zn-Cu-rich deposits occur in the Chisel sequence. The Watts and Talbot deposits, located east-southeast of the town of Snow Lake lies in the eastern portion of the Flin Flon-Snow Lake Greenstone belt and is a stratabound accumulation of sulphides that precipitated in a depositional environment similar to the base metal deposits of the Snow Lake mining camp.
The Lalor deposit and its associated 1901 satellite zone are interpreted as a gold enriched volcanogenic massive sulphide (“VMS”) deposit that precipitated at or near the seafloor in association with contemporaneous volcanism, forming a stratabound accumulation of sulphide minerals. The depositional environment for the mineralization is similar to that of present and past producing base metal deposits in felsic to mafic volcanic and volcaniclastic rocks in the Snow Lake mining camp. The deposit appears to have an extensive associated hydrothermal alteration pipe.
The Lalor VMS deposit is isoclinaly folded and flat lying, with zinc mineralization beginning at approximately 600 metres from surface and extending to a depth of approximately 1,400 metres. The mineralization trends about 320° to 340° azimuth and dips between 30° and 45° to the northeast. It has a lateral extent of about 1,400 metres in the north-south direction and 780 metres in the east-west direction. Sulphide mineralization is pyrite, sphalerite and chalcopyrite. The current interpretation suggests the deeper copper-gold lens tends to have a much more linear trend to the north than the rest of the zones. Gold and silver enriched zones occur near the margins of the sulphide lenses and in local silicified footwall alterations. These silicified areas often correlate with disseminated stringer chalcopyrite, pyrrhotite and pyrite, whether together or independent of each other. This footwall gold mineralization is typical of VMS footwall feeder zones with copper-rich disseminated and vein style mineralization overlain by massive zinc-rich zones. The gold bearing lithologies remain open down plunge to the north and northeast.
The depositional environment for the mineralization at Lalor and the 1901 deposit are similar to present and past producing base metal deposits in felsic to mafic volcanic and volcaniclastic rocks in the Snow Lake mining camp. The deposits appear to have an extensive associated regional hydrothermal alteration.
The mineralized envelopes are shallow dipping, with zinc mineralization defined to date beginning at approximately 600 m from surface and extending to a depth of approximately 1,100 m. The mineralization trends about N320° to N340° azimuth and dips between 30° and 45° to the north. It has a lateral extent of about 1,400 m in the north-south direction and 800 m in the east-west direction.
Sulphide mineralization is dominated by pyrite and sphalerite. In the near solid (semi-massive) to solid (massive) sulphide sections, pyrite occurs as fine to coarse grained crystals ranging one to six millimetres and averages two to three millimetres in size. Sphalerite occurs interstitial to the pyrite. A crude bedding or lamination is locally discernable between these two sulphide minerals. Near solid coarse-grained sphalerite zones occur locally as bands or boudins that strongly suggest that remobilization took place during metamorphism.
Disseminated blebs and stringers of pyrrhotite and chalcopyrite occur locally within the massive sulphides, adjacent to and generally in the footwall of the massive sulphides. The hydrothermally altered rocks in the footwall commonly contain some very low concentrations of sulphide minerals. Some sections of massive pyrrhotite occur, but these tend to give way to pyrite-sphalerite-dominant zones.
The top two lenses of the stacked base metal zones at Lalor (referred to as Zone 10 and 11) and lenses 1 and 3 at the 1901 deposit have higher grade zinc and iron content. Lenses located lower in the stratigraphy and coded as Zones 20, 30, 31, 32, 40 and 42 at Lalor have moderate to high zinc grades hosted in near solid sulphides containing higher grade gold and locally appreciable amounts of copper.
The footwall gold mineralization is typical of any VMS footwall feeder zone with copper-rich, disseminated and vein style mineralization overlain by a massive, zinc-rich lens. The fact that the footwall zone is strongly enriched in gold suggests a copper-gold association which is comparable to other gold enriched VMS camps and deposits (Mercier-Langevin, 2009). Some of the footwall zones tend to be associated with silicification and the presence of gahnite. These zones are often characterized by copper-gold association and are currently interpreted as being associated with higher temperature fluids below a zone of lower temperature base-metal accumulations.
Gold and silver enriched zones occur near the margins of the zinc rich sulphide lenses and as lenses in local silicified alteration. Remobilization is illustrated in some of the gold-rich zones by late veining that is restricted to the massive lenses.
Some of the footwall zones tend to be associated with silicification and the presence of gahnite. These zones are often characterized by copper-gold association and are currently interpreted as being associated with higher temperature fluids below a zone of lower temperature base-metal accumulations.
General observations of the known gold zones indicate areas which are coarse-grained and porphyroblastic in nature are gold poor, while fine grain siliceous (± veins ± sulphide traces) and strained looking stratigraphy tend to be gold rich. However, the intensity and style of alteration can vary strongly over short distances and may suggest that the alteration was forming discordant stockwork like zones that are now strongly transposed in the main foliation (Mercier-Langevin, 2009).