Summary:
Fluorite in the St. Lawrence veins is mostly massive and coarsely crystalline, filling veins from wall to wall. Close to the vein walls, the fluorite is commonly finely banded. In the larger veins, fluorite surrounds sharp fragments of granite. The most common gangue mineral is quartz, occurring in the material locally known as “blastonite”, which is brecciated fluorite cemented by a mixture of microcrystalline quartz and fluorite, as mentioned above. Nodular forms of fluorite and calcite are common in larger veins mainly along the footwalls (Williamson, 1956). Calcite occurs in nearly all the fluorite veins. Barite, galena, sphalerite, and hematite are common, while chalcopyrite, pyrite, bornite, covellite, and chalcocite are less common (Harris, 1995a). Sulphides (mainly galena and sphalerite) are more common near the contacts of the veins, or where the vein cuts into the country rocks. It is possible that sulphide content of the veins may increase in the deeper parts of the veins (Van Alstine, 1948).
The fluorspar veins at St. Lawrence are complex structures along fault zones within the St. Lawrence granite and related porphyries. Commonly, the faults exhibit evidence for multiple episodes of movement. Typically, the veins have a high-grade core, and irregular amounts of mineralized brecciated granitic material along one or both walls. Three major types of fluorite veins are recognized, as follows:
• Low-grade veins, with an average width of 5 m to 7 m and grades ranging from 35% CaF2 to 70% CaF2, such as the Director, Blowout, Tarefare, and Blue Beach North and South veins.
• High-grade veins, with an average width of one metre or less and grades averaging 95% CaF2, such as the Lord and Lady Gulch, the Iron Springs and Canal veins.
• Other veins that occur in the peripheral region, which have significant barite intergrown with fluorite. Examples are Meadow Woods, Lunch Pond, Clam Pond, and Anchor Drogue veins.
The Blue Beach North (BBN) Vein is approximately 2.5 km long and extends from the surface to almost 400 m below the surface. It is situated in the eastern part of the property, very close to the western boundary of the town of St. Lawrence, trends northwestward (Azimuth 310°), and dips steeply (75° to 85°) to southwest. From southwest to northeast, the general sequence of rocks intersected in the Alcan 1984 drill holes as well as the BML 1999 drill holes in the area of the BBN Vein are: SLG, blastonite, fluorite vein, and tuffisite. In general, the mineralized zones are grouped into two groups:
• A wider zone at the hanging wall (Main Zone), which ranges in thickness from 2.5 m to 7.5 m, and averages 4.55 m.
• A footwall zone (FW zone), which has an average thickness of 2.75 m.
The Tarefare (TF) Vein is approximately 1.8 km long, and ranges from 0.6 m to 14.0 m in horizontal thickness, with an average thickness of 2.4 m. It is situated approximately 3.5 km southwest of the BBN Vein, and extends from the surface (at the southern end) to 500 m below the surface (near the northern end of the vein). It is oriented north-westerly (Azimuth 325°) and is subvertical or dipping steeply to the northeast. In general, the mineralized intersections are grouped into three groups: the Main Vein, HW Zone, and FW Zone, as noted above. The FW Zone contains the Blowout Vein and other subsidiary narrow veins.
The vein structure contains three subparallel veins, which are the Grassy Gulch/Tarefare Vein, Blowout Vein, and Hope Vein, and has been traced for approximately 2.5 km (Howse et al., 1983). The TF Vein contains abundant blastonite accompanied by crystalline, locally banded massive fluorite with gangue of quartz and calcite and lesser amounts of sulphides (Harris, 1995b).
The Blowout Vein is situated close to (10 m to 25 m east of) the TF Vein, as noted above. It is subvertical, and near the southern part it dips moderately to the northeast. The habit of mineralization is similar to that at the TF Vein, with occasional high-grade intersections.
Other fluorite veins in the St. Lawrence area that have had production in the past include:
• Director Vein: this is the largest vein, and past production has largely come from this vein. It extends in excess of 2 km along strike, ranges 0.3 m to 30 m in thickness, and extends more than 400 m in the vertical dimension. Alcan archive records indicate that at the time of closure of the mine, the remaining reserves of the Director Mine were approximately 2 million tonnes at an average grade of 56% CaF2. Mining operations, however, were suspended mainly because of excessive water inflow at the underground working areas.
• Hookey Branch Vein: Grey, yellow, green, and purple-coloured vein with galena, traced for more than 600 m along strike. It is 1.5 m to 4 m thick, and is close to the Director Vein.
• Canal Vein: This vein has been explored by a shallow shaft, close to the Director Vein. Alcan archive records indicate that this vein may be up to 2.5 m thick.
• Valley Vein: Grey, red, green, and purple-coloured vein with galena, barite, blastonite, and calcite. This vein was intersected in 2008 by hole BBS-103, which indicates an average thickness ranging from 1.0 m to 2.5 m and extending approximately 270 m along strike.
• Compressor Vein: This vein was discovered in 1999 as a result of BML’s drilling program. It consists of several short sections (15 cm to 25 cm) of high-grade fluorite alternating with granite and granite breccia, and a wider zone (1.17 m) of calcite with brecciated granite and fluorite. Average grade over the 2.72 m intersection is 38.9% CaF2.