Summary:
The Croinor Property deposit is hosted by the synvolcanic Croinor Property sill, which has a general attitude of N295° and dips north at 50°–65°. The sill ranges from 60 to 120 m in thickness over a strike length of approximately 3 km. It can be observed on several outcrops and is intersected by numerous drill holes. Near the Croinor Property deposit, the sill is in direct contact with pyroclastic units. Elsewhere, the sill is in contact with fragmental volcanic rocks, and sometimes with massive volcanic rocks. Generally, the northern and southern contacts of the sill with the enclosing volcanic rocks are clearly observed by the sudden appearance or disappearance of pyroclastic fragments. The northern and southern contacts are generally foliated parallel to the regional S2 schistosity. They are strongly foliated to sheared in places, but practically undeformed in others.
The Croinor Property deposit gold-rich lenses are made of quartz-carbonate-tourmaline-pyrite veins, altered pyritic host rock material, and/or tectonic breccia (pyritic host fragments within a quartz-carbonate tourmaline-pyrite vein). These mineralized lenses are spatially controlled by reverse-oblique shear zones that crosscut and displaces both the lenses and its dioritic host. A hydrothermal alteration halo surrounds these structures. Zoning begins with an epidote-chlorite envelope that gradually changes into a chlorite carbonate zone closer to the shear. Within the shear structure itself, the host rock has undergone extensive alteration characterized by a sericite-ankerite-pyrite assemblage. Several types of veins have been identified including shear veins, brecciated quartz-tourmaline veins, quartz-tourmaline-carbonate veins, quartz tourmaline veinlets, tourmaline veins, tension veins, and tectonic breccia. The veins consist of quartz, tourmaline and carbonates with minor amounts of pyrite, chalcopyrite and native gold.
Several types of veins have been identified by previous authors, including Chénard and Turcotte (2004). Gaborit (1988), who mapped the Croinor deposit outcrop in detail, observed two types: veins parallel to the main shear (“shear veins”) and subhorizontal tension veins. The veins are generally composed of quartz, tourmaline and carbonates with minor amounts of pyrite, chalcopyrite and native gold. The veins vary from a few centimetres to several metres in thickness and plunge weakly toward the east. Based on a structural analysis, Gaborit (1988) showed that the shear zones and both types of veins are related to the same phase of deformation. Moreover, Chénard and Turcotte (2004) also encountered mineralized tectonic breccias.
Shear veins
Chénard and Turcotte (2004) noted that the shear veins are oriented parallel to shears and range from a few centimetres to several metres in thickness. These veins consist of quartz, tourmaline and carbonate with very little sulphides. The percentage of pyrite does not exceed 3% and it seems to be closely associated with tourmaline. Tourmaline occurs in the form of fine millimetre-scale veinlets and/or needles. Gaborit (1988) claims to have locally observed native gold in quartz, generally near contacts with host rocks or with tourmaline and chalcopyrite (<1%) associated with the quartz. These veins show variable degrees of deformation. They are boudinaged, folded and brecciated. The wall rocks to the veins are regularly bleached and, occasionally, brecciated.
Brecciated quartz-tourmaline veins
Chénard and Turcotte (2004) described these veins to be metric to decimetric in thickness and composed of grey or milky white quartz with varying quantities of tourmaline veinlets and/or needles. They contain less than 20% altered and mineralized diorite fragments. These fragments are subangular and range from 1 cm to 1 m in size. The fragments are leached, silicified, pyritized and carbonatized (calcite, ankerite). They contain 1% to 15% fine- to coarse-grained auriferous pyrite (0.3 cm - 2 cm). Fuchsite is occasionally present in the fragments with pyrite. In veins containing only quartz, pyrite is generally absent, and few carbonates are present, while quartztourmaline veins contain small amounts of pyrite (<2%). Pyrite borders and/or is present inside tourmaline veinlets. There seems to be a close relationship between tourmaline, fuchsite and pyrite. The host rocks are usually leached and pyritized (<15% pyrite).
Quartz-tourmaline-carbonate veins
Chénard and Turcotte (2004) described these veins as ranging from 10 cm to 1 m in width. They are composed of white quartz, massive tourmaline and/or tourmaline veinlets and carbonates (calcite, ankerite). Tourmaline and carbonates are generally more abundant along vein wall rocks. The percentage of tourmaline is usually less than 35%. In most cases, these veins contain traces to 3% fine pyrite, often associated with tourmaline.
Quartz-tourmaline veinlets
Chénard and Turcotte (2004) described the quartz veins to be millimetric to centimetric, whereas the tourmaline veinlets are usually millimetric. The density of these veins varies from 1% to 10%. Milky white quartz veinlets contain less than 1% tourmaline. Pyrite is generally absent in these veins, but their wall rocks contain auriferous pyrite (1%–15%), especially in silicified zones. Elsewhere in the diorite, trace to 3% disseminated pyrite is observed. Quartz-tourmaline veinlets are often folded, boudinaged and discontinuous.
Tourmaline veins
Chénard and Turcotte (2004) noted that tourmaline veins are rarely observed. These veins are generally 1 m to 10 m thick. They consist of more than 80% massive tourmaline with less than 20% quartz. Tourmaline is massive but may occur in the form of veins and needles in the presence of quartz. In some cases, these veins may be brecciated and contain 5%–25% leached and pyritized diorite fragments. Fragments contain up to 15% fine- to coarse-grained pyrite, sometimes cubic. Associated with tourmaline, trace to 10% medium- to fine-grained pyrite is regularly present. In drill hole CR-02-78, Chénard and Turcotte (2004) saw an example of a metre-scale tourmaline vein. This vein returned gold grades up to 9.7 g/t Au.
Tension veins
Chénard and Turcotte (2004) noted that even though tension veins have been well documented around shear zones, their identification in drill core remains difficult. The only clue that can confirm the presence of tension veins is the fact that tourmaline and/or quartz grow perpendicular to the vein contacts. The tension veins were injected into subhorizontal low-angle tension fractures (Gaudreau et al., 1988). According to Gaudreau et al. (1988), these veins dip to the southeast, but Gaborit (1988) observed north-northeast dips. The veins are generally arranged en echelon (Gaborit 1988). The subhorizontal tension veins contain milky white quartz with very little carbonates and rare tourmaline. The veins are thin (less than 15 cm) and have a very short extension. The wall rocks are pyritized and contain 1% to 5% auriferous pyrite (Gaborit, 1988). Gaborit (1988) reports the presence of visible gold in these veins.
Tectonic breccia
Gaudreau et al. (1988) observed an association between the tectonic breccias and faults and/or reverse shear zones. These tectonic breccias contain angular, leached and pyritized diorite fragments, similar to host rocks and ranging in size from 1 cm to 50 cm. The quartz-tourmaline matrix represents up to 80% of the breccia volume. Diorite fragments are strongly silicified, sericitized and ankeritized. Fuchsite is visible in some fragments. The fragments contain 1% to 15% fine- to coarse-grained auriferous pyrite crystals or aggregates. Chénard and Turcotte (2004) characterized these breccias as the main gold- bearing structures within the Croinor Sill, where gold grades are the most consistent and may exceed 30 g/t Au in some cases.