Summary:
The Spanish Mountain gold deposit is classified as a sediment-hosted vein (SHV) deposit, as defined by Klipfel (2005).
Gold mineralization occurs as two main types:
• Disseminated within the black, graphitic argillite. This is the most economically significant form. Gold grain size is typically less than 30 µm, and is often, but not always, associated with pyrite. Disseminated gold has also been associated with quartz veins within faults zones in the argillite.
• Within quartz veins in the siltstone/tuff/greywacke sequences. It occurs as free, fine to coarse (visible) gold and can also be associated with sulphides including galena, chalcopyrite and sphalerite. Highest grades have come from coarse gold within quartz veins.
Disseminated gold within the argillite units is by far the most economically important type of mineralization, and occurs in multiple stratigraphic horizons. From drill core, elevated gold content has been noted within fault zones as well as quartz veins within fault zones. However, the influence of fault zones in relation to the gold content of the deposit is not certain.
Examination of 15 representative core samples of disseminated gold in thin section work by Ross (2006) has concluded the following:
• Native gold (electrum) was identified in four samples, and it occurred as inclusions and fracture fill in pyrite, on crystal boundaries between pyrite crystals and in the gangue adjacent to pyrite. It is very fine grained <20 µm, and generally <5 µm. It is associated with equally fine-grained chalcopyrite-galena-sphalerite, which occurs in all the same habits. All the mineralized samples occurred in variably carbonaceous mudstones/siltstones to fine-grained greywackes, with quartzcarbonate-pyrite veinlets and disseminations. There is no clear indication from this study that the gold is preferentially associated with any habit of pyrite (i.e., disseminated or veinlet, euhedral or subhedral). The deformation state (i.e., degree of cataclastic deformation) of the host rock does not appear to be significant, at least not on the thin section scale; however, a larger scale relationship to position on fold limbs should not be ruled out.
Although a lesser component, quartz veins carrying free gold have yielded the highest grade individual samples on the Property. These veins tend to occur in the more competent facies such as siltstone and tuff/greywacke. The veins are discontinuous on surface and exhibit a strong nugget effect. The veins have been followed with confidence for about 40 m on the Main Zone. Gold is often associated with base metals in these veins. In particular, sphalerite and galena and chalcopyrite are commonly associated with free gold. Geochemically, the base metals are insignificant, but mineralogically they are a good indicator of gold mineralization. It is thought that gold and base metals may have been re-mobilized into these veins.
These veins typically cross cut all foliation fabrics and thus appear to have been emplaced late in the tectonic history. From work done by geological mapping and on oriented core data, it is known that the veins generally strike between 010° and 050°, and dip at various angles to the southeast and northwest. Several “blow out” veins, which are 1 to 5 m in thickness, have been identified on the Main Zone.