Summary:
The Björkdal Property, containing both the Björkdal mine and the Norrberget deposit.
Deposit Type
Björkdal
The predominant source of ore at Björkdal is contained in a package of anastomosing, sheeted quartzveins. This epigenetic vein network appears to be structurally controlled and consists of more than one thousand sub-parallel quartz veins that typically strike 030° to 090° from true-north. Such strong structural-geological influences over geometry of any quartz vein hosted mineralisation clearly suggests a strong spatial and temporal relationship with orogenic/tectonic processes (i.e., mesothermal/greenstone gold systems). In contrast, the mineral associations with gold mineralisation, and the large alteration signature of the Björkdal area, could alternatively suggest that host depositional mechanisms are responsible for the mineralisation at Björkdal as there are some similarities with skarn and/or porphyry systems.
A much smaller, yet prolific source of ore at Björkdal is observed in strongly altered lenses of skarn. Skarnification occurs commonly within the Mine, especially in the limestone/marble unit where it occurs as discreet patches and lenses, these lenses typically measure 200 m to 400 m along strike, 100 m to 200 m down dip and are usually no more than 10 m thick. However, similar calc-silicate alteration has taken place in areas where local shearing has affected the volcanoclastic host rock. The altered rock texture appears sheared and mottled to a varying degree; locally the rock can have a folded appearance. In places where the skarnification is the strongest, the precursor rock texture has been completely overprinted.
Norrberget
Primary mineralisation at the Norrberget deposit is observed to be associated with amphibole alteration bands and veinlets, and where mafic tuffs and volcaniclastic rocks are interbedded and in contrast to what is observed at Björkdal. The mineralisation is preferentially emplaced where there is a structural change to the rock such as at lithological contacts, altered bands and where shearing interacts with the interbedded sequences, due to the changing rheological characteristics of the unit. The abundance of pyrrhotite and pyrite appears to be controlled by specific lithology types within the volcaniclastic package which can indicate a differing redox based upon temperature change and fluid evolution.
The mineralisation at Norrberget is limited spatially to 50 m stratigraphically below the lower marble contact, which is believed to be a result of the cooling and redox changes of the fluid as it passes through the units.
The gold is very fine-grained and rarely visible. Where gold grains have been observed, they are found to lie on the boundary or in interstitial material between grains. Elevated gold grades are mostly found in areas with little to no pyrite.
Mineralization
Björkdal
Gold Mineralization The Björkdal gold deposit is a lode-style, sheeted vein deposit hosted within the upper portions of the Skellefte Group sediments. To date, the deposit has been outlined along a strike length of approximately 2,100 m, across a width of approximately 3,600 m, and from surface to a depth of approximately 800 m. Gold is found within quartz veins that range in thickness from less than a centimetre to more than several decimetres. These veins are usually observed with vertical to sub-vertical dips and strike orientations between azimuth 000° and azimuth 090°, with the majority of veins striking between azimuth 030° and 060° (true north). Veining is locally structurally complex, with many cross-veining features observed and thin mineralized quartz veinlets in the wall rocks proximal to the main quartz veins.
Gold-rich quartz veins are most often associated with the presence of minor quantities of sulphide minerals such as pyrite, pyrrhotite, marcasite, and chalcopyrite. Associated non-sulphide minerals include actinolite, tourmaline, and biotite. Scheelite and bismuth-telluride compounds (i.e., tellurobismuthite and tsumoite) are also commonly found within the gold-rich quartz veins and are both excellent indicators of gold mineralization.
Gold occurs dominantly as free gold, however, gold mineralization is also associated with bismuthtelluride minerals, electrum, and pyroxenes. Silver is seen as a minor by-product of the Björkdal processing plant, however, very little is known about its deportment within the deposit, although it is assumed to be associated with electrum in the mineralization.
Skarn Mineralization
Skarnification occurs commonly within the Mine, especially in the limestone/marble unit where it occurs as discreet patches and lenses. These lenses typically measure 200 m to 400 m along strike, 100 m to 200 m down dip and are usually no more than 10 m thick. However, similar calc-silicate alteration has taken place in areas where local shearing has affected the volcanoclastic host rock. The altered rock texture appears sheared and mottled to a varying degree; locally the rock can have a folded appearance. In places where the skarnification is the strongest, the precursor rock texture has been completely overprinted. The skarnified rock has been divided to prograde and retrograde phases based on their dominant mineralogy. Prograde skarn is light green and is dominated by clinopyroxene patching with partial to complete breakdown of the pyroxene patches to amphiboles (actinolite/tremolite), chlorite, calcite, and, to minor degree, serpentine and talc. The retrograde skarn is finer grained and darker green in colour than the prograde skarn and consists primarily of amphiboles, chlorite, and calcite (Figure 7-8). It is likely that the retrograde skarn represents patches of alteration where the calc-silicification did not progress as far as it did in the prograde skarn. The limestone can also be dolomitic and silicified as well as containing irregular quartz patches, quartz veins, and overprinting calcite veins.
Shearing is a known mechanism of skarnification. The skarnification here is most likely due to fluid influx where shears and faults interacted with the limestone/marble unit or calcite banded volcanoclastic rocks. The limestone/marble unit is predisposed to accommodate strain and be exploited by structures (both large and smaller scale) due to the rheological difference between the limestone/marble and the surrounding volcanic and volcaniclastic rocks. It is more ductile, prone to folding on varying scales and the calcium carbonate is reactive enough to interact with infiltrating fluids and more importantly, provide calcium for the calc-silicification. The large scale structures are interpreted to function as channels for the fluids that alter the host rocks in the Björkdal area. Where the Björkdal shear or its smaller conjugate faults intersect with the limestone/marble unit, the retrograde skarnification and low grade gold mineralization can occur (Figure 7-9). Where two or more structures interact with each other and the limestone/marble unit, the skarnified lenses consist of prograde skarn and carry higher grades (e.g., Lake Zone north skarn lens).
Norrberget
The mineralization at Norrberget is stratabound within an interbedded altered volcaniclastic package that sits unconformably below a 30 m to 40 m thick marble unit. Gold mineralization has been observed up to 50 m below this contact. Gold mineralization is principally hosted in an amphibole-albite banded alteration and is also common where volcaniclastics are interbedded with crystalline tuff units. Theses alteration bands vary between one centimetre and 50 cm in thickness, are typically fine to medium grained and appear to be sheared. Trace sulphides and minor quartz/carbonate are associated with the bands.