Summary:
The sulphide enrichment gold deposit model best describes the mineralization of the Bradshaw Deposit.
The sulphide enrichment deposit model is characterized by a dominance of sulphide minerals over quartz veins, and is localized in shear zones adjacent to rheologically differing mafic to ultramafic volcanic rocks of tholeiitic petrochemistry. Mineralization typically comprises native gold associated with disseminated to massive arsenopyrite and vein hosted pyrite and arsenopyrite in silicified chloritic and sericitic schists, within a broad zone of potassium metasomatism and wall rock sulphidation (disseminated pyrrhotite and pyrite). Carbonatization of the wall rocks is a less conspicuous feature than silicification. Pervasive silicification and silicate alteration minerals developed within the shear zone consists of quartz, albite, chlorite, actinolite, tourmaline and amorphous carbon, suggesting a dominance of silicic and potassic alteration.
The Bradshaw Deposit comprises a geological Main Zone and several lesser Hanging Wall Zones. Gold mineralization in the Main Zone occurs primarily within a fractured and brecciated altered horizon previously interpreted as a shear zone in hanging wall basaltic flow rocks at or near the contact with steeply north-dipping (85°) footwall ultramafic rocks to the south.
Within the geological Main Zone, higher-grade gold mineralization is localized along the footwall of the horizon, termed the MZ1 Zone (previously referred to as M1 Zone - Harron, 2006) and occasionally along the hanging wall of the horizon, termed the MZ2 Zone (previously referred to as M2 Zone - Harron, 2006). Both gold mineralized zones appear to rake steeply to the east based on current drill data. Their variation in widths may reflect tectonically controlled shoots or boudinage structures.
Sporadic, anomalous to lower-grade gold mineralization is present between these subzones. Pervasive silicification, minor quartz-ankerite veining, hematite staining and presence of tourmaline generate a recognizable mauve to pinkish-grey hue for the mineralized zone. Total sulphide content of the mineralized horizon varies from 3-30% with occasional 2-5 cm wide bands of massive arsenopyrite and pyrite. Most of the sulphide component in the Main Zone is in the form of seams, bands and clots of sulphides accompanied by zones of heavy disseminations of 5-15% sulphides over 5-10 cm core lengths. The largest concentrations of arsenopyrite correspond to the highest gold concentrations. Visible gold is not a feature of this type of mineralization. Some late stage fracturing and brecciation of the mineralized horizon has caused varying amounts of sulphide remobilization (Roussain, 2004).
Similar mineralization forms multiple structures believed to be subparallel to the strike and dip of the Main Zone and are referred to as hanging wall zones as they are located immediately north of the main zone. They are highly silicified zones accompanied by intense bleaching, brecciation and quartz flooding, tourmaline, 5- 10% pyrite and arsenopyrite. The overprint of silica flooding and white quartz veining makes the hanging wall zones appear different from the Main Zone but the gold is associated with the sulphide component as in the main zone. As in the Main Zone, higher concentrations of arsenopyrite give rise to higher gold values. A total of six such parallel structures (HWZ1 to HWZ6) have been identified in locations and are significant contributors to the total number of ounces of gold contained within the overall Bradshaw Deposit.
To date, the deposit has a drilled strike length in excess of 950 m, trending N070-080°E, and has been tested to a depth in excess of 1,000 m. The horizontal width of the geological Main Zone varies from 2 to 22 metres. The Bradshaw zones are from 1 to up to 15 metres in horizontal width and average 2 to 3 metres. The deposit remains open along strike and at depth.
Overburden depth along the strike length of the deposit ranges from 10 to 16 metres and averages approximately 12 metres deep.