The geology of the Berringa Goldfield consists of north-south trending Ordovician metasediments that are tightly folded. A series of north south trending, steeply west dipping reverse faults cut across the strata. Gold mineralisation is primarily associated with the west dipping faults as structurally controlled gold-quartz lodes in the eastern and western limbs of the folds. Baragwanath (1907) subdivided quartz veins into three categories: saddle reefs, fault reefs and spurs. The main line of historic workings occurs over a strike of 2.2km, located approximately one km to the west of the regional scale Berringa Fault. d’Auvergne (2002) subdivided the Berringa goldfield into three geographical zones, each characterised by its own mineral control (Figure 3.2);
- The Northern Zone includes the Birthday Tunnel mine which produced 73,474oz at an average grade of 6.2 g/t Au. The upper mine levels worked saddle and spur reefs on a major anticline. The lower mine levels worked fissure reefs on the western leg of the anticline. The structures pitch gently northward.
- The Central Zone includes the Southland Mine (including Berringa Prospecting Syndicate), the Kangaroo Mine and Williams Fancy Mine. Combined the mines produced 107,345oz at an average grade of 8.35 g/t Au. The northern end of the central zone is characterised by relatively flat pitching structures trending south pitching to the south. The Kangaroo Mine worked saddle reefs on the main anticline. The Williams Fancy Mine worked saddle reefs in the upper mine levels and western leg fissure lodes at depth.
- The Southern Zone includes the Birthday and South Birthday Mines which collectively produced 112,432 oz at an average grade of 12.6 g/t Au. The geology of the southern zone is distinctly different to the central and northern zones of Berringa goldfield. The mines worked fissure reefs on the western leg of an anticline and the ore body pitches to the south. There is no known development of saddle reefs as in the northern and central zone mines. Folding is open with only one fold pair recognised across the extent of the historic workings (d’Auvergne, 2002).Figure Long section (west) of Berringa Goldfield zones
The previous operators of the Berringa Gold Project, Ballarat Goldfields NL and Lihir Gold Limited developed a geological model for the ongoing exploration and development of the project. The geological model hosts similarities to the Ballarat Gold Project in that an upright Anticlinal fold dominates the field, with a relatively simple westerly dipping west limb leading to the Berringa Syncline to the west.
The eastern limb of the anticline is more complicated, with a series of parasitic folds plunging in and out of the eastern limb along strike. In addition there is an apparent doming structure along the main Berringa Anticline resulting in a 20o southerly plunge to the south of the field, and a 25o northerly plunge to the north of the field.
A series of steep westerly dipping reverse faults intersect through the fold hinges, at irregular vertical intervals. These are understood to be the conduits for the mineralising fluids with historical ore shoots being developed upon:
- The intersection of the reverse faults and the fold hinges forming breached fold hinges with variable quartz content ranging from massive quartz to wide quartz spur zones.
- Western legs/limbs of the Anticline, these were typically mined as narrow bodies of higher grade mineralisation, and maybe additionally influenced by lithology.
- Intersection of west dipping reverse faults and favourable lithology on the eastern limb of the Berringa Anticline.
In addition a further complication to the geology of the Berringa Goldfield results from the presence of east-west cross course faults, which throughout their traverse of the goldfield have displaced the fold axes and reef faults along strike. Figure below displays the fold geometry and steep westerly dipping reverse fault.