Summary:
The Aukam graphite deposit has the characteristics of a vein-style graphite deposit. The graphite is predominantly concentrated in veins, with only minor disseminated graphite occurring in the host rock. It is believed that the graphite formed by precipitation from a CO2 – rich hydrothermal fluid channeled along the shear zone, and possibly related to an intrusion at depth.
The graphite in the Aukam area is hosted by altered Garub Sequence granite that is exposed in an erosional window in the Nama cover (Goldfields, 1987b). In the vicinity of the graphite occurrence, the granite is coarse grained, variably altered (see below) and generally shows a well-developed foliation with an easterly to south easterly trend and a south to southwest moderate to steep dip.
Schist and gneiss, both of which are quartz and biotite dominant, well foliated rocks, also occur in the vicinity of the graphite occurrence. The gneiss, which is distinctly banded, forms discrete positively weathered outcrops to the east and west of the main workings. Both lithologies appear to be intruded by the granite, primarily as narrow interfingering apophyses or dikes.
Late pegmatite veins, which are widespread in the area, cut the altered granite in places above the workings. Dolerite dikes also cross cut the granite in the vicinity of the graphite workings.
Alteration is most apparent in the granite and includes K-feldspar, sericite, epidote and kaolinite (de Kock, 1935, Goldfields 1987b). There appears to be a zonation of alteration of the granite from a general, widespread kaolinization over an area of at least 450m x 100m, within the shear zone, to intense epidotization in the immediate vicinity of the graphite veins and lenses. Hematite –limonite is also often found closely associated with the graphite mineralization, typically adjacent to the massive lenses as a halo and as stockwork type veins in places. Local concentrations of red/pink garnet also occur in the granite.
Mineralization
Graphite mineralization at Aukam occurs as massive lenses and veins, and as disseminated patches mostly associated with strong alteration. It is hosted by an east-west trending shear zone traceable on surface for about 400 metres along with mapping indicating the mineralization may extend for four kilometres.
Alteration and mineralized are believed to be caused by CO2-rich fluids channeled by the shear zone (Goldfields 1987b). De Kock (1935) speculates that the graphite was introduced into fissures and fractures by hydrothermal solutions subsequent to the intrusion of the granite since the granite is strongly altered in the presence of graphite. This suggests a hydrothermal fluid responsible for the alteration of the granite and the precipitation of graphite, possibly due to a redox reaction with iron oxides hematite and limonite that often accompany significant occurrences of graphite.
Goldfields (1987b) suggested that the hydrothermal fluids may be related to an intrusive at depth to the east of the deposit along the extension of the shear zone. The presence of a strong magnetic anomaly is given as evidence of the intrusive.