Summary:
The geology of the greater Arcadia area is dominated by greenstone lithologies of the Arcturus Formation of the Harare Greenstone Belt (HGB). These greenstones are encircled and intruded by a variable suite of granitic rocks, the oldest of which may have been intruded at a similar time with the youngest felsic volcanic rocks of the belt. There is also some evidence for a small remnant area of gneissic basement to the greenstone belt.
The HGB takes the form of a complex refolded synform structure, which outcrops in two major limbs.
The E-W trending Arcturus Limb occupies a broad band across the centre of the area, and to the west of the city of Harare this passes via a fold closure into the N-S trending Passaford Limb which is contiguous northwards to the greenstones of the Bindura area.
The main HGB lithological units comprise mainly meta-basalts, banded iron formations, metaandesites, serpentinites, dolerites and the lithium bearing pegmatites that also host beryl, tin and tantalite amongst others.
Four different rock types are found on the Arcadia and Green Mamba claims. These are metabasalt, pegmatite, dolerite and quartz veins, listed from relatively oldest to youngest. The pegmatites comprise the lithium bearing rocks and are of immediate relevance to the Project.
Pegmatite outcrops are coarse to very coarse grained, white to greyish white in colour and blocky in appearance with some oxidation along joint planes. However, natural pegmatite outcrops are rare. This is because they contain high proportions of feldspar which readily weather to clay minerals such as kaolinite and hectorite. The pegmatite mineralogy is dominated by feldspars (mostly albite), lesser quartz and muscovite. The contained lithium bearing minerals include petalite, spodumene, eucryptite and rare lepidolite.
The ore bodies that constitute the reserve show local variation, notably in the proportions of the two main lithium ore minerals; spodumene and petalite. However, there is actually little overall lithological variation among the ore bodies, which are essentially lithium rich quartz-feldspar pegmatites. Detailed XRD and petrographic studies have deduced that the ore bodies consist on average of:
- 45% various feldspars, notably albite;
- 30% quartz;
- 19% lithium minerals, predominantly petalite and spodumene, with an average weight ratio of 2:1;
- 4% muscovite, in solid solution with subsidiary amounts of lepidolite.
The pegmatite bodies are also concentrically zoned. The outer-most zones are the wall and aplitic zones, which are fine grained and comprise mostly feldspar, quartz and muscovite with some rare lepidolite. The intermediate zone is the widest of all the zones and has a coarse to very coarse-grained texture. Furthermore, the intermediate zone hosts petalite and spodumene, along with feldspar and quartz. The centre-most zone is referred to as the core zone, which is dominated by coarse grained quartz and muscovite. However, the core zone is not always developed. It is important to note that the thickness of each zone varies, depending on the thickness of the entire pegmatite body.