Summary:
The area of EPL 5047 is underlain by rocks of the NNMP with the lithology of EPL 5047 comprising units from the Gordonia Sub-province, which is separated from the Richterveld Sub-province by the north-westerly trending PSZ. Although the most prominent feature of EPL 5047 is the PSZ, numerous other structural zones can be noted with predominantly northeast and east-west trends. The host rocks comprise a volcano-sedimentary sequence intruded by a coarse-grained gabbroic rock.
The main lithologies comprise volcanic rocks, chlorite schists and phyllites, all metamorphosed to varying degrees.
A pegmatite is defined as “an essentially igneous rock, commonly of granitic composition, that is distinguished from other igneous rocks by its extremely coarse but variable grain size or by an abundance of crystals with skeletal, graphic, or other strongly directional growth habits. Pegmatites occur as sharply bounded homogenous to zoned bodies within igneous or metamorphic host rocks.” (London, 2008).
Pegmatites are defined by several geological, textural, mineralogical and geochemical parameters, and are broadly classified as either simple/common or complex based on the presence or absence of internal zonation. Simple/common pegmatites are unzoned, poorly fractionated and thus usually un-mineralised. Complex pegmatites often contain potentially economic concentrations of mineral/elements (including lithium, tantalum, niobium, tin, beryllium and REE).
The Rare-Element classes are subdivided based on composition into three broad families based on other petrological, paragenetic and geochemical data:
• Lithium-Caesium-Tantalum (LCT);
• Niobium-Yttrium-Fluorine (NYF) and;
• Mixed LCT – NYF families.
The Swanson pegmatites are classified as LCT pegmatites.
It should be noted that pegmatites often occur as a combination/hybrid of the subtypes listed, but with one or two of the minerals dominating over the other(s).
Simmons (2021) points out that attempts to relate pegmatite types or subtypes to magma genesis or tectonic regimes as has been attempted in granite classifications are not satisfactory. He also notes that the classification fails to address the possibility of pegmatites forming by direct anatexis.
Rare-Element pegmatites are often intruded into metamorphic rocks where the peak metamorphic conditions attained are upper greenschist to amphibolite facies (London, 2008) and have temporal and spatial associations with granitic plutons. Most pegmatites occur in swarms or pegmatite fields and occupy areas ranging from tens to hundreds of square kilometres; they may be associated with a discrete granite source around which they are systematically distributed, from the least fractionated granite to the most highly evolved pegmatites are the greatest distance from the granite source (London, 2008); however, this is not always the case. The possibility of pegmatites forming by direct anatexis of the host rock should also be considered.
With increasing fractionation, there is also often an increase in the complexity of the internal pegmatite zonation. The most highly evolved distal pegmatites are usually the most complexly zoned and associated with potentially economic concentrations of the elements and associated minerals identified above.
Pegmatites may vary from a few metres to hundreds of metres in length with variable widths ranging from <1 m to tens of metres wide and may have simple to complex internal structures. A number of different internal units may be present within a pegmatite based on differences in mineral assemblage, modes and textures. These may include zones of primary crystallisation forming more or less concentric shells (asymmetric zonation also common), complete or incomplete, from the margin inwards; replacement bodies formed at the expense of pre-existing units with or without lithologic and/or structural control; and fracture fillings associated with primary zones or replacement units.
The main rock forming minerals in a granitic pegmatite include quartz, mica (muscovite and biotite) and feldspar. Other minerals may occur in economic concentrations and include, but are not limited to, various lithium minerals, beryl, tourmaline, cassiterite, coltan, topaz, garnet and various rare-earth minerals.
The Tantalite Valley pegmatites belong to the LCT family of pegmatites and can be classified as a mixture of the spodumene, lepidolite and albite-spodumene subtypes of complex type, lithium subclass, rare-element class pegmatites.
The pegmatites on the Swanson Property are not zoned, with the exception of isolated instances, and are banded to massive, are dominated by quartz, sugary albite and muscovite, and exhibit variable concentrations of tantalite, spodumene and lepidolite.