Summary:
Deposit Type
The Nampala deposit is classified as a turbidite-hosted orogenic gold deposit that has been significantly influenced by tonalitic intrusions. This style of mineralisation is characteristic of greenstone belts worldwide and is typically associated with deformed and metamorphosed sedimentary sequences, often intruded by late-stage granitoids. The Nampala Property exhibits many key features of orogenic gold deposits, including its geological setting, structural control, alteration assemblages, and likely fluid composition.
Mineralization
Gold mineralisation at the Nampala Mine is divided into a Main and an East Zone. The turbidites hosting the Main and East gold zones of the Nampala Mine are oriented NNE and dip steeply ESE. In the Main Zone, these are composed of thick, interbedded greywacke, siltstone, and shale sequences. A graphitic shale horizon, which is not gold-bearing, separates the Main and East Zones. The East Zone turbidites include interbedded sandstones (arenite and gritstone), with gritstones comprising coarse lithic fragments suspended in a quartz-feldspar matrix.
The dominant hydrothermal alteration in both the Main and East zones of the Nampala Mine is marked by pervasive carbonatisation and silicification, with disseminations of pyrite and arsenopyrite accompanied by chlorite and kaolinite. Extensive saprolitic weathering is evident across the Property, affecting all lithologies to depths that can exceed 100 m before giving way to unaltered fresh rock. Overlying this, a thick layer of residual lateritic soil and duricrust, at times exceeding 10 m in thickness, blankets the region, rendering outcrops scarce.
Gold mineralisation is primarily hosted within competent, coarse-grained turbiditic units, specifically greywackes and siliceous sandstones. These lithologies, characterised by their brittle nature, facilitated the fracturing and subsequent vein formation. Gold occurs predominantly within structurally controlled tension quartz vein systems and stockworks that exploit these fractures and associated zones of enhanced porosity.
The tonalite intrusion, enveloped by lamprophyres, also contains mineralised quartz veins that share a similar orientation with those observed in the metasediments, suggesting a common structural control on mineralisation. Although the lamprophyre intrusions exhibit limited mineralisation, confined primarily to their margins, they appear to influence the spatial distribution of gold significantly. This is evidenced by the preferential concentration of gold mineralisation in the metasediments proximal to the lamprophyre contacts, supported by lithological competency contrasts and geochemical gradients.
Conversely, shear zones are predominantly developed within the more ductile, often graphitic, shales. These shear zones, however, are typically barren of significant gold mineralisation.
The structures and quartz vein system within the Main Zone are structurally and lithologically controlled and can be classified into three distinct mineralised structural domains, aligned along a north-south axis that are separated by at least two brittle faults: Northern Domain, Central Domain, Southern Domain.
Northern Domain
The Northern Domain is marked by NE–SW oriented mineralised envelopes containing two distinct quartz vein sets:
1. Moderately mineralised, sub-vertical en-échelon veins: These white to smoky lenticular veins trend ENE–WSW, dip steeply to the SSE, and are typically 10 m to 20 m long and 10 cm to 20 cm thick.
2. Conjugate stockwork of narrower, shallow-dipping veins: This stockwork consists of vertically stacked white veins and veinlets trending E–W.
The stockwork in the Northern Domain is primarily confined to arenite and sandstone beds and coarse-grained greywackes, forming decametric sigmoidal (augen-shaped) envelopes.
Central Domain
The Central Domain consists of an envelope containing three quartz vein sets:
1. Subvertical en-échelon white veins trending ENE–WSW and dipping steeply to the SSE. These veins range from 10 cm to 60 cm in thickness.
2. Flat, undulating veins that dip shallowly (25° to 30°) to the SSW and SSE. 3. Conjugate stockwork veins that consist of stacked smoky and white veins striking roughly N100 and dipping 50° to the SSW.
The Central Domain’s stockwork is denser than in the Northern Domain. The Nampala Mine’s highest-grade mineralisation occurs where stockworks overprint the en-échelon veins in both domains.
Southern Domain
The Southern Domain exhibits the following characteristics:
1. Quartz vein envelopes oriented approximately 020°N: This envelope is truncated by N–S structures or terminates at stratigraphic contacts. Veins propagate in coarse sediments along the intrusive contact and hornfels, where a less dense stockwork of quartz-carbonate veinlets and stringers, with disseminated sulphides, develops.
2. Anastomosing sub-vertical shear corridors: Oriented NNE–SSW. These corridors converge near the south wall of the mine pit. Mineralisation and hydrothermal alteration occur predominantly in more competent coarse-grained greywackes and sandstones, bound by less competent mudstone and siltstone. 3. Conjugate en-échelon tension veins: These veins are confined to coarse sediments, with flat veins dipping 25° to 30° to the S–SSE (2–3 m wide and <5 cm thick) and narrower sub-vertical veins with a steep W dip (60° to 70°). Sigmoidal shapes are often observed in the west-dipping veins.
Alteration
The dominant hydrothermal alteration in both the Main and East zones is marked by pervasive carbonatisation and silicification, with disseminations of pyrite and arsenopyrite accompanied by chlorite and kaolinite (clay minerals). The alteration exhibits a clear zonation pattern radiating outward from quartz veins. Sulphides, primarily fine (submillimetre) pyrite and arsenopyrite, are predominantly disseminated. These are concentrated within silicate-carbonate alteration halos in the surrounding wall rock and the quartz-carbonate veins. Silicification and arsenopyrite content appear slightly more pronounced in the East Zone than in the Main Zone, suggesting a higher intensity of alteration in this area.