Finsch has an age of 118 ± 2.8 Ma (Smith et al., 1985), and is a Group-2 kimberlite (Smith, 1983a; Fraser and Hawkesworth, 1992). The geology of Finsch Mine in the open-pit and on the 348 m underground sampling level was described by Clement (1982). The pipe was emplaced through a thick sequence of Proterozoic, Griqualand West sequence sedimentary rocks comprising dolomites, banded iron formation and shales. Preserved within the pipe are large fragments of Karoo-aged sediments, lavas and dolerite which were clearly present at the time of emplacement, but have been subsequently eroded away.

Clement defined eight kimberlite varieties in the mine which he called F1–F8. F1 was described as being the most significant volumetrically and was classified as a diatreme-facies TKB. This variety hosted numerous very large blocks of down rafted Karoo basalt lava and sedimentary rocks. The F5 variety was also described as TKB, but occurred within a satellite portion of the pipe called “the precursor”. The other varieties were classified as hypabyssal kimberlite or kimberlite breccia.

Exploration of deeper levels of the mine found that the F8 variety increased in size with depth, and that the lithiccontent of the F1 declined with depth. Mapping of deeper levels has also revealed the presence of steeply dipping bedding planes within both the F1 and F8 varieties. Ekkerd et al. suggested that the F1 and F8 could be the same kimberlite, varying only in lithic content and alteration.

Mining method: Underground sub-level cave.

Production has been from the 4.8 Ha, 4-level Upper Block 5 SLC (sub-level cave), which has been operating at steady state since 2019. It predominately produces highly commercial diamonds of +five carats and gem-quality smaller diamonds.

In extension of the mine plan to 2030, the Board has approved the Lower Block 5 3-level project which is expected to commence during FY 2023 with production contributions from FY 2025 onwards.

Petra has put in place a ‘grade and draw’ strategy to cut off the waste and prevent further progression. The drilling and blasting of production rings ensures that the ingress of waste and fines can be arrested timeously, allowing the next ring to be accessed to produce cleaner ore.
This approach aims to ensure the planned extraction can continue for FY 2023 at an optimal grade recovery.

The implementation of the 78-Level Phase 2 extension to the current cave will also assist in providing clean ore for the remaining life of the current sub-level cave and enable a smooth transition to the new, lower block 5 sub-level cave from FY 2025 onwards.

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