In June 2024, Tasman Mining Ltd., a wholly owned subsidiary of Endura Mining Pty Ltd, acquired OceanaGold Corporation’s stake in the Snowy River (Blackwater) project. Following the transaction, Tasman Mining became the 100% owner and operator of the Snowy River project.
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Summary:
The Blackwater Mine is situated in a hilly, dissected area of partially exposed Greenland Group rocks overlain by fluvioglacial terrace deposits and recent colluvium and alluvium.
The deposit is a continuation of the Birthday Reef that underlies historical mine workings of the Blackwater Mine.
The Birthday Reef is an orogenic mesothermal quartz vein-hosted gold deposit situated in the western limb of a regionally significant anticline, proximal to the anticline hinge. Mineralised veins tend to strike sub-parallel with the regional geological structure. The deposit is hosted within a sequence of Ordovician age folded turbidite sediments (The Greenland Group).
The Reefton Goldfield lies in the area known as the West Coast Basin and Range Province, which is divided into three broad northerly-trending belts that terminate in the south and east against the Alpine Fault (Cooper, 1974). The Western Belt comprises early Palaeozoic quartz-rich rocks of the Greenland Group, within which lies the Reefton Goldfield. The Central Belt contains a mid-Cambrian to early-Ordovician volcanic island arc assemblage and the Eastern Belt consists of younger sedimentary rocks from lower Ordovician to early-Devonian in age.
The Reefton Goldfield is hosted by late Cambrian to early Ordovician (circa 500Ma) Greenland Group sedimentary rocks which form the basement rocks in the district. These are interbedded, massive to thinly-bedded (1-20m thick), quartz-rich sediments comprising gradational psammitic (greywackesandstone) and pelitic (argillite-mudstone) rock types. They are interpreted as a proximal turbidite succession derived from the erosion of a mature continental landmass, which lay to the east and southeast.
The Greenland Group sediments are moderately deformed and have undergone a late Silurian to mid-Devonian (438-395Ma), low-grade metamorphic event. This event was initiated by east-west compression resulting in regional folding and metamorphism to lower-greenschist facies, with illite clay facies predominating. Folds are close to tight, upright, with north-south trending fold axes and display a single pervasive and penetrative steeply-dipping, axial-planar cleavage.
Devonian, Triassic, Cretaceous and Tertiary sedimentary sequences overlie the Greenland Group rocks in the Reefton Goldfield. These sedimentary rocks occur in a belt along the western margin of the Greenland Group and also as downthrown fault-bound basins lying on the basement rocks. These basins are Tertiary in age and formed in response to Alpine block faulting. The younger sediments are not as strongly deformed or metamorphosed as the basement rocks, although their beds commonly dip at up to 35º, probably through rotation of the down-thrown blocks.
Mineralisation
Gold mineralisation at the Blackwater Mine is hosted within a quartz vein (reef) where about 70-80% of the gold is present as native gold, commonly occurring on the laminated host rock inclusions, with the remainder occurring as refractory gold locked in the lattice of pyrite and arsenopyrite. Sulphides comprise up to 1% by volume of the vein and besides pyrite and arsenopyrite, include minor stibnite and rare chalcopyrite and molybdenite.
The Reefton Goldfield mineralisation has important similarities, and is probably co-genetic and coeval to, the mineralisation at Bendigo and Ballarat in Victoria, Australia. In both Goldfields, mineralisation occurs within Ordovician sediments and is associated with the later stages of folding and thrust faulting.
Gold-bearing fluids arose from depth along highly deformed, fold-related corridors generated by high fluid pressures associated with regional metamorphism and deformation. Mineralisation occurred when these fluids precipitated gold-arsenopyrite-pyrite-stibnite, carbonate and quartz in brittle-ductile fractures.
Most of the gold-bearing quartz veins in the Reefton Goldfield are arranged along a linear belt that runs north-south through the Greenland Group sequence. This suggests the presence of a deep-seated structure that has tapped a large reservoir of mineralised fluid.
Fluid stability data from fluid inclusions in the quartz veins and the low salinity nature of the fluids, suggests that the mineralisation was probably derived from metamorphic devolatisation of the sediment pile, although the possibility of an igneous source, or component, cannot be entirely discounted.