Summary:
The Bowdens Silver Project is situated on the north-eastern margin of the Lachlan Fold Belt. The deposit is on the southern edge of 7km wide volcanic caldera complex.
The Bowdens Deposit is a low to intermediate sulphidation epithermal base-metal and silver system hosted in Carboniferous aged Volcanic rocks and Ordovician aged sediments.
Local Geology
The geology comprises of flat-lying mid-Carboniferous Rylstone Volcanics intruding into and erupted over Ordovician sequence of sediments called the Coomber Formation.
The Rylstone Volcanics range from 10 to more than 700 metres thick. Near the deposit thickness typically range from 50 to 250 metres.
Mineralisation outcrops on its southern extreme and plunges shallowly to the north along the contact between the Rylstone volcanics and Coomber Formation. The volcanics unconformably overlie the eastern side of the northwest trending Northern Capertee Rise and are themselves unconformably overlain by the Permian to Triassic aged shallow marine to alluvial sedimentary rocks of the Sydney Basin. (Biggs, Klein and Madayag, 2024).
Mineralisation
Mineralisation includes veins, breccias and fracture fill veins within tuff and ignimbrite rocks, and semi massive veins, breccias and fracture fill in siltstone, shale and sandstone.
Mineralisation is overall shallowly dipping (~15 degrees to the north) with high-grade zones preferentially following a volcanic intrusion. There are several vein orientations within the broader mineralized zones including some areas of stock-work veins.
Mineralisation throughout the Deposit is interpreted to have been deposited at low temperatures, often emplaced within a halo of earlier silica – pyrite over breccia, which has clearly acted to impede porosity and create overpressures within a lower fluid pressure regime. Hydrothermal fracturing formed preferentially subparallel to bedding and volcanic stratigraphy and packages of steep, randomly anastomosing veins.
The deposit mineralogy consists of polyphasal pyrite, sphalerite, galena, arsenopyrite, marcasite, silver-bearing and silver minerals (e.g., acanthite, pyrargyrite, proustite, pearcite, polybasite, stephanite, Ag-rich tetrahedrite, freibergite, tetrahedrite and tennantite) with lesser chalcopyrite and gold. The system displays vertical and horizontal zonation, with the thermal gradient increasing at depth and to the southwest. Semi to massive sulphides of high-iron sphalerite–pyrite–galena–chalcopyrite are developed in the upper basement units (Bundarra Zone), along with anomalous gold, whereas the Aegean Zone at depth in the northeast consists of only polybasite/pearcite. Gold and copper are deposited more prevalently at depth and in the south of Bowdens associated with galena–chalcopyrite overprinting pyrite–sphalerite. Ag–Bi sulfosalts (new minerals related to matildite) have also been observed at depth (Lay 2019).
Ample textural evidence suggests that failure was due to local low permeability and hydrothermal sealing permitting local overpressures (Carter, 2023). Flat lying faults with slicken fibers, and centimetre-scale black bands of cataclasite material, highlight the occurrence of repeated micro seismic events, characteristic of fluid injection stimulating brittle failure (Cox, 2020).
The zinc dominant mineralisation primarily occurs in the Bundarra Zone as stacked, flat-lying to moderately dipping zones of veins, breccias and fracture-fill sulphides … within siltstones, shales and sandstones of the Coomber Formation.
The gold-dominant mineralisation is associated with an increase in zinc, lead, and sulphur … across the Rylstone Volcanics and Coomber Formation basement contact, as well as within the Gully fault.
Gold occurs as irregular to partly formed grains about 5–50 µm in size. It fills fractures in early-stage iron lead and zinc sulphides and rims later-stage copper and lead sulphides. There are also small amounts of gold introduced late within quartz and carbonate veins.
Structure
Fault control which has led to the localisation of vein networks consist of numerous west dipping listric faults, of which the Eastern and Gully Faults host the Bowdens Deposit. The Eastern Fault is characterised by strong deformation, sericite-clay alteration, and silica flooding (as well as Prices Gully Fault), while the Gully Fault is characterised by more fault gouge and clay milling.
Locally the stratigraphy and orebody geometry is controlled by NNW trending faults and crystal tuff dome. A dacite intrusive and later mineralising fluids have ascended these conduits and cross faults with mineralisation depositing principally in the hanging wall of the Eastern fault, along geological contacts and brittle host units of welded tuffs and ignimbrites.
Alteration
Alteration within the Rylstone Volcanics at Bowdens consists predominantly of an argillic assemblage of clays (illite-smectite) sericite, silica, adularia and a wide range of carbonates (i.e. calcite, ankerite, rhodochrosite), representative of a steam heated blanket (Le Wang 2019), with a peripheral propylitic assemblage of chlorite and hematite. The Bowdens alteration and mineralisation events consisted of initial fluidised brecciation of silica-pyrite-adularia followed by progressive alteration phases of clay-carbonate (Lay 2019). These alteration styles consist of wall rock replacement and open-space filling dominated by quartz and adularia, followed by phases dominated by sulphides, then carbonates and finally clay minerals.
Dimensions
The open-pit Mineral Resources at Bowdens have an approximate extent of:
• 1,050m east-west;
• 1,250m north-south;
• From surface to 340m below surface.