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Location: 1 km E from Stawell, Victoria, Australia
PO Box 265StawellVictoria, Australia3380
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Sulfide gold mineralization at Stawell is typical of orogenic gold deposits.Stawell is hosted by quartz-rich turbiditic sedimentary rocks (Albion Formation) that overlap a thick pile of tholeiitic basaltic lavas (Magdala Basalt). It is suggested that the host-rock (Stawell Facies) was originally a turbiditic sedimentary rock that was hydrothermally altered in response to seawater interaction with the hot basaltic pile. Subsequent regional greenschist metamorphism and ductile deformation culminated in the formation of the Magdala mineralized system and produced a complex pattern of hydrothermal alteration.There are three mineralization styles at Stawell, being Magdala (separated into west and east flanks), Golden Gift and Wonga. The Magdala and Golden Gift ore types are hosted within the Magdala Volcanogenic. Within the Magdala Deposit there are three main ore types; Central Lode, Basalt Contact Lodes, and Magdala Stockwork Lodes. The East Flank mineralization introduces a new ore type: The Hampshire Lode.Central Lode mineralization was a significant production source from Magdala early in the mine’s history. It is a quartz-rich shear lode ranging from 0.5m to 10m in width and generally dips 55° to 65° to the west with a total strike length of 4km and a down-dip extend of one kilometer. The overall structure is mineralized economic shoots that vary from 20m to 30m in strike up to 200m to 350m in strike. Free gold in the quartz is associated with pyrite, arsenopyrite and recrystallized pyrrhotite. Average mined grade for Central Lode is 4.0 g/t Au – 7.0 g/t Au.Basalt Contact Lodes are located parallel to the Magdala Basalt and in ‘waterloo’ or reentrant positions. They are typically 2m wide and are represented by arrays of quartz sulphide tension veins immediately adjacent to the Volcanogenic Basalt contacts. Sulphides include pyrrhotite, arsenopyrite and pyrite and occur as alteration selvages on tension vein margins. The main alteration mineral is stilpnomelane, resulting in its dark color. The mineralization is isolated to the Magdala Volcanogenic package with none present in the adjacent Magdala Basalt. Ore shoot lengths range between 50m and 450m. The average mined grade for Basalt Contact Lodes is 4.0 g/t Au – 9.0 g/t Au.The Magdala Stockwork Lodes are situated above major basalt noses and can be described as a hybrid between Central and Basalt Contact Lodes. They consist of large quartz tension vein arrays with arsenopyrite and pyrrhotite dominant sulphide mineralization. The strike extent is limited to 40m to 50m and limited vertically to between 30m and 50m. Average mined grade for Magdala Stockwork Lodes is 4.0 g/t Au – 7.0 g/t Au.Unlike the Magdala Deposit there is only one identifiable ore type in the Golden Gift and this is termed the Golden Gift Stockworks. Though there is only one discernible ore type in the Golden Gift, the Golden Gift Stockworks contain a spectrum of all Magdala styles. Typical widths range from 8-12m up to 30m and the strike extents of shoots range between 150m and 400m. Areas of highest gold grades and largest widths are situated above major basalt noses which are present in most ore bodies. Quartz content is generally below 25%. Mineralization includes abundant recrystallized pyrrhotite and coarse grained arsenopyrite, pyrite and visible gold. Average mined grade is 4.0 g/t Au – 10.0 g/t Au.The Aurora B Zone sits on the East Flank of the Magdala Basalt. Like the West Flank, mineralized gold lodes occur within altered sediment off the contact of the Magdala Basalt ‘dome’. However, the host rock appears to differ and as a result so does the style of gold mineralization. The Fest Flank sediment hosted ‘Volcanogenics’ protolith appears to consist of siliceous siltstones, mudstones and calcareous sandstones and gold mineralization has a close association with sulphides (Aspy, Py, Po). Unlike the West Flank sediments, the Eastern Flank includes a magnetite-rich banded unit. This East Flank protolith was most likely siliceous and ferruginous chemical sediment. The magnetite banding is likely the result of interfingering of Fe-rich sediments (granular Fe-oxide particles and minor pyrite) with siliceous sediments. The term ‘BIF’ has been used in the logging codes to represent this due to visual similarities to banded iron formations. The Aurora B gold mineralization was observed to differ from the sulphide-replacement-style mineralization observed on the Western Flank. The Eastern Flank of the Magdala Basalt is distinguished by a significant reduction of chlorite (which is predominantly limited to vein selvages), and a near absence of muscovite. The Aurora B mineralization is hosted within a strongly altered silica-sulphide altered siliclastic sedimentary unit that contain centimeter to meter scale sub-planar layers dominated by magnetite-chlorite-carbonate alteration. The West Flank mineralization, however, is hosted in quartz lodes and within Fe enriched mudstone (The “Stanwell Facies” or Volcanogenics – Mine term).
