Summary:
The Tomingley gold deposits are interpreted as orogenic gold systems positioned within a major structural zone. This style of deposit is well documented globally with the more significant example in Australia being the Archean greenstone belts of the Yilgarn Craton in WA.
Roswell
The Roswell deposit is hosted in the Mingelo Volcanic Formation, a strongly deformed and hydrothermally altered Ordovician aged belt of volcanics that are predominantly andesitic volcaniclastic breccias, lesser sandstone/siltstone units, lavas and black mudstones. The volcanics are overlain by the younger Cotton Formation siltstones.
The mineralisation at Roswell is primarily hosted by two ‘brittle’ volcanic units (monzodiorite and andesite) as per the structural setting observed at the Tomingley gold deposits. These volcanics host structural zones generated by a competency contrast between the ‘brittle’ volcanics and ‘ductile’ volcaniclastic sediments.
Mineralisation is characterised as similar to the Tomingley gold mineralisation, with quartz carbonatepyrite-arsenopyrite veins hosted in phyllic altered volcanics. These sheeted quartz veins are orientated as steep east dipping, striking approximately 10o east of north, and are typically constrained within the volcanic units. The mineralisation has been defined by drilling over a strike length of approximately 600 metres and remains open to the north and at depth. The higher grade mineralisation occurs in the southern section, proximal to and truncated to the south by a regional NW trending structure named the Rosewood Fault. The San Antonio deposit is a continuation of the mineralised zone to the south of the fault. The Rosewood Fault is of a similar orientation to the structure that dextrally displaces the Caloma deposits from the Wyoming deposits, positioned in the centre of the Tomingley ‘gold camp’.
The mineralisation at the Roswell Deposit is displaced by three approximately 4 metres thick dolerite dykes dipping steeply to the NNE, striking WNW. The dolerites postdate the gold mineralisation.
Weathering of the mineralised bedrock has developed a saprolitic clay profile extending approximately 35 metres from the base of alluvium to fresh rock. The mineralised bedrock lies beneath a Cainozoic alluvium overburden between 30-55 metres thick.
San Antonio
The San Antonio deposit is similar to Roswell being hosted in the Mingelo Volcanic Formation, a strongly deformed and hydrothermally altered Ordovician belt of volcanics that are predominantly andesitic volcaniclastic breccias, lesser sandstone/siltstone units, lavas and black mudstones. The volcanics are overlain by the younger Cotton Formation siltstones.
The San Antonio deposit is located south of the Roswell deposit, dextrally displaced and separated by the Rosewood Fault. The drilling has defined north to north-northeast striking, sub vertically dipping, deformed and attenuated andesitic volcanic and volcaniclastic stratigraphy. The mineralisation at San Antonio is primarily hosted by three ‘brittle’ volcanic units as per the structural setting observed at the Tomingley gold deposits. These volcanics host structural zones generated by a competency contrast between the ‘brittle’ volcanics and ‘ductile’ volcaniclastic meta-sediments. Litho-geochemistry has identified a phosphorous enriched (apatite bearing) andesite 70m to 80m thick with a 10m wide brecciated basal contact that hosts the majority of mineralisation in the northern half of San Antonio. The andesite thins to a thickness of less than 20m to the south and is offset by the west-northwest trending ‘Kenilworth Fault’. The fault appears to rotate the volcanic stratigraphy sinistrally 15o to the east. South of the Kenilworth Fault, intruding near or along the margins of the andesite are two thin, less than 10m thick, feldspar phyric dacitic intrusions. These intrusions are invariably mineralised and are the focus to the mineralisation in the southern section of the deposit. A third volcanic unit, approximately 50m thick, is a monzodiorite sub-volcanic sill that is positioned west of the andesite in a similar stratigraphic position as observed at the Roswell deposit. The monzodiorite appears only mineralised north of Kenilworth Fault forming mineralised lodes with strike lengths of up to 200m. This unit remains poorly defined by drilling due to the restricted access of the Newell Highway.
Mineralisation is characterised as Tomingley-style orogenic gold mineralisation, with quartz carbonatepyrite-arsenopyrite veins hosted in phyllic altered volcanics and volcaniclastics. These sheeted quartz veins are orientated from steeply east dipping to vertical forming sigmoidal shapes, striking approximately north-south to 15o east of north, and hosted within subvertical dipping stratigraphy. The mineralisation has been defined by drilling over a strike length of approximately 1,000m and remains open at depth. The more significant sized and higher grading zones of mineralisation are hosted in the andesite in the north and in the dacitic intrusives in the southern sections of San Antonio.