Summary:
The Tomingley deposits show many similarities to orogenic gold deposits such as alteration, mineralisation and vein style and composition. Each of the deposits has its own structural nuances, however, mineralisation is dominantly hosted within the subvolcanic sills, competent lavas or along their immediate contacts with volcaniclastic metasediments.
Alteration appears multiphase with repeated cracking, crushing, veining and sealing, leading to heterogeneous, patchy alteration and discontinuous narrow veinlets. Alteration is characterised by a bleaching white mica (muscovite)–carbonate (ankerite)–albite-silica-(± chlorite) as pervasive replacement of the host rock around strong quartz + carbonate (ankerite) + pyrite + arsenopyrite ± albite veining. The veining typically forms subparallel sheets up to 10 m thick and spaced typically 20 - 30 m apart within the sills and lavas and along their contacts.
Multiple phases and recrystallisation of pyrite and arsenopyrite occur early in the paragenesis. Late fractures in earlier pyrite and arsenopyrite have served as nucleation sites for the precipitation of gold which occurs within or disseminated near the selvages of the quartz - ankerite vein assemblage. Rare sphalerite and chalcopyrite are syngentic with gold mineralisation.
The Tomingley Gold Extension Project permits Alkane to extend gold mining operations to the San Antonio and Roswell resources immediately south of the existing mine.
Roswell
he Roswell deposit is positioned north of a regional northwest trending structure termed the Rosewood Fault. This fault, originally identified in the aeromagnetic data, appears dextral and is of a similar orientation to the structure that dextrally displaces the Caloma deposits from the Wyoming deposits at Tomingley.
The drilling at Roswell has defined a fault bounded section of volcanic stratigraphy covered by 30 m to 55 m of alluvial clays and sands. The faulted subvertical volcanic stratigraphy is rotated from striking north to striking north-northeast. The mineralisation appears to be hosted by two different volcanic units - monzodiorite and andesite - within a coarse grained volcaniclastic package generating structural zones by a competency contrast between the ‘brittle’ volcanics and ‘ductile’ volcaniclastics.
The stratigraphy at Roswell comprises immature volcaniclastic sandstones and conglomerates with lesser siltstones/mudstones. More evolved, fine grained plagioclase phyric multi-phased andesite lavas are slightly magnetic and hosts a significant proportion of the gold mineralisation. In thin section, the andesite lavas have abundant tiny apatite needles within the plagioclase, accounting for the slightly elevated phosphorous concentration in comparison to the other volcaniclastics and lavas within the stratigraphic package.
Intruding into this volcanic package east of the andesite lavas, is a monzodiorite that appears to have the same petrographic qualities as the subvolcanic sills that host the majority of the mineralisation at the Tomingley deposits, with the exception that it has a holocrystalline texture suggesting it is likely a deeper intrusive. A second, smaller and porphyritic, monzodiorite intrusive was identified west of the andesite lavas at depth by the recent deeper drilling.
The mineralisation at Roswell is characterised as typical quartz-carbonate-pyrite-arsenopyrite veins hosted in phyllic altered volcanics. The mineralised zones range from 2 m to 30 m wide and as stacked tension veins, sometimes becoming more of a stockwork within the andesite host.
San Antinio
The San Antonio deposit is also positioned south of a regional northwest trending Rosewood Fault. A second significant structure named the Kenilworth Fault is oriented west-northwest and transects the southern section of the San Antonio deposit.
The mineralised bedrock lies beneath a Cainozoic alluvium overburden between 20m - 60m thick. Weathering of the mineralised bedrock has developed a saprolitic clay profile extending approximately 35 metres from the base of alluvium to fresh rock.
The stratigraphy at San Antonio comprises immature volcaniclastic sandstones and conglomerates with lesser siltstones/mudstones. More evolved, fine grained plagioclase phyric multi-phased andesite lavas are slightly magnetic and hosts a significant proportion of the gold mineralisation. In thin section, the andesite lavas have abundant tiny apatite needles within the plagioclase, accounting for the slightly elevated phosphorous concentration in comparison to the other volcaniclastics and lavas within the stratigraphic package.
Intruding into this volcanic package west of the andesite lava, is a monzodiorite that appears to have the same petrographic qualities as the subvolcanic sills that host the majority of the mineralisation at the Tomingley deposits. This unit is poorly constrained by drilling with only a few drill holes testing along strike from mineralisation defined within the central portion of this unit. This favourable unit for hosting mineralisation is open along strike.
A second type of intrusive, feldspar phyric dacites, are identified only in the southern portion of San Antonio and intrude along or proximal to the contacts of the andesite. Although less than 10m thick where intersected these intrusives are usually strongly mineralised.
The mineralisation at San Antonio is characterised as quartz-carbonate-pyrite-arsenopyrite veins hosted in phyllic altered volcanics and volcaniclastics. The mineralised zones range from 2m to 20m wide and form as sheeted sigmoidal tension veins, sometimes becoming more brecciated and shear hosted along the eastern contact with the andesite host.
The mineralisation at San Antonio can be displaced by an occasional 1m - 3m thick postmineralisation dolerite dykes. The dolerite dykes in the region are Devonian aged and have a similar orientation, dipping steeply to the north-northeast.