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.
Wyoming 1
Gold mineralisation at Wyoming 1 is distributed both around and within a small (40 metres by 100 metres near surface, broadening at depth), roughly elliptical, subvertical, feldspar ± augite phyric sill. The deposit has been separated into distinct mineralised zones: the ‘porphyry’ zone; contact zone; hangingwall zone; and the ‘footwall’ zone.
The hangingwall zone appears stratigraphically controlled by a thin fine grained carbonaceous mudstone striking north-northwest and is the only defined mineralisation not having a close spatial relationship with a porphyritic sill host. The high grade mineralisation appears to have a strong ore shoot control plunging 40° to the south-southeast. Parasitic fold axes measured in drill core parallels this direction.
The footwall zone is an apparent extension of the hangingwall zone on the western fold limb. The zone of mineralisation is characterised as poddy quartz veining up against the Cotton Formation contact. The gold mineralisation does not extend into the Cotton Formation.
The ‘376’ and ‘831’ zones were originally interpreted as east–west high grade gold zones truncating and transecting the sill. The ‘contact’ zone was interpreted as a zone of mineralisation focused on the north-eastern contact of the sill. There is currently no evidence from pit mapping for the separation of ‘831,’ ‘376’ and ‘contact’ structures as interpreted from exploration data. Grade control has now combined all three zones as a high gold grading ellipse shaped ‘contact’ zone wrapped and focused on the north and north-eastern margin of the sill.
The ‘porphyry’ zone of mineralisation is dominated by a stockwork-like vein system of irregular silicification (locally described as ‘mushy quartz’) however, planar veins have a pervasive west-northwest strike.
Caloma 1
The Caloma 1 (previously known as just Caloma) deposit is hosted within two moderately west dipping (steepening at depth) porphyritic sills up to 60 m thick separated by thin metasiltstone units. Gold mineralisation is focused within a shallow to moderate west dipping sheeted vein system that approximately parallels the strike of local stratigraphy. The lodes terminate at the Cotton Formation contact in the west and tends to ‘horsetail’ when in contact with the volcaniclastic sediments in the east. The sheeted veins are dislocated by an east– west dyke swarm transecting the deposit, slightly rotating the stratigraphy and mineralisation in between dykes. The Caloma 1 mineralisation to the north is terminated near a northnorthwest trending structure where sheeted veining is rotated to moderately north dipping. To the south mineralisation has rotated 30°, striking northwest along a porphyritic sill— metasediment contact. Mineralisation linking Caloma 1 with Caloma 2 appears to be restricted within or adjacent to a narrow, steeply southwest dipping volcaniclastic sediment unit and is poddy in nature.
Caloma 2
The stratigraphy at Caloma 2 has a distinctive east–west orientation in contrast to the northnorthwest orientation at the adjacent Caloma 1 deposit. This dramatic change in trend is interpreted to be associated with folding along the major northwest trending fault which dislocates stratigraphy from Wyoming 3 to Caloma 2. The linking stratigraphy between Caloma 1 and Caloma 2 dips steeply to the southwest, aligning itself with this major structure to the south. Two dolerite dykes crosscut the stratigraphy and mineralisation and also have late off sets. Synformal folding has been observed within the oxide zone of the Caloma 2 pit.
The mineralisation at Caloma 2 is constrained to the east by shallow west dipping sheeted veins and to the west by moderately north dipping en-echelon vein sets. Mineralisation appears to dilate when in contact with a northern bounding volcaniclastic sediment unit. At depth there is evidence for a reverse saddle reef like structure associated with the closure of a moderately west plunging synform.
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.