Summary:
Myhree, Boundary, Jones Find, Fingals East deposites: The style of mineralisation is Archaean orogenic gold.
Myhree Mining Centre
Myhree (open pit)
A well-developed laterite zone, up to 25m thick, sits above the local sequence which consists of (from footwall to hangingwall): komatiitic ultramafic, interlayered dolerite and volcaniclastic sediments (with variable black shale lenses), komatiitic ultramafic, and then into polymictic conglomerate with intruded dolerite lenses.
The footwall of Myhree is characterised by a zone of heavy chlorite alteration within the footwall ultramafic, generally following the trend of the ultramafic-diorite contract. Abutting the chlorite alteration is a package of highly silicified rocks generally consisting of the diorite and volcaniclastic sediments. The silicification can however cross into the footwall ultramafic, producing a bleached and highly competent rock. Above the silicified zone, the hangingwall ultramafic is characterised by talc-carbonate alteration. Finally, the conglomerate also appears to be highly silicified.
The main orientations are NW-SE dipping north and north-south dipping west. Mineralisation terminates on a bounding NS fault, with no mineralisation observed on the eastern side. Both structures have no evidence in the drilling of a reduction in competency.
Boundary (open pit)
The Boundary deposit is hosted within NNE trending, west dipping units of altered ultramafics that sit below ~20m of overburden including ~10m of pisolitic laterite. These units range from Komatiite to peridotite and have been intensely altered by silica, carbonate and chlorite, with little relic textures visible. Occasional intense fuchsite alteration is also present. Mineralisation is associated with areas of increased shearing, with associated increases in sulphides in quartz veins. Thin zones of black shale are present in areas of high strain. The deposit is crosscut by NW trending faults which seem to both dislocate mineralised trends, as well as act as zones of higher grade gold concentration.
Majestic Mining Centre
Majestic (underground)
Majestic is hosted within a quartz diorite on the western margin of the Juglah Monzogranite. The quartz diorite is relatively equigranular and contains up to 10% quartz. The quartz diorite has been intruded by porphyritic dykes that somewhat bound the main zone of mineralisation.
A deep weathering profile of ~30-40m (down to 60m in places) exists across the deposit and displays weak supergene mineralisation above 35m that sits directly below a stripped zone of mineralisation.
Majestic is dominated by generally NS, steeply west dipping structures. Within these structures two plunges have been identified, both within drill core measurements and grade distributions:
• Gentle north to gentle south plunge identified within vein intersections containing sulphides, alteration contacts and progressively higher gold grade cut-offs.
• A moderate southwest plunge within veins that contain various silicate infill minerals, alteration contacts, lithological contacts, shears, sulphide bearing veins, late faults and areas of moderately elevated gold grades.
• Two styles of mineralisation are observed, an earlier biotite-pyrite wash and a later state bleaching (albite-silicapyrite). Features of the two styles include:
- Biotite-pyrite mineralisation: Spatial association with porphyritic dykes; Elevated gold generally associated with increase in pyrite content; Increased biotite fractures/brecciation indicate elevated gold.
- Albite-silica-pyrite mineralisation: Elevated gold and copper associated with increased pyrite content; Commonly associated with quartz-sulphide veining with albite alteration halos; Later stage non-mineralised albite-silica alteration overprint mineralised veins.
• Based on fluid inclusion work, mineralising fluids are thought to be derived from a magmatic derived fluid source.
Jones Find (open pit)
Jones Find comprises a north-striking, steeply west dipping sequence of sheared mafic-intermediate intrusive rock. The host is locally foliated and schistose. Widespread alteration comprises silica-carbonate, whilst localised zones of silicasericite-kaolinite are observed as well as silica-sericite-chlorite(-biotite). The weathering in the area is typically to depths of 20-35m and displays evidence of supergene enrichment.
Locally, the granitoid exhibits intense shear related deformation, which is associated with alteration haloes of up to 100m in width. The following styles of gold mineralisation have been recognised at Jones Find:
• Narrow vein gold hosted within quartz veins, associated with shearing and biotite/sericite alteration. Most old workings are developed in these zones.
• Quartz-biotite-clay ± albite alteration associated with anomalous gold assays ranging from 0.1 g/t Au to 0.4 g/t Au.
• Supergene enriched gold mineralisation within the saprolitic zone.
Crown (open pit)
Crown is characterised by a lack of topographical relief and is covered by recent alluvium and colluvium up to 4m deep. The area is dominated by altered granodiorite which hosts the mineralisation. To the immediate south of Crown, the mineralisation appears to be truncated by the ENE trending Celebration dyke that forms part of the Widgiemooltha Supersuite.
Alteration over Crown appears to be middle green schist facies regional metamorphism which has generated new assemblages of plagioclase-actinolite-biotite-quartz-clinozoisite-ilmenite-sphene-chlorite-sulphides (pyrite, chalcopyrite, pyrrhotite). Mineralisation is associated with narrow quartz sulphide veins and sulphide veinlets that trend roughly north south and dip steeply to the west. Mineralised structures are observed at surface.
Fingals Mining Centre
Fingals Fortune (open pit)
The Fingals Fortune deposit is situated on the western limb of the Mt Monger anticline, dipping at ~30-40 degrees to the southwest.
Three brittle deformation events are identified within the deposit:
• Well-developed bedding parallel thrusts striking ~340° and dipping 30°-40° to the west are strongly associated with mineralisation. The thrusts are associated with quartz veining that boudinages, resulting in variable thickness of the shear zones from 1 to 6m. Flat lying tensional structures with associated quartz veining occur between the thrusts.
• Sinistral subvertical oblique-slip shear zones striking 345° form an en-echelon system. These form narrow subvertical shear zones that overprint the thrust zones.
• EW set of brittle faults striking 70° and dipping 50-80° to the north. Faulting of this orientation is believed to occur between the historic north and south pits at Fingals Fortune.
Mineralisation is predominantly hosted in the highly sericite altered felsic porphyry, with historic pit sampling indicating that gold is generally hosted within quartz veining. Limited lower grade gold may potentially occur within the alteration zone.
Mineralisation has strong structural controls, with the reactivation of the bedding parallel thrusts during the sinistral obliqueslip shear event thought to be the mineralising event. This has resulted in three main orientations to mineralisation:
• Moderately shallow westerly dipping veins following the bedding parallel thrust structures.
• Flat lying to shallow south dipping veins following the tensional structures.
• Unconfirmed narrow subvertical veins following the sinistral oblique-slip shear zones (Mc Gahren, 2015).
Thicker zones of mineralisation are generally observed where the thrust zones intersect the flat lying structures.
Fingals East (open pit)
The Fingals East deposits are situated on the eastern limb of the Mt Monger anticline, dipping at ~40-50° to the SE. HighMg pillow basalts host a coarse-grained magnetic dolerite sill, with shearing along the margin and within the dolerite hosting the mineralisation.