Summary:
Maldon Goldfield Project comprises of the Union Hill Mine and Nuggety Mine.
The Maldon Goldfield lies within the central part of the Bendigo-Ballarat Zone of the Lachlan Fold Belt, defined by the regional north-trending, west-dipping Avoca Fault to the west and the Mount William Fault to the east. Major goldfields often occur about 3-5 km west of major thrust faults, and the Maldon Goldfield is located some 5 km west of the Muckleford Fault, the largest in the Bendigo Zone.
The Eaglehawk-Linscott’s Reef structure comprises a series of quartz reefs which outcrop discontinuously between the Maldon Railway Station in the south and Union Hill in the north. Linscott’s reef is linked to Eaglehawk reef by a duplex-like structure at the northern end of the Union Hill open pit. The Linscott’s fault-reef zone is typically 2-10 m wide. The arrays comprise discontinuous lenses of massive quartz, laminated quartz, and quartz-metasediment breccia. Locally, zones of quartz-filled tension gashes (spur veins) up to 50 m wide are associated with the west-dipping segment. The latter are common in the Union Hill open-cut mine.
Wall rocks within and close to the reef structures show evidence of hydrothermal alteration. Biotite alteration is the most abundant and distinctive alteration phase. Carbonate alteration is also present. The biotite alteration overprints an earlier Muscovite alteration, which is associated with abundant, arsenopyrite. Reef structures at Maldon typically exhibit strong sulphide alteration as disseminations and veinlets in halos up to 2m around reefs. Sulphides commonly encountered include; pyrite, arsenopyrite, chalcopyrite, galena, sphalerite.
Union Hil
The Union Hill Mine is part of the Eaglehawk-Linscott’s reef system. The reef extends north-south over a 2.5km strike length and is steeply dipping. The reef can be divided into eastern and western reefs that are interpreted to be separated by a fault. The reefs have different characteristics. The eastern reef is laminar and predictable but low grade. In contrast the western reef is irregularly mineralised in en-echelon quartz veins.
The Eaglehawk reef is not uniformly mineralised. Within the reef, mineralised zones are known as shoots. The most recently mined shoot is known as the Alliance South. This shoot dips vertically, is 90m below surface and has been defined over a strike length of approximately 330m, has a down dip extension of 280m and a horizontal width of up to 10 meters.
Auriferous quartz reef systems in the Maldon Goldfield are associated with D3-D7 structures and exhibit a complex history of quartz vein formation. The metasedimentary sequence and reef systems were intruded by the Late Devonian (370 Ma) sub-volcanic Harcourt Batholith, quartz porphyry, diorite and basaltic lamprophyre dykes. The Harcourt Batholith has a 3-4 km contact metamorphic aureole that forms an embayment that dips gently to the southeast. Most major gold-producing reefs lie within the cordierite isograd. The most significant of the felsic dykes locally is referred to as the 'Elvan' dyke, and is known to traverse the Maldon goldfield from the north to the south and beyond. Quartz veining is commonly located along the margins of the 'Elvan' dyke, and in places was economically mined for gold.
Gold commonly occurs in stylolitic to slickolitic seamy quartz and to a lesser extent in massive quartz, quartz hornfels breccias and extensional vein arrays (‘spurs’). Recrystallisation and possible further addition of quartz veins probably occurred during D5-D6.
Gold mineralisation at Maldon shows an association with arsenopyrite and minor amounts of other base metal sulphides, largely pyrite, galena, sphalerite, chalcopyrite and marcasite in common with other Central Victorian Goldfields. However it is also associated with native bismuth, bismuthinite (Bi2S3), maldonite (Au2Bi), joseite [Bi3Te(Se,S)], several other unidentified tellurides, pyrrhotite, loellingite (FeAs2) scheelite, stibnite and molybdenite and has an unusually high pyrrhotite to pyrite ratio.
Gold-bismuth mineralisation in hydraulic breccias of quartz and hornfels appear to overprint arsenopyrite-pyrrhotite-pyrite mineralisation. Auriferous quartz vein samples from Linscott’s Reef are reported to contain two phases of gold. One with a silver content of 8% similar to other Central Victorian gold deposits, the other was very pure with less than 1% silver and the gold was closely associated to bismuth. The variable replacement of large, twinned arsenopyrites by loellingite and pyrrhotite is thought to be the result of the interaction with granite related fluids. A nickeliferous, euhedral phase of arsenopyrite, distinct from the altered arsenopyrite, is closely associated with gold-bismuth mineralisation and it is thought that later contact metamorphism redistributed the gold and bismuth from maldonite and modified large twinned crystals, of loellingite in wall rock alterations to arsenopyrite and pyrrhotite.
Union Hill Prospect incorporates the Linscott’s and Eaglehawk reefs, both associated with the German Anticline, a relatively large east-dipping fold (~68° E) in the area. A major low angle, northwest dipping reverse fault cuts the sequence and has an estimated throw of 25 m. Eaglehawk reef is associated with two folds at Alliance South prospect, the German Anticline and the German Syncline to the east. The anticline is overturned to the west, has axial planar dips of ~75° E and in places has parasitic folds developed in the east limb. The western lode has steep east to west dips and is sub-parallel to the west limb of the German Anticline but weakens at higher levels and becomes bedding-parallel. The eastern lode dips moderately to steeply west and lies on the Eaglehawk structural surface, crosscutting bedding and continuous with the lode intersected in the upper flexure. Two weaker lodes occur in the footwall of the eastern lode but do not show continuity throughout the drilled area.
Nuggetty Reef
The Nuggetty Reef is associated with the Nuggetty syncline and is a fault-related vein array formed in a breached synclinal hinge. The syncline hinge dips very steeply east and it has been transposed by the reef structure. The reef has a prominent, steeply east-dipping fault, which is sub-parallel to the east limb of the Nuggetty syncline and bounds the reef to its east. The bulk of the historically mined reef is associated with the hinge and west limb of the Nuggetty syncline. Quartz veining within and close to the east wall is mainly shear and bedding parallel. To the west, the quartz vein array comprises dilatational massive, laminated, breccia and spurry quartz. Quartz veining is bedding parallel and massive seamy dilatational breccia and spurry quartz veining. Ore shoots are associated with subtle dextral flexures in reef.
The Nuggetty Reef comprises fault associated quartz reefs that discontinuously outcrop between Five Ways to the south and the granite contact to the north. It forms a 490 m long, north-northwest to north-northeast trending line of outcrop. Down plunge the reef is known from diamond drilling to extend some 500 m south of Five Ways. The known down-dip extent of the Nuggetty Reef is approximately 250 m at the Tarrangower Nuggetty Alliance Company shaft (TNAC), and 200 m in the Five Ways region. Gold production was predominantly from the main (eastern) shoot associated with the east reef. Further production was from smaller shoots associated with the west reef and a flat linking spur reef connecting the two structures.