The Maldon Gold Operation is centred around the town of Maldon and consists of exploration and mining licences overlying the historic Maldon, Wehla, Amherst and Dunolly Goldfields.
Union Hill Mine
Centennial mining licence MIN5146 contains the Union Hill Mine and overlies the Maldon town site and most of the main producing reefs of the Maldon Goldfield, which between 1856 and 1926 produced over 1.7 million ounces of gold (Table 1 & Figure 5).
Table 1. Historical gold production from major reefs on the Maldon Goldfield.
Figure 5: Interpreted Geology of the Maldon Goldfield (GSV, 1:250 K Seamless Geology).
The metasedimentary sequence and reef systems were intruded by the Late Devonian sub volcanic Harcourt Batholith, which comprised at least three separate, dominantly granodiorite intrusive phases. K-feldspar-cordierite-andalusite-sillimanite-biotite grade contact metamorphic grade (M2) was attained in aureoles associated with the batholith (Gregory, 1994).
Quartz porphyry, diorite and basaltic lamprophyre dykes also intrude the Ordovician sequence. A quartz porphyry dyke appears to have intruded at least one of the major reef systems at Maldon. Evidence of post-granitic deformation includes reverse and strike-slip faulting, some of which is associated with the intrusion of an ultramafic dyke (Ebsworth and Krokowski de Vickerod, 2002).
Orogenic Gold deposits in the Western Lachlan Fold Belt, including the Bendigo Zone, which hosts the Maldon Goldfield and MIN5528, typically occur in quartz vein systems occupying brittle-ductile structures concentrated in regional anticlinoria. The distribution of some gold deposits is also thought to be controlled by deep first order faults. Gold may also be hosted in hornfels and altered dykes proximal to structurally controlled quartz vein systems. Auriferous quartz reef systems in the Central Maldon Shear Zone are associated with D3-D7 structures and exhibit a complex history of quartz vein formation (Ebsworth and Krokowski de Vickerod, 2002).
A more comprehensive overview of the geology of the Central Maldon Shear Zone is described in Central Maldon Goldfield, 1:5,000 Map Area, VIMP Report 75, Ebsworth and Krokowski de Vickerod, 2002.
The Maldon Goldfield is on the Maldon Central Shear Zone and includes the Union Hill underground mine, which has a decline developed for several kilometres parallel to the Maldon Central Shear Zone to the Alliance South Deposit (Figure 6).
Figure 6: Interpreted 3D view of the Maldon Central Shear Zone showing mining targets.
In the June Quarter 2017 pre-development work commenced at Union Hill to deepen the existing main decline below the 1080 level to access interpreted extensions to the Alliance South Shoot (Figure 7). At the end of the September Quarter 2017 the decline reached the Alliance South Zone on the 1060 level. The mine plan is to undertake long hole stoping of payable reef. First batch of low grade ore from the 1060 strike drive was delivered to Porcupine Flat Mill for treatment in October 2017.
The mine plan is to further develop the Alliance South Shoot from the 1060 to the 1020 levels as well as to follow up on other development opportunities around Linscotts, Eaglehawk and Ladies reefs.
A total of seven exploration targets for potential underground development have been identified along the Maldon Central Shear Zone (Figure 6). The current decline is within 500m of the key targets, excluding Nuggetty, with potential for modern exploration tools and diamond drilling to locate nearby ore shoots that were missed by historical mining.
The Nuggetty Mine was one of the highest grade gold mines in Victoria, having produced an estimated at 301,000 ounces from 50,000 tons, an average grade of 184 g/t (~ 6 oz/t). A recent mining study has returned a positive economic assessment of the proposed Nuggetty Underground Mine, 2km north of Union Hill. The Nuggetty mining lease was recently renewed and the Company is moving ahead with the necessary approvals for mining.
Figure 7: Proposed decline and level development at Union Hill to access the Alliance South Shoot.
In 2010, Octagonal Resources Pty Ltd recommenced a decline to access the Alliance South Shoot at the southern end of the Eaglehawk Reef, which been discovered earlier by Alliance Resources Limited. In 2014 the Alliance South shoot was intersected with some mining undertaken on the 1100 and 1080 levels, including a trial long hole stope by A1 Consolidated Gold Ltd in 2015.
Geology and Mineralisation
The Union Hill Mine lies within the northern extremity of the Central Maldon Shear Zone (Figures 5 and 6), located within the central part of the Bendigo Zone of the Lachlan Fold Belt.
The stratigraphy is composed of substantial volumes of strongly and complexly deformed (folded and faulted), thick-bedded turbidites of the Ordovician Castlemaine Group which have undergone metamorphism to lower greenschist facies and have been intruded by Devonian granites, producing contact metamorphic aureoles up to a few hundred metres wide.
The host rocks consist of Ordovician (Lancefieldian) turbiditic metasediments of the Castlemaine Group (490-450 million years, Cas and VandenBerg, 1988) which have undergone several major deformational events, reflecting a regional thin skinned style tectonic history. The dominant regional structures are tight, isoclinal, north-south trending asymmetric F2 folds with a pervasive axial planar cleavage and occasional small parasitic folds. Regional greenschist facies metamorphism was contemporaneous with F2 folding (Ebsworth and Krokowski de Vickerod, 2002).