Mandalay Resources Australia Pty (“MRA”), formerly Australian Gold Development, is a private Australian Corporation that operates Costerfield. MRA owns 100% of the voting securities of its sole subsidiary, Mandalay Resources Costerfield Operations Pty.
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Summary:
Deposit Types
Narrow vein, antimony-gold and gold-only lodes are the targeted deposit styles at the Costerfield Property. Economic lode material consists of either a ‘typical’ gold-bearing quartz and carbonate with massive stibnite (for example, the Augusta C, D, and E Lodes, N Lode, Cuffley and Youle), or gold-only quartz and carbonate veining as seen in the Shepherd system.
The mineralised shoots are understood to be structurally controlled, typically by the intersection of the lodes with major cross-cutting, gouge filled fault structures and shears. Notable west to northwest dipping thrust faults typically bound the mineralisation packages at the Costerfield Property but can become significantly mineralised themselves along the fault planes. Shallower and dominantly west dipping thrust faults, typically at very low angles or even parallel to bedding with a laminated quartz component, link between the larger order thrust faults. The link faults can also offset the vertical lode structures up to 50 m in an east–west sense. This structural framework leads to the subvertical, north–south extensional veining seen in the Augusta, Brunswick, Kendall and Shepherd systems, along with the moderately west-dipping fault reactivated deposit at Youle.
Property Mineralisation
The Costerfield Property lies within a broad gold-antimony province mainly confined to the Siluro-Devonian Melbourne Zone of Victoria. The narrow quartz-stibnite-gold veins of the Melbourne Zone are mesothermal to orogenic in nature and are a product of a 380–370?Ma tectonic event. Gold in Central Victoria is believed to have been derived from the Cambrian greenstones that underlie the entire province at depth; however, the?origin of the associated antimony has been less studied. Significant portions of the local area are obscured by alluvium and colluvium deposits, which have been washed over the surrounding flood plains by braided streams flowing east off the uplifted Heathcote Fault Zone. Some of this alluvial material has been worked for gold but workings are small-scale and limited in extent. Most of the previously mined hard rock deposits were found either outcropping or discovered by trenching within a few metres of the surface.
The mineralised structures in the Costerfield Property, which typically dip steeply east or west (Augusta, Brunswick, Kendall, Shepherd), or moderately west (Youle) are likely to be related to the formation of the Costerfield Dome and the subsequent development of the Moormbool Fault. The main reef system(s) appear to be developed in proximity to the axial planar region of the Costerfield Dome or hosted in reactivated west-dipping thrust faults.
The economic mineralisation at the Costerfield Property occurs in a north–south corridor that includes the Costerfield, Brunswick and Augusta zones. The moderately west to steeply-dipping quartz-stibnite-gold veins have thicknesses ranging from several millimetres to 1 metre, and extend over a strike of at least four kilometres. The vein systems are centred in the core of the doublyplunging Costerfield Anticline and are hosted by unmetamorphosed (anchizone) Costerfield siltstones. Individual veins can persist for up to 800 m along-strike and 300 m down-dip.
The mineralogy of the vein contents and mineral proportions differ from vein to vein throughout the Augusta, Cuffley, Brunswick and Youle lodes. However, the texture and chronological order of each vein/mineral generation remains remarkably consistent across all lodes.
The Costerfield Property lodes are typically anastomosing, en echelon style, narrow-vein systems, which dip from 25–70° west to 70–90° east. Mineralised shoots are observed to plunge to the north when structurally controlled and south when bedding controlled.
The mineralisation occurs as single lodes and vein stockworks associated with brittle fault zones. These bedding and cleavage parallel faults that influence the lode structures range from sharp breaks of less than 1 mm to dilated shears up 3 m wide that locally contain fault gouge, quartz, carbonate and stibnite.
Deposit Mineralisation
There are two main types of mineralised lodes found on the Costerfield Property. Typically they consist of stibnite dominant lodes and gold only lodes. The stibnite dominant lodes vary from massive stibnite with microscopic gold to quartz-stibnite, with minor visible gold, pyrite, and arsenopyrite. The stibnite is clearly seen to replace quartz, and gold can also be hosted by quartz. Costerfield’s gold-only veins typically consist of single-generation quartz-carbonate matrix hosting free gold, usually sub-millimetre grains but rarely up to 3–4 mm across with minor pyrite and arsenopyrite. The best grades in gold-only veins often are associated with antimonybearing sulfosalts such as tetrahedrite, bournonite and boulangerite. Depth of emplacement appears to be the major control on the abundance of stibnite; the gold-only vein systems are generally found below the level of or at the base of stibnite-bearing lodes. Systems considered ‘gold only’ include the bulk of the Shepherd veins (the southwestern portion of Shepherd contains some significant stibnite), and the Sub-King Cobra Fault West veins.
A variety of accessory minerals are associated with the mineralisation, including pyrite, arsenopyrite, aurostibite, pyrrhotite, muscovite, sphalerite and galena within the vein. Wallrock alteration minerals are predominantly pyrite, arsenopyrite and ferroan carbonate spotting, surrounded by a broader, visually cryptic halo of muscovite replacing phengite. Small crystals of barite and bournonite are often seen in chlorite-coated joints near the lodes.