VIMP Report 71 - Tarnagulla goldfield, central Victoria 1:10 000 map area report

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Abstract This report describes the geology of the Tarnagulla Goldfield (together with the Llanelly and Waanyarra goldfields). It gives a comprehensive account of the geological history, Palaeozoic and Cainozoic rock units, and the structural, economic and mining geology. It also presents a brief mining history, outlines the regional geological setting and geomorphology, and summarises previous geological work and mineral exploration. The exploration potential of the goldfield is also presented. Particular emphasis is placed on a presentation of modern data on the structural and economic geology from the Poverty-line-of-reef (PLR), which forms the basis for a new torsional and strike-slip interpretation of reef formation. The en echelon fault/quartz reef or ‘lines’ formed during torsional/transpressional regional deformations spanning the Late Ordovician–Middle Devonian. There appears to be a strong correlation between the stress/strain geometries calculated for these deformation events and the location of high-grade gold mineralisation at the Tarnagulla Goldfield.

Alluvial gold was discovered in the Tarnagulla (Sandy Creek) area in 1852. Soon after, gold bearing quartz reefs were recognised and underground mining gathered pace in the mid-1850s. The total recorded production (incomplete) for the Tarnagulla Goldfield (inclusive of the Llanelly and Waanyarra fields) is estimated to be 561,000 oz of gold (17,400 kg). The Poverty reef dominated the field for high grade and gold yield. From 1853–1866 360,000 oz (11,197 kg) of gold were mined from the “Bonanza Shoot” of 122,000 tonnes (an average head grade of 92 g/t), at the north end of the Poverty reef. Reef Mining N.L. discovered a second high-grade shoot at the south end of the Poverty reef in 1994. The vertical shoot, named the Nick O’ Time Shoot (NO’T) has been mined from the top (70 m below surface) to a depth of 250 m via a decline, for a yield of 57,400 tonnes @ 29.1 g/t; 53,000 oz (March, 2000).

Ordovician Castlemaine Group metasediments have been deformed over a protracted period and a number of separate deformation (folding and faulting) events can be recognised. A major division can be made between structures formed prior to and after intrusion of the Early Devonian Tarnagulla pluton and associated dykes. Folding of the Castlemaine Group during regional deformation in the Late Ordovician-Early Silurian was accompanied by formation of an axial-plane slaty cleavage and parallel, spaced, thrust faults which breach the folded sequence. Plunge reversal of folds from moderate north to moderate south occurs at the regional (anticlinoria and synclinoria) and local scales, and one such reversal coincides with the high-grade Bonanza shoot in the Poverty reef. Folds and cleavage are overprinted by several generations of younger structures (faults, crenulations, veins), whose relative timing (D2–D7) has been interpreted using overprinting criteria. The overprinting fault structures are crucial to a full understanding of the formation and subsequent evolution of the quartz-reef systems which host gold mineralisation across the Bendigo Zone, and these are discussed in detail.

The Poverty Line of reef (PLR) and other reef lines in the goldfield each comprise a number of discrete, west-dipping reef segments, systematically offset in a left-stepping (or sinistral) en echelon pattern. The overall line of reef trends NNE-N at an angle to the general structural grain of the goldfield. The segments have associated R synthetic Riedel shears that overall have a spiral geometry (they become more shallowly-dipping with depth), and appear to have formed in response to reverse displacement (west-over-east) along the fault-line, with a component of sinistral strike-slip displacement. Metasediments in the footwall of the NO’T shoot transversely buckle into steeply plunging folds or sigmoidal ‘rolls’.

The overall line of reef lies approximately subparallel to the interpreted trace of the Stawell–Melbourne Terrane boundary of White and Chappell (1988) and Chappell et al. (1988). The Poverty reef (PR) is a steeply west dipping, ellipsoidal, gently (15–20°) south-plunging quartz segment. The segment has three distinct, structurally different domains along its length; a central, low grade domain of massive quartz, and two marginal domains of high grade, more deformed quartz (the NO’T and Bonanza ‘shoots’) at each end of the reef. Gold–sulphide mineralisation in the PR is concentrated in the marginal domains, associated with R Riedel splays off the reef and subtle 3-D flexural bends. The contacts between the three domains are steeply plunging, consistent with an interpretation of the NO’T and Bonanza high grade quartz shoots as pull-apart structures formed during torsional (dip-slip and strike-slip) shear along the reef. In plan view the NO’T shoot has dextral asymmetry with respect to the central domain, while the Bonanza shoot has sinistral geometry. The reef system consists of several R synthetic Riedel shears that are three-dimensionally, spirally deformed (shallowing at bottom) and show a sinistral, reverse pattern. Metasediments in the footwall of the NO’T shoot transversely buckle into steeply plunging folds or sigmoidal ‘rolls’.

Within the shoot domains, laminated quartz and dissolutional stylolitic fractures and seams are auriferous. In the NO’T stylolites and slickolites in en echelon tail veins reveal a mix of a sinistral and dextral geometry. In NW and SE margins of the NO’T quartz core, stylolites suggest that solution transfer operated under a sinistral transpression regime. Complex but narrow alteration halos exist around reef structures. In the NO’T shoot, the gold–sulphide mineralisation occurs in three different structural settings. A vertical zoning of the gold–sulphide mineralisation is also apparent.

Economically important, high-grade sulphide and gold mineralisation is spatially and temporally related to the development and reactivation of the footwall and hangingwall fault structures and associated laminated, stylolitic and brittle fractured and brecciated quartz. The degree of deformation and structural preparation for incoming fluids was different in the central and marginal domains. At least two separate phases of sulphide mineralisation post-date the syn-sedimentary pyrite in the wall rocks and pre-date carbonates associated with the albite-quartz veins. The ‘D’ quartz-infilled fault and albite–quartz faults and veins cross-cut, displace and overprint the high-grade footwall of the PR, and therefore provide an upper timing constraint on the pre-magmatic sulphide–gold mineralisation. The carbonates which fill open spaces in the albite–quartz veins are undeformed, and a lack of associated stylolites also constrains the upper limit of the magmatic gold–sulphide mineralisation.

The presence of the gold–sulphide mineralisation in three overprinting sets of structures suggests that mineralisation has been a prolonged process, involving a complex interplay between structural preparation, the host rocks, and discrete pulses of metamorphic and subsequently magmatic fluids under changing P–T conditions. The economically important gold is interpreted to have been introduced along with sulphides and quartz-veining before intrusion of the granite, but magmatic fluids may have scavenged later gold from the crust (Foster et al., 1998) and may have interacted with and remobilised pre-existing gold–sulphide mineralisation.

The goldfield lies within the south east corner and east side of the Tarnagulla Pluton, which appears to have been intruded into an environment of dextral shear. Dextral movements on faults pre- to syn-date the pluton and can probably be attributed to the Stawell–Melbourne Terrane boundary.

Bibliographic reference Krokowski de Vickerod, J., Cuffley, B., Evans, T., 2001. Tarnagulla Goldfield, Central Victoria, 1:10 000 Map Geological Report. Victorian Initiative for Minerals and Petroleum Report 71. Department of Natural Resources and Environment.