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HARCOURT NORTH

Overview

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The Harcourt North project area is centred on the Harcourt Granodiorite between the Castlemaine and Bendigo Goldfields. The project consists of one large tenement application area: EL007218 with an area of approximately 122 km2. Two target concepts have been defined: 

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  • Target A – Primary target; repeat of Bendigo and Castlemaine GF structural setting and mineralisation immediately beneath a new interpretation of thin-skin granodiorite geometry; 

  • Target B – Secondary target; Pilcher’s Bridge GF located within the granodiorite northern hornfels envelope

 

See Figures 1, 3 and 4 below for EL007218 location, structural setting and target area locations.

Figure 1:Exploration Licence application area and location plan.

Exploration Target and Geological Concept

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The EL application lies directly between, and along strike of two of the great historical goldfields of central Victoria; the Bendigo field which produced over 22 Moz from 1851 to the 1950’s, and the Castlemaine field which produced more than 5.6 Moz over its life. These goldfields combined have produced over 35 % of Victoria’s gold (Willman, 2005).

 

The Goldfield Structural Setting

The structural setting of both fields is similar with local variations in fold, fault and quartz vein style. Willman (2005) interprets the broad structural architecture as consisting of:

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  • A Major Intrazone listric fault marking the east boundary of the Castlemaine Group turbidite thrust sheet (Whitelaw Fault and it’s interpreted southern extension south of the Harcourt Granodiorite, the Taradale Fault)

  • A Major Intrazone fault west of the goldfields (Muckleford Fault)

  • The Goldfield Domain – mineralised fault/fold network with most historical gold production

  • Minor Intrazone Fault marking the eastern boundary of the Goldfield Domain (the Shicer Gully Fault at Castlemaine, the equivalent not yet mapped at Bendigo)

  • Eastern Domain – narrow zone of minimal or no gold production between the Goldfield Domain (and Minor Intrazone Fault) and the Whitelaw/Taradale Faults. Characterised by steep, west dipping fold enveloping surface, increasing S1 cleavage intensity and increase in metamorphic grade.

 

Overall, both Goldfield Domains are characterised by tight F1 folds striking north (Castlemaine) to north-north-west (Bendigo) with sub-horizontal enveloping surfaces to the folds within the goldfield domain, and laterally continuous bedding parallel faults and low-displacement, strike-parallel faults controlling auriferous quartz-vein development (Willman, 2005).

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Importantly, the geographic centres of production of each goldfield coincide with structural culminations (shallow doming) in the host anticlines. This association is best developed in the Bendigo Goldfield where all the major mineralised folds plunge gently north and south away from the central part of the goldfield (10-15 degrees plunge). At Castlemaine, most of the field plunges north at approximately 10 degrees with a culmination mapped at Wattle Gully Anticline, the most productive mineralised fold that lies in the centre of the goldfield (Willman, 2005).

 

Figure 2 below (from Willman 2005), illustrates the interpreted structural architecture in cross-section. Figures 3 and 4 show the goldfield locations defined by the distribution of mapped production centres and the location of major structural elements and their relationship to the goldfields and the Harcourt Granodiorie.

Figure 2: Schematic structural setting in cross-section through the Bendigo/Castlemaine Goldfields (Willman, 2005).

The Harcourt Granodiorite

The Bendigo and Castlemaine Goldfields and their host package of Castlemaine Group turbidites are stitched by the approximately east-west trending, kidney shaped, Devonian Harcourt Granodiorite. The emplacement model and geometry for this pluton has to date been interpreted as the classic diapir type or reverse teardrop, with the corresponding assumption of significant vertical thickness, probably in the order of many kilometres. As a result, no exploration for gold mineralisation between the two goldfields within the boundary of the Harcourt Granodiorite has been undertaken. 

Recent work by Neil Phillips of Melbourne University has shown from field observations that large, Devonian-age plutons in eastern Victoria can have a flat, tabular geometry with emplaced vertical thickness of <1,000m and in some cases much less than this. The basal contact with underlying sediments has been mapped locally in some cases (Phillips, 2017). This geometry can be explained by the fracture transport model of pluton emplacement (Cook-Gordon mechanism) whereby rapid upward vertical transport of magma along steep fault conduits stops at a rheological boundary and flattens laterally forming thin, laccolithic intrusions. 

 

Exploration Model

Cygnet Resources proposes that the Harcourt Granodiorite was emplaced via the fracture transport model. Gridded state gravity data (Figure 3) shows that the potential field response of the granodiorite is not uniform; the western and eastern ends of the ‘kidney’ are characterised by deep gravity lows relative to the surrounding turbidite package and the gravity low lobes are separated by a ridge of higher density of similar response to the turbidites. The gravity ridge within the granodiorite corresponds spatially to the Bendigo/Castlemaine Goldfields trend. 

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It is possible that the deep crustal Whitelaw/Taradale and Muckleford Faults, stitched by each lobe respectively, were the primary feeder conduits for granodiorite magma and that this has produced a local thickening of crystallised magma at each lobe and a corresponding gravity low. Lateral migration of magma inboard of each fault conduit may have been restricted by a zone of increased silica, shorter fold wavelengths or other differential physical properties associated with the goldfield corridor, leading to a thinner vertical thickness of granodiorite in the central zone.

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The state airborne magnetics data (Figure 4) shows a well-defined zone of narrow, positive magnetic lineaments bisecting the granodiorite and coincident with the gravity ridge and the goldfield trend. These magnetic features may represent structural lineaments in the underlying Castlemaine Group if the granodiorite is very thin or possibly more magnetic dykes near the base of the pluton, concentrated here due to structural influence from the underlying sediment package. The eastern boundary of the magnetic lineament zone corresponds with the strike extension of the Castlemaine Field Shicer Gully Fault.

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The exploration model is dependent on two assumptions; one, that the granodiorite is of limited vertical thickness such that Castlemaine Group turbidites are within practical drilling and mining depth, and two that a repeat of the goldfield pattern exists along strike between the two fields. In support of the second assumption, both goldfields domal geometries plunge towards the pluton centre at shallow angles; a midpoint between the two culminations lies on the magnetic linear zone central to the granodiorite margins (Figures 3 & 4). Both goldfields are 12-14 km in strike length; corridor strike length beneath the granodiorite is approximately 12 km, suggesting enough room for another culmination and full field repeat if fold wavelength is relatively consistent along strike.

Figures 3 & 4: Bendigo/Castlemaine Goldfields relative to structural setting, and granodiorite extent over gravity (top) and airborne magnetics (bottom). 

In addition, many gold deposits in central Victoria give geochemical and petrophysical indications of Devonian granite-related overprint and upgrading of pre-existing orogenic-style slate-belt gold mineralisation (e.g. Maldon, Drummond/Belltopper, and recent work at Cappers Prospect at the north end of the Castlemaine field). If a repeat of Bendigo/Castlemaine exists immediately below the granodiorite base, the potential for the same upgrade processes to have occurred must be considered a possibility.

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Proposed Exploration Program 

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The primary target zone along strike and between the goldfields has received no modern exploration attention, therefore a proposed program must commence at first principles:

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  • Mapping of the granodiorite with emphasis on crystal size (evidence of quenching), distribution and concentration of xenoliths and rafts of country rock, fault and fracture orientation.

  • Stream sediment sampling, rock-chip sampling and east-west soil survey lines across strike using Niton analysis may detect As-Au-Sb-Zn anomalism as leakage through pluton fractures and joints from mineralisation below the base and help to define the goldfield corridor geometry and priority areas for drill testing.

  • Modelling of 2D magnetic flight-lines and gravity data may help to determine pluton thickness along the field strike.

  • Application of detailed ground gravity and the passive seismic method to model base pluton geometry.

  • Drill testing; any intersection of Castlemaine Group sediments below the granodiorite within the field corridor would be considered initial proof of concept. Further drilling if warranted mineralised structures.

 

References

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Willman, C. (2006) Regional structural controls of gold mineralisation, Bendigo and Castlemaine goldfields, Central Victoria, Australia. Miner Deposita (2007) 42: 449–463

Phillips, G. N. (2017) Determining the shape of an intrusion: the Tallarook granite. Applied Earth Science, Volume 126, 2017 - Issue 1

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