The shallow epithermal gold-silver-copper mineralising system at the Mt Carrington project provides a number of indications that the system is potentially the distal expression of a buried intrusion source that may host a Cu-Au porphyry deposit. These factors are summarised below, and presented in Figures 1 to 7.
Prospecting and small scale mining of copper has been undertaken in the Mt Carrington region since the 19th century. Significant copper occurrences are noted on the main Mining Leases particularly to the south-west of the defined gold and silver Resources, where historical surface and shallow underground copper mining was undertaken at the Pioneer, Gladstone Hill and All Nations Mines.
White Rock interprets that a gold-silver-copper-zinc metal zonation exists on the central Mining Leases, and also on several regional mineralised systems in the Drake Volcanics. On the central Leases the zonation comprises distal Ag(-Au) deposits in the north-east (e.g. Lady Hampden) transitioning to Au-Zn(-Ag) dominant deposits (e.g. Kylo and Strauss) in the centre of the leases, to Au-Cu deposits further south-west (e.g. Mt Carrington) to Cu deposits (e.g. All Nations, Pioneer and Gladstone Hill). The zonation implies increasing temperature of mineralised fluids and proximity to a porphyry source at depth to the southwest (Figure 1).
Historical surface and shallow underground copper mining was carried out on the All Nations, Pioneer and Gladstone Hill copper prospects in the 1800’s, predominantly mining thin quartz-chalcopyrite(Cu) veins. It is considered by White Rock that these veins are analogous to ‘D-veins’ found marginal to porphyry Cu-Au deposits, (first described by in the porphyry deposit literature by Gustafson and Hunt in 1975).
The southwest area of the Leases encompassing the All Nations, Mt Carrington and Gladstone Hill mines also contains an extensive sub-horizontal supergene copper blanket developed at shallow depths above the zones of quartz-chalcopyrite D-veins (Figures 3, 5 and 6). The supergene blanket is a layer of semi-continuous secondary copper mineralisation ranging from 1 to 20 metres in thickness, and ranging in grade between 0.1% and 3% Cu, over 1 metre to 50 metre downhole drill intersections.
A number of major northeast trending structures also exist in this area, and these are interpreted to be the focus for the widespread ‘D-vein’ type quartz-chalcopyrite veins concentrated in this southwest area (Figures 2 and 3). Supergene enrichment of the copper in the D-veins is the likely source of the extensive supergene blanket of copper. In places where the D-veins come to surface the supergene zone is upgraded in size and tenor with oxidation extending down structure up to 50m and the localised supergene blanket extending out at greater thicknesses for short distances. Limited drilling of the supergene copper blanket has been completed by previous explorers and White Rock, with significant copper intersections recorded, as shown in Table 2.
The D-veins themselves represent valid high-grade Cu vein targets at interpreted depths of 50 to 300m below surface. Limited drill testing of the D-veins has resulted in the detection of significant copper mineralisation (as shown in Table 1) though the size and frequency of known veins to date have not prioritised these as an exploration target for previous explorers. Similarly while the supergene blanket is extensive, the grade and thickness did not highlight this target as a viable stand-alone economic target for previous explorers.
The understanding of the setting of the shallow copper mineralisation at Mt Carrington presents a compelling exploration case for deeper drilling to test the porphyry-hosted copper-gold potential on the central Mining Leases and regionally. It is considered that while the economic potential for copper-gold mineralisation associated with a porphyry may be at considerable depth (>500m), the potential for high grade copper-gold veins (‘D-veins’) extending upwards from the porphyry source also presents a strong secondary target.
Early in 2015 White Rock completed an initial phase of deep penetrating electrical geophysics and drilling to better understand the potential for deep porphyry associated copper mineralisation. Two lines of MIMDAS defined a large intense chargeability feature to the west of the Central Lease and several additional intense though smaller chargeability features linking the large chargeability anomaly with the surface expressions of copper mineralisation (Figure 7). A program of three drill holes tested the most prominent chargeability anomalies. Each of the drill holes intersected extensive disseminated pyrite alteration of sufficient intensity and distribution to explain the chargeability response. Importantly, the alteration is interpreted to be part of a large halo of pyrite-illite-silica alteration that encompasses a number of high-level intrusive volcanic bodies, including porphyries, within an extensive and significant hydrothermal system. This style of alteration is typically observed elsewhere within the outer and upper portions of a mineralised copper-gold porphyry and/or intrusive related gold system. The alteration and chargeability anomalies are open to the north, west and south, and open at depth, presenting significant potential for future mineralisation discovery.
White Rock is using the information from the initial program to plan follow-up studies to help define vectors towards copper rich targets within the extensive alteration system.
Figure 1: Location of deep porphyry copper target on the Mt Carrington central Mining Leases. There is a broad zonation from distal silver mineralisation at Lady Hampden and Silver King in the east, to the proximal gold mineralisation at Kylo, Strauss and Guy Bell, and the historic copper occurrences at Gladstone, All Nations, Carrington and Pioneer interpreted to represent the surface expression of structural controls to a copper porphyry mineralised source at depth.
Figure 2: Mt Carrington Mining Leases copper zone – IP chargeability depth slice at approximately 200m below surface with the interpreted major “D” vein structural controls hosting copper mineralisation. The location of highly chargeable bodies (red) is interpreted to map variability in alteration related to a porphyry source juxtaposed by structural controls and a complex history of intrusions.
Figure 3: Mt Carrington Mining Leases copper zone – surface copper anomalism (soil geochemistry) and sub-surface supergene copper mineralisation (drillholes) coincide with the extent of primary copper mineralisation in “D” vein structures. A significant Molybdenum-Bismuth-Indium anomaly in soil geochemistry also provides a residual immobile signature indicating a primary intrusive target at depth, bound by the All Nations structural trend containing “D” vein copper mineralisation (see Figure 4).
Figure 4: Mt Carrington Mining Leases copper zone – examples of copper mineralisation (quartz-chalcopyrite “D” veins) from the All Nations trend intersected in KYDD001 at 53.4m and 54.6m.
Table 1: Examples of drill intersections of primary copper mineralisation hosted in quartz-chalcopyrite veins analogous to “D” veins from the All Nations trend. The “D” veins are interpreted to extend vertically above the margins of copper porphyry mineralisation. Note all interval lengths are downhole and drillhole KYDD001 was drilled down-dip, therefore the intersections reported are not indicative of true widths. All tabled results have been previously released by Rex Minerals Ltd (ASX: RXM) and White Rock Minerals Ltd (ASX: WRM). Refer to the ASX website for RXM announcements dated 30 March 2009, 28 September 2009 and 28 March 2010. Refer to the ASX website for WRM announcement dated 3 February 2009, and for the White Rock Minerals Ltd Prospectus dated 30 September 2010 (Goldner and Associates Independent Geologists Report [August 2010]).
Table 2: Examples of assay results for supergene copper mineralisation in the Gladstone prospect area drilled by White Rock Minerals Ltd and earlier explorers. In some cases the supergene copper mineralisation overprints relict primary copper “D” vein mineralisation. All tabled results have been previously released by White Rock Minerals Ltd (ASX: WRM). Refer to the ASX website for WRM announcement dated 3 February 2009, and for the White Rock Minerals Ltd Prospectus dated 30 September 2010 (Goldner and Associates Independent Geologists Report [August 2010]).
Figure 5: Mt Carrington Mining Leases copper zone – Gladstone Prospect cross sections (refer figure 3 for location) displaying White Rock drillholes GHDD001 – 005, and historical drilling. The interpreted shallow supergene copper zone shown in yellow pinches and swells laterally, with a supergene overprint also developed down zones of more intense primary quartz-chalcopyrite veining.
Figure 6: Plan view showing location of drill holes (WCDD001, 002 and 003) with respect to copper soil anomalism and the chargeability anomalies located immediately west of the known Mt Carrington gold-silver deposits and copper prospects.
Figure 7: Vertical projection of drill holes targeting MIMDAS IP chargeability cross sections – Line 1 (1400N) and Line 2 (1100N). High chargeable response in pink, low response in blue.
Figure 8: Schematic illustration of mineralisation styles associated with an intrusive source at depth beneath a volcanic caldera setting. White Rock conceptualise that the mineralisation styles at Mt Carrington are all linked to an intrusive source that could include porphyry copper mineralisation that is relatively proximal to the high level epithermal gold-silver mineralisation defined in the current Resources.