Drilling Upgrades Porphyry Copper Target at Nankivel
- Extensive alteration halo intersected with modest copper intersections to date
- Mineralogy and geochemical vectors point to a large and undrilled porphyry copper target southwest of Nankivel Hill
- Coincident 2km x 500m IP geophysical anomaly starting at about 150m depth
- Further drilling requires access negotiations
Careful analysis of both the diamond and RCP drilling assisted with petrology and multi-element pathfinder geochemistry under consultant advice has positively modified the target model from the preliminary high-level epithermal setting previously interpreted (IVR ASX announcement 26th April 2017) to a shallow undrilled porphyry target extending from the advanced argillic breccia outcrop on Nankivel Hill about 2km to the southwest.
The drill results have focussed targeting on a large Induced Polarisation (IP) chargeability anomaly delineated by the late 2016 survey on 400m spaced lines (IVR ASX announcement 15th March 2017). The upgraded potential warrants renegotiation of current heritage restrictions to the revised target area.
Investigator Resources Managing Director John Anderson said "Our porphyry copper target interpretation has been logically modified and upgraded by the recent drilling and the application of state-of-the-art models and exploration science, including pathfinder geochemistry. The alteration haloes and geochemical signatures around porphyry copper deposits are being increasingly understood, and we now have high-integrity evidence that suggests we are within drilling reach of a potentially large porphyry copper system immediately southwest of Nankivel Hill at a depth shallower than previously projected.
This location is currently not accessible for drilling, so our immediate priority is to review the heritage boundaries with the Native Title Holders, with whom we have worked for many years and enjoy a good relationship. The bulk of the revised Nankivel target is under subdued topography compared with the adjacent hill," Mr Anderson said.
He said Investigator is a pioneer in identifying the potential for porphyry mineral systems on the southern Gawler Craton, and continues to apply rigorous industry-leading exploration science typically the cutting-edge applied by major companies.
"This approach minimises the usual exploration risk and provides the best discovery opportunities for our shareholders. As Investigator continues to drill and undertake collaborative research with the State Geological Survey and universities, several lines of evidence point to a transitional relationship between the IOCG deposits of the Olympic Dam belt and the emerging porphyry systems of which Nankivel is a prime example. This expands the copper potential in the southern Gawler Craton where Investigator has taken a strong tenement position," Mr Anderson added.
Recent RCP Drilling and Results
The previously interpreted shallow epithermal target following the diamond drilling was investigated with twenty slimline reverse circulation percussion (RCP) holes (PPRC421 - 440) drilled during last June with an average depth of 117m and generally on a dip of -60 degrees (Plan - see Figure 1 in the link below). As well as testing shallow flat-lying IP anomalies, two holes were positioned to test as close as possible to the northern margin of a strike- and depth-extensive strong chargeability anomaly that extends southwest from under Nankivel Hill (see Figure 2 in the link below - note this is an oblique view with the southern holes drilling over but not reaching the IP anomaly) that was otherwise inaccessible to drilling under current heritage restrictions (see Figure 1 in the link below).
The details of the drill holes are provided in Table A (see the link below). The intersections for the gold, copper, zinc and lead assay results are summarised in Tables B, C, D and E (see the link below) respectively.
Refer to Appendix 1 for 'TABLE 1 (see the link below): Peterlumbo Tenement, Nankivel Diamond and Reverse Circulation Drilling Results, July 2017 - JORC 2012', information relating to the compliance of the 2012 edition of the JORC Code. This includes Section 1 - sampling Techniques and Data and Section 2 - Reporting of Exploration Results.
The revised porphyry copper target interpretation encompassing the drilling results is summarised in plan and cross-section in Figures 3 and 4 (see the link below). The cross-section shown by the trace in the plans of Figures 1 and 3 (see the link below) was selected as the most appropriate one to demonstrate the alteration and geochemical vectors as detailed in Figures 5 to 8 (see the link below) in support of the interpreted target. It captures a broad window including the traces of holes (from northwest to southeast) PPRC438, PPDH157, PPRC430, PPDH155, PPRC426, 436, 431 and 437.
The initial target of advanced argillic alteration was only intersected in a few holes indicating a small deep root zone rather than a broad shallow cap. The drilling intersected a persistent flat-lying and copper-anomalous supergene zone with visible covellite (see Figure 5 in the link below). This is now interpreted to be the source in part of the initially targeted shallow IP anomaly. The plus 500ppm copper intersections (Table C) (see the link below) are low order with a best intersection of 4m @ 1,236ppm Cu in the supergene zone. They are significant however for vectoring towards the adjacent porphyry copper target with the elevated copper dominantly in the area of the initial phyllic intersection of PPDH155 in holes PPRC426, 430, 431, 436 and 437 (see Figures 1 & 3 in the link below).
The RCP drill program was extended to twenty holes owing to the widespread intersection of phyllic alteration in the majority of the holes including strong pyrite mineralisation indicative of a large robust hydrothermal system. The best pyrite intersections clustered along the southern side of the drill coverage in holes PPRC431, 437, 436, 426, 433, 432, 424, 428, 429 and 440. The geochemical measure of pyrite is sulphur assays as shown in Figure 6 (see the link below). The sulphur assay represents about half the pyrite content and this indicates pervasive pyrite contents of 5% to 10% on the southern side of the drilling and continuing untested beyond PPRC437. As initially recognised in the intense alteration and breccias of PPDH155, the amount of pyrite is considered to reflect a large hydrothermal system.
Pathfinder element geochemistry using ICP-MS techniques has become a valuable tool in using drill assays to define vector directions which remotely point to the targeted potassic core and copper mineralisation of porphyry copper systems (e.g. Halley, S., Dilles, J.H. and Todsal, R.M., Footprints-Hydrothermal alteration and geochemical dispersion around porphyry copper deposits: Society of Exploration Geologists Newsletter, v.100, p1-17).
Metals associated with copper such as bismuth (Bi) and molybdenum (Mo), even at low levels, define broader and zoned halos enabling the prediction of proximity to the copper core. The values and gradual increases in these metals at Nankivel indicate a shallow porphyry copper core is possibly present a few hundred metres south of the current drilling pattern. The assay levels for these metals are shown on the sections of Figures 7 and 8 (see the link below) with the right levels and progressive increase to the south.
Bismuth (see Figure 7 in the link below) in particular shows a steady increase pervasively achieving the accepted porphyry-proximity threshold of greater than 1ppm Bi between PPDH155 and PPRC426 with the maximum value of 29.2 ppm Bi in the most southern hole PPRC437.
Molybdenum (see Figure 8 in the link below) shows patchy assays over the porphyry-proximity threshold of 5ppm Mo. However the best Mo values (maximum 26.4ppm Mo) are again in the most southern hole PPRC437, suggesting the start of an inner molybdenum shell closer to the copper core.
The results were assessed in plan with the distribution of maximum bismuth and accompanying tellurium values in each hole shown in Figure 3 (see the link below). They show a robust vector towards the large IP chargeability anomaly modelled to start 150m to 200m below the surface and interpreted to be the extension of the strong sulphide content in the adjacent drilling. This is consistent with the most southern hole of the cross-section, PPRC437, also showing the best copper, gold, bismuth and molybdenum results.
Overlying scattered outcrops of sericite-altered volcanics and metasediments also have elevated bismuth values. An outcrop of graphitic metasediment is located at the west end of the IP anomaly where it correspondingly reaches the surface. Graphitic metasediment occurs in the basement throughout the region such as at Paris and Thurlga. This is not considered to be the source of the main IP anomaly which has a different orientation to the local stratigraphic trend. The samples of the two holes (PPRC424 and 428) adjacent to the outcrop contained pyrite but no graphite and show clear signs of hydrothermal mineralisation in the bismuth and tellurium geochemistry (see Figure 3 in the link below).
An experienced consultant petrologist is assisting with the characterisation of the intrusive types and porphyry alteration mineralogy and zoning in the drill holes. The alteration patterns are summarised for the drill holes in Figures 5 to 8 (see the link below). The strength of the Nankivel hydrothermal system is indicated by the degree of phyllic alteration overprinting the broader propylitic alteration of multiple intrusions. It also implies a high exposure to hydrothermal fluids. The petrology reduced the extent of the potassic alteration previously interpreted to narrow zones near the bottoms of holes PPDH155 and 157. This also indicates proximity to the potassic and copper core and is more in keeping with the standard porphyry model and the revised shallow porphyry target.
A date was published in May by the State Geological Survey for the outcropping alunite alteration on Nankivel Hill of 1586+/-8 million years (Reference: Nicholson et al., A Mesoproterozoic advanced argillic alteration system: 40Ar/39Ar thermochronolgy from Nankivel Hill, Gawler Craton. Report Book 2017/00011, Department of Premier and Cabinet, Government of South Australia). This date supports the Investigator model that the Paris and Nankivel mineralisation are close to Olympic Dam age. There are local and regional ramifications for new discovery success in the southern Gawler Craton where Investigator continues to build a strong tenement position (see Figure 12 in the link below).
A propylitic-altered granodiorite from PPDH147 is also being dated by the GSSA with preliminary results supporting the correlation of the early Nankivel felsic and mafic intrusives with the St Peters Suite of subduction granites. This implies a long intrusive history for the Nankivel complex that is now permissive under a new overseas concept for late porphyries such as Nankivel to form at the same time as the non-subduction Olympic Dam mega-event.
Relation to the Paris silver deposit
Additional supporting evidence has come from the geometallurgical work underway at the Paris Silver Project with contributions from Scott Halley, with the geochemical characterisation of three suites of dykes intersecting and bounding Paris (e.g. Hafnium:Zirconium plot in Figure 9 in the link below). These data show the fluorite-bearing central dyke, considered to have caused the brecciation and silver mineralisation, is the latest and most fractionated. This dyke is correlated with a number of interpreted northeast dykes that show close associations with the main prospects and pathfinder metal patterns like molybdenum (see Figure 10 in the link below). The similar settings of Nankivel and Paris at the intersection of northeast dykes (pink on Figure 10 in the link below) with the same NW-SE structure (dashed blue line) is further support for the Nankivel porphyry copper IP target.
It is a reasonable interpretation that the lateness, fluorine-content and northeast trend of the mineralising Paris dyke correlates the Paris mineralisation with the brecciated alunite-altered dyke and topaz alteration on Nankivel Hill along with the fluorite topaz alunite alteration at the top of the phyllic zone in PPDH155 and the late fluorite veins in PPDH157. A drill sample of the central Paris dyke is being dated by the collaborative ARC Linkage project examining the spectrum of deposit styles in the Gawler Craton to establish if the dyke and the Paris mineralisation are also close to Olympic Dam age.
Applying the same Hafnium:Zirconium plot to all the assays for the Nankivel diamond core holes shows evidence of multiple intrusive phases not readily discernible otherwise owing to the intense alteration. A central Paris dyke signature is not evident yet; however the equivalent cupola of a late fluorine-bearing and porphyry mineralising phase is likely to remain undrilled under the Nankivel IP anomaly within the core of the interpreted zoning pattern (see Figure 4 in the link below).
As well as moving the Paris silver project forward, Investigator's objectives are to drill the Nankivel IP target and extend IP surveying to the interpreted additional porphyry target areas at Nankivel West and Helen (Figure 10).
The key activity at Nankivel will be to resurvey the heritage envelope around Nankivel Hill. Investigator has positive relationships with the Native Title Holders with an Exploration Agreement in place. The bulk of the revised Nankivel target is under subdued topography compared with the adjacent hill. The Nankivel West and Helen target areas are already approved for drill access after prior heritage surveys.
Investigation of the Company's large datasets will continue for targeting and research purposes.
Interest in the epithermal, skarn and porphyry potential of the Nankivel Hill area originated with the discovery of the alunite-silica-haematite-altered rhyolite breccia outcrop in 1990. MIM Exploration mapped the extent of illite, dickite, pyrophyllite and alunite alteration and undertook RCP drilling (NAN001 - 003) with limited assaying on the hill in 1995/6 under heritage approvals at the time.
Following the Paris epithermal silver discovery in 2011 with a current resource standing at 9.3Mt @ 139g/t silver for 42Moz of contained silver, Investigator established the copper-gold-silver potential of the Nankivel intrusive complex with the discovery of the Helen skarn in 2014 and proceeded to drill around the margin of the complex assisted by government collaborative PACE funding, including PPRC334 and 335 as the nearest accessible areas to Nankivel Hill. These intersected propylitic-altered magnetic monzogranite and granodiorite.
After acquiring adjusted heritage approvals, diamond Hole PPDH147 and RCP holes 403, 404 and 405 (see Figure 1 in the link below) were drilled late in September/October 2016 under government collaborative PACE funding to test a magnetic anomaly and vein extensions near the advanced argllic outcrop at Nankivel Hill for porphyry, skarn and epithermal mineralisation.
Monzodiorites and granodiorites were intersected with extensive magnetic propylitic and demagnetised pyritic-phyllic alteration, the latter with anomalous copper and gold values. This encouraged suspension of the drilling while an IP survey was undertaken late in 2016 over a large demagnetised area of the Nankivel intrusive complex.
A selection of IP chargeable anomalies were tested by four diamond holes in March-April this year with one hole PPDH155 intersecting a broad pyritic-phyllic zone including a shallow kaolinite cap with evidence of relict alunite, topaz and bornite/chalcopyrite inclusions in pyrite. This was correlated with the advanced argillic alteration on Nankivel Hill and an east-west high-sulphidation target was proposed to extend for 2km along flat-lying IP and EM anomalies, the latter derived from a single airborne EM survey flown by CSIRO for water studies. The assumption of a shallow epithermal cap implied a deeper porphyry target nominally at 700m depth or more under the standard zoning model for porphyry copper deposits. The revised porphyry target based on alteration and geochemical data is much shallower at around 150m depth and has the geophysical support of a large IP anomaly.
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About Investigator Resources Ltd
Investigator Resources Limited (ASX:IVR) is a metals explorer with a focus on the opportunities for greenfields silver-lead, copper-gold and nickel discoveries offered by the emerging minerals frontier of the southern Gawler Craton on South Australia’s northern Eyre Peninsula.
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