Redefining a historic high-grade zinc-gold mine as a bulk-tonnage open pit operation
- Major development project with significant base metal-gold open-pit potential, with orogenic gold and base metal mineralisation defined which is very similar to other large deposits in the region
- Large mineralised system confirmed by initial drill results (announcement, 26 April 2017)
- Historic mine site, extensive workings
- Several kilometres strike
- Broad mineralisation intercepts
- Unsampled mineralised drill core
- Main resource on freehold land owned by Ardea
- Bulk tonnage Exploration Target defined
- Lewis Ponds Main Zone and Tom’s Zone main shaft mullock sampling by Ardea returned assays of 9.9–12.1g/t gold and 272–539g/t silver, possible epithermal mineralised affinity
The breakdown for the full Lewis Ponds resource categories is as follows:
|Resource Category||Quantity (Mt)||Zn(%)||Cu (%)||Pb (%)||Au (g/t)||Ag (g/t)|
|Total Mineral Resource||6.62||2.4||0.2||1.4||1.5||69|
Bulk tonnage model for exploration and development
At Lewis Ponds, “the mineralisation is accepted as Volcanogenic Massive Sulphide (VMS) type” (Prospectus, page 46). The mineralisation as described is multiple lenses (1, 2 and 3) of massive stratabound base metal sulphides hosted within the Anson Formation pyritic siltstones. Exploration has historically been on the basis of a narrow high grade underground mining model, and has generated a mineral resource totaling 6.6Mt at 1.5g/t gold and 2.4% zinc estimated at a 3% ZnEq cut-off grade (refer Prospectus Table 3.2 for full description of resource status).
As noted in the Prospectus (page 44), Lewis Ponds “occurs on the Lewis Ponds Fault; a subsidiary fault to the Godolphin Fault. The region is well known for being prospective for a variety of deposit types, especially VMS deposits and orogenic gold deposits.”
Having now completed regional geological orientation programs in the Lewis Ponds area, it is clear that Lewis Ponds has a significant potential for orogenic gold deposits of the style of Regis Resources’ McPhillamys deposit, located “15km south along the Godolphin Fault” from Lewis Ponds (Prospectus page 44).
As well as the traditional zinc-dominant VMS model, there is now a requirement to additionally evaluate Lewis Ponds as a bulk tonnage orogenic gold-base metal system, with the clear mining model being McPhillamys (resource 73Mt at 0.93g/t Au at 0.4g/t Au cut-off grade, Regis Resources, 31 March 2015).
There are very clear geo-metallurgical associations at Lewis Ponds, possibly representing separate mineralising events, in order of abundance:
- Dominant Zn-Pb-Ag-Au (stringer dominant but significant massive sulphide interbands).
- Au-Ag only (which is poorly sampled, thus difficult to quantify as an orogenic gold system).
- Minor Zn-Pb only.
- Rare Cu only, always in the footwall.
Lewis Ponds mineralisation is dominantly a 20-50 metre wide stringer (shear vein) system with 2-10 metre thick bands of massive Zn-Pb-Ag-Au mineralisation able to carry sub-grade stringers, with typically 2-5 m internal waste bands.
The mining implications for Lewis Ponds are:
- Drill intercepts are required to be bulked into broad lower grade intervals of the McPhillamys style, using a 1% ZnEq cut-off (approximately 0.5g/t AuEq).
- Open-pit bulk mining is more likely to be feasible, given the favourable Lewis Ponds strip ratios and consistent shallow high-grade mineralised occurrences.
At Lewis Ponds, the bulk tonnage potential is confirmed in Prospectus Figure 3.3 (refer gold-equivalent calculation), with the following gold-equivalent intercepts on Section 940mN representative of the overall Lewis Ponds mineralised system.
- Western Lode
- TLPRC-02 90m at 1.02g/t AuEq from 18m
- SLP-7 27m at 1.59g/t AuEq from 62m
- TLPD-02W 30m at 2.44g/t AuEq from 271m
- Central Lode
- LPRC-32 54m at 1.72g/t AuEq from 26m
- TLPD-40 37m at 0.92g/t AuEq from 199m
- Eastern Lode
- TLPD-40 68m at 0.97g/t AuEq from 57m
Having now completed field reconnaissance, it is felt that Lewis Ponds is better evaluated as a base metal-gold stringer system. Table 2 summarises the more significant “bulk-tonnage” intercepts at Lewis Ponds. More detailed drill intercept data has been previously published by TriAusMin Limited in ASX announcements between 2002 to 2014 (Prospectus page 71) and these reports may be obtained from Ardea.
From reviewing available drill core at site and archived core photographs, it is clear that extensive zones of sericite-pyrite alteration (potential gold mineralised host) have not been sampled. There is thus a requirement to cut and sample this core (assuming the individual core is still available), with the potential to establish “McPhillamy-style” mineralised systems.
The Ardea intention is to drill a single orientated core hole on Sections 1000mN, 800mN, 600mN and 400mN, and all available core on these 200 metre spaced sections will be re-logged, re-photographed and assayed.
This program will act as a control to allow a resource update based on bulk-tonnage parameters. The known high grade zone extends from Section 300mN to Section 1500mN and ultimately all historic drill holes within this zone will require re-logging and assay. This requirement was not anticipated within the Prospectus work program and has arisen through field studies subsequent to November 2016 Prospectus lodgment.
Field exploration results
Whilst siting drill collars for the initial proposed Ardea drill program, a suite of samples was collected as a first pass in defining Lewis Ponds geo-metallurgical types (GeoMet study). At the old Lewis Ponds Main Zone and Tom’s Zone main shafts, the main producers in the field, it was apparent that the dominant shaft mullock at both locations (600m apart along strike) is a distinctive white vuggy quartz-pyrite “sinter” (which appeared to have the appearance of epithermal style mineralization).
|Location||Sample No||Au (g/t)||Ag (g/t)||Zn (%)||Pb (%)||Cu (%)||As (ppm)||Sb (ppm)|
|Lewis Ponds Main Zone shaft||LP01300||9.9||539||0.05||1.6||0.30||1030||2650|
|Tom’s Zone shaft||LP01301||12.1||272||0.09||2.7||0.01||90||74|
The high-grade Au-Ag with anomalous As-Sb is consistent with epithermal style mineralisation, and the low base metal values are not indicative of a VMS setting at the shaft areas.
Base metal and gold production is unknown for the Lewis Ponds shaft. Toms produced 30,000 tonnes of pyrite ore for sulphuric acid production. It is puzzling that such high grade precious metal mineralisation remains at both locations as shaft mullock and was not treated in the historic operation.
This mineralisation style was not predicted within the Prospectus and is of significant economic potential. In particular, “Gold Stringer” is a geo-metallurgical type recognized in Ardea’s work but not described in historic Lewis Ponds records.
Drill approval environmental submissions are being prepared, with Department of Resources and Energy approvals anticipated in time for February 2017 drilling.
|Hole ID||East (local)||North (local)||From (m)||To (m)||Width (m)||Au (g/t)||Ag (g/t)||Cu (%)||Pb (%)||Zn (%)||ZnEq¹||AuEq²||m.ZnEq³||Geo Met|
Geo-Metallurgy (GeoMet) – PS Polymetallic Stringer, PM Polymetallic Massive, CS Copper Stringer (no intercepts are significant), GS Gold Stringer (no intercepts are significant)
|Metal prices US$ (21 Dec 2016)||2617||5488||2177||1133||16|
¹ Zinc Equivalent Estimate
|Recovery for ZnEq calc||100%||80%||80%||90%||80%|
|ZnEq recov multiply factor||1.000||1.678||0.665||1.253||0.016|
² Gold Equivalent Estimate
|Recovery for AuEq calc||80%||80%||80%||100%||80%|
|AuEq recov multiply factor||0.575||1.205||0.478||1.000||0.011|
³ m.ZnEq = intercept width x ZnEq value
Scoping study level financial model for a 1.5Mtpa open-pit with base metal float circuit indicates 1.6% ZnEq is a suitable break-even cut-off grade.
In consideration of the broad mineralised intercepts over a strike length of 1.15km between Section 350mN and Section1500mN, the initial Lewis Ponds Exploration Target is estimated at 15 - 25Mt at 2.2 - 3.7% ZnEq (1.2 -2.0g/t AuEq)⁴ (see Table 2 for values used in defining zinc and gold equivalents). The estimated breakeven mining grade is 1.6% ZnEq.
⁴ An Exploration Target is a term used within the JORC2012 Code for an estimate of the exploration potential of a mineral deposit. As used in this release the stated Exploration Target is based upon the parameters described in the text, however the potential quantity and grade is conceptual in nature and there is insufficient information to estimate a Mineral Resource and it remains uncertain if further exploration will result in the estimation of a Mineral Resource in this area of drilling.
The potential quantity and grade is conceptual in nature, and there has been insufficient exploration based on the “bulk-tonnage” concept to estimate a mineral resource and it is uncertain if further exploration will result in the estimation of a “bulk tonnage” mineral resource.
The system is open north and south along strike within areas of historic workings and soil geochemical anomalism. Significantly, there are extensive runs of historic drill core with no assays at all or only base metal assays (i.e. no gold assays).
Commonly within the Lewis Ponds lode envelopes, the runs of non-assayed material correspond to core photography which clearly shows the core is altered and likely mineralised. These intervals are ascribed nil grade within intercept calculations, meaning the Exploration Target grade is likely under-estimated.
Funds raised under Ardea’s initial public offer will be used, amongst other things, to test the exploration target through diamond drilling on 100 metre spaced sections over the next 12 months.
Historic TriAusMin Lewis Ponds announcements
|File Date||File Size|
|TriAusMin Commences Diamond Drilling Program at Mt Nicholas in New South Wales, Australia||25 Mar 2014||29 KB|
|TriAusMin Expands Open Pit Potential at Lewis Ponds Project in Australia||15 Dec 2011||767 KB|
|TriAusMin Commences Drilling on Lewis Ponds Base and Precious Metal Project, New South Wales, Australia||3 Nov 2011||227 KB|
|TriAusMin Moves Forward with $3 Million Program at Woodlawn and Lewis Ponds Projects in Australia||23 Feb 2011||24 KB|
|Lewis Ponds Project Presentation||Nov 2010||332 KB|
|Targets Identified for Exploration at Lewis Ponds Gold Project||8 July 2010||89 KB|
|Tri Origin Commences Airborne Geophysical Survey of Lewis Ponds||23 Feb 2010||45 KB|