The Ravensthorpe Gold Project is located at a key and highly prospective intersection of two globally significant mineralised orogenic belts:
- The Yilgarn Craton, at the southern limit of the Archaean Southern Cross Superterrane – host to over 7.6Moz of gold endowment (Southern Cross/ Marvel Loch) and over 250,000t of nickel endowment (Forrestania)
- The Albany-Fraser orogenic belt, host to the 8Moz gold Tropicana orebody and the 330,000t nickel Nova deposit
As may be expected at the intersection of two significant mineralised belts, occurrences of economic mineralisation within the Project’s tenements are extensive. Within tenure under ACH ownership there are:
- + 1Moz AuEq of gold in resource plus copper and silver
- Previous mines, now under care & maintenance or historically operated, that produced nickel (Rav 8), copper (Mt Desmond and Mt Benson), together with the suite of historic mines at Kundip the subject of current reported resources (above)
- Numerous geochemical and coincident geochemical-geophysical anomalies of gold and gold/copper mineralisation representing walk up drill targets
- Occurrences and/or drill intercepts indicating targets of potential economic interest for manganese, cobalt, antimony/silver and lithium
In the immediate surrounds of the Project’s tenure there are two contemporary mines:
- Galaxy Resources’ Mt Cattlin lithium-spodumene mine, targeting production of 135,000t of lithium concentrate per annum
- The Ravensthorpe Nickel Project, owned by First Quantum Minerals Ltd, until recently producing approximately 35,000t of nickel annually
Numerous historic mining operations are also found in the area, including the Eldverton Copper Mine and Ravensthorpe Copper Smelter, which were Western Australia’s largest single source of copper production in their day.
Discovery in the Proterozoic
At the time of the discovery of the Trilogy deposit by Homestake in 1997, Homestake consultants wrote in the foreword to an internal unpublished report:
“There are several lines of evidence which suggest that the Ravensthorpe Camp deposits [Kundip, etc] and the Trilogy discovery are related not only spatially, but possibly temporally:
i. The mineralogy was simple, pyrite-chalcopyrite. An unusual assemblage for the Yilgarn Craton. For example, outside of the SW corner of the Yilgarn there are only two Cu-Au occurrences in the Yilgarn Craton;
ii. All of the local Cu-Au camps are located within 2km – 5 km of the margin of the Ravensthorpe Tonalite or its deeply buried equivalent;
iii. Almost all of the Cu-Au camps are located on NE trending structural corridors; commonly mimicked by Proterozoic dyke swarms; and
iv. These camps occur on the southern margin of the Yilgarn Craton in a particular structural array that appears to link the brittle break-up of the Craton margin and the deposition of the Proterozoic rocks.
The possible implications of this are that the deposits in the Archaean host rocks and Trilogy might really be of the same age. Either the Kundip, Ravensthorpe Camps might be basement feeder zones to now exhumed Proterozoic mineralisation or that Trilogy might be a dismembered fragment of Archaean mineralisation in an Archaean host rock. The other attractive possibility is that the Ravensthorpe area hosts two ages of Cu-Au mineralisation: one at 2.6 billion years (the typical age in Archaean rocks) plus a younger (1.2-0.6 billion year [note Trilogy is now dated approx. just before 1700Ma]) Proterozoic episode relating to major metallogeny in an extensional basin regime – perhaps above a long lived (1.5 billion year) mantle underplating event.
Experience suggests that if the latter is the case then there might be some world class deposits in the area. If the former is the case, then a large array of deposit styles may be present in the province since what is now exhumed must only be a portion of the total deposits in the area”.
Trilogy was discovered in what was previously considered to be unprospective Proterozoic sediments (the Mt Barren Group), and at a time well before the discovery of Tropicana and Nova led to a better understanding of the prospectivity of these craton edge collision zones.
Homestake ultimately exited the Project tenements in 2000 unable to justify further exploration at the polymetallic Trilogy deposit as a global gold producer however ACH notes the following in relation to the opportunity that remains to make significant discoveries with the Project tenements:
- There was a protracted hiatus in greenfields exploration at the Project driven by an almost exclusive focus by Tectonic on brownfields exploration post Homestake’s departure to bolster mineable inventories at Kundip
- The discovery of Tropicana in 2001 and Nova in 2011 in the Albany Fraser orogen
- Multiple geochemical anomalies were generated by Silver Lake between 2012 and 2015 which have experienced no systematic bed rock testing
- The increasing weight of academic research into the collision edge Craton margin zones of the Albany-Fraser Orogen and their importance to metallogeny
- The eventual consolidation of the majority of the relevant Archean and Proterozoic host rocks by Tectonic, then Silver Lake Resources and now ACH, especially in the Mt Barren Group and Bandalup Pools area
The collision zones at the immediate edge of the Yilgarn Craton are now emerging discovery “hot spots” with numerous strategic land positions and corporate plays unfolding as academic and company research begins to unlock the belt’s history and potential.
In the Archaean sequence at Kundip and at the numerous regional gold/copper exploration anomalies and bedrock targets, variously sized oxide gold (+/-silver) mineralised positions sit atop narrow (massive sulphide) vein and breccia lode positions in the fresh rock. ACH interprets the mineralised veins and shears to be part of a discharge zone beneath the sea floor that fed potential massive-sulphide deposits that were emplaced at higher stratigraphic levels and more than likely have been eroded away or dislocated by faulting.
The present mineralised lodes therefore are similar to the feeder zones of VMS systems. In addition to Kundip, the gold/copper or copper deposits known at Mount Cattlin, Mount McMahon and the historically significant Elverdton Copper Mine are likewise potential feeder zones and together with Kundip, display the “clustering” characteristic of deposits in VMS belts surrounding in this case the Manyutup Tonalite.
ACH believes further feeder zones are possibly present and untested on its regional tenements at Ravensthorpe with the Old Gregg, Mumbles and Meridian targets examples in the immediate vicinity of the town. The following figure shows a stylised representation of the geological setting in the Archaean during the mineralisation event and how it would correspond with the current geological plan of the Ravensthorpe area.
Upon acquisition of the Project in 2012, SLR commenced a holistic review of the entire Project area which highlighted many encouraging exploration targets. It initiated a program of prioritising and testing these targets. In order to facilitate ongoing exploration, the region was broken up into several “sectors” based on broad metal groups, many of which still have not had any detailed follow-up. Two of these sectors align with ACH’s stated strategy of progressing gold-copper oxide and copper-gold-silver primary mineralisation. These are as follows:
Sector 1 – gold only, in the north western portion of the region. This sector hosts many historical small shafts within the Annabelle volcanic sequence. Extensive soil sampling and some geophysical surveys have been carried out in the area. The Meridian prospect is the most advanced and has returned a number of significant drill intercepts including:
- JHP006: 3m @ 27.73g/t Au
- JHP001: 5m @ 5.93g/t Au
- ANP007: 4m @ 4.44g/t Au
Sector 2 – copper-gold, northwest of Kundip and including areas around historic copper and gold mining. The area hosts numerous historical workings and the largest previously mined copper deposit known as Elverdton. The most advanced prospects in this sector are Ard Patrick, PLP, Fed and Mt McMahon. The best drill intercepts from each area are:
- 5m @ 65.36g/t Au (Ard Patrick)
- 2m @ 2.83% Cu (PLP)
- 0.7m @ 21.7g/t Au (Fed)
- 1m @ 3.16% Cu (Mt McMahon)
The Phillips River Mineral Field was until recent times, the largest copper producer in Western Australia, producing over 33% of both the tonnage and value of copper produced in the state up until 1954. When production began to wane in the early 1920s the Ravensthorpe area had produced over 95,000 tonnes of copper ore worth more than 0.5 million GBP at the time.
The Mt Desmond/Eldverton share of this production total at that time was some 47,000 tonnes grading 8.8% Cu.
Eldverton enjoyed a second period of production between 1957 and 1971 producing a further 740,000 tonnes at 1.4% Cu and 0.8 g/t gold from underground mining. In total the mine produced 14,250 tonnes of copper and just under 23,000 oz of gold.
The Mt Desmond- Eldverton line of lode extends over 850 metres in strike hosting copper/gold mineralisation in a 20-30 metre wide shear zone containing mafic greenstone rafts offset by later cross cutting faults. Historic records suggest that the presence of parallel mineralised structures on the main line of lode cannot be ruled out with a drill hole on the 1050ft N cross section recording 40 feet (12.2 m) at 2.1% copper, seemingly not exploited above the old 2 Level (76 metres below surface).
Mt Desmond is located on a granted Mining Lease and was the subject of a previously calculated inferred resource of 324,000 tonnes @ 2.1% Cu and 0.9g/t Au.
The mine achieved historic recoveries of >92% Cu and 85% Au by flotation only and has numerous encouraging intersections drilled after mining ceased in the 1950s such as:
- ELV 12 – 3 metres @ 13.8% Cu and 4.3g/t AU
- DER01 6 metres @ 2.45% Cu and 9.7g/t AU
- DER05 5 metres @ 2.04% Cu and 1.0g/t Au
Further to the north again, the PLP prospect (and old mine immediately north of Mt Desmond) recorded surface drill results of 4.0 metres @ 2.8% Cu and 1.13g/t AU and 2.0 metres @ 2.8% Cu and 2.6g/t Au. Although not part of the current Life of Mine for the RGP, Mt Desmond offers an obvious target for extensional exploration work to firm up on the possibility of either open pit cutbacks over the old workings or future underground access to exploit the copper rich mineralisation.
As an example of the style of regional targets generated by SLR, the Old Greg target, located mid-way between the Kundip mine area and Ravensthorpe, serves to highlight the quality of the untested portfolio of discovery opportunities.
The tenement hosting this target was acquired by Tectonic in 2011, not long before deal discussions with SLR commenced in 2012. A strong VTEM anomaly is associated with an interpreted northwest striking structure. The target sits within the Manyutup Tonalite and displays coincident gold (ppb) anomalism and associated weak copper anomalism (>130ppm Cu) identified from soils. Peak gold anomalism records 94ppb Au, with a zone >15ppb Au registering dimensions of approximately 600m x 400m. This is of similar scale to the footprint of the preferred open pit shell at the Kaolin deposit which contains over 165,000oz of gold.
ACH believes that the regional discovery opportunities presented by targets such as Old Greg, Mumbles/Meridian and others are in fact further VMS feeder zone clusters capable of delivering a significant addition to the mineable inventory at grades of copper/gold similar to the high grade shoots recorded at Harbour View and Flag. Essentially each individual target area could deliver:
- a Kaolin sized deposit, that is 150-200koz of gold each
- areas such as Meridian/Mumbles could represent a Kundip sized collection of deposits, namely camps of 0.5-0.7Moz of gold
Likewise, it is also possible that the extensive, but to date generally sub-economic massive pyrite/sulphide occurrences in the adjacent Chester Formation of the Carlingup Terrane to the east could be examples of more distal sedimentary exhalative, sediment hosted, stratiform massive sulphides. Exploration in this and the adjacent Hatfield Formation has been far from systematic and offers significant discovery upside.
The regional tenements at the RGP have excellent potential for further Trilogy style lead/zinc/silver mineralisation, specifically on ACH’s holdings over the Proterozoic sediments of the Mt Barren Group.
In regions with sedimentary exhalative style mineralisation, it is often the case that multiple deposits occur within the prospective host sequence rather than single deposits in isolation. Exploration to date using soil geochemistry and RAB/AC drilling as the primary tools has identified a number of prospective corridors warranting follow-up such as the Queen Sheba trend, Bandalup Gossan, Railway, No Tree Hill and Woodstock targets.
To the immediate south of Trilogy, extensive zinc anomalies have been identified by soil sampling and ACH is currently executing a low cost, potentially high impact gravity survey program to help with target definition and drill planning.
Further to the east, previous exploration discovered potentially economic quantities of zinc mineralisation in first pass bedrock drilling at the Bandalup Gossan / Pools prospect. This prospect straddles the contact between the mid-late Proterozoic Mt Barren Beds of the Albany-Fraser Orogen and the Hatfield Formation of the Archaean, Ravensthorpe Greenstone Belt.
MPI/Outokumpu, Homestake and then Anglo American all explored this area to investigate the extensive base metal anomalism centred on the Bandalup Pools Prospect. These groups were searching for large Sediment Hosted Lead-Zinc deposits. MPI drilled BGD1 that intersected 36.6 metres at 1.09% Zn and 1.07% Pb whilst Anglo American drilled 12 RC holes and recorded abundant pyrite and pyrrhotite in all holes hosted by carbonaceous siltstone and volcanoclastic sandstone. AS follow up, 2 diamond tails were planned however when one of the RC holes intersected semi-massive to massive pyrite-pyrrhotite mineralisation over almost its entire length a third tail, was completed to ascertain the style and significance of mineralisation. The first hole BDRCD001, was drilled beneath an historic hole (BGD-1), to test down dip continuity. Anomalous results were as follows:
- 11m @ 0.8% Zn and 1.16% Pb from 194-204m in volcanoclastic sandstone including 2m @ 5.17% Pb and 2.46% Zn from 194-196m and 1m @ 1.62% Zn from 200-201m
- 45m @ 1.32% Zn and 0.79% Pb from 226-271m in graphitic siltstone and breccia including 2m @ 1.28% Zn from 239-241m, 3m @ 3.09% Pb and 4.9% Zn from 246-249m and 1m @ 1.23% Zn from 250-251m
Anglo American also tested a selection of galena samples from veins hosted by late brittle veining. The Pb isotope analysis confirmed that the Pb at Bandalup Pools had been sourced from a low Pb Archaean source and then concentrated during the Proterozoic.
Exploration during the 1960s and 1970s revealed the presence of nickel-sulphide mineralisation in the Carlingup Terrane, at several localities east of Ravensthorpe. The targets were designated a “RAV” code and number. RAV 8 progressed from an original indicated resource of 249 900 t averaging 1.95% Ni as referenced by Witt (1998) via a BFS Study figure (1999) of 164,933 tonnes at 5.83% to be mined via open pit and underground methods by the Project’s previous owner, Tectonic Resources for a total of 457,997 tonnes at 3.47% Ni for 15,886 tonnes of nickel metal.
RAV 8, and several other prospects described by Marston (1984), occur along the strongly deformed north eastern limb of the Maydon Syncline. Disruption of the original sulphide orebodies into several smaller, separated deposits during regional deformation has made exploration difficult, and mining generally uneconomic to date.
The deposits are contained in serpentinized dunite and peridotite that have been metamorphosed to tremolite rich assemblages and talc-forsterite(-anthophyllite) rock. Talc-carbonate schist occurs in widespread deformation zones. The RAV 8 deposit consisted of minor massive to brecciated sulphide ore within a larger shoot of disseminated sulphides. Disseminated sulphides in talc-forsterite assemblages (matrix ore) occur in triangular interstices between prismatic metamorphic olivine crystals. The ore shoot plunges 30o southeast, approximately co-linear with the main tectonic lineation in this area. Disseminated ore contains up to 2% Ni, and massive-sulphide ore up to 18% Ni. The main sulphides are pentlandite and pyrrhotite with minor pyrite and chalcopyrite.
At Bandalup Hill, nickel was also found in lateritic deposits up to 80 m thick, which went on to become the Ravensthorpe Nickel Project, initially developed by BHP Billiton in 2007 at a cost of US$2.2B, before being sold to First Quantum Minerals in 2010. To date this project has produced approximately 23kt of nickel metal each year and as at the last reported date had 221 Mt at 0.62% Ni of mineral reserves for 1.3Mt of nickel metal.
In March 2018, ACH concluded a transaction with Galaxy Lithium Australian Limited (GLAL), a subsidiary of Galaxy Resources Ltd (ASX: GXY) whereby Exploration Licences E74/379, E74/399 and E74/406 were sold to GLAL. As part of this transaction, ACH retained the rights to all minerals other than Lithium and Tantalum on these tenements.
The Mt Chester Manganese Prospect is located within granted mining lease M74/163, approximately 9km from Ravensthorpe and 180km from the Port of Esperance. Manganese mineralisation was initially discovered circa 1903 by the Geological Survey of Western Australia and an adit was completed prior to 1909 to intersect the deposit. This generated a small amount of material for testing as a fluxing agent for a nearby Copper smelter. At the time, the deposit was categorised as “structural,” being a deposit where “the formation of manganese has been controlled or localised by such structures as folds.” The adit was subsequently mapped and sampled during the 1980s by Metana Minerals NL, with a report recording 15m at 17.7% Mn from the sampling. No other work is recorded in the adit.
In the 2000s Pioneer Resources conducted mapping and surface rock chip sampling along the projected outcrop of the relevant manganese rich rocks. From the completed program, 21 surface samples were taken from the manganese horizon over 500m of strike. Sixteen (16) of the samples assayed above 30% Mn, to a maximum value of 49% Mn. The manganese mineralisation remains open to both the north and south of the area sampled. Pioneer had earlier completed XRD determinations of manganese samples from the adit that confirmed that the dominant manganese minerals are the highly sought-after pyrolusite and cryptomelane. A further eighteen (18) of the surface rock chip samples returned assays above 50% Fe, thought to be a parallel haematite mantle overlying regionally extensive pyrite beds.
- Assays were completed by Ultratrace Laboratories Perth
- Assay technique: XRF- Fusion
- Sample locations by GPS