The economic geology of the so called Ring of Fire area in the James Bay lowlands of northern Ontario is centred upon two principal domains, namely the Archaean granite-greenstone terrain with potential for gold, base metals, volcanogenic massive sulphides and diamond deposits, and the younger Ring of Fire layered igneous complex (ROF) with potential for chromite and platinum group elements-nickel-copper (PGE-Ni-Cu) deposits. This article is restricted to the ROF and focuses on its potential for new discoveries.

Backed by the knowledge of its consultants who have industry experience on the Bushveld and Great Dyke layered igneous complexes in southern Africa, and consulting assignment experience on the Stillwater mine in the United States, the Marathon-PGM, Shakespeare and Lac des Îles properties in Canada, the Arctic Platinum project in Finland, the Pechenga complex of the Kola Peninsula in Russia and the Voskhod chromite project in Kazakhstan, Micon has been able to play a pivotal role in the advancement of exploration on the ROF. It was the first independent consulting company to produce an NI 43-101 compliant Technical Report on the ROF chromite mineralization, to interpret the geometry of the ROF and to recognize the potential for stratiform PGE-Ni-Cu discoveries within the complex.

Current and Future Discoveries

The earliest discovery of significance within the ROF was the Eagle’s Nest PGE-Ni-Cu deposit by Noront Resources Ltd. in 2007. This was followed by the Big Daddy chromite discovery by the then joint venture partners Spider Resources Inc., KWG Resources Inc. and Freewest Resources Canada Inc. Several other chromite discoveries have since been made along a highly magnetic linear peridotite unit which stretches for approximately 15 kilometres from Black Thor in the northeast to Blackbird in the southwest.

Apart from work on the Eagle’s Nest deposit, since 2008 most if not all exploration efforts/drilling programs have been focused on chromite. Micon believes that there remains significant potential for further PGE-Ni-Cu and chromite discoveries and, possibly, vanadium and iron. However, it is new PGE-Ni-Cu discoveries, as opposed to additional new chromite discoveries, that will enhance the economic outlook for the ROF area.

In the quest for new discoveries in the ROF, the essential ingredients are a broad based knowledge of the geological environments and mineralization genetic models associated with layered igneous complexes, combined with the ability to apply effective geochemical tools in the location of PGE-Ni-Cu enriched zones which, unlike chromite, are not visually identifiable in drill core or surface exposures. It is important to note that within igneous complexes, PGE-Ni-Cu deposits can be stratabound (syn-genetic) or non-stratabound, with the latter typically arising from rejuvenated igneous activity post-dating the complex or occurring as contact metamorphic deposits.

Most, if not all, layered igneous complexes are differentiated into a lower ultramafic zone and an upper mafic zone. However, despite this broad similarity, each complex is unique and variations in deposit types and styles should be expected when exploring in a new environment – hence, the need to think outside the box as exploration programs proceed.

Where to Look Within the Ring of Fire

Unlike most other large layered igneous complexes such as the Stillwater, Bushveld and Great Dyke complexes, the ROF has been subjected to a complex structural history culminating in it being transposed into a sub-vertical position. A generalized stratigraphy of the ROF as interpreted by Micon is presented in the figure below.

The numbers 1, 2 and 3 represent the chromite horizons which Micon has designated as CrZ1, CrZ2 and CrZ3, respectively.

Generalized Stratigraphy of the ROF

The tilting and subsequent erosion of the ROF complex to a sub-vertical position has resulted in the various mineralized horizons being brought to surface thereby giving a variety of mining options.

PGE-Ni-Cu Potential

Within a layered igneous complex, PGE-Ni-Cu mineralization may occur in ultramafic units, for example the Main Sulphide Zone of the Great Dyke and the Ferguson Reef of the MunniMunni complex, or within mafic units, as in the Merensky Reef of the Bushveld complex, the J-M Reef of the Stillwater complex and the API/APII (Ala-Penikka) zones of the Penikat intrusion in Finland. Thus exploration for PGE-Ni-Cu in the ROF should target both the mafic and ultramafic units.

Assimilation of country rocks is an important mechanism for initiating the exsolution of immiscible sulphide liquids in contact-type deposits. The contact of the ROF with the older basement rocks on the west side is an exploration target for deposits analogous to the Platreef in the Bushveld complex of South Africa.

Detailed geological modelling completed by Micon shows that the Eagle’s Nest deposit is parallel to and wholly detached from the layered series of the ROF and is therefore not within a conduit feeder of the ROF as previously postulated. However, the dyke hosting the deposit and the ROF complex most likely originate from the same deep-seated magma chamber. It is probable that more of these discordant pipe-like deposits remain to be discovered. Interestingly, the occurrence of the Eagle’s Nest deposit is analogous to the dunite pipes of the Bushveld Complex which are mined for PGE-Ni-Cu – the Driekop and Mooihoek pipes emplaced in the Upper Critical zone, and the Onverwacht and Twyfelaar pipes in the Critical and Lower zones, respectively.

Chromite

Chromite deposits in igneous complexes are universally associated with the ultramafic units. Micon believes that the ROF has at least three main chromite horizons or zones. The uppermost chromite zone (CrZ1) is traceable from the Black Thor deposit in the northeast through Black Creek, Big Daddy, and Black Horse to Blackbird 2 east zone (BB2-1) in the southwest over a continuous strike length of approximately 15 kilometres. The second chromite zone (CrZ2), represented by the Black Label in the northeast (located about 200 m to the west of Black Thor) and the Black Bird 2 central zone (BB2-4/5/6) about 15 kilometres to the southwest, is a strong candidate for new discoveries in the intervening ground between these extremities. The third chromite zone (CrZ3) has been identified in the Blackbird area immediately to the east of the Blackbird 1 deposit as the Blackbird 2 west zone (BB2-2/3) and may extend as far northeast as CrZ1 and CrZ2. All three chromite horizons are parallel to each other but the separation distance between the zones may vary along strike and dip due to structural disturbances.

CrZ2 and CrZ3 are less compact than CrZ1 and may be difficult to recognize as discrete horizons because of brecciation and/or tectonic dislocation and the presence of xenoliths. Nonetheless, these are easily discernible where drilling is closely spaced in the Blackbird area.

Titanium and Vanadium

Finally, the ROF also has potential for discovery of titanium and vanadium deposits, analogous to those found in the Bushveld complex. Exploration has been initiated in the mafic units located to the east of the Black Thor deposit and anomalous mineralization has already been noted.