Largely overlooked in the central and western Gawler Craton is a large slab of Archean to earliest Paleoproterozoic crust with komatiites stretching for hundreds of kilometres, recent hits of +1% nickel and a gold deposit of more than one million ounces. There are big crustal shear zones, mantle tapping structures and arguably the best new airmag and MT data in the country.
If this slab of crust (known as the Mulgathing Complex) was in WA it would have explorers crawling all over it, so why is it off the industry’s radar? The fact it is not part of the Yilgarn certainly doesn’t help, nor does an historically patchy understanding of the (almost entirely concealed) stratigraphy.
The Geological Survey of South Australia has taken a big step in rectifying the latter with a new stratigraphic classification of the Mulgathing Complex, unravelling many of the puzzles and linking the now-much-better-defined units to their economic potential.
The new study was produced by recent GSSA recruit, Dr Megan Williams, who integrated 30 years of literature and made many trips to the Tonsley core library, along with GSSA’s senior principal research scientist Anthony Reid. It is GSSA’s most important report on the Mulgathing Complex since 1993.
The Mulgathing Complex has three main packages: undifferentiated Gawler Craton/eastern Mulgathing Complex; Harris Greenstone Domain (which includes the Lake Harris komatiites); and the Christie and Wilgena domains. Along strike from the Christie Domain towards the southwest lies the Fowler Domain, which includes rocks of the Mulgathing Complex as well as younger intrusives.
Western Areas triggered a pegging rush in the Fowler Domain just under a year ago after reporting very encouraging results from a maiden three-hole drill campaign at the Sahara prospect, including up to 1.38% nickel and the interpretation of an elongate chonolith intrusive system. (The age of the mafics hosting the Sahara nickel-copper mineralisation is not known, and there is a real possibility they are part of the Mulgathing Complex). Western Area resumed drilling resumed in February, targeting higher grade extensions along strike.
Exploration in the Mulgathing has been (and continues to be) dominated by the search for gold. There are at least two styles of gold deposit; metamorphosed gold mineralisation as seen at the +1 Moz Challenger deposit, and vein-style gold associated with granites such as the deposits at Tarcoola.
Gold is still an attractive target, not least because the Mulgathing Complex is massively underexplored, but the future of the mining industry surely lies with battery metals such as nickel and copper. Why would you not make it a priority to generate new nickel targets in the Mulgathing Complex, where there is already plenty of indications of nickel coming up from the mantle and perhaps the most overlooked stretch of komatiites in the world?
The most important point to make about the Mulgathing being off the industry’s radar is the fact it is so under-explored. As the report points out “of the 22,886 drillholes over the Mulgathing Complex region (many of which do not penetrate the cover sequences), only 853 are diamond drillholes, 638 of which are at Challenger mine and surrounds.” There is public file drill core (59 diamond drill cores in the South Australia Drill Core Reference Library with significant (>5 m) intersections of Mulgathing Complex stratigraphy), of which only a handful have been studied in any detail.
Other explorers have had a go in the past. Vale was active in Lake Harris Greenstone Belt a few years ago and is still interested, renewing two permits last year (EL 6578 and 6579) about 25 km south and southwest of Tarcoola. Endeavour Gold drilled komatiites near Lake Harris and still holds most of the ground at the eastern of the Belt. The company quit the project after finding no more than elevated nickel readings. (This was co-funded under South Australia’s PACE program, so all the results and drill core will be open file).
Megan said the komatiites in the Lake Harris Greenstone Belt are commonly found with felsic units that are generally mapped as a metavolcanic known as the Kenella Gneiss. But there are locations where the komatiites are interbedded with metasediments, which is a major distinction in terms of their prospectivity. The accepted nickel mineral system for komatiites relies on the supply of sulphur from metasediments.
“Endeavour’s search area had more granitic lithologies adjacent to the komatiites than the metasediment. But there’s about 300km of strike length of those komatiite-bearing units. Lake Harris is actually quite a small area so there’s certainly potential elsewhere to find a different felsic package adjacent to the komatiite,” Megan said.
Another important aspect of the new study is the answers it has provided about mafic rocks across the Mulgathing Complex, particularly in the north around Mt Christie. “We have a much better understanding of the story up there where there are potential nickel-bearing rocks,” Megan said.
The bigger picture about the prospectivity of South Australia’s komatiites is an interesting one. Anthony said the Lake Harris Greenstone Belt is about 150 to 200 million years younger than the komatiites of the Yilgarn and similar systems in the Superior Craton in Canada. Does that matter? Anthony suggests it is neither here nor there when we are talking about rocks of about 2.7 Ga in the Yilgarn versus about 2.5 Ga for the Harris Greenstone Belt. He also argues nickel-bearing komatiites at 2.5 Ga might not be a feature anywhere else in the world because this part of Archean is very poorly preserved around the world.
“This particular time interval is poorly preserved globally, and that’s because wherever you go these rocks have been reworked by younger Proterozoic systems, for example the Sask Craton in Canada, which is locally exposed in the Reindeer Zone of the Trans Hudson, as well as regions within the North China Craton. For whatever reason, these terranes don’t seem to have survived in the same way as the Yilgarn or the Pilbara cratons for example.”
Anthony said the Mulgathing Complex was more structurally complex than the Yilgarn, which was another factor working against it, but the new stratigraphic study was a major step forward in helping explorers deal with that.
“One of the difficulties of working in the area has been the stratigraphy. But if you drill a hole out there now, you can pretty much say what the rock is, based on Megan’s work. I think that’s a huge step forward. There hasn’t been a single point of truth since the Archean chapter of the 1993 Bulletin of the Geology of SA by Sue Daly and Mark Fanning. The new report has full stratigraphic descriptions, complete descriptions of all the units we know so far with our best guess of their age, their composition, and mineral potential. This really is a landmark publication for these rocks and we look forward to following this up with more work on the geology of this region,” Anthony said.
Not the Yilgarn, but komatiites at Lake Harris, South Australia. Photo credit: Anthony Reid