Targeting gold from Afar - Stratex International investigate the African Rift Valley
Could the African Rift Valley be the new Patagonia for gold deposits? David Hall of Stratex International plc reports.
Discovery rates in the gold industry are falling short of what is needed to sustain the market in the long term, and the issue is being compounded by companies focusing their efforts on near-mine and brownfield targets rather than investing in greenfield exploration. Coupled with recent cuts in investment, the exploration and discovery pipeline is going to be even more seriously affected in the future.
Bonanza gold deposits make attractive targets because of their potential to yield high rates of financial return. And there is a strong argument for concentrating the search for such base metaldeficient low-sulphidation gold veins in rift settings that are characterised by bimodal volcanism (both mafic and felsic magma from the same magma chamber). This case has been made by economic geologist Dr Richard Sillitoe to identify new deposits and also to attempt to define new gold districts.
However, hydrothermal alteration associated with rift-related low-sulphidation veins is minimal, which seriously hampers both their direct and indirect recognition. Three of the larger recent discoveries at Cerro Vanguardia and Esquel (Argentina) and El Peñón (Chile) are only partly exposed and were not recognised until eventual discovery from the late 1980s onwards. We can add the more recent discoveries of Cerro Moro, Las Calandrias and Cerro Negro to this list. Exploration for bonanza gold deposits is even more problematic because, realistically, future discoveries are likely to be blind, with no surface expression.
Investigating in the Rift
With these factors in mind, reviewing such rift environments and areas of hot spring activity is essential. Working with Dr Sillitoe in the African Rift Valley, Stratex International has been exploring this concept in a region without known gold mineralisation.
The triangular-shaped Afar Depression covers an area of approximately 200,000 square kilometres in the Afro-Arabian Rift System, which extends south from Syria and passes through the Jordan Valley, the Dead Sea, the Red Sea, the Afar Depression and the East African Rift, before terminating in southern Africa. The Main Ethiopian Rift, the southern Red Sea and the western Gulf of Aden lie within the Afar Depression forming a rift-rift-rift triple junction between the Nubian, Somalian and Arabian Plates. The central part of the Afar Depression is dominated by lowland plains corrugated by horsts and grabens, and rare local high relief peaks representing shield volcanoes. It can be divided into northern, eastcentral, south-western and south-eastern regions on the basis of similar structural trends. The volcanism is strongly bi-modal, akin to known gold bearing regions such as Patagonia.
Although an initial field visit to the Lakes district of the Main Ethiopian Rift in March 2009 found nothing of significance, research in a library in Addis Ababa revealed extensive geothermal work in the Tendaho graben by the Aquater group from Italy. Numerous hot spring occurrences were defined and in a somewhat different geological setting from that of the Main Ethiopian Rift (MER). The Tendaho graben is a distinct northwest–southeast trending feature conjugate to the MER trend. In October 2009, during a visit to the region, we discovered an outcropping of epithermal gold mineralisation. Subsequent to the discovery of the Megenta gold system, Stratex undertook further research and targeting in the Afar Depression. Research showed that epithermal potential in similar rocks had been defined by the USGS in 1997 in the neighbouring Republic of Djibouti.
Three distinct settings with potential for low-sulphidation epithermal gold mineralisation became apparent in the Afar Depression. All are closely associated with the compositionally bimodal, basalt-rhyolite suite and extreme extension that characterises the less than four million-year-old evolution of the Afar Depression. These early discoveries prompted Thani Ashanti, the Middle Eastern joint venture between AngloGold Ashanti and Thani Dubai Holdings, to enter into an option with Stratex. Drilling in 2011 at Megenta confirmed epithermal gold mineralisation at depth. Widespread gold was intersected blind below the sinter terrace, with values up to 19.5g/t Au over 0.7 metres and 43.5 metres grading 0.67g/t Au.
In 2010, the Blackrock prospect was discovered. A prominent rib was recognised close to the road from the Allana Potash camp, comprising chalcedonic silica veining with abundant calcite and an initial sample returned 2.3g/t Au with 61ppm As. Immediately, based on examination of Google Earth images, other potential areas of silica ribbing were identified and incorporated into a new licence application. Regional work subsequently defined new mineralisation in the Black Water and Magdala Zones south of the discovery vein, with impressive outcropping of epithermal veins over a cumulative strike of 14 kilometres. It is interesting to note that prominent epithermal quartz veins outcrop in much the same way as observed in Patagonia.
The Black Water zone was prioritised for detailed mapping and sampling following encouraging early-stage results, and five substantial northwest-southeast striking vein systems have now been identified. One of these has been traced for 1,600 metres and the other systems are between 700 and 1,100 metres long. Three of them have been systematically channel-chip-sampled, although scree cover and poor outcrop on two systems meant that continuous sampling was not always possible. Importantly, of the initial 339 samples collected, 107 returned values of greater than 1g/t Au, and 276 (81%) assayed greater than 0.1g/t Au. Only three samples returned non-detectable gold.
Stratex commenced a scout 5,000 metre drill programme at Black Water and again highly encouraging gold mineralisation was intersected with classic colloform banded epithermal veins with good vertical and strike continuity. Gold values overall are sub-economic but higher grades such as 15.95g/t Au over 0.25m and 4.97g/t Au over 0.5 metres indicate potential at depth. Both drilled systems are high level, and from the textures and alteration it is apparent that deeper drilling is required.
Stratex intends to undertake this in first quarter of 2013. The project has achieved what it set out to: discover a whole new gold province. This is Patagonia 20 years ago. Despite the high level in the systems drilled to date, the textures and presence of sinter show us that potential is open at depth, and this is the next area to be tested. Drill testing such vein systems can be frustrating, given the sporadic nature of results due to the typical lode characteristics of the higher grade portions of veins of this style. Despite this characteristic, similar vein systems such as those in northern Nevada, USA, as well as relatively recent discoveries in Patagonian Argentina, have been proven through perseverance in drilling and many are now mines. Stratex hopes that a similar approach will see the Afar Depression develop in much the same way Patagonia has.
For more information, contact David Hall, +44 (0) 207 830 9650