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POSITION MAGAZINE – SPECIAL ISSUE ON MINING
Innovation in Mineral Resource Exploration from Hyperspectral Imaging A mining boom means great opportunity, easier financing and expansive land openings. However it also means greater competition for all of these things. Mining companies seek competitive edges even in boom-time and one of the main ways to achieve this is technological innovation on the front-end of exploration. Every prospect is ultimately drilled in order to provide proof of resource, but getting to the point of drill locations is accomplished in many ways – some high tech, some low tech.
“The cost of reconnaissance drilling, primarily RAB and aircore has escalated due to rising fuel and labour costs. Similarly the availability and the costs of geologists to map project areas has seen rapid escalations in cost and in recent years a diminishing skill base,” laments Peter Schwann of Aruma Resources Ltd., Perth. A natural outgrowth of these issues is to use airborne spatial imaging, particularly hyperspectral, to distill large areas of interest into smaller land packages for further more expensive and time-consuming assessments (eg. point geochemical sampling and ultimately drilling). The world of advanced spatial imaging has many components used in both the exploration and development phases of
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mining. These imaging techniques can range from simple use of Google Earth as a proxy for directed, high spatial resolution photography to things like LiDAR surveys for detailed terrain modeling. Airborne hyperspectral imaging (such as the Australian sensor HyMap) has been in use by the mining industry since the mid-1990’s, and has increasingly been employed as a means of economic, large-area, district-reconnaissance for ore-indicative primary lithology and secondary alteration patterns. Hyperspectral was initially used at finer scales (necessitated by, among other things, early in-ability to visualize large GB-sized datasets with 1990’s computing power). Areas in the several hundreds of square kilometer range were routinely mapped at resolutions of 10-20 m. But improved computing power and advanced algorithms for geocorrection and material identification have made mapping and visualization of many thousands of square kilometers at resolutions of 3-5 m, a reality in the last ~5 years.
Figure 1. The 128-band HyMap hyperspectral scanner co-mounted with the four-band, Vexcel Ultracam in light aircraft.
Of course satellite-based multispectral mapping from sensors such as the US/Japanese instrument ASTER that flies on
the Terra platform or the original Landsat series of sensors, cover many thousands of square kilometers a day. However, it is the spectral fidelity of airborne hyperspectral (generally 100+ bands) that ultimately sets mineral mapping from these sensors apart from those maps derived from multispectral instruments (generally in the 5-15 band range). So too does the differences in spatial resolution, where thematic multispectral satellites image at resolutions of ~30m while airborne instruments image in the ~1-5 m range.
Further innovation has recently been realized in the form of co-acquired high-resolution airborne hyperspectral data with orthophotographs and derived Digital Terrain Models (DTM). The draping of thematic data on terrain models is not new, but merging thematic data (eg. Hyperspectral-derived mineral alteration maps) with precisely colocated 1 m terrain topography derived from photography flown at the same time is a big step forward in product sophistication. It allows for ready visualized relationships between the spatial-mineralogicaltopographic features of the earth’s surface. Figure 1 shows both the HyMap scanner and the Ultracam co-mounted in light aircraft.
Similar to more sensitive or higher resolution geophysics, airborne hyperspectral plus high resolution terrain modeling has provided mining companies with a competitive edge via technical innovation. This article samples this innovation by looking at three active, mineral resource exploration areas in Western Australia including gold exploration in the Northern Goldfields, uranium and REE exploration in the Yilgarn and iron ore development in the Pilbara. Airborne mineral mapping was undertaken by Australia’s HyVista Corporation using the HyMap airborne
hyperspectral scanner (128 bands in the 4502500 nm spectral region at ~3 m spatial resolution) and a co-flown Vexcel Ultracam large format digital camera (see www.hyvista.com for more details). •
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Large exploration areas are distilled down to prospectsized land packages Specific mineralogy related to ore-genesis, even in highly weathered areas, is absolutely identified and geolocated Primary spatial image and terrain data is merged with secondary exploration datasets (geophysics, geochemistry) to derive tertiary products and models of potential ore systems
Gold Exploration in the Eastern Goldfields: Laverton East Project, WA There have been many airborne hyperspectral surveys in the Eastern Goldfields of Western Australia (eg. The 1:100 000 Kalgoorlie spectral geology mapsheet, T.Cudahy/CSIRO [http://c3dmm.csiro.au/kalgoorlie/kalgoorli e.html]).In fact, this is a district that has seen a hundred plus years of exploration and development – and yet new discoveries are being made every day. Areas previously surveyed with ‘traditional’ technologies (eg. Field mapping, multi-spectral imaging, ground-based gravity, magnetic and electrical) are now being re-surveyed with advanced technologies like hyperspectral and high resolution airborne geophysics. As an area with less than 10% outcrop, the Goldfields present the challenge of revealing geological and geochemical features and processes that the human eye cannot see and which satellites are incapable of tracking.
Figure 2. Mineral mapping of Laverton East with location of hyperspectral-directed RAB holes
The Laverton East gold prospect is one such project located approximately 20km northnortheast of the town of Laverton in the Mt. Margaret District and lies on the eastern margin of the Laverton-Karonia greenstone belt of the Archean Yilgarn Craton. Explored in the modern era since the late 1960’s with prolific auger drilling, sediment sampling and basic ground geophysics, modern assessments of the district have only begun in the last ~ten years with higher resolution airborne geophysics in the early 2000’s (Placer) and airborne hyperspectral in 2010 with the exploration team from Aruma Resources Ltd.
Mineralisation at the project area is thought to be largely shear-hosted with dominant north-northwest trending structures hosting most anomalous gold in the form of narrow, discontinuous veins. Intrusive mafic to ultramafic bodies are also common along shear zones and also host mineralization within pressure shadows and dilatants zones. Most of the project area is deeply weathered and dominated by lateritic duricrust, colluviums and alluvium. Coincidentally, gold enrichment of ferruginous laterite profiles near weak primary sources is also common and can create large anomalous Au haloes.
The HyMap survey flown over Laverton East in 2010 and subsequent processing revealed 16 separate mineral species and mapped a comprehensive set of alteration assemblages expected in this precious-metal lode/vein deposit. More specifically, a zone of goethite anomalism was mapped in HyMap data that is identical to gold anomalism defined by rock chip analysis elsewhere in the project (see Figure 2). This goethite map was used to site locations for a RAB program in 2012; results are encouraging to-date with ore-grade Au values in one location so far.
Managing Director of Aruma Resources Ltd., Peter Schwann summed up the company’s use of HyMap at Laverton East and other Aruma prospects, “Using a suite of indicator minerals has allowed the large area Exploration Licenses (of up to 600 km2) to be quickly mapped and evaluated at a cost equivalent to minimum expenditure. This allows focused second phase exploration to be undertaken on smaller selected areas of interest in a short time period.” Subsequent drill locations have been sited based on the HyMap-derived mineral maps and will be explored later in 2012 and 2013.
Uranium exploration in the weathered, northern Yilgarn: Yeelirrie, WA
The terrain of the northern Yilgarn craton in Western Australia is particularly weathered, even by Australian standards. Such weathering has previously been deemed a difficult obstacle to overcome in hyperspectral surveys. However its’ status as a prime target for uranium explorers led HyVista to overfly one of the most wellknown Uranium districts: Yeelirrie. Though already under production by BHP Billiton, a hyperspectral survey over both the known deposit and regional ground was flown in 2010. Originally discovered in 1972 by WMC Resources, this primarily carnotite ore body takes the form of mineralized calcretes formed above Archean granites and greenstones. The ore zones themselves form within the valley paleodrainage systems, but are difficult to map in the field as outcrop is poor; anywhere from 1 to 2 meters of sandy loam and clay overburden are generally present. Nevertheless, an alteration assemblage indicative of type uranium ore for Yeelirrie was successfully mapped in several locations (namely saponite clays forming above the uranium-ore containing dolomite calcrete units)(see Figure 3).
Figure 3. Mineral mapping in Yeelirrie district demonstrating ability to identify and map mineral assemblages, in the presence of intense weathering, indicative of calcrete-hosted Uranium.
This hyperspectral survey demonstrates the ability to map not only the ore-hosting rock in a weathered environment, but also to track these same unique assemblages further afield into un-tested ground. Such mapping is difficult and time-consuming with other traditional mapping methods. As detailed by the Aruma Resources exploration group, “A geology team with aerial photography and reasonable ground access would require approximately a day per 10km2, so a 200km2 lease would be some 20 days at a total cost of greater than $60,000 which would be some 100% more than HyMap mapping. Using the HyMap and groundtruthing would allow for focused ground investigations significantly reducing field time and cost.”
Indicative iron-ore primary mineralization and secondary alteration was identified and mapped with the HyMap data which was flown in 2010 at a resolution of 3m and covered an area of approximately 100km2. The oxides goethite and hematite and the clays, illite and kaolinite are mapped extensively through the pit, on the tailings piles and regionally, while distribution of chlorite is seen solely in the surrounding country rock (see Figure 4). The high hematite concentrations in the Dale Gorges and Joffre members can be seen in green throughout the 3D map in Figure 4 as well as the more restricted occurrences of goethite shown in red, (primarily from the hydration of hematite ore).
The ability to co-acquire airborne hyperspectral data and colour orthophotography (with subsequently derived digital terrain models) is a growing market sector. Mineral alteration from hyperspectral coupled with high resolution photos and terrain brings thematic data into the detailed, vertical space and is particularly well-suited to projects entering development, projects in the mid-life of a pit and/or projects nearing the end of economic development and requiring environmental due-diligence.
Conclusions
3D Geological Mapping in the Pilbara: Mt. Whaleback, WA
Located near the town of Newman, WA, BHP Billiton’s Mt. Whaleback iron ore pit was targeted for a co-flight of both HyMap and HyVista’s Vexcel Ultracam. This pit was chosen as it is the highest grade hematite deposit in the Hammersly and the largest, single, continuous iron ore deposit in all of Australia. Mt. Whaleback iron ore is hosted by the Brockman Iron Formation and is predominately hematite, goethite and martite (with primary magnetite).
Beyond the obvious implications for increased ability to grade iron ore with accurate, fine-scaled mineralogy, the combination with true-colour 15cm orthophotography and 1 m derived DTM’s, leads to a new set of derived, measured variables. This includes but is not limited to, more accurate ore body thickness calculations, mine engineering and planning improvements (eg. bench and highwall construction/monitoring/reclamation relative to mapped lithology and alteration) and specific mineralogic data for heap leach pad and tailings piles exploitation, management and remediation.
While many exploration models and toolkits have not changed much in the past 50-60 years, the innovations in airborne spatial imaging and airborne geophysics promise to help miners re-discover, re-evaluate and bring new mineral resource to market in an increasingly competitive atmosphere. Hyperspectral imaging is just one tool being used by current explorationists, but one that provides the ability to economically evaluate large tracts of land in small amounts of time.
Figure 4. A 3D mineral map showing the distribution of primary and secondary ore mineralization and alteration at the Mt. Whaleback iron-ore pit in WA. The mineral data from the HyMap is overlain on 15cm, true-colour orthophotos that have been in-turn overlaid on a 1m DTM.