Precompetitive Mineral Mapping Products of the Broken Hill 2002 Airborne HyMap Survey
The objective of this 2009 project between the Geological Survey of NSW and CSIRO was to add pre-competitive exploration value to the original 2002 airborne HyMap survey data collected from three areas over the Broken Hill Domain (Fig. 1 Courstey of PIRSA) by providing a suite of publicly-available mineral abundance and mineral composition maps. This project is part of a larger vision of CSIRO’s Mineral Down Under Flagship (MDU) and the newly established WA Centre of Excellence for 3D Mineral Mapping (C3DMM) to generate a publicly-available, seamless 3D mineral map of the Australian continent based on emerging satellite, airborne) and drill core logginghyperspectral technologies in collaboration with the government geosurveys. These Broken Hill Block mineral maps, spanning 4000km2 at 3.5m pixel resolution, thus build on similar
projects with other geosurveys (including Geoscience Australia) in Queensland in 2008 and Western Australia in 2005.
The high grade Proterozoic rocks of the Broken Hill Block are a challenge for HyMap type data as the wavelengths detected are best suited to OH-bearing minerals. Prograde metamorphism reached granulite facies (red line – facing image) with only a small area in the north below the sillimanite-muscovite (blue line) isograd. There is growing evidence that the region may have experienced more than one post-prograde (re)hydration event. The map of white mica distribution (Fig. 2) is thus a function of later hydrothermal fluid activity and is largely constrained to north of the published Proterozoic retrograde staurolite (yellow) isograd. The colours of this mineral map relate to the level of Tschermak substitution in the white mica, which are consistent with field spectral measurements (coloured dots). The red tones are Si-rich (Al-poor) “phengite” and the blue tones are Si-poor (Al-rich) muscovite. The airborne/field data show that the white mica absorption is >2190 nm indicating that paragonite is not present, which has been supported by XRD analysis of the 50 field samples. Areas not mapped as white mica, such as in the south, comprise kaolinite that has developed over in situ rocks and some transported materials. Al-smectite (possibly also mixed layer kaolin-smectite) is developed in (but not all) drainage lows, which can be useful for mapping soils Tenosols.
Retrograde shear zones (RSZ) typically comprise a foliation of white mica. However, both the K radiometric and the white mica content maps (Fig 3b) of the Broken Hill region show that these RSZ (outlined in black/white) are relatively poor in K and white mica content compared to the surrounding country rock.
The mineral maps show that the RSZ, especially those closer to the edges of the Broken Hill Block like southeast of Stephens Creek Reservoir (Fig. 3), are characterised by phengitic mica (Fig. 3c, red tones). These phengitic zones also extend beyond the interpreted boundaries of the RSZ and are often spatially associated with Thakaringa-style epigenetic vein deposits (magenta diamonds). These RSZ also show a white mica chemical gradient along their length (Fig. 4) which can be interpreted as a physicochemical evolution of the related hydrothermal fluids as they moved along these pathways. However, whether these represent a “cooler” Si-rich fluid invading into the Broken Hill Block (from all sides) or a “hotter” Si-poor fluid being expelled from the Broken Hill Block is for further investigation.
Over 20 GIS-compatible mineral maps were generated for each of the three Broken Hill Blocks as JPG and GeoTIFF products, which can be ftp-downloaded from the C3DMM webpage and the Auscope portal. Each product, which can be up to 1.5 Gbytes each, has associated metadata/descriptions including: the methods how each was generated; what they mean in terms of mineralogy; and what other effects complicate their accuracy.
It is expected that these new mineral products will provide valuable information for Broken Hill exploration that complements other pre-competitive geoscience data available from the region.
Director, Western Australian Centre of Excellence for 3D Mineral Mapping
CSIRO Exploration and Mining
Telephone number: (08) 6436 8630
Facsimile number: (08) 6436 8555