Student exercise Interpretation of visible-near to shortwave infrared reflectance drill core data from a channel iron deposit in the Hamersley Province (WA) Introduction Channel iron deposits (CIDs) are the second most important source for iron production in WA, after the bedded iron deposits (BIDs). CIDs developed along tertiary palaeochannels, where these crosscut banded iron formations (BIFs) or for several kilometres downstream, above metabasalt and metasedimentary rocks (Morris and Ramanaidou, 2007). CIDs consist of a mixture of Fe-(oxyhydr-)oxide pelletoids and ferruginised wood fragments, cemented in a goethite matrix. Detrital components, including quartz grains, BIF fragments or clays are rare. Many of the pelletoids are ooids, with a hematite or goethite core, surrounded by onion-like goethite shells (Fig. 1). 1
Transcript
Student exercise
Interpretation of visible-near to shortwave infrared reflectance drill core data from a channel iron deposit in the Hamersley Province (WA)
Introduction
Channel iron deposits (CIDs) are the second most important source for iron production in WA, after the bedded iron deposits (BIDs). CIDs developed along tertiary palaeochannels, where these crosscut banded iron formations (BIFs) or for several kilometres downstream, above metabasalt and metasedimentary rocks (Morris and Ramanaidou, 2007). CIDs consist of a mixture of Fe-(oxyhydr-)oxide pelletoids and ferruginised wood fragments, cemented in a goethite matrix. Detrital components, including quartz grains, BIF fragments or clays are rare. Many of the pelletoids are ooids, with a hematite or goethite core, surrounded by onion-like goethite shells (Fig. 1).
Fig. 1 Typical CID mixture of Fe-(oxyhydr-)oxide pelletoids and ferruginised wood fragments, cemented in a goethite matrix (From Morris et al., 1993)
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CIDs throughout the Hamersley Range have a similar stratigraphy within the palaeochannel, as shown in Fig. 2 for the palaeochannel hosting the series of Yandi CID.
Fig. 2 Stratigraphy of the Yandi CIDs in the Marillana Creek (from Kneeshaw in Ramanaidou et al., 2003)
The CID can vary internally between different ore types of which some pure endmember forms are depicted in Fig. 3. Vitreous goethite received its name thanks to its typical shininess. It is a massive form of goethite, often (but not always) associated with silica and its reflectance spectrum has a characteristic wide crystal field absorption beyond 900 nm and a steep reflectance slope between 1400 and 1800 nm. Ochreous goethite is a more common form of goethite, being often powdery and having a characteristic strong yellow colour. Ochreous goethite is very often associated with AlOH-type clays, like for example kaolinite. This form of goethite is distinguished in the USGS library as limonite. The ‘original’ CID can have many different appearances, but should always be built up by goethite (and mostly hematite) pelletoids, mixed with ferruginised wood fragments in a goethite matrix.
In the lower CID at Yandi (Fig. 2), ochreous goethite is extensively developed and believed to a product of weathering below the water table. Vitreous goethite at Yandi is more often developed closer to the surface as a hardcap covering the iron ore body (Ramanaidou et al., 2003).
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Fig. 3 Common ore types in CID
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Kaolinite is the most common clay in CIDs. In many of the CIDs both well- and poorly-crystalline kaolinite are developed (Fig. 4). Well-crystalline kaolinite (upper spectrum in Fig. 4) is thought to be the result of in-situ weathering of basement rocks or neoformation of kaolinite below the water table in more acidic conditions. Poorly-crystalline kaolinite (lower spectrum in Fig. 4) can be the result of neoformation in alkaline conditions (above the water table) or transport of kaolinite. Distinctions of well- and poorly-crystalline kaolinite have proven to be powerfull indicators for delineating palaeochannel boundaries (Haest et al., 2012).
References
Haest, M., Cudahy, T., Laukamp, C., and Gregory, S., 2012b, Quantitative mineralogy from visible to shortwave infrared spectroscopic data: (II) 3D mineralogical characterisation of the Rocklea Dome channel iron deposit in Western Australia: Economic Geology, v. 107.
Morris, R. C., and Ramanaidou, E. R., 2007, Genesis of the channel iron deposits (CID) of the Pilbara region, Western Australia: Australian Journal of Earth Sciences, v. 54, p. 733-756.
Morris, R. C., Ramanaidou, E. R., and Horwitz, R. C., 1993, Channel Iron Deposits of the Hamersley Province, AMIRA Project P75G - Restricted Report 399R, CSIRO Exploration and Mining, p. 208.
Ramanaidou, E. R., Morris, R. C., and Horwitz, R. C., 2003, Channel iron deposits of the Hamersley Province, Western Australia: Australian Journal of Earth Sciences, v. 50, p. 669-690.
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Fig. 4 Kaolinite crystallinity, dependent on intensity of absorption at ~2165 nm (intensity ~ crystallinity)
Exercise 1: annotate spectra
- Identify minerals and classify in function of lithology or ore type- Order the spectra in function of depth, starting from 1 at the surface - Indicate position of drill core on the profile in Fig. 2
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Exercise 2
TSG file called RKD_5-7-9 contains hyperspectral diamond drill core data for RKD 5, 7 and 9 at 1 cm spatial resolution. These drill cores are aligned E-W along a profile through a CID. Coordinates, RL and bottom of drill hole depths are provided in Table 1.
Table 1 Drill hole data
- Create in TSG scalars to extract the mineralogical information from the hyperspectral data, using the profile function
- Plot extracted parameters as a function of depth for each drill hole in the scatterplot window (select specific drill hole using the class masking tool)
- Group data for all drill cores manually in a profile (possibly using powerpoint, although any real drawing program will do)
- Delineate the different zones in the channel
An example of what is expected is shown below + describe in detail why you put the boundaries at a certain depth:
TSG file called RC_data contains hyperspectral RC drill core data for 180 holes, sampled at a 1 m resolution. Auxiliary data include easting and northing for all drill cores, allowing to plot any results up as a map in TSG using the scatterplot and full XRF assays to validate the hyperspectral results.
- Extract the mineralogy from TSG, copy-processing the scripts from the RKD_5-7-9 TSG file
- Validate the hyperspectral-based mineralogy results against the provided XRF data, if possible
- Map the CID channel at the surface (scope in scatterplot limited by depth interval 0-1 m), in particular the palaeochannel boundary and if possible the local mineral content. Can you recognise zones of high clay/carbonate/Fe-(oxyhydr-)oxide abundance?
- Experiment with maps at different depths to explore for buried CID ore and see how this ore zone at depth corresponds to your palaeochannel outline mapped at the surface.
