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Fosterville Gold Mine, Victoria

Situated approximately 20km north-east of Bendigo in Central Victoria, Fosterville is an established gold mine operated by Perseverance Exploration.  The deposit is hosted in a sequence of interbedded Ordovician sandstones and shales (turbidites) and consists of  a series of approximately north-south striking structurally controlled ore bodies. Extraction of gold from oxide material is currently being carried out using heap leach. Mineralisation continues into the sulphide zone.

An initial PIMA survey was carried out on 50 grab samples from grade control drill spoil in the pit. Assay data from this grade control drilling was compared with the spectral data. Although the assay data was taken from 5 metre composite samples and there was only poor control on the PIMA samples, there appeared to be a relationship between gold and the position, and shape of the AlOH absorption feature between around 2196 and 2218nm.  Further work indicated that illite composition could be correlated with the distribution of mineralisation.

To better understand the spatial relationships between illite compositions and gold, the analysis of over 700 located grade control assay pulps was carried out. The spectra in the Fosterville dataset come from these grade control sample pulps.

 Download Fosterville TSG File
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Download the Fosterville zip file to your PC and then unzip it. There are three files in the zip file, Fosterville.tsg, Fosterville.bip and Fosterville.ini, all three files are required to be in the same directory for you to successfully open the TSG file and view the Fosterville data.

The Fosterville file is a deceptively simple data set because there are only 2 principal mineral groups recognised in the spectral data (illite/muscovite and kaolinite). However, the dataset is particularly challenging because of the apparent lack of significant variation in mineralogy throughout these samples. In fact it is very easy to discard a dataset like this if only traditional analysis techniques such as visual interpretation are used. Unfortunately, this is almost certainly true of many datasets analysed prior to the availability of software like TSG. It is not until these data are analysed as an integrated and spatially related dataset that the significance of the spectral data becomes apparent.

This data set is useful because it demonstrates how many of TSG's analysis functions and display graphics can be used to resolve a wealth of invaluable mineral information directly related to alteration and the distribution of the target mineralisation.

Here are a series of steps that you might like to follow which demonstrate a variety of TSG analysis features using the Fosterville dataset. 

  •     Start up TSG

  •     Open up the Fosterville tsg file. The file has been set up already and should open up with the TSG screens already prepared for the following steps.  The set up information is saved in the .ini file associated with the data set. If you are using a copy which has been changed since its creation you may need to set up some of the displays yourself.

  •     Examine a selection of the spectra in the Spectrum screen. You will notice that the data are fairly monotonous consisting primarily of spectra dominated by illite group minerals with generally weaker proportions of kaolinite. All kaolinite here is weathering related.

  •     Go to the Scatter plot screen, if not already set up you can build each graph as they are described.

  •     The Scatter plot screen should be set up to show 8 plot windows in 4 columns and 2 rows . We will refer to these Plot Windows as PW1 to 8 from top left to bottom right. When using the Scatter plot screen it's best to have TSG maximised on your screen desktop. If your screen resolution is still too low to see the data clearly try working with fewer (eg 2) plot windows and scroll through them using the small up and down arrows next to the Subs onscreen controls.

  •     Remember at any stage you can pop up the floater window to examine the spectrum of any of the data points displayed.

PW1 to 5 are all spatial plots and use X = Eastings, Y = Northings, with the data points coloured by different scalars.

 PW1 - this plot shows the spatial distribution of Au (ppm) across the dataset. X = Eastings, Y = Northings and Aux = Au. The colour of the Au scalar is scaled between 0 and 2 ppm. To change the colour scale click the RH mouse button and select Scale from the pop up menu, which will pop up the Scale dialogue.  This lets you set the scale for the x and y axes as well as the scalar used for the data point colours.

 PW2 - this plot shows the spatial distribution of variations in the wavelength of the main AlOH absorption feature (Aux = Wave_AlOH). This scalar is calculated from a spectral profile, in PW2 it is colour scaled from 2206 to 2210nm. To see how this scalar was calculated select the scalar column and then select Edit/add/delete > modify scalar from the RH mouse pop up menu in the Log screen.

 PW3 - this plot shows the spatial distribution of a feature parameter scalar (feature 2196) which reads the wavelength of absorption features in the vicinity of 2196nm (paragonites).

Calculating values from feature parameters often works well when the target absorption is not the dominant local feature. This is frequently the case for AlOH absorptions associated with paragonitic micas at Fosterville, as the AlOH absorption is dominated by the kaolinite and the 2196nm paragonite feature is usually only evident as an inflection. Simply calculating the wavelength of the dominant AlOH feature (see PW2) tends to give intermediate values influenced by any AlOH minerals present (kaolinite, phengitic and paragonitic illites at Fosterville). Although the feature parameter may not identify the paragonite with a 100% success rate, because of the large number of samples in this dataset and because they are all located we can still get a strong indication of the distribution of these illites with respect to the Au mineralisation in PW3.  Therefore, the spatial distribution of the short wavelength (paragonitic) illites in PW3 clearly shows the spatial relationship between Au mineralisation (PW1) and the distribution of this illite phase.

When working in environments containing a number of minerals possessing coincident or nearly coincident diagnostic absorption features it is important to develop analysis or processing strategies that can be used to separate and identify these minerals. Here at Fosterville we have a number of AlOH phyllosilicates, some alteration related (paragonitic illites), some related to unaltered country rock (phengitic illites), some widely spread (muscovites) and  some weathering related (kaolinite).

PW4 and 5 are both parameters designed to map kaolinite. Its often useful to try out a couple of different processing strategies targeted at a particular mineral and then to compare the results. The similar spatial distribution of data in PW4 and 5 tends to indicate that these parameters are both mapping kaolinite distribution quite effectively. These results can be checked further by using the Floater window to check some of  the sample spectra.

The parameter used in PW4 (feature 2160) is a feature parameter measuring the depth of the secondary kaolinite AlOH absorption near to 2160 nm. The parameter used in PW5 is a parameter that measures the slope of the 2160nm feature, this is less than 1 for kaolinitic samples and greater than 1.02 for those samples with negligible kaolinite. This second plot visually maps the kaolinite distribution very well, and makes a good comparison with the Au plot (PW1), basically the elevated Au zones are where there is negligible kaolinite (shown in dark blue).

PW6,  and 7 are all scatter plots where the X axis is a spectral profile scalar representing the wavelength of the dominant AlOH feature (Wave_AlOH). The Y axis in these plots is a spectral profile scalar representing the depth of the dominant AlOH feature (DepthAlOH). In these samples kaolinite has the effect of  increasing the brightness or albedo of the sample and the relative depth of the AlOH absorption feature, as a result kaolinite rich samples tend to have higher Y values in these plots. Because the wavelength of the kaolinite AlOH absorption does not vary to any great extent samples with higher kaolinite proportions tend to cluster around 2206 to 2208nm (X axis). Conversely the mass of samples spread across the X axis with lower Y values tend to be dominated by illites of varying composition (paragonitic, muscovitic and phengitic).

PW6 uses TSA illite weight to colour the data points, here cooler colours indicate less illite (more kaolinite in these data), as expected the higher illite weights are found towards the base of the plot.

PW7 show a similar trend however this time the auxiliary scalar is the "feature 2160" scalar used in PW4 to map kaolinite. Here the hotter colours indicate stronger kaolinite responses towards the top of the plot.

PW8 is a histogram of AlOH wavelengths for all samples, illustrating broadly the three influences on the AlOH wavelengths on these samples: the large middle peak is dominated by kaolinite, the small peak near 2205nm represents those kaolinitic samples mixed with the paragonitic illite, and the weak peak near 2211nm is associated with the phengitic illites mixed with kaolinite.  These can be checked using the Floater window.

Finally, you may like to look at how the data points in the scatter plots relate spatially to their position in the E-N plots.  To do this click the RH mouse button to get the pop up context menu, select Class browse/edit and then turn on the lasso option. You can then select clusters of data points with the lasso and see where those samples occur in the other plots.

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TSG™ is developed by the CSIRO Earth Science and Resource Engineering (CESRE) Division, Sydney Australia. Copyright 2007 CSIRO.  TSG is marketed and distributed under license by AusSpec International.  Web site design by Sasha Pontual (AusSpec) and Jon Huntington (CSIRO CESRE).  Questions or problems regarding this web site should be directed to enquiries@thespectralgeologist.com .  TSG™, TSA™, HyLogging™, HyLoggerTM and HyChipsTM are trademarks of the CSIRO,  TerraSpecTM and FieldSpecTM are trademarks of Analytical Spectral Devices, PIMATM is a trademark of Integrated Spectronics,  PosamTM is a trademark of JOGMEC,  ENVITM is a trademark of ITT Visual Information Solutions.
Last modified: 28-Jan-2010.