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Nov 2002 - Finding the Key

Finding the key – it is often simply in the detail

There are many occasions in which interpretation is required to understand or identify geological formations, structural formations, or even profitability and we have noticed an increasing use of observing, recording or re-interpreting the detail to successfully solve such interpretations or “find the key”.

Our initial observance of underground “detail” was on a visit a number of years ago to Triako’s Mineral Hill mine and their “wallpaper” with each heading mapped in geological detail and posted on the walls of their operations room. Triako apparently still create their wallpaper, but do not digitize or scan in the headings because they advance so quickly, however, those detailed records have been extremely useful in creating their recent 3d-models and sections.

More recently (December 2001) on a visit to Hill 50’s Star mine we saw the same mapping of headings being done. Hill 50 went through a phase of thinking they could bulk-mine Star for an average grade of just over 4g/t, but when they embarked on the plan about December 2000, grades instead dropped to about 3.5g/t. The reason is in the detail. As shown in figure 1, Star’s geology is complex.

Figure 1 Level Plan of Hill 50’s Star Mine

Although Star often has visible gold, its average grade is about 4.5 to 4.7g/t from an average of about 5 different ore zones which are based on a number of different vertical pods of grade in 5 different directions which resulted in the original “star-shaped” open-cut. Star can attain its expected average grade (it was in fact about 4.8g/t in September 2001), but only through detailed face mapping resulting in selective extraction.

Detailed face mapping of development could have made the delineation of ore boundaries easier for the reopening of Independence Gold’s Long mine, or provided the extensions to MIM’s Hilton, which were recently extended by development. Detailed mapping tends to be used more for identifying and tracking high grade shoots as was shown in a presentation on RIO’s Peak gold mine in NSW at a recent Sydney conference in October 2002, where the backs underground are grid sampled between the drives across a level to pin-point the high grade shoots and their direction. Such attention to detail of the shoots aided in the delineation of their new deeper Perseverance high grade orebody pod.

Mapping of high grade shoots is not solely restricted to underground mining. On a visit to Anglogold’s Sunrise open-cut after the Diggers Conference in August 2002, we saw the mine undertaking detailed floor mapping to identify their high grade shoots, and using plastic location pipes within the ore to be blasted so that post the blast, the high grade shoots’ positions could be accurately delineated and mined appropriately.

Detailed mapping also does not have to be restricted to geology. On a recent visit to Aquarius’ Mimosa platinum mine in Zimbabwe in July, we saw the geologists geotech mapping the hangingwall for the positions and orientation of angular blocks to try and identify them and provide appropriate support as required to prevent (if possible) the blocks from falling out of the hangingwall.

On that visit to Mimosa we saw a different use of observing detail. It is well-known that the platinum orebodies of the Merensky Reef in South Africa are easy to identify with chromite bands and pegmatoid orebody, and spotty norites with mottled anorthosite hangingwalls and footwalls. However, the Great Dyke of Zimbabwe is characterized by an MSZ or main sulphide zone in which copper and nickel values increase to a peak (usually coincident with the PGE values) and then drop out. One of the major problems has been the physical identification of the economic ore zone, as illustrated by Zimplats re-working Hartley and finding about 30% of the stopes were off-reef in uneconomic parts of the orebody. Looking at the ore it can be difficult to identify the pin-head sized sulphides, yet alone an increase or decrease in them.

However, it can be done. Mimosa have been mining their orebody for about 6 years’ now and their geologists have a 94% accuracy in identifying the position of the MSZ, and in the remaining 6%, they are about 15cm out. Whether the technique is applicable to the Hartley complex or even Helix’s Munni Munni remains to be seen. However, Mimosa uses three marker horizons to identify the MSZ on preferably freshly broken rock, namely : (1) the presence of quartz ellipsoid shapes with the long axis in line with the dip of the orebody, and which are about 50cm above the MSZ, (2) the pin-head sized sulphides increase to a point (the MSZ) and then literally drop out or do not appear.

Finally (3) the rock has two matrix crystalline pyroxenes, namely white-green needle-shaped augite and coarse brown-sugary bronzite. The augite dominance occurs in the hanging wall and mixes in the middle around the MSZ before becoming bronzite-dominant in the footwall. In the mixing zone, sometimes orb-shaped oikocrysts are seen of up to 1cm diameter augite enclosing bronzite.

In the Great Dyke, the nickel and copper values are important in determining the economic section of the orebody. While it is common to measure open-cut areas or blocks underground according to their profitability (allowing for existing and required development) in gold mines, it was with interest that we noted that MIM had extended the life of its Hilton mine by using the concept of the profitability of ore blocks.

However, it also has to be recognized that a focus on detail is not applicable to all situations. In Selwyn’s presentation at the Sydney conference, they complained of having too much detail available in their move towards a larger reserve/resource lower grade operation, in contrast to its earlier history which was focused (pre-Arimco and Australian Resources) on apparently a profitable selective mining operation concentrating on higher grades and less staff.

The move away from detail in Selwyn’s case, however, appeared to be the exception in the Sydney conference, with the majority in our opinion, focusing on increasing detail to “find the keys” applicable to their orebodies.

Disclosure and Disclaimer : This article has been written by Keith Goode, the Managing Director of Eagle Research Advisory Pty Ltd, who has a Proper Authority with State One Equities, and with his associates, either has or expects to have interests in most of the stocks in this article, This e-mail address is being protected from spambots. You need JavaScript enabled to view it . The opinions expressed in this article should not be taken as investment advice, but are based on observations by the author. The author does not warrant the accuracy or completeness of any information and is not liable for any loss or damage suffered through any reliance on its contents.

  • Written by: Keith Goode
  • Friday, 01 November 2002