Drilling deep - diamond drilling

Materials World magazine
,
6 Nov 2014

Diamond drilling to obtain core from an orebody is preferred at early to intermediate stages of exploration. Professor Tom Blenkinsop, Cardiff School of Earth and Ocean Sciences, offers an introduction to dealing with the core and examines how new approaches in structural analysis could aid future exploration projects.

Diamond drilling to obtain core from an orebody is preferred at early to intermediate stages of exploration. Professor Tom Blenkinsop, Cardiff School of Earth and Ocean Sciences, offers an introduction to dealing with the core and examines how new approaches in structural analysis could aid future exploration projects. 

Diamond core drilling is used to probe the contents of known ore deposits and potential sites. By withdrawing a small diameter core of rock from the orebody, geologists can analyse the core by chemical assay and conduct petrologic, structural and mineralogical studies of the rock. The process uses mobile drilling rigs with diamond tools to obtain cores, which are typically 5–6cm in diameter. Drill holes can be many hundreds of metres long and they can be drilled from surface or underground. Recovering coherent, continuous core is difficult in rocks that have been deformed – a common characteristic in some types of mineral deposit. Another challenge is the almost universal tendency of drill holes to deviate from their initial orientation, due to the anisotropic (directionally dependent) nature of most rocks. Drill holes need to be planned to take deviation into account, or even to be redirected during drilling. Successful diamond coring therefore requires skill and experience.

Core retrieved from drilling is a direct record of the underlying geology, including any mineralisation. While geophysical and geochemical methods can tell us a lot about rock properties in the drill hole independently of core, the nature of structures – especially discontinuities – can rarely be ascertained remotely. For example, depositional layers (bedding) in a sedimentary rock could be confused with faults or strong tectonic fabrics. Incorrect identification of these features would have adverse effects on prediction of orebody locations.

In addition, folds, which may control the position of ore, can only be analysed in core. In most orebodies that show some control by structures such as faults, shear zones, folds or fabrics, core is essential for understanding how the orebody is affected by the structures (see below). Possibly even more important is that core contains the most significant evidence of how the orebody formed.


Improving the process

There is pressure to minimise the extent of core drilling because of the expense of core retrieval, and current practices for analysing drill core have some problems. As the core is being retrieved, it can be orientated relative to the drill hole. This step gives invaluable information about the orientation of structures (for example, folds and faults) that might affect the orebody. However, systems for measuring structural orientations commonly vary from one deposit to another and from one mining company to the next. Critical measurements of folds, lineations, shear directions and vorticity vectors (see diagram) are often overlooked in much logging. This may be because of lack of familiarity with methods to deal with these features, lack of standardisation, and/or lack of awareness of the relation between these methods and the more common methods for planar features, such as bedding and faults.

The orientation of a structure is generally measured relative to the drill hole. This is converted into true geographical orientation in a further stage of analysis. Additional problems are caused when this step is carried out later than, and independently from, recording of the structural measurements. This delay prevents interactive hypothesis testing and discourages checking of anomalous readings for errors.

Once core has been logged, it is normally cut in half, so that one part of the core can be sent for assay. The remaining core is harder to work on and preserves less information than full core, so it is imperative to extract as much structural information as possible on the first examination. This requires a comprehensive and unified approach.

Such a unified system has recently been developed to deal with these problems. The system, called Cor!, economises the effort of logging core, expands the range of structures that can be routinely measured and delivers final results rapidly.

This system:

  • allows all kinds of structural information to be measured at the same time and in the same manner, for example with a template or a core protractor
  • provides true geographical orientations of structures simultaneously with their measurement relative to the drill hole
  • prevents the need for later retrieval of stored core for further measurements
    introduces a set of common principles and practices that can be widely applied to structural core logging
  • allows half core to be dealt with when necessary

 

The key to the system is an innovation that permits the use of angular measurements relative to the orientation mark on the core for both planar and linear structures. These are called beta angles, and they are combined with alpha angles in current, widely-employed methods for dealing with planar structures, such as faults or bedding planes. These measurements are simple and swift to make, using either core protractors or templates. The unified system extends the use of beta angles to measure lineations, fold hinges, shear directions and vorticity vectors. For some specific features (for example, fold hinges), simple linear measurements with a tape measure or a ruler are also made. The system relies on existing tools, and extends methods that are in use on many mine sites. An additional advantage of this system is that geographical orientations of structures can be derived in real time as measurements are made, via a macro-enabled spreadsheet.

Future significance

New developments in applying coiled tube drilling methods to the minerals industry (as pioneered by the Deep Exploration Technologies CRC in Australia, for example) may reduce the amount of core drilling in exploration programs in future. Paradoxically, this increases the importance of a unified system of analysis, since any core that is retrieved will become more valuable. It is also possible that existing material may need to be re-examined, which can be accomplished readily by this system, since it also works well with half-core.

One of the main challenges of applying structural analysis to mineral deposits is to demonstrate its financial value. This may be partly because structural geology is sometimes regarded as an arcane and specialised subject, even by mineral deposit experts. The challenge is to simplify the complexities of the system and at the same time demonstrate its impact and worth.

Remote methods such as downhole imaging may be devised to collect more structural information in the future. However, these methods could also use some of the principles of unified core analysis. An interesting possibility is that remote methods could be calibrated to analyse for specific structures in a particular deposit using a limited amount of core. As mineral deposits are increasingly explored for in areas of little or no outcrop, these new techniques will become more important and may also be relevant to cores that are extracted from the ocean floor.

Diamond coring is expensive, so alternative, more economical methods need to be considered. Unfortunately, the cheaper alternative of reverse circulation drilling, which can provide some insights into rock type, does not reveal structural details such as those mentioned above, because no core is retrieved. Remote methods of analysis are increasingly used in the hard rock mining industry, but they still cannot get to grips with structural details. A potential problem with any core-based method may be the reliability of the orientation, but this can now be overcome by combining analysis with borehole imaging techniques.

Diamond drill core is still an irreplaceable exploration asset. Given its cost and potentially decreasing availability in future, it is all the more important to make the best use of data from core. A unified system is needed that allows comprehensive, detailed information to be collected quickly and efficiently from new or existing drill core, and realises the full significance of mine and regional scale exploration programmes.