10 Things to Remember when Collecting Structural Data from Drill Core

In the mineral exploration industry, diamond core drilling provides the opportunity to collect structural data relating to a target or deposit. This improves knowledge and understanding of geological and mineralisation controls of a target, with an outlook to creating inclusive informative models from combined surface mapping, geophysics and down-hole lithology and geochemical assay data.

Below we provide ten key points to remember when collecting structural data from drill core:

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To minimise errors during the core acquisition and preparation process, the project geologist should research and select the most appropriate down hole survey and orientation tool for a particular drill programme.

Consider the magnetic strength of the rocks being drilled when deciding on a down hole survey tool. If the rocks being drilled have low magnetism then tools that use magnetometers such as the REFLEX EZ-Shot®, and Camteq Proshot are appropriate. If the rocks are magnetic then a tool that uses accelerometers or a non-magnetic gyro such as the REFLEX Gyro® and DeviflexTM would be appropriate.

Methods available for orienting core fall broadly into two categories 1) mechanical methods (including the spear, Ezy-MarkTM and Ballmark®) 2) digital methods (including the REFLEX ACT and the Coretell ORIshot). Some are more appropriate for particular conditions. Take the following into consideration:


  •       The condition and competency of the rock expected down hole.

  •       The accuracy and reliability of the data recovered, and the potential for a check back by the rig geologist.

  •       Ease of use, which can also affect the accuracy of the data.

  •       The cost of the method. Cheaper methods are not cost effective if they result in greater loss of data.

  •       The time cost associated with drilling down-time.

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Drill core preparation is required prior to the collection of structural data from drill core. Here are some guidelines:

  •       Never change convention of marking the orientation line on either top or bottom of core.

  •      Use a channel greater than 7m in length, preferably much longer (~30m) when slotting together the core, this allows at least 3 orientation marks to be ‘lined-up’.

  •       Reference both forwards and backwards between orientation marks.

  •     A dashed line should be used where there are poor joins between pieces of core or where orientation marks do not match.

  •       Meter marks should be marked on after orientation of the core

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Each site has its own conventions when it comes to the structural logging process. Some conventions to know from the offset include:

  •       Whether the orientation line is on the top or bottom of the core.      

  •      Conventions for the recording strike and dip measurements for example dip/dip direction, strike/dip using the right hand rule, strike/dip using the left hand rule, strike/dip with quadrant.

  •       Whether the β reading is taken from the top or bottom of the structure ellipse.

  •       Knowing the logging codes for your project, most companies use different codes and it is important to use those created for specific projects.

  
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Structural features can be measured and recorded by using one of two methods: 

  • Positioning the core in its true orientation, measuring the strike and dip of a planar feature, or azimuth and plunge for a lineation, e.g. using Coremap™ or rocket launcher.

  • Taking measurements of core angles, measuring the alpha-beta (α-β) of a planar feature, or gamma-delta (γ-δ) for a lineation e.g. using core protractors or Kenometer. 



There are of course advantages and disadvantages to using each instrument, details of which can be found in an earlier SJS blog.

When deciding which method to use, consider the following:

  • How easy it is to visualise and interpret data during collection the measurements, α-β data need to be converted to meaningful strike and dip readings to understand geologically.

  • Accumulative potential error in setting up the device, placing core in the device and taking readings.

  • Cost.

  • Time to take a reading.

  • Whether readings can be taken from half core.

  • Whether rocks are magnetic.

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It is absolutely vital to get a handle on structural terminology and codes. Common issues with codes include:

  • Use of collective nouns e.g. foliation and shear. Foliation can describe a number of layered features including cleavage, schistosity and gneissic banding. Shearing can describe a host of deformation intensities, from mylonite to fibres on a small fault.

  • Too many codes to describe similar features e.g. moulin axis and mineral elongation.

  • Too few deposit-relevant codes such that different features (some of which would be mineralisation significant) are logged as the same feature, e.g. bedding and sulphide banding.


To combat these problems, a deposit or site-specific code sheet that is practical and intuitive should be designed by site geologists who are familiar with the geology of the deposit.

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First and foremost identify and measure relevant features.

Do not follow a rigid process such as “collect one structural reading per metre” – taking readings for the sake of having data must be avoided. Collecting a large volume of data and understanding the structure of an area do not usually go hand in hand.

Often a large volume of data can obscure important features.

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Lineations are often undervalued and rarely measured relative to planar features. Some reasons for this include:

  • Lineations are not immediately obvious when studying core, they occur on the stubs of pieces of core.

  • On-site geologists are unaware of lineations.

  • γ-δ measuring tools are not present within the core yard, or on-site geologists are unsure of how to use them.


Lineations often seen in core include:

  • Mineral lineations - commonly plunge sub-parallel to ore shoots, therefore can be essential for the prediction of ore body shape and orientation.

  • Fold axis - taken from small-scale folds in core can be used to predict the plunge of large-scale folds.

  • Bedding-cleavage intersection lineation - can tell you about the geometry of folding.

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It is common within the mining and exploration industry for a company to collect a huge amount of α-β data only to file them away in the geological database where they are seldom, if ever used.

The first step in ‘making the data useful’ is converting α-β to strike and dip/ dip and dip direction. There are several ways of doing this, but all methods require the following knowledge of how the data were collected:

  • Position of orientation line on core (top-of-hole or bottom-of-hole).

  • The orientation of the drill hole at the depth the measurement was taken.

  • How the measurement was taken (i.e. taking the β reading from the top or bottom of the structure ellipse).


Common software that can convert α-β data include Micromine® and GeoCalculator©. The conversion can also be done via manual plotting on a stereonet (see upcoming blog posts).



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Make the structural data you are collecting useful, to yourself, your fellow geologists and to management, to add value to the project you are working on.

There are some things that can be made part of the structural logging process:

  • Create interpretive illustrations of structural relationships throughout the logging process; this could include having a pictorial column in the log sheet to draw important features, compiling summary diagrams at the end of each diamond hole accompanying a memo and schematic cross-sections of each drill line.

  • Create stereonets: they can be used for analysis to detect trends and patterns of structural measurements, also combining them with diagrams allow an interpretive picture of the relationship between structures and mineralisation.

  • Use 3D packages to display and interpret structural data, including using it to aid the interpretation of lithological and mineralogical boundaries. 


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There is no reason why the same geologist can’t take control of the entire structural workflow, from collecting measurements in the core yard to plotting and interpreting data in 3D. A number of errors can easily be introduced if different geologists are involved at different stages of the workflow, some of which have been highlighted in this blog. This includes not fully understanding and utilising structural features linked to mineralisation in the modelling process. In other words, vital knowledge and information can simply get lost if the workflow is disjointed.









SJS Resource Management 10-Jun-2014
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