Core logging 101

Step one, mark some meterage on core

Start with the first box and measure one meter intervals starting from the tags. Mark with permanent marker and avoid overwriting something good. Each section between tags should be 3 meters. Moving broken pieces of core together as you are measuring will give you an accurate count. If the numbers don’t add up – you need to decide if significant or insignificant – either way it should be recorded.

A tag off by 10cm is not overly bothersome, off by half a meter or more and it is.  Usually discrepancies are either driller error or core loss. If recording Rock Quality Designation (RQD) – this will have already been done for you by a technician. 

Core boxes for NQ core are built to hole 15 feet – the typical length of a core barrel. We like to measure things in meters so 15 feet of core = 4.57 meters. So each core box holds approximately 4.5 meters of core. Now when the drillers tag the meterage, they are doing so by sight – mistakes do happen. If tags don’t align as they should – there is likely a problem.

Marking on the core itself is done with markers or lumber crayons. Each colour will identify what it is in reference to. One colour for lithology, one for structure, one fore sampling and maybe one for geotechnical – be consistent and if taking pictures – note what the colours mean.

Logging the lithology

Before you start logging core you should have some idea of the basic rock units in the area. Unless you are in completely unexplored area there should be some report, field work or map which gives you an idea of what rock types you will encounter. Start with generics and try to get specific especially around units of interest. Make note of what to expect before starting to log.

Put as many core boxes as you can on whatever logging tables you have. The top left corner will be the top-most part of the core in the hole, the bottom right will be the bottom most point. This is true of a single box or 1000 boxes if setup properly. A quick visual check will tell you if they are setup right as the drillers tags will form nice diagonal lines as shown below.

The Geology of the core consists of differentiates different colours, textures, grain sizes as well as increase/decrease mineralogy in the basic core.  Whetting the core will make differences in colour and texture stand out much more than dry core. 

Systematically divide the core out into its basic component units. Marking the beginning/end of any colour, textural, alteration and lithology. These contacts will be sharp or gradational, choose symbols to denote gradational or sharp contacts. If there is a sharp contact, mark the angle of the contact and note and alteration. 

You may need multiple passes to divide and mark-up the core lithology properly. Some changes are more significant than others and as you become more accustomed to a particular locations core, you will start grouping insignificant units. Pay particular attention to areas around known ore bodies as these numbers will be used (sometimes) for resource calculations. 

What you are logging is any time there is a change – mark it. It may be the same rock type but its harder, or a slightly different colour. Make note with crayons on the core.

Groupers vs separators

Groupers are core loggers who will tend to group similar units more readily, especially in known ore zone areas. If you know where an ore zone is the stuff outside becomes of less important. The advantages to this type of logging is speed, one can log a lot of core by only prioritizing known good zones and grouping known un-mineralized zones.  

The grouper still notes and samples everything mineralized outside the known zones but the bulk of the logging time is in and around known ore zones.

Separators pull out every change in core lithology, mineralogy and structure. Their logs are highly detailed and this type of logging is a must for new logging areas without well defined ore zones. It also allows for potential new ore zones to be identified more readily in the future. This method however is very time consuming. A grouper style logger will log two to five times the amount of core in the same time frame as the separator.

 

A happy medium is between both, typically tho when starting a new logging area, one will start a separator then as they become comfortable move into the grouper. Extremes on both ends are non-desirable as someone who separates to much will quickly become overwhelmed with backlog. Grouping to much may result in missing some key lithologies that may become important in the future.

Log some alteration

In most cases in hard rock core logging alteration is a large part of the mineralogy. It tells you where the rock has changed – like from fluid movement or contact with a heat source. This is especially true in vein style gold deposits as we have here in NW Ontario. The alteration of the right type usually will denote mineralization and ore zones.  Silicification and sericitic alteration tend to be the go to alteration in Beardmore/Geraldton Gold, with a specifically altered iron formation being the go to ore body at Musselwhite mine.

Alteration is most easily noted by colour or hardness changes.

Sericitic alteration – makes the core paler in most cases. Silicification makes the core harder in most cases (More quartz). Talc alteration makes the core softer, chloritic/fuschite alteration makes things green.

Sometimes the texture changes with the mineral assemblage, at Musselwhite the iron formation alters to biotite-chlorite signifying a potential ore body.

Note changes in colour, hardness and even texture for alteration, as well as any changes in mineralization accompanying it.

Structure, structure and more structure

Logging structure is probably the harder thing to log – especially if you have small scale features.

Large structure components like Fault Gouges are easier, lithological contacts are easy, however small micro-faulting and micro-structure may be difficult to discern.

The Geology of the core consists of differentiates different colours, textures, grain sizes as well as increase/decrease mineralogy in the basic core.  Whetting the core will make differences in colour and texture stand out much more than dry core.

Systematically divide the core out into its basic component units. Marking the beginning/end of any colour, textural, alteration and lithology. These contacts will be sharp or gradational, choose symbols to denote gradational or sharp contacts. If there is a sharp contact, mark the angle of the contact and note and alteration.

You may need multiple passes to divide and mark-up the core lithology properly. Some changes are more significant than others and as you become more accustomed to a particular locations core, you will start grouping insignificant units. Pay particular attention to areas around known ore bodies as these numbers will be used (sometimes) for resource calculations.

What you are logging is any time there is a change – mark it. It may be the same rock type but its harder, or a slightly different colour. Make note with crayons on the core.