Core logging 101

Core logging is the systematic recording and measuring of as much information as possible/required to determine the lithology (rock types), mineralogy, potential geological history, structure and alteration zones through a tiny piece of cylindrical rock drilled and removed from a potential mineral deposit.

Considered the primary method of determining grade, size and mineability of a potential mineral deposit drill cores are relied on by exploration and mining companies around the world.

This is Not meant to be a step by step guide, more of an overview of the procedures, More technical procedures and informtion will be provided and linked as it is prepared.

The Basics

You need to know where your core fits into the world.

About drill core

Drill core comes in a array of sizes, each with its own purpose as well as pros and cons. The most common drill core used in North America is NQ sized. This provides a nearly 2 inch in diameter core meaning it is big enough to see most structures, provide good sized samples yet thin enough to be economically drilled quickly in multiple hard rock types and to great depths.

BQ core (smaller) tends to still be used for some exploration as the drills are smaller/lighter, it still provides an OK sample size and is relatively cheap to drill. BQ is also dominant in underground drilling due to its smaller drilling apparatus.

HQ and larger cores are great at shallow massive sulphide type deposits where they provide a great sample size.

From Major Drilling: 

Core name Core size (Inches/mm Hole Size (Inches/mm)
1.064 / 27.0 1.890 / 48.0
BQ-BQ-U 1.438 / 36.5 2.360 / 60.0
NQ-NQ-U 1.875 / 47.6 2.980 / 75.7
HQ 2.500 / 63.5 3.782 / 96.0
PQ 3.345 / 85.0 4.827 / 122.6
NQ-3 1.775 / 45.0 2.980 / 75.7
HQ-3 2.406 / 61.1 3.782 / 96.0
PQ-3 3.270 / 83.0 4.827 / 122.6

Know where your hole is going

Knowing where your hole starts is important, but just as important are the direction its going (azimuth), what angle its being drilled at (dip) and its projected length (EOH).

All drill records will have an azimuth, dip and length recorded – these are values needed to actually drill the hole. The azimuth is just a direction to turn the rig. Someone will set-up foresights or backsights (or both) so the driller can line up the drill rig to. They then elevate the drill head so that the drill is angled at the correct “Dip”. This is the angle down from the surface the drill is drilling. The length of hole can change dynamically as the hole is drilled.

Down hole surveys

As any drillhole proceeds, the action of the drill and the properties of the rock will usually divert the drill as it proceeds. No drill core is perfectly straight.

To record variations in dip, azimuth or both, some sort of downhole survey tool is needed. These tools are dropped down the holes at pre-determined intervals and readings of dip/azimuth at that physical location in the hole are recorded. They are then retrieved and drilling continues or is adjusted based on those values.

Record dip, azimuth and downhole surveys/intervals in the header of the log. Also useful to know units and if using true north or magnetic north for azimuth.

Drills will tend to flatten and turn to the right – flatten because the pressure from the rocks and right due to the direction the drills typically rotate as they drill.

Know where your hole is

This first thing to know and record about your core is its location in the world.

Hole Location in the old days was recorded using a local mine grid system which basically used a man made grid actually cut into the minesite (stakes, pins,surveys) and each hole would be referenced from one known point from which they were physically measure out.

Drill hole locations like -700W, 200N would indicate the drillhole location is -700 units W (or 700 units E) and 200 units North from some point usually referenced in a map/report.  The units used will vary and are usually included in the log but not always.

In addition, the cardinal directions (N/S/E/W) may not actually be as mine grids tend to align themselves along known geology or features. So a large regional fault may run through a property, from that the mine grid is parallel.

Real World example from Northern Empire minesite in NW Ontario:

This Map shows a mine grid (light blue) with drill holes as labelled. The direction of the grid lines run approximately 72 degrees which corresponds with local geological features. The log coordinates for hole 86-4 in the original 1984 log are 775E 20S, and hole 86-1a is 700E 21S. The shown locations are exact GPS coordinates (to within 50cm) of the actual hole.

Things to note:

  • The grid overlain is ideal assuming a flat surface, the original grid was physically measured on the ground – this will lead to errors.
  • N/S/E/W are relative
  • Physically measure locations introduce error (topography changes primarily) In this case hole 86-4 is off by approximately 5 meters.

Today with a simple store bought GPS accurate to within two meters and more expensive ones accurate to millimetres, GPS has been the goto for recording hole locations.

When recording a hole location with a GPS, make sure to note the coordinate system (Projection, Datum) as well as error – otherwise great error can be introduced from future work.

Example coordinate notes:
Datum set to WGS84
Projection: Lat/Long
Units: Decimal Degrees

Generically you should have WAAS or DGPS turned on (Increases accuracy), have at least four satellites and note the GPS rendered error. (Vertical and horizontal error usually vary greatly with horizontal much more precise than verticle.

Lets start logging some core

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.

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.

core blocks align in picture

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.

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.

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 microfaulting and micro-structure may be difficult to discern.

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

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.


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 prioritising 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.


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.


Sampling your core and logging the samples

The most important part of your logging would be the areas which do or potentially mineral bearing. Marking off a sample is done in an easily distinguishable marker/crayon and should cover the mineralized envelope as well as contact areas.

A potential sampling area, usually denoted by a mineralogy and alteration change, is made up of three parts.

  1. The Main sample area (Vein, mineralized zone, Alteration)
  2. The Upper Contact (Just before the sample area)
  3. Lower contact (Just after the sample area)

The Main sample area is marked to start at the upper and lower contacts of the Vein/alteration, mineralized envelope. These sample or samples (depending on length of zone) are sampled by predetermined maximum sample lengths and each individual sample should be homogeneous in content (all quartz vein, all wall rock, all one type/density of mineral). You should mark a cut line down the individual sample for the cutters to cut – should run in such a way that both halves of the core are equal.

Continuing the sample out on either end of the mineralized envelop is just a matter of taking pre-determined sized samples on either side of the contacts for a meter or two (usually decided before hand).

Each and every sample should be individually logged as per primary/secondary mineralization, grain sizes of minerals, habit of minerals, percentage of minerals as well as alteration, colours and of course veining if appropriate. This is usually done for primary and secondary dominant mineralization where minerals are usually sulphides (Po, Py, As, Cp) as well as actual precious metals/minerals you my be looking for. Percentages of these minerals should be estimated and recorded.

Parallel Mineralized envelopes

When logging mineralized zones, multiple parallel zones may be present. In this case, with potential mining widths in mind you may want or need to sample potential rock between zones. If you sample a 30m wide zone, then have 5m of barren, then another 30m zone – it would be best to sample right through. If you two 1m zones separated by 30m, probably not important to sample everything in between.

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