What is drill core?
Drill core is a solid cylinder of rock cut from deep underground. It is the most direct look a geologist gets at a deposit before anyone digs, and most resource estimates rest on it.
How core is cut
A diamond drill turns a hollow bit set with industrial diamonds. The bit grinds a ring of rock away and leaves a column in the middle, which slides up into an inner tube. With wireline drilling the inner tube is winched up inside the rods every few metres without pulling the whole string out. The driller marks each run with a block showing the depth, and the core goes into trays in order.
Core sizes
| Size | Core diameter | Hole diameter | Typical use |
|---|---|---|---|
| BQ | 36.5 mm | 60 mm | Deep holes, where a thinner string reaches further |
| NQ | 47.6 mm | 75.7 mm | The most common exploration size |
| HQ | 63.5 mm | 96 mm | Broken ground, near-surface holes, more sample |
| PQ | 85 mm | 122.6 mm | Metallurgical test work, geotechnical holes |
Standard wireline sizes; a hole often starts in a larger size and steps down as it goes deeper.
Logging, splitting and assaying
- Recovery and RQD: the length of core recovered against the length drilled, and how broken it is.
- Geological log: rock type, alteration, veins, the minerals that carry the metal, and the depth of each change.
- Structure: on oriented core, the angles of veins, faults and contacts, which tell geologists how the zone is tilted.
- Photographs of every tray, wet and dry.
- Sampling: the core is sawn in half along its length (sometimes quartered), and one half goes to the laboratory in intervals of about 1 to 2 m. The other half stays in the tray as a record.
- Assay: the laboratory crushes and pulverises the sample and measures the metal, for example by fire assay for gold.
Rock quality designation (RQD) is the total length of core pieces 10 cm or longer, as a share of the run. In a 3 m run where the pieces of 10 cm or more add up to 2.25 m, RQD = 2.25 ÷ 3 = 75%. Geotechnical engineers use it to judge how strong the ground is; broken, low-RQD zones are often faults.
The virtual core shed lays out real trays from South Australian holes, photographed at the state core library, with the laboratory assays and the HyLogger minerals lit along each row. Pull a tray and log it yourself.
The assays become the intervals you read in a release (see how to read drill results). Many geological surveys keep core libraries where old core can be inspected, such as South Australia's Drill Core Reference Library.
Core or chips?
Reverse circulation (RC) drilling returns chips, not core. It is quicker and costs less per metre, so companies often use RC to find and outline a zone, then diamond holes to confirm the geology, measure structures and collect metallurgical samples. Releases should say which method each hole used.
Questions
What is a core sample?
A cylinder of rock recovered by a hollow drill bit set with industrial diamonds. In mining it is logged by geologists, photographed, usually sawn in half, and one half is sent to a laboratory for assay while the other is kept.
What is the difference between diamond drilling and RC drilling?
Diamond drilling cuts a solid core that keeps the rock's texture, structure and contacts. Reverse circulation (RC) drilling smashes the rock into chips that are blown up the inside of the rods. RC is faster and cheaper; core gives more information and is usually needed for resource estimates and metallurgy.
What are NQ and HQ core?
Standard wireline core sizes. NQ core is about 47.6 mm across and HQ about 63.5 mm. Larger PQ core (about 85 mm) is used where more sample is needed, smaller BQ (about 36.5 mm) for deep holes.
What is core recovery?
The length of core recovered as a share of the length drilled. Broken or soft ground can be lost, and low recovery makes the assays less reliable. The JORC Code's Table 1 asks companies to report it.
Read next
How to read drill resultsResources and reserves
Sources
- JORC Code, 2012 Edition, Table 1: sampling techniques and drilling (Joint Ore Reserves Committee)
- South Australia Drill Core Reference Library (Department for Energy and Mining)
An explainer of how mining companies report their work, written from the public codes and rules linked above. It describes the documents and does not rate any company, project or result. Not investment advice.
Keep exploring
See it on real projects
All mining storiesThe world runs on rockAll mineralsMining by country
Pictures: Timna in de Negevwoestijn; rotspartij "de zuilen van Salomo", Poll, Willem van de / Europeana (Public Domain Mark 1.0); Chiquicamata Mine, Chile, NASA/METI/AIST/Japan Space Systems, and U.S./Japan ASTER Science Team (Public domain (NASA media, no copyright)).