Lithology logging is the systematic identification and recording of rock types encountered in drill core, interval by interval, from collar to end-of-hole. For each interval you record the rock name, grain size or crystal size, texture, colour (on both fresh and weathered surfaces), primary mineralogy, fabric, and the nature of the contact with the next unit. It is the first pass on the core — the foundation every other log (alteration, mineralisation, structure) is built on — and the single most important dataset in a geological database.
This guide covers how to do it: what each field means, how to make consistent identifications at the logging table with minimal equipment, how to define interval boundaries, and the mistakes that corrupt lithology data downstream.
If you haven’t yet read How to Log Drill Core, Step by Step, start there — it places lithology logging in the full sequence (it’s Step 6, after recovery, RQD, and photography are done).
Why lithology is the foundation pass — and why it must come first
Every downstream product of a drill program hangs off lithology: the geological interpretation, the resource model, the stratigraphic cross-section, the assay context, and the alteration and mineralisation logs. Assay intervals only make geological sense if you know what rock they cut. Alteration descriptions only make sense relative to a host rock. Contacts in the geological model only hold if they were picked correctly in the log.
That’s why the lithology pass comes before alteration, mineralisation, and structure. It defines the intervals that the other passes describe. And it’s why consistent, complete lithology logging is worth the time — thin or inconsistent lithology data propagates errors into every model built from it.
The CIM Mineral Exploration Best Practice Guidelines are explicit on this point: all geological information collected in drill hole logs should use a standard geological legend that is consistent across the property, and logging software should capture information in a consistent, functional, and secure way suitable for use in subsequent phases of evaluation. Inconsistent lithology is not a minor issue — it means different geologists have effectively mapped different geology, and the database reflects that confusion.
What you record for each lithology interval
A complete lithology log entry covers six things for every interval. None are optional.
Rock name and rock type
The rock name is the primary field. It should be a specific geological term drawn from the agreed code dictionary — not a free-text description. Granodiorite, not “granite-ish.” Andesitic tuff, not “volcanic-looking stuff.” Every name in the log should map to a code and a definition in the project legend, agreed and locked before drilling began.
The three rock families and how they commonly appear in exploration drilling:
| Family | How it forms | Typical examples in drill core |
|---|---|---|
| Igneous | Cooling of magma (intrusive = slow, extrusive = fast) | Granite, granodiorite, diorite, gabbro, basalt, andesite, rhyolite, tuff, porphyry, volcanic breccia |
| Sedimentary | Deposition and lithification of particles or chemical precipitates | Sandstone, siltstone, mudstone, shale, conglomerate, limestone, dolomite, chert |
| Metamorphic | Recrystallisation under heat and/or pressure | Schist, gneiss, quartzite, hornfels, marble, phyllite, slate |
Within igneous rocks, the presence and pattern of phenocrysts (large crystals in a finer matrix) is often the most important observation — porphyritic texture is a key ore-deposit vector in many systems. Record the phenocryst mineralogy and size as well as the matrix groundmass.
Grain size and crystal size
Grain size is reported differently depending on the rock family.
Clastic sedimentary rocks use the Wentworth scale (or a simplified version):
| Term | Approximate size |
|---|---|
| Clay / Mud | < 0.004 mm |
| Silt | 0.004–0.063 mm |
| Fine sand | 0.063–0.25 mm |
| Medium sand | 0.25–0.5 mm |
| Coarse sand | 0.5–2 mm |
| Granule / Pebble | 2–64 mm |
| Cobble / Boulder | > 64 mm |
Igneous and metamorphic rocks use crystal size:
| Term | Crystal size |
|---|---|
| Fine-grained (aphanitic) | < 1 mm (crystals invisible to the naked eye) |
| Medium-grained | 1–5 mm |
| Coarse-grained (phaneritic) | 5–30 mm |
| Pegmatitic | > 30 mm (cm-scale crystals) |
In igneous rocks, grain size tells you the rate of cooling — fine-grained rocks cooled quickly (extrusive or shallow intrusive), coarse-grained rocks cooled slowly (deep plutonic). That’s directly useful for targeting the edges of a system or tracing depth relationships in a drill hole.
A 10× hand lens is the standard tool. Carry it at the logging table.
Texture and fabric
Texture describes the arrangement of grains or crystals and their relationship to each other. Key descriptors:
- Igneous: equigranular (all crystals similar size), porphyritic (large phenocrysts in a finer matrix), vesicular (gas bubbles), amygdaloidal (vesicles filled with secondary minerals), glassy/vitric, pyroclastic/fragmental
- Sedimentary: massive (no internal structure), laminated (< 1 cm beds), thin-bedded (1–10 cm), graded (coarse-to-fine within a bed), cross-bedded, bioturbated
- Metamorphic: foliated (planar fabric), lineated, gneissic banding, granoblastic (equant interlocking crystals with no preferred orientation)
Fabric — any preferred orientation of crystals, grains, or clasts — is worth recording even in the lithology pass, because it affects how you measure structures later.
Colour (fresh and weathered)
Record colour on both a fresh broken surface and the weathered core surface. They are often very different, and consistency matters more than precision.
On a fresh surface, you see the primary colour of the rock without oxidation masking it — useful for correlating between holes. On a weathered surface (the as-drilled exterior of the core), you see how the rock presents when oxidised, which is relevant to alteration and weathering interpretation.
Use descriptive terms from a consistent colour list: light grey, dark grey, pink, off-white, pale green, buff, brown, cream. Some programs use the Munsell Rock-Color Chart (115 colour chips with alphanumeric codes such as N7 for light grey) for full standardisation; others rely on a simplified project colour list. Either approach works — the critical thing is that everyone on the project uses the same system.
Do not leave colour as “grey” when you can be more specific. “Light grey, fine-grained granite” is exportable data. “Grey granite” forces everyone downstream to assume.
Primary mineralogy
Estimate the modal percentage of each primary mineral visible in the rock — that is, the proportion by volume, estimated visually. Standard notation uses percentages: Qtz 35%, Kfsp 30%, Pl 25%, Bt 8%, Hbl 2% for a typical granodiorite.
You don’t need to be precise to the per cent — broad estimates (< 5%, 5–15%, 15–30%, 30–50%, > 50%) are usually sufficient for geological interpretation. What matters is consistency: use the same mineral abbreviations throughout the project (project code dictionary) and err toward listing more minerals rather than fewer.
Tools that help at the table:
- 10× hand lens: reveals crystal size, cleavage, and mineral habit
- Dilute HCl (10%): fizzes on carbonates (limestone, dolomite, carbonate-altered rocks); a weak or slow fizz indicates dolomite vs a strong, immediate reaction for calcite
- Streak plate: reveals mineral colour independent of surface alteration (pyrite = greenish-black; hematite = red-brown; magnetite = black)
- Magnet: identifies magnetite and other magnetic minerals instantly — important in iron oxide copper-gold (IOCG) and other systems where magnetic susceptibility is a vector
Grain angularity and sorting (sedimentary rocks)
For clastic sedimentary rocks, add two more observations: grain angularity (angular, sub-angular, sub-rounded, rounded, well-rounded) and sorting (well-sorted, moderately sorted, poorly sorted). These constrain the depositional environment and help correlate units between holes.
Contact types — where one interval ends and the next begins
A lithology interval closes when the geology changes. The contact between two units is as much a data point as the rock description itself — record it carefully.
| Contact type | What it looks like | Why it matters |
|---|---|---|
| Sharp | A clear, traceable line between two units; less than 1 cm transitional zone | Depositional break; may indicate unconformity or erosion |
| Gradational | The rock transitions over > 1 cm; no single contact line | Continuous environmental change; log the transition zone as its own interval |
| Intrusive / Chilled margin | One rock clearly cuts the other; the intrusion may show a finer-grained margin (chilled) | Places the intrusion after the host; chilled margins can be assay targets |
| Faulted / Sheared | Gouge, breccia, slickensides, or a zone of disruption at the contact | Structural control on mineralisation; the contact itself is logged as a structural feature |
| Unconformable / Erosional | Evidence of erosion or non-deposition at the contact (truncated beds, basal lag) | Sequence boundary; may control ore-trapping |
Always pick contacts where they actually occur in the core — not at the nearest round depth. If the contact is at 127.3 m, log it at 127.3 m, not 127.0 m. A contact forced to a round number compounds as an error when the geological model is built.
How do you define interval boundaries in practice?
Work top-to-bottom through the core tray. When you see a change in rock type, grain size, texture, or colour that represents a real geological boundary, that’s where your current interval ends and the new one begins.
What counts as a new interval?
- A change in rock name (granodiorite to basalt intrusion)
- A significant change in grain size (coarse-grained to fine-grained granite)
- A change in texture that represents a new unit (equigranular to porphyritic)
- A change in colour that reflects a change in primary lithology — not alteration (alteration changes get their own log)
- Any obvious structural contact (fault, shear zone)
What does NOT require a new interval?
- Normal gradational variation within a single unit
- An alteration halo that doesn’t change the rock name (log it in the alteration pass)
- A vein (log it in the structure/vein pass)
- A short patch of core loss or drilling disturbance mid-interval
Minimum interval length is project-specific but is typically 0.1–0.5 m. Very short intervals (say, a 5 cm intrusive dyke cutting a host) are worth capturing — they often mark important events in the deposit history — but record them accurately and note in the comments if the interval is too short to sample.
Building and using a code dictionary
The single most important lithology decision is made before the first hole is drilled: agree the rock-name codes with the whole geology team, lock them, and enforce them throughout the project.
A code dictionary maps every code to a rock name, a short definition, and typically an example photograph:
| Code | Rock name | Definition |
|---|---|---|
| GDI | Granodiorite | Medium-to-coarse granular intrusive; Qtz > 20%, plagioclase dominant over K-feldspar |
| GRP | Granite porphyry | Porphyritic texture; K-feldspar or quartz phenocrysts > 5 mm in an aphanitic to fine matrix |
| AND | Andesite | Fine-grained volcanic, intermediate composition; plagioclase ± hornblende ± pyroxene |
| SST | Sandstone | Clastic; medium to coarse; quartz-dominant; visual porosity present |
| SCH | Schist | Strongly foliated metamorphic; biotite ± muscovite ± garnet fabric visible |
The dictionary prevents the single most common and damaging lithology error: the same unit being logged as GDI, granodiorite, Granodiorite, and gran dio by four different geologists. Those variants don’t merge in a database — they become four rock types where there is one.
When a rock type appears that isn’t in the dictionary, add it to the dictionary immediately, with a definition, and notify the team. Never invent a one-off code and use it only in one hole.
Common lithology logging mistakes
Using free text instead of codes. Descriptions in free-text fields are almost impossible to query or cross-section. Codes are searchable; prose is not.
Assigning contacts at round depths. A 0.3 m systematic shift in every contact means a systematic error in every geological model built from the log. Pick contacts where you see them.
Describing colour inconsistently. “Grey” in one hole, “dark grey” in another, and “light grey” in a third — for the same unit — creates apparent lithological variation that doesn’t exist. Agree colour terms across the team and stick to them.
Logging alteration as lithology. Pervasive silicification, argillic alteration, or potassic alteration can change a rock’s appearance dramatically. Log the original (or inferred) protolith in the lithology field, and log the overprinting alteration in the alteration pass. Confusing the two hides vectors to mineralisation.
Not logging texture in porphyritic units. The phenocryst type, size, and percentage in a porphyry system are often the most important targeting information in the hole. “Qtz-Kfsp porphyry, 20% Qtz phenos 5–8 mm, 15% Kfsp phenos 3–5 mm” is a datum. “Porphyry” is not.
Skipping the weathered/fresh colour distinction. Colour on a weathered (oxidised) surface can look completely different to colour on a fresh break, especially in oxide zones. Record both every time.
FAQ
What is lithology logging? Lithology logging is the systematic identification and recording of rock types in drill core, interval by interval, capturing the rock name, grain size, texture, colour, mineralogy, and contact type for each unit intersected. It is typically the first geological pass made on core — before alteration, mineralisation, or structural logging — because it defines the intervals that every other dataset hangs off.
What’s the difference between grain size and crystal size? Grain size applies to fragmental (clastic) sedimentary rocks and describes the diameter of individual grains or clasts — classified on the Wentworth scale from clay through to boulder. Crystal size applies to igneous and metamorphic rocks and describes the size of mineral crystals formed by cooling or recrystallisation. Both are measured visually using a hand lens, comparing against a grain-size chart.
How do you pick lithology contacts in core? A contact is placed at the depth where the geology changes — where a different rock type, a significant change in grain size, or a clear fabric boundary can be seen in the core. Contacts should be picked at their actual position to the nearest 0.1 m, not rounded to the nearest whole or half metre. The nature of the contact (sharp, gradational, intrusive, faulted) is recorded alongside the depth.
Why does the code dictionary matter so much? Because geological databases query on codes, not intent. If the same rock unit appears under three different codes or spellings, it will be treated as three separate rock types in any cross-section, strip log, or resource model. A locked code dictionary, agreed before drilling begins, ensures every geologist on the program logs the same unit the same way — which is the precondition for any inter-hole correlation.
What tools do you need to identify rock types at the core table? The essentials are: a 10× hand lens (for crystal/grain size, mineralogy, and texture), dilute hydrochloric acid (10% HCl, to identify carbonates), a streak plate (for mineral colour), and a magnet (for magnetite and other magnetic minerals). A grain-size comparator card and a colour reference list or Munsell Rock-Color Chart round out the kit. A UV lamp can also be useful where fluorescent minerals (scheelite, some carbonates) are expected.
How Blue Butterfly handles lithology logging
Lithology logging is only as good as the system that enforces it. In Blue Butterfly, every lithology field is backed by a locked code dictionary: rock names, grain sizes, texture terms, contact types, and colour codes are all dropdown selections drawn from the agreed project legend. There’s no free-text rock-name field to diverge from — the same code means the same thing across every geologist, every hole, and every campaign.
Contacts are entered at the depth they occur (to 0.1 m precision), intervals auto-close without gaps or overlaps, and the validated lithology data renders directly into a depth-scaled strip log and exports to modelling software without an intermediate spreadsheet step. When a new rock type needs to be added to the dictionary, an administrator updates it project-wide — not hole by hole.
Lithology logging shouldn’t be the place where good geological judgement disappears into a messy database. Blue Butterfly is built so the log you write in the core shed is the model-ready dataset on the other end.
Sources
- Geology Hub — Lithologging – Borehole Cores and Sampling Techniques (overview of lithology logging; field parameters; contact types; importance in mineral exploration): https://geologyhub.com/lithologging-borehole-cores-sampling-techniques/
- CIM — Mineral Exploration Best Practice Guidelines (2018) (standard geological legend consistent across the property; logging software for consistent capture; recovery noted on logs; sampled intervals respecting geological contacts): https://mrmr.cim.org/media/1080/cim-mineral-exploration-best-practice-guidelines-november-23-2018.pdf
- RockMass — A Guide to Core Logging for Rock Engineering (grain size classification; crystal size; colour recording; texture description at the logging table): https://rockmass.net/files/core_logging_guide.pdf
- USGS — Standardized Method for Logging Drill Core at the Idaho National Laboratory, Idaho (standardised colour, texture, composition, and alteration description; Munsell colour chart use; fresh vs weathered surface distinction): https://pubs.usgs.gov/publication/tm5E1/full
- USBR — Engineering Geology Field Manual, Chapter 4: Classification of Rocks and Description of Physical Properties (rock classification; grain and crystal size; texture; hardness; weathering descriptors; standard logging components): https://www.usbr.gov/tsc/techreferences/mands/geologyfieldmanual-vol1/chap04.pdf
- AAPG Wiki — Core Description (contact types: gradational, interbedded, sharp; unconformities; intrusive contacts; facies description): https://wiki.aapg.org/Core_description
- RockMass / Mineral Size Classification — Grain Size Sedimentary/Metamorphic/Igneous Rocks (size ranges; Wentworth scale for sedimentary; crystal size terminology for igneous and metamorphic): http://rockmass.net/files/mineral_size_classification.pdf
- Geology.com — The Acid Test for Carbonate Minerals and Carbonate Rocks (HCl reaction; calcite vs dolomite response): https://geology.com/minerals/acid-test.shtml
- Munsell — Geological Rock-Color Chart (115 colour chips; alphanumeric code system; use on wet or dry samples): https://www.forestry-suppliers.com/p/77333/30811/munsell-geological-rock-color-chart
- ScienceDirect Topics — Core Logging (from/to depths; colour; grain size; texture; structures; mineral composition; alteration; rock name as primary logged fields): https://www.sciencedirect.com/topics/earth-and-planetary-sciences/core-logging