Geological mapping records the rocks, structures and alteration exposed at surface and turns them into a map and a model of what lies beneath. In mineral exploration it comes before geophysics and drilling, because it tells you where to look and why. In Tanzania, where deep weathering and soil cover hide most bedrock, good mapping combines satellite data, field traverses, pits, auger holes and geochemistry into a single interpretation that drilling can then test.
Why Geological Mapping Comes First
Drilling is the most expensive part of early exploration, and a drill hole only tests the ground it passes through. Mapping decides where those holes go. It identifies the rock units, contacts, faults, folds and alteration zones that control mineralisation, and it builds the structural framework that every later dataset is interpreted against. A drilling programme planned without a map is a set of guesses; one planned from a sound structural interpretation is a set of tests.
Mapping Scales and What Each Delivers
| Scale | Stage | Traverse spacing | What it delivers |
|---|---|---|---|
| 1:250,000 – 1:100,000 | Regional desktop and reconnaissance | Satellite and existing maps; selected field checks | Belts, major shear zones and intrusions; licence area selection |
| 1:50,000 – 1:25,000 | Licence-scale mapping | About 1–5 km | Rock units, major structures, alteration trends; target areas for geochemistry |
| 1:10,000 – 1:5,000 | Prospect mapping | About 100–500 m, plus pits and trenches | Mineralised trends, structural geometry; first drill targets |
| 1:1,000 – 1:500 | Detailed and pit mapping | Continuous exposure, trenches, workings | Vein and shear geometry, plunge of ore shoots; drill hole design and resource model input |
Start from what already exists. The Geological Survey of Tanzania publishes Quarter Degree Sheet geological maps at 1:100,000 and holds airborne geophysical and geochemical datasets that save months of regional work.
Remote Sensing and Satellite Geology
Multispectral imagery
Landsat 8/9 and ASTER multispectral imagery are the standard tools for regional mapping in Africa. Band ratios and colour composites highlight clay, iron oxide and other alteration minerals and help separate rock types across large areas at 1:250,000 to 1:50,000 scale. ASTER's shortwave and thermal infrared bands discriminate mineral groups that visible imagery cannot. Both datasets are freely available.
Radar and elevation data
Sentinel-1 radar and SRTM or ALOS elevation models show landforms regardless of cloud cover, which matters in Tanzania's wetter highlands. Shaded relief viewed from several sun angles reveals lineaments (faults, shear zones, dykes and fold hinges) that often control where gold is found. Lineaments are interpretations, not facts, until they are checked on the ground.
Mapping Through Tanzanian Cover
Most of the Lake Victoria and Lupa goldfields have little outcrop. Bedrock is hidden under deep lateritic weathering profiles, transported sands and the black clay soils (mbuga) of low ground. Mapping here relies on more than walking outcrops:
- Ironstone and quartz float mapped carefully can trace banded iron formations and quartz vein trends under shallow cover.
- Artisanal workings are free exposure. Pits and shafts show vein orientation, host rock and weathering depth, and local miners often know which structures carry gold.
- Pitting, trenching and auger drilling reach saprolite, where the original rock texture and structures are still visible even though the rock has weathered to clay.
- Soil and termite mound geochemistry detects gold and pathfinder elements from bedrock below the cover.
- Geophysics, especially magnetics, maps rock units and structures that cannot be seen at surface. See geophysical surveys in East Africa.
Structural Controls on Gold in Tanzania
Most Tanzanian gold is structurally controlled, so structural mapping is where exploration value is created or lost.
- Lake Victoria Goldfields. Archaean greenstone belts of the Tanzania Craton, including Geita, Sukumaland, Musoma-Mara and Kahama-Nzega, host gold in shear zones and in banded iron formations where folding and faulting created traps. Mapping fold hinges and shear intersections is central to finding ore shoots.
- Lupa Goldfield. Around Chunya and Makongolosi, gold sits in quartz veins and shear zones in Palaeoproterozoic rocks of the Ubendian belt, often associated with granitic intrusions. Vein orientation and the structures that cut them control where gold concentrates.
- Ore shoots plunge. In shear-hosted deposits, the richest gold commonly forms shoots that plunge within the structure. Measuring lineations and fold axes lets drill holes intersect the shoot rather than the barren part of the shear.
At prospect scale, the mapper systematically records strike and dip of foliation, veins and contacts, plus lineations and fold plunges, and captures them digitally in QGIS-based tools or similar field apps so the data flows straight into the model.
From Map to Drill Targets and a Resource
- Integrate the data. Mapping, geochemistry, geophysics and existing workings are combined into one interpretation of where mineralisation should be.
- Rank targets. Each target gets a geological reason, not just an anomaly: the right host rock, the right structure, supporting geochemistry.
- Design drilling. Hole orientation follows the mapped structures, so holes cross mineralisation at a useful angle. RC drilling tests broadly and cheaply; diamond core confirms structure and grade. See drilling services in Tanzania.
- Build the 3D model. Surface mapping and drill logs are combined into a 3D geological model with geological modelling software, which is revised as each hole is logged.
- Estimate the resource. A JORC or NI 43-101 resource estimate is only as good as the geological model beneath it. Domains are built from mapped and logged geology, not from grade alone. See mining technical consulting.
Geotechnical Characterisation
Mapping and core logging also feed mine design. Rock Quality Designation (RQD), fracture frequency, joint set orientations and point load strength from core are the inputs to open-pit slope and underground excavation design. In Tanzania's deeply weathered profiles, the depth and shape of the transition from soft saprolite to fresh rock is one of the most important design parameters, and it varies sharply along strike.
Where We Work
Frequently Asked Questions
How long does geological mapping take?
It depends on area, scale and access. A licence-scale reconnaissance programme is typically a few weeks of fieldwork plus interpretation; detailed prospect mapping with pitting and trenching takes longer. Rainy seasons and access in low-lying mbuga areas often set the real schedule.
Can I skip mapping and go straight to drilling?
You can, but it usually costs more. Without a structural interpretation, holes are often drilled parallel to mineralisation or into the wrong structure. Mapping is a small fraction of drilling cost and makes every metre drilled more informative.
Is satellite mapping enough on its own?
No. Satellite and radar data are excellent for regional framework and targeting fieldwork, but lineaments and alteration signals must be checked on the ground before they are trusted, especially under Tanzanian soil and laterite cover.
What does a small-scale miner need from geological mapping?
Mainly an understanding of the structure being worked: the orientation of the vein or shear, where it is richest, and where it continues. That guides where to sink the next shaft and avoids following barren ground. A PML holder rarely needs regional mapping, but a detailed map of the workings and nearby structure is often very good value.