You hold ground on a rift margin. There are hot springs, steep fault scarps and patches of bleached rock, and you need to decide whether this is a base metal system or just hot water. Active rift geothermal fields and ore deposits share ingredients, but most geothermal activity does not make ore. After reading this you should be able to do three things. You can judge whether a fluid is chemically capable of carrying metals. You can pick out the fault segments where fluid is most likely to have been focused. And you can build a simple, checkable ranking from free data.

The principle: hot water, permeable faults, and a reason to drop metal

In plain terms, water sinks into the crust, is heated by magma or the geothermal gradient, and rises again along fractured zones. On the way it dissolves metals from the rocks it passes through. It deposits them where conditions change, for example where it cools, boils, mixes with cooler groundwater or reacts with wall rock. An ore deposit needs all three steps: a metal source, a pathway, and a trap.

The technical detail matters because it decides which springs are worth following up.

Fluid chemistry. Zinc, lead and copper are carried mainly as chloride complexes (metal ions bonded to dissolved chloride) in saline, near-neutral to slightly acidic, reduced fluids. Alkaline, carbonate-rich waters are generally poor carriers of these metals. Rift fluids differ from place to place. Thermal brines in Kenya and northern Tanzania are all sodium carbonate type waters, while those in Ethiopia are sodium chloride type. In the northern Tanganyika Rift, two types of hydrothermal end-member fluid, a NaHCO₃ type and a NaCl type, have been identified. The NaCl fluids may derive from mixing between meteoric water and a deep-seated basement brine. The sulfides at the vents of the bicarbonate type are enriched in As and Tl, which are useful pathfinder elements. A pathfinder is an element that is easier to detect than the target metal and travels with it.

Metal source. Rift volcanics and basement can both supply metals. A study of Western Rift lakes concluded that meteoric waters derived their metal and salt load on passage through volcanic wall rock, helped by volcanic CO₂. It is an older paper, but the mechanism is a useful one to test.

Pathways. Fluid flow is structurally controlled. Seismic work at one Kenyan volcanic complex found steeply dipping faults that play a critical role in controlling the movement of geothermal fluids. Rifting also creates new faults and reactivates older ones, continually forming pathways for hydrothermal fluids. Old basement shear zones reactivated under rift stress are therefore prime candidates. The best sites are usually fault intersections, dilational jogs (steps where the fault opens as it slips) and relay ramps between overlapping segments. These are places where permeability stays open and fluids can mix.

Workflow: from fault map to ranked targets

1. Map the structures. Use a free digital elevation model such as SRTM or Copernicus DEM. Produce hillshades from at least four illumination azimuths so you do not miss faults parallel to the light. Add geological survey maps and any open aeromagnetic data. Separate rift-parallel normal faults from older basement fabrics.
2. Locate the structural traps. Digitise fault intersections, jogs and relay zones. Record strike and, where known, dip.
3. Classify the fluids. Compile spring analyses from published surveys. Plot chloride, sulphate and bicarbonate on a ternary diagram to separate chloride-rich from bicarbonate-rich waters. Silica and Na/Li geothermometers estimate reservoir temperature. One evaluation reported uncertainty close to ±20 °C for such relationships, so treat them as approximate.
4. Map alteration from satellite. ASTER has six bands in the 1.6–2.43 µm shortwave infrared (SWIR) region. Al-OH minerals such as muscovite, kaolinite and alunite absorb near 2.2 µm. Mg-OH minerals and carbonates absorb near 2.33 µm, coincident with ASTER band 8 (2.295–2.365 µm). Published ratios include (4 + 6)/5 for argillic and (5 + 7)/6 for phyllic zones. Sentinel-2 band 12 and Landsat 8 band 7 also sample roughly 2.2 µm, but they are broad bands and cannot separate individual minerals.
5. Sample for pathfinders. Take stream-sediment or soil samples along and across the ranked structures. Analyse for Zn, Pb, Cu, As, Tl and Mn.
6. Integrate. Score each fault segment on structure, fluid type, alteration and geochemistry. Rank the segments and visit the top ones first.

Worked example (illustrative only)

This is a generic, hypothetical setting and not a real project. Imagine a 12 km rift-margin fault zone cutting older basement. Digitising gives four fault intersections and two dilational jogs. Compiled spring data show five springs. Three are sodium-chloride type at an estimated 110–130 °C, and two are sodium-bicarbonate type at lower temperatures. All three chloride springs lie within 500 m of a mapped intersection or jog.

A SWIR ratio image highlights two Al-OH anomalies, each about 1 km across. One sits on a jog with a chloride spring and the other on a plain fault segment with a bicarbonate spring. A first-pass score out of 4 (one point each for structural trap, chloride fluid, Al-OH anomaly and pathfinder anomaly) might look like this:

• Jog A: trap, chloride fluid, Al-OH anomaly, and elevated As and Zn in soil. Score 4, first priority.
• Intersection B: trap and chloride fluid, but no anomaly in either remote sensing or soils. Score 2, needs mapping.
• Segment C: Al-OH anomaly and bicarbonate fluid, no trap. Score 1, low priority. Kaolinite here may simply be weathering.

The score does not prove anything about mineralisation. It tells you where to spend the first field week.

Common mistakes and limitations

• Treating any hot spring as a metal signal. Many rift waters are bicarbonate-dominated. Confirm the fluid type before reading anything into metal potential.
• Confusing present activity with ore-forming activity. Modern springs show that fluids are moving now. They do not show that metals were concentrated in the past. Older, reactivated systems may be more relevant, and they can be buried under young volcanic or sedimentary cover.
• Over-reading band ratios. Al-OH absorption also comes from weathering clays, soils and lake sediments. Dry vegetation, iron staining and dust also disturb ratios. Some ASTER scenes have SWIR data problems, so check the provider's notes for your acquisition date.
• Trusting DEM lineaments blindly. Lava flow edges, drainage lines and erosion scarps mimic faults. Verify with field measurements or geophysics.
• Sampling bias. Springs are found where people go, not where fluids are. Absence of data is not absence of fluid.

To check your results, run a field spectrometer or XRD (X-ray diffraction) on a few samples to confirm the minerals behind each anomaly. Repeat spring sampling in different seasons. Ask whether the ranking still holds if you drop any single data layer. If it collapses, it rested on one layer.

Key points to remember

• Ore needs a metal source, a pathway and a trap. Rift heat and faults supply the pathway easily, so check the other two.
• Chloride-bearing fluids carry base metals far better than sodium-carbonate waters, and East African rift fluids vary between the two.
• Fault intersections, jogs and reactivated basement structures are the best places to look.
• ASTER SWIR ratios map Al-OH and Mg-OH alteration minerals, not metals. Confirm them on the ground.
• A transparent scoring scheme, tested by removing one layer at a time, is more useful than a single striking anomaly.

Sources

• (PDF) Geothermal systems along the East-African Rift (researchgate.net)
• Hydrothermal activity in the Northern Tanganyika Rift, East Africa ... (sciencedirect.com)
• Hydrothermal origin of metals in some East African Rift Lakes (link.springer.com)
• Potential geothermal reservoir systems in the Kenyan Great Rift ... (sciencedirect.com)
• Frontiers (frontiersin.org)
• Geothermal activities in the Main Ethiopian Rift: Hydrogeochemical ... (sciencedirect.com)

About Orex — Orex is a mineral exploration intelligence platform based in Mwanza, Tanzania. We combine satellite remote sensing, elevation-derived structural analysis and open geoscience data to help explorers, licence holders and investors focus their fieldwork on the ground that matters.

Want to see fault structures and intersection targets on your area of interest — for free? Install GoldRadar on your phone or desktop: it maps lineaments and automatically flags fault intersections derived from satellite elevation data, giving you a structural framework for preliminary exploration before you spend a dollar on the ground.