Drill Target Generation Using Structural Geology

Drilling is the most expensive step in most exploration programmes, and a drill hole that misses its target by a few degrees of azimuth or dip can tell you nothing at all. The best insurance against wasted metres is a sound structural interpretation. Structural geology explains how rocks have been deformed, where fluids were channelled, and in which direction mineralised zones are likely to continue at depth. This is especially important for orogenic gold, base-metal veins, and many other deposits whose location is controlled by faults, shears and folds.

This article sets out a practical workflow for turning structural observations into ranked, well-designed drill targets.

Why Structure Controls Mineralisation

Hydrothermal fluids move through rocks wherever permeability is highest. Faults, shear zones, fractures and fold hinges provide the pathways. Mineral deposition occurs where fluid pressure, temperature or chemistry change abruptly, and these changes are most pronounced in specific structural settings.

In orogenic gold systems, for example, fluids rose along crustal-scale shear zones and deposited gold in subsidiary structures. A widely accepted concept is that repeated fluctuations in fluid pressure during fault slip, sometimes described as fault-valve behaviour, allowed fluids to surge into dilatant sites and precipitate quartz veins and gold. The result is that high-grade zones are not scattered randomly. They occupy particular positions along and within the structural network.

Step 1: Assemble and Integrate the Data

Target generation begins by collecting every available dataset and putting it into a single geographic information system.

• Geophysics. Airborne and ground magnetics are the foundation for structural interpretation because they reveal faults, shears, dyke offsets, folds and lithological contacts, often beneath cover. Radiometrics, gravity and electrical surveys add information on lithology and alteration.
• Remote sensing and topography. Satellite imagery and digital elevation models highlight lineaments, drainage anomalies and alteration.
• Geological mapping. Outcrop observations of rock types, contacts, foliation, veins and faults provide the ground truth.
• Geochemistry. Soil, stream and rock chip anomalies show where mineralising fluids have left a chemical imprint.
• Historical workings and drilling. Artisanal pits and old drill logs mark where mineralisation exists and provide orientation data.
• Previous reports. Even incomplete historic work can contain valuable structural measurements.

At this stage, the aim is to find agreement between datasets. A target supported by several independent lines of evidence is far more credible than one supported by a single anomaly.

Step 2: Collect Quality Structural Data in the Field

Field measurements convert interpretation into evidence. Structural geologists record:

• Orientation of planar features such as foliation, bedding, veins, joints and faults, expressed as strike and dip or dip direction and dip.
• Orientation of linear features such as mineral stretching lineations, fold axes and slickenlines, expressed as plunge and trend.
• Shear sense indicators, such as offset markers, asymmetric porphyroclasts or drag folds, which show the direction of movement.
• Vein characteristics, including thickness, mineralogy, texture, vein sets and crosscutting relationships that reveal the order of events.
• Alteration and mineralisation associated with each structure.

Good practice includes using a calibrated compass-clinometer, recording magnetic declination correctly, taking enough measurements for statistical reliability and storing data in a structured database. Oriented diamond core, where core orientation tools are used, provides valuable three-dimensional structural data from depth.

Step 3: Establish the Deformation History

Rocks in orogenic belts usually record several phases of deformation. Stereographic projection, using stereonets, helps to group measurements into populations and to identify fold axes, principal stress directions and the relationships between structures.

The explorer asks several questions. What were the major deformation events and their ages relative to each other? Which structures were active when gold-bearing fluids were circulating? What was the direction of shortening or shearing at that time? Answering these questions reveals which fault and fracture orientations were favourably oriented to open, and therefore which were likely to host the richest mineralisation.

Step 4: Apply Kinematic Analysis to Find Dilatant Sites

Kinematic analysis uses the direction of movement on faults and shear zones to predict where space opened up. In a simplified strike-slip shear zone, for instance, bends or step-overs in the fault trace can create either extension or compression depending on the direction of slip. Extensional sites become sinks for fluid and vein formation, while compressional sites tend to be tight and unproductive.

The following structural settings are commonly associated with mineralisation in shear-hosted systems.

Structural setting	Why it is favourable	What to look for
Fault jogs and releasing bends	Extension opens space for veins	Changes in strike of a shear that match the slip direction
Fault and shear intersections	Fluid pathways converge	Crossing structures visible in magnetics or mapping
Fold hinges and saddle reefs	Layer-parallel slip opens space at the hinge	Folded banded iron formation or contrasting layers
Contacts between competent and incompetent rocks	Strain partitions and fractures concentrate	Granite or iron formation against schist or sediment
Fractures in competent units	Brittle failure creates stockwork veins	Dense vein swarms in massive volcanic or intrusive rock
Splays off major shears	Subsidiary structures host high-grade shoots	Branching structures from regional lineaments

These templates guide interpretation but must always be tested against local data. Not every jog is mineralised, and the best targets are those where favourable geometry coincides with geochemical or geophysical anomalies.

Step 5: Understand the Plunge of Mineralised Shoots

Many deposits are not flat sheets but elongated shoots that plunge in a specific direction. The plunge often parallels the intersection of two structural elements, a mineral lineation or a fold axis. Determining the shoot direction is critical, because drilling along strike at shallow depth may repeatedly miss a shoot that plunges steeply away.

Stereonets can be used to calculate the intersection of vein and foliation planes, and existing drill results can be projected along the likely plunge. This step often turns a handful of old artisanal pits or shallow holes into a coherent exploration model that predicts where deeper mineralisation should lie.

Step 6: Build a Three-Dimensional Model

Modern implicit modelling software allows geologists to construct three-dimensional models of faults, lithological units, alteration and grade shells from drill and surface data. These models make it easier to visualise how structures interact, to test alternative interpretations and to plan holes that intersect the target at the best angle.

Even a simple model made from cross-sections and plans can be valuable in early-stage projects. The key is to keep interpretations consistent with all data and to document the assumptions behind them.

Step 7: Define and Rank Targets

A target is a defined volume of ground where evidence suggests the presence of mineralisation. For each target, explorers should document:

• The geological concept being tested, for example a shear zone intersection beneath a soil gold anomaly
• The supporting evidence from geology, geophysics, geochemistry and structure
• The expected size and style of mineralisation
• The depth at which it should be intersected
• The level of confidence and the main risks

Targets are then ranked using a consistent scoring system. Criteria may include strength of geochemical anomaly, structural favourability, geophysical response, proximity to known mineralisation, and ease and cost of access. Ranking allows limited budgets to be directed to the best opportunities first and provides a transparent basis for decisions.

Step 8: Design the Drill Holes

Drill hole design translates structural understanding into practical drilling instructions. Key principles include:

• Intersect the target as close to perpendicular as practical. This gives true thickness and avoids overstating or understating width. Where veins dip steeply, holes are usually angled to cross them at a high angle.
• Account for hole deviation. Holes drift from their planned path, so the expected deviation should be considered, particularly in deep holes.
• Test multiple structural orientations. Where the dominant orientation is uncertain, early holes can be oriented to test more than one possibility.
• Plan fences of holes. Drilling along sections spaced at regular intervals produces data that can later support geological modelling and resource estimation.
• Consider access and environment. Slopes, land ownership, water bodies and sensitive areas affect pad locations and approval requirements.
• Set clear objectives for each hole. Each hole should answer a specific question, such as whether a structure continues at depth or whether a geochemical anomaly is explained by bedrock mineralisation.

An initial programme should be staged, with a review after the first holes so that interpretation can be refined before more metres are drilled.

Common Pitfalls

Typical mistakes include relying on a single dataset, using too few structural measurements, ignoring regional deformation history, drilling at an unfavourable angle for convenience, treating artisanal workings as the full extent of mineralisation and failing to update the model as new data arrive. Another is the temptation to drill to confirm a favoured model rather than to test it. The most successful explorers remain open to alternative interpretations.

Conclusion

Structural geology turns scattered observations into a coherent picture of how a mineral system works. By integrating geophysics, mapping and geochemistry, applying kinematic reasoning, determining shoot plunges and ranking targets objectively, explorers can design drill holes that give the best chance of discovery. In greenstone belts such as those of western Kenya, where gold is closely controlled by shear zones and folds, a rigorous structural approach is one of the most cost-effective tools available.