D1 · Publication Volume 17

Lithological and Mineralisation Domains

stationarity, geological continuity, and hard and soft boundaries

Learning objectives

By the end of this lesson, the learner should be able to define domains from geological purpose and continuity; distinguish lithological, structural, alteration and mineralisation partitions; assess stationarity assumptions; select hard, soft or transitional boundary representations; avoid circular grade-driven interpretation; and validate domains against evidence, alternatives and downstream use.

A domain is not merely a closed shape. It is a claim that locations inside share stated geological characteristics or continuity relevant to a task. Different tasks may require different domain systems. The modeller must make the membership rule explicit before geometry is constructed.

Domain purpose and geological membership

Start with the variable or process the domain is meant to organise. Lithological domains may follow material identity and stratigraphic relations. Structural domains may group orientation or deformation style. Alteration domains may represent reaction assemblages or intensity. Mineralisation domains may represent a geological process, continuity style or thresholded response, but should not be defined solely to make a later estimate convenient.

Write inclusion and exclusion rules using observable or defensible criteria. Specify hierarchy, precedence and whether domains are exhaustive and mutually exclusive. A location may belong to one lithological domain and one alteration domain simultaneously; forcing both into one combined code can create a fragmented, unstable partition.

Continuity and stationarity assumptions

Stationarity means that a chosen statistical description is sufficiently stable within a domain for the intended analysis. It is not guaranteed by a closed wireframe. Test distributions, trends, orientation, support and spatial continuity. A domain can be geologically coherent but statistically heterogeneous; conversely, similar distributions can occur in geologically unrelated bodies.

State whether continuity follows stratigraphy, structure, veins, replacement fronts, weathering or another process. Compare directions of geological continuity with any downstream spatial-continuity model. If evidence is sparse, preserve the geological hypothesis and avoid overstating stationarity.

Hard, soft and transitional boundaries

A hard boundary asserts a discontinuity or strict membership change across a surface. A soft boundary permits continuity or information sharing across it. A transitional boundary represents gradual or uncertain change over a zone. Boundary behaviour is a modelling decision that should follow process and evidence.

Do not infer “hard” solely because two codes differ. A logging convention may create an abrupt categorical break in a gradual geological transition. Conversely, a subtle contact may be a real barrier. Store boundary type and confidence as attributes of the relationship between domains, not as an undocumented property of the mesh.

Thresholds, circularity and support

Grade or property thresholds can help describe a response but risk circularity when the same data define, fit and validate a mineralisation domain. Separate geological indicators from the target variable where possible. Test alternative thresholds and compare how they change volume, continuity, included support and downstream decisions.

Account for sample support. A short high value and a long moderate interval do not constrain the same volume. Do not place contacts at assay boundaries without checking recovery, sampling, analytical quality and geological context. Preserve below-detection and missing values rather than converting them into false boundary evidence.

Domain hierarchy and overlapping concepts

Use persistent domain objects and relations. A parent stratigraphic package may contain structural subdomains; a mineralised horizon may cross alteration classes; a weathering profile may overprint several lithologies. A hierarchy or multi-axis classification is clearer than proliferating combination codes.

Define precedence only where a downstream representation requires one category per location. Record the derivation rule and preserve the underlying memberships. This allows a future use to regroup domains without reconstructing geology from an opaque integer code.

Synthetic worked example

The fictional horizon contains synthetic assay values and alteration observations. A first envelope uses one fixed threshold and connects every above-threshold interval, producing a thin bridge across the central fault. Geological review shows that the intervals belong to different structural blocks and that the bridge lacks alteration and orientation support.

Three models are compared: a threshold-only envelope, a structurally separated geological domain and a transitional probability zone. The structurally separated model best represents the evidence used for the immediate decision, while the transitional zone communicates uncertainty near sparse contacts. The threshold-only model remains as a sensitivity case rather than being deleted.

Domain design separates geological membership, continuity assumptions and hard, soft or transitional boundaries.
Domain design separates geological membership, continuity assumptions and hard, soft or transitional boundaries.

Practice and review checklist

For one domain system, document:

  • purpose and target variable or process;
  • geological inclusion and exclusion criteria;
  • hierarchy and overlap rules;
  • continuity directions and stationarity tests;
  • boundary type, evidence and confidence;
  • sample-support limitations;
  • sensitivity to threshold or coding changes; and
  • which data were used for construction versus validation.

Ask whether a reviewer could reconstruct the membership logic without seeing the final coloured volume.

Decision implications and integration

Publish the domain dictionary, relationship graph, boundary attributes, geometry version and derivation rules together. Downstream estimates or simulations must cite the exact domain version and support transformation used.

A domain change is material when it changes membership, topology, volume, continuity assumptions or boundary behaviour. Such a change requires revalidation, not only a new colour legend.

Sources