Many geologists consider the modelling of faults in mineral deposits to be extremely important. For resource geologist, faults may be the most important features they model. (We recently saw an excellent example of fault analysis by Ron Reid from a resource geologist’s perspective.) Indeed faults can represent boundaries where grade on one side may be unrelated in continuity on the other side (termed "hard boundaries" in geostatistical terminology), so in these special cases there is no doubt that modelling of faults is important. But that's about the extent to which some geologists think about faults and, unfortunately, they never consider what the faults actually represent and how they got there.
Faults are probably the most emphasised features of many mineral deposits. I know this because I see this repeatedly when I look at geological maps generated by companies. If one pattern of lithologies or grades seen in one drillhole fence does not match up with that in an adjacent drillhole fence, geologists commonly draw a fault to explain away the problem. In such cases, the fault:
- is almost always interpreted as vertical,
- interpreted as parallel to the two drillhole fences, and
- there is commonly no physical evidence of a fault as none of the drillholes actually intersects this phantom fault!
In other words, the fault probably only exists in the mind of the interpreter.
Faults are convenient
Faults are very convenient and easy-to-use excuses, so there can be a tendency to interpret their presence far too often, regardless of their existence!
After modelling hundreds of deposits, I personally feel that the importance and significance of faults have been exaggerated in the minerals industry. The attention placed on faults has shifted the attention from what I consider to be relatively more important features. There is a feature that goes hand-in-hand with faulting, but do you know what this other feature is? Read on – all will be revealed!
What is a Fault?
The other issue that I think is significant can be revealed by simply asking the question, "What is a fault?"
This may be a strange question to ask, but what I am questioning is why do faults form in the first place? If all geologists are familiar with the processes of fault formation they will be able to understand why I believe the importance of faults has been completely overemphasised in the minerals industry.
The image above is a diagrammatic representation of strain. Strain in (a) represents homogeneous strain and (b), the more natural looking inhomogeneous strain.
This version of the diagram comes from the textbook
Structural Geology: principles, concepts and problems by Robert Hatcher Jr (1995, Prentice Hall 2nd Ed), but you can find similar diagrams in almost any structural text in the earlier chapters on strain. This diagram represents the first-principles of what strain is, and I always refer to these diagrams in my head when I interpret mineral deposits.
Inhomogeneous strain is shown in (b), but the right and left images in (b) are slightly different. The one on the left has a fault (shown with arrows), and the right image that is labelled "Z" does not exhibit any faulting (at least not at this scale anyway). The state of blocks X and Y can explain what faulting is. The state of strain in these two blocks are different, and they are sufficiently different such that the ductile strains of the two blocks are incompatible with each other. Once you develop strain incompatibilities between two volumes, the consequence is to develop a discontinuity between the two contrasting states of strain. This dislocation that develops between the two states of strain is called a fault.
The boxer analogy
Maybe the above explanation is a little complicated, so I will explain using a non-geological analogy.
Imagine, the blocks X and Y are a magnification of a patch of skin of a boxer's forehead. The boxer has been hit by the opponent’s glove in patch X. This distorts this region X very suddenly, but Y remains relatively undistorted. If the deformation in X passes a certain threshold, there will be a tear that develops between Patch X and Y, and that will manifest itself as a gash in the skin. This gash effectively develops because of the two contrasting states of strain between the two patches of skin, and the two states of strain are incompatible with each other.
Faults are the result of strain incompatibilities in the wall rocks
The strict definition of a fault is "a crack in the earth's crust resulting from the displacement of one side with respect to the other", but this explanation does not provide a reason why these displacements can occur, and often it is what had happened in the wall rocks.
The state of strain of the wall rocks is what most industry geologists ignore because they are usually transfixed with the faulting!
When you look at faults from a genetic perspective, it places a whole new light on the faults that you may be mapping or trying to model in your deposit or prospect. For one thing, you will try to seek a mechanical explanation for faults that you are trying to model. Also, the above explanation clearly ties the strain states of the blocks on either side of the fault to the genesis of the fault itself. In fact, when you look at blocks X and Y, the fault itself is simply a small part of a much larger story. By focusing on the fault as being the primary story and ignoring the state of strain in X and Y, you might actually be missing out on the really important story. Also volumetrically, the fault is only a thin surface, whereas the rock material that surrounds the fault effectively 100% of the volume, so again this is another reason why the strain state of wall rock should be carefully examined.
The reason why the fault is there in the first place may actually be recorded in the adjacent wall rocks, but what do many geologists invariably do when they interpret or examine faults?
They:
- don't think why the fault developed, and
- fail to understand the state of strain in the wall rocks!
A much better practice is to consider the whole picture i.e. the fault, the damage zones, and the overall state of strain of the wall rocks.
Geologists will have to adjust their thinking about this issue. Unfortunately, the software companies, who make the products that geologists use, don’t help the situation because they rarely ever see this as an issue, much to the detriment of the industry as a whole. To them a fault is just a boundary surface, and nothing much more.
A software company that has some structural integrity
A software that is heading in the right direction, at least from the point of view of fault modelling, is
3DMove. This software is developed by Midland Valley which was founded by experienced structural geologist
Dr Alan Gibbs. This is one of very few software products on the market that ties together the wallock strain history and fault development. It recognises the connection between wall rock deformation and fault development, so if you are developing an interpretation of a fold and thrust belt, check out this software. The modelling concept is well ahead of mining packages which ignore fault history and the failure to see the relationship between rock deformation and fault development. The shortcoming of 3DMove is that it really only applies to fold-thrust sequences with clearly definable stratigraphy, but the concept itself teaches us some important lessons that appear to be generally lacking in software products that dominate the minerals exploration and mining industry.