DOCUMENTING KINEMATICS IN EXPLORATION AND MINING PROJECTS – WE NEED TO DO BETTER!

Structural geology is top of the pile when it comes to forming ore deposits. In the exploration and mining industry, there are many things we do really well, and many things we don’t, with regard to documenting structural geological relationships. One of the things we don’t do well is the resolution of kinematics. Understanding the kinematics of structures should be a fundamental skill, and the documentation of the interpreted kinematics should be a fundamental task. They aren’t.

Knowing the movement history of your structures over time is as important as collecting good quality planar and linear data. Sadly, the collection of linear data is something else we don’t do well, and commonly don’t do at all. But I digress. This post is about documenting kinematics. However, very very very few companies capture this information, and virtually none have the facility to record it in their databases, other than as a comment.

Apparent movement and the resultant interpretations vary depending on the direction of viewing and on the dip of the structures. Apparent movements are a function of not knowing the orientation of the movement vector and can develop as shown in the figure below.

Structures are never planar, so they can go from hosting an apparent reverse sense to hosting an apparent normal sense if the dip direction changes. The incorrect terminology will hinder the correlation of structures. For example, the same fault is less likely to be correctly linked if it is called a reverse fault at one location and a normal fault at another one. I’ve posted on this before and unashamedly repeat the diagrams here. Clearly, the same fault is present in each of the diagrams. However, the interpretations vary markedly, depending on location.

To resolve the movement sense, we require well developed kinematic indicators and the orientation of the movement vector. The photo collage posted below shows asymmetric foliations in gold-chlorite schist (bottom left), asymmetric distribution of chalcopyrite-bearing strain shadows adjacent to gold-bearing pyrite (top right), and asymmetric gold-bearing pyrite grains in non-coaxial shears (top left and bottom right). These are just a few of the many kinematic indicators that don’t get documented.?

To document kinematics unambiguously in our database, we need to divide the kinematic information into two separate columns – one for movement in plan and one for movement in section. We then need to say which way one block has moved relative to the other e.g. E-block-down, S-side-up, top-to-the-north etc. To document structures as reverse or normal is not good practice, because this apparent movement can change over short distances. So, someone’s reverse fault can be the same as someone else’s normal fault, as shown above.

The division of kinematics into consistent relative motions allows for filtering of the information. For example, it may only be S-side-up structures that accommodated movement in a particular deformation that are mineralised. Similarly, if we see movements nominated in both columns, we know we are dealing with oblique movement at that point and we can check to see if there is a lineation/movement vector that supports this.

I have included an extract from one of my spreadsheets, showing kinematics documented in plan (Sin = sinistral) and section (S-dn = south-side-down) in the box on the right-hand side of the figure.?

At the end of the day, people may differ on how to capture this information. However, currently 99% of databases haven’t even gotten to that point. It’s something we really should improve on.