by Luca Smeraglia, PhD, independent structural geology consultant and structural geologist at the National Research Council of Italy
As a structural geologist born and raised in Italy, most consulting and research projects associated to structural geology are supplied by the oil and gas industry. With only a few economic geology courses taught in Italian universities and one active mineral exploration project in northern Italy, for a young geologist the mining industry recalls ancient mineral workings during the Roman Empire, the golden era of mines in Sardinia during the 70s and 80s, and the exotic far lands of Africa, Australia and Canada.
Therefore, as expected, I started my career working on projects focused on understanding structural controls exhibited by fault zones during hydrocarbon migration. However, I soon realized that the traces of paleofluid flow within faults, and broadly speaking within the Earth’s crust, are mineral veins. While wandering for structural mapping projects across multiple countries, veins associated with different geological structures immediately captured my interest. In the summer of 2019, a conversation with a structural geology consultant working in the mining industry opened my eyes. I understood that all the previous experience gained in mapping structurally controlled veins and geological structures can be converted and applied to mineral exploration with just a little reskilling.
Structural geology can be perceived as a dark science by some mineral explorers, especially those working dominantly with quantitative geochemical data. A structural sketch can be seen as a fancy paint on a dirty field book in a world of coding and data science. However, most mineral deposits are structurally controlled. A few basic concepts in structural geology can be simply learned by anyone after a small amount of training, and used in order to better focus on exploration, drilling, and resource exploitation. Here, I would like to share with you, using several field examples from different projects around the world, six concepts to keep in mind when exploring structurally controlled mineralization.
1. Be respectful of old structures
While working on a project in Germany, a single-layered calcite vein up to 10 m (32.81 ft) thick and 30 m (98.43 ft) high suddenly appeared within a limestone open pit. As you can see from Fig. 1a-b, the vein is oriented parallel to a well-developed joint set. Fluid pressure reopened pre-existing joints and precipitated a vein parallel to the main joint set. Keep in mind that old structures can be easily re-activated and mineralization precipitated. Therefore, when looking into a new geological terrane, always start to understand the orientation of old structures. They can be re-opened during evolving geological time if tectonic stresses are optimally oriented with respect to them.
2. Dilation, dilation, and dilation
Faults are not straight railways across an empty desert. They bend, curve, arrest, and interact with each other in order to find their way home, losing displacement at their tips. If two undulose surfaces move apart, a new space is created, and fluid can find a good conduit for movement. Dilation commonly occurs along the undulose trace of strike slip faults. However, prospective dilational sites can occur also along the dip of normal faults, such as shown in Fig. 1c from the Ionian fold-and-thrust belt in Greece. Always look at fault bends to see if dilation occurred.
3. Competence matters
Stiff rocks can be fractured, and minerals can precipitate within fractures. Soft rocks can behave as a plastic material and muffle tectonic stresses as a pillow. In Central Oman, shale layers are alternated with stiff limestone beds within Triassic rocks (Fig. 1d). Limestones are veined, shales are not. This principle can be applied also within ancient volcano-sedimentary rocks within greenstone belts. They seem like hard rock, but, because volcanic-intrusive rocks are stiffer compared with the nearby schists, the former can be easily fractured and veined. Always look at the stiffness contrasts among your local stratigraphy. The mille-feuille cake effect is served.
4. Linkage occurs not only on LinkedIn
Faults want to interact, but interaction is not always easy, like among humans. Therefore, within dilational sites (see point 2) veins and fractures can be easily created. Oman is a wonderful place, where you can go looking for small-scale linkage structures between strike-slip faults. Keep in mind that concepts I refer to as RR and LL rules can be applied from regional-to outcrop-scale. When a set of right-stepping (R) and en-echelon right-lateral (R) strike-slip faults occur, dilation is created within the fault overlap zone (Fig. 2a). The same applies when a set of left-stepping (L) and en-echelon left-lateral (L) strike-slip faults occur (Fig.2b). If you see in map view right-stepping and right-lateral strike-slip faults (RR) and/or left-stepping and left-lateral (LL) strike-slip faults, have a look in the area of fault overlap, because you could find some interesting veins and/or dilation sites with breccias.
5. Do not undervalue the small structures
We all want that thick and long high-grade vein. Unfortunately, main faults do not always carry thick mineralization. Luckily, main faults are always associated with a cohort of minor and sometimes undervalued structures, such as en-echelon veins oriented at a low angle with respect to a principal thrust from Omani Mountains (Fig. 2c). When drilling across the main shear zone, it is possible to miss the minor mineralized structures that are subparallel to the drill holes. Always have a look around the major structures to find the small ones with different orientations.
6. Bending and stretching
Saddle reefs within fold hinges are widely known for hosting ore shoots along the fold axis. However, it is always worth to emphasize this concept. This is also valid for synclines as shown in Fig. 2, where the hinge of an open syncline from the Ionian fold-and-thrust belt of Greece is intensely fractured and mineralized by calcite. While anticline hinges are the perfect example of saddle reef structures, do not undervalue gentle syncline hinges and try to test them.
All the previous examples are certainly an incomplete list of structurally-controlled mineralization. However, these simple concepts can be easily learned by everyone, tested in the field, and also while reviewing drill cores, using new eyes and perspective. Structural geology trains you to think in three and even in four dimensions if you are keen to collect data, especially if you resolve the cross-cutting relationships between different vein generations. But this might be the topic for another piece. Times are now exciting for structural geologists wishing to pursue a career into mineral exploration and who want to keep working as field geologists.
Read Issue 19 here:
