On a smaller scale they can help identify planes of weakness and channels in which fluids may have deposited economic minerals. On a larger scale they can help the geologist to reconstruct the stresses that the rock has been exposed to which may lead to other important regional or local structures.
Descriptions of an ore mineral deposit or project area often refer to geologic structures or features that are important to the deposit. Indeed, we have often written about structurally controlled mineralization in our mining company press release reviews. While hydrothermal activity and ore-bearing fluids are linked to most economic mineral deposits, the paths that these fluids take are invariably the paths of least resistance: structural features like folds and faults.
So why should you care?
Mining companies will often refer to these structural features when describing their deposits. Here are some examples from real mining company news releases:
- “drill intersected significant mineralization in the hanging wall zone”
- “hinge zone drilling encounters high grade gold mineralization”
- “encountered high-grade gold values in the upper limb portion of the fold structure”
- “the axial plane of a large scale anticline fold axis which plunges at a shallow angle to the north”
- “High grade gold intersected in multiple holes along the strike of the axial plane of the anticline continues to support our geologic model”
Some of these descriptions probably don’t belong in a press release, but geologists are often hesitant to over simplify in a way that presents the data as inaccurate. Let’s review the two main structural features that may be described in a geologic report or news release.
A fault is the displacement of rock due to fracturing and movement along a plane. Large scale fault systems relating to modern earthquakes are probably most familiar, but faults are common features on large and small scales.
Tectonic activity along plate boundaries is the usual culprit for large scale structural features creating large scale fault systems, folding and mountain belts. Many mineral deposits occur in ancient terranes that are no longer tectonically active but include structural and geologic features that reveal a more violent history.
In very old terranes rock may be structurally deformed and metamorphosed by the high temperatures and pressures exerted at depth.
The greenstone belts that host many mineral deposits in Canada and Europe are deformed from these types of processes. Though the deformation takes place deep within the crust, erosion and tectonic activity may eventually expose these rocks at the surface.
Examples of fault-controlled mineralization include world-class gold camps of Timmins, Ontario and Val d’Or Quebec. Large scale fault systems became channels for gold-bearing fluids through the region creating many high-grade, high tonnage deposits.
Mineral deposit description will often refer to the hanging wall or footwall zones relating to mineralizations. These terms refer to the position of the rock in relation to the fault plane and are illustrated to the left.
In structurally deformed terranes large and small scale folding and faulting will occur concurrently.
Much of the structural deformation that occurs within rock formations can be described as folding. Folds may not be as readily apparent as in the photo included with this article: In fact, it is often only through careful analysis of small scale rock textures such as mineral orientations or big-picture geophysical anomalies can a fold structure be identified.
In general terms, an anticline is a convex fold with the youngest beds on the outside of the curve. A syncline is is the opposite with the youngest beds on the inside. But unless the relative ages of the folded rocks can be determined, these labels cannot be used. The various terms help geologists to describe the structure accurately and in a standardized way, but the descriptions can often be confusing. The diagram to the right may help explain some of the terminology related to structural folds
Ultimately fold structures are important for two reasons. On a smaller scale they can help identify planes of weakness and channels in which fluids may have deposited economic minerals. On a larger scale they can help the geologist to reconstruct the stresses that the rock has been exposed to which may lead to other important regional or local structures.
Identifying a fold structure associated with mineralization can help the geologist plan new drill and exploration targets.