Structural Geology

Structural controls on mineralization are evident in almost every type of ore deposit.  It is therefore important to recognize different types of structural features which are present in rocks, and how the development of these structures can influence ore deposition either directly or indirectly.  Natural forces, such as heat and pressure, can occur on any scale, large or small.  The same forces can cause “deformation” of the rocks, which includes:

That such large scale forces exist is evidenced by the fact that many sedimentary rocks known to have been horizontal at the time of deposition are now tilted to any angle and in some cases are completely overturned.  The action of these large scale forces results in the formation of folds and faults in rocks, which may be visible on the on a small scale, such as in hand specimen or outcrop.  However, when present on a large scale these features may not be so obvious.  To identify these features on a large scale, geologists must measure the geometry of the rocks on the outcrop scale and plot the information on a map.  The patterns which emerge can indicate the presence of these large scale features either at the surface or underground.  The method used is to measure the strike and dip of the rocks, which are two vectors used to describe the geometry of planar features of rocks using an imaginary horizontal plane as a reference. 

Strike:  direction of a line formed by the intersection of the inclined surface (bedding, fault plane, etc...) and an imaginary horizontal plane (Figure 10 – 1).  The direction of the line is expressed as the bearing, which is the angular difference between true north and the strike line.  The standard system to describe the direction is the “azimuth” system.  In the azimuth system true north has an azimuth of 0, due east has an azimuth of 90, due south has an azimuth of 180, and due west has an azimuth of 270.  The bearing of the strike shown in Figure 10 – 1 is due north, or 0.

Dip:   angle of the inclined surface below the imaginary horizontal plane.  The dip is always measured perpendicular to the strike line (Figure 10 – 1).

Folds

Folds result from plastic deformation, ie, deformation which does not rupture or fracture the rocks, but instead causes them to permanently bend.  Plastic deformation most often occurs well below the earth’s surface, where conditions of high heat and pressure allow the rocks to behave in this manner.  Folds are easiest to recognize in sedimentary or metamorphic rocks where some type of layering or fabric is discernable.  Every fold has the following components (Figure 10 – 2):

Figure 10 – 2.  Fold axis, axial plane, and fold limbs.

Folds which have both limbs dipping away from the fold axis are called anticlines; folds which have both limbs dipping towards the fold axis are called synclines (Figure 10 – 3).   When an anticline is uplifted and eroded, older rocks are exposed near the fold axis and younger rocks are exposed away from the axis.  When a syncline is uplifted and eroded, younger rocks are exposed near the fold axis and older rocks are exposed away from the axis (Figure 10 – 3). 

 

figure 10-3 pending permission

Figure 10 – 3.  Anticline and syncline (modified from Putnam, 1973).

Each type of fold can be further classified by the relationship of the axial plane to the limbs.   For example, observe the four types of anticlines shown in Figure 10 – 4:

The same terminology and geometry of fold axes shown in Figure 10 – 4 also applies to synclines.

Faults

Faults are breaks in rocks where slippage occurs.  The surface where the slippage occurs is called the “fault plane”.  The fault plane can form at any geometric orientation from horizontal to vertical.  The orientation of the fault plane is defined by measuring the strike and dip, just as with any other planar feature in rocks.  If the fault plane is vertical, the fault is called a vertical fault.  The relative motion which can occur includes:

Dip slip faults which dip less than 90 degrees are further defined by the relative displacement of the blocks on each side of the fault plane.  The block of rock which occurs above the fault plane is called the “hanging wall”.  The block which occurs below the fault plane is called the “footwall” (Figure 10 – 5).

Strike slip faults typically have near vertical fault planes, and since the displacement is parallel to the strike of the fault plane, there generally is no hanging wall or foot wall.  Strike slip faults are defined by the relative motion of the block on the opposite side of the fault from the point of observation.  For example, if the relative motion on the opposite side of the fault is to the left, it is called a “left-lateral strike slip fault”.  If the relative motion on the opposite side of the fault is to the right, it is called a “right-lateral strike slip fault” (Figure 10 – 5). 

Figure 10 – 5.  Faults.

Evidence of Faulting

Direct evidence of faulting can often be difficult to locate due to the effects of weathering at the surface which tend to obscure the features which develop when rocks rupture and slide past each other.  Geologists seek several types of features which provide direct evidence of faulting:

Indirect evidence of faulting can also be present.  This type of evidence may include the juxtaposition of two map units which are usually not contiguous, such as two sedimentary rock formations of different ages, or a intrusive in sharp contact with a country rock instead of containing a hornfels or skarn zone in between.  Geologists also examine topographic maps and aerial photographs for linear features on the surface.  Lastly, aeromagnetic anomalies or other linear aeromagnetic features can be indicative of large scale fault structures.

Joints

The cracks or fractures found in most rocks exposed at the surface are called “joints”.  Joint spacing is often rather consistent within a specific rock type in a specific environment.  For example, fine-grained rocks tend to have close-spaced joints, while coarse-grained rocks tend to have wide-spaced joints.  The jointing pattern within a specific rock type is sometimes so consistent that it can often be a useful aid for geologic mapping. 

Joints form in several different ways.  One way is by deformation of the rocks, such as folding.  These types of joints are particularly common in the apical region of folds.  Joints can form by contraction in mud which dries, and be preserved when the mud becomes lithified into a mudstone.  Joints can also form in volcanic or subvolcanic rocks during cooling (also a result of contraction).  In this situation, joints form perpendicular to cooling surfaces, and generally form columns with five or six sides (called “columnar jointing”).  Another type of jointing develops when cracks form parallel to the topographic surface, called “sheeting”.  These types of joints result when uplift and erosion removes the confining pressure of the overlying rock layers.  As a result the rocks rebound and tend to break into slab-like layers.

Structural Controls on Mineralization

Nearly all hydrothermal deposits exhibit some degree of structural control on mineralization.  Structures (fractures, faults or folds) which form prior to a mineralizing event are referred to as “pre-mineral” (Figure 10 – 6).  Geologists are keenly interested in pre-mineral structures because these structures influence the localization of ore by hydrothermal fluids utilizing these pathways.  By mapping these structures and projecting the geometry in the subsurface, new ore deposits may be discovered.  Structures which form after a mineralizing event, and hence may be responsible for offset or removal of mineralized zones, are referred to as “post-mineral”.  In some cases the formation of structures and mineralization appear to be nearly synchronous (Figure 10 – 7).  In these situations, shearing was probably ongoing during the mineralization event.  This is evidenced by ore minerals localized along a fault plane which are deformed.

Fractures and fault zones provide excellent pathways for hydrothermal fluids to circulate through.  Open-space filling has long been recognized as the primary method of vein formation.  The formation of breccia and gouge due to the grinding action of the rocks adjacent to the fault plane increases the ‘structural porosity’, which in turn increases the permeability.  Under certain conditions, breccia or gouge may itself provide the host for mineralization.  Intersections of structural features often are better locations to prospect for mineralization, especially where the structures are high angle.  It is thought that the intersection of high angle structures provides pathways for fluids from deep sources to move closer to the surface.

Figure 10 – 6.  Fracture systems in rocks overlying an igneous intrusion.   A & B:  radial fractures above a circular intrusion. C & D:  longitudinal fractures above an elliptical intrusion (from Emmons, 1937).

Figure 10 – 7.  Cross section of Ft. Knox granite-hosted gold deposit, Fairbanks District, Alaska, showing late-stage shear zones containing high grade gold mineralization ( 1.0 ounce per ton) (after Bakke, 1991).