The mineral system concept has seen a growing interest in recent years. Particularly in Australia, where it is applied to exploration for ore deposits hidden under cover. This concept (covered in detail by McCuaig & Hronsky, 2014) explains that for significant ore deposits to form, a combination of favourable geological elements must come together (figure 1).
Figure 1: Elements that control ore genesis, according to the mineral system concept (McCuaig & Hronsky, 2014)
Fertility, geodynamics (during formation of ore deposits) and preservation are (generally) similar over relatively large regions. The favourable whole-lithosphere architecture tends to be more localized, because it relies on the presence of fundamental lithospheric flaws. These flaws are sometimes called ‘fundamental basement structures’, ‘large-scale ore-controlling structures’, or ‘metallogenically important trans-lithospheric structures’ and they form key pathways for vertical migration of mineralized fluids through the crust. According to the mineral system concept, sizable mineral deposits tend to form in the vicinity of such fundamental basement structures. This means that within regions that are known (or expected) to be fertile and where a favorable transient geodynamic setting has occurred, mapping the locations of these fundamental lithospheric flaws is the key to reducing the size of target area(s) where exploration should be focused.
A well-known example of these large-scale ore-controlling structures is the NE-SW structure that runs through Antamina, in Peru (figure 2). Other examples of world-class deposits where similar types of structure have been mapped include: Grasberg, OK Tedi and Porgera (New Guinea, see e.g. Garwin et al., 2005), the Kalgoorlie Super Pit (Yilgarn Craton, West Australia, see e.g. Mole et al., 2012) and many others in Australia (McCuaig & Hronsky, 2014). And possibly the most famous examples are the Carlin & Battle Mountain-Eureka Trends in the Great Basin of the western US (Grauch et al., 2003).
Figure 2: Region around Antamina mine, Peru (yellow arrow). The strike-extensive structure that runs through the Antamina deposit causes significant deformation along its trace. This includes the area 200 km along strike to the NE from Antamina (blue box), within the Sub-Andean Fold-Thrust Belt, where an obvious change between deformation geometries on either side of the structure occurs. (Elevation data used: AW3D30 DEM)
Large-scale ore-controlling structures have a number of common characteristics. They are generally:
- Strike-extensive. When mapped, these structures are significantly longer than the average fault in the same region.
- Depth-extensive (they often reach the lithospheric mantle) with relatively steep dips (as imaged in geophysics), which means they are generally quite straight in map view.
- Commonly juxtapose distinctly different basement domains.
- Multiply-reactivated during different phases of deformation. The sense of movement of these structures upon reactivation depends on the orientation of the prevailing stress field, so they often undergo variable senses of movement throughout their very long history.
- Vertically-accretive growth histories. Undisturbed younger sedimentary/volcanic deposits (or obducted rocks) can cover the structure during periods of inactivity, but upon reactivation of the underlying structure, such deposits tend to deform along its’ trace. When this deformation succeeds, the basement structure propagates upwards and continues its’ vertical accretion (figure 3).
- These are not necessarily obvious structures at or above the level of mineralization, they can be ‘cryptic’ (hard to see at current erosional levels). The reason for this is that deformation close to the surface is only a reflection of the most recent movement(s) along the structure. In most cases the deeper basement, which has seen larger/multiple/variable movements, isn’t exposed at surface. For this reason, these structures may easily be missed by geological surveys, or only mapped along part of their trace, as they are difficult to recognize when out in the field. But they are often the most important structures to try and map, as they are fundamental for targeting mineral exploration.
Figure 3: Example of upward propagation of a fundamental basement structure through the deformation of overlying undeformed units upon reactivation of the structure. (McCuaig & Hronsky, 2014)
Figure 3 shows the upward propagation of a fundamental basement structure after the deposition of sediments and/or volcanic rocks above it in a tectonically inactive environment. The figures explain why the movement of these large faults often seems relatively small and their surface expression cryptic. While the example in figure 3 is valid in many cases, it is important to realize that, during periods of basin formation, fundamental basement structures are often active rather than inactive. The (oblique) extensional stress field under which the formation of most basins initiates is more likely to reactivate large, pre-existing weaknesses (if their orientation is favourable), than it is to form new ones. The vertical (or oblique) movement along fundamental basement structures during deposition of sediments in the newly formed basin causes changes in thicknesses and facies in the sedimentary package. These changes follow the traces of the underlying (reactivated) structures that control them (figure 4).
Figure 4: Examples changes in thickness (left) and facies (right) of newly deposited volcanic/sedimentary strata during early stages of basin formation. (Adapted from McCuaig & Hronsky, 2014)
If such a basin, with changes in the sedimentary package across fundamental basement structures, undergoes further deformation at a later stage in geologic history, the changes in sediment thickness & facies will have an effect on the deformation style and geometry. If the basin undergoes compression (or transpression), the changes in the sedimentary column lead to mappable changes in frequency of faults, differences in wavelengths of folds (visible as changes in distance between fold axes and dip of flanks of synclines and anticlines) and changes in depth, lithology & thickness of detachment levels (again causing changes in frequency, amplitude & geometry of folds, or in more extreme cases changes from thick-to thin skinned tectonics). The location of the change in sediment thickness and/or facies is the location of the fundamental basement structure, as this is where the changes in the stratigraphic column occur. Working backwards, mappable lineaments (or linear zones) along which changes in the geometries of young faults and folds occur are a reflection of significant underlying changes at depth, and thus indicate the presence of fundamental basement structures.
As the location of fundamental basement structures is the main element of the mineral system that helps to reduce the size of target areas (where exploration should be focused), the ability to recognize them is important. Currently, their presence is often confirmed through tectono-stratigraphic reconstruction and geophysics. The recognition through reconstruction of a detailed tectono-stratigraphic framework relies on sudden changes in facies and thickness within the sedimentary package of a basin, due to activity of basement structures during their deposition (as explained above). The detection through geophysical methods depends on activity along the basement structures juxtaposing different lithologies (close to the surface and/or at depth), each with different physical characteristics, which are reflected in the geophysical data.
Another very effective and low-cost method for the recognition of vertically accretive structures is geological mapping using satellite data. A top-down view over a large region provides a different perspective, an overview of the area which allows for the detection of mappable elements that are easily missed by geologists while out in the field. Examples of such elements are changes in the frequency of faults and changes in wavelengths of folds along strike, as well as lineaments along which these changes occur. The NE-SW oriented structure that crosses Antamina shows good examples of these mappable changes, some of which occur in the area around the mine. Individual anticlines, synclines and thrusts disappear, or bend, across the trace of the structure, as a result of changes in the underlying stratigraphic package (figure 5). When zooming out further, it becomes clear that the entire Marañon fold-thrust belt steps to the right around the NE-SW fundamental basement structure (figure 6).
Figure 5: Trace of fundamental basement structure in the area around Antamina. Changes in frequency and orientation of synclines, anticlines and faults change across the structure (Love et al., 2004)
Even more profound than the changes near Antamina are those in the surface geology 200 km to the NE (figure 5). East of the 76°W meridian (black N-S line through purple box in figure 6) the NE-SW fault runs through a river valley that separates two parts of the Sub-Andean Fold-Thrust Belt, each with a distinctly different geometry. On the northern side, the older lithologies (Mesozoic) are exposed in the western part of the uplifted range, while folded Cenozoic sediments crop out along the eastern limit of the up-thrusted block. To the south of the NE-SW valley, the bulk of the up-thrusted Mesozoic lithologies are exposed at surface in the eastern part of the block, while to the west mainly Cenozoic units are found in outcrop. Another clear difference is the presence of a significant west-verging west verging thrust to the north of the fundamental basement structure, while no thrust with that vergence has been mapped to the south. As a result of the difference in resistance against erosion of the various lithologies found at surface, elevation data clearly reflect the changes in deformation geometry, across this vertically accretive basement structure (figure 2).
Figure 6: Red arrow indicates location of Antamina, in a zone where the (green colored) Marañon Fold-Thrust Belt steps to the right. Purple box in NE indicates same area as blue box in figure 2, where the geometry of the Sub-Andean Fold-Thrust Belt completely changes from N to S, across the NE-SW fundamental basement structure. Excerpt from Geological Map of Peru (scale 1:1.000.000), published by INGEMMET Peru.
The parts of fundamental basement structures that do not show significant displacement at surface (schematically shown in figure 3) also commonly form subtle lineaments, which extend for 10s – 100s of kms. These can often be mapped with satellite data too; in some parts they are more evident in the elevation data, while in other areas they are more clearly visible on the imagery, depending on the nature of the subtle deformation of the youngest lithologies.
The resolution of freely available imagery and elevation data will generally be sufficient to map these fundamental large scale structures. Such satellite datasets can be obtained easily, directly from archive, with choice from a variety of imagery options (e.g. Sentinel 2 data – up to 10m resolution, and Landsat and ASTER, both up to 15m) and elevation datasets (e.g. SRTM and AW3D30 – both 30m resolution). These datasets provide a uniform, unbiased, global coverage at a higher resolution than any other global datasets, which makes satellite data excellent for use in continental/province scale exploration programs.
While these fundamental structures extend over large distances, their surface expression can be very subtle, as explained above. Recognizing and mapping these structures can be quite a challenge, and one needs a skilled structural geologist with ample, relevant experience, who can translate subtle geomorphological changes into valuable structural information. As the data itself is free, the cost of this method is low, when compared to other methods for recognizing the presence of fundamental basement structures. This makes it ideal to help determine areas where exploration should be focused.
Garwin, S., Hall, R and Watanabe, Y., 2005, Tectonic Setting, Geology, and Gold and Copper Mineralization in Cenozoic Magmatic Arcs of Southeast Asia and the West Pacific. in Hedenquist, J., Goldfarb, R. and Thompson, J. (eds.), Economic Geology 100th Anniversary Volume, Society of Economic Geologists, p. 891-930.
Grauch V.J.S., Rodriguez B.D., Bankey V., and Wooden J.L., 2003, Geophysical and isotopic constraints on crustal structure related to mineral trends in north-central Nevada and implications for tectonic history. Economic Geology 98-2, p. 269-286.
Love, D.A., 2004, The Lithologic, Stratigraphic, and Structural Setting of the Giant Antamina Copper-Zinc Skarn Deposit, Ancash, Peru. Economic Geology 99-5, p. 887-916
McCuaig, T.C.and Hronsky, J.M.A., 2014, The mineral system concept: the key to exploration targeting. Society of Economic Geologists Special Publication 18, p. 152-175
Mole, D. R., Fiorentini, M. L., Thebaud, N., McCuaig, T. C., Cassidy, K. F., Kirkland, C. L., Wingate, M. T. D., Romano, S. S., Doublier, M. P., Belousova, E. A., 2012, Spatio-temporal constraints on lithospheric development in the southwest-central Yilgarn Craton, Western Australia. Australian Journal of Earth Sciences. 59-5, p. 625-656
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