July 25, 2017
The role of core orientation in Mining Exploration and Planning
Structural information is critical at all stages of exploration, planning and mining. Core orientation provides information on system geometry, potential plunge components, and the orientation of potential faults that could offset mineralization. But most of all, it provides the context for other data sets which allow for a better interpretation of geochemistry, mineralogy, short wave infra-red (SWIR) and geophysical data.
Core orientation has historically not been used in exploration or drill out of bulk-tonnage deposits (e.g., porphyry systems) as the grade distribution was taken to be truly disseminated, hence the orientation data wouldn’t matter. In a cost-constrained environment, geologists have opted for having orientation capability on site, but only using it when they intersect structural domains of interest. The problem with this approach is that most of these types of deposits have limited outcrop hence detailed structural information is not known. Since this is the case, continuous core orientation measurements throughout these types of deposits provide statistically meaningful data on orientations of hydrothermal elements observed during drilling. Until you know or have a high degree of certainty about the geometry, the spatial arrangement of the system and geology, everything is based on assumptions. Core orientation is also critical in more complex geology environments as well as sediment-hosted systems and gold vein deposits, where geometries become the most important part of further targeting.
Core orientation is also critical in more complex geology environments as well as sediment-hosted systems and gold vein deposits, where geometries become the most important part of further targeting.
Importance of Oriented Core When Exploring for Porphyry-Type Deposits
Currently, most exploration for porphyry copper systems occurs in areas covered by post-mineral volcanic and sedimentary sequences (e.g., Southern Peru and Northern Chile, British Columbia, Western USA). Detailed structural information is generally scarce at early exploration stages because outcrop is scarce. Oriented drill holes provide the perfect opportunity to measure the orientations of hydrothermal elements that influence the interpretation of surface chemistry, lithological and structural elements which in turn influence the construction of geological models. The volume of data available from downhole structural measurements allows for statistically meaningful observations about the ‘dominant’ orientation of hydrothermal elements as opposed to relying on spare outcrop data. Geologists frequently assume regional structural features influence deposit location and shape in some way. Reliable structural data in unweathered rocks allows explorers to test hypotheses regarding the relationships between mineral deposit formation and regional features used in the targeting stage.
RESOURCE DEFINITION AND MODELING PHASE
Core orientation increases the reliability of structural modeling where structures place hard boundaries on ore domains. Measurement of slip vectors on fault surfaces may allow for the prediction of the offset direction of mineralized blocks. Core orientation allows geological input to be variographically used in grade interpolation modeling. An unconstrained variogram merely looks at the distribution of grade in each sample relative to those around it. When the orientation of structures that host mineralization is known (veins, beds, faults), then the variogram can be guided to yield a more geologically plausible result. It is also possible to recognize and predict differences in the variography of different metals if these reside in different stages of a hydrothermal paragenesis where each stage has different preferred orientations as seen in decoupled distribution and variography of Cu and Mo in many porphyries (e.g. El Teniente, Bingham, Chuiquicamata, Quebrada Blanca).
Importance of Oriented Core When Exploring for Sediment-Hosted Deposits
In sedimentary terranes, orienting core is paramount. Everything depends on the geometries of the geology. Lower cost drilling technologies like RC or aircore are often used when trying to find a certain shallow rock type under cover, or testing large shallow areas for mineralization with many holes (e.g. following up a soil geochemistry anomaly). This type of drilling can be used to define if there is mineralization or if there is this certain rock type X present where it is supposed to be, but when more information is required to understand the geology in greater detail or if the deposit of interest is deeper, then diamond core drilling is required. Once the decision to drill diamond core is made, there is only a marginal increase in cost to have the core oriented, relatively speaking. It should be an easily justifiable investment given that it will provide the additional structural information needed to spatially tie multiple datasets together.
At an early exploration stage, it is possible to plan drill holes using only surface information and/or in geophysical datasets if available. Sometimes when outcrop is sparse, the initial drill hole may reveal that geology is dipping the opposite way, folded/faulted, etc. Having oriented core provides access to hard data on the geometry, which can be used to refine the plans of subsequent holes so that more cost-effective exploration can be conducted. It is not uncommon for geologists exploring in these types of terranes to adjust the orientation of planned holes on the fly based on the structural results from the previous hole drilled. For old rocks (most big sedimentary deposits are Neoproterozoic), the geology can be very complicated, making the need for structural measurements essential. Although, in flat-lying, undeformed deposits, it is theoretically possible to get away with drilling vertical holes at an early stage and monitoring the intersection of the bedding with the core axis to check it is always approximately 90° and therefore flat assuming the drill hole can remain perfectly vertical which can be difficult to maintain.
There is a large amount of information that can be obtained from core orientation including typical structural measurements of bedding, or the direction of mineralization-hosting veins. However, it is possible to get more detailed data like defining the direction of displacement of a fault plane from drill core and then define which direction to place another hole to have a chance of hitting that displaced block. In folded rocks, the bedding and foliation intersection can be used to determine in which direction to expect fold closure. This approach can be very useful in sedimentary deposits strongly controlled by folding (e.g. migration of fluids up into an anticlinal trap).
RESOURCE DEFINITION AND MODELING PHASE
For a resource drill-out of a sedimentary project, core orientation is absolutely essential. At this stage, the cost vs. the value is immaterial. Whether the mineralization is along bedding, along foliation, confined to veins within a unit etc, there is always a need for orientation measurements to aid in understanding the deposit for further resource definition, 3D modeling and even geometallurgy (location/geometry of hard vs soft units, dense vs less dense units etc).
This also applies for vein mineralization in sediments, where the orientation of ore filled veins must be understood. For example, when ore-bearing veins are sub-vertical, drilling a series of vertical, non-oriented holes would have a high chance of missing them.
Importance of Oriented Core at Mine Design Phase
A detailed understanding of structural feature orientations discovered during the exploration and drill out phases of mineral exploration is a critical data set that can also be used for geotechnical studies to inform mine design. Structural features like fault zone veins and bedding planes are key contributors to physical rock properties that are later geotechnically measured and used as controlling parameters at mine design. The earlier information is known about these structural features, the better, especially when it comes to making sound economic decisions related to mining a mineral deposit.
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