Mineral Deposit Sampling Techniques

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Mineral Deposit Sampling Techniques

source https://www.ausimmbulletin.com/feature/increasing-value-through-best-practice-sampling-quality-assurance-and-quality-control-in-gold-mining/

Rock Sampling
Trench/Adit Mapping
Core Sampling / Drilling
    see also Diamond Drilling (in field skills...)

sample lines

Rock Sampling

Rock sampling reveals the true potential of an area for containing a mineral deposit. An anomaly in a rock sample from bedrock has had no effects of secondary dispersion, so the location of the sample is the location of the source. A rock sample anomaly will provide much more valuable information about the location of the mineral deposit because its source is within the mineralizing system, ie, it helps delineate the zone of primary dispersion. However, this applies only to rock samples collected from bedrock. Rock samples of float (rock material suspended in colluvium with no indication of proximity to the bedrock source), talus, glacial material, etc... give no indication of location of the source, so even if they are highly mineralized, they are of limited value. Rubble (rock material suspended in colluvium and due to consistency or other information suggests proximity to the bedrock source) in some cases may be worthwhile to sample.
Several different types of rock samples are collected for mineral exploration. Most importantly, rock samples are collected to determine the concentration of metals, including both the major and trace metals. This type of sample is most commonly referred to as a “geochem” sample. Trace metal values are often useful as “pathfinders”, which means they are closely associated with the metal of interest and may occur within a halo surrounding the mineralization of interest.
Rock geochem samples are collected in different manners depending on the goal of the sampling.

The principle types include:
Several other types of rock samples are sometimes collected to help interpret the history of mineralization in an area, to better understand the relationships between different ore minerals, or to determine more detailed geochemistry. These types of samples are often collected to evaluate the mineralization in a regional context, or to compare the mineralization with models which might apply to a given situation. Although they can might be costly, the information they provide can be invaluable. Some of these sample types include:
Petrographic Samples

SIMs sample

Trench/Adit Mapping

Trench or adit mapping is the process of creating a geologic map, which shows the geology of the floor and walls of the trench or adit. Adit mapping emphasizes mapping of the walls more than the floor because the floor is often poorly exposed due to the presence of a layer of debree which results from blasting and mucking. Trench mapping emphasizes floor mapping because:
  1.  the floor is usually scraped as clean as possible with a dozer or backhoe, and
  2. because floor mapping shows a “map view”. Trench or adit mapping always involves setting up a base line using a tape. Meter marks are then painted or flagged and labeled.
  3. The base line and distance marks are then drawn to scale on the map page to facilitate mapping. Often the same base line is used to accomplish a chip channel sampling program.
trench detail

One approach is to first draw the outline of the floor, which will be oriented with respect to true north and drawn to scale. The geology of the floor is then mapped just as an ordinary geologic map is made. The corner of the trench or adit matches the edges of the strip showing the geology. This is the “map view” (looking straight down) of the geology of the floor. The edges of the “strip map” represent the two bottom corners of the trench. The walls of the trench or adit are mapped adjacent to the strip map such that the right wall is mapped as if looking at the vertical on the right, and the left wall is mapped as if looking at the vertical wall on the left. These can be labeled to indicate they represent the geology of the walls, even though it is usually obvious. This gives a 3-D perspective of the geology, which greatly facilitates the interpretation of the geometry of features. For example in determining the dip of layers, faults, joints, etc... on the floor of the trench, it is useful to show where the feature trends as it intersects the adjacent walls. Structural measurements can be put directly on the map, in notation form next to the appropriate distance mark.
Another simpler approach used to make mapping more rapid is to sketch the floor outline at a standard, average width and not worry about the exact width. The outline is drawn parallel to the edge of the map sheet without regard to actual geographic orientation. The azimuth of the axis of the trench or adit floor is carefully measured and noted on the map. If the trench or adit contains bends, then the new orientation is noted at the appropriate distance mark on the map.
The alteration style can be added to one side or the other of the map if desired. The alteration can be mapped using colors, patterns or other designators, in the same way the rock types are mapped.

Example of Trench  map oriented to true north.

Example of Trench 5 map with trench axis parallel with map page edges.

Core Sampling

Diamond drilling has directly resulted in the discovery of many minable orebodies that would otherwise have gone untapped. Before the introduction of mainstream diamond drilling, mining was still primarily dependent on finding outcrops of rock, with little information available about ore concentrations below the surface. Diamond drilling allows the removal of solid cylinders of rock (core) from deep within the earth.
core samples

Drill Structure
Diamond core drilling is so called because it uses a ‘diamond bit’. This drill bit is composed of group of small, industrial grade diamonds set into a metallic, soft matrix. As the ground is drilled, this matrix will wear away and expose more diamonds.
This is then attached to a drill rod, which is around 10 foot in length, and then more sections of pipe can be attached to the top of this so a greater depth can be drilled. The depth that is drilled to is estimated by the number of rods attached to the top of the drill rod.
Inside the drill rod, a core tube is attached to a cable via a latching mechanism. The core tube is lifted to the surface using the cable, so the solid core can be removed.
There are two primary types of diamond drilling-rotary drilling and wineline drilling. Rotary drilling is used primarily for bore hole drilling, whereas wineline drilling is used for solid core sampling.
There a five standard tube sizes associated with wire line drilling. These are as follows:
diamond drill bit (cuts the rock core)

The drill size used depends on the desired core diameter and the desired depth of drilling, and the wider the diameter of the tube, the more power that is required to drive the drilling.
A drill bit with industrial diamond cutters in a matrix. Image Credit: http://www.sandia.gov

Core Extraction
To extract core, the drill rod rotates the diamond bit, spinning it into the ground. As the drill bit bores through the rock, solid rock is taken into the circular opening at the end of the bit, into the core tube, and can then be recovered at the surface as it piles up. Once the core is recovered at the surface it is broken along natural fractures and stored in core trays to await analysis. A standard core tray can hold around 3m of core.

drill rig

To keep the drill moving through the rock smoothly, the drill must be well lubricated using water to prevent overheating or sticking.
For optimum core extraction, the driller must listen to the drill to evaluate subsurface conditions. To keep drilling efficient, the rotation speed, pressure and water circulation must be strictly monitored.
Sometimes when drilling in highly fractured zones, overheating can occur due to a stuck bit. This issue is usually counteracted by the injection of mud or sawdust to plug fractures in the rock.

Diamond core finds its primary function in the exploration mining sector. It is usually one of the last stages of exploration, during which the orebody is delineated in three dimensions. This will determine whether the prospect is economically viable. Using a diamond drill rig, long vertical sections of core can be extracted from deep in the ground, which can then be analysed at the surface by geologists.
The core can then be analysed using a wide range of petrologic, structural and mineralogical techniques to determine whether the potential mining site is economically viable.
Extracted core is first washed and macroscopic features are logged by an exploration geologist. The core is then cut and representative samples are sent for chemical analysis.
Health and Safety
Mine sites always have inherent safety risks and diamond drilling rigs are no exception. While operating a diamond drill rig, correct protective gear must be worn at all times. Steel toe cap boots, hearing protectors, gloves, and goggles are some of the typical pieces of protective clothing needed.
When approaching an active drill rig, care must be taken to catch the drillers’ attention before stepping into the work area, as not to surprise the operator.
Furthermore, as diamond drilling is primarily an exploration technique, the conditions in which it is carried out are often harsh and so due diligence needs to be taken whilst in these environments. For example, if working in desert conditions, enough water and shelter must be provided for the drillers to prevent dehydration and heat stroke.