Basic Type of Rock Sampling in Geology

Rock Sampling-Introduction

A sampling of minerals deposits is an important aspect of geological Exploration. A geological sample is a representative unit of soil, rock, or ore that is selected from a larger mass or volume of exposures and serves as an example of that larger body. The main purpose is to determine the quality or composition of a material. According to Boxter and Park, Rock sampling is defined as The process of taking a small portion of an article such that the consistency of the portion shall be the representative of Whole.

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, it helps delineate the zone of primary dispersion. This applies only to rock samples collected from bedrock because Rock samples of float (rock material suspended in colluvium) talus, glacial material, etc. give no indication of the location of the source, so even if they are highly mineralized, they are of limited value addition in sampling. Rock samples are collected to determine the concentration of metals, including both the major and trace metals. 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.

Type of Sampling

Grab Sampling

A grab sampling is an important rock sampling method from a confined area. It can be a single piece of rock. These are the most common types of samples collected. The sample usually consists of a single piece of rock, or chunks, which are representative of a specific type of rock or mineralization.

Bulk or Composite Sampling

A composite sample consists of small chips of uniform rock material collected over a large area. These are the ideal “representative” samples. The procedure is to collect small pieces of rock over a large ore deposit and to make the sample homogenous. A composite sample might be collected to determine the background values of trace elements in a particular type of rock or to determine if ore grade mineralization is present over a large area. Bulk sample refers to the few tonnes of samples collected either from the trench, pits, and channels or from the run of mine. It is a costly process of taking very large samples, Often the bulk sample will be a composite of material from development drifts and raises.

Channel Sampling

It is the most accepted method of sampling which is best suited to bedded, banded, and vein-type of deposits, this method consists of cutting channels across the face of exposed ore and collecting resulting chips, fragments, and dust from each channel to make a sample. In some cases, the channel may also be cut along a line making a small angle with the thickness of the deposit. The process involves the following steps-

• The exposures are cleaned first and remove the unwanted materials are. If necessary the rock face may be trimmed at cutting points of the deposit.
• The location of the channel is marked by a set of parallel lines.
• The inner part is excavated material is collected, Pulverized carefully and the sampling site is labeled. The cutting carefully depends on the ore body or metal deposit. This provides a great deal of information about the width and other aspects of the geometry of a mineralized zone.

Petrographic Sampling

Petrographic sampling is to conduct thin-section petrographic analysis of the rock, which is the identification and evaluation of the minerals comprising the rock by using a microscope equipped with both plane and polarized light. A thin section is made of the rock, which is a paper-thin slice of the rock mounted on a glass slide.

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Different minerals have different optical properties when the plane light or polarized light is transmitted through the thin section. Textural relationships also become apparent, which provides information about the order of crystallization (paragenesis). The procedure is to cut a flat side and use special epoxy to glue the piece of rock called a plug, to the glass slide. Thin a special trim saw cuts off the part opposite the glass. Then the rock wafer is polished with special grinders to achieve the desired thickness. The thickness must be very precise to compare the optical properties with a known standards.

A slight refinement of grab sampling in which the material to be sampled is spread out flat and channeled in one direction with a shovel, and the material for the sample is taken at regular intervals along the channel. The procedure is repeated with several other channels in different directions until a sample of the proper size has been secured. Also called channel sampling.

Also read- Type of Chemical Bonds in Minerals

Core Sampling

Diamond drilling has directly resulted in the discovery of many minable orebodies. The introduction of mainstream diamond drilling is a great evolution in the mining industry. Before the invention of drilling excavation of minerals is 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.

Diamond core drilling is used as a ‘diamond bit’. This drill bit is composed of a 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 feet in length, and then more sections of pipe can be attached to the top of this so a greater depth can be drilled. 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 wireline drilling. Rotary drilling is used primarily for borehole drilling, whereas wireline drilling is used for solid core sampling. There a five standard tube sizes associated with wireline drilling.

Core samples then are recovered at the surface as it piles up. Once the core is recovered at the surface it is broken into two half, one half is crushed to the powdered, and the second half is preserved for future reference. The crushed powered sample is again divided into two half, one half is preserved and the second half set to the lab for chemical analysis. A standard core tray can hold around 3m of the core.

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Error in Sampling

Various factors imposed numerous errors in sampling and its analysis. There are mainly two types of errors- Random and Systematic errors. The error in sampling is happen due to the following reason-

• Improper spacing in samples
• Insufficient number of samples
• Improper location of samples
• The incorrect weighting of the assay
• Failure to recognize ore, thus leaving it unsampled
• Improper chemical analysis and salting