Sampling of Mine Workings: Methods, Workflow and Quality Control

Sampling of mine workings provides data on the composition and distribution of valuable components in the rocks and ores exposed by a working. It is more than collecting material: it covers justification of sample locations and methods, geological documentation, sample preparation, laboratory analysis and evaluation of the results, including their quality.

For a project, sampling refines the boundaries of mineralised zones, supports geological interpretation, enables ore-quality control during mining and informs further investigation. There is no single method: the approach and parameters depend on the geology, the type of working and the decision the data must support.

Where sampling is carried out and why

Surface and underground workings

At surface, trenches, costeans, pits and accessible open-pit benches are sampled. In trenches, material is taken along the floor or the lower part of the wall in bedrock, across the full width of the orebody outcrop; samples are tied to chainage and an instrument survey of the working. In pits, sampling follows the wall at set depths, which shows how composition changes with depth, for example in a weathering profile. On pit benches, intervals are located relative to the survey and the geological sketch of the face.

Underground, faces and walls of drives, crosscuts and adits are sampled. The position of the orebody relative to the working is critical. A crosscut intersects the orebody across strike, so sampling its wall gives a complete intersection of the mineralised zone. A drive developed along strike allows successive faces to be sampled to trace variability along the orebody, while its walls expose only part of the thickness.

Exploration and grade-control sampling

Exploration sampling characterises mineralisation — boundaries, grades and variability — and provides data for the geological model. Grade-control sampling supports day-to-day control during mining: refining ore contours, ore quality and dilution. Samples from exposed rock, drill core and broken ore are obtained under different conditions. An exposure shows the geometry of the orebody, but its surface may be contaminated or weathered. Core characterises rock along the hole axis, away from the working. Broken ore is already mixed, and its link to a point in the rock mass is lost. Such data can only be compared with these limitations in mind.

Principal methods of sampling mine workings

The method is selected to suit the geology and the decision the data must inform. No method suits all ores, workings and project stages equally.

Channel sampling

Material is collected along a channel of defined cross-section, cut by hammer and chisel or by saw on the surface of the working. The channel is oriented in the direction of greatest grade variability, typically across the strike of the orebody. The chosen width and depth are maintained along the whole length, and all material is collected. If softer zones enter the sample preferentially while harder material breaks away, the sample is biased. The method works well on orebodies of relatively consistent structure; on fractured rocks of varying hardness, the cross-section is harder to maintain and sampling errors increase.

The channel is sectioned at geological boundaries: orebody contacts and changes in lithology or mineralisation style are not combined in one sample. Interval length and sample mass are set according to the objective and the nature of the ore. Sample length along the surface of the working is not the true thickness: where the channel crosses the orebody at an angle, the orientation of the orebody and of the channel is used to calculate true thickness. There is no universal channel cross-section or sampling interval — these are set by the work programme.

Chip-layer sampling

In this method, known in Russian practice as zadirka, a layer of set depth is removed from the whole exposed area of the orebody or from a defined panel. It covers a larger volume than a channel and is used mainly for narrow veins and veinlets, where a channel may not yield representative material. The method is labour-intensive and requires control of the layer depth. It is not automatically more accurate: on uneven surfaces and rocks of varying hardness, sampling errors persist.

Point and specimen sampling

Point sampling is the systematic collection of small portions of material on a set grid across the surface of the working, combined into one sample. Specimen sampling takes an individual piece to study mineral composition, physical properties or geochemistry. The weakness of both methods is representativity: low mass and visual selection create a risk of collecting mainly visibly rich material. Point samples help assess mineralisation quickly and specimens help study the material, but such data are generally insufficient for quantitative conclusions on the average grade of an interval.

Bulk sampling

A bulk sample is a large volume of material, up to all the broken rock from an advance of the working. It is taken for metallurgical testwork and where the valuable component is extremely unevenly distributed. Unlike a routine sample, a bulk sample requires justification of the location and mass: the site must represent the ore type being studied. At deposits with coarse gold, greater mass reduces the error associated with rare large particles. However, a larger volume does not remove all errors: if material comes from the wrong domain, or too small an aliquot is taken during sub-sampling, bias remains.

Selecting the method and sample parameters

Selection is based on the shape and thickness of the orebody, the size of mineral grains and of particles of the valuable component, grade variability, rock strength and fracturing, access to the surface and the purpose of analysis. Blasthole and other methods are added where justified. Cross-section, interval length and sample mass are confirmed by the work programme and, where possible, by trial sampling on site.

How sampling of mine workings is carried out

Work runs from defining the task to dispatching samples to the laboratory, and each stage has its own output. Tools and parameters are defined by the project-specific procedure.

Geological documentation and site preparation

Inputs include a plan of the working tied to the mine survey, rock descriptions, contacts, mineralisation and the position of the orebody. Before sampling, the geologist sketches the wall or face, records contacts, structures and alteration zones, and takes photographs with scale and location. These materials are used to select sampling locations and orientation.

The surface is prepared for sampling: where a contaminated or weathered layer distorts composition, it is removed, and the programme specifies in advance when this is done and how it is recorded. Access to the working follows the site's safety procedures and requires authorisation from its responsible personnel.

Marking out intervals and sample locations

Orientation, boundaries and sample numbers are set by the geologist responsible for documentation, in line with the work programme. Boundaries follow lithological contacts and changes in mineralisation; different geological units are not combined in one sample without justification. Below is a hypothetical layout of a channel on the wall of a crosscut intersecting a quartz vein.

The values are hypothetical. Intervals within the mineralised zone are shorter because contacts, not a fixed spacing, define their boundaries; the outer samples in host rock show where mineralisation fades out.

Sample collection and field control

During collection, the chosen channel or panel geometry is maintained and all material is collected, preventing loss of flying chips and contamination by extraneous rock. A particular risk is preferential collection of soft or visibly rich material, which distorts grades. Sample mass is checked by weighing and compared with the mass expected from the geometry; deviations from the programme — short collection, a change of direction, collapse of part of the wall — are recorded in the sampling log.

Labelling and dispatch of samples

Labelling starts with a unique number; each sample is placed in a durable bag with a ticket. The minimum location record includes the working or level, the position and limits of the interval, the date, the sampling method and the responsible person. Samples are dispatched under a signed transmittal at each stage, from the working to the laboratory. Numbers are checked against the log and the laboratory submission before dispatch, when discrepancies are easiest to correct.

Sample preparation and laboratory analysis

At the laboratory, the sample is dried, crushed, split and pulverised to an analytical aliquot according to the agreed protocol, then analysed and reported. The preparation protocol and analytical method are selected for the material and the elements of interest. Sample preparation and analysis are carried out by accredited laboratories; Minerals Operating prepares the laboratory submission, inserts control samples into the stream and reviews the results.

Sample representativity and QA/QC

A sample is representative if its composition reflects the sampled area with sufficient accuracy for the purpose of the study. Precise analysis cannot correct a bias introduced during field collection: the laboratory accurately measures what it receives. The main causes of bias are incorrectly defined boundaries, an uneven channel cross-section, incomplete collection, mixing of intervals and location errors.

Control samples and data verification

Control samples serve different purposes. Certified reference materials check analytical accuracy, blanks detect contamination during preparation and analysis, and duplicates assess precision at the stage at which they are taken. A field duplicate — a second sample from the same interval of the working — reflects material variability and sampling error. Coarse-crush and pulp duplicates characterise sample preparation, and internal laboratory checks characterise analysis; where necessary, part of the samples is sent to a second laboratory for umpire assays. Table 1 of the JORC Code (2012 Edition) treats sampling techniques, sample preparation, assay quality and sample security as matters for disclosure, but it does not prescribe a sampling procedure for a particular deposit.

Control results are assessed batch by batch as they are received. When a deviation occurs — a reference material outside acceptable limits, a signal in a blank, poor duplicate agreement — the batch and documentation are reviewed and the cause is established: transposed numbers, a laboratory failure or a sampling error. Depending on the case, re-assay, re-preparation or resampling is then carried out. Insertion rates, tolerances and acceptance criteria are set by the project programme.

What the client receives

The deliverables are defined by the contract and typically include:

• the sampling log and sample registers;
• interval location plans and geological sketches;
• photographs of sampled surfaces;
• laboratory certificates;
• QA/QC results with conclusions;
• the geological sampling database.

The sample number in the database allows the working, interval, date, sampling method and sampler to be traced. The data are used to refine geological boundaries, interpret grades, build sections and models and make decisions on further exploration. Data from different methods are compared with regard to their quality and the conditions under which they were obtained. Sampling of mine workings is not in itself an estimate of Mineral Resources or Ore Reserves but one of the data sources for it.

Work organisation for clients based in Almaty

Minerals Operating carries out geological exploration and technical consulting. For sampling, this may include programme design, geological documentation of workings, organisation and supervision of sample collection, QA/QC, database management and interpretation of results; the scope at each site is defined by contract. Analysis is performed by accredited laboratories, with which the preparation protocol, turnaround and data format are agreed. The client's office may be in Almaty while the licence area lies in Almaty Region or elsewhere in Kazakhstan: the programme and interpretation are discussed with the office, and fieldwork is carried out on site.

Cost and timing depend on access to and condition of the workings, the volume and method of sampling, rock properties, logistics, the analytical programme and QA/QC. For an estimate, please provide the site location, the types and length of workings, available geological data and the purpose of sampling.

Frequently asked questions

How does sampling of workings differ from core sampling?

In a working, the sample is taken from an exposure and tied to the wall or face by the mine survey, with the orebody geometry visible. Core is tied to the hole axis by downhole survey data, and its quality depends on core recovery and the splitting method. Results are therefore not automatically interchangeable; their comparability is tested, for example, where a working and a drill hole intersect the same orebody.

Can the same interval and mass be set for all samples?

Parameters depend on geological boundaries, material heterogeneity and the purpose of the study. A uniform interval that crosses contacts mixes ore and host rock. Specific values are valid only for the documented conditions of a particular deposit.

Is a laboratory certificate sufficient to assess sampling quality?

No. A certificate confirms the analytical result for the material received but does not show how the sample was taken. Field documentation, traceability from the working to the certificate and control results across the entire chain are required. The grade of a single sample says nothing about its representativity.