Coal projects that require unique considerations be taken into account when it comes to disclosure of drill sampling procedures, quality control measures, data verification and the adequacy of the data to support resource estimation in compliance with NI 43-101.
NI 43-101 standards and best practice guidelines: data verification using geophysical logs when sampling coal
Companion Policy 43-101CP references the following guidance documents for coal:
- Geological Survey of Canada Paper 88-21: A Standardized Coal Resource/Reserve Reporting System for Canada
- Guidelines specific to coal in CIM Estimation of Mineral Resources and Mineral Reserves – Best Practice Guidelines, which also references
- ASTM standards Volume 05.06 – Coal and Coke
These documents provide best practice guidelines on drillhole sampling procedures that should be considered when disclosing coal sampling results under NI 43-101. Specific disclosure requirements under NI 43-101 include:
- Exploration Information (s.3.3(1)(c)): the quality assurance program and quality control measures applied during the execution of the work being reported on.
- Data verification (s.3.2 (a) and (b)): a statement of whether or not a qualified person has verified the data disclosed, including the sampling, analytical and test data underlying the information; and a description of how the data was verified.
- Item 11 of Form 43-101F1 Technical Report: sample preparation methods and quality control measures employed before dispatch of samples to an analytical or testing laboratory.
Geophysical logs: Down-hole geophysical logs are an essential part of the quality control measures employed during coal sampling, before dispatch of samples for analysis. Variations in coal quality, such as high-ash beds that are difficult to detect visually, can be discerned from geophysical logs. Geophysical logs from nearby drill holes can ensure equivalent intervals are sampled consistently in each of the cored seams.
Core recovery: Coal is one of the most difficult materials to core and sample because of its heterogeneity and brittle nature. Core recovery of 90 per cent to 95 per cent in the zone of economic interest would be considered acceptable for most mineral deposits but could yield misleading data for coal projects. Crushed core or core loss can cause an incorrect estimation of the coal interval. A core logger can determine that core is lost from the drilled interval, but without a geophysical log, it is not possible to determine whether or not the lost core is coal. Core recovery should be determined by comparing the total length of recovered core with the total length of the coal seam, as determined from the geophysical logs.
QA/QC During sampling: Separating the coal samples into coal plies and partings based on geophysical logs during core sampling is best practice because a rock parting may be observed on the geophysical log, with no evidence of it in the core log. In addition, a parting may have been misidentified by the core logger, or not recovered at all during drilling. It is not good practice to sample two coal plies together and an internal parting separately. Individual coal plies and partings should be sampled independently, except for thin (<20 cm) partings that would not be separable during mining. Proper full seam composites can then be calculated during the modelling process. If desired, plies can be sampled together at the laboratory by combining the remains of the individual plies and analysing the coal quality.
The geophysical log is an important part of the data verification process. Under NI 43-101, “data verification” means the process of confirming that data have been generated with proper procedures, have been accurately transcribed from the original source and are suitable to be used.
When properly calibrated, the geophysical log is much less subject to human error than a visual core log. Inconsistencies between geologists’ descriptions, errors in the positioning of the core in boxes, lost core, et cetera, all introduce errors into the core log. These errors can be mitigated by the use of the geophysical log as the unbiased umpire when logging the core.
The authors consider it best practice to have the core logged by the geologist using the geophysical log as a reference to determine core recovery and appropriate sample intervals, and to identify significant lithological changes appropriate for modeling of the coal seam thickness and quality, and the thickness and position of rock strata.
Ron Parent (left) is a principal geologist with AMEC Americas Limited, and has over 20 years experience in coal projects, primarily in Canada and Colombia. AMEC is a leading international consulting, engineering and project management company.Greg Gosson is the technical director of Geology & Compliance for AMEC Americas Limited. He is a frequent speaker on mining technical disclosure standards at mining industry forums.