A Geostatistical Approach to Optimal Drilling and Sampling Design: A Case Study of the Obuasi Gold Deposit in Southern Ghana

Casmed Charles  Amadu; Gordon  Foli; Emmanuel Daanoba  Sunkari

doi:10.4314/just.v40i1.1258

Casmed Charles Amadu Department of Earth Science, CK Tedam University of Technology and Applied Sciences
Prof. Gordon Foli Geological Engineering Department, Kwame Nkrumah University of Science and Technology
Emmanuel Daanoba Sunkari (PhD) Department of Geological Engineering, University of Mines and Technology

DOI: https://doi.org/10.4314/just.v40i1.1258

Keywords: Obuasi underground mine, optimal drilling interval, gold grade distribution, ore deposit, variogram analysis, Geostatistics

Abstract

This paper demonstrates the application of geostatistical techniques to the design of optimum drilling/sampling density, to an epithermal mixed vein- and disseminated sulphide type gold deposit. The Obuasi Mine is situated within the Ashanti gold belt of Ghana. The mine has past production and current reserves exceeding about 1,200 tonnes (t) of gold. The deposit is a typical shear zone type, characterised by non-uniform sulphide mineralisation distribution and boudinage auriferous quartz reefs having sharp boundaries. The main objectives of this study were; to determine the distribution of gold grades, and to use geostatistical approach to determine the optimum drilling and sampling intervals for the underground mine. The data for the study come from Block 1, Kwesi Mensah Shaft of the mine. It comprises of two data sets, diamond drill core and channel samples from cross-cuts. 507 drill hole cores were sampled at intervals of between 1.0 m, and 149 crosscuts, from which a total of 114488 channel samples were obtained and assayed by Atomic Absorption Spectroscopy (AAS) method for gold (Au). Statistical analysis showed that gold grade data had multiple-population characteristics. To analyse for spatial structure of gold mineralisation, 3D semi-variograms were computed in the 3 principal directions. Variability of the Au grades within the deposit were revealed, and nugget effect, C₀ defined as 3.7 (g/t)² and ranges (a's) ranged from 3.7, 16.1 to 27.5 in the across-strike, along strike, and down-dip directions, respectively. Sidewall channel sampling interval of 1 to 1.5 m across strike (used in crosscut and reef drives backs), and 30 m drill hole spacing for the down-dip section are adequate. Drill hole spacing in sections perpendicular to the strike of the deposit should be between 30 to 50 m, depending on the size, complexity of mineralization and structure of the orebody.

Author Biographies

Casmed Charles Amadu, Department of Earth Science, CK Tedam University of Technology and Applied Sciences

Casmed Charles AMADU is a Lecturer at the Department of Earth Science, Faculty of Earth and Environmental Sciences, CK Tedam University of Technology and Applied Sciences, Navrongo, Ghana. He holds a PhD in Geological Engineering from KNUST. He spent sixteen (16) years as an Exploration/Mine Geologist at several gold and base metal projects in West and North Africa. His research interest is the application of Geochemistry and Geophysics in mineral exploration, imaging of sub-surface fractures, ore deposit geology, structural geology (in the structure and tectonics of Precambrian greenstone belts), and geostatistical characterisation of sub-surface heterogeneity.

Prof. Gordon Foli, Geological Engineering Department, Kwame Nkrumah University of Science and Technology

Gordon FOLI is an Associate Professor in the Geological Engineering Department at Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana. His background is in Geology, with 19 years mining industry experience and 12 years of university teaching. He holds a PhD in Geological Engineering from KNUST and specialised in Arsenic remediation in mine drainage, mineral exploration and resource delineation, mining environmental management, ISO 14001 Environmental Management Systems (EMS) implementation and auditing.

Emmanuel Daanoba Sunkari (PhD) , Department of Geological Engineering, University of Mines and Technology

Emmanuel Daanoba SUNKARI (PhD) is a Lecturer at the Department of Geological Engineering of the University of Mines and Technology, Ghana. His research aims to contribute significantly to the understanding of the petrology, genesis, and geodynamic setting of hydrothermal ore deposits using multidisciplinary and multi-scale approaches, supported by fieldwork and various high-precision analytical techniques.