Blasthole Sampling for Grade Control in Open Pit Mine at Batu Hijau Porphyry Copper-Gold Deposit, Sumbawa-Indonesia

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

Blasthole Sampling for Grade Control in Open Pit Mine at Batu Hijau Porphyry Copper-Gold Deposit, Sumbawa-Indonesia

Johan Arif, Eddy Priowasono, Dini Rachmawati, Onie Handayani & Agung Naruputro

Mine Geology Department,
PT Newmont Nusa Tenggara, NTB - Indonesia.

Abstract

The principal objective of an optimised grade control procedure is to maximise the financial return from short-term mining of a bench, taking into account the related costs of sampling. Any interpretation of ore outlines based on poor quality sample data (regardless the sample density) will produce sub-optimal results and cause misclassification of ore and waste will leads to financial loss. Most of open mine operations using blasthole sampling as a tool for grade control purposes. Blastholes are drilled in the mine benches with the aims of placing charges for blasting. At Batu Hijau mine, typical hole size is 31 cm in diameter, which create a meter high by 3 meter cone of cutting weighting about 2.5 tonnes. Blast hole sampling is not conducted during operation but after the drill rig has moved on by a small sampling crew. Currently, the sampling methode is not an auto sampling, but conducted by the sampler crew using spear pipes. The pipe has a 6-cm diameter and sharpened on a 45-degree angle on the one end. Prior sampling the subdrill is removed from the surface of the cone then 12 stabs are taken around the pile (six opposite direction). The total weight of the each blasthole sample is between 6-10 kg, which contained in calico bags. The method is simple and highly efficient; a four persons sampler crew can take 100 samples in about 4 hours.

In this paper we report the results of blasthole sampling study counducted at Batu Hijau mine using various non-auto sampling tools. The aims were to compared the available techniques and define the ‘best practice’ for blasthole sampling. Six sampling methods were tested at more than 30 blasthole piles representing the various lithology, mineralisation, alteration, grade and physical condition of the piles. The methods are namely spear pipe, gilson splitter, under the deck, auger and cone and quarter. In comparing the results, cone and quarter method was assumed as the best representation value of the cutting piles, since it has the largest amount taken from the pile. The study results indicated that the spear sampling is the best options available. It is a simple and efficient method that collects a reasonable accuracy. The recommendations were emphasized on closer supervision on samplers as well periodic re­training on sampling method and also reminding the importance of sample quality. The blasthole cutting piles were easily destructed by water, hence twenty four-hours sampling was highly recommended especially in wet seasons thus removing rainfall as source of sampling error.