“THE CASSIAR STORY”

Mine Planning

G. Scott Zimmer, Mine Engineer, Cassiar Asbestos Corporation Limited, Cassiar, B.C.

Abstract

Mine planning at Cassiar consists of an annual cycle of projection, control, evaluation and revision following com- pletion of annual diamond drilling during the summer months. The inherent difficulty in evaluating fibre grade and distribution, combined with a high-wall phase or push-back mining system, requires continuous modification to designs and schedules based on a cumulative evaluation of equip- ment performance and prediction versus production. To expedite the planning sequence, a mini-computer system has been introduced which allows emphasis to be placed on optimizing designs, exempting personnel from manual cal- culations and tabulations.

Introduction

SINCE 1971, Cassiar has been carrying out an exten- sive diamond drilling program to further define the limits of the orebody in terms of tonnage and grade. Evaluation of drilling data indicated sufficient ore of suitable quality to warrant an ongoing expansion of operations and facilities. In order to provide a base for sound decision making, Cassiar’s mine planning pro- cedures have undergone a substantial change since 1973 with the review of old procedures and the intro- duction of a mini-computer.

Mine planning at Cassiar can be summarized as an annual cycle consisting of ore and waste projection, control, evaluation and revision. This entails the re- vision of the ore reserve figures after the evaluation of the annual drilling program, a sub-division of re- serves into mining increments, engineering control and direction of mining operations, a reconciliation and evaluation of production with initial projections and, finally, a modification of reserves based on the pro- duction evaluation in preparation for use in conjunc- tion with new drilling data to begin the annual cycle again.

G. Scott Zimmer was born in the U.S.A. and educated at Michigan Technological University from 1962 until 1966, majoring in geological en- gineering. He joined United Keno Hill Mines Limited in 1967 as senior ex- ploration geologist. From 1969 until 1970, he was employed as a staff geol- ogist in the consulting firm of R. G. Hilker Limited in Whitehorse. In 1970, he joined Conwest Exploration Com- pany Limited as a staff geologist, progressing to resident geologist in charge of the White- horse office. He joined Cassiar Asbestos Corporation Limited in 1972 as mine geologist and became mine engi- neer in 1978. He is a Member of the CIM and a Fellow of the Geological Association of Canada.

Keywords: Cassiar Mine, Mine planning, Drilling, Samp- ling, Pit design, Reserve calculations, Scheduling, Com- puters.

There are three fundamental problems to consider in medium- to long-range planning. Two of the diffi- culties are fibre related; the third is due to a mining method commitment.

The first fibre-related problem is the difficulty of developing a rapid and consistent method of deter- mining fibre content to an absolute or standard base. A new test mill system is currently being installed, but until the system is operational, all “assay” data used in mine planning are based on C.C.R.G. (Corrected Core Reading Grade). The relationship of C.C.R.G. with R.M.G. (Recoverable Mine Grade) is entirely dependent on mill efficiencies.

The second fibre-related difficulty is in assigning a rock value ($/ton) to future ore for evaluation pur- poses. Cassiar produces different fibre grades with values ranging from $320 per ton to $3,620 per ton. It is theoretically possible to produce nearly all of these products from any ore grade. Accordingly, derivation of rock value is again a function of mill efficiency and relies heavily on historical product distribution and market demands.

The third constraint is a commitment to a push- back or phase mining concept due to an ever-increasing hanging-wall depth [currently 1000 ft (304.8 m) ver- tical and increasing to at least 1600 ft (488 m) over the mine life], which imposes severe restrictions on ore availability and flexibility, requiring critical ore and waste scheduling, as well as constant evaluation to ensure ore continuity between phases.

Reserve Calculations and Pit Design

Mine planning procedures have been simplified as much as possible in order to produce a general flow sheet (Fig. 1). The foundation for planning consists of a master set of bench plans covering the entire ore- body at mining bench intervals: 45 ft (13.7 m) in waste and 30 ft (9.1 m) in ore. These plans carry lithology, macro-structure, a fibre to no-fibre inter- polated (to 3.0% C.C.R.G.) contact and high-grade zones within the “ore” limits. The fibre-bearing zone on each bench within the 3.0% C.C.R.G. contour is sub-divided into blocks, nominally 100 ft (30.5 m) by 100 ft (80.5 m), based on drill-hole intersections. Computer input of block areas, C.C.R.G. grade and C.C.R.G. length distribution for each bench produces a master grade-block model for the fibre-bearing zone which is independent of economic cut-off grade, mining limits and topography. Details of master model de- velopment are covered under “‘Computer Applications’. The up-dated master model provides the base for economic evaluation and mining projection of the ore- body.

Prior to establishing pit limits, a costing exercise is carried out to define the economic cut-off grade (cur- rently established at 3.0% C.C.R.G.). This exercise can be complicated due to the probability that much of the low-grade material below a given cut-off grade would necessarily be mined as waste due to access considerations.

Also prior to establishing or revising pit limits, structural geology must be evaluated to derive an optimum slope design for various zones of the proposed pit. It should be noted that one of the advantages of phase mining is that intermediate phases may be treated as trial slopes so that design parameters can

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