ABSTRACT:

In situ stress measurements made between 1200 and 1500 meters depth at Minnova Inc.'s Ansil deposit resulted in highly unusual in situ stress tensors seemingly unrelated to the normal regional field stresses observed in the Canadian Shield. Further analysis of the field and numerical model data suggests that the massive sulphide orebody is behaving as a soft inclusion relative to the brittle host formations. Finite element analysis was used to obtain a preliminary calibration to the measured field stress values and thereby to induce the most probable upper bound for the far field stresses.

1 INTRODUCTION

An in situ stress measurement program was conducted during the winter of 1988 at the Ansil mine, located 500 km northwest of Montreal, Quebec, Canada. The massive sulphide deposit is located between 1200 and 1500 meters below surface. It has an ellipsoidal mushroom shape varying in length from 100 to 140 meters in the upper Zone. The flattened tail of this "mushroom" is 15 meters thick and has a length of 365 meters, dipping at 50 degrees east. A peculiarity of the orebody is the almost 90 degree rotation between the strike of the upper and lower zones. The core of the orebody is rich in chalcopyrite (CuFeS2) and pyrrhotite (FeS) while the outside shell contains mostly sphalerite (ZnS) and pyrite (FeS2). Initial far field stress assumptions were based on published regional data, (Herget, 1982, 1987). The vertical stress was assumed to increase linearly with depth based on an estimated average density of 2700 kg/cu.m. The average horizontal stress was considered to be extremely high based on measurements made elsewhere in the region. Herget's relationship (Figure 1) results in a horizontal to vertical stress ratio of 2.6 at a depth of 1200 meters, (σv = 32 MPa; σ HMAX = 84 MPa).

2 HISTORY OF THE ANSIL DEPOSIT

The Ansil copper deposit was discovered in 1981. A 1500 meter deep circular production shaft was sunk in 1984 and drill drifts were driven on the 7, 8 and 9 levels for final orebody delineation. The circular concrete lined production shaft is 6.71 meters (22 ft) 0.D. and 5.49 meters (18 ft) I.D. The total cost for developing the project to the production stage was $65 million Can. The ore zone is situated between a footwall rhyolite and a hangingwall andesite. Each formation is locally chlorite altered adjacent to the orebody and contains roughly three joint sets widely spaced. Index testing of the key rock units results in the average elastic properties listed in Table 1. Drill core RQD's suggest that the massive sulphide can vary from very good, (RQD = 100%), to very poor, (RQD = 20%). As more exposure of the ore has become available it has been observed that, although it is only lightly jointed, (one horizontal joint set), the material itself is often very weak and can be broken by hand. The sulphide was observed to break along small pre-existing fractures that often appeared rusty.

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