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Case histories |
1 Department of Geology University of Wales, PO Box 914, Cardiff CF1 3YE, UK
2 Vieille-Montagne Sverige, Centrumvägen, S-690 42 Zinkgruvan, Sweden
The use of geostatistics in ore evaluation studies is based on the relationship between point values and their spatial distribution. When an orebody is deformed the syn-depositional spatial relationship between control points is altered and it is extremely difficult to represent these points on a single plan suitable for ore reserve estimation. To overcome this problem, deformed orebodies could be dissected into a number of individual units and analysed separately, but this results in a reduction of control points available for analysis. An alternative to orebody dissection is to unroll the deposit so as to return control points back as close as possible to their original relative spatial locations.
This paper describes attempts to reconstruct the Nygruvan orebody structurally at Knalla mine, Zinkgruvan, Sweden, which is operated by Vieille-Montagne Sverige. The deposit is located in the Bergslagen geological province of south-central Sweden, and has features in common with stratiform volcanogenic massive Zn-Pb sulphides and sediment hosted exhalative Zn-Pb deposits. Nygruvan is essentially a thin (5–20 m) tabular deposit of large lateral extent, dipping at 60–80° to the northeast, which was folded and faulted during the Svecofennian orogeny.
Current ore resource evaluation methods employ the polygon method for global resources using diamond drill information, and the extrapolation of chip sample data between mine levels to produce reserves of mining blocks. These methods do not take account of orebody structure, assuming it to be a planar structure of variable thickness.
Structural reconstructions of thrust belts and deformed basins are usually achieved by drawing sections in the plane containing the movement vector. Sectional reconstruction methods do not, however, give consideration to transverse movements. If strain is triaxial, a two dimensional restoration using cross sections is not appropriate, instead a three dimensional method should be used. To date little work has been done on the development of a suitable method applicable to deformed orebodies
A small area of the mine has been selected for preliminary studies and a variety of manual methods applied in order to develop a suitable computerized restoration algorithm (these include two dimensional sectional and triaxial methods). A new Datamine unrolling package, which will be tested on this deposit, is described. A geostatistical study will be undertaken to generate a block reserve model once the deposit has been unrolled satisfactorily.
