The Role of Orebody Geological Controls in Mine Planning and Design
Orebody shapes, types, composition, and origins are inseparably linked to the geological history of the region in which they occur. The same geological history dictates their stress environment. The geology of ore deposits is thus complex. Modern theories of ore deposit classification are based on genesis and environments of deposition rather than form, texture and the mineral contents. While these classification systems are important in economic geology they are not critically relevant in mining and geomechanics.
Classification of orebodies based on shape/structure aid in their safe and economic extraction, and is of more interest to exploration geologists, mining and rock mechanic engineers. The shape classification system differentiate sheetlike deposits (e.g. syngenetic seams, les-shaped deposits, veins) which are two-dimensional, stocks which are 3-dimensional (e.g. metasomatic replacement bodies) and impregnations or disseminated deposits. Orebody structural controls can be divided into regional and local/detailed. Local structural controls such as fissures, shear zones, folding, faulting and dykes refer to the detailed structures associated with the orebody or sections of an orebody.
The existing state of virgin stress in a rockmass is the cumulative product of events in the rockmass geological history. Orientation of principal stresses is severely influenced by both regional and local geological structures. The geometric relationship between an orebody and the in situ state of stress dictates how safe and economic the orebody can be extracted. Specifically, it dictates the stability of the excavations used in the extraction of the orebody. The shape of an orebody and its local structural controls can render whole or part of the orebody subjected to adverse loading. Changes in local orebody geometry and immediate geology is not often given the desired attention during mine planning and sequencing and in geomechanics. Recent studies show that about 50% to 70% of mining problems are linked to geology. These problems include inadequate use of geology as input in geomechanics design and in mine planning and sequencing.
This paper discusses the importance of geological controls in geomechanics and mine planning and design using case examples. In particular, the influence of geological controls on the in situ stress state and the potential consequences on mining is emphasized.