Geological Domaining and Resource Estimation–A Discussion

摘要

Interpreted geologic domains provide the geologic architecture of many current models used tornestimate the potential economic resources in mineral deposits. The spatial extent of each domain isrnusually defi ned by a three-dimensional wireframe or triangulated surface.rnThe set of geologic wireframes forming part of a resource model might generally be called a spatialrninterpretation of some set of geologic features coincident with the mineralisation, based primarilyrnon logged and mapped properties of the samples and other exposures of the mineralisation. Therninterpreted spatial distribution of rock type, alteration and grade of drill hole samples aboverna particular grade threshold are commonly used criteria in constructing the geologic domainrnwireframes. Frequently, the grade threshold is closely related to the anticipated economic cut-offrngrade.rnThe boundaries or spatial limits of different domains are most commonly defi ned as ‘hard’ orrn‘soft’. A ‘hard’ boundary usually means no dependent relationship of any kind is defi ned between thernproperties of samples inside a particular domain wireframe and those samples outside the wireframe.rnA ‘soft’ boundary commonly implies that some kind of dependent relationship does prevail betweenrnthe properties of samples inside and outside the particular wireframe. Many ways are used to describernthose relationships, few of which are based on spatial analysis of the continuity of sample attributesrnacross the domain boundaries.rnIn local estimation procedures such as ordinary block kriging, ‘hard’ boundaries act as a total barrierrnto the infl uence of samples on either side of the boundary depending on the location of the block beingrnestimated. In some instances, ‘semi-permeable’ boundaries are used in which the samples inside arndomain wireframe are allowed to infl uence the estimates outside but not the reverse. This can lead tornmany undesirable effects in estimates of block grades.rnThis paper discusses the value of this approach to geologic domaining from a viewpoint of modelrncomplexity, as well as in relation to the implications for both linear and non-linear estimation methodsrnand contrasts it with alternatives. It also explores some important consequences of using ‘hard’ andrn‘soft’ geological boundaries with the commonly used modelling methods: excessive smoothing ofrnestimates near domain boundaries and conditionally biased estimates of grade at the global and localrnscale.