By Ignatious Kamwanje
Mineral deposit models are important in quantitative resource assessments since the grades and tonnages of most deposit types are significantly different and mineral types occur in different geological settings that can be identified from geological maps. Mineral deposit models are the keystone in combining the diverse geoscience information on geology, mineral occurrences, geophysics, biogeography and geochemistry used in resource assessments and mineral exploration. Well-designed and constructed deposit models allow geologists to know from observed geological environments the possible mineral deposit types that may exist and allow economists to determine the possible economic viability of these resources in the region. Thus, mineral deposit models play the central role in transforming geoscience information to a picture that is more useful to policymakers.
Probably the most important part of synthesizing mineral deposit models is the planning stage, in which consideration of the purpose and possible uses of the models should determine the character of the models. Ideally, deposit models would provide the necessary and sufficient information to discriminate possible mineralized environments, types of known deposits from each other, and mineral deposits from mineral occurrences. Ore deposit Models used in assessment include descriptive models, Grade and tonnage models, deposit density models, economic model and quantitative models.
- Mineral Resource Estimation
Most resource estimates are now undertaken by the use of computer-based block modelling methods. For these block models to be valid, it is best practice that they are constrained by the controlling geological elements of the deposit, such as geological boundaries, structure and the spatial distribution of grades and other attributes. These controls are derived from the geological interpretation carried out by the resource geologist during the resource modelling process. Geological interpretation initially involves the use of all relevant data to define the domain boundaries, which differentiate the various zones of mineralization in the deposit. These domain boundaries are incorporated into the computer model using such tools as wireframes, triangulation and gridding. The geologist then undertakes statistical and variogram analysis of grade data within these domains. This analysis provides the geologist with a measure of the spatial distribution of grades within each domain as in search radius and a means of defining an appropriate set of parameters which can be used in block grade estimation. The resource model is thus built by interpolating these domains with blocks and grades which should accurately reflect the geologist’s understanding of spatial distribution of grades within each domain. Visual checking of the model against the relevant data is essential, as is progressive updating and refinement of the model as new data becomes available. As the resource database expands through feasibility to mining, the need for updated geological interpretation to control the resource model is no less important than during the exploration phase of the project. Without this continuing geological control the model ceases to be a valid base on which to make mine planning decisions.
Resource estimation is used to determine and define the ore tonnage and grade of a geological deposit block model that is developed by a resource geologist. There are different estimation methods that are used for different scenarios and this depends on the ore boundaries, geological deposit geometry, grade variability and the amount of time plus cash that is available. A typical resource estimation involves the construction of a geological and resource model with data from various sources. In a computer generated resource model, the important and significant steps of resource estimation are:
- Development, standardization and data validation of the database.
- Plotting of sections and geological modeling.
- Geostatistical analysis using the necessary tools and Q-Q Plots.
- Ore block modeling and estimation.
2. Geological Modeling
A geological model consists of a 3Drepresentation of an ore body that is constructed by resource geologist on the basis of the knowledge of the known deposit using such methods as reconnaissance/geological surveys, geophysical surveys, drillhole lithologs and assays. A geological model that represents the spatial locations and extents of rock types or ore types is an essential input for mineral resources evaluation and mine planning and, as such, affects all subsequent stages of the mining process. The typical workflow for assessing mineral resources consists of grouping the rock types or ore types into geological domains in which the quantitative variables of interest e.g. and/or geomechanical geochemical, geometallurgical, are assumed to be homogeneously distributed and then interpolating these variables within each domain using geostatistical analysis techniques.
Geological modeling is a key step prior to geostatistical estimation or running simulations for a chosen frequency of the grades within a mineral deposit. Although alteration, mineralization and lithological aspects should be considered in determining the geological model (domaining) for interpolation, common practice consists in contouring the grades and generating grade shells. Within each shell the grades are considered homogeneous and can therefore be interpreted as a realization of a stationary random function, allowing variogram analysis modeling, kriging or conditional simulation.
An orebody model serves as the geological basis of all resource estimation and it starts with a critical review of existing drill hole and surface or underground sample data as well as maps and plans with current geological interpretation. Drill hole or sample databases are set up to suit all the quantitative and qualitative information necessary to build a resource model. The creation of a geological model may include the following steps:
- 3D orebody modeling
- Sectional, longitudinal, 3D and multi-seam modeling
- Geostatistical, variography analysis of composites with spatial continuity
3. Block Model Estimation
Once the geological modeling is completed, the geological envelopes are divided into block models. Subsequently, the estimation of these blocks is done from compositing that are point measures of the grade of ore in the rock. A block model is a simplified representation of an ore body depositand its surroundings that can be thought of as a stack of computer-generated “bricks” that represent small volumes of rock in a deposit (ore and waste). Each brick, contains estimates of data, such as elemental grade, bulk density and other geological or engineering entity values.Several different mathematical methods can be used to do the estimation depending on the desired degree of precision, quality and quantity of data and of their nature.
4. Resource Block Model
Block modelling helps the geologist determine shape, dimensions, and surface details by making 3D models. By making a series of block models in different shapes and sizes it gives the resource geologist a feel for the product. The block model is created using geostatistics and the geological data gathered through drilling of the prospective zones of mineralization. The block model is essentially a set of specifically sized “blocks” in the shape of the mineralized orebody. Although the blocks all have the same size, the characteristics of each block differs from one another. The grade, bulk density, rock type and confidence are all unique to each block within the entire block model. Once the block model has been developed and analyzed, it is used to determine the ore resources and reserves (with project economics considerations) of the mineralized orebody. Mineral resources and reserves can be further classified depending on their geological confidence. This discussion will not dwell on the reserve and resource types because they were once discussed in the previous articles.
5. Block Model Softwares or Brands
The most common block model types encountered in the mining industry are, Surpac, Micromine, Datamine, Vulcan,MineSight,Leapfrog,Mine plan and Rockworks especially in coal, industrial minerals like aggregates and other stratabound economic mineral deposits just to mention but a few.