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Computer application in a mining project.

In the early 1970s the Brazilian Government decided to increase mining activity in the country by offering geological data from its National Department of Mineral Production to large mining companies. Using some of this information, Rio Capim Caulim was formed in mid-1992 to carry out exploration of areas located in Para State, east of the Tocantins River. Economic kaolin mineralisation was encountered and the resulting project is scheduled to start production during 1995.


The orebody is located in a region of smooth terrain, with several plateaus consisting of low benches and stepped terraces. The hydrology is dominated by the Capim River which originates in the southeast of the State of Para. The river is about 600 km long, running in a south-north direction until it meets the Guama River.

Kaolin lenses are located in the Barreiras Formation of Pliocene age covered by Quaternary sediments throughout the Capim River valley. The stratigraphy observed during exploration, from top to bottom, is:

* Silt-sand top soil, of a creamy colour below the vegetation layer, becoming stained at depth, bearing loose blocks of ferruginous lateritic pisolites;

* Varigated clays, in different hues (red, yellow, violet and brown) usually presenting stains of kaolin material, with intercalations varying from a few centimetres to 2 metres of red sand of variable granulometry;

* White kaolin, sometimes slightly rose or cream-coloured, with minimal sand content, generally hard at the top. The middle layers are normally of whiter material;

* Argillaceous sandstone, a kaolinic red sand of variable granulometry, presenting at times a cross-section stratigraphy and an alternation with red, violet or yellow clay, and beds of conglomerate hardened kaolin gravel.

Geological research was conducted through 1,018 boreholes and drillholes totalling 22,052 m of drilling. Samples taken to the laboratory were analysed for the following factors: crude brightness, grit fraction (+325 mesh), brightness of the degritted fraction, viscosity (Brookfield & Hercules), and abrasiveness.

Ore reserves were initially calculated using the classic method of polygonal areas of influence and the results presented in the research programme's final report with the approval of the National Department of Mineral Production.

For the purpose of increasing the level of information on the ore and therefore having more data for the mining plan, the reserves were re-evaluated by geostatistical methods.

Reserves evaluation by geo-statistics

To build the database, borehole and drillhole information was transferred to a computer. Software was specially developed to manage the DBF format files. It was written in xBase (Clipper 5.01) and named Geodata. Through specialised routines, the data was transferred to the microLYNX Plus software with which the reserve evaluation studies were done.

The main variables for the study were as follows:

* ALVNATU = Brightness of the crude kaolin;

* RESIDUO = Percentage above 325 mesh;

* ALVDESA = Brightness of the degritted fraction minus 325 mesh.

The area's topography available in 1:2000 scale digitised maps was digitised into the computer. In order to increase detail and topography confidence, the co-ordinates (x,y and z) of each borehole or drillhole collar was also added to the topographic files. Using the triangulation algorithm, the digital topography surface was remodelled.

Geologic interpretation

The delineation of the orebody followed two basic criteria. The first refers to the determination of mineralised thickness (vertical limits), while the second refers to the orebody's horizontal limits.

Both criteria were applied with the assistance of the above-mentioned software, which allowed the determination of the vertical sections. The projection of the boreholes and drillholes with zone of interest variables classified by colour, provided the necessary elements for on-screen geologic interpretation. This was accomplished by the use of a mouse and the 3-D interactive features of microLYNX Plus.

Ore modelling

Due to the ore lithology, it is usual to adopt one of the following solutions:

* 2-D model using the grade accumulation concept;

* 3-D model for blocks of the same size.

However, in this case a third model was chosen based on blocks with constant base and variable heights. The values of footwall and hangingwall of each block were automatically established from the triangulated surfaces.

The previously interpreted East-West sections were used to generate the top and base zones of the waste and mineral layers. The top and bottom points of the hangingwalls and footwalls were transferred to different files. To obtain a complete 3-D interpretation of the surfaces, triangulation of these files was made in the same way as in the topographic modelling. As the generation of these surfaces was based on the section interpretation, a perfect collar adherance was attained.

Several models were defined using 20m x 20m base blocks, with extensions compatible to the kaolin lenses. The system uses the origin of this model and the blocks' base dimensions and expands the centroid upward and downwards to the hangingwall and footwall intersections of each layer. This is done in order to generate the blocks with variable heights. It is interesting to note that this method allows the modelling of an unlimited number of layers, which may be part of the final model. In this particular case, there were two layers, one of overburden and one of mineral in distinct horizons.

Geostatistical analysis of


In order to establish the spatial correlation necessary for the kriging estimates, the main variables of the zones of interest were modelled using semi-variograms.

The experimental semi-variograms were calculated in 3-D. Because no significant anisotropy was detected, a global variogram was adopted for the horizontal plane (at 180). On the other hand, a strong geometric anisotropy with drift was verified in the vertical plane. It was then decided to adopt the semi-stationary hypothesis, allowing the application of linear random kriging to be conducted in 3-D.

All the experimental variograms were adjusted for spherical nested schemes, suggesting the existence of two structures, a small one and a large one, as shown in Table 1.
                   Table 1: Variographic analysis
                  Variographic parameters


Orientation  Nugget  Structure   Model     Range  Threshold
             (%GE)2                          (m)    (%GE)2

Horizontal/    4.0       1      Spherical    140      15.0
global                   2      Spherical   1100      15.2

Vertical       4.0       1      Spherical    2.6      15.0
directional              2      Spherical    4.2      15.2

Variable          Variographic parameters

Orientation  Nugget  Structure   Model     Range  Threshold
              (%)2                           (m)      (%)2
Horizontal/    0.5       1      Spherical    170       7.0
global                   2      Spherical   1100       5.7
Vertical       0.5       1      Spherical    2.8       7.0
directional              2      Spherical    3.8       5.7

Variable          Variographic parameters

Orientation  Nugget  Structure   Model     Range  Threshold
             (%GE)2                          (m)   (%GE)2

Horizontal     4.0       1      Spherical    140      13.2
global                   2      Spherical   1100      14.4
Vertical/      4.0       1      Spherical    2.6      13.0
directional              2      Spherical    4.2      14.4

Kriging estimates of block


The contents of the variables in zones of interest were estimated using the 3-D linear kriging method and the results of the variographic analysis. The errors related to the content estimates were considered compatible with:

* The level of detail of the research data;

* The dimensions of the blocks used;

* The precision required at the present project stage.

Mine planning

The mining of the mineral from the kaolin lenses will be done by the strip mining method, which consists of the following operations:

* Deforestation and preparation of the mining areas;

* Overburden removal and exposure of mining areas;

* Mineral excavation and transportation;

* Reclamation of degraded areas.

Bulldozers will be used for the deforestation and preparation. Motor-scrapers will be used for the overburden removal. The ore will be removed by trucks and backhoes. Reclaiming will be managed through the use of bulldozers and regegetation will be carried out with intensive usage of native specifies of plants.

The strips will be 48m wide, except for the initial box cut which will be 60 m wide. Strip mining involves a great number of simulations that take into consideration the following factors:

Postion and geometry of the initial box cut;

Strip geometry (orientation, dimension and sequence of advance).


The numerous tasks in mine planning previously conducted on drawing boards can be speeded up and enhanced considerably by the use of computers, in particular the calculations and graphic displays. The time saved in these tasks can be re-allocated to analysis and planning.
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Author:Nader, Bek; Valadao, Charles S.; Ferreira, Antonio F.
Publication:Mining Magazine
Date:May 1, 1994
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