GEMS ComparisonofPolygonalandBlock

Comparison of Polygonal and Block Model Reserving Techniques in Gemcom A Case Study on a Thin Reef Deposit Pieter-Jan Grbe1 and Warren P. Johnstone2 1. Terra-Search Geological Consultants (Pty) Ltd. 2. Gemcom Africa (Pty) Ltd.

WHITE PAPER

*Gemcom Software was acquired

by Dassault Systmes, the

3DEXPERIENCE Company, in July

2012. It is now known as GEOVIA.

Gemcom GEMS Comparison of Polygonal and Block Model Reserving Techniques in Gemcom

Copyright 2011 Gemcom Software International Inc. Page 2 of 10

Synopsis This paper compares the results derived from two sets of reserve reports. The two sets of figures represent reserves of a tabular orebody calculated independently using polygonal and block model reserving techniques in Gemcom software. The objective of this case study is to demonstrate that the use of a block model to calculate reserves is not only as accurate, but more effective than the more traditional approach of polygonal reserving. This was achieved by applying polygonal reserving techniques to an orebody best suited to this approach by virtue of relatively uniform geometry (attitude and thickness) and grade distribution. The LG6 Chromitite Layer within the Bushveld Igneous Complex is an ideal example of such geometrical consistency.

The two methods made use of exactly the same drillhole dataset and the same orebody solid model. For the purpose of this paper the tonnage and grade (single element) were calculated for each solid. The resulting tonnage and grade estimates are remarkably similar suggesting that the methodology applied in both techniques is sound. However a closer examination of the assumptions reveals that the block model technique is more comprehensive. This finding is complimented by the speed, ease-of-use and downstream applicability of the block model module in Gemcom.

Introduction The comparison presented in this case study was prompted by the widely held assumption that polygonal reserving is best suited to the reserve calculation of thin seam orebodies stretching over a few kilometers along strike and down dip. In contrast, block modelling has often been perceived to be applicable to more bulky, massive orebodies and by inference, less effective when applied to thin seam orebodies.

The orebody under discussion is the LG6 Chromitite Layer mined at Waterval West Mine (Fig. 1). Waterval West Mine is a producing mine, mining within the limits of the Bushveld Complex. The mine is situated just to the east of Rustenburg Town in North West Province, South Africa. The mine is one of three chromitite producing mines supplying the ferrochrome furnaces of Alloys Division of Xstrata South Africa. Xstrata is listed on the London Stock Exchange and prescribes to the Australasian Code for the Reporting of Mineral Resources and Ore Reserves. It is imperative for Xstrata to use an accurate and reliable method for calculating reserves in order to present annual reserve statements.



Geological Setting & Orebody Geometry The Bushveld Complex is a large igneous complex consisting of differentiated ultra-mafic to felsic igneous rocks. The rock units are psuedo layered, dipping gently towards the center of the complex1. The complex covers an area of 450km west to east and 300km north to south (Fig. 1).

Figure 1: Geological Setting of Waterval West Mine

The Bushveld Igneous Complex contains at least 18 distinctly named chromitite layers, these occur in 3 groups e.g. a Lower-, Middle- and Upper Group and are part of the Rustenburg Layered Suit (Fig. 2). The group in which each layer occurs is reflected in the prefix of the layer name (e.g. LG1 LG7, MG1 MG4 and UG1 UG2).

The LG6 (Lower Group No 6 Chromitite Layer) and the MG1 Chromitite Layers are the two economic chromitite layers currently mined. The Waterval West Mine produces charge chrome from the LG6 Chromitite Layer. The layer consists of two chromitite sub-layers (LG6 and LG6A) separated by a middling pyroxenite. The layers are mined as a package using trackless mining methods. For the purpose of this exercise only the LG6 is modelled.



Figure 2: Stratigraphy of the Rustenburg Layered Suite

The LG6 Chromitite is dipping at 13 with a thickness averaging 0.98m over a strike distance of 3,600m. Certain geological features complicate the continuity of the chromitite layers by creating discontinuities in the layering faults, potholes, transgressive dykes and intrusive Iron-Rich Ultramafic Pegmatite bodies (IRUPS).



Geological Modelling Geological data were incorporated into Gemcom from surface drilling and underground mapping. Aeromagnetic survey data were also available.

The Gemcom Workspace for the Waterval Project consists of typical drillhole information organized as follows:

HOLE-ID FROM TO THICKNESS CR FE SI CR:FE SG

The drillhole data in the Gemcom Workspace was used to present two sets of point data, the LG6 FROM intersections and the LG6 TO intersections. These points were used to create two surfaces representing the top and bottom contacts of the LG6 Chromitite Layer. Both surfaces were expanded to cover the entire mine license area. The two surfaces are very close together in relation to the strike and dip extent of the surfaces. The Global Coordinates (XY Plane) selection under the TIN Plane Selection menu proved the best way to create the surfaces with the least resulting errors. The two non-intersecting surfaces were stitched together to create a solid representing the LG6 Chromitite layer. The solid were then validated by means of a Report Volume/Size of Single Solid and through the Check Solid/Surface Validity routine to make sure no solid problems will be encountered later with the block modelling.

Four categories of clipping polygons were constructed to clip the primary solid into areas representing continuous mine-able reef. The first boundary, starting from the outside working towards the center, was the mine license boundary. The second set of clipping polygons were all the geological boundaries represented by the out-crop and a major dyke traversing the mine license area. The final clipping polygon was the mined-out area, which had to be clipped from the solid reef area. These clipping polygons were used to clip the LG6 reef solid to 6 individual solid blocks, which were separated by the above-mentioned features. These polygons were converted and saved to the polygon workspace for the polygonal reserving.

Methodology The overall aim of the reserving exercise was to report the in-situ reserves for the mine with the corresponding chrome (Cr2O3) grades.

The alternative reserving methods were both applied from first principals but differed substantially in approach. For polygonal reserving a new polygon workspace needed to be created in Gemcom, whereas for the block modeling method a seam or layer solid and a block model project needed to be created. The two procedures are described below:

Polygonal Reserving

It is highly recommended that the whole process be thought through and planned before the exercise is started. Carefully create all necessary polygons and save them into the polygon workspace. Plan the sequence in which the polygons will be clipped against each other (e.g. mine boundary, geological boundaries and mined-out boundaries).

Once the correct polygon and working outlines have been setup, the following procedural steps are necessary in Gemcom:

Create reserve polygons from drillhole data stretching over the mine boundaries (save as Reserves).

Clip & Close Reserves polygons against boundary polygon outline (save as Res-1)

Clip & Close Res-1 polygons against geological boundaries (save as Res-2).

Clip & Close Res-2 polygons against mined-out boundaries (save as Res-fin).



Figure 3 shows the final polygons resulting from the clip and close procedure. These polygons are saved in the polygonal workspace which in the case of this study is an ACCESS Database.

To calculate the reserves, export (copy) the data to an Excel spreadsheet and manipulate the data to the required format e.g. totals for tonnages, weighted averages for grades etc.

Figure 3: Plan View of Reserve Polygons in Gemcom.

Block Modelling

The same polylines were used to define the reserve block boundaries for both polygonal and block model reserve calculation techniques. The polylines demarcated the mined-out areas, the mine license boundary and the boundaries of all the main areas rendered unmineable by unfavorable geological conditions.

Once the correct drillhole intersect selection options have been setup, the following procedural steps are necessary in Gemcom:

Create from and to surfaces from drill hole intersects.

Create solid from two non-intersecting surfaces.

Clip solid with mine boundary, geological boundaries and mined-out clipping polygons.

Validate resulting solids.

Once the solids have been created and validated the exercise can progress to the creation of a block model project for the orebody.

Create a block model project, which completely covers the orebody in 3D space.

Update the block model folder from solids.

Populate the block model with grade using an appropriate kriging profile.



Once the block model has been populated with rocktype, density, percentage and grade data, the final procedural steps are to report the tonnages through the volumetric menu in Gemcom as follows:

Set-up all profiles and formats for the reserve report.

Define source parameters (geology solids for rock code, block model for grades and block model for density).

Create reserve report.

Figure 4: Plan View of Block Model populated with Cr2O3 Grades.

The colour profile indicates variable grade distribution.



Results Polygonal Reserving

Area 2 774 325 m2

Tonnage 11 081 647 tonne

Cr203 Grade (weighted average) 41.74%

Block Model Reserving

Area 2 663 722 m2

Tonnage 11 027 808 tonne

Cr203 Grade (weighted average) 41.37%

Summary

Parameter Polygonal Reserves Block Model/Solids Reserves % Difference

Tonnage 11 081 647 tonne 11 027 808 tonne - 0.49%

Cr2O3 Grade 41.74% 41.37% - - 0.89%

The following input data was exactly the same for both calculation methods:

Drillhole data

Reserve Block boundaries

SG (Specific Gravity for tonnage calculation)

Discussion The end results are remarkably similar. The input data used was exactly the same so any expected or resulting differences cannot be explained by the data used. If there were supposed to be a difference in the result it would have to be the reserve calculation method, which in this comparison is not the case.

However, it could be argued that every cell or block in the block model represents all the data points (drillhole intersects) contained within the search radius of the kriging parameters and not dependant on one data point only, as is the case in the polygonal reserves. The block model reserve is more representative of the data set as a whole at any given point in the solid.

The close result of the totals for the tonnage and grade could be a function of the fact that the data set is viewed globally and local differences and trends have not been compared.

The close result in the comparison of the two methods in this exercise will not necessarily be the same in other data sets. This is probably the result of the uniform geometry (attitude and thickness) and grade distribution observed in the LG6 Chromitite layer at Waterval West Mine.



The validity of the solids that are used is very important and care should be taken to minimize complex polylines, which dissect and over-complicate polygons. Despite this, the system was capable of supporting the numerous small polygons resulting from the intricate boundary line of the mined-out area at Waterval West Mine (See Fig. 3).

In Competent Person reserve estimation reports, care should be taken to describe methodologies and critical assumptions utilized in the reserve modelling. A number of published papers and reserve-reporting guidelines strongly recommend that block model results be checked against raw and composited data3. This check can be done by comparing the block model against drillhole data.

The CIM Best Practices Guidelines3 recognizes the fact that models can be generated by other methods (cross-sectional, polygonal etc.) with or without the use of software. It assumes however that reserve estimations that are included in feasibility studies will be created through block modelling.

Duke and Hanna2 also suggest that for the validation of a resource block model there is no substitute for comparing between the model and the relevant borehole data plotted systematically on a full set of plans and sections.

The comparison made in this exercise presents an appropriate method of evaluating thin seam block models through the polygonal method which represents real data e.g. real drillhole thicknesses and grades.

Conclusions From the results of this exercise, the following conclusions can be drawn:

There is no appreciable difference between the results of the two methods.

The block model method applied to the thin seam deposit in this case study, proved to be valid and accurate.

Spatial trends relevant at the scale of mining can be distinguished from the block model plots. These trends are not discernable in the polygonal reserve plots.

Polygonal reserving can be an effective way to validate primary reserving through block modelling.

In summary the tonnage and grade estimates are remarkably similar suggesting that the methodology applied in both techniques is sound. However a closer examination of the assumptions reveals that the block model technique is more comprehensive. This finding is complimented by the speed, ease-of-use and downstream applicability of the block model module in Gemcom.



Acknowledgements The authors would like to thank Xstrata South Africa (Pty) Ltd for permission to use the data and findings presented in this paper.

References 1. Eales, H. V. A. (2001). FIRST INTRODUCTION TO THE GEOLOGY OF THE BUSHVELD

COMPLEX and those aspects of South-African geology that relate to it. Popular Geosciences, Series 2. The Council for Geosciences, Geological Survey of South Africa. pp36-38.

2. Duke, J. and Hanna, P. (2003). Computer-based Resource Estimation In Accordance With The 1999 JORC Code. ECS International (Pty) Ltd. (2003). http://www.ecsi.com.au/

3. Estimation Best Practices Committee of the Canadian Institute of Mining and Metallurgical and Petroleum, (2002). ESTIMATION OF MINERAL RESOURCES and MINERAL RESERVES BEST PRACTICE GUIDELINE. http://www.narural-resources.org/

This document gives only a general description of products and services and except where expressly provided otherwise shall not form part of any contract. Changes may be made in products or services at any time without notice. Copyright 2011, Gemcom Software International Inc. Gemcom, the Gemcom logo, combinations thereof, and Gemcom GEMS are trademarks of Gemcom Software International Inc. All other names are trademarks, registered trademarks, or service marks of their respective owners.

The solutions you expect. From the people you trust.

GEMS ComparisonofPolygonalandBlock

Documents

Transcript of GEMS ComparisonofPolygonalandBlock