Geological Modelling

GEOLOGICAL MODELLING AND EVALUATION OF NICKEL LATERITE DEPOSITS 1 Graeme Lyall 

Abstract The development of geologically realistic resource models for nickel laterite deposits is hindered   by their extensive geometry and complex grade characteristics. Thickness modeling with digital terrain surfaces is appropriate for straightforward examples, however, more complex deposits will require additional enhancements. Examples are included to illustrate methods that have been employed on deposits evaluated by Anglo geologists in South American. Grade interpolation should take cognizance of the characteristic presence of vertical trend profiles and the multivariate behavior of the variables that are to be estimated. This is of particular importance if simulation exercises are to be performed. The tabular geometry and presence of grade trends may lead to problems when using kriging algorithms as the interpolation method. An example showing how this problem can be minimized is provided. Most of the techniques described were developed using the DATAMINE software package. The versatile nature of this software was beneficial in developing the innovative tools used in these studies. Generalised Nickel Laterite Profile Nickel laterite deposits form by surface weathering and leaching processes in tropical and sub-tropical climates.  Typically, these phenomena result in three main mineralized units (laterite, saprolite and hard rock), which can be pictured on the cross section in the figure below. Characteristic vertical trends in nickel and iron grades are also shown. The unweathered fresh rock at the base has a dunitic to peridotitic composition, of which the principal constituents are approximately 40% SiO 2, 35% MgO and 8% Fe. Nickel grades in this unit are sub-economic.   The laterite unit is characterized by Fe enrichment (>30%) and SiO 2 and MgO depletion (generally both < 10%).  The highest Ni grades are encountered within the saprolite zone, which shows compositions between fresh rock and laterite.

Typical cross section

100 m

Laterite Saprolite

Fe

Ni

Fresh Rock  Vertical Grade profiles

1

Anglo American Chile – Assistant Manager of Mineral Resource Division

Geological Modeling in DATAMINE Technique for generating “optimal” drillhole coding In most laterite deposits, fairly abrupt grade boundaries are observed between the Ferich laterite unit, the underlying Ni-rich saprolite and the low Ni grade hard rock at the base, as can be observed from the grade profiles in the figure above. The identification of these zone boundaries is commonly a manual process done on the inspection of the drill hole grades. Moreover, considering that some of these deposits may cover extensive areas and the number of drill holes are often of the order of several hundred and sometimes in the thousands, this manual process can be a significant task. To alleviate this, an automatic method was developed using DATAMINE processes to identify the “optimal” intercept for each mineralised unit. In summary, this involves an iterative compositing procedure based on previously established cut-off grades for each unit. The process defines the top and bottom of continuous “mineralised” intercepts and identifies the optimal interval for each unit. The optimal composite interval will include samples falling below the established cut-off grades only if the contained metal (above cut-off) in sample extensions to the interval exceeds the loss of contained metal (below cut-off) in the waste samples. Both Fe and Ni cut-offs can be considered. The figure below illustrates the results of this process. Geological zonation using optimal composite algorithm High Fe Laterite Ni-rich saprolite

Ni-rich saprolite Waste Included

Waste Not Included

Hard rock base Hard rock base i N % 9 . 0

e F % 5 3

i N % 9 . 0

e F % 5 3

The above procedure proved very useful during evaluation studies for carried out on extensive drill hole data that required a relatively rapid evaluation. On reviewing the results, the “optimal compositing” procedure provided intercepts almost identical to the manual determinations. Loma de Niquel The modeling of the laterite-saprolite and saprolite-hard rock interfaces is best done by generating thickness models for the above units, by this way avoiding cross-overs with the surface topography. For the Loma de Niquel deposit, cross sections are interpreted in DATAMINE at 50 metre intervals. These are then used to generate 2D thickness data spaced at 10 metre intervals along each section. Additional thickness data are provided from the drill hole intercepts and from horizontal delineations indicating the areal extents of the mineralized unit (thickness=0). All three data types are used to interpolate 2D thickness models on a 5x5 metre grid. A surface elevation model is also generated on the same 5x5 metre grid. These are illustrated in the figure below.

Thickness Data

Gridded Surface Elevation

Drill hole intercept Cross section interpretation Limits of unit – thick=0

Gridd Gridded ed La Later terit ite e thic thickn knes ess s

Gridd Gridded ed Sapro Saprolilite te Thic Thickne kness ss

From the 2D grid model containing surface elevation, laterite  and saprolite thickness, the elevation of the laterite-saprolite and saprolite-hard rock interfaces can be calculated. Digital terrain models are then generated using the gridded models together with the data used for the interpolation (cross sections, drill hole intercepts and horizontal limits). These are used to develop block models followed by grade estimates. The figure below illustrates these procedures.

Cross section interpretation

DTM’s generated using thickness models

Block Modelling and grade estimation

A more complex example An example of a more complex deposit where a total of five units have been recognized in the vertical profile is shown in the figure below. In addition, these units are often discontinuous meaning that in many cases not all of the units will be present.

Vertical Profile Outcropping waste Acid Ore  I nter na nall W as astt e  Basic Ba sic Or e 

Hard Rock Base

Thickness modelling techniques were also used on this deposit to generate the surface wireframes for the base of each unit (see figure below) and the geological block model.

Wireframe Surfaces Outcropping waste Acid Aci d Or e  I nter nal Wast e  Basic Ba sic Or e 

Barro Alto At Barro Alto in Brazil the laterite development can be classified into two main types. Flat lying areas (ETO and PTO areas) show typical laterite profiles similar to the Loma de Niquel deposit, however, approximately half the mineralisation is characterized by much thicker and complex profiles (WTO areas) cross-cut by sub-vertical sub-vertical chalcedonic chalcedonic and internal waste bodies as is shown in the cross sections below. WTO

ETO SE

NW

OVERBURDEN ACID ORE BASIC ORE ORE CHALCEDONY WASTE

Acid ore - SiO2/MgO SiO2/MgO>2.5 >2.5 Basic ore - SiO2/MgO