Ore Deposit Modeling

7/28/2019 Ore Deposit Modeling

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DRILLING METHODS

ORE DEPOSIT MODELING

Objectives - The student will be able to:

Explain a geologic model. Describe the purpose of a geologic model. List three models used to calculate the weighted average grade of a

deposit.

Explanation

The ultimate objective of all exploration is to predict, with the greatest accuracypossible, the shape, distribution, and concentration of ore before miningbegins. This involves the creation of a geologic model. The purpose is to use a

three dimensional model to calculate the quantity and quality of the oredeposit. Modeling looks at the three dimensional perspective or an ore deposit,by combining drill section information with surface information. The informationis examined along both horizontal slices (plan maps) and vertical cross sections(drill sections). The geologic model is one interpretation of thesubsurface. Different interpretations can lead to drastically different results(Figure F15), so great care must be taken in choosing the best model.

Part of the purpose of ore deposit modeling is to predict the geology. This isdone by extrapolating outward from known sample data into areas which arenot yet drilled. Modeling involves prediction of the grade (concentration) andtonnage (weight) by using a geometric model. Volume formulas for the specificgeometric shapes can be used to calculate the volume. For example, a veindeposit could be estimated using a “slab” shape, where: volume= length x width x thickness. Geologic modeling for mine planning is typically based on a large amount of drilldata (close-spaced drill holes). Drill holes are generally placed along a paralleldrill lines, to make the calculation of the volume easier. A square grid isconvenient, but not essential. The chance for misinterpretation is greatlydecreased by more drilling, but this is generally also followed by some type of excavation, usually trenching at the surface or drifting underground, to further

decrease the risk or error in estimating the resource.

Tonnage and grade calculations are done using geostatistical methods, whichare most easily done with a computer. However, even the computer programsrequire input of geologic model before a calculation is made. After the volumecalculation is complete, the volume is converted to weight by multiplying thevolume by the “tonnage factor”. The tonnage factor states the density of the oreand host rock, in terms of cubic feet per ton. Various methods can be used tocalculate the grade of the material, but simple arithmetic averaging of thesample values usually provides a reasonably good approximation.

http://www.dmtcalaska.org/exploration/ISU/unit4/u4vocab2.html#Extrapolating





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Figure 15: A) Drill intercepts interpreted to represent large, laterallycontinuous ore bodies. B) Same drill intercepts interpreted to representdiscontinuous, small scale ore bodies.

Block Models

Block models are the simplest of all models which can be used (Figure

F16). The method is as follows:

1. Draw the outline of the ore body on a vertical drill section and planmap. Then draw a series of stacked, blocks which encompass and bestapproximate the shape of the ore body. Rectangular blocks are theeasiest to use, but other shapes can be used as long as the area can becalculated. Use a consistent block thickness of at least 20 feet. For aninclined, tabular shaped deposit, a single inclined block may sometimesbe used.

2. Draw the blocks on a plan map, one layer at a time, using a different planmap for each layer. The midpoint between adjacent drill lines is used asthe boundary between adjacent blocks.

3. Measure the area of each ore block in a map view. For a simplerectangular block this is the length X width (in feet).

4. Multiply the area of each block times the thickness of the block (asdetermined graphically in the drill section) to calculate the volume of theblock (in cubic feet), ie, Volume = Length X Width X Thickness

5. Divide the volume by the “tonnage factor” (cubic feet per ton) to obtainthe total tonnage for each block.

6. Calculate the average grade (arithmetic mean value) of each block byaveraging the values of all samples within the block.

7. Calculate the overall “weighted average grade” for the entire deposit byusing the formula:

Block 1: Block 2: Block 3 (total tons x

grade) +

(total tons x

grade) +

(total tons x

grade)= "Weighted Average

Grade"Sum of tonnages for all blocks



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A B

Figure F16: A) Block model based on geologic interpretation of a dipping orebody. The volume and grade of each block is summed for the total. B) Blockmodel based on vertical drill section, with ore blocks A, B, C and D, based ondrill intercepts.

Polygon Models

The polygon model is more appropriate for scattered, or bulk mineabledeposit. The polygon model establishes a polygon shape around each drill hole(in plan view), which is within a specified “area of influence”, or maximumdistance, from each hole (Figure F17). The boundaries of each polygon

correspond to the mid-point between two adjacent drill holes. Ore intervals areset to a specified thickness, and established in advance. The intervals,sometimes referred to as “benches” are chosen to correspond to bench miningelevations. The volume of each polygon can then be determined, although thisis generally done with sophisticated computer programs.

The average value for the polygon is calculated for each polygon by deriving thearithmetic mean value for all the values within each polygon’s depth range(thickness). Then the weighted average grade can be determined in the samemanner as used for the block model, by summing the tonnage of all polygonsfor a specified drill hole, and then dividing this into the sum of the individual

resource calculations (grade x tonnage) for each block.



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Figure F17: Polygon map of drill holes showing perpendicular bisectors andaverage grades for polygons.

Triangular Models

The triangular method is similar to the polygonal method. It calculates thevolume of a triangular-shaped prism formed between three adjacent drill holes(Figure F18). Like the polygonal method, bench levels are specified. Grade

determinations for the prisms are determined differently than the polygonmodel. In the triangular model, the grade is determined by averaging the valueof the three values at the corners of each triangle.

Figure 18: Triangular model showing drill hole locations at corners of triangles, and average value for the holes. The grade for each triangular prism-

shaped area is found by averaging the three values at the corners.



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TECHNICAL MINING TERMS

Grab Sample: A grab sample is a sample of rock material from a confined

area (< 1 foot across). It can be a single piece of rock.

Composite Sample: A composite sample consists of small chips of uniformrock material collected over a large area (> 5 feet across).

Chip Channel Sample: A chip channel sample consists of small chips of rockcollected over a specified interval. Samples are collected systematically toprovide a representative value for each interval. Samples are usually collectedin succession along a line laid out on a bedrock exposure. The method isvaluable because if mineralization is present, the width of the mineralized zonecan be determined.

High Grade Sample: A high grade sample consists of obviously mineralizedrock material which has been “high graded” or separated from less mineralizedor non-mineralized rock material.

Pitch: Angle between the horizontal and any linear feature. Synonymous with“rake”. Used to describe the linear orientation of an “ore shoot”, which is alinear zone of relatively higher grade ore within the plane of a vein.

Intercept: The depth range of a feature of interest (chemical, mineralogical,etc..) found in a drill hole in which the upper and lower boundaries are specified.

Intersect: To meet and cross at a point.



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Collar Location: The location of the mouth or opening of a drill hole or mineworking.

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Ore Deposit Modeling