UNIVERSITY OF DAR ES SALAAMCOLLEGE OF NATURAL AND APPLIED SCIENCES (coNAS)

GEOLOGY DEPARTMENTCOURSE GY 413- MINING GEOLOGY

ORE BODY MODELS: What are they and challenges in creating ore body models

MAGOHE Stephen-2011-04-03217

CONTENT:

INTRODUCTION

-meaning

-application

-relevance

OBJECTIVE

ACTIVITY

-modelling.

-challenges of ore body models.

RESULTS

CONCLUSION & RECOMMENDATIONS

REFFERENCES

INTRODUCTIONMeaning;

Ore body models- are computerized representations of portions of the earths crust based on geological and geophysical observations made on and below the earths surface. Ore body models are numerical equivalent of a 3-D geological map complemented by description of physical quantities in the domain of interest.

Application:

Mining industry –defining the volume and mineral concentration ( hence value of mineralization)

Mineral exploration industry- geometry and placement of mineral deposits in the subsurface of the earth

Energy industry (oil and gas) –determination of fluid saturation

Natural resources and natural hazards management.

Industrial relevance:

mineral deposits that are deemed to be economic may be developed into a mine.

In oil and gas industry

- the models are used by reservoir engineers to identify which recovery options offer the safest and most economic, efficient and effective development plan for a particular reservoir.

-calculation of hydrocarbon saturation.

helps in delineating P.A.F & N.A.F in the sulfide hosted deposits for environmental purpose.

OBJECTIVE:

the main objective of ore body modelling is to better understand the shape and make predictions as to further activity in the field.

Others includes:

•Predicting with greatest accuracy as possible the shape , distribution and concentration of the ore before mining begins.

• To interpret the subsurface since the information involved is examined along both horizontal slices (plan maps) and vertical cross sections (drill sections).

• To predict the continuity of the subsurface geology by extrapolation from known sample data into the areas that are not yet drilled.

•ACTIVITY:1. MODELLING:

DRILL HOLE COMPOSITING:

Compositing- is the standard processing technique for regularizing the length or vertical height of desurveyed drill hole samples.

It involves the sample assay data combined by computing weighted average over longer intervals to provide a smaller number of data with greater length for use in developing the resource estimate.

-compositing is usually a length weighted average (ie. Usually within fixed length intervals within a compositing ‘zone’ field.

In ore body modelling compositing involves two processes:

I. Composite down drill holes.

II. Composite over benches.

DRILL HOLE

DRILL HOLE PROCESSES

COMPOSITE DOWN DRILL HOLES COMPOSITE OVER BENCHESCompositing computation is applied to have total thickness and unique grade value for each drill hole

Field under investigation is divided into parallel horizontal levels and parts of drill holes corresponding to these level are composited instead of compositing the entire hole.

Summary_ differences

DDH COMPOSITE

BENCH COMPOSITE

•Modeling utilizes topographic information and drill hole database

Stage 1; Gps data;

Frequently and representatively recorded GPS values and aerial views.

Helps to visualize the photo realistic views of the topography.

Stage 2: DH data;

The process begins with database building, the database is composed of; spatial coordinates of drill hole, geological formations that they intersect + depth of formations and their assay values. (remember database structure)

Then

-Data extension on a network & gridding is done.

Procedures ( building a 3D model)

1.It all begins with an empty chunk of a 3-D space on a paper or computer screen.

2.Letter on the drill hole information is brought to that space just as it exist in the real world.

Figure: The drill hole collars are displayed in the design window as points using colored circle symbols as shown in the image

Plan view of BH intersection to the ore( source SHANTA GOLD)

3.the 3-D space will then be divided into zones representing different rock types & different zones of mineralization.

The boundaries of these zones are typically 3-D wireframe surfaces and solids or series of polylines . At this point the location size and shape of the deposit are loosely defined.

• To define this and determine the grade it will be necessary to develop a block model.

• A block model is composed of rectangular blocks or cells , each of which has attributes such as grades , rock types , average density , resource classification , oxidation codes e.t.c

• Block models are constructed based on wireframes and drill hole information files created previously.

• the resultant model will be used in estimation of grade into the model cells.

• A parent cell is the largest cell allowed in the model, the size of these cells are defined by the user and should be based on several factors such as DH spacing, Mining method and geological structure hosting the ore.

Figure: BLOCK MODEL

• There are different ways in determining what value a block will hold. A rock type of a block might be assigned based on the zone it falls in from previous wireframe work.

• block values such as grades are typically assigned based on some weighted averaging scheme using the surrounding drill hole grade values.

• block sizes vary from model to model, a typical block size is about 25% of the average drill hole spacing for x and y dimensions. It is also equal to the projected mine bench height for the two directions

Summary:

To produce a 3D ore body model you basically need:

• mapping information.

• drill holes information.

• quality composition (assays)

• Mine design information

• survey data that enables surface modelling.

.

Geological map , pit design and assays informationSource: ANGLO GOLD ASHANTI-Nyankanga pit floor mapping

Section 50200

Bore hole information-laboratory assays

Section 50170 Section 50170

Bore hole information-LITHOLOGY obtained from logging

A closer view of how the bore holes intersects the ore body

CHALLENGES OF ORE BODY MODELLING;

i . Geological modelling software ( a worst scenario)

-designed by statisticians who knows little about geology

- applied by the geologists / engineers who knows little about statistics.

software used for geological modelling should be capable to integrate with geological database to streamline the modelling process.

ii. Capturing the true complexity of mineral deposits that matches the field conditions.

To address the variation between planned and actual production; Mining organizations finds it a challenge to balance creating a mine model that matches the field as accurately as possible.

ii. Limited number of drill hole data .

This leads into the ore mineralization to be interpreted as either discontinuous, small ore body, pockets/lenses, or large laterally continuous ore body.

iv. Modelling the nugget effect (mine splays effect)

Coarse gold mineralized portions always produces very high assays, and our first response is excitement at the thought of high grade deposits . This will in turn affect the geological interpretation process during modelling because the nugget values will spread and affect adjacent blocks showing a large area with high grade contrary to the reality. It should be noted that nugget effect defines the variability at very short distance, so nugget effect should be interpreted using the closest data available.

v. SMU and INTERNAL WASTE.

When the block to be modelled doesn’t qualify the minimum block size the surrounding perimeters are included so as to meet the conditions for a minimum block size.

If it happens the surrounding perimeter is a waste dominated it will lead into dilution ( edge dilution).

Material inclusion in the midst of the ore will be included in a block during modelling leading into low block grade due to internal dilution.

All these sorts of dilutions are inevitable because at this point it is better mining ore and its waste at low grade than sending the ore to the waste damp.

vi. Smoothening perimeter shapes.

Sharp angles should be avoided during modelling to perimeter design. This is inevitable for site mining reasons. Though inevitable it has an effect that it causes ore loss because the parts omitted to smoothen the perimeter is an ore lost.

RESULTS:

3D model that defines the shape , distribution and concentration of the ore body.

CONCLUSION & RECOMMENDATION:

Mining companies today are under increasing pressure to boost output from their existing mines and to bring new projects quickly. Fundamental to the long- term performance on an operation is the geologic modelling and mine planning.

To maximize mine profitability , planners must create mine plans that match the field as accurately as possible. Not surprisingly , this can be difficult task, and getting it wrong can result in large unforeseen cost and significant lost revenue opportunities.

REFFERENCES:

- Challenges and trends for geological modelling & visualization- A. Keith Turner, 7th Dec, 2005

- Geological Modelling Introduction – Dr. Inna Overeem,(community surface dynamics modelling system, UNIVERSITY OF COLORADO at builder) sept, 2008.

- Gy 413 lecture notes, Dr. Kinabo (geology department UDSM)2008

- Geological model of the Netherlands- Geological survey of Netherlands , Oct., 2005

- Mine to mill campaign industrial attachment geological report(pp.29)- S. Magohe , 2014

- Geita gold mine production manuals.

- Snowden manual- Viv Snowden – 17 may 2009

GRACIAS!!!!