Aspects of resource geology in Archaean orogenic gold deposits

Roberts, Lucy Sarah (2005) Aspects of resource geology in Archaean orogenic gold deposits. PhD thesis, James Cook University.

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Abstract

Within the mining industry, all phases of the operation are dependent on the resource model. The focus of the mining operation is a model, which is often based on incomplete or flawed data. An orebody is commonly a poorly understood geological entity, and the resource model is generated from samples taken from this orebody. The inherent variation caused by geological complexity within the orebody can cause errors in the resource model. In order to reduce errors, the grade distribution of the deposit needs to be quantified, the sampling and resource evaluation protocols need to be optimised, and the level of inherent risk within the model needs to be identified and managed. Two gold deposits (Golden Pig and Marvel Loch) in the Southern Cross Greenstone Belt of the Yilgarn Craton were studied to determine the effect that these factors have on the resource model.

Fractal analysis was applied to the Taurus orebody of the Golden Pig deposit. The grade distribution within the Taurus shear zone could be split into two different domains using the concentration-area method of fractal analysis. As the majority of fractal methods are two-dimensional, the technique was modified to determine the fractal dimensions of the deposit in three dimensions. This is the first study known to have applied fractal techniques to a mining based dataset in three dimensions. The grade distribution of the Taurus shear zone features two fractal dimensions, separated by a "threshold" value of 7 g/t. Geological interpretation of the Taurus shear zone indicates that two mineralising events are present, one overprinting the other. The geological evolution of the shear zone is thought to have produced the two differing fractal dimensions, with the "low" grade distribution due to low grade, more pervasive mineralisation, and the "high" grade distribution relating to cross cutting high grade tension veins. Sampling protocols were optimised for four different mineralisation styles: Haddons and Taurus orebodies from Golden Pig, and Boulder and East orebodies from Marvel Loch. The application of Gy sampling theory was shown to reduce errors within the processing of samples of broken rock. Heterogeneity tests were used to determine the sampling constant (K) of Gy theory value for each of the orebodies, and thereby to propose an optimised sampling regime. Gy nomograms were used to determine whether each of the protocols would breach "Gy's Safety Line". A constant sample size of 1 t for each of the mineralisation styles was found to minimise the fundamental sampling error to acceptable levels, with the total error never exceeding ±10%. Comparing the orebodies showed that the K value of the deposit correlated with the percentage coarse gold within the deposit. Resource evaluation techniques were also optimised for four different mineralisation styles: the Haddons and Taurus orebodies from Golden Pig, and the Undaunted and Sherwood orebodies from Marvel Loch. Resource evaluation techniques used both estimation and simulation, and all case studies included an optimised estimation method. In three of the case studies, conditional simulation models are also presented. Quantitative Kriging Neighbourhood Analysis (QKNA) was used to determine a suitable search area for the individual orebodies. Each of the orebodies was modelled using a variety of techniques and search areas, and comparisons were made between the global mean grade of the model with the declustered mean grade of the input data, individual blocks from the model, and trend plots and grade tonnage curves of the individual models. The Haddons case study was also reconciled with a known bulk sample grade to illustrate the suitability of the modelling technique. The optimised evaluation technique is different for every orebody, illustrating how a blanket approach to resource evaluation is unsuitable. The financial risk from incorrectly modelling variograms for the Haddons orebody was quantified. Variograms were modelled using an experimental variogram and there is an inherent risk that variograms maybe modelled incorrectly. The case study presented here aims to quantify the financial risk involved in modelling the variogram parameters (the nugget effect and range) incorrectly. The Haddons case study illustrates how modelling the nugget effect incorrectly by 20% can result in a Moderate financial loss, but is Very Likely to occur, which is deemed to be a high risk situation. Modelling the range at slightly longer lengths than the "true" range also results in a financial loss, but the loss is smaller, being classified as Minor. This scenario is also Very Likely to occur. The level of risk involved in a single decision made at the beginning of the resource evaluation procedure can be seen to have significant impacts throughout the project. Financial modelling using the incorrect block models illustrates how a discrepancy in the variogram modelling could affect whether the deposit is developed into a mine or not. As all of the deposits studied in this thesis are classified as Archaean Orogenic Gold Deposits, the specific conclusions may only be pertinent to this style of mineralisation. Even within this single style of mineralisation, different resource evaluation approaches are needed for each orebody. Gaining a more thorough understanding of the grade distribution, optimising sampling and resource evaluation protocols, and identifying inherent risks are important in each case and for all mineralisation styles. Good management of all factors involved in the production of a resource model is vital if unnecessary errors, which can have major financial consequences, are to be avoided.

Item ID: 43745
Item Type: Thesis (PhD)
Keywords: Archean; geology; gold deposits; gold mineralisation; Golden Pig deposit; Marvel Loch deposit; orebodies; orebody modeling; orebody modelling; orogenic; resource model; Southern Cross Greenstone Belt; WA; Western Australia; Yilgarn Craton
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Appendices B-E (data) are not available through this repository.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Appendix A: Roberts, Lucy S. (2004) Resource estimation comparisons: preliminary results from Haddons Lode, Golden Pig Mine, Western Australia. In: Proceedings of Mine and Resource Geology EGRU-NQ AusIMM Symposium, pp. 129-148. From: Mine and Resource Geology EGRU-NQ AusIMM Symposium, 2 April 2004, Townsville, QLD, Australia. Appendix A: Roberts, L.S., Dominy, S.C., and Nugus, M.J. (2003) Problems of sampling and assaying in mesothermal lode-gold deposits: case studies from Australia and North America. In: 5th International Mining Geology Conference Proceedings, pp. 387-400. From: 5th International Mining Geology Conference, 17-19 November 2003, Bendigo, VIC.

Date Deposited: 15 Jun 2016 01:37
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040306 Mineralogy and Crystallography @ 33%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 34%
04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 33%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 100%
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