the application of a deposit tectogenesis in pit slope geotechnical engineering: a case example

�Slope Stability Santiago Chile, November 2009

The application of a deposit tectogenesis in pit slope geotechnical engineering: a case example

The role of the tectonic stress associated with the mineralized systems has a profound influence on structural regime of the deposit, and determines the structural controls on mineralization and the host rock deformation. Often the focus in structural geology is on the deformation events leading up to the time of mineralization. These events clearly precondition the ground into which the mineralization is deposited. The structural pattern around mineralized systems however is significantly affected by the tectonic stresses prevailing at the time of the mineralization. In this paper the authors will show the relationship between the Tectogenesis of the deposits and the prevailing structural regime. Understanding the tectogenesis allows practitioners to anticipate the structural orientation of faults and shears most particularly at the pit scale and to develop a geometric model of the deposit to assist in pit slope design, as this must be consistent with the tectonic regime. Much of the information can be readily determined from carefully collected and interpreted stereonets supported by the analysis (tectogenetic analysis) of tectonic processes during a deposit formation. It is possible to ascertain “missing data” resulting from biased data collection. Often the faults and shears will have a fractal relationship. This can lead to order of magnitude prediction of where these features should occur. A practical example will be shown for a major nickel project where these features were identified.

Abstract

P.M. Dight

Australian Centre for

Geomechanics

The University of Western

Australia

W.V. Bogacz

Archon Resource Technologies

and Strzelecki Metals Ltd

INTRODUCTION

In the geological literature, “genetic models” of primary metalliferous deposits can be classified into two categories: those where structural control and tectonic deformation is not a factor, and those that are dominated and developed due to tectonic factors and the accompanying specific tectonic regime which controls the placement of the mineralization and its distribution within the geological sequence (host rock) of deposit. This paper is about deposits that have undergone tectonic deformation and where tectonic deformation has significantly contributed to the formation of the deposit, including the placement of the mineralization with the patterns of mineralized zones controlled by a tectonic regime active during the mineralizing processes.

As a rule and certainly for the deposit(s) presented in this paper this deformation developed during a specific late stage tectonic event(s) governing the orebody rock wall preparation and emplacement in the process of the deposit formation. This is why the host rock and the ore system (of any structurally controlled deposit) represent mutually depended tectonic genesis and as a consequence, the geometric pattern of structures involved in the deposit formation. The scale ranges from a deposit to local and to microscopic scale (i.e. it is a fractal relationship). Nevertheless, the variety of structural features and the complexity of the tectonic processes, and often the lack of clarity in understanding the role and timing of the tectonic deformation in the origin of orebodies and the control on mineralization, can limit the practical application of structural geology to the conceptual thinking in addressing and identifying solutions to geotechnical problems and mine development designs.

The interpretation of the relationship between tectonic deformation and the distribution of mineralization in a context of tectonic genesis

indicates that the positioning of the mineralization is integrally combined with the overall deposit tectonics controlling its formation (tectogenesis). These developed or were generated within a tectonic regime existing during the mineralization stage(s) processes. A degree of this deformation normally increases close to and within the orebody. This was observed in a majority of deposits analyzed including but not restricted to Au, Cu, Ni, Mo, U and Zn-Pb. The impact of structures developed in such a progressive regime on rock geotechnical properties, slope design and mine stability could be potentially significant.

This paper is designed to provide a basic understanding of tectogenesis and tectogenetic analysis as outlined in Figures 1 and 2. This is an alternative method to traditional thinking applied when investigating tectonic deformation of deposits (Bogacz, 2, 3, 18 and 19). The methodology has been successfully applied to the understanding of the nature of ore systems/orebodies and structural controls on mineralization for numerous deposits in Australia and worldwide, including the largest metalliferous ore systems in Australia such as: Telfer (Au-Cu) in the Proterozoic Patterson Orogen (Bogacz, 4), Mt Keith (Ni) of the Agnew-Wiluna Greenstone Belt of the Yilgarn Craton (Bogacz and Cadman, 5,

� Santiago Chile, November 2009 Slope Stability

Bogacz, 3), Argyle diamond deposit of Kimberley region (Bogacz and Dight, 6), Gibraltar (Au) Mine (Dight, 7), Sunrise Dam (Au) Mine (Dight, 8) and Chariot (Au) Mine (Dight, 9). In the exploration the application of tectogenetic analysis has allowed identification of deposits, e.g. the Arenal (Au-Ag) deposit in grass root environment in the northern Uruguay (Paydirt, 10) or uranium-rich Paralana Mineral System, which includes the large Mt Gee (U) deposit within the Mt Painter Inlier of Curnamona Craton, South Australia (Bogacz, 11, Bogacz et al., 12).

Figure 1 – Deposit’s tectogenesis and its practical application – conceptual diagram (after Bogacz, 18, 19)

Figure 2 – Application of structural geology and implication of tectogenesis to predictive thinking (after Bogacz, 18, 19)

CONCEPT OF TECTOGENESIS

Metalliferous deposits, regardless of whether they are considered as originally structurally controlled or not, can be described as an association of the host rock and the mineralization. The distribution of the mineralization, including the positioning of high grade zones within the host rock, determines the ore system (orebody). Any ore system displays its own unique geometry, but as a rule, it is closely linked or follows the geometry and the pattern of specific tectonic structures confined within major tectonic features which normally determine the reach (limits) of orebody. In most cases, tectonic structures to which the mineralization is confined display secondary development. As a consequence, structures and tectonic processes controlling the formation of mineral systems represent tectonic features formed at a later stage compared to the original structural geometry created during the host rock tectonic evolution and metamorphic recrystallisation.

If the tectonic evolution of the host rock represents pre-existing rock wall preparation type processes, then tectonic deformation controlling emplacement of the mineralization into favorable, mostly rejuvenated, structural settings would be associated with a specific and separate stress regime during the processes forming the mineralization. This regime can be explained by tectogenetic analysis (Bogacz, 3).

Tectogenesis (tectogenetic analysis) is an integrated method of understanding of deposits, particularly the processes leading to their tectonic development from a regional and deposit to local and also microscopic scale, which has recently been positively defined by ore petrogenesis studies (Bogacz and Kucha, 1). Tectogenetic analysis allows productive structures to be separated from those which were non-productive and not active


(passive) during the orebody/deposit forming processes. Tectogenesis is a predictive method allowing identification of areas with the potential to host structurally controlled mineralization. A powerful element of the method is to develop deposit scale predictions of dominant structures and structural geometry/trends together with localized structures corresponding to the tectonic development that has led to the formation of the deposit.

TECTOGENETIC ANALYSIS

Tectogenetic analysis is designed to provide information on a uniform interpretation of tectonic deformation and tectonic genesis of any ore system for any deposit (Bogacz, 3). Among numerous aspects of this analysis, the following are investigated in particular:

The specific structural regime during the formation of a deposit in which mineralization is, or could be confined.The deformation mechanism(s) responsible for the formation, geometry and tectonic boundaries of a deposit, including the structures and tectonic processes controlling the ore system.The geometric, geomechanical (e.g. shear, extensional) and kinematic variability of tectonic structures propagating and/or controlling the host rock deformation and mineralization development.The structural geological factors and stress regime during the deposit (host rock) and orebody formation.

Tectogenetic analysis can also be described as an assessment of the host rock geomechanical regime which generated the tectonic deformation environment, which was then favorable for the formation of the ore system. When correctly applied, a consistent relationship between tectonic deformation of the deposit and the pattern of mineralization-carrying structures can be identified. If the formation of this model could be linked with the structural geological factors, e.g. the surrounding granite or other magmatic body up-doming, or larger basement fracture activity, the explanation of origin (tectonic genesis) for the tectonic deformation and corresponding orebody model, which is the mineral (ore) system tectogenetic model, could be determined. If achieved, such model can be easily applied from a deposit exploration stage, everyday geotechnical work and mine design through to development and mine expansion stages (after Bogacz, 19).

SIMPLE INTERPRETATION

Some PrinciplesMany of the principles of tectogenesis can be identified in simple testing. The example given here is from a direct shear test Figure 3.

The initial fracturing is predominantly extensional until the shear displacement overrides the initial fracturing Figure 4. Hence the strain is critical to understanding the tectogenesis. Examples of such structure patterns can be readily recognised in nature and experimental work, and were reported since the early 20th century (Figure 5a, after Ramsay and Huber, 20; Figures 6b and c, after Cloos, 21, 22).

Figure 6 shows the structural controls for the Mt Keith deposit (the subject of the case study shown in this paper) where the tectonic features have

a dominant role in the formation of the deposit then on the pit wall stability. Another example of using simple mechanical models to understand the structural controls can be found using a UCS test on brittle rock (Dight, 15) case the structural pattern is very similar to a deposit of volcanic origin. Using such examples help to understand the brittle behaviour of many systems at the time of mineralization and the expected pattern of structures. Hence there is a close relationship/interference between the orientation of the dominant shears, the extensional features and the stresses applied at the time of mineralization. The stress orientations have also been observed to still hold a similar orientation today (Dight, 16).

••

•

•

Figure 3 – A simple direct shear test model – note the principal stresses, these are unique

Figure 4 – The structures that are initially created during shearing, predominantly extensional


Figure 5a – Observation of structural movement in a rock mass (after Ramsay and Huber, 20)

Figure 5b – Clay experiment. Shearing and feather-jointing in the course of simulation of a strike-slip fault (after Cloos, 21, Figures 6 and 7)

Figure 5c – Feature joints (after Cloos, 22)

ExAMPLES OF TECTOGENETIC INTERPRETATION

Tectogenetic analysis has been applied in the interpretation of more than 300 deposits in various geological, tectonic and geotechnical regimes/conditions in Australia and worldwide. The results are overwhelmingly consistent: application of tectogenesis allows the explanation of dominant and relevant to the mineralization/orebody formation and deformation sets of structures, their geometric, kinematic and tectogenetic relationship, all leading to the understanding of tectonic processes which took place within the deposit.

This allows not only identification of structures controlling the orebody/deposit formation, but also their separation from pre-existing and post-dating structures. Below are presented a few simple examples of tectogenetic analysis/interpretation.


In Figure 6 on the left side is the interpreted geology and geometry of the nickel Mt Keith orebody illustrated in section (after Hopf and Head, 14). Tectogenetic analysis indicated that deposit’s nickel mineralization is structurally controlled, including at deposit scale. This is schematically presented in the diagram on the right side.

Figure 6 – Left: Interpreted geology and geometry of the Mt Keith orebody in section (after Hopf and Head, 14) Right: The deposit’s tectogenetic interpretation (after Bogacz, 3)

Figure 7 is a photo and the interpretation of tectonic setting/structure sets and tectonic processes explaining the observed tectonic deformation. These include flexural- slip mechanism of folding/movement, i.e. reverse-slip movement on the bedding surfaces and corresponding (not accidental) concurrently developing specific stes of shares and faults.

All these structure sets contribute to the shortening of the geological sequence and in the past, sliding of rock mass in particular along the bedding surfaces and bedding parallel clay-rich horizons into the pit was a major geotechnical problem.

Figure 7 – Structural setting (top) and tectogenetic interpretation (bottom diagram) in the Leigh Creek coal mine (South Australia)

In more detail, explanations of a local geometric and kinematic setting from Tarcoola gold mine (South Australia) and from the Golden Mile gold mine in Kalgoorlie (Western Australia) are shown. These lead to the understanding of tectonic processes and contributing to the understanding of a deposit scale tectogenesis.


At Tarcoola (Figure 8) the structural deformation presented is developed in the host rock due to a dominant role of the bedding-foliation parallel normal dip-slip movement along a low-angle dipping zone/structure. This zone/structure clearly determines two deformation domains: weakly deformed on the top and intensely deformed on the bottom. The bottom domain had taken most of tectonic stress and therefore bedding-foliation is rotated to a steeper attitude, acuate tending to be sigmoidal in shape as a result of its splitting off of the contact zone. There are developed small drag folds (e.g., in the bottom right corner of the photo) indication a reverse slip kinematics.

At Golden Mile gold mine (Figure 9) intensive normal dip-slip tectonics in the host rock is commonly observed. An example of the interpretation of a small scale sigmoidal tectonic structure, initially prepared based on pre-existing two (west and east dipping) sets of fractures sets, formed as a result of late stage extensional normal-dip-slip shearing processes is shown. Indications of a normal dip-slip tectonic regime (σ

1 steep or vertical) in various scales are commonly observed throughout the Golden Mile and structures related to this

regime certainly influence open-cut mine geotechnical properties/stability.

Figure 8 – Tarcoola gold mine (South Australia); structural controls in the underground face

An interpretation of quartz controlled mineralization at the Fraser open-cut gold mine, Southern Cross, Western Australia (Figure 10), provides an additional example for ‘sigmoidal’ development of structures controlling mineralized zones. Note the offsets and gross of the mineralized structures in the open-cut are focused in a specific lode/body with regard to the main trend of quartz reefs and shears. Important is a dextral movement along the main reef (left part of the photo) because this kinematics allowed the development of extensional lower order offset structures. North-South elongation of the ‘sigmoidal’ lode is determined by the location and kinematics of the main reefs.

Figure 9 – Golden Mile gold mine, Kalgoorlie (Western Australia); interpretation of a ‘sigmoidal’ structure developed due to normal dip-slip shearing in extensional tectonic regime


Figure 10 – Fraser Gold mine, Southern Cross (Western Australia) – note the offsets and gross of the mineralization focused in a specific ‘sigmoidal’ lode structure

APPLICATIONS IN GEOTEChNICAL ANALYSIS

An initial investigation at an Australian mine (Mine A) was undertaken because there was doubt about the grade distribution. The investigation concluded that there were structural controls and that these could be related to the deposit formation, hence the deposit could be intrusive of nature (Bogacz and Cadman, 5); in 1996 it was not popular to consider that nickel deposits could be intrusive (e.g. Hill et al., 13, Hopf and Head, 14); it was not until 2005 that this was recognised “officially” as a possible model for Ni deposits in Western Australia (e.g. Rosengren et al., 23).

Figure 11a is a published diagram of the structural controls inferred for the deposit. The initial model had the deposit as an extrusive deposit that had been subject to sinistral strike slip shear (e.g. Hopf and Head, 14). Figure 11b shows that the structural controls also apply to the distribution of grade (shown in plan) thus confirming doubts with regard to the application of the extrusive model.

The defect data for Mine A can be seen on stereonets in Figure 12. The data has been simplified for presentation. It would be expected from a review of the defect data that there is possibly a fold with a plunge axis at 25° to the SW. However in a brittle environment the concentration of poles in the SW appears to be associated with the presence/interference of shear structures, including deposit scale west dipping structures (on south wall –left side of the photo, Figure 13 (Bogacz and Cadman, 5)).

Figure 11 – a) Geotechnical model Mine A, northern part; b) deposit’s grade distribution; both plan view


These significantly contributed to the deposit formation and the geometry of the Mine A orebody, which is schematically illustrated on diagrams in Figure 6, i.e. ‘split’ area of the top part of the orebody (left diagram) and its marginal areas on the contact with the main bottom and top shears potentially limiting the orebody development (right diagram). Corresponding to this shear activity are derived/second order structures, which are represented by the poles distribution on stereonets in Figure 12. The concentration of poles in the SW part of the stereonets is approximately the normal to the axis of the maximum principal stress direction. Stress measurement undertaken using acoustic emission shows similar orientations. These orientations also have an impact on slope stability where crushing is observed in SE corners of the pit and toppling in the SW and NE corners of the pit (Dight, 17).

In Figure 14 then is the tectogenesis interpretation of major shear structures at Mine A deposit shown in plan alongside a simple direct shear test. The pattern of structures is very similar. The dominant structural controls strike across the pit at an acute angle to the pit wall (approximately 20°) and cause significant structure impacts on stability.

Figure 12 – a) Pole, b) trace stereonets, and c) interpreted stress field pattern for Mine A

The important aspect of the geotechnical application of tectogenesis as it moves the structural concepts into the field. The tectogenetic model of structural features indicates a particular location and signature spacing which when understood can be used to predict location (Dight, 7, 8 and 9). If the expected pattern can be recognised, information in blind spots resulting from directional bias can be anticipated and where appropriate can lead to appropriately designed investigations. There are strong correlations between the structural features identified and the stress regime with particular reference to defect morphology which helps in the interpretation of structural data. Lastly it means much more information can be extracted from good field data acquisition than is traditionally the case.

Figure 13 – Mine A – a deposit scale west dipping major shear structures; early stage open-cut, south wall


Figure 14 – Deposit scale geotechnical model and tectogenetic interpretation of major shears in plan view

CONCLUSIONS

The distribution of mineralization within the host rock determines the ore system (orebody) of any primary deposit. Understanding of tectonic evolution of the host rock and ore genesis identification alone is not sufficient to explain the origin of metalliferous and other (e.g. diamond) structurally controlled deposits. This is because the tectonic deformations controlling the deposit’s mineralization display a unique timing and development. The understanding of the ore system can be achieved by application of a tectogenetic analysis.

Tectogenesis (tectogenetic analysis) is an integrated method of understanding of deposits from a regional and deposit to local and microscopic scale, where it is positively defined by petrogenesis. This method also allows the separation of productive structures from non-productive and not active (passive) structures during the orebody/deposit forming tectonic processes. Tectogenesis allows predictability of potential locations of orebodies within areas of mineralization. Also allows prediction of local dominant structure geometry and patterns, and lower order structures corresponding to their formation in the overall tectogenetic setting(s) of deposits as parts of a deposit scale tectonics.

Tectogenetic analysis can be described as an assessment of the tectonic geomechanical regime during mineralization. It has been applied to the interpretation of orebodies recognized as non-structurally controlled, and those that are structurally controlled. The results are compelling that in both cases structural deformation is critical in explanation the geometry of the ore mineralized system, the distribution of high grade or barren zones and in establishment of the link between tectonic deformation and the ore system. This allows the generation of an ore system tectonic genesis (tectogenetic) model.

The application of tectogenetic criteria or analyses, as an alternative method of structural investigations, allows the implementation of a conceptual tectogenetic model. A feature of the model is the ability to predict of position of the mineralization. The tectogenetic model is easy to implement in the everyday work of a professional geological and mining team at any stage of exploration, geological assessment of the deposit and mine development.

The creation and implementation of a tectogenetic model of the ore mineralized system has a significant impact on:

The generation of exploration targets and mine expansion.Providing better targets for drill hole locations.Improving the reserve calculations (and can be used in conjunction with variography).Targeting better grade control.Improving grade reconciliation.Providing a unifying framework for geotechnical parameter assessment.

••••••

�0 Santiago Chile, November 2009 Slope Stability

In this way, a much more efficient use can be made of the expenditure for exploration and development.

Tectogenetic analysis provides a key to the chaos surrounding mineralized systems.

Implication in open-pit geotechnical engineering are significant as the method of Tectogenetic Analysis and application of Tectogenesis thinking allow prediction of structures and often their location which are critical in a mine assessment in terms of their geometry, potential location and geotechnical importance. Determination of main (first order) structures and prediction of areas with presence of lower order structures, their variability and intensity of development/concentration throughout the open-cut in this method is predictable. Thus, the method and the understanding of Tectogenesis provide important criteria for optimal open-pit and underground mine design and geotechnical engineering tool.

REFERENCES

1. W.V. Bogac and H. Kucha, “The genesis of the Mt Gee uranium deposit and the Paralana Mineral System, Mt Painter Inlier, Curnamona Craton, South Australia: an example of the application of Tectogenesis and Petrogenesis in the investigation of metalliferous deposits”, 2009 (in preparation).

2. W.V. Bogacz, “Tectogenesis of metalliferous deposits, or how to understand and use structural data in mine development and exploration”. Australasian Institute Mining and Metallurgy, Perth, Talk, 8 February, 1999 (abstract).

3. W.V. Bogacz, “Metalliferous deposits: Tectogenesis and mineralisation controls”. In: Piestrzynski, A (ed), Mineral Deposits at the Beginning of the 21st Century, 6th Biennial SGA-SEG Meeting, Krakow, Poland, 26-29 August 2001, Krakow.

4. W.V. Bogacz, “The tectogenesis of the Telfer gold-copper ore system in the Proterozoic Paterson orogen, north western Australia”. Annales Societatis Geologorum Poloniae, 74: 95-121.

5. W.V. Bogacz, W. and J. Cadman, “Structural study of the “Mine A” deposit for mine development and exploration purposes”. BFP Consultants, Unpublished Report, 1996.

6. W.V. Bogacz and P.M Dight, “Extensional Tectonic Deformation and Tectonic genesis Model of the Deposit”. Report prepared for Argyle Diamond Mines – BFP Consultants, Unpublished Report 2002.

7. P.M. Dight, “Ground support: Predicting when to make the change”. In: Ground Support in Mining and Underground Construction, Proceedings of the Fifth International Symposium on Ground Support, 28-30 September 2004, E. Villaescusa and Y. Potvin, Eds, A.A. Balkema Publishers, 2004, 499-506.

8. P.M. Dight, “Structural controls for the Sunrise Dam Mine.” BFP Consultants, Unpublished Report 2005.

9. P.M. Dight, “Stability of the Chariot Mine”. BFP Consultants, Unpublished Report 2005.

10. Paydirt Article “The finding of Arenal Deposit, Uruguay”, Paydirt, March 2004.

11. W.V. Bogacz, “The Uranium-Rich Paralana Mineral System – Myth and Truth – North Flinders Ranges, South Australia”. In: Australia’s Uranium, AusIMM Conference, Adelaide, Australia, 10-11 July 2006: 35-36.

12. W.V. Bogacz, A. Younger, H. Kucha and A. Piestrzynski “The tectogenesis of the uranium-rich Paralana Mineral System at Mt Painter Inlier, South Australia”. In: SGA Meeting Dublin 22-24 September 2007.

13. R.E.T. Hill, S.J. Barnes and C.S. Perring, “Komatiite volcanology and the volcanogenic setting of associated magmatic nickel deposits”. In: Proceedings Nickel ‘96 (Eds: E.J. Grimsey and I. Neuss.) Australasian Institute Mining and Metallurgy, 1996, 91-95.

14. S. Hopf and D.L. Head, “Mt Keith nickel deposit”. In: Geol. Austral. and P N G, Min. Dep. (Eds: D.A. Berkman and D.H. Mackenzie). Australasian Institute Mining and Metallurgy, 1998, 307-314.

15. P.M. Dight, “Impact of Structural Model on Geotechnical Interpretation”. ACG course on open pit stability (P.M. Dight ed) Unpublished 2009.

16. P.M. Dight, “Determination of Insitu Stress from Oriented Core”. ISRM Conference on Insitu Stress. Trondheim 2006.

17. P.M. Dight. “Pit wall failures on ‘unknown’ structures”. SAIMM Conference on Open Pit Mines. 2006.

��Slope Stability Santiago Chile, November 2009

18. W.V. Bogacz, “Tectonic genesis of metalliferous deposits: developing an integrated tectonic deformation and mineralisation model”. In: Applied Structural Geology in Mineral Exploration and Mining, AIG International Symposium, Kalgoorlie, Australia, 23-25 September 2002, 22-25.

19. W.V. Bogacz “Application of structural geology in the formation of an orebody model”. ACG presentation. Open pit stability, P.M. Dight, Ed, Unpublished 2002.

20. J.G. Ramsay and M.I. Huber. The techniques of modern structural geology. Academic Press. 1983.

21. H. Cloos, Experimenten zur inneren Tektonik, Zentralblatt für Mineralogie und Paleontologie 1928, S.615.

22. H. Cloos, Einführung in die Geologie: Berlin, Borntraeger 1936, S.236 Abb. 187.

23. N.M. Rosengren, S.W. Beresford, B.A. Grguric and R.A.F. Cas An intrusive origin for the komatiitic dunite-hosted Mount Keith disseminated nickel sulphide deposit. In Economic Geology, January 2005 v. 100; No. 1; 149-156.

the application of a deposit tectogenesis in pit slope geotechnical engineering: a case example

Documents