Effects of Hydrothermal Alteration on the Geomechanics of Degradation at the Bagdad Mine, Arizona

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Authors Coutinho, Paulo

Citation Coutinho, Paulo. (2020). Effects of Hydrothermal Alteration on the Geomechanics of Degradation at the Bagdad Mine, Arizona (Master's thesis, University of Arizona, Tucson, USA).

Publisher The University of Arizona.

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Download date 05/10/2021 14:56:11

Link to Item http://hdl.handle.net/10150/648603

EFFECTS OF HYDROTHERMAL ALTERATION ON THE GEOMECHANICS OF DEGRADATION AT THE BAGDAD MINE, ARIZONA

Paulo Coutinho

______

A Thesis Submitted to the Faculty of the

DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING

In Partial Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

WITH A MAJOR IN MINING, GEOLOGICAL, AND GEOPHYSICAL ENGINEERING

In the Graduate College

THE UNIVERSITY OF ARIZONA

2020 2

Acknowledgements

I would like to extend my sincere gratitude to my advisors, Dr. John Kemeny and Dr. Isabel Barton, for their crucial role in guiding me through my graduate studies. I would also like to thank my thesis committee members, Mr. Keith Taylor, for his valuable support and enlightening comments.

Dr. Sergio Castro Reino for his resources and experience involving rock mass degradation.

Furthermore, I would like to thank my coworkers at Freeport McMoRan Inc. for providing me geology and geological engineering knowledge pertaining to the Bagdad deposit, and Call &

Nicholas for their constructive criticisms regarding my modeling techniques. Finally, I would like to extend a special thank you to Mr. David Streeter for his guidance in running the geotechnical analysis at the University of Arizona.

Table of Contents

List of Illustrations ...... 5 List of Tables ...... 7 Abstract ...... 8 Chapter 1 – Introduction ...... 10 Background ...... 10 Rock Mechanics and Mineralogy ...... 10 Types of Failures and Quantifying Failure Conditions ...... 12 Modeling How Rock Mass Characteristics Affect Slope Stability ...... 15 Porphyry Deposit Geology and Alteration and Relationship to Geomechanics ...... 16 Geology of the Study Site ...... 20 Pregnant Leaching Solution Infiltration ...... 25 Chapter 2 – Literature Review ...... 26 Chapter 3 – Problem Definition and Methodology ...... 31 Geotechnical Tests ...... 35 Rock Characterization and Short-Wave Infrared Spectrometry (SWIR) Data ...... 38 Petrographic and SEM Characterization ...... 40 Slope Stability Finite Element Model ...... 41 Chapter 4 – Results ...... 44 Geotechnical Tests ...... 44 Shortwave Infrared Imagery (SWIR) Data ...... 49 Petrographic Characterization ...... 51 Slope Finite Element Modeling ...... 62 Chapter 5 – Conclusion and Future Work ...... 72 Recommendations for Future Projects ...... 73 Chapter 6 – Reference ...... 74

List of Illustrations

Figure 1: Diagram illustrating different circular failure mechanisms. From left to right: slope failure, toe failure, and base failure...... 14 Figure 2: Outcrop illustrating the change in joint spacing with type of hydrothermal alteration between a zone of intense sodic-calcic alteration (A) and a zone of intense potassic alteration with a propylitic overprint (B) at the Yerington district (Nevada). Joint spacing increases in the zone of sodic-calcic alteration...... 18 Figure 3: Cross-section of an idealized porphyry alteration zoning system (Berger et al., 2008; Lowell and Guilbert, 1970)...... 19 Figure 4: Geographic location of the Bagdad mine in the United States...... 21 Figure 5: Schematic illustration of typical vein formed during multiple stages of alteration. The

white zone consists of vein of quartz-sulfide-Cu (±Carbonate) with fixation of SiO2 released due to hydration, carbonatization, and sulfidation of silicates. The green zone is the proximal zone of sulfide (sericite, ± Carbonate) with fixation of S and Cu. The yellow zone is the intermediate

zone of carbonate (±chlorite, some sulfide) with CO2 fixation. Finally, the red zone is the distal zone with chlorite (±carbonate, less sulfide content) and fixation of H2O...... 22 Figure 7: Geologic Map of the Bagdad Pit with local structures separated into West (1), North (2), East (3), and Southeast (4) where geotechnical test and thin section samples were collected based on map from Rathkopf et al., 2017...... 23 Figure 6: Geologic map indicating the major formations and structures around the Bagdad area. (Rathkopf et al., 2017 and sources therein)...... 24 Figure 8: Illustration of the same core from the Grasberg mine, 3 years apart. The RQD of the section below 102 m changed through time, probably due to the oxidation of pyrite in the core generating sulfuric acid...... 32 Figure 9: Diagram illustrating the workflow for this project...... 34 Figure 10: Example of Load Cell Versa stress through time for the Brazilian test...... 36 Figure 11: Samples soaking under PLS and tap water for 5 days...... 40 Figure 12: Example of meshed slope and boundary conditions along slope...... 42 Figure 14: UCS data from the Bagdad mine displaying their average (green) and -1-standard deviation (red). Samples with XRD and Strain Gauges are broken out as shown...... 46 6

Figure 15: Tensile strength data from the Bagdad Mine displayed with the average line (green) and the -1 standard deviation line (red)...... 48 Figure 16: The effective vertical stress compared to the effective confining pressure using the average uniaxial compressive strength of all data points...... 48 Figure 17: Example of hydrothermal alteration as a potential source of degradation, as the alteration of primary micas to chlorite causes the core to break along the alteration zone...... 50 Figure 18: Illustration of SWIR spectra of montmorillonite (A) and muscovitic-illite (B) from the East wall, and phengite (C) and muscovite (D) from the Southeast wall...... 51 Figure 19: Overview of 30 μm thin section from samples from the Southeast wall. A (PPL) and B (XPL) illustrate the arrangement between a vein-controlled mineralization and the groundmass. C (PPL) and D (XPL) show the edge of a naturally formed fracture with a hydrothermal phengite vein. E (PPL) and F (XPL) indicates the fracture density along sericitized zoned plagioclases...... 52 Figure 20: Illustration of hydration curve indicated with the red arrow prior to (A) and after (B) soaking the quartz monzonite samples from the Southeast wall in PLS solution...... 53 Figure 21: Backscattered electron imaging of chloritized biotite from Figure 19 (C and D) with chemical composition of biotite (top), chlorite, and phengite (bottom)...... 55 Figure 22: Backscattered electron imaging of sericitized plagioclase from Figure 19 (E and F) with chemical composition of phengite (top), and plagioclase zones (mid and bottom)...... 56 Figure 23: Relationship between UCS and mineral ration related to the degradation of biotite and plagioclase...... 59 Figure 24: Relationship between density and mineral ration related to the degradation of biotite and plagioclase...... 61 Figure 25: Hoek and Brown method using SRF for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall. These illustrations indicate how rock mass degradation may affect slope stability using parameters from Table 7...... 68 Figure 26: Hoek and Brown method with no SRF applied for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall using parameters from Table 7...... 69 Figure 27: Hoek and Brown method using SRF for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall using layers (green line) with different RQD and joint spacing data for the RMR parameter from Table 8...... 70 7

Figure 28: Hoek and Brown method using no SRF for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall using layers (green line) with different RQD and joint spacing data for the RMR parameter from Table 8...... 71

List of Tables

Table 1: Mineral assemblage associated with different hydrothermal alteration (Seedorff et al., 2005)...... 17 Table 2: Number of samples tested for each analytical method...... 33 Table 3: Uniaxial compressive strength data with additional analytical characterization methods and their location in the pit...... 45 Table 4: Tensile strength data and their location in the pit...... 47 Table 5: Moisture content and specific gravity of sample from the Southeast wall...... 49 Table 6: Parameters used for the RS2 model...... 63 Table 7: RMR Rating used for RS2 models using the Hoek-Brown failure criterion considering only the hydrothermal alteration...... 65 Table 8: RMR Rating used for RS2 models using the Hoek-Brown failure criterion simulating PLS-rock interaction...... 65 Table 9: Data collected through RS2 indicating how the slopes properties change due to rock alteration...... 66

Abstract

The objective of this study is to characterize how mineralization, hydrothermal alteration, and weathering affect slope stability in porphyry mines using samples from Freeport-McMoRan’s

Bagdad Mine, Arizona. Bagdad, an open-pit mine 240 miles northwest of Tucson, is a copper-

molybdenum porphyry system characterized by the presence of chalcopyrite and molybdenite as

ore minerals and silver as a by-product. Multiple alteration episodes accompanying mineralization

emplaced sulfide, oxide, carbonate, and sheet silicate minerals in the country rock and in the

porphyry, potentially affecting their strengths and friction angles and hence the maximum

allowable slope angle. To analyze the potential effects, I investigated rock pairs of the same

lithological type, but varying extents of hydrothermal alteration (sericitic and argillic) through

geotechnical tests, petrography, scanning electron microscopy (SEM), shortwave infrared (SWIR),

and imagery analysis. Continuous expansion of open pits within the mining industry due to ore

price shift causes companies to excavate through areas that have previously been used for other

purposes like mineral processing. Therefore, multiple mines throughout the world happen to have

the mutual issue of pregnant leaching solution (PLS) leaking through their pit wall since they may excavate through ancient leach pads. The PLS infiltration and interaction with the rock mass may

be a source of degradation since it can affect rocks’ initial fabric, mineralogy, and texture. I also

analyzed data and performed experiments on rocks affected by PLS under controlled environments

to determine the effect of the PLS on rock strength. Although the results for the hydrothermally

altered rocks show a correlation between chlorite-biotite ratio and uniaxial compressive strength

(R2 = 0.98), the existing data and models still leave considerable uncertainty about the magnitude

and mechanism(s) of rock mass degradation due to alteration, as well as due to pregnant leaching

solution (PLS) exposure. Hydrothermal alteration showed unclear but potentially deleterious 9 effects on rock mass stability. On the other hand, no evidence for laboratorial experiment with PLS alteration was identified in the samples.

The rock strength data obtained from the geotechnical tests were input into a model to determine the effects of alteration, especially chloritization, on geomechanical degradation. Continuum

Finite Element predicted how the excavation stability shifted due to changes in the rock mass associated with its degradation using the Hoek and Brown failure criterion using a generic slope.

The results show potentially significant decreases in excavation stability due to rock mass degradation related to hydrothermal. The PLS-induced alteration models indicated potential changes of stability assuming different rock mass ratings (RMR) for Rock Quality Designation

(RQD) and joint spacing. However, because of the unclear relationship between alteration and rock strength, the principal conclusion of this work is that more research will be necessary to further explore the tentative link between alteration type and geotechnical characteristics. The modeling results are meant to provide a general idea of the effects of material degradation on the behavior of the general slopes, not to recreate the detailed realities.

Chapter 1 – Introduction

Background

Rock Mechanics and Mineralogy

Slope stability research is an interdisciplinary field that overlaps, but is not limited to, geology,

soil mechanics, and rock mechanics. It mainly seeks to understand the behavior of rock or soil masses that may be stable or unstable depending on their material properties, excavation geometry, and the presence, abundance, and properties of large-scale discontinuities that include joints, faults and bedding planes. Assessing rock mass stability is crucial to the mining industry, as well as to civil engineering, since it relates directly to operating costs and safety constraints. For open pit mines to achieve their most profitable mining, they must maintain a slope angle shallow enough to maintain acceptable factors of safety, but steep enough to minimize the amount of material that has to be excavated and moved.

A significant amount is known about slope stability based on previous research and abundant case studies (e.g. reviews by Armstrong and Kemeny, 1991; Chen et al., 2017; Ferrier et al., 2016;

Griffiths and Lane, 1999; Lyons-Baral and Kemeny, 2016). However, active research is still very important as many aspects of slope stability remain unquantified or poorly understood. This is particularly the case for topics such as time-dependence of failure, the effect of mineralogical and lithological parameters, and strength degradation due to mineral alteration, which is the topic of this thesis.

Certain features of rock slopes provide qualitative clues to their potential movement. Rock type

exerts a major influence. Different rocks have different unconfined compressive strength,

cohesion, friction angle, tensile strength, triaxial, and shear strength values, due to differences in 11

fabric, mineralogy, and texture. Thus, slopes with different rock compositions will behave

differently. Unaltered and unweathered igneous rocks, for example granitoids, are normally strong.

Certain metamorphic rocks, for example schists, and certain sedimentary rocks, for example shale,

are generally weaker and more prone to instability, as well as being highly anisotropic. Weathering

of any rock type generally results in a strength reduction, and even granitic rocks, when highly weathered, have demonstrated significant stability problems. A second important feature is the

orientation of discontinuities relative to the orientation of a slope. Discontinuities or combinations

of discontinuities that “daylight” into a slope are much more prone to make a slope unstable. The

shear properties of the discontinuities are also important, and features such as faults containing

clays, or foliation in metamorphic rocks containing soft minerals, are more prone to cause slope

problems than those with substantial roughness. Porosity is another important factor. Water

content within the rock mass affects its movement, since pore pressure can help a rock mass

overcome frictional resistance to sliding. Water content may be a source of strength loss when

dealing with unconsolidated material. In the case of strong igneous and metamorphic rocks, the

low porosity generally keeps water content too low to significantly influence the intact rock

strength but increases in pore pressure along discontinuities can be a major source of slope displacement. Sedimentary rocks can have a significant porosity, and pore pressure buildup in these rocks is one of the most important factors in slope stability calculations.

In addition to filling pores, water can also contribute to geomechanical weakness through interactions with swelling minerals. The crystallographic structure of clays consists of stacked tetrahedral-octahedral sheets separated by an interlayer with exchangeable ions. In some clay types such as smectite, water will absorb into the interlayer between sheets, and cause expansion of the structure up to 250% of the original width of the unit cell along the c-axis. The hydration of 12 anhydrite to gypsum is also accompanied by a 61% volume increase (Wittke, 1990; Yilmaz, 2001).

The substantial changes of volume from clays and gypsum can be trivial contributors to micro- fracture growth and rock strength, but if the rock contains high proportions of gypsum or swelling clays, or even a relatively small amount that is localized along fractures, the overall negative impact on slope stability can be substantial.

Types of Failures and Quantifying Failure Conditions

Geotechnical failure is a complex and important topic, especially in modern and future mining.

The advance of technology in the mining industry has made it possible for mining operations to strip deeper ore while maintaining safe high wall slope stability conditions, even as increased depth and higher slope angles add to the risk of slope failure. Geotechnical personnel need to know the conditions that drive failure mechanisms, how they started, their dimensions, and how they will propagate through time in order to avoid damages not only to operations, but also and especially to the personnel at the mine.

Some causes of slope instability have long been understood. These include, but are not limited to, erosion, rainfall, seismic events, geological features, loading, excavation, slope height, and bench angle. More recently, advances in computation have allowed numerical modeling of slope failure in rock masses to provide additional insights into the types, risks, and behavior of rock mass failures. Generally, they have been classified into four different failure types: plane, wedge, circular, and toppling. Each one of them has specific features reviewed by (Martin, 1990; Hoek and Bray, 1978; Adhikary et al., 1996; Cruden, 1989; Jiang et al., 2013; Pariseau and Voight, 1979;

Pritchard and Savigny, 1990; Wang et al., 2012; Yang et al., 2012; Zhang et al., 1989). 13

The plane slope failure consists of a daylighting sliding plane striking subparallel to the slope face.

For failure to occur under the influence of gravity and discontinuity friction alone, the sliding plane

dip must be greater than the plane friction angle. The upper end of the sliding surface must intersect

the upper portion of the slope or end in a tension crack so that the sliding block has a release

surface.

Wedge failures are characterized by a rock mass that slides along two planes of weakness that cut

through the slope and intersect each other inside the rock mass. According to Piteau and Martin

(1982), the failure may involve the central block either sliding along the intersection of the two planes, sliding along only one of the two planes, sliding and rotating on one plane prior to separation across the other one, or progressive raveling in the case of a highly jointed rock mass.

Wedge failures are among the most important type to consider in designing the geometry of an

excavation slope and are particularly prone to time-dependent failure. If a rock mass is gradually

decreasing its strength and friction angle due to time-dependent weathering and degradation

processes, it is possible that there will be a proportional increase in wedge failures due to the

associated changes in friction angle and cohesion.

Circular failures are more likely to happen along weathered, heavily fractured rock masses. The

sliding surface is the line with the least shear resistance to slide. Generally, the joint spacings and

block sizes comprising the rock mass are significantly smaller than the rock mass size. Within the

category of circular failures, three subtypes are distinguished: slope failure, toe failure, or base

failure (Figure 1). 14

Figure 1: Diagram illustrating different circular failure mechanisms. From left to right: slope failure, toe failure, and base failure.

Toppling failure describes the rotation of blocks along a base, similar to how dominos knock one

another down. Toppling failures can be divided into two types: block toppling and flexural

toppling. Block toppling occurs when two joint sets, one orthogonal to the other, dip steeply

towards the slope face, forming blocks of rock that rotate through failure. Flexural toppling occurs

when the slope fails due to columns of rocks that are separated by steeply dipping discontinuities.

The risk of any of these failures is quantified by the factor of safety, a parameter used to classify

a slope as stable or unstable. Factor of safety is defined by the ratio between the forces holding and the forces driving the slope failure (Equation 1). For failure in general, the shear stress at a

location in a rock mass or along a discontinuity must be more than the shear strength at that

location:

= (1) 𝜏𝜏 𝐹𝐹𝐹𝐹𝐹𝐹 𝑓𝑓 τf is the shear strength or stress calculated from a numerical𝜏𝜏 or kinematic model.

Another way to calculate the factor of safety is through Strength Reduction Factor which adjusts

the strength parameters along the slope until it is unstable (FOS < 1). 15

Modeling How Rock Mass Characteristics Affect Slope Stability

Once the relevant rock and discontinuity characteristics such as porosity, discontinuity density, strength, friction angle are known, their exact impact on slope stability can be quantitatively evaluated by the use of a numerical simulation such as the Finite Element Method (FEM) and the

Discrete Element Method (DEM). These are widely used by geotechnical engineers to predict slope displacement and changes in factor of safety, and to design slope angles, buttresses, and other mitigation processes to improve slope stability.

Of the available modeling methods, FEM has advantages over other approaches to slope stability modeling, as discussed by (Griffiths and Lane, 1999; Hammah et al., 2004) and others. In FEM modeling of slopes there are multiple variables, including, but not limited to, (1) rock type along the slope and (2) selection of failure criterion. Included in the rock type are crucial inputs such as unit weight, Poisson’s ratio, Young’s Modulus, axial effective principle stress strength, and confining effective principal stress. For the failure criterion the inputs are the rock mass’s

Geological Strength Index (GSI), the intact rock constant (mi), and the disturbance factor (D)

(Equations 4 through 7).

100 = exp (2) 28 14 𝐺𝐺𝐺𝐺𝐺𝐺 − 𝑚𝑚𝑏𝑏 𝑚𝑚𝑖𝑖 � � − 𝐷𝐷

100 = exp (3) 9 3 𝐺𝐺𝐺𝐺𝐺𝐺 − 𝑠𝑠 � � − 𝐷𝐷

1 1 = + (4) 2 6 −𝐺𝐺𝐺𝐺𝐺𝐺 −20 15 3 𝑎𝑎 �𝑒𝑒 − 𝑒𝑒 �

= + + 𝑎𝑎 (5) ′ 3 1 3 𝑐𝑐𝑐𝑐 𝑏𝑏 𝜎𝜎 𝜎𝜎′ 𝜎𝜎′ 𝜎𝜎 �𝑚𝑚 𝑐𝑐𝑐𝑐 𝑠𝑠� 𝜎𝜎

Where σ1 and σ3 are the axial and confining effective principal stresses, σci is the UCS gathered through geotechnical tests, mb is the rock mass reduced value of the intact rock constant, and s

and a are constants based on the rock mass features.

Porphyry Deposit Geology and Alteration and Relationship to Geomechanics

Since the research described in this thesis was performed at a porphyry deposit, a short introduction

to the deposit type is in order. Porphyries are among the world’s most important deposits of copper,

tin, tungsten, and molybdenum, and can contain gold and silver as well. This deposit type is

characterized by igneous intrusions typically of granitic to intermediate composition, with a

porphyritic texture and a group of characteristic alteration and mineralization styles. Reviews are

provided by Seedorff et al. (2005) and Sillitoe et al. (2010), among others.

Briefly, magmas derived from partial melting of the subducting plate rise through the overlying,

denser crust. As pressure and temperature decrease, the magma begins crystallizing and exsolving

fluids, typically rich in chloride. These salty fluids circulate through and around the magma,

scavenging metals, but eventually build up fluid pressure at the top of the magma chamber due to

continued exsolution. When the accumulated fluid pressure exceeds the strength of the overlying

rock, the rock shatters and the fluids escape and expand into the fractured cupola. Upon expansion

they cool, precipitating metal sulfide ores as the solubility of the metals they carry decreases. This

process leads to the formation of deposits that are typically high in tonnage, but lower in grade

compared to other deposit types (Anderson, 1948, 1950; Anderson et al., 1955; Wilkinson and

Kesler, 2009; Seedorff et al., 2005; Singer et al., 2008; Kelser and Wilkinson, 2004). This original 17 hypogene mineralization typically forms 6-10 km below the surface. If uplifted and exposed through weathering or faulting, sections of the hypogene orebody can be subjected to supergene alteration by oxygenated groundwater, which dissolves the hypogene sulfides and reprecipitates them as oxide and carbonate ores, sometimes after downstream transport. The sulfide assemblages, ore mineralogy, rock types, mineralization styles, and other characteristics vary among different porphyry deposits (Nielsen, 1968).

The complex high-temperature hydrothermal fluids that form porphyry hypogene ores also change the initial mineral assemblage of the original host rock when in contact with it. The alteration is usually variable throughout the deposit since a whole range of different fluid compositions and temperatures may react with the same rock type. The alteration products depend on the fluid composition, the host rock composition, the pressure and temperature of the environment, the fluid-rock ratio, and other geological factors. In some situations, the rocks can be overprinted by various alterations due to subsequent reinjections of fluids from different stocks (Figure 2 and

Table 1).

Granitoid Mineralogy Alteration Kf Plag (Ab) Bi Qz Hbl Mt K-silicate ~ ±Kf -/sBi ~/+ sBi +/~ Na Ab Ab Chl(±Rut) +/- Chl(±Rut) Rut Na- Ca Na(-Ca) Olig(±Epid) Olig(±Epid) Act(±Tit) -/+ Act(±Tit) Tit Ca Ca Plag±Gt±Epid Ca Plag±Gt±Epid Cpx(±Tit) -- Cpx(±Tit) Tit Sericitic Ser Ser Chl/Ser±Py +/~ Chl/Ser±Py Rut(Leuc) Advanced Argillic ALS/ Pyrophyllite ALS Py/ALS/Dickite +/~ Py Rut Propylitic Rf Ab+(Epid/Ser/Cal) Chl(±Rut) ~ Act( / Chl) (±Rut) Hm(±Py) Table 1: Mineral assemblage associated with different hydrothermal alteration (Seedorff et al., 2005).

B Figure 2: Outcrop illustrating the change in joint spacing with type of hydrothermal alteration between a zone of intense sodic-calcic alteration (A) and a zone of intense potassic alteration with a propylitic overprint (B) at the Yerington district (Nevada). Joint spacing increases in the zone of sodic-calcic alteration. 19

Figure 3: Cross-section of an idealized porphyry alteration zoning system (Berger et al., 2008; Lowell and Guilbert, 1970).

The types of alteration involved in porphyry formation were described by Lowell and Guilbert

(1970), among others, and each mineralogical product of the different alteration types can be found

in Table 1. The mineralogy and the alteration types may not be consistent throughout the deposit

since the alteration product depends upon the fluids and host rocks specific to the deposit, and are commonly overprinted by multiple hydrothermal pulses (Beane and Bodnar, 1995; Creasey, 1959;

Hemley et al., 1967; Rose, 1970; Schwartz, 1947).

Most commonly, a subset of the original silicates such as feldspars, pyroxene, and amphiboles, degrade into clays, chlorite, sericite, and other phases. Most of these alteration minerals are weaker than their original progenitors, particularly the clays. Hypothetically, therefore, extensive alteration could change the overall rock strength, making it weaker and more likely to degrade through time. Further alteration, such as the type associated with subsequent crystallization of

secondary sulfides, oxides, and carbonates in supergene settings could also cause degradation over

time, also affecting rock mass strength. Alternatively, a few alteration styles such as silicification 20

can result in rocks that are harder and stronger than their unaltered or differently altered

equivalents.

The impact of alteration on geotechnics is largely unconstrained but likely considerable. Figure 2

shows an example from an exploration trench near a porphyry deposit in Nevada where differences

in alteration style result in substantially different joint spacing. The exposure shows different

alteration types and grades lead to a fracture density that is considerably higher along zone A

compared to zone B, giving zone A a lower Geologic Strength Index (GSI) value (Hoek et al.,

2000; Marinos and Hoek, 2001).

Geology of the Study Site

The Bagdad Mine is a porphyry copper-molybdenum open-pit mine located in west-central

Arizona, 100 miles northwest of Phoenix and 60 miles west of Prescott (Figure 4), with annual production of 218 million recoverable pounds of Copper as of 2019. Its recoverable proven and probable mineral reserves consist of 2.5 Gt at 0.32% copper and 0.02% molybdenum (Freeport-

McMoRan 10-K Form, 2019) making it one of the largest copper-molybdenum deposits in the

United States. Mineralization includes hypogene chalcopyrite, molybdenite, and other sulfides, along with supergene chalcocite, chrysocolla, malachite, and azurite.

Figure 6 and Figure 7 show the geologic map of the Bagdad area. The local geologic setting is a syncline of Proterozoic metavolcanics and metasediments dismembered by multiple fault systems that were intruded by Mesoproterozoic plutonic rocks ranging from gabbro to rhyolite in composition. It was formed between 80 and 55 million years ago during the Laramide orogeny with rock of quartz monzonitic composition which was later intruded by Precambrian igneous formations which are also part of the Yavapai Series. The Yavapai Series consists of lava flows, 21

pyroclastics, intrusive and sedimentary rocks that have been metamorphosed to meta-volcanics, meta-intrusives, schist, quartzite and slate. After the Yavapai Series, multiple intrusions happened in this region such as the silicic King Peak rhyolite, several mafic intrusions, silicic alaskite porphyry and the Lawler Peak granite. During the Tertiary, the rocks were eroded forming the Gila

Conglomarate formation that is afterwards followed by rhyolite domes and basaltic lava flows.

(Figure 6) (Anderson et al., 1955; Rathkopf et al., 2017; Titley 1982; Barra et al, 2003).

Figure 4: Geographic location of the Bagdad mine in the United States.

The copper-molybdenum mineralization is mostly related to the early quartz monzonitic-

granodioritic stocks that later were intruded by NE-oriented bodies of monzogranite and

granodiorite. Small zinc and lead deposits occur at coeval breccia bodies of rhyolitic and granitic

compositions. Supergene leaching, oxidation, and enrichment occurred throughout the Tertiary

until the area was overlain by the Gila Formation and basalt flows (Anderson et al., 1955; Rathkopf

et al., 2017). The largest mineralized stock in Bagdad is the quartz monzonite, whose textures 22

range from porphyritic to seriate. The dominant minerals are quartz, plagioclase ranging from

calcic oligoclase to andesine, biotite, hornblende, sphene, magnetite, apatite, and zircon. The

quartz monzonite and other rock units are cut by a series of major structures trending dominantly

northwest (Figure 7).

Bagdad’s hypogene orebody is divided into three zones based on pyrite content. The central part of the deposit consists of <1 weight percent pyrite, while the surrounding zone has 1-4 weight

percent pyrite. The outer zone consists of >4 weight percent pyrite. The highest content of

chalcopyrite tends to be through the intermediate pyrite zone and decreases inwards and outwards.

Figure 5 illustrates how a vein system looks like and where the sulfides are located relative to the

gangue minerals.

Figure 5: Schematic illustration of typical vein formed during multiple stages of alteration. The white zone consists of vein of quartz-sulfide-Cu (±Carbonate) with fixation of SiO2 released due to hydration, carbonatization, and sulfidation of silicates. The green zone is the proximal zone of sulfide (sericite, ± Carbonate) with fixation of S and Cu. The yellow zone is the intermediate zone of carbonate (±chlorite, some sulfide) with CO2 fixation. Finally, the red zone is the distal zone with chlorite (±carbonate, less sulfide content) and fixation of H2O.

Figure 6: Geologic Map of the Bagdad Pit with local structures separated into West (1), North (2), East (3), and Southeast (4) where geotechnical test and thin section samples were collected based on map from Rathkopf et al., 2017. 24

Most of the chalcopyrite ore in Bagdad occurs as quartz-chalcopyrite-pyrite veinlets with K- feldspar and biotite selvages. Molybdenite crystallized later and mostly occurs with quartz in veinlets with k-feldspar and accessory chalcopyrite, magnetite, and pyrite (Anderson et al., 1955;

Barra et al., 2003; Rathkopf et al., 2017).

Figure 7: Geologic map indicating the major formations and structures around the Bagdad area. (Rathkopf et al., 2017 and sources therein).

Multiple types of hydrothermal alteration are present throughout the deposit, potassic and sericitic

alteration being the most common. The potassic alteration in Bagdad consists of secondary

orthoclase replacing primary feldspars in vein envelopes up to 5mm thick, and igneous biotite and

hornblende replaced by secondary shreddy biotite. The potassic alteration is not restricted to any

lithology, but is mostly associated with the molybdenite veinlets, and it has different intensities

over the deposit (Rathkopf et al., 2017). Sericitic alteration is less common than potassic alteration 25

and consists of sericite replacing K-feldspar and plagioclase, and sericite-chlorite replacing biotite.

An overprinting, less widespread and less intense propylitic alteration episode turned some of the

biotite into chlorite (Anderson, 1948; Anderson et al., 1955). This study used samples with mainly

sericitic alteration, which produced an assemblage of sericite-molybdenite-quartz-orthoclase- pyrite-chalcopyrite.

Pregnant Leaching Solution Infiltration

While the open pit excavations are generally quite stable, chronic instability is a problem in some

mines, in particular sections of pit walls. The exact causes of movement are typically complex, but probable factors include geological structures, rock mass strength, and pore pressure. For those mines with large, unlined run-of-mine leaching operations, the infiltration of acidic pregnant leach solution (PLS) in the slope is also a potential factor. In addition to increasing pore pressure, ongoing PLS infiltration may also further alter and weaken the rock mineralogy, depending on the conditions, chemistry, and mineralogy of the rocks. The water contained in PLS and rainfall may also be increasing the rate of sulfide mineral weathering in the rocks, which generates additional sulfuric acid that furthers rock degradation observed in core samples (Figure 8).

Chapter 2 – Literature Review

Several previous studies have examined the effects of hydrothermal alteration on rock mass

degradation in multiple environments, including non-porphyry systems and all sorts of hydrothermal alteration types. Watters et al. (1995) reported that argillic alteration in phonolites and andesites decreases not only rock strength, but also joint cohesion, joint friction, and unit weight. They found that argillic alteration decreased the uniaxial compressive strength (UCS) of phonolite from 8,500 psi to 500 psi, the joint cohesion from 43 psi to 14 psi, the joint friction from

40º to 22º, and the unit weight from 150 lb/ft³ to 130 lb/ft³. For the same alteration grades and types, the andesite UCS decreased from 18,000 psi to 1,000 psi, the joint cohesion from 38 psi to

10 psi, the joint friction from 38º to 13º, and the unit weight from 155 lb/ft³ to 125 lb/ft³. From a slope stability standpoint, the argillic alteration indicated that a less altered slope of 450 ft. high would start to see instability (defined as Factor of Safety of 1) when the slope angle is steeper than

60º, while a more altered slope of the same height would start to see instability with a slope angle any steeper than only 40º.

Similarly, Kohno et al. (2012) examined the changes of intact rock strength in rocks subjected to forced wetting, ignoring the effects on joint properties and using point load strength index and uniaxial compressive strength correlations. The findings showed that the relationship between both tests is UCS = 16.4 Is(50) in forced-dry states and UCS = 16.5 Is(50) in forced-wet states in soft rocks, defined as samples with uniaxial compressive strength below 3,625 psi. The correlation was only possible in soft rocks with high porosity, since strong low-porosity rocks could not be wetted.

Coggan et al. (2013) used UCS to compare changes in intact rock strength due to hydrothermal alteration. They segregated samples into 5 levels of alteration intensity from fresh (1) to completely 27 altered (5) and measured their density, porosity, kaolinite content, P-wave velocity, UCS, and elastic modulus. Intense alteration correlated with lower UCS, primary wave velocity, density, and

Schmidt Hammer rebound in granite samples. For the Schmidt Hammer Rebound Number, the least altered samples registered values above 50 while completely altered samples registered values below 10. The densities of fresher samples were roughly above 156 lb/ft³ while completely altered samples were roughly 120 lb/ft³. The Primary-Wave velocities for fresher samples were roughly 5,000 m/s while those of more altered samples were between 2,000 and 3,000 m/s. Finally, the UCS of fresher samples were mostly above 17,000 psi while those of more altered samples were mostly below 8,700 psi. All in all, the data collected by Coggan et al., 2013 indicated that the argillic alteration and corresponding increase in kaolinite content reduced the intact rock strength (Coggan et al., 2013).

Watters et al. (1995), Kohno et al. (2012), and Coggan et al. (2013) illustrated how rocks degrade due to hydrothermal alteration. However, another potential cause of degradation is the effect of clay content on pore pressure, since the clays formed by hydrothermal alteration can reduce permeability and prevent efficient water flow through or across the fractures. To address this issue,

Sullivan (2007) studied the effects of clays in pit wall depressurization. His findings indicated that the clays led to low permeability preventing rapid depressurization (Sullivan, 2007).

These studies show that hydrothermal alteration is not only a concern for geologists, but also matters to geotechnical and geomechanical engineers. Nevertheless, mineral alteration has not played a significant role in most previous studies of rock mass stability analysis and modeling.

Alteration outside the hydrothermal environment, for instance by PLS infiltration, is also potentially crucial but also largely unstudied. Even though the leaching acid is intended for copper- 28 bearing minerals, it also affects other minerals present in the rock, but whether the PLS-related geotechnical issues at sites relate to acid-mineral reactions weakening the rock, or to increased pore pressure, remains unknown.

Either or both would be a plausible explanation. The impact of pore pressure on slope stability has been discussed above. From a leaching perspective, Baum (1999) studied the effects of PLS exposure on gangue minerals in leaching experiments. He described the breakdown of acid- unstable gangue phases into more acid-stable ones, for instance the breakdown of chlorite into montmorillonite. The reactions took place over a matter of months to a few years at ambient conditions, so decades of PLS infiltration could easily have a similar or greater effect. Quartz monzonite, which is as common rock in porphyry systems, has a moderate reaction to PLS, reacting more than felsic rock types but less than more mafic ones. In the case of quartz monzonite, new minerals likely to form from the effects of PLS could include phases such as gypsum, montmorillonite, kaolinite, iron oxides, and biotite. The precipitation of these minerals could affect rock strength much like those formed by hydrothermal alteration.

Considerable geotechnical information can also be obtained through LiDAR (Light Detection and

Ranging) instruments, which collect a point cloud, a dense set of points or laser reflections that accurately depicts the geometry of a surface such as a rock slope. Each point has information about its surface location (x,y,z) as well as information about the surface color and reflective intensity

(r,g,b,i). Point clouds can be obtained from LiDAR scanners, drones equipped with cameras, or other methods. Important aspects of point clouds include the density of points (spacing between points) and the accuracy of each point in representing the actual location of the surface. In general both are higher for parts of the slope closer to the scanner. 29

Kemeny et al. (2015) described multiple geotechnical applications for LiDAR and point clouds,

including characterization of overhanging slabs and other important features of road cuts and

slopes. Similarly, Enge et al. (2014) and Hubbard et al. (2012) employed geophysics to

characterize underground reservoirs through boreholes and point cloud (Enge et al., 2007; Hubbard

et al., 2013). Griffith et al. (2014) tied LiDAR based DEM to subsurface stresses applying it to an

underground coal mine. Similarly, data collected through LiDAR by Lyons-Baral (2012) indicated

that the subsurface structure of Coronaro Cave, Arizona, was influenced by the regional topography (Griffith et al., 2014; Griffiths and Lane, 1999; Lyons-Baral, 2012). In this case, cave stress numerical modeling using the high-resolution point clouds indicated failing joints while a lower resolution underground model was not able to reproduce the high-resolution results. With the advance of automation, it is possible to extract fracture data through point cloud automatically as described by Chen et al. (2017). Chen et al. (2017) developed a cutting-edge methodology calculating the normal vectors for every point cloud, using Ransac Shape Detection method, collect intersecting segments data, and using a Floodfill algorithm to adhere true block data (Chen et al.,

2017). This is another project that could only be successfully accomplished due to the use of high- resolution point clouds. The point cloud approach is crucial for the mining industry and has a long tradition in the geotechnical data acquisition (Kemeny, J., Combs, J., Lyons-Baral, 2015; Lyons-

Baral, 2012; Lyons-Baral and Kemeny, 2016; Walton et al., 2016). The point cloud processing constraints depend on software and hardware parameters, but with the most recent advances, it is possible to process large amounts of data with only a personal computer.

The purpose of the research described in this thesis is to understand the effects on rock strength of hydrothermal and PLS-related alteration, and to illustrate by numerical modeling how they can 30 affect slope stability, using rock mechanics tests and alteration information from the Bagdad mine and assumptions on how PLS would affect generic slopes.

Chapter 3 – Problem Definition and Methodology

General degradation of rock masses around pit walls over time, and the observed movement, could be related to sulfide minerals weathering to sulfuric acid which attacks other minerals in the rock, and/or due to pore pressure from rainwater and PLS, PLS infiltration causing additional acid alteration, and movement along fault sets (Figure 8).

To answer this question, this project aims to (1) evaluate changes of rock strength due to hydrothermal and artificially PLS-induced alteration, and (2) use numerical modeling to quantify the rock mass degradation and related slope movement using Bagdad’s samples since Bagdad’s lithology and alteration are typical of porphyry copper deposits.

The methodology for (1) consisted of describing the mineralogical, textural, and chemical features of core samples and tying them to their geomechanical properties. First, sections of drill cores that broke naturally along veins or fractures related to hydrothermal alteration were selected and examined, to determine the intensity of alteration of igneous minerals to hydrothermal minerals that could diminish rock quality designation. Petrographic analyses in 10 different thin sections obtained from samples from multiple parts of the pit were conducted, followed by analysis using a JEOL 6010LA benchtop Scanning Electron Microscope (SEM). of fractures formed during the geotechnical tests, as well as on samples artificially soaked in PLS solution and water after the scan to determine clay mineralogy and detect swelling clays. Finally, I applied this data to a

Continuum Finite Element Model using a generic slope to observe the effects of degradation along a slop (Figure 9). 32

Figure 8: Illustration of the same core from the Grasberg mine, 3 years apart. The RQD of the section below 102 m changed through time, probably due to the oxidation of pyrite in the core generating sulfuric acid.

In addition, 4 samples of X-Ray Diffraction (XRD) data were obtained, which enabled the quantification of the rocks’ sheet silicate contents and correlations with their intact strength and

density. Secondly, I conducted Point Load, Brazilian, Uniaxial Compressive Strength, Triaxial,

Rock Shear, Moisture Content, and Specific Gravity tests on samples throughout the pit (Table 2).

The goal was to examine how the overall intact strength of rocks compared throughout the pit with

the same or similar lithology and alteration characteristics. Short-Wave Infrared (SWIR) scanning

was carried out on the surfaces

The effects of alteration derived from the petrographic, XRD, SEM, and SWIR analyses, and the

geomechanical properties from the rock mechanics tests, were inputs into a finite element model

to quantify the effects of mineral alterations in rock mass stability. In this continuum finite element model, different alteration zones were represented by rock parameters taken from the results of the geotechnical tests on samples of the corresponding type and extent of alteration and degradation.

Each element of the triangulated FEM mesh tracked the stress-strain state of the rock mass, and its

properties were degraded accordingly to their degree of alteration degree of each zone. The

progressive deformation and failure predicted by the model were correlated with the data collected

from samples with different degrees of alteration using RS2 by Rocscience.

Data Acquisition Number of Samples Point Load 5 Brazilian 45 UCS 29 UCS with Strain Gauges 5 Triaxial 8 Moisture Content 5 Specific Gravity 5 P-wave velocity, Poisson’s Ratio, Young’s modulus from Point Load 5 Shear Test 1 Petrographic thin section examination 10 SWIR analyses 16 Table 2: Number of samples tested for each analytical method. 34

Geomechanical Properties Geological Properties

RQD Analysis Field Work

Geotechnical Short Tests Wave Infrared

PLS Alteration Alteration Character- Method ization

Finite Element Model

Figure 9: Diagram illustrating the workflow for this project.

Geotechnical Tests

Point Load

The point load test applies a pointed axial load to both sides of the specimen, and upon failure, the

point load strength index is calculated. In equation 8 and 9, Is, P, De, and Is(50) stands for Point

Load Strength Index, applied load at failure, Standard Equivalent Diameter, and the Point Load

Strength Index Corrected for 50 mm diameter cores, respectively.

= (6) 𝑃𝑃 𝑙𝑙ₛ 2 𝐷𝐷ₑ = (7) 1.967 𝐷𝐷ₑ 𝑙𝑙ₛ ₍₅₀₎ � � ⁰·⁴⁵ The loading stress is recorded after the sample fails parallel to the axis.

Uniaxial Compressive Strength

The Uniaxial Compressive Strength (UCS) test applies a distributed force across the end of the

sample instead of a point load along its axis. Strain gauges were glued on the sample surfaces to

measure elastic properties at the same time. UCS is calculated as the peak of the collected

force/deformation or stress/strain curve (Bieniawski, 1975; Broch and Franklin, 1972) (Equation

10). It is related to the intact rock cohesion, Co, through the formula:

2 = (8) 1 𝐶𝐶₀𝑐𝑐𝑐𝑐𝑐𝑐∅ 𝑈𝑈𝑈𝑈𝑈𝑈 Where is the internal angle of friction, which is −determined𝑠𝑠𝑠𝑠𝑠𝑠∅ by conducting triaxial tests at

different∅ confining stresses. 36

Disk Tensile or Brazilian Test

The Brazilian or Disk Tensile tester applies a load to the top and bottom curved sections of the sample and interrupts the load automatically as soon as the failure happens (Figure 10). The tensile strength (T) is calculated through equation 11 using the Applied Load (P), Sample Thickness (L), and Sample Diameter (D).

2 = (9) 𝑃𝑃 𝑇𝑇 𝜋𝜋𝜋𝜋𝜋𝜋

Figure 10: Example of Load Cell Versa stress through time for the Brazilian test.

Triaxial Strength

The triaxial test is similar to the UCS test, but has a confining stress in addition to the applied axial load. The sample is inserted in a rubber tube that seals it completely. The sealed sample is placed into the SBEL CT-500 load frame, which applies a confining compressive stress. The confining stress is held constant while the axial load is increased until failure. The final principal stress can be calculated through Equation 12. 37

( ) = (10) ( ²) 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺 𝑙𝑙𝑙𝑙 𝜎𝜎₁ The axial load consists of the same procedure𝐴𝐴𝐴𝐴 used𝐴𝐴𝐴𝐴 for𝑖𝑖𝑖𝑖 the UCS test and also produces a stress-

versus-time graph showing failure when the stress drops substantially. A series of triaxial tests at

different confining stresses are conducted in order to determine the strength increase due to an

increase in confinement, which is related to the internal angle of friction. For example, the mi

parameter can be determined by assuming the Hoek and Brown intact failure criterion through triaxial tests.

Small-Scale Direct Shear test

All the laboratory tests described above measure the intact rock properties. However, equally important for modeling purposes is to determine the shear properties of pre-existing joints and fractures, done with the direct shear test. Unlike destructive geotechnical tests such as Point Load,

UCS, Brazilian, and triaxial tests, the Small-Scale Rock Shear test consists of applying normal and shear forces to naturally formed fractures. A test consists of applying a constant normal stress and then increasing the shear stress until slip occurs along the fracture. The test is repeated multiple times under different normal stresses to determine the friction angle and cohesion associated with the fractures. The direct shear test is non-destructive, meaning that multiple shear tests at different normal stresses can be conducted on a single sample (Barton and Choubey, 1977).

Primary and Secondary Seismic Wave

The Primary (P) and Secondary (S) wave test procedure is also a non-destructive geotechnical test.

This test measures the material’s P and S wave speeds, which can be correlated with the elastic

properties, Young’s modulus and Poisson’s ratio. The machinery consists of an oscilloscope with

transmitting and receiving transducers, which the sample is placed in between. The transducers 38

measure the time for a wave to travel, which when divided by the distance yields P and S wave

velocities. The P-wave testing allows comparisons between UCS and P-wave velocity, and

between the point load index and P-wave velocity.

Moisture Content and Specific Gravity

The moisture content and specific gravity tests were carried out on left-over pieces of core samples

used for the aforementioned geotechnical tests. The samples were roughly 2 inches by 1 inch,

irregular, and chosen to be typical of the general mineralogical composition of the core samples.

Moisture content was assessed by weighing samples after heating each in an oven for 24 hours at

100 ºC and comparing its weight afterward to its pre-heating value.

Rock Characterization and Short-Wave Infrared Spectrometry (SWIR) Data

The initial rock characterization analysis was based on conventional techniques such as hand

sample and petrographic 30 μm thin section examination. However, neither of these techniques is

suitable for identifying clays and other fine-grained phyllosilicates. To characterize the

hydrothermal minerals that led to the potential loss of cohesion and strength resulting on the rock

degradation, and to characterize the mineralogy in greater detail, this project examined the samples

through Short-Wave Infrared (SWIR) spectrometry. SWIR spectrometry measures the change in

reflectance and absorption of light as a function of wavelength, which is a proxy for mineralogy

(Clark, 1999). The SWIR analysis consists of a shortwave infrared measurement ranging from 0.4

– 2.4 μm wavelength (Ferrier et al., 2016), recording reflections based on the wavelength. It is a

crucial tool to identify, in particular, the Al-OH bearing minerals, which are usually associated with argillic alteration; for instance, kaolinite features prominent peaks at 2.2 μm. Other hydroxyl- metal bonds, such as Mg-OH and Fe-OH, are also detectable, although Si-O and others absorb outside the SWIR wavelength range (Clark, 1999). 39

SWIR analyses were performed with an ASD Systems TerraSpec Pro Analyzer, using a 1-cm- wide contact probe. For mineral identification, the spectra were processed by The Spectral

Geologist (TSG) software by Spectral Geoscience. Backgrounds were normalized against a

SpectraLon white reference prior to scanning the sample. The scanned locations along the samples consisted of natural fracturesand failure planes, where the samples broke due to Point Load,

Brazilian, UCS, Triaxial and shear tests. These represented the mineralogy of the rock as a whole and the mineralogy along the fractures.

X-Ray Diffraction

XRD uses patterns in the X-ray scattering caused by the regularly spaced ions in the crystal lattice to identify mineral lattice parameters, which are a proxy for mineral type. In this project, I examined correlation between XRD indicators of alteration grade and UCS in order to determine how alteration affected UCS and what UCS parameters to use for the Hoek-Brown failure criterion in modeling. Minerals detectable by XRD and considered indicators of alteration grade include kaolinite and other clays, calcite, and chlorite.

Pregnant Leaching Solution Degradation

To supplement the petrographic and SEM search for evidence of PLS-related degradation, core samples were soaked in PLS for 5 days and scanned them with SWIR before and after submersion, comparing the change against SWIR measurements of one control sample soaked in tap water and one left in air for an equivalent time (Figure 11). 40

Figure 11: Samples soaking under PLS and tap water for 5 days. Petrographic and SEM Characterization

Ten thin sections of variably altered quartz monzonite, 5 from the East wall and 5 from the

Southeast wall along zones 3 and 4 of Figure 6, respectively, were examined under the

petrographic microscope to characterize the extent, types, and mineralogy of alteration. Afterwards

they were examined with a JEOL 6010LA scanning electron microscope (SEM) to look for

evidence of chemical change consistent with mineral-fluid reactions. Evidence of chemical reactions related to hydrothermal alteration would include the replacement of igneous feldspars by clays, the replacement of pyroxene/amphibole by biotite, the replacement of biotite by chlorite, and the presence of veins and veinlets. Evidence of chemical reactions related to PLS-rock would include clay compositions along fractures suggestive of highly acidic low-temperature solutions through fractures; mineralogical or textural differences between quartz monzonite samples; or changes in overall rock/mineral chemistry with distance from natural fractures that would suggest acid leaching of the rock close to the fractures. 41

Slope Stability Finite Element Model

A rock mass failure analysis was conducted using the RS2 software by Rocscience to evaluate the

effect of material degradation on slope stability. The goal was to develop a methodology in which

open pit mines could use to identify changes in stability and correlate them with extent of alteration. It would be important to delineate which sections of the slope could be affected by this degradation process. The Rocscience RS2 simulation were conducted assuming intact rock properties and simulations assuming Hoek and Brown rock mass properties, with no explicit jointing, to predict localized failure along the benches and to determine the factor of safety for large scale circular failure to occur. Table 9 gives all the material properties used for the simulations, including the strength properties, elastic properties, and the in-situ stress assumptions.

Insofar as possible, these parameters were set to values obtained from actual laboratory or field measurements. In many cases, these were not available, so the modeling should be considered preliminary and approximate.

The cross-sections used for the RS2 modeling were generic cross-sections to exemplify how a major slope would behave under different alteration criterions. The same cross-section was used to describe not only the different degrees of alteration slope, but also to simulate how the slope would behave if the RQD and joint spacing shifted from top to bottom due to potential PLS infiltration. The overall slope angle is 45°, the bench angle is 55°, the bench height is 10 meters, and the slope height is 600 meters. The boundary conditions consisted of fixed boundaries at the bottom, “roller” boundaries at the sides, and free boundaries on top to accurately simulate slope movement. The mesh used for the slopes were created with 6 noded triangles (Figure 12). 42

Figure 12: Example of meshed slope and boundary conditions along slope.

The modeling approach used a generalized Hoek and Brown model for the RS2 simulations with

Bieniawski’s RMR used to calculate GSI for different degrees of alteration along rock masses, assuming RMR could be interchanged for GSI in the model. Two of the five main parameters of the RMR system, intact rock strength and joint condition, were determined from the results of the geotechnical experiments and field work. Because of the lack of data, the values for two others,

RQD and joint spacing, were assumed, but not constant, for the alteration grades. For instance, for

2 profiles, the RQD will change based on potential PLS-rock interaction. A single water rating, the fifth parameter, was used for both experiments to minimize variability. The other parameters necessary for the Hoek and Brown criterion include the mi value for quartz monzonite, which in this project was it was gathered through RocData by Rocscience using the triaxial data from laboratorial tests, and the disturbance value D of 0.5. This analysis in RS2 then gives regions of failure that would be expected under the assumed properties and stress conditions, as well as a factor of safety for circular failure calculated using the strength reduction technique (SRF). 43

Separate intact friction angles for unaltered and altered rocks were not determined due to the lack

of geochemical data, and so an average value was calculated. Similarly, separate values for RQD

and joint spacing for different degrees of alteration rock were not calculated. Material parameters that were varied by alteration or location in the pit included the UCS and joint condition.

Chapter 4 – Results

Geotechnical Tests

For UCS and point load tests, a subset of results is shown in Figure 13 with P-wave velocities as

measured prior to the strength testing. The samples used for the point load test are from the

Southeast corner while the ones used for UCS tests with strain gauges are from the East wall. The

results of the correlations between UCS and P-wave velocity, and between point load and P-wave velocity, are shown in Figure 13. No comparison of UCS and point load is possible due to the different samples used for each. The results show that there is a fairly good correlation between point load strength and P wave velocity (R2=0.8483), though based on only 5 samples, but there is

a poor correlation between UCS and P-wave velocity (R2=0.2679) based on only 6 samples. Better correlations might emerge with additional testing and geochemical data.

If the weaker point load tests in Figure 13 represented samples with increased alteration, the tentative positive correlation with the P-wave velocity indicates that mineral alterations reduce the stiffness/elasticity of the rock. However, there are many other factors that could account for the variation in rock strength in the results shown in Figure 13, without XRD or petrographic data to verify the extent or intensity of alteration in the samples.

This project included 34 UCS tests on samples from across the pit (Table 5). Measured strengths ranged from 2,261 psi to 32,238 psi, with an average of 15,771 ± 8,451 psi. Since rock samples tested were quartz monzonite, it is reasonable to assume that the weaker strengths are associated with the higher grades of alteration, although this must be considered tentative in the absence of petrographic data on the same samples. This assumption was used for the numerical models. 45

Table 5 shows that there was no observable correlation between rock strength and pit wall region or depth. (Table 5 and Figure 14).

Sample Number Uniaxial Compressive Strength (psi) String Gauge or XRD Data Alteration Location BGD-E-UCS-1 7,657 --- Quartz Sericite East BGD-E-UCS-2 20,184 --- Quartz Sericite East BGD-E-UCS-3 7,966 --- Quartz Sericite East BGD-E-UCS-4 25,882 --- Quartz Sericite East BGD-E-UCS-5 13,975 --- Quartz Sericite East BGD-E-UCS-6 22,034 --- Quartz Sericite East BGD-E-UCS-7 10,801 --- Quartz Sericite East BGD-E-UCS-8 15,211 --- Quartz Sericite East BGD-E-UCS-9 12,391 --- Quartz Sericite East BGD-E-UCS-10 6,971 --- Quartz Sericite East BGD-E-UCS-11 4,460 XRD Quartz Sericite East BGD-E-UCS-12 15,622 --- Quartz Sericite East BGD-E-UCS-13 32,818 XRD Quartz Sericite East BGD-E-UCS-14 18,419 --- Quartz Sericite East BGD-E-UCS-15 15,102 Strain Gauge Quartz Sericite East BGD-E-UCS-16 12,844 Strain Gauge Quartz Sericite East BGD-E-UCS-17 17,624 Strain Gauge Quartz Sericite East BGD-N-UCS-1 14,945 Strain Gauge Quartz Sericite North BGD-N-UCS-2 7,790 Strain Gauge Quartz Sericite North BGD-N-UCS-3 9,920 Strain Gauge Quartz Sericite North BGD-SE-UCS-1 12,661 --- Quartz Sericite Southeast BGD-SE-UCS-2 26,525 --- Quartz Sericite Southeast BGD-SE-UCS-3 24,581 --- Quartz Sericite Southeast BGD-SE-UCS-4 7,412 --- Quartz Sericite Southeast BGD-SE-UCS-5 13,246 --- Quartz Sericite Southeast BGD-W-UCS-1 23,969 --- Quartz Sericite West BGD-W-UCS-2 28,617 --- Quartz Sericite West BGD-W-UCS-3 18,293 --- Quartz Sericite West BGD-W-UCS-4 22,046 --- Quartz Sericite West BGD-W-UCS-5 2,261 --- Quartz Sericite West BGD-W-UCS-6 10,922 XRD Quartz Sericite West BGD-W-UCS-7 22,418 --- Quartz Sericite West BGD-W-UCS-8 18,424 XRD Quartz Sericite West BGD-W-UCS-9 12,238 --- Quartz Sericite West Table 3: Uniaxial compressive strength data with additional analytical characterization methods and their location in the pit. 46

Considering the large scatter of the dataset, it is complicated to segregate rock strength from

different portions of the pit and its alteration grade without geochemical data.

Figure 13: UCS data from the Bagdad mine displaying their average (green) and -1-standard deviation (red). Samples with XRD and Strain Gauges are broken out as shown.

The results of Brazilian tests showed a wide range of tensile strengths from 897 psi to 2561 psi

among the 45 samples. The average tensile stress is 1416 ± 410 psi (Table 6 and Figure 16).

Similarly to the UCS data, there is no correlation pit region and tensile strength to tie to alteration grade.

Friction angle and cohesion were obtained through shear and triaxial testing. One small scale rock shear test and the test data yielded 33° for the friction angle, 4.42 psi for the cohesion, and an estimate of 4 – 6 for the JRC. Triaxial tests of samples throughout the pit were conducted at confining stresses of 750, 1,000, 1,500, 2,000, and 3,000 psi and the results are given in Figure 16.

A least squares fit through the data gives a best fit line with a slope of 8.2458 and an intercept of 47

13,540 psi, yielding a friction angle of 51.6°. The average of the UCS obtained from triaxial tests conducted is 16,357 psi, with an intercept of 13,540 (Figure 16) based on a highly scattered dataset

(R2=0.4039). It is not possible to correlate intact friction angle with alteration grade since the mineralogy of the triaxial samples shown in Figure 16 was not measured. The triaxial data was inputted in Rocscience RocData and the value indicated an mi of 29 which will be implemented in the FEM and is close to bibliography reviews in mi values for granitoids (Hoek and Bray, 1978).

Sample Number Tensile Strength (psi) Alteration Location BGD-E-BR-1 1058 Quartz Sericite East BGD-E-BR-2 1314 Quartz Sericite East BGD-E-BR-3 926 Quartz Sericite East BGD-E-BR-4 1233 Quartz Sericite East BGD-E-BR-5 918 Quartz Sericite East BGD-E-BR-6 1298 Quartz Sericite East BGD-E-BR-7 1323 Quartz Sericite East BGD-E-BR-8 1200 Quartz Sericite East BGD-E-BR-9 1291 Quartz Sericite East BGD-E-BR-10 1271 Quartz Sericite East BGD-E-BR-11 1726 Quartz Sericite East BGD-E-BR-12 1185 Quartz Sericite East BGD-E-BR-13 1331 Quartz Sericite East BGD-E-BR-14 1713 Quartz Sericite East BGD-E-BR-15 1744 Quartz Sericite East BGD-E-BR-16 1390 Quartz Sericite East BGD-E-BR-17 1270 Quartz Sericite East BGD-E-BR-18 1012 Quartz Sericite East BGD-E-BR-19 1715 Quartz Sericite East BGD-E-BR-20 2367 Quartz Sericite East BGD-E-BR-21 2292 Quartz Sericite East BGD-E-BR-22 1877 Quartz Sericite East BGD-E-BR-23 1101 Quartz Sericite East BGD-E-BR-24 1154 Quartz Sericite East BGD-E-BR-25 1119 Quartz Sericite East BGD-E-BR-26 1010 Quartz Sericite East BGD-E-BR-27 897 Quartz Sericite East BGD-SE-BR-1 2012 Quartz Sericite Southeast BGD-SE-BR-2 1448 Quartz Sericite Southeast BGD-SE-BR-3 1541 Quartz Sericite Southeast BGD-SE-BR-4 1266 Quartz Sericite Southeast BGD-SE-BR-5 2561 Quartz Sericite Southeast BGD-W-BR-1 1194 Quartz Sericite West BGD-W-BR-2 1102 Quartz Sericite West BGD-W-BR-3 1233 Quartz Sericite West BGD-W-BR-4 2129 Quartz Sericite West BGD-W-BR-5 1355 Quartz Sericite West BGD-W-BR-6 2030 Quartz Sericite West BGD-W-BR-7 1652 Quartz Sericite West BGD-W-BR-8 1034 Quartz Sericite West BGD-W-BR-9 1407 Quartz Sericite West BGD-W-BR-10 1766 Quartz Sericite West BGD-W-BR-11 1068 Quartz Sericite West BGD-W-BR-12 1001 Quartz Sericite West BGD-W-BR-13 1190 Quartz Sericite West Table 4: Tensile strength data and their location in the pit. 48

Figure 14: Tensile strength data from the Bagdad Mine displayed with the average line (green) and the -1 standard deviation line (red).

Uniaxial Compressive Strength and Triaxial Data 60000

50000 y = 8.2458x + 13540 R² = 0.4039 40000

30000

20000

Effective Vertical Stress (psi) Stress VerticalEffective 10000

0 0 500 1000 1500 2000 2500 3000 3500 Effective Confining Pressure (psi)

Figure 15: The effective vertical stress compared to the effective confining pressure using the average uniaxial compressive strength of all data points.

The measurements of moisture content and specific gravity (Table 7) were used to characterize how sericitic alteration can affect moisture content as well as specific gravity. Based on both data sets, it is possible to conclude that the data set from the Southeast wall, in particular, does not present major changes in specific gravity and moisture content when compared to the UCS data indicating that the changes of strength may not be due to degradation but other aspects of rock

strength such as anisotropy.

Sample Depth (ft.) Moisture Content (%) Specific Gravity BGD-SE-UCS-1 52 0.84 2.47 BGD-SE-UCS-2 100 0.64 2.47 BGD-SE-UCS-3 248 0.21 2.35 BGD-SE-UCS-4 300 0.34 2.40 BGD-SE-UCS-5 380 0.47 2.38 Table 5: Moisture content and specific gravity of sample from the Southeast wall.

Shortwave Infrared Imagery (SWIR) Data

Chlorite and biotite alteration were identified by hand sample examination along the quartz

monzonite core samples tested for geotechnical tests (Figure 17). The alterations are locally

fracture-controlled, suggesting a relationship to fluid flow through the fracture system. It is also

possible to spot through Figure 17 that the core broke through a hydrothermally altered zone indicating that it is possible that hydrothermal alteration can affect RQD similarly to what was

illustrated in Figure 2 and Figure 8.

Figure 16: Example of hydrothermal alteration as a potential source of degradation, as the alteration of primary micas to chlorite causes the core to break along the alteration zone.

The SWIR results showed that the minerals along the failure zones are mainly kaolinite, muscovite, montmorillonite, muscovitic-illite, phengite, montmorillonite with phengitic-illite, alunite, and albite (Figure 18). Since none of these minerals forms at low temperatures, it is most likely that all of the minerals present along the natural fractures were formed due to hydrothermal alteration and not weathering processes. All of them could affect rock strength and, thus, rock mass stability. 51

A B

C D

Figure 17: Illustration of SWIR spectra of montmorillonite (A) and muscovitic-illite (B) from the East wall, and phengite (C) and muscovite (D) from the Southeast wall.

Petrographic Characterization

Mineral Associations and Reactions

The general mineralogy of the thin sections consists of plagioclase, potassic feldspar, quartz,

biotite, phengite, epidote, sericite, pyrite, chalcopyrite, calcite, apatite, zircon, and chlorite with a

variety of different textures, including holocrystalline, porphyritic, equigranular, with grains ranging from fine to medium. Crystals are anhedral to subhedral and hypidiomorphic (Figure 19).

The matrix is fine quartz and feldspar grains between relatively larger biotite, chlorite, plagioclase,

and potassic feldspar crystals. The most common alteration consists of biotite turning into chlorite

(Figure 19 C and D, and plagioclase turning into sericite and calcite (Figure 19 A and B). Alteration

is more intense close to the hydrothermal veinlets in envelopes emanating from each veinlet. There 52 was no observed change in mineralogy with distance from natural fractures. Finally, no supergene mineralization nor porosity, were spotted throughout the thin sections.

Bio + Chl A Bio + Chl B

Feldspar + Ser Feldspar + Ser

Qtz Qtz

Cal Cal

20 μm 20 μm

Felds + Ser C Felds + Ser D

Phen Phen Bio + Chl Bio + Chl Qtz Qtz

Feldspar + Ser Feldspar + Ser 20 μm 20 μm

Plag + Ser Bio + Chl E Plag + Ser Bio + Chl F

Qtz Qtz Plag + Ser Plag + Ser

20 μm 20 μm

Figure 18: Overview of 30 μm thin section from samples from the Southeast wall. A (PPL) and B (XPL) illustrate the arrangement between a vein-controlled mineralization and the groundmass. C (PPL) and D (XPL) show the edge of a naturally formed fracture with a hydrothermal phengite vein. E (PPL) and F (XPL) indicates the fracture density along sericitized zoned plagioclases. 53

Pregnant Leaching Solution Potential Degradation

The SWIR results indicated that soaking quartz monzonite samples in the PLS solution did not affect the sample spectra except for adding a slight hydration peak, which is observed on both samples submerged under PLS and water. (Figure 20). The samples’ density did not show statistically significant change after soaking. However, it is possible that some changes in mineralogy or density would have occurred over more time than the 5-day course of the experiment.

The lack of evidence for PLS-induced alteration in the rock is surprising. Since quartz monzonite

has major acid-consuming minerals, such as chlorite, biotite, calcite, and Ca-feldspar, the mineralogy should be affected by PLS infiltration. One possibility is that the lack of porosity and high fracture spacing make it difficult for the acidic solutions to reach the target minerals. Reaction kinetics are also slow at low temperatures, and it is possible that detectable mineralogical and chemical change will occur in future as PLS exposure continues.

A B

Figure 19: Illustration of hydration curve indicated with the red arrow prior to (A) and after (B) soaking the quartz monzonite samples from the Southeast wall in PLS solution.

Scanning Electron Microscope (SEM)

The SEM data in Figure 22 confirmed the petrographic observation of the chloritization of biotite and the presence of micro-fractures induced by the alteration. Apatite and quartz were also identified as potential alteration products. The vein-filling mineral is phengite, which was also spotted with SWIR. It appears that the minerals were formed due to hydrothermal alteration since all of them form above 100ºC and would not be able to crystallize under weathering conditions.

Figure 22 and Figure 23 illustrate the sericitization of an anorthite crystal whose zoning is crosscut

by a fracture which is filled by phengite. It indicates that the last stage of feldspar growth happened

before micro-fracturing and the episode of sericitic alteration that formed the phengite. Feldspar is

a relatively strong mineral whose growth should not appreciably weaken the rock, but its

conversion to phengite could diminish rock strength considerably.

Figure 20: Backscattered electron imaging of chloritized biotite from Figure 19 (C and D) with chemical composition of biotite (top), chlorite, and phengite (bottom). 56

Figure 21: Backscattered electron imaging of sericitized plagioclase from Figure 19 (E and F) with chemical composition of phengite (top), and plagioclase zones (mid and bottom).

X-Ray Diffraction (XRD)

In porphyries, minerals such as chlorite, epidote, calcite, and some clays form only by

hydrothermal alteration, and their abundance in the rock can therefore serve as a proxy for

alteration grade. This can be normalized for variation in rock characteristics by ratioing the

abundance of these alteration minerals to the abundance of their igneous precursors, for instance

taking the ratio of remaining chlorite to biotite. While this ratio should be used with caution since

biotite can be a product of potassic alteration as well as igneous formation, in general, a ratio of

more than 1 can be construed to mean that the rock contains more altered minerals than primary

minerals (Figure 24 and Figure 25). The Overall Alteration Ratio was calculated by dividing the sum of the calcite, chlorite, and total clays content by the sum of the biotite and plagioclase contents.

Relationship between UCS and Chlorite and Biotite Ratio 35000

30000

25000

20000

15000

10000 y = -31024x2 + 32875x + 26133 R² = 0.9857 5000 Uniaxial Compressive Strength (psi) 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Chlorite/Biotite

Relationship between UCS and Calcite to Plagioclase Ratio 35000

30000

25000

20000

15000

10000 y = -251620x + 31490 R² = 0.4169

Uniaxial Compressive Strength (psi) 5000

0 0 0.02 0.04 0.06 0.08 0.1 0.12 Calcite/Plagioclase

Relationship between UCS and Total Clays to Plagioclase Ratio 35000

30000

25000

20000

15000 y = -167411x + 52459 R² = 0.3098 10000

5000 Uniaxial Compressive Strength (psi)

0 0 0.05 0.1 0.15 0.2 0.25 0.3 Total Clays/Plagioclase

Relationship between UCS and Overall Alteration Ratio 35000

30000

25000

20000

15000

10000 y = -126696x + 68377 R² = 0.7104

Uniaxial Compressive Strength (psi) 5000

0 0 0.1 0.2 0.3 0.4 0.5 0.6 Calcite, Chlorite, Total Clays / Biotite, Plagioclase

Figure 22: Relationship between UCS and mineral ration related to the degradation of biotite and plagioclase.

The relationships between chlorite-biotite ratio, calcite-plagioclase ratio, total clays-plagioclase ratio, and overall alteration ratio indicate that there may be an inverse correlation between the precipitation of hydrothermal minerals and rock strength, although this is tentative since the correlations are based on only four data points. The decline in UCS correlates best with chloritization of biotite (R2 = 0.98), although other factors likely contribute since minimal effect

on strength is likely to emerge from chemical change in a mineral that is roughly 5% of the bulk

rock composition. 60

Relationship between Density and Chlorite and Biotite Ratio 168

167

166

165

164 Density (pcf)

163 y = -5.3203x + 170.34 R² = 0.4231 162

161 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Chlorite/Biotite

Relationship between Density and Calcite to Plagioclase Ratio 168

167

166

165

164

Density (pcf) 163 y = -58.921x + 167.2 162 R² = 0.4919

161

160 0 0.02 0.04 0.06 0.08 0.1 0.12 Calcite/Plagioclase

Relationship between Density and Total Clays 168

167

166

165

164

Density (pcf) 163

162 y = -1.5295x + 171.82 R² = 0.5947 161

160 0 1 2 3 4 5 6 7 8 Total CLays (%)

Relationship between Density and Overall Alteration Ratio 168

167

166

165 y = -8.1812x + 167.06 164 R² = 0.0637 Density (pcf)

163

162

161 0 0.1 0.2 0.3 0.4 0.5 0.6 Calcite, Chlorite, Total Clays / Biotite, Plagioclase

Figure 23: Relationship between density and mineral ration related to the degradation of biotite and plagioclase.

The intensity of alteration correlates with density less than with UCS by all measures. Comparisons

of UCS and alteration proxies indicate that the chloritization of biotite may be driving the

geomechanical degradation. This is surprising given that biotite makes up a much smaller

proportion of the rock than feldspar and is therefore less critical to maintaining rock strength and

given that the strength difference between biotite and chlorite is less than that between feldspar

and clay. Possibly the chloritization of biotite is simply one manifestation of a group of alteration

reactions that have the cumulative effect of weakening the rock. Another possibility is that the data

for kaolinite / clay is confused by the derivation of some kaolinite from muscovite rather than

feldspars, meaning that a high-kaolinite rock could still have substantial intact feldspar to provide

geomechanical strength. K-feldspar may also be a product of potassic alteration, which is common

in the quartz monzonite unit. Therefore, it is not possible to determine based on the XRD data if

the K-feldspar may influence the rock strength as well.

The tentative conclusion from the UCS-XRD comparisons is that the hydrothermal alteration, as best represented by chloritization of biotite and associated reactions, is driving the loss of strength and cohesion.

Slope Finite Element Modeling

This section describes four simulations that were conducted using Rocscience RS2, two

simulations with two levels of alteration degradation, and two simulations considering the effects

of PLS in these two rock types. In-situ stress conditions were due to gravity along with the

assumption that the pre-mining horizontal stress at any depth is equal to the vertical stress,

horizontal/vertical in-situ stress ratio = 1. To analyze the effects of alteration degradation, a pair

of models were made using two sets of parameters: samples with chlorite content higher than the

biotite content (chlorite > biotite) and chlorite content lower than the biotite content (chlorite < 63 biotite). The Generalized Hoek and Brown rock mass strength criterion was used with the parameters shown in Table 6.

Model Parameters Chlorite > Biotite Chlorite < Biotite Density (lb/ft3) 162 167 Poisson's Ratio 0.295 0.21975 UCS (psi) 11,269 32,818 GSI 54.5 66.5 Ei * 106 (psi) 9.1235 8.25975 Table 6: Parameters used for the RS2 model.

The density and uniaxial compressive strength used for the models are the average of the samples with chlorite > biotite and chlorite < biotite (Figures 23 and 24). The Poisson’s Ratio and intact

Young’s modulus (Ei) used for the model were assumed based on different sets of samples. The samples with UCS values above average were assumed to have the same elastic properties, which is the average value of all the elastic properties in this zone. The same assumption was applied to the samples with strength values within the average and -1 standard deviation lines and below the

-1 standard deviation line (Figure 14). The Rock Mass Rating (RMR) was calculated based on the five parameters given in Bieniawski (1989) (UCS, RQD, joint spacing, joint condition, water). It was then assumed that the Geologic Strength Index (GSI) is equal to the RMR, since both systems have the same scale from 0 to 100. GSI was then used in the Hoek and Brown rock mass criterion as show in Equations 2-5.

Based on the table in Bieniawski (1989), the RMR rating for the UCS parameter is chosen to be 7 for chlorite > biotite and 14 for chlorite < biotite. Separate values for chlorite > biotite and chlorite

< biotite for the RQD and joint spacing parameters were not available. For the RQD parameter, an average value of 46 was used for both chlorite > biotite and chlorite < biotite, based on information provided by Freeport-McMoRan, giving an RMR rating of 10.5. For joint spacing, a range between 64

60-200 mm was chosen for both, giving a rating of 10. The condition of discontinuities rating was

assumed to be 20 for chlorite > biotite and 25 for chlorite < biotite based on field work. The

groundwater was assumed to be wet for both rock types, giving a rating of 7. Based on these five

parameters, the total RMR for chlorite > biotite and chlorite < biotite are 54.5 and 66.5, respectively (Table 7). Note that these two models are not meant to take into account the effects of PLS infiltrating the rock mass. This is taken into account with two additional models described below.

To take into account the potential effects of infiltration of PLS, two additional RS2 models were conducted. In these models, it was assumed that PLS will cause a decrease to both the RQD and

the joint spacing. The other parameters in the RMR system are kept the same as in the previous

non-PLS simulations. The RQD assumptions used for profiles simulating the potential PLS-rock

interaction were segregated in 5 horizontal sections where the RQD changes every 100 meters from top to bottom, assuming higher alteration in the upper layers where concentrations of PLS are higher. The RQD is spaced by a factor of 20% where the top section has an RQD of 0% and the bottom section is 80%. The average joint spacing in each layer was assumed to vary proportionally to the RQD. The joint condition rating was assumed to be 20 for chlorite > biotite and 25 for chlorite < biotite based on field work. The groundwater was assumed to be wet for both simulations due to groundwater and PLS, the rating assumed was 7. For all cases, I assumed Intact

Rock Constant (mi) of 29 based on the lab data. A Disturbance Factor of 0.5 was used (Table 8).

UCS (psi) RQD Spacing of Discontinuity* Condition of Discontinuity* Groundwater* Slope RMR Data Condition Rating Data Rating Data Rating Data Rating Data Rating Rating (psi)

Chlorite > 60 - 200 Highly Weathered 11,268 7 46 10.5 10 20 Wet 7 54.5 Biotite mm Walls

Chlorite < 60 - 200 Slightly Weathered 32,818 14 46 10.5 10 25 Wet 7 66.5 Biotite mm Walls

* Data assumed based on field work. Table 7: RMR Rating used for RS2 models using the Hoek-Brown failure criterion considering only the hydrothermal alteration.

Spacing of Condition of UCS RQD Groundwater Discontinuities Discontinuities RMR Slope Condition Rating Data (psi) Rating Data Rating Data Rating Data Rating Data Rating Highly 11,268 7 0 1 < 60 mm 1 20 Wet 7 36 weathered walls Highly 11,268 7 20 6 60 - 200 mm 6 20 Wet 7 46 weathered walls Chlorite > Highly 11,268 7 40 9 60 - 200 mm 9 20 Wet 7 52 Biotite weathered walls Highly 11,268 7 60 12 200 - 600 mm 10 20 Wet 7 56 weathered walls Highly 11,268 7 80 16 0.6 - 2 . m 15 20 Wet 7 65 weathered walls Slightly 32,818 14 0 1 < 60 mm 1 25 Wet 7 48 weathered walls Slightly 32,818 14 20 6 60 - 200 mm 6 25 Wet 7 58 weathered walls Chlorite < Slightly 32,818 14 40 9 60 - 200 mm 9 25 Wet 7 64 Biotite weathered walls Slightly 32,818 14 60 12 200 - 600 mm 10 25 Wet 7 68 weathered walls Slightly 32,818 14 80 16 0.6 - 2 . m 15 25 Wet 7 77 weathered walls Table 8: RMR Rating used for RS2 models using the Hoek-Brown failure criterion simulating PLS-rock interaction. The results of the four simulations are now described. First, we look at the non-PLS simulations, one run for chlorite > biotite and one for chlorite < biotite. It should be pointed out at the start of this section that the modeling results are meant to provide a general idea of the effects of material degradation on the behavior of a generic slope. Unfortunately, many assumptions have been made in the simulations as described above, and some of these assumptions restrict the accuracy of the results. To analyze the results, we show plots of contours of maximum plastic shear strain (Figures

25 and 26), as well as maximum values of maximum plastic shear strain and maximum principal stress at the toe of the slope (σ1) where bench failure may occur, as well as minimum value of the 66

minimum principal stress at the top of the mesh (σ3) where tension cracks are likely to form (Table

9). Figure 26 shows contours of maximum plastic shear strain for the two cases. It shows that in both cases, tension-like failures are occurring near the top back of the slope. In the more altered case (chlorite > biotite), localized failure near the toe of the slope is predicted to occur, due to the combination of high stresses and a weaker rock mass. This localized bench failure does not occur for the less altered case (chlorite < biotite). We also consider the Shear Strength Reduction method

(SSR) to determine the factor of safety for a large circular failure to occur in these two cases. Using this method, RS2 systematically reduces the shear strength properties until a large-scale failure is produced. This results in a Strength Reduction Factor (SRF) to produce a large circular failure.

Figure 25 shows the results for the two alteration types. It indicates a stress reduction factor of 3.2 for the more altered case (chlorite > biotite) and 5.0 for the less altered case (chlorite < biotite).

The more altered rock mass shows three large circular failures occurring, while the less altered rock mass shows a single large circular failure from the top to the bottom of the slope (Table 9).

SRF for PLS-Rock Max. σ1 along Min. σ3 Slope Large Max. Shear Total Alteration the slope toe along the Condition Circular Plastic Strain* Displacement (ft.)* Simulation (psi)* surface (psi)* Failure Chlorite > No 3.20 3.49 x 10-3 1,870 - 4.35 0.9 Biotite Chlorite < No 5.00 5.55 x 10-4 3,900 - 62.35 0.6 Biotite Chlorite > Yes 2.80 2.72 x 10-3 2,059 - 0.44 0.9 Biotite Chlorite < Yes 4.77 2.56 x 10-4 3,857 - 11.60 0.5 Biotite *Data gathered with no SRF applied. Table 9: Data collected through RS2 indicating how the slopes properties change due to rock alteration.

Now we discuss the results of the two simulations that consider the effects of PLS. The plots of

contours of maximum plastic shear strain are shown in Figures 27 and 28, along with values at

specific locations for the maximum principal stresses and plastic shear strain in Table 9. The 67

additional degradation due to PLS results in a reduction in both the Strength Reduction Factors for

both rock mass types (chlorite > biotite and chlorite < biotite) to values of 2.8 and 4.77 respectively.

The more altered rock mass with PLS also shows some additional bench failure in the lower parts of the slope, but this does not occur for the less altered rock mass. Tension cracks near the top of the slope are not as apparent in both rock mass types with the PLS assumptions, possibly due the highly reduced rock mass strength in the top layer. It should be noted that these simulations did not take into account the possible increase in fluid pore pressure due to PLS infiltrating the rock mass. 68

Figure 24: Hoek and Brown method using SRF for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall. These illustrations indicate how rock mass degradation may affect slope stability using parameters from Table 7. 69

Figure 25: Hoek and Brown method with no SRF applied for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall using parameters from Table 7.

Figure 26: Hoek and Brown method using SRF for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall using layers (green line) with different RQD and joint spacing data for the RMR parameter from Table 8.

Figure 27: Hoek and Brown method using no SRF for chlorite > biotite (top) and chlorite < biotite (bottom) along the generic pit wall using layers (green line) with different RQD and joint spacing data for the RMR parameter from Table 8.

Overall, the results of the modeling with both methods illustrate how the weakening of rock masses by hydrothermal alteration can contribute to slope failures in open pits, even without further alteration or pressure increases from PLS infiltration. It is important to point out that these simulations do not reflect the behavior of a specific mine but are intended to simulate a

methodology that can be used if mine sites are dealing with PLS-rock interaction. 72

Chapter 5 – Conclusion and Future Work

This project aimed to describe samples that are degraded due to hydrothermal alteration and PLS

infiltration, to quantify alteration grade by chlorite, calcite, clays and other relatively weaker mineral phases, to correlate alteration grade to rock strength, and finally to input the results into a

Finite Element numerical model of the effects of alteration on slope stability. While sampling and analytical uncertainties render the results tentative, a comparison of geotechnical tests and XRD data showed a likely correlation between stronger alteration and weaker rock. This was supported by the results of the geotechnical analyses, which indicated a potential negative correlation of chloritization and associated hydrothermal alteration with compressive strength, although with too few data points to be definitive.

Although in theory, reaction with acid PLS should alter gangue minerals in ways that would weaken the rock, no corresponding chemical or mineralogical effects were observed in intact rock by SWIR. Further projects with SEM and optical petrography could take place to characterize this matter.

The FEM analysis using the Hoek and Brown failure criterion indicated that the Maximum Shear

Plastic Strain, σ1, σ3, and total displacement also changed as a function of alteration grade and potential PLS-rock reaction, with slopes having chlorite > biotite more likely to fail than slopes

with chlorite < biotite. Given the low concentrations of chlorite in the rock, it is probable that the

rock mass degradation in practice results from additional alteration associated with chloritization

rather than chloritization itself.

Although many aspects of this remain to be studied, the results of this project show some

indications that hydrothermal alteration and PLS infiltration can both decrease highwall stability. 73

This research indicated the benefits of coupling geochemical and geotechnical data to describe and model rock mass degradation and its effects in mines.

Recommendations for Future Projects

As emphasized above, the results of this research project show interesting possible connections between hydrothermal alteration and rock mass degradation but are tentative only. To be placed on a more rigorous basis, further work linking alteration and degradation should begin with geotechnical tests on well-characterized samples of disparate alteration types and grades. This would enable shear tests along fractures with different alteration minerals, simulating loss of cohesion and friction angle as a function of mineralogy. After this, various types of numerical modeling could use the data to simulate failures for the most-altered and least-altered scenarios.

The full effect of PLS infiltration on rock mass strength remained elusive in the methods used in this study. To better gauge it, a thorough experimental approach is recommended. It would require well-characterized sample pairs representing the same rock type and alteration type and intensity.

One of each pair would be subjected to immediate geotechnical tests while the other would be soaked in PLS for some months, then would be re-characterized and tested in turn. Methods developed for metallurgical evaluation of oxide ores should be considered. This would show what, if any, mineralogical and chemical changes occurred due to PLS infiltration and how they affected rock mass integrity.

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