effect of weathering and alteration on point load and slake durability indices and the ...

216 Pages·2009·11.59 MB·English

EFFECT OF WEATHERING AND ALTERATION ON POINT LOAD AND
SLAKE DURABILITY INDICES AND THE CHARACTERIZATION OF THE
DEBRIS FLOW AT THE QUESTA MINE, TAOS COUNTY, NEW MEXICO

by

Gertrude Fobiah Ayakwah

Submitted in partial fulfillment of the requirements for the
Degree of Master of Science in Mineral Engineering
with Specialization in Geotechnical Engineering

New Mexico Institute of Mining and Technology
Department of Mineral Engineering

Socorro, New Mexico
May, 2009
This thesis is dedicated to God almighty for seeing me through, to my Mum Agnes Adjeley
Fumey, my dad Emmanuel Okyere-Boakye Ayakwah and my siblings for their prayers and
support during the writing of this work.
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Abstract
Point load strength (Is50) and slake durability (ID )indices provide a measure of the
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strength and durability of rock fragments. These are also related to the alteration intensity
and weathering of the materials. Samples were collected from the rock piles, alteration
scars, and debris flows at the Questa mine with the purpose of examining relationships
between Is50 and ID , mineralogy, chemistry, weathering, hydrothermal alteration, and
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other geotechnical parameters. The Is50 from the various rock piles ranges from 0.6-8.2
MPa and the ID ranges from 80.9-99.5%. The Is50 and ID results from the samples
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collected indicate that, the samples from the debris flows are in average stronger (average
Is50= 4.0 MPa and ID = 98.4%) than the rock-pile samples and that the alteration scar
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samples are in average weaker (average Is50 = 2.8 MPa and ID = 89.2%) than the rock-
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pile samples. However, most of these rocks are strong in terms of their Is50 and ID .
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The debris flows studied are similar to the Questa rock piles in terms of lithology,
slake and point load indices, friction angle and particle size distribution. The profile
studied is not a typical weathered profile. This is because it was observed that there are
no systematic variations in the composition of the samples collected from the profile, and
this indicates that the debris flow was formed by several different flood events with
slightly different sources. The results of the geochemical and geotechnical
characterization indicate that, the samples collected from the debris flow are similar to
each other and do not show signs of decreasing weathering from the top of the profile to
the bottom. The cementation of the debris flow was found to be similar to the ones found
in the rock piles and is formed by oxidation of sulfide minerals producing sulfates and
iron oxides. The debris flows are therefore well cemented, even below the surface.
Finally, the slake durability measurements of rock pile material collected from the hot
zones were found to be similar to the measurements obtained for other materials in the
Questa area. They also indicated high durability.
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Acknowledgements

This study owes its success to several people who contributed in various ways. However,
some key personalities require special mention.
My heartfelt gratitude goes to my research supervisors; Dr. Virginia McLemore
and Dr. Ali Fakhimi, for their encouragement, guidance and constructive criticisms which
enabled me to produce this research work on time.
I also thank Chevron Mining Inc. for funding this research work.
I wish to also than the Questa Rock Pile Weathering Stability Project team for
their assistance throughout the sampling and data analysis period of this project.
To Dave Jacobs of Chevron Mining Inc, Ariel Dickens, Kojo Anim, Frederick
Ennin, Samuel Nunoo, Dawn Sweeney, Kelly Donahue and Erin Philips, I say a big thank
you for your help in the laboratory work and data analysis of this work.
I express my profound gratitude to Dr. Navid Mojtabai, my academic advisor who
has been a father and a consular to me since the day I applied for admission to this
honorable institution and also offered a lot of encouragement.
Finally, to Yirrah family in Virginia, Carilli family in Socorro, my aunties Diana,
Edith, Charlotte, Vida in New Jersey for the love, encouragement and their prayers
during hard times in my academic life.

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Table of Contents 
List of Figures ..................................................................................................................... v
List of Tables ..................................................................................................................... ix
1.0 INTRODUCTION ................................................................................................. 1
1.1. Background ...........................................................................................................1
1.2 Thesis Overview ...................................................................................................2
1.3 Project Background ...............................................................................................3
1.4 Thesis Objective and Scope ..................................................................................4
1.5 Description of study area ......................................................................................5
1.5.1 Location ..........................................................................................................5
1.5.2 History of Questa Mine ...................................................................................6
1.5.3 Mine Features..................................................................................................8
1.5.4. Mine geology and mineralogy ......................................................................12
1.6 Rock mass and intact rock strength ....................................................................17
1.6.1 Rock durability and slaking ..........................................................................18
2.0 EFFECTS OF WEATHERING AND ALTERATION ON POINT LOAD AND
SLAKE DURABILITY INDICES OF QUESTA MINE MATERIALS, NEW
MEXICO…... .................................................................................................................... 22
2.1 Introduction .........................................................................................................22
2.2 Alteration and weathering of the Questa rock piles ............................................24
2.3 Field and Analytical Methods .............................................................................29
2.3.1 Sampling .......................................................................................................29
2.3.2 Laboratory Analysis ......................................................................................30
2.4 Results .................................................................................................................38
2.5 Discussion ...........................................................................................................43
2.6 Conclusions .........................................................................................................55
3.0 CHARACTERIZATION OF QUESTA DEBRIS FLOWS ................................. 57
3.1 Introduction .........................................................................................................57
3.2 Definitions...........................................................................................................58
3.3 Background .........................................................................................................60
3.4 Sampling and analytical methods .......................................................................63
3.5 Description of the debris flow profile .................................................................66
3.6 Results .................................................................................................................68
3.7 Discussion ...........................................................................................................70
3.8 CONCLUSIONS.................................................................................................83
4.0 HOT ZONE STRENGTH STUDY ...................................................................... 85
4.1 Introduction .........................................................................................................85
4.2 Background .........................................................................................................86
4.3 Methods...............................................................................................................87
4.4 Results .................................................................................................................88
4.5 Discussion ...........................................................................................................91
4.6 Conclusion ..........................................................................................................94
5.0 CONCLUSIONS AND RECOMMENDATIONS ............................................ 96
5.2 References Cited .................................................................................................98
APPENDIX A TEST RESULTS ............................................................................... 105
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APPENIDX B. SUMMARY RESULTS OF QUESTA MATERIALS USED IN THE
STUDY. .......................................................................................................................... 125
APPENIDX C. SUMMARY COMPARISM STATISTICS OF THE STRENGTH and
DURABILITYCLASSIFICATION FOR QUESTA MATERIALS. ............................. 197

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List of Figures 
Figure 1.1: Location map of the chevron molybdenum mine and vicinities. ......................6
Figure 1.2: Map of chevron mine site showing mine rock piles and mine features. ...........9
Figure 1.3: Conceptual geological model of GHN rock pile, as interpreted from surface
mapping, detailed geologic cross-sections, trenches, drill holes, construction method and
observations during reclamation of GHN (McLemore et. al., 2008a) ...............................11
Figure 1.4: Geologic map of Questa – Red River Vicinity (Ludington et al., 2004.) .......13
Figure 1.5: Map of the alteration scars at the Questa Mine. The red circles shows where
samples were taken from, for point load and slake durability test. ....................................17
Figure 2.1: The point load strength equipment in use (sample under test), showing contact
cones, the loading and measuring systems (display gage) and samples to be tested. ........32
Figure 2.2: Typical irregular samples after point load strength test showing their planes
of failure……….. ...............................................................................................................33
Figure 2.3: slake durability equipment during usage comprising of the test drums and the
motor for rotation. ..............................................................................................................36
Figure 2.4: A typical brushed sample, each piece weighing between 40 to 60 g before the
slake durability test is performed. ......................................................................................36
Figure 2.5: A typical sample after slake durability test showing bigger and smaller pieces
of rock fragments after the test. .........................................................................................37
Figure 2.6: Histogram plot of point load strength index at the rock piles location (GHN,
SSS, SSW, MID and SPR). ................................................................................................42
Figure 2.7: Histogram plot of slake durability index at the rock piles location (GHN,
SSS, SSW, MID and SPR). ................................................................................................42
Figure 2.8: Scatter plot of Slake Durability Index and Point Load Index vs. distance from
outer edge of GHN rock pile. The weathering intensity was confirmed by petrographic
analyses, especially textures, as described by McLemore et al. (2008a). See Figures 1.2
and1.3 for location of trenches and layers in GHN where samples were obtained.
Appendix B includes a summary of the description of these samples. ..............................44
Figure 2.9: Point load strength index values for the rock piles, alteration scars and debris
flows. The average point load strength index for each location is shown with a red circle.
The number of samples for each location and the standard deviation are shown in
parentheses. PIT samples are unweathered drill core samples of andesite and rhyolite
(Amalia Tuff) of various hydrothermal alteration intensities. See Figure 1.2 for location
of rock piles. See Figures 1.2 and 1.3 for location of trenches and geologic units in GHN
where samples were obtained. Appendix B summarizes the location and description of
these samples. ....................................................................................................................45
Figure 2.10: Slake durability index values for the rock piles, alteration scars, and debris
flows. The average slake durability index for each location is shown with a red circle.
The number of samples and the standard deviation for each location are shown in
parentheses. PIT samples are unweathered drill core samples of andesite and rhyolite
(Amalia Tuff) of various hydrothermal alteration intensities. See Figures 1.2. and 1.3 for
location of rock piles and location of trenches and geologic units in GHN where samples
were obtained. Appendix B summarizes the location and description of these samples. ..46
Figure 2.11: Variation in slake index, point load and alteration (QSP, Propylitic and
Argillic) of the Questa rock materials. See Figure 1.2 for location of rock piles where
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samples were obtained. Appendix B summarizes the location and description of these
samples……….. .................................................................................................................47
Figure 2.12: Point load strength index values for different lithologies (Amalia, Andesite
and Intrusive) which includes drill core and outcrop samples. The average point load
strength index for each lithology is shown with a red circle. The number of the samples
and the standard deviation are shown in the parentheses. See Figure 1.2 for location of
open pit. Appendix B summarizes the location and description of these samples. ...........48
Figure 2.13: Slake durability index values for different lithologies (Amalia, Andesite and
Intrusive) which include the drill core and outcrop samples. The average slake durability
index for each lithology is shown with a red circle. The number of samples for each
location and standard deviation are shown in the parentheses. See Figure 1.2 for location
of open pit. Appendix B summarizes the location and description of these samples. ......48
Figure 2.14: Variations between slake durability index, point load index, mineralogy, and
chemistry. The mineralogy and chemical analyses were performed on splits of the same
sample set that were used in the geotechnical testing and represent the mineralogy and
chemistry of the sample tested by geotechnical methods. See Figure 1.2 for location of
rock piles. Appendix B summarizes the location and description of these samples. .........50
Figure 2.15: Variation in slake index and point load index of the Questa rock materials. 51
Figure 2.16: Variations between slake durability index, point load index, friction angle,
and residual friction angle. The friction angle was determined on the fine-grained matrix
from the same location as the samples tested for slake durability and point load, which
were determined on larger rock fragments. See Figure 1.2 for location of rock piles. See
Figure 1.3 for location of trenches in GHN where samples were obtained. Appendix B
summarizes the location and description of these samples. ...............................................52
Figure 2.17: Variation in slake index, point load index and paste pH of the Questa rock
materials. See Figure 1.2 for location of rock piles. See Figures 1.2 and 1.3 for location of
trenches and geologic unites in GHN where samples were obtained. Appendix B
summarizes the location and description of these samples. The upper part of the figure is
for the all the other rock pile location and the analogs except the GHN whereas the lower
part is GHN….. ..................................................................................................................54
Figure 2.18: Variation in slake index, point load and simple weathering indices (SWI) of
the Questa rock materials. See Figure 1.2 for location of rock piles. See Figures 1.2 and
1.3 for location of trenches in GHN and geologic units where samples were obtained.
Appendix B summarizes the location and description of these samples. The upper part of
the figure is for the all the other rock pile location and the analogs excluding the GHN
whereas the lower part is GHN. .........................................................................................55
Figure 3.1: Photograph of Goat Hill debris flow. Boxes show location of collected
samples. Collected samples consist of a bulk grab of rock material stored in 5 gallon
buckets and includes matrix (soil) and rock fragments. ....................................................67
Figure 3.2: Variations of slake index, friction angle, and point load index and percent
gravel with depth from the base of the debris flow profile. No observed trend of
parameters with depth. .......................................................................................................71
Figure 3.3: Variations of paste pH, paste conductivity, water content and dry density with
depth from the base of the debris flow profile. No clear trend was observed between the
parameters and depth. ........................................................................................................72
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Figure 3.4: Gradation curves for the sieve analysis on the individual samples from the
debris flow profile. .............................................................................................................73
Figure 3.5: Variations of FeO and Fe oxide minerals with depth from the base of the
debris flow profile. No clear trend of parameters with profile. .........................................74
Figure 3.6: Variations of total feldspar (K-feldspar+plagioclase) with depth from the base
of the debris flow profile. No clear trend of parameters with profile. ...............................74
Figure 3.7: Variations of selected trace elements with depth from the base of the debris
flow profile. No clear trend of trace elements with profile. ...............................................75
Figure 3.8: Variations of sulphur and SO4 with depth from the base of the debris flow ..75
Figure 3.9: Variations of sulphur/sulphate ratio with depth from the base of the debris
flow profile. No clear trend of sulphur/sulphate with profile. ...........................................75
Figure 3.10: Variations of geochemical and mineralogical parameters on the X-axis and
sample location along the profile from base to top on the y-axis. No clear trend of
parameters with profile. .....................................................................................................76
Figure 3.11: Clay mineralogy XRD scans for the debris flow weathering profile. I = illite,
C = Chlorite, S = smectite, K = kaolinite, J = Jarosite. .....................................................77
Figure 3.12: Backscattered electron microprobe image showing a cemented grain
consisting of small hydrothermally-altered phenocrysts within MIN-GFA-0001 sample.
The cement consists of clay minerals (illite), jarosite, and Fe oxides. The numbered
points represent points for mineral chemistry. ...................................................................79
Figure 3.13: Backscattered electron microprobe image showing well cemented grains of
hydrothermally-altered phenocrysts within MIN-GFA-0001 sample. Illite, jarosite and Fe
oxide crystals are cementing the rock fragments. The cementation is similar in chemistry
and texture as that found in the GHN rock pile. The numbered points represent points for
mineral chemistry. ..............................................................................................................79
Figure 3.14: Backscattered electron image (BSE) of a soil sample from GHN rock pile
showing rock fragment and associated fine-grained matrix material. Note the similarity in
texture of the cementation of rock fragments in this image compared to the image in
Figure 3.13. The fine-grained matrix consists of clay minerals and gypsum. ...................80
Figure 3.15: Backscattered electron microprobe image showing well-cemented
hydrothermally-altered phenocrysts within MIN-GFA-0006 sample. Illite, jarosite, Fe
oxide and feldspar crystals are cementing the rock fragments. The numbered points
represent points for mineral chemistry. .............................................................................81
Figure 3.16: Backscattered electron microprobe image showing hydrothermally-altered
phenocrysts within MIN-GFA-0006 sample. Illite, jarosite, Fe oxide and kaolinite
crystals cementing the rock fragments. The numbered points represent points for mineral
chemistry…….. ..................................................................................................................81
Figure 3.17: Sample location along the profile, sample photos and microprobe images
along with sample type and strength of cementing agents. ...............................................82
Figure 4.1: Location of venting drill holes and surface vent area (SGS-JMS-0001). Blue
indicates drill holes drilled in 1999 that contain monitoring instruments for temperature,
O and CO . Red indicates drill holes and surface vent area that do not contain
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temperature and gas instrumentation and are sites monitored by the New Mexico Tech
team………….. ..................................................................................................................86
Figure 4.2: Temperature log of drill hole SI-50 from Sugar Shack South rock pile. ........89
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