SME Annual Meeting
Feb. 22-Feb. 25, 2009, Denver, CO
Preprint 09-018
THE EFFECT OF WEATHERING ON PARTICLE SHAPE OF QUESTA MINE MATERIAL
S. Nunoo, New Mexico Inst. Of Mining and Tech., Socorro, NM
V. T. McLemore, New Mexico Bureau of Geology and Mineral Resources, Socorro, NM
A. Fakhimi, New Mexico Inst. Of Mining and Tech., Socorro, NM
G. Ayakwah, New Mexico Inst. Of Mining and Tech., Socorro, NM
al., 2005). Sphericity (described as spherical, needle-like, tabular, and
flat) refers to the similarity of a particle to a sphere with equal volume.
Roundness/angularity describes the degree of abrasion of a particle as
shown by the sharpness of its edges and corners. It is expressed by
Wadell (1932) as the ratio of the average radius of curvature of the
several edges or corners of the particle to the radius of curvature of the
maximum inscribed sphere or to one-half the nominal diameter of the
particle. Smoothness/roughness refers to the texture of the surface of
the particle.
ABSTRACT
The shape of particles has a significant effect on the shear
strength and deformational characteristics of granular materials.
Weathering can change the grain shapes. For example, more
weathered sands tend to be rounder regardless of particle size.
Particle shape analysis was performed on samples collected from the
Questa mine rock piles, a debris flow and the pit alteration scar. In
order to increase the validity of the particle shape analysis, 4 persons
including 2 geologists and 2 mining engineers were involved in
describing the particle shapes. The results of this analysis indicate that
rock fragments in the collected samples are mainly subangular,
subdiscoidal and subprismoidal. The sphericity and angularity of the
rock fragments of the older analogs are similar to those of the rock
piles. This suggests that short-term weathering (<100 years) has not
noticeably changed the particle shapes at the test locations. Rock piles
made of more angular particles are more stable compared to rock piles
with rounded particles.
The purpose of this paper is to present results from a study to (1)
determine the particle shape of some of the Questa mine rock pile and
analog materials and (2) investigate the effect of weathering on the
shape of the particles. Analog materials are from sites in the vicinity of
the Questa mine that are similar in composition and weathering
process as the rock piles, but are older than the rock piles. Processes
operating in the natural analogs share many similarities to those in the
rock pile, although certain aspects of the physical and chemical system
are different (Graf, 2008; Ludington et al., 2004). The alteration scar
(sample QPS-SAN-0001) and debris flow (sample MIN-SAN-0001) are
considered natural analogs for future weathering of the rock piles
because they have undergone hydrothermal alteration, weathering,
and erosion since they were formed and could represent the future
weathering of the rock piles. The pit alteration scar is younger than
0.24 ± 0.12 Ma (40Ar/39Ar age of jarosite, Virgil Lueth, written
communication, November 2008) and the debris flow is younger than
4220 + 40 years (14C age of wood from the debris flow, Virgil Lueth,
written communication, November 2008).
INTRODUCTION
Research studies have shown that the shape of particles can
significantly modify the shear strength (i.e. friction angle) and
deformational characteristics of granular materials. Das (1983)
reported that friction angle for medium dense sandy gravel and
medium dense sand have values ranging from 34º to 48º and 32º to
38º, respectively, due to difference in particle shape. Morris (1959)
studied the effects of particle shape on the strength of aggregate
material and concluded that “perfectly spherical particles give the
weakest aggregate, whereas chunky aggregates offer increase in
strength up to a point where the very roughness imposes limiting
conditions of density (or void ratio), where after additional irregularity of
shape limits the obtainable density and the strength falls off.”. Cho et
al. (2006) concludes that the decrease in particle sphericity and/or
roundness leads to increase in the constant volume critical state
friction angle. Critical state friction angle is the minimum (i.e. safest)
friction angle.
LOCATION AND SITE DESCRIPTION
The Questa molybdenum mine, owned and operated by Chevron
Mining Inc. (formerly Molycorp, Inc.) is located approximately five miles
east of Questa, Taos County, New Mexico. The mine is in the Taos
range of the Sangre de Cristo Mountains of the Southern Rocky
Mountains near the edge of the Rio Grande rift in north-central New
Mexico (Fig. 1). The mine is on southward facing slopes and is
bounded on the south by Red River (elevation of approximately 7500
ft) and on the north by the mountain divides (elevation approximately
10,750 ft). Mining operations first began at the site in the 1916 and
open pit mining was conducted between 1965 and 1983. During that
period, overburden rock associated with development of the open pit
was deposited on nearby steep hill slopes, resulting in nine rock piles
(Fig. 1). The rock material making up the rock piles are referred to in
older literature as mine waste, mine soils, overburden, sub-ore, or
proto-ore, but do not include the tailings material that consists of nonore waste remaining after milling. Robertson (1982) described mine
rock piles as some of the largest man-made structures at a mine by
volume and height.
The shape of particles reflects the material composition, the
release of the grains from the matrix, the formation history of the
particles, transportation of the particles, depositional environments of
the material, and mechanical and chemical processes acting on the
particles, including weathering (Cho et al., 2006). Weathering is the set
of physical and chemical changes, up to and including disintegration of
rock by physical, chemical, and/or biological processes occurring at or
near the earth’s surface (e.g., in the vadose zone within approximately
300 ft of ground surface at temperatures less than or equal to
approximately 70°C) that result in reductions of grain size, changes in
cohesion or cementation, and change in mineralogical composition
(modified from Neuendorf, et al., 2005). Weathering can change the
particle shape. For example, more weathered sands tend to be
rounder regardless of particle size (Cho et al., 2006), mostly due to
mechanical abrasion during transport. Particle shape of sediments and
sedimentary rocks is described by the following three parameters: form
or sphericity, roundness/angularity and smoothness/roughness
(Krumbein, 1941; Barrett, 1980; Powers, 1982; Dodds, 2003; Oakey et
SAMPLE DESCRIPTION
The samples collected for this study were from the surface of two
rock piles (Sugar Shack West, Spring Gulch) and from within the
interior of the Goathill North rock pile and consisted of a
heterogeneous mixture of rock fragments ranging in size from ~0.1 m
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SME Annual Meeting
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to <1 mm in diameter in a finer-grained matrix. The talus/scree deposit
from the alteration scar is a rock fall or landslide material that formed
when the natural slope of the scar slid, possibly as a result of heavy
rainfall. The Goathill debris flow is a heterogeneous mixture of
sediment that was deposited by a slurry of water and sediment during
flood erosion of the alteration scars (Ludington et al., 2004). Most rock
fragments within the sample exhibit varying degrees of hydrothermal
alteration (Appendix 1) and have been exposed to weathering since
the construction of the rock pile (approximately 25-40 years).
Petrographic descriptions and the mineralogy are summarized in
Appendix 1. Samples for the particle shape analysis were sieved from
the original samples and were >2 mm (no. 10 sieve) in size. Samples
for point load and slake durability testing were 4-10 cm in size.
weathering in these samples include 1) change in color from darker
brown and gray in less weathered samples (original color of igneous
rocks) to yellow to white to light gray in the weathered samples, 2)
increase in abundance of jarosite, gypsum, Fe oxide minerals and
soluble efflorescent salts, and low abundance to absence of calcite,
pyrite, and epidote in weathered samples, and 3) dissolution textures
of minerals (skeletal, boxwork, honeycomb, increase in pore spaces,
fractures, change in mineral shape, accordion-like structures, loss of
interlocking textures, pits, etching) within weathered samples. In
addition, a simple weathering index (SWI) was developed to
differentiate the weathering intensity of Questa rock-pile materials
(Appendix 1). Five classes (Appendix 1) describes the SWI
classification for the mine soils at the Questa mine based on relative
intensity of both physical and chemical weathering (SWI=1, least
weathered to SWI=5, most weathered).
Figure 1. Questa rock piles and other mine features.
Figure 2. The effect of particle shape on friction angle of sand (Cho et
al., 2006 with permission from ASCE). Open circles and closed circles
are for sand with sphericity greater than 0.7, and sphericity lower than
0.7, respectively.
BACKGROUND
Cho et al. (2006) studied the effect of particle shape on internal
friction angle, as shown in Figure 2. Open circles are sand grains with
sphericity >0.7 and closed circles are sand grains with sphericity <0.7.
The plot shows a negative correlation between internal friction angle
and roundness. As roundness varied from 0.1 (very angular) to 1 (well
rounded), the internal friction angle decreased from approximately 40º
to 28º. Sphericity (described as spherical, needle-like, tabular, and flat)
refers to the similarity of a particle to a sphere with equal volume.
Roundness/angularity describes the degree of abrasion of a particle as
shown by the sharpness of its edges and corners (Powers, 1982;
Dodds, 2003).
METHODOLOGY
Sample Collection and Sample Preparation
In order to study the particle shape of the sampled Questa rock
piles and analog materials, five sieve sizes (2 inch, 1 inch, ½ inch,
No.4 and No. 10 sieves) were employed to separate the particles or
rock fragments into different sizes. Samples were collected from the
upper most part of the Goathill North, Spring Gulch, and Sugar Shack
West rock piles, the Goathill debris flow, and a talus/scree deposit
within the Pit Alteration Scar at the Questa mine. Some of the sample
locations were selected near locations where in-situ direct shear tests
were performed (Fakhimi et al., 2008). Note that two samples from the
Sugar Shack rock pile and three samples from Goathill North rock pile
were collected at different locations. Twenty grains were visually
selected from the retained material on each sieve in order to obtain the
entire range of particle shapes observed. For some samples,
particularly of the larger size fractions, less than 20 particles were left
on the sieves. Some particles retained on other sieves with smaller
opening were used to compensate for shortage in the number of larger
particles and to make the total selected rock particles equal to 100 out
of each sample. Table 1 shows the number of particles and their sizes
from each sample that was used for particle shape analysis.
In summary, there appears to be a general agreement in the
literature that the more angular the particles, the higher the shear
strength, if all other factors remained constant (Cho et al., 2006; Leps,
1970; Marsal et al, 1965). Materials with higher shear strength tend to
be more stable. This is because more angular particles form an
interlocking that results in a more stable slope than more rounded
particles.
Chemical weathering throughout the world is based upon the CO2
system, where the dissolution of feldspar to form clays is the most
important chemical reaction (Drever, 1997; Price, 2003). However, in
the Questa rock piles, dissolution of pyrite, calcite, and to a lesser
extent chlorite, illite, and other silicate minerals are more important
chemical reactions that results in 1) dissolution as seen in water
seeping from the rock piles and 2) the precipitation of gypsum, jarosite,
soluble efflorescent salts, and Fe oxide/hydroxide minerals. These
reactions can occur within years to hundreds of years, until the source
of sulfur is consumed. It is difficult, but possible to distinguish between
pre-mining hydrothermal alteration and post-mining weathering by
using detailed field observations and petrographic analysis, especially
using microprobe analyses (Appendix 1). Some of the indications of
Shape Measurement
Sphericity and roundness can be estimated visually using a
comparison chart (Fig. 3). These charts make it easier to examine the
influence of particle shape on geotechnical properties (Powers, 1982;
Cho et al., 2006). More sophisticated modeling techniques using
Fourier, fractal, or image analyses are available (Clark, 1987; Hyslip
and Vallejo, 1997; Smith, 1999; Bowman et al., 2001; Sukumaran and
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SME Annual Meeting
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

Ashmawy, 2001; Alshibli and Alsaleh, 2004), but were not used in this
study, because these methods were not part of the scope of the
project.
Table 1.
analysis.

Samples and the particle sizes used for particle shape




Angular – Particles have sharp edges with no round edges
Subangular – Particles have more sharp edges than round
edges
Spherical –Particles are similar to a ball or a sphere in three
dimensions
Discoidal – Particles are similar to a disc in one dimension
Subdiscoidal – Particles are somewhat similar to the
discoidal description
Prismoidal – Particles are similar to a prism with long or
needle-like shapes
Subprismoidal – Particles are somewhat similar to the
prismoidal description
Based on the chart in Figure 3, the sphericity and roundness of
each grain was determined and the results were analyzed as
described in the
Point Load Test
The point load test is a simple test for estimating rock strength.
The equipment consists of a loading frame that measures the force
required to split the sample and a system for measuring the distance
between the two contact loading points. The point load test can be
performed on samples with different shapes (Broch and Franklin,
1972). All samples were classified according to the classification index
in Table 2.
Table 2. Point load strength index (Is50) classification (Broch and
Franklin, 1972).
According to Folk (1955), an error associated with particle shape
analysis by visual methods is that a grain that seems to be subangular
to one individual can seem subrounded to another individual because
each individual has a different perspective of visualizing things. He
also emphasized that the error in terms of roundness was significant
as compared to the sphericity of the grains. In order to limit the errors
described by Folk (1955), four individuals, including two geologists and
two mining engineers, described the particle shapes using the chart
(Fig. 3) developed by Powers (1982). Each individual examined the
same set of particles. This approach reduced the bias in the
description of particle shapes based on an individual visual inspection.
Slake Durability Test
The slake durability test was developed by Franklin and Chandra
(1972) and recommended by the International Society for Rock
Mechanics (ISRM, 1979) and standardized by the American Society for
Testing and Materials (ASTM, 2001). The purpose of the test is to
assess the influence of physical weathering on rocks as simulated by
subjecting rocks to dry and wet cycles in a rotating drum, thereby
measuring their resistance to wear and tear and breakdown. Durability
of rocks can be described as the resistance to deterioration under
physical weathering conditions over time. Slaking is defined as the
extent of swelling of rocks containing clay minerals when in contact
with water (Franklin and Chandra, 1972). The slake durability index
(ID2) is a measure of durability and provides quantitative information
on the mechanical behavior of rocks according to the amount of clay
and other secondary minerals produced in them due to exposure to
climatic conditions (Fookes et al., 1971). All samples were classified
according to the classification index in Table 3. For each test, 10 rock
fragments weighing between 40 to 60 grams were used.
RESULTS
Figure 3. Comparison chart for estimating particle shape and
roundness (Powers, 1982).
Sample descriptions are summarized in Appendix 1. The samples
represent a range of lithologies and weathering intensities as
determined by petrographic and electron microprobe analyses, color,
paste pH, presence or absence of pyrite, calcite, gypsum, and jarosite.
There is no apparent relationship to particle shape with lithology.
The definitions used for description of particle shapes are as
follows:


Subrounded – Particles have more round edges than sharp
edges
Rounded – Particles have round edges with no sharp edges
Based on the visual comparison method (Powers, 1982),
sphericity and roundness of the rock fragments from the samples were
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obtained (Appendix 2). Figure 4 shows the results of particle shape
analysis for sample MIN-SAN-0001. Figure 4a suggests that
irrespective of particle size, the sphericity of a particle can be
described as subdiscoidal and subprismoidal. In Figure 4b, the
angularity of particles is shown; the majority of the particles are
subangular to subrounded. Note that in this specific sample, the finer
particles are more angular. Similar bar graphs for the other samples
have been provided and are reported in Appendix 2.
Figure 5 shows the overall distribution of roundness and
sphericity for all eight samples. The bar graph in Figure 5 for each
sample was obtained by taking the particle shape analysis for all the
100 particles without considering the particle size. From Figure 5, it is
clear that the majority of the particles are subangular. Note also that
subprismoidal and subdiscoidal particles are predominant in each
sample. It is interesting to note that the maximum number of spherical
particles is in MIN-SAN-0001 (the sample collected from the Goathill
debris flow). The reason for more spherical particles in the debris flow
is probably a result of these materials being partially transported by
water (Ayakwah et al., 2008).
Table 3. Slake durability index classification (Franklin and Chandra,
1972).
Figure 5. Overall distribution of sphericity and roundness class for all
samples.
Table 4 summarizes the slake durability results. The slake
durability values of the samples ranged from 89% to 99%. Similar
results were obtained by Viterbo (2007) and Gutierrez et al. (2008).
Figure 6 illustrates the scatter of the slake durability index of the rock
piles and analogs. Sample descriptions, including lithologies, are in
Appendix 1.
Table 4. Summary of slake durability results.
(a)
Table 5 summarizes of point load strength results indicating point
load strength in the range of 0.6 to 5.0 MPa. Each point load test value
in Table 5 is the average of six or more measurements. The point load
test results (Fig. 7) suggest that the rock fragments are strong except
for rock fragments from sample GHN-KMD-0017. The high point load
strength and slake durability of rock pile and analog materials indicate
that there is no change in particle strength as a result of weathering.
Similar results were obtained by Viterbo (2007) and Gutierrez et al.
(2008). The higher the rock fragment’s strength, the greater their
resistance to weathering.
(b)
Figure 4. (a) Distribution of sphericity and (b) distribution of roundness
of particles in sample MIN-SAN-0001.
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SME Annual Meeting
Feb. 22-Feb. 25, 2009, Denver, CO
samples are mainly subangular, subdiscoidal and subprismoidal. Note
that the sphericity and angularity of the rock fragments of the analog
materials are similar to those of the rock piles. This suggests that there
is no relationship between particle shape and short-term weathering
(<100 years) at the test locations Rock piles made of more angular
particles are more stable compared to rock piles with rounded
particles.
Figure 6. Slake durability of samples from rock piles and debris flows.
Table 5. Summary of point load test results.
Figure 8. A photo of the material from the surface of a Questa rock
pile showing the angularity of the rock fragments compared to a
spherical ball 50 mm in diameter.
ACKNOWLEDGEMENT
This project was funded by Chevron Mining Inc. (formerly
Molycorp Inc.), the New Mexico Bureau of Geology and Mineral
Resources (NMBGMR and the Department of Mineral Engineering,
both at the New Mexico Institute of Mining and Technology (NMT). We
would like to thank the professional staff and students of the large
multi-disciplinary field team for their assistance in the fieldwork and
data analyses. We would also thank American Geological Institute for
giving us the permission to use Powers, 1982 chart. David Jacobs,
Dirk van Zyl, and Solomon Ampim reviewed an earlier version of this
paper and their comments are appreciated. Interpretations and
conclusions expressed in this paper are not necessarily those of
Chevron Mining Inc.
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CONCLUSION
Particle shape analyses were performed on samples collected
from the Questa rock piles and two analog materials (the Goathill
debris flow and a talus/scree deposit within the Pit Alteration Scar).
Most rock fragments selected exhibit varying degrees of hydrothermal
alteration (Appendix 1) and have been exposed to weathering since
the construction of the rock pile (approximately 25-40 years).
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durability indices indicate that, although some of the samples are
weathered, the rock fragments are quite strong and have a similar
range in slake durability and point load indices (Tables 4 and 5). The
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SME Annual Meeting
Feb. 22-Feb. 25, 2009, Denver, CO
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APPENDIX 1
Brief descriptions of the samples, including indications of
weathering are below. The lithology, hydrothermal alteration, and
mineralogy are in Table 1-2. Explanation of the SWI (Simple
Weathering Index) is in Table 1-3. Samples for the slake durability and
point load measurements tested rock fragments of the predominant
lithology (MIN-SAN-0001-intrusive, SPR-SAN-0001-andesite, SSWSAN-0001-andesite,
SSW-SAN-0005-andesite,
QPS-SAN-0001andesite, GHN-KMD-00017-andesite, GHN-KMD-0055-andesite, GHNKMD-0095-rhyolite).
Sample MIN-SAN-0001 (MIN-SAN-0002 is a split) was collected
from the top of the Goathill debris flow (UTM 452369E, 4059919N,
zone 13). The sample is dark brown to gray in color. The particle size
ranges from gravel to clay size. The rock fragments are poorly
cemented in a clay matrix, and there appears to be iron staining on
some of the rock fragments. In outcrop the sample is well graded (poor
sorting). In the sample there is no trace of chlorite, epidote, or pyrite,
although there are relict eroded pyrite cubes that have been replaced
by jarosite (Fig. 1-1) in some rock fragments. The sample has a paste
pH of 3.53, low NP (Neutralizing Potential, -1.87), and SWI of 3
(moderately weathered). The relatively moderate gypsum + jarosite
(3.2%, Table 1-2) is consistent with a moderately weathered intensity.
Figure 1-2. Overview image of rock fragments with soil matrix
adhering to the larger rock fragments.
Sample SSS-SAN-0005 (SSW-SAN-0006 is a split) was collected
from the top of the Sugar Shack West rock pile (UTM 453975E,
4060822N, zone 13). The hand sample consists of cobble to clay-size
particles, light brown in color, and is well graded (poor sorting). It has
some plasticity with no cementation. The sample has a paste pH of
2.4, low NP (Neutralizing Potential, -2.53), and SWI of 4 (weathered).
The relatively high gypsum + jarosite (6%, Table 1-2) is consistent with
a weathered intensity.
Sample QPS-SAN-0001 (QPS-SAN-0002 is a split) was collected
from Questa pit scar (UTM 454146E, 4062551N, zone 13). The hand
sample is well graded (poorly sorted) material with brown coloration. It
has some plasticity and cemented with gypsum. The sample has a
paste pH of 2.84, low NP (Neutralizing Potential, -0.52), and SWI of 4
(weathered). The relatively high gypsum + jarosite (5%, Table 1-2) is
consistent with a weathered intensity.
Sample GHN-KMD-0017 was collected from Unit I, trench LFG006, bench 9 (UTM 4062143.2N, 453695.9E, zone 13. The sample
was located approximately 2 ft east of the outer edge of the rock pile.
Sample GHN-KMD-0017 is a select, bulk sample of a layer that
consists of soil matrix and rock fragments that vary in size from 3
inches to less than 1 mm. It is pale yellow, oxidized, poorly-sorted,
medium consistency, and high plasticity. Chlorite is found as rare
green individual grains. Gypsum is found as primarily rounded milky
grains with some euhedral clear crystals. Pyrite is found as small cubic
crystals within rock fragments and soil matrix. The sample has a paste
pH of 2.19, low NP (Neutralizing Potential, 0.73), and SWI of 4
(weathered). The relatively high gypsum + jarosite (5.5%, Table 1-2) is
consistent with a weathered intensity.
Figure 1-1. Highly altered quartz-rich clast (darker areas) with relict
pyrite cubes replaced by jarosite (brighter areas).
Sample SPR-SAN-0001 (SPR-SAN-0002 is a split, fig. 1-2) was
collected from the top of the Spring Gulch rock pile (UTM coordinates
4062285N, 455255E, zone 13). In hand sample, the sample is brown
in color. The particle size for the rock fragments ranges from cobble to
clay and there appears to be minor Fe oxide staining on the outside of
some fragments. The sample is well graded (poor sorting). The
primary cement in this sample is Fe-oxides with minor amounts of clay.
Pyrite crystals have numerous small inclusions of apatite and quartz.
The sample has a paste pH of 3.98, high NP (Neutralizing Potential,
18.96), and SWI of 2 (least weathered). The abundance of gypsum
(2%) with no jarosite is consistent with a least weathered intensity.
Sample GHN-KMD-0055 was collected from Unit I, trench LFG007, bench 12 (UTM 4062146.5N, 453676.5E, zone 13). Sample GHNKMD-0055 is a select, bulk sample of a layer that consists of soil
matrix and rock fragments that vary in size from 3 inches to less than 1
mm. It is pale yellow, coarse sand to gravel, and well graded (poor
sorting). Edge of rock fragments shows Fe oxide and clay-rich material
adhered to the edge of the sample. Pyrite cubes within the rock
fragments are relatively fresh (Fig. 1-3). The sample has a paste pH of
4.27, low NP (Neutralizing Potential, -15.03), and SWI of 4
(weathered). The relatively low gypsum + jarosite (2%, Table 1-2)
suggest a weathering intensity of moderately weathered.
Sample SSW-SAN-0001 (SSS-SAN-0002 is a split) was collected
from the top of the Sugar Shack West rock pile (UTM 453682E,
4060534N, zone 13). The hand sample consists of cobble to clay-size
particles, is light brown in color, and is well graded (poor sorting). It is
non-plastic with local pockets of high plasticity with moderate
cementation. The sample has a paste pH of 2.9, low NP (Neutralizing
Potential, -17.23), and SWI of 4 (weathered). The relatively high
gypsum + jarosite (6%, Table 1-2) is consistent with a weathered
intensity.
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Sample GHN-KMD-0095 was collected from Unit C, trench LFG008, bench 18 (UTM 4062118.6N, 453656E, zone 13). Sample GHNKMD-0095 is a select, bulk sample of a layer that consists of soil
matrix and rock fragments that vary in size from 3 inches to less than 1
mm. It is yellow brown, coarse sand to gravel, and well graded (poor
sorting). The sample has a paste pH of 2.73 and SWI of 4 (weathered).
The relatively low gypsum + jarosite (1.2%, Table 1-2) suggest a
weathering intensity of moderately weathered.
GHN-KMD-0055-31-01 Unit I
Figure 1-3. BSE image of fresh pyrite in rock sample. Brightest areas
on image are pyrite grains, which are of uniform brightness and display
distinct grain margins.
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Table 1-1. Summary of sample preparation for specific laboratory analyses. XRF–X-ray fluorescence analyses, XRD–X-ray diffraction analysis, ICP–
Induced-coupled plasma spectrographic analysis, NAG–net acid producing tests, ABA–acid base accounting tests.
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Table 1-2. Description of samples. Hydrothermal alteration and composition of rock fragments was determined by petrographic techniques using a
binocular microscope and electron microprobe. The mineral composition was determined by a modified ModAn technique using petrographic analysis,
clay mineralogy by X-ray diffraction, and whole rock chemistry (McLemore et al., 2009). Phyllic or QSP (quartz-sericite-pyrite) alteration consists of
quartz, sericite (a form of illite), and pyrite. Propylitic alteration consists of essential chlorite (producing the green color), epidote, albite, pyrite, quartz,
carbonate minerals, and a variety of additional minerals. Argillic or clay alteration consists of kaolinite, smectite (montmorillonite clays), chlorite, epidote,
and sericite.
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Table 1-3. Simple weathering index for rock-pile material (including rock fragments and matrix) at the Questa mine (McLemore et al., 2008a).
The SWI accounts for changes in color, texture, and mineralogy due to weathering, but it is based on field descriptions. Some problems with this
weathering index are:







It is subjective and based upon field observations.
This index does not always enable distinction between pre-mining supergene hydrothermal alteration and post-mining weathering.
The index is developed from natural residual soil weathering profiles, which typically weathered differently from the acidic conditions within
the Questa rock piles and, therefore, this index may not adequately reflect the weathering conditions within the rock piles.
This index refers mostly to the soil matrix; most rock fragments within the sample are not weathered except perhaps at the surface of the
fragment and along cracks.
The index is based primarily upon color and color could be indicative of other processes besides weathering intensity.
This index was developed for the Questa rock piles and may not necessarily apply to other rock piles.
Weathering in the Questa rock piles is an open not a closed system (i.e. water analysis indicates the loss of cations and anions due to
oxidation).
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APPENDIX 2
Figure 2-3.
0001)
Figure 2-1. Particles from Debris Flow Sample (MIN-SAN-0001)
Particles from Pit Alteration Scar Sample (QPS-SAN-
(a)
(a)
(b)
Figure 2-2. Distribution of a) sphericity b) roundness of particles from
the debris flow sample.
(b)
Figure 2-4. Distribution of a) sphericity b) roundness of particles from
the Pit Alteration Scar sample.
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Figure 2-5. Particles from Sugar Shack West Sample (SSW-SAN0005)
Figure 2-7. Particles from Spring Gulch Sample (SPR-SAN-0001)
(a)
(a)
(b)
Figure 2-8. Distribution of a) sphericity b) roundness of particles from
the Spring Gulch rock pile sample.
(b)
Figure 2-6. Distribution of a) sphericity b) roundness of particles from
the Sugar Shack rock pile sample (SSW-SAN-0005).
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Figure 2-9. Particles from Sugar Shack West Sample (SSW-SAN0001)
Figure 2-11. Particles from Goathill North Sample (GHN-KMD-0017)
(a)
(a)
(b)
Figure 2-10. Distribution of a) sphericity b) roundness of particles from
the Sugar Shack rock pile sample (SSW-SAN-0001).
(b)
Figure 2-12. Distribution of a) sphericity b) roundness of particles from
the Goathill North sample (GHN-KMD-0017).
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Figure 2-13. Particles from Goathill North Sample (GHN-KMD-0055)
Figure 2-15. Particles from Goathill North Sample (GHN-KMD-0095)
(a)
(a)
(b)
Figure 2-16. Distribution of a) sphericity b) roundness of particles from
the Goathill North sample (GHN-KMD-0095).
(b)
Figure 2-14. Distribution of a) sphericity b) roundness of particles from
the Goathill North sample (GHN-KMD-0055).
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