Geologic Processes Affecting the Chemistry, Mineralogy, and Acid Potential on Particle Size Fractions: Examples from Waste Rock Piles in New Mexico, USA
Virginia T. McLemore
New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM, 87801, ginger@gis.nmt.edu
PURPOSE
• To describe the differences in mineralogy, chemistry, and acid potential between different particle size fractions from different materials
• To discuss some of the geologic, geochemical, pedological or man‐influenced processes that controlled these changes between the different particle sizes
IMPORTANCE OF UNDERSTANDING COMPOSITIONAL DIFFERENCES IN SIZE FRACTIONS
• Help plan and assess reclamation procedures • Compare trace‐element concentrations in mined versus undisturbed areas
• Determine background concentrations • Provide background data that can assist with the planning of future mining operations
IMPORTANCE
• Determine the best size fractions for prediction tests, such as humidity cell tests and understand the composition of the size fractions
Humidity cell
testing
IMPORTANCE
To understand shear strength results
Shear strength is defined by the
internal friction and cohesion within
granular material and is in part
controlled by particle size
Shear Force
Shear
Zone
Shear Force
LOCATION OF STUDY AREAS
Dam at San Miquel on the Pecos River.
Pecos River
area—stream
sediments
draining a VMS
deposit
Numerous discrete and diffuse seeps
occur along the base of the mine waste Pecos River below Willow Creek and
pile.
the Pecos mine.
Pecos Samples
The finer-size fractions are
typically the smallest
proportion of the sample by
weight (generally, <25% in the
<2 mm fraction)
Size fractions
verses metal
content—Pecos
stream
sediments
Size fractions
verses metal
content—Pecos
stream
sediments
Size fractions
verses metal
content—Pecos
stream
sediments
Hillsboro—porphyry Cu deposit with veins, carbonate hosted Pb‐Zn deposits
The finer-size fractions are
typically the smallest
proportion of the sample by
weight (generally, <25% in the
<2 mm fraction)
Hillsboro—increase in metals with decrease in
grain size (vein rock pile, FAAS, Munroe,
1999)
Hillsboro—decrease in Cu and increase in
Pb with decrease in grain size (vein rock
pile, FAAS, Munroe, 1999)
Hillsboro—increase in Cu, As with decrease
in grain size (carbonate-hosted Pb-Zn rock
pile, FAAS, Munroe, 1999)
Questa project—
porphyry Mo deposit
Top (fine-grained,
WR1)
Intermediate-gradational
(WR2)
Toe (coarse-grained,
WR3)
Sampling a
trench
gypsum
jarosite
pyrite
weathering
Shallow Slope Failure After Rain on Sugar Shack West
Questa rock piles—poorly‐graded or well‐graded sandy gravel with small percentage of fines
Total Feldspar
GHN-JRM-0001
GHN-JRM-0002
GHN-KMD-0088
MIN-AAF-0001
MIN-AAF-0004
MIN-SAN-0002
QPS-AAF-0001
QPS-AAF-0003
QPS-AAF-0005
QPS-AAF-0009
QPS-SAN-0002
SPR-SAN-0002
50
40
30
20
10
0
0.000
0.200
0.400
0.600
0.800
1.000
1.200
Pyrite
Particle Size Fractions (inches)
As the size
fraction
decreases Kfeldspar, pyrite
decreases
6
GHN-JRM-0001
GHN-JRM-0002
GHN-KMD-0088
MIN-AAF-0001
MIN-AAF-0004
MIN-SAN-0002
QPS-AAF-0001
QPS-AAF-0003
QPS-AAF-0005
QPS-AAF-0009
QPS-SAN-0002
SPR-SAN-0002
5
4
Pyrite %
K-feldspar + Plagioclase %
60
3
2
1
0
0.000
0.200
0.400
0.600
0.800
1.000
Particle Size Fractions (inches)
1.200
Gypsum vs. Size Fraction
6
5
Gypsum (%)
4
GHN-JRM-0002
MIN-AAF-0001
QPS-AAF-0001
SPR-SAN-0002
SSW-SAN-0002
3
2
1
0
1
0.75
0.5
0.187
0.0787
Size Fraction (inch)
Gypsum decreases in concentration in the
finer size fractions
Sulfide
3.50
3.00
GHN-JRM-0001
GHN-JRM-0002
GHN-KMD-0088
MIN-AAF-0001
MIN-AAF-0004
MIN-SAN-0002
QPS-AAF-0001
QPS-AAF-0003
QPS-AAF-0005
QPS-AAF-0009
QPS-SAN-0002
SPR-SAN-0002
S%
2.50
2.00
1.50
1.00
0.50
0.00
0.000
0.200
0.400
0.600
0.800
1.000
1.200
Partcle Size Fractions (inches)
High sulfide concentration in the larger size
fractions
Paste pH
8
7
GHN-JRM-0001
GHN-JRM-0002
GHN-KMD-0088
MIN-AAF-0001
MIN-AAF-0004
MIN-SAN-0002
QPS-AAF-0001
QPS-AAF-0003
QPS-AAF-0005
QPS-AAF-0009
QPS-SAN-0002
SPR-SAN-0002
Paste pH
6
5
4
3
2
1
0
0.000
0.200
0.400
0.600
0.800
1.000
1.200
Particle Size Fractions (inches)
As the size fraction decreases the
paste pH decreases in some samples,
but not all
Calcite
1.2
GHN-JRM-0001
GHN-JRM-0002
GHN-KMD-0088
MIN-AAF-0001
MIN-AAF-0004
MIN-SAN-0002
QPS-AAF-0001
QPS-AAF-0003
QPS-AAF-0005
QPS-AAF-0009
QPS-SAN-0002
SPR-SAN-0002
Calcite %
1
0.8
0.6
0.4
0.2
0
0.000
0.200
0.400
0.600
0.800
1.000
1.200
Particle Size Fractions (inches)
Calcite variable with size fraction sizes
NP
30.00
GHN-JRM-0001
25.00
GHN-JRM-0002
NP (kgCaCO3/t)
20.00
GHN-KMD-0088
MIN-AAF-0001
15.00
MIN-AAF-0004
10.00
MIN-SAN-0002
5.00
QPS-AAF-0003
QPS-AAF-0001
QPS-AAF-0005
0.00
-5.000.000
QPS-AAF-0009
0.200
0.400
0.600
0.800
1.000
1.200
QPS-SAN-0002
SPR-SAN-0002
-10.00
Particle Size Fractions (inches)
NP decreases in smaller size fractions
indicating that as the grains become smaller
the amount of neutralizing minerals reduces,
thus reducing the capacity to neutralize acid
FACTORS CONTROLLING PARTICLE SIZE COMPOSITION
Primary igneous crystallization, pre‐
mining hydrothermal alteration and weathering
GHN-NWD-0002-02
Pyrite cubes
Clay
Alteration—This is an intensely altered andesite rock
as evidenced by the presence of hydrothermal clay in
the hachured-looking areas. Also, pyrite cubes can be
seen. FOV is 2.5 mm
The breaking up of the rock material during blasting, hauling, and dumping during mining also contributes to differences in size fractions Weathering—In rock piles, the fine‐grained soil matrix is weathered, while interiors of rock fragments are not These are typical weathering textures. Note the lack of weathering of the rock fragments. GHN-NWD-0024-30-05
Gypsum
Gypsum cementation in soil sample. FOV
2.5 mm Cementation=cohesion
GHN-KMD-0051-31-07
Fe-oxide
GHN-KMD-0051-31-07
Questa rock piles—Fe oxides (weathering) cement grains on the exterior and interior of rock fragment
Bright areas represent Fe-oxide cement in rock fragment
Precipitation of weathered minerals, such as gypsum, along fractures and veins, which increase the mineral volume and breaks apart the rock Other minerals can armor the original crystals and prevent further weathering of sulfides
Ore material can be more friable and less cemented, compared to other more resistant, typically silicified material that can form the wall rock in many districts CONCLUSIONS
• Primary igneous crystallization, premining hydrothermal alteration and
weathering, and post-mining blasting,
hauling, dumping, and emplacement into
the rock pile and subsequent weathering
affect the composition of each size
fraction
CONCLUSIONS
• The finer-size fractions are typically
the smallest proportion of the sample
by weight (generally, <25% in the <2 mm
fraction)
• Paste pH values decrease from the
coarser-size fraction to the finer-size
fraction
• The sulfur concentrations are higher
and the NP lower in the finer sizes than
larger particle sizes
CONCLUSIONS
• Weathering can be more pronounced in
the finer-size fraction than the
coarser-size fraction
– Amoring
– Cementation
Acknowledgements
• Funding
– U.S. Bureau of Reclamation (cooperative agreement no. 3‐FC‐40‐
13830)
– U.S. Geological Survey
– Chevron Mining Inc. (formerly Molycorp, Inc
– NMBGMR
• M.S. theses
– Eric Munroe
– Gabriel Graf
– John Morkeh
• Professional staff and many students who worked on these projects (Nelia Dunbar. Lynn Heizler, Lynn Brandvold, Kelley Donahue, Virgil Lueth)