IMPORTANCE OF UNDERSTANDING COMPOSITIONAL DIFFERENCES IN SIZE FRACTIONS

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@nmbg.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 maninfluenced 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

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 pile. Pecos River below Willow Creek and 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). In some samples, the concentrations of metals increase in the smallest size fractions, but not all.

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). The concentrations of metals generally increase in the smallest size fractions.

Hillsboro increase in metals 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 Sampling a trench Top (fine-grained, WR1) Intermediate-gradational (WR2) Toe (coarse-grained, WR3)

Questa rock piles poorly-graded or well-graded sandy gravel with small percentage of fines

Total Feldspar K-feldspar + Plagioclase % 60 50 40 30 20 10 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 0 0.000 0.200 0.400 0.600 0.800 1.000 1.200 Particle Size Fractions (inches) Pyrite 6 As the size fraction decreases K- feldspar, pyrite decreases Pyrite % 5 4 3 2 1 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 0 0.000 0.200 0.400 0.600 0.800 1.000 1.200 Particle Size Fractions (inches)

Gypsum vs. Size Fraction 6 5 Gypsum (%) 4 3 2 GHN-JRM-0002 MIN-AAF-0001 QPS-AAF-0001 SPR-SAN-0002 SSW-SAN-0002 1 0 1 0.75 0.5 0.187 0.0787 Size Fraction (inch) Gypsum decreases in concentration in the finer size fractions

Sulfide S % 3.50 3.00 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) 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 High sulfide concentration in the larger size fractions

Paste ph Paste ph 8 7 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) 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 As the size fraction decreases the paste ph decreases in some samples, but not all

Calcite 1.2 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) 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 variable with size fraction sizes

The finer-size fractions are typically the smallest proportion of the sample by weight (generally, <25% in the <2 mm fraction). The concentrations of metals generally increase in the smallest size fractions. Mineral concentrations change as well.

FACTORS CONTROLLING PARTICLE SIZE COMPOSITION

Primary igneous crystallization, pre-mining hydrothermal alteration and weathering

The size of the original primary igneous minerals during crystallization, often in a fine-grained groundmass, results in minerals of different sizes that are subsequently liberated from the rock fragments Some of these minerals, such as biotite and hornblende, are more susceptible to weathering and liberation from the host rock than other, more resistant minerals, such as quartz

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

Pre-mining hydrothermal alteration and weathering can result in the replacement of larger primary silicate minerals by smaller hydrothermal clay minerals These replacement clay minerals can remain in the larger size fractions until liberated during mining and dumping into the rock piles or subsequent physical weathering

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, resulting in different size fractions within the waste rock pile

CONCLUSIONS Primary igneous crystallization, pre-mining 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

Funding Acknowledgements 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)