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Scientific Investigations Report 2008-5063

Prepared in cooperation with the U.S. Bureau of Land Management

Net Acid Production, Acid Neutralizing Capacity, and Associated Mineralogical and Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado

By Douglas B. Yager, LaDonna Choate, and Mark R. Stanton

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Scientific Investigations Report
2008-5063 PDF (22.5 MB)
Abstract

This report presents results from laboratory and field studies involving the net acid production (NAP), acid neutralizing capacity (ANC), and magnetic mineralogy of 27 samples collected in altered volcanic terrain in the upper Animas River watershed near Silverton, Colo., during the summer of 2005. Sampling focused mainly on the volumetrically important, Tertiary-age volcanic and plutonic rocks that host base- and precious-metal mineralization in the study area. These rocks were analyzed to determine their potential for neutralization of acid-rock drainage.

Rocks in the study area have been subjected to a regional propylitic alteration event, which introduced calcite, chlorite (clinochlore), and epidote that have varying amounts and rates of acid neutralizing capacity (ANC). Locally, hydrothermal alteration has consumed any ANC and introduced minerals, mainly pyrite, that have a high net acid production (NAP). Laboratory studies included hydrogen peroxide (H2O2) acid digestion and subsequent sodium hydroxide (NaOH) titration to determine NAP, and sulfuric acid (H2SO4) acid titration experiments to determine ANC. In addition to these environmental rock-property determinations, mineralogical, chemical, and petrographic characteristics of each sample were determined through semiquantitative X-ray diffractometry (Rietveld method), optical mineralogy, wavelength dispersive X-ray fluorescence, total carbon-carbonate, and inductively coupled plasma–mass spectrometric analysis.

An ANC ranking was assigned to rock samples based on calculated ANC quantity in kilograms/ton (kg/t) calcium carbonate equivalent and ratios of ANC to NAP. Results show that talus near the southeast Silverton caldera margin, composed of andesite clasts of the Burns Member of the Silverton Volcanics, has the highest ANC (>100 kg/t calcium carbonate equivalent) with little to no NAP. The other units found to have moderate to high ANC include (a) andesite lavas and volcaniclastic rocks of the San Juan Formation, west and northwest of the Silverton caldera, and (b) the Picayune Megabreccia Member of Sapinero Mesa Tuff along the western San Juan caldera margin. Sultan Mountain stock, composed of granitoid intrusive rocks, was shown to have low ANC and moderate NAP.

Sequential leachate analyses on a suite of whole-rock samples from the current and a previous study indicate that host rock composition and mineralogy control leachate compositions. The most mafic volcanic samples had high leachate concentrations for Mg, Fe, and Ca, whereas silicic volcanic samples had lower ferromagnesiun compositions. Samples with high chlorite abundance also had high leachable Mg concentrations. Trace-element substitution, such as Sr for Ca in plagioclase, controls high Sr concentrations in those samples with high plagioclase abundance. High Ti abundance in leachate was observed in those samples with high magnetite concentrations. This is likely due to samples containing intergrown magnetite-ilmenite. Whole rocks having high trace-element concentrations have relatively high leachate trace-element abundances. Some lavas of the San Juan Formation and Burns Member of the Silverton Volcanics had elevated Zn-, Cd-, and Pb-leachate concentrations. Manganese was also elevated in one San Juan Formation sample. Other San Juan Formation and Burns Member lavas had low to moderate trace-element abundances. One sample of the pyroxene andesite member of the Silverton Volcanics had elevated concentrations for As and Mo. Most other pyroxene andesite member samples had low leachate trace-element abundances.

Mine-waste-leachate analyses indicated that one mine-waste sample had elevated concentrations of Cu (1.5 orders of magnitude), Zn (1 order of magnitude), As (1 order of magnitude), Mo (1.5 to 2 orders of magnitude), Cd (1 to 2 orders of magnitude), and Pb (2 to 3 orders of magnitude) compared to whole rocks. These data indicate the importance of whole-rock geochemistry or leachate analyses prior to using igneous outcrops or talus for mine-waste remediation projects. Also, if ANC units have modest trace contaminant abundances compared with mine waste targeted for cleanup, then these ANC materials could still be useful to consider for remediation projects.

New magnetic susceptibility measurements indicate that the high ANC pyroxene andesite member of the Silverton Volcanics (previous study) also has the highest median volume magnetic susceptibility. Other units with representative high ANC outcrops or talus (Burns Member lavas, tuffs of Eureka Member of Sapinero Mesa Tuff, Picayune Megabreccia Member, and San Juan Formation) have variable but generally moderate to low magnetic susceptibility. Sultan Mountain stock has a moderate, mean magnetic susceptibility.

 

Version 1.0

Posted June 2008

  • Plate PDF (75.5 MB)
  • Map showing acid neutralizing capacity and net acid production of Animas River watershed rocks near Silverton, Colorado.

Suggested citation:

Yager, D.B., Choate, LaDonna, and Stanton, M.R., 2008, Net acid production, acid neutralizing capacity, and associated mineralogical and geochemical characteristics of Animas River watershed igneous rocks near Silverton, Colorado: U.S. Geological Survey Scientific Investigations Report 2008–5063, 63 p.



Contents

Abstract

Introduction

Previous Work

New Work for this Study

Study Area

Geologic Setting

Mid-Tertiary Volcanism

Mineralization and Subsequent Erosion Events

Rock Types of Interest for Potential Acid Neutralization Capacity

Precambrian Rocks

Paleozoic Sedimentary Rocks

Mesozoic Sedimentary Rocks

Tertiary Sedimentary Rocks

Tertiary Igneous Rocks

Regional Propylitic Alteration

Quaternary Surficial Deposits

Acid-Generating and Acid-Neutralizing Minerals

Acid-Generating Mineral Reactions

Acid-Neutralizing Minerals

Methods

Sample Preparation and Analyses

Net Acid Production Test

Acid Neutralization Capacity Test (Acid Titration)

Sequential Extraction Procedure

Rock Physical Properties (Magnetic Susceptibility)

Results

Net Acid Production

Sulfide-Bearing Samples

Nonsulfide-Bearing(?) Samples

Acid Neutralizing Capacity

Eureka (Unit Tse) and Picayune Megabreccia (Unit Tsemb) Members

San Juan Formation (Unit Tsj)

Burns Member (Unit Tb)

ANC and NAP Unit Comparisons

Sequential Leachate Results (Whole Rocks)

Magnesium

Aluminum

Potassium

Calcium

Titanium

Iron

Manganese

Nickel

Copper

Zinc

Arsenic

Strontium

Molybdenum

Cadmium

Barium

Lead

Sequential Leachate Results (Grand Mogul Mine Waste)

Magnesium

Aluminum

Calcium

Titanium

Iron

Manganese

Nickel

Copper

Zinc

Arsenic

Strontium

Molybdenum

Cadmium

Barium

Lead

Rock Physical Properties (Magnetic Susceptibility)

Summary

ANC and NAP

Burns Member (Unit Tb) and Picayune Megabreccia Member (Unit Tsemb)

San Juan Formation (Unit Tsj)

Sultan Mountain Stock (Unit Tig)

Sequential Leachate Studies (Whole Rocks)

Major Elements

Mineralogic Control on Leachate Compositions

Trace-Element Leachate Abundances

San Juan Formation (Unit Tsj)

Burns Member (Unit Tb)

Pyroxene Andesite Member (Unit Tpa)

Mine-Waste-Leachate Compositions

Magnetic Susceptibility and Corresponding ANC, NAP, and Leachate Rock Properties

Pyroxene Andesite Member (Unit Tpa)

Burns Member (Unit Tb)

Eureka Member (Unit Tse)

Sultan Mountain Stock (Unit Tig)

San Juan Formation (Unit Tsj)

Conclusions

Acknowledgments

References Cited

 


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