U.S. Geological Survey
Open-File Report 02-454
By Robert Ayuso and Nora Foley
Detailed mineralogical, geochemical and radiogenic isotopic studies of iron-sulfide and secondary iron oxy-hydroxide minerals in natural bedrock in coastal Maine and New Hampshire test the link between arsenic-rich sulfide minerals in bedrock and secondary oxy-hydroxide minerals. Samples were selected from over 70 bedrock localities, including 22 within the regionally extensive and sulfide-mineral-rich Penobscot Formation and 10 associated with mineral deposits from coastal New Hampshire and Maine, and coupled with data from drill core collected at several sites including areas where well waters contain anomalous As abundances (e.g., Northport, ME). The data were used to establish a diversity of primary and secondary mineralogical hosts for arsenic in bedrock of this part of New England. The studies show that bedrock mineralogy is critical to contributing arsenic to groundwater and suggest a number of mineralogical pathways for arsenic that define weathering processes. The studies show that lead isotopic compositions of the sulfides and iron oxy-hydroxides overlap and establish a genetic link between the sulfides and secondary minerals. The data and interpretive results were presented at Arsenic in New England -- A multidisciplinary Scientific Conference, Manchester, New Hampshire, May 29-31, 2002, sponsored by the New Hampshire Consortium on Arsenic, are available in abstract and poster (full size = 84 by 36 inch sheet) formats.
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Pb Isotopes, Arsenic Sources and Enrichment Pathways Linking Sulfides from Mines and Unmineralized Rocks to Secondary Iron Oxides, Coastal New England [8.9-MB PDF file measuring 84 x 36 inches]
Mineralogical Pathways for Arsenic in Weathering Meta-Shales: An Analysis of Regional and Site Studies in the Northern Appalachians [14.7-MB PDF file measuring 72 x 36 inches]
By Robert Ayuso1, Nora Foley1, Joseph Ayotte2, Ann Lyon1, John Burns1, Robert Marvinney3, Andrew Reeve4, and Gilpin Robinson1.
1U.S. Geological Survey, Reston, VA 201921
2Pembroke, NH 032752
3Maine Geological Survey, Augusta, ME 043333
4Dept. of Geological Sciences, University of Maine, Orono, ME 044014
Sulfide and secondary iron oxy-hydroxides minerals (n = 56) collected from mines and sulfidic meta-shales were analyzed for their lead isotopic compositions and for trace elements in an effort to test the link between arsenic-rich sulfide minerals and secondary oxy-hydroxides presently forming along rock surfaces, joints, crevices, fractures, etc. This study is part of a detailed mineralogical and geochemical analysis of iron-sulfide minerals in natural bedrock (e.g., the sulfide-rich Cambrian-Ordovician Penobscot Formation) and drill cores in coastal Maine and New Hampshire. Weathering of pyrite, pyrrhotite and other sulfide minerals generates acid and releases metals (e.g., Pb, Cu, As, Co, Ni) that are then sequestered in secondary minerals (e.g., ferrihydrite, goethite, scorodite, jarosite, and natrojarosite, rozenite, and melanterite). The iron oxy-hydroxide minerals constitute the ideal substrates for sorption reactions involving As, Pb, and other metals in solution. Our study shows that lead isotopic compositions of the sulfides and iron oxy-hydroxides overlap and establish a genetic link between the sulfides and secondary minerals.
Lead isotopic compositions were determined by thermal ionization mass spectrometry using bulk samples and acid-leached samples of galena, arsenian pyrite, pyrrhotite, löllingite, cobaltite, and arsenopyrite. Lead isotopic compositions range as follows: 206Pb/204Pb = 18.073-19.489; 207Pb/204Pb = 15.539-15.675; 208Pb/204Pb = 37.947-39.102 and plot as nearly vertical fields on standard uranogenic and thorogenic lead diagrams. Lead isotopic compositions of secondary minerals, including goethite, jarosite, and natrojarosite had the following ranges: 206Pb/204Pb = 18.356-21.945; 207Pb/204Pb = 15.595-15.839; 208Pb/204Pb = 38.169-39.162. The isotopic compositions plot on uranogenic and thorogenic lead diagrams as broad and steep fields that extensively overlap the field of the sulfides but extend to more radiogenic compositions. The similarity of isotopic compositions provides evidence that fluid-mineral reactions leading to the decomposition of the sulfides released metals and arsenic and imprinted the lead isotopic signatures of the sulfides on the secondary minerals. Arsenic and lead contents in the sulfides vary widely. For example, in the Penobscot Formation, pyrites range from ~20 ppm to ~2000 ppm arsenic and ~10 ppm to >400 ppm lead, but pyrites from the mine areas are substantially higher in arsenic and lead and range to thousands of ppm. Bedrock occurrences represented by bulk rock samples also have a wide range in arsenic and lead contents. For example, in the Northport area, Maine, where As in groundwater is elevated, arsenic contents as high as 730 ppm have been found in rocks of the Penobscot Formation. Arsenic and lead contents of the secondary minerals are also highly variable but characteristically closely match the ranges found in the sulfides and bulk rock samples. Although the isotopic compositions of the sulfides can account for most of the variations in the secondary minerals as a result of weathering, a more radiogenic lead component could be present in the secondary minerals but its identity has not been determined precisely. Possible contributions from anthropogenic sources cannot be disregarded.
By Nora K. Foley1, Robert A. Ayuso1, Joseph D. Ayotte2, Nicole West1, Jeremy Dillingham1, Robert G. Marvinney3, Andrew S. Reeve4, and Gilpin R. Robinson, Jr1.
1U.S. Geol. Survey, Reston, VA 201921
2Pembroke, NH 032752
3Maine Geol. Survey, Augusta ME 043333
4Dept. of Geol. Sciences, Univ. of Maine, Orono, ME 044014
Concern about arsenic-bearing groundwaters in New England has caused examination of possible sources in the local bedrock. Detailed mineralogical analyses of iron-sulfide minerals from over 70 bedrock localities, including 22 within the regionally extensive and sulfide-mineral-rich Penobscot Formation and 10 associated with mineral deposits from coastal New Hampshire and Maine, coupled with data from drill core collected at several sites including areas where well waters contain anomalous arsenic abundances (e.g., Northport, ME), establish a diversity of primary and secondary mineralogical hosts for arsenic in bedrock. Reactions involving arsenic minerals and either groundwaters at low pH or in bicarbonate fluids at near-neutral pH probably control arsenic contents in groundwater in the region. Bedrock mineralogy is critical to contributing arsenic to groundwater and suggests a number of possible mineralogical bounds on the pathways for arsenic that help define weathering processes.
Primary arsenic-bearing minerals identified include pyrite (max. 4 wt.% As in FeS2), pyrrhotite (max. 0.5 wt.% As in Fe1-xS), löllingite, realgar (?), cobaltite, arsenopyrite, cobaltian arsenopyrite (max. 8.4 wt.% Co), and tennantite. Supergene minerals that constitute intermediate mineralogical sources include orpiment and arsenolite-like minerals, Co-Ni-arsenates (?), calcium-arsenates (rauenthalite, phaunouxite?), scorodite (FeAsO4.2H2O) and secondary arsenopyrite, pyrite, and marcasite. Pyrite, the most abundant iron-sulfide mineral in many of the rocks, is a primary host for arsenic in subeconomic mineral occurrences (e.g., volcanic-associated massive sulfides, metamorphic-gold, and Carlin-gold deposit types in the region). In meta-shales, coexisting pyrrhotite, cobaltite, and arsenopyrite constitute a probable source for high arsenic contents (e.g., Penobscot Fm.).
Weathering of pyrrhotite in the Penobscot Fm. results in (1) complex mixtures of pyrite + marcasite, and (2) iron oxy-hydroxides and secondary salts, such as ferrihydrite, rozenite and melanterite. Weathering of pyrite, löllingite, realgar (?), and arsenopyrite and other sulfide minerals in these settings causes the production of acid and release of trace metals, including As, Co, Ni, Pb, etc., which can sorb on iron oxy-hydroxide substrates. An alternate pathway to consider is the oxidation of arsenopyrite or other arsenic-bearing minerals to produce iron oxides and release sulfur and arsenic, which under specific conditions may produce arsenolite or orpiment. Subsequent leaching of the goethite + arsenolite or orpiment assemblages by bicarbonate-bearing fluids could release arsenic into the groundwater system. Löllingite occurring at mineralized localities oxidizes to scorodite and some iron oxy-hydroxides products. Calcium-arsenates are thought to form at some sites, possibly by the reaction of acidic, arsenic-bearing waters with calc-silicate substrates. The presence of calcium-arsenates also suggests a process whereby (1) As is liberated from bedrock by direct interaction between anaerobic HCO3- groundwaters and arsenic-minerals and (2) is subsequently re-precipitated at low pH. When solubility is controlled by calcium-arsenate, calcium in solution suppresses the solubility of arsenic, however, the long-term ability of these minerals to sequester arsenic is untested.
This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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