%0 Conference Proceedings %B Abstracts with Programs - Geological Society of America %D 2010 %T Arsenic in central Massachusetts bedrock and groundwater %A McTigue, David F. %A Stein, Carol L. %A Brandon, William C. %A Joseph P Kopera %A Keskula, Anna J. %A Koteas, G. Christopher %K #StaffPubs %K alteration %K arsenic %K arsenides %K arsenopyrite %K Ayer Granodiorite %K BEDROCK %K central Massachusetts %K chelmsford granite %K Devonian %K dilation %K discharge %K dissolved materials %K drinking water %K Eh %K fractures %K General geochemistry 02A %K geochemistry %K granites %K ground water %K igneous rocks %K joints %K massachusetts %K metals %K metamorphism %K meteoric water %K overburden %K Paleozoic %K petrography %K plutonic rocks %K pollutants %K reduction %K solubility %K solution %K sulfides %K theoretical models %K United States %X Across the New England "arsenic belt," groundwater arsenic (As) concentrations often exceed the EPA's 0.01-mg/L drinking water standard. In overburden groundwater at a site within this belt in north-central Massachusetts, As has been reported at levels up to 7.6 mg/L. Bedrock at the site consists of Silurian Central Maine Terrane metasediments intruded by the Devonian Ayer granodiorite and Chelmsford granite. Exchange of hydrothermal fluids between these lithologies during intrusion and later deformation, faulting, and metamorphism resulted in crystallization of arsenic-bearing minerals, including arsenopyrite. Quaternary deglaciation and unloading dilated joint systems in the bedrock, allowing increased exposure of the mineralogy to meteoric water. Several arsenopyrite alteration products (e.g., scorodite), of varying solubilities, precipitated on fracture surfaces and along grain boundaries between major phases. In the emerging conceptual model for this site, groundwater is recharged in bedrock uplands and moves downgradient through the fracture network, becoming increasingly reducing as it moves along a flow path. Arsenic dissolved from arsenopyrite and arsenic-bearing alteration phases in bedrock remains in solution until the groundwater discharges to lowland areas hydraulically downgradient. In these adjacent lowlands, glacial sand and gravel overburden lies above the bedrock. When the reducing water reaches more oxidizing conditions, As-sorbing hydrous ferric oxides (HFO) precipitate out on the aquifer solids, resulting in accumulation of As in the deep overburden aquifer. A large landfill at this site, now closed and capped, imposed reducing conditions, and As is mobilized into groundwater by reductive dissolution of the HFO. The presence of elevated As in groundwater is consistent with arsenic-bearing phases generated in granitoids at depth during regional metamorphism, which were subsequently altered, and are being solubilized at present by the circulation of shallow groundwater through varying redox environments. This scenario is supported by geochemical and petrographic studies of the granitoids and the occurrence of the highest groundwater and soil arsenic concentrations in the adjacent deep overburden. %B Abstracts with Programs - Geological Society of America %I Geological Society of America (GSA) : Boulder, CO, United States %C United States %V 42 %P 216 - 217 %8 2010/11/01/ %@ 00167592 %G eng %U https://gsa.confex.com/gsa/2010AM/finalprogram/abstract_182430.htm %N 55 %! Abstracts with Programs - Geological Society of America %0 Conference Proceedings %B Abstracts with Programs - Geological Society of America %D 2010 %T Evidence for arsenic-mineralization in granitic basement rocks, Ayer Granodiorite, northeastern Massachusetts %A Koteas, G. Christopher %A Keskula, Anna J. %A Stein, Carol L. %A McTigue, David F. %A Joseph P Kopera %A Brandon, William C. %K #StaffPubs %K acadian %K arsenic %K arsenides %K arsenopyrite %K Ayer Granodiorite %K Berwick formation %K fractured materials %K geochemistry %K granodiorites %K Igneous and metamorphic petrology 05A %K igneous rocks %K lower Paleozoic %K massachusetts %K Merrimack Synclinorium %K metals %K metamorphic rocks %K metamorphism %K metasedimentary rocks %K metasomatism %K Middlesex County Massachusetts %K migration of elements %K mineralization %K Mineralogy of non-silicates 01C %K northeastern Massachusetts %K orogeny %K Paleozoic %K plutonic rocks %K pollutants %K pollution %K pyrite %K sulfides %K United States %X Core samples of the Ayer Granodiorite along the eastern margin of the Merrimack Belt in northeastern Massachusetts host a series of sulfide and oxide phases that resulted from interaction with sulfide-bearing meta-sedimentary host rocks. Euhedral arsenopyrite grains are found with ilmenite, apatite, and REE phosphates in zones that generally mimic the intersection between a gneissic fabric and a relict magmatic foliation. Arsenopyrite crystals are typically elongate with this lineation. Euhedral to subhedral pyrite crystals have also been observed, but are localized to areas without As-bearing phases. Micro-fractures that parallel either a steep NW-striking joint set or gently-dipping sheeting joints are commonly filled with interwoven calcite cements and As-bearing Fe-oxides. Surface coatings of major fracture sets are also characterized by Fe-As-rich rinds that host micron-scale sub-angular particles of quartz, feldspars, and phyllosilicates. Where micro-fractures are most concentrated, sulfide-bearing minerals are less common; however, subhedral to anhedral arsenopyrite grains do occur along some open micro-fractures. These crystals preserve lobate grain boundaries and are associated with As-bearing Fe-oxide-rich coatings along adjacent fractures. The presence of 1) pyrite, 2) arsenopyrite associated with phosphates, and 3) As-bearing fracture coatings suggests multiple stages of mineralization. We propose that intrusion-related fluid-rock interaction associated with heating of nearby sulfide-bearing schists of the Berwick Formation during Acadian orogenesis may have provided the necessary constituents for growth of sulfide phases in the Ayer. It appears that Late Devonian greenschist facies metamorphism and metasomatism led to mineralization that generated arsenopyrite and accompanying phosphates; however, the role of the cross-cutting Clinton Newbury Fault Zone as a conduit for hydrothermal fluids may also be important. Lower temperature As-bearing Fe-oxide and calcite coatings on open fractures surfaces may be associated with a change from lithostatic- to hydrostatic-pressures during post-glacial regional uplift. This mineralization appears to be synchronous with intense microfracturing that post-dates all other mineralization. %B Abstracts with Programs - Geological Society of America %I Geological Society of America (GSA) : Boulder, CO, United States %C United States %V 42 %P 160 - 160 %8 2010/03/01/ %@ 00167592 %G eng %U https://gsa.confex.com/gsa/2010NE/finalprogram/abstract_169998.htm %N 11 %! Abstracts with Programs - Geological Society of America