31-4 Investigation of Pb, Zn, and Cd Mineralogy and Mobility in Fluvial Sediments Impacted by Legacy Mining in a Mountain Watershed

Session: Environmental Geochemistry and Health (Posters)

Poster Booth No.: 30

Presenting Author:

Christopher Mills

Authors:

Mills, Christopher T¹, Morrison, Jean M², Walton-Day, Katie³, Kane, Tyler J⁴, Campbell, Kate M⁵, Chapin, Thomas P⁶, Croteau, Marie-Noële⁷

(1) U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, DENVER, CO, , (2) U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO, USA, (3) U.S. Geological Survey, Colorado Water Science Center, Denver, CO, USA, (4) U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO, USA, (5) U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO, USA, (6) U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO, USA, (7) U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Moffett Field, CA, USA,

Abstract:

Legacy mining impacts often include metal-bearing mine waste dispersed in fluvial sediments downstream of historical mining and milling sites, which can persist long after point-source remediation and affect ecosystem recovery. Understanding the extent, mineral hosts, and mobility of dispersed metals is needed to assess lingering impacts.              

We investigated Pb, Zn, and Cd concentrations, mineralogy, and mobility in mining impacted fluvial sediments along an approximately 4 km reach of Tomichi Creek downstream of the Akron Mine (Colorado, USA). The mine and mill were active from the 1880s to1950s (primary commodities Pb, Ag, Zn, Cu, and Au) and were remediated in 2016. We compared automated mineralogy (AM), a high-throughput scanning electron microscopy-based technique that characterizes sample mineralogy, with sequential extraction (SE) which assesses element mobility by targeting increasingly recalcitrant phases. Sample types included bed sediments, suspended sediments, and floodplain alluvium.

Pb, Zn, and Cd concentrations were consistently highest in the fine-grained fraction (<0.64 um) of both bed and suspended sediments. Fine-grained Cd and Zn concentrations were highly correlated (Cd/Zn=0.005±0.001) and increased twofold from immediately below the mine to downstream sites, where they ranged from 14-26 and 2500-4000 mg/kg, respectively. Sediments contained 0.05-0.9 weight % sphalerite (ZnS) contributing an estimated 18 to >100% of total Zn. Sequential extraction results showed that sulfide plus residual fractions contained 51±5% of total Zn. Both methods indicate substantial Zn and Cd hosted in phases more labile than sphalerite.

Fine-grained Pb concentrations were 1000-3500 mg/kg. Cerussite (PbCO₃) was more abundant (0.00-0.42 weight %) than galena (PbS; 0.00-0.12 weight %) and accounted for 4 to >100% of total Pb, though it did not correlate well with Pb released in the carbonate SE fraction. However, a phase of Pb associated with Fe was also identified by AM and its abundance correlated well with Pb released in the Fe/Mn oxide SE fraction. A weathering profile of mill concentrates in floodplain alluvium exhibited major variations in Pb, Zn, and Cd mineralogy and mobility with depth and may represent a continuing stream sediment source. The profile provided further opportunity to compare AM and SE methods and results will be presented. Our findings highlight the complementary strengths of AM and SE for evaluating dispersed mine-waste mineralogy and metal mobility in mining-impacted watersheds.

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