INVESTIGATING ARSENIC MINERALOGY IN HISTORIC MINE WASTE FROM THE IDAHO-MARYLAND MINE (GRASS VALLEY, CALIFORNIA) USING SEM-EDS

We are investigating the arsenic (As) mineralogy in mine tailings from the Idaho-Maryland Mine (Grass Valley District), the second largest historic underground lode gold mine in California. We are analyzing 15 petrographic thin sections composed of mixtures of tailings, naturally derived sediment, and organic matter sampled from the mine’s storage site (sieved to <250 µm). We are using several micro-analytical techniques (Scanning Electron Microscopy–Energy Dispersive Spectroscopy (SEM-EDS), microRaman, and Electron Microprobe Analysis (EMPA)) to characterize sample mineralogy and As mineral residence. The purpose of this work is to improve understanding of the relationship between As bioaccessibility and As mineralogy, and to evaluate human health risks from ingestion. Results from SEM-EDS analyses are reported here.

A TeScan© VEGA-3 SEM (25 kV, beam intensity 15) with 2 Oxford© EDS detectors was used to image and chemically map large areas (<134 mm² @ 200x mag). We analyzed single-element maps, phase maps, and spectra collected from individual As-bearing grains.

Our analysis identified ~10 unique As-bearing phases. Two commonly occurring, high-As (>20 to ~44 wt% As) phases had low modal abundance: arsenopyrite (FeAsS; 47% of samples) and As-Fe oxide (20% of samples)—these phases never co-occur in any samples. FeAsS grains always had micrometers-thick weathering rinds containing As. Phases with low levels of As (<1 wt%) were most common (60% of samples, representing >99% of As-bearing pixels); Fe co-occurred in all As-bearing phases. SEM-EDS did not detect As in pyrite (ideal formula: FeS₂; detection limit of ~0.3 wt%), although pyrite can contain several wt% As (Foster & Kim, 2014. Rev. Min. Geochem. v. 79. pp. 257-369).

Arsenic bioaccessibility measurements can be influenced by rare, highly bioaccessible phases and/or common, low bioaccessibility phases. Our analytical approach allows us to evaluate arsenic mineral residence on a sample by sample basis. We will complement these analyses with microRaman and EMPA data and compare with As bioaccessibility measurements from the same samples. SEM-EDS automated mapping can be a valuable tool to characterize the mineral residence of trace elements of concern, and to improve understanding of human-health risk based on mineralogy.