WEATHERING OF SULFIDE ORES BY RAINWATER IN AN ARID ENVIRONMENT: AN EXAMPLE FROM THE MARISCAL MINE, BIG BEND NATIONAL PARK, TEXAS

Mariscal Mine in Big Bend National Park is situated near the northern end of Mariscal Mountain, which is a plunging anticline formed during Laramide compression.  Cinnabar was last mined in 1943 from deposits formed at the contact between altered volcanic rocks and limestone.  Several studies have focused on the potential of Hg release into the environment.  However, the ore deposit is also enriched in Fe, Cu, Pb, and Zn.  The sulfides of these elements are much more soluble in water (log K > -16) than cinnabar (log K = -39.5), and may pose an environmental hazard of their own.  This region of Texas gets most of its precipitation during the late Summer/early Autumn.  However, owing to high evaporation rates, transport of the dissolved load is limited by their eventual reprecipitation with the available ligands (e.g., sulfide, sulfate, carbonate).

Samples were collected of limestone, altered volcanics, calcine from tailings piles, and sediment from a major arroyo that drains the tailings pile.  Sediment samples were collected at regular intervals along the length of the arroyo.  The sediment was sieved into different grain size fractions and analyzed for major and trace elements by ICP-OES.  Leaching experiments were conducted in tubular reaction vessels, into which limestone, volcanics, and a thin layer of mixed sulfides (sphalerite, pyrite, chalcopyrite, galena, chalcocite, stibnite) were packed, in the same stratigraphic arrangement as observed at the mine.  Simulated rainwater (carbonic acid with pH=5.7) was dripped through the column, and the leachate was filtered and collected, and analyzed by ICP-OES.

Arroyo sediments adjacent to the calcine pile contain high abundances of Zn (300-400 ppm), but less than 50 ppm Cu.  Further downstream, sediments become enriched in Cu (up to 220 ppm), especially in the fine fraction, Zn remains high, but Fe gradually decreases.  Leaching experiments resulted in increasing concentrations of Fe, Zn, and Cu with increasing water to rock ratios.  The low abundances of Fe in arroyo sediments might reflect its early removal from solution by soluble CO₃^2-, in agreement with theoretical calculations.  These results show that weathering of sulfide ores, even in arid environments, can lead to the dispersal of metals, albeit limited in areal extent.