AMERICAS — Reclamation of California mercury deposits a

Previous mining of mercury deposits in the northern California Coast Range mercury belt has become the source of environmental geochemical concerns.

Occurrence of metals in the surrounding rock, soil, sediment, water and air has become a problem which is complicated by continued deposition of mercury from active gas vents and hot springs in the deposits.

One mine in the area, the inactive Sulphur Bank mine, once the fifth largest mercury producer in the U.S. (130,000 flasks), is now an Environmental Protection Agency Superfund site. The ore occurs in a young andesite flow (less than 44,000 years) and Cretaceous shales and graywacke. Reclamation efforts consist of keeping mine tailings and mine waters from entering nearby Clear Lake, a major recreational area.

However, the mitigation efforts are complicated by the ongoing deposition of mercury and associated elements from hot springs and gas vents in the open-pit site.

The pit is filled with acidic water hosting elevated concentrations of boron, ammonia, chloride and other elements derived from the venting of hot springs at the bottom of the lake.

Mercury vapor from gas vents is venting from open-pit walls. This vapor is in part fixed as cinnabar by native sulphur generated from bacterial reduction of sulphate; the remainder is discharged into the atmosphere. Other components of the vapor include carbon dioxide, methane, hydrogen sulphide and helium. Elevated levels of mercury in soils adjacent to the mined area reflect both naturally occurring mercury derived from the gas vents and mercury released during mining activity.

Furnacing was the primary method of mercury recovery, during which process mercury vapor was released into the atmosphere.

Meanwhile, in the Sulphur Creek mercury-gold district, located about 20 km northeast of the Sulphur Bank mine, tailings and excavations contribute metals to the drainage basin. However, the major amounts of metals are from precipitates that are derived from numerous hot springs in and adjacent to the mercury and gold deposits. The mercury-gold deposits consist of subhorizontal, high-grade veins (typically greater than 1 oz. per ton) hosted in Cretaceous adularized mudstone and serpentinite altered to silica-carbonate.

Hydrocarbons are common within the veins, and the highest gold contents occur in amber-to-black hydrocarbon-bearing quartz displaying froth, or spongy-type bubbly quartz vein textures.

Sediments and precipitates from the Sulphur Creek hot springs consist of a fine-grained black silt and clay composed of smectite, chalcedony, calcium-magnesian carbonate, pyrite, magnetite, rutile, cinnabar and gold. Anomalous concentrations of trace metals in the precipitate and sediment mud include gold (maximum value, 1.3 ppm), tungsten (50 ppm), cobalt (150 ppm), nickel (1,500 ppm), chromium (3,000 ppm) and titanium (3,000 ppm). Mercury content of the hot spring fluid is anomalously high — 10-20 ppb — and indicates that only part of the mercury is deposited in the subsurface and that mercury in solution directly enters waters in the drainage basin. Cinnabar grains up to 8 microns in diameter enter the drainage basin and are dispersed downstream. Other elements with elevated contents in the high chloride (2.9%), alkaline, hot-spring fluid include lithium (maximum content 16 ppm), titanium (600 ppb), bromine (210 ppm), antimony (100 ppb) and tellurium (25 ppb).

Extremely high concentrations of tungsten are present in the hot-spring fluids — 3,200-7,400 ppb — and are some of the highest known naturally occurring concentrations of tungsten in water.

In these alkaline fluids, tungsten is probably present as tungstate anion. Only part of the tungsten is deposited near the hot-spring vent. The remainder enters waters in the drainage basin, and elevated levels of tungsten — 260-680 ppb — occur in the stream water up to several kilometres from the hot-spring discharge site.

In the Sulphur Creek district, active hot springs contribute particulate mercury and solutes of mercury, tungsten and other metals to waters in the drainage basin, and previous mining activity contributes only a minor component. Environmental baseline studies that measure the effects of mining activity also need to address natural processes associated with ore formation. — By James Rytuba and William Miller and taken from a recent “Mineral Resources Newsletter” of the U.S. Geological Survey, Denver, Colo.