Bald Mountain study shows uses of baseline data

The public often reacts to proposed mining development as though it will degrade and pollute a pristine wilderness, but anyone capable of a little geochemical reflection knows that mineral deposits can do a perfectly good job of releasing metals into the environment on their own.

It follows that an environmental baseline study should be near the top of the things-to-do list for anyone trying to develop a mine. Comprehensive baseline data on the concentration of chemical species in the surrounding soils, surface water, groundwater and air may later turn out to be the only source of proof that a mining operation has handled its wastes responsibly.

One excellent example of the virtues of early baseline studies comes from the Bald Mountain massive sulphide deposit in Aroostook Cty. in northeastern Maine, where a study by the U.S. Geological Survey shows that naturally occurring levels of chemical species can be high around mineral deposits.

Development at Bald Mountain, currently held by Black Hawk Mining (BHK-T), is on hold; a prefeasibility study in 1999 concluded that initial development would only be economic at a gold price of US$350 or higher. Black Hawk has since taken a $4.5-million writedown on the carrying value of the project.

Four zones

Bald Mountain is a volcanic-hosted massive sulphide deposit where four zones of mineralization have been defined. Supergene (surface) weathering has left an upper leached cap where only the precious metals are left in paying quantities — just over a million tonnes grading 4.5 grams gold and 103 grams silver per tonne. Beneath that cap is a 2.2-million-tonne resource that the same supergene processes enriched in copper, bringing the grade up to 2.78%. The supergene copper zones are made up mainly of pyrite, chalcocite, covellite and enargite.

The primary mineralization below the gold and copper bodies is zoned too, with low-grade zinc mineralization to the south and a large primary copper body at depth. The primary massive-sulphide mineralization is chiefly pyrite and pyrrhotite, plus chalcopyrite, sphalerite and arsenopyrite.

The primary zinc deposit has 12 million tonnes of 2.3% zinc, and the primary copper zone, 20 million tonnes with an average grade of 1.6% copper.

Black Hawk bought the property in 1995 from Boliden (BOL-T), for US$2 million and a 2% net smelter return royalty. Boliden had spent about US$30 million drilling off the mineralization before deciding that Bald Mountain did not fit the company’s economic requirements.

Black Hawk had developed a plan to mine the leached cap for gold and silver, then (as long as economics were favourable) take the supergene copper mineralization for solvent extraction-electrowinning. It started the permitting process in 1996, and in 1997 the USGS began its baseline study on the area’s ground and surface waters, comparing groundwater in wells drilled into the deposit with surface water and with water from wells up the groundwater gradient from Bald Mountain.

Groundwater affected

The USGS researchers (geologists Robert Seal and David Haffner, mineralogist Nora Foley and geochemist Allen Meier) found that the deposit itself had significantly affected the groundwater system without any human interference. Iron, base metals, arsenic and sulphate all showed major increases near the mineralization.

Total base metal concentrations in the groundwater (defined as the sum of the concentrations of copper, zinc, nickel, cobalt, cadmium and lead) ranged from 2.5 to 152.5 micrograms per litre in monitoring wells in and around the deposit, but only 5.7 micrograms per litre up-gradient and 0.4 to 4.1 micrograms per litre in surface waters.

Similarly, arsenic — a major concern because of its toxicity and because the Bald Mountain hypogene zones carry arsenopyrite — was present in concentrations of 2 to 430 micrograms per litre near the deposit, but at much lower concentrations in surface water, and not detectable up-gradient.

The researchers’ work on the solution-chemistry of the system showed that groundwater was largely in equilibrium with the mineral assemblage — silica and iron oxides and hydroxides — left in the leached cap zone. But it also showed that the waters could still potentially dissolve away more metal from heavy-metal mineral species.

While Bald Mountain has yet to have any impact on the metal markets, there can be little doubt it has been having a definite impact on the environment ever since the retreat of the last ice sheets.