Acid-neutralizing potential of minerals in intrusive rocks of the Boulder batholith in northern Jefferson County, Montana

Experimental studies show that fresh granitic rocks of the Boulder batholith in the
Boulder River headwaters near Basin, Montana have significant acid-neutralizing
potential and are capable of neutralizing acidic water derived from metal-mining related
wastes or mine workings. Laboratory studies show that in addition to the acidneutralizing
potential (ANP) of minor amounts of calcite in these rocks, biotite, tremolite,
and feldspars will contribute significantly to long-term ANP.
We produced 0.45 micrometer-filtered acidic (pH = 2.95) leachate for use in these
ANP experiments by exposing metal-mining related wastes to deionized water in a
waste:leachate ratio of 1:20. We then exposed these leachates to finely-ground and sized
fractions of batholith rocks, and some of their mineral fractions for extended and repeated
periods, for which results are reported here. The intent was to understand what reactions
of metal-rich acidic water and fresh igneous rocks would produce.
The reactions between the acidic leachates and the bulk rocks and mineral fractions
are complex. Factors such as precipitation of phases like Fe-hydroxides and Alhydroxides
and the balance between dissolved cations and anions that are sulfate
dominated complicate analysis of the results. Research by others of acid neutralization by
biotite and tremolite attributed a rise in pH to proton (H+) adsorption in sites vacated by
K, Mg, and Ca. Destruction of the silicate framework and liberation of associated
structural hydroxyl ions may contribute to ANP. Studies by others have indicated that
the conversion of biotite to a vermiculite-type structure by removal of K at a pH of 4
consumes about six protons for every mole of biotite, but at a pH of 3 there is pronounced
dissolution of the tetrahedral lattice.
The ANP of fresh granitic rocks is much higher than anticipated. The three bulk
Boulder igneous rock samples studied have minimum ANP equivalent to about 10-14
weight percent calcite. This ANP is in addition to that provided by the 0.36-1.4 weight
percent calcite present in these samples. The total rock ANP is thus equivalent to that of
many sedimentary rocks that are generally believed to be among the most efficient for
attenuation of acidic waters.
The long-term ANP contributed by biotite, tremolite, feldspars, and possibly
unidentified minerals in these rocks, as well as calcite, are all important with regard to
their natural remediation of degraded water quality originating from Fe-sulfide rich
mineral deposits and the associated mine wastes and acid-mine drainage water.