UC Riverside

In situ mining of seafloor massive sulfide (SMS) deposits will have consequences thus far not quantified. On land, interaction of mined sulfide minerals with surface and groundwaters yields acid mine drainage. Pulverization of SMS on the ocean floors will produce highly reactive sulfide mineral surface areas, leading to the localized potential for seafloor acid generation. Chalcopyrite (CuFeS₂) is one of several ore minerals found in SMS deposits whose oxidation kinetics need to be quantified to estimate the significance of acid production.

To constrain the oxidation rate of chalcopyrite in seawater, the initial rate experimental method was employed and combined with the isolation method to derive a rate law. Data collected from batch reactor experiments without abundant precipitates (pH <4.5), between 7°C and 25°C, and P_O2 from 0.10 to 0.995 atm were incorporated into the rate law. The molal specific rate law is:

Rsp = - 10^-9.38(P_O2)^1.22(H+)^0.36

Chalcopyrite oxidizes slowly in seawater relative to other sulfide minerals like pyrrhotite (Fe_1-xS), so data from this study establishes a minimum rate of abiotic SMS weathering by oxidation. The slow rate of oxidation of chalcopyrite observed here has positive implications for seafloor mining. Not only will this sulfide not be the main culprit for acid production, but the copper ore will arrive at the surface with minimal dissolution and loss of metal value. Constraining the oxidation rates of individual sulfide mineral species will be useful in modeling SMS mining repercussions, as well as quantifying rates of natural chemical weathering in the oceans over geologic time. This information will be applicable to interpreting the Cu/Fe ratios of VMS deposits.

The potential for local acid generation can be viewed as a microcosm of the global problem of ocean acidification caused by dissolution of anthropogenic atmospheric CO₂. Data show sulfide mineral oxidation rates increase with lower pH, implying that a worldwide drop in ocean pH may amplify the dissolution of SMS deposits, changing the marine ecosystem.