Geochemical evolution of flooding mine waters in a zoned, sulfide-hosted ore deposit, Summit Valley mining district, Butte, Montana

The chemistry of ground water is a function of several variables including the composition of the recharge water, the hydrogeologic properties of the aquifer, and the mineralogic composition of the aquifer material. The chemistry of water depends, in part, on the history of that water; each component of the hydrologic cycle imparts a change, permanent or temporary, on the chemistry and is carried with the water as it moves through the hydrologic cycle. This study evaluates the evolution of water chemistry through a zoned sulfide deposit. Atmospheric oxygen and carbon dioxide provide the source and sink for near-surface waters and mineral assemblages for various components of the sulfide deposit. Each of the four groups of minerals (sulfides, alteration minerals, secondary minerals, and carbonate minerals) interact with water with varying degrees as influence by other processes such as oxidation/reduction, buffering, dissolution, and precipitation. While it is possible to determine the concentration and nature of complexes through analytical techniques, the large number of complex species makes it impractical, particularly at low concentrations. Geochemical modeling provides a means of accounting for the dominant species possible for most solutions.; This investigation considered the influence of local mineralogy, mine geometry, and the mine-flooding water-balance to evaluate the variation in water chemistry within the mines of the Summit Valley mining district in Butte, Montana. The mines in this are have been active at various times over the past 130 years; underground mining ceased in 1982 and ground water was allowed to flood the workings; water levels in the workings are still rising at present. Each mine, and in some cases, each set of workings within the zoned porphyry-copper orebody has unique mineralogy. Mineralogic information was examined to construct a general model of mineral assemblages for each mine. Detailed maps of the underground workings were used to determine mine volumes, connections between mines, and to give weight to the potential influence of minerals from difference parts of the mine. The mine geometry and its position in the flooding system were used to identify potential sources and the likely chemistry of water entering each mine. Mines that represented different positions within the ore body and for which sufficient information could be obtained where selected to construct the models.; The results of the geochemical modeling were used to determine flow paths during the early and late period of flooding as well as identify sources of water entering the underground workings. The early period of flooding was marked by a considerable volume of surface water entering the workings via a nearby open pit. The source of this water, an acid leaching operation, was identified through modeling the mass balance of iron and copper in the receiving waters. The evolution of water chemistry became more stable and predictable after the initial effects of this surface water and flow paths were identified through geochemical modeling.