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MEMORANDUM 4& <br /> AdrianBrown <br /> 1. Oxidation by atmospheric oxygen. When atmospheric oxygen contacts pyrite it causes oxidation, <br /> converting the sulfide to sulfuric acid, and releasing soluble iron (this is the process which produces acid <br /> rock drainage -ARD).The products of oxidation are generally removed by dissolution in vertically <br /> infiltrating precipitation, and ultimately pass into the saturated groundwater system and flow to exit the <br /> diatreme either at the natural overflow points(pre-mining) or via the drainage tunnels (during-and <br /> post-mining). Sulfur removal by this mechanism is almost entirely restricted to locations above the <br /> water table, as there is essentially no access of atmospheric oxygen to the sulfides below the water <br /> table. In geological time there is introduction of dissolved oxygen and possibly dissolved ferric ions in <br /> the infiltrating water, but these oxidant fluxes are very small due to the low aqueous concentrations of <br /> both. <br /> 2. Leaching by infiltrating precipitation. Soluble sulfur species (particularly gypsum and anhydrite)can be <br /> directly removed from the locations above the water table due to leaching by infiltrating precipitation <br /> water.The infiltrating water dissolves the sulfates and rapidly reaches chemical saturation with respect <br /> to sulfate.This sulfate-saturated water moves down through the bedrock above the water table, and <br /> enters the(water)saturated bedrock. Because the seepage is by then sulfate-saturated, it dissolves no <br /> further gypsum or anhydrite below the water table. <br /> Accordingly, sulfur is removed in geologic time predominantly above the water table, and so it is reasonable to <br /> expect that sulfur content of the rock will be lower above the water table,which can be used as a marker for the <br /> presence of the water table during geologic time. <br /> The sulfur concentration in the diatremal rocks has been computed as a function of elevation, and the results <br /> are presented in the left-hand panel in Plate 10.As can be seen,the average sulfur concentration increases <br /> slowly from approximately 0.5%at ground surface to about 1.5%at about elevation 9,700 ft amsl,which is <br /> interpreted as being the highest elevation of the geologic water table (for locations where data is available). It <br /> then declines slightly through elevation 9,650 ft amsl, before increasing rapidly to more than 2%by elevation <br /> 9,400,which is interpreted as being the lowest elevation of the geologic water table. <br /> Based on sulfur,the natural water table in the diatreme is interpreted to be between elevation 9,400 ft amsl and <br /> 9,700 ft amsl, depending on location within the diatreme. <br /> Carbon <br /> Carbon is present in the Cripple Creek diatreme rockmass as calcite (calcium carbonate)and,to a lesser extent, <br /> dolomite (calcium magnesium carbonate).These carbonates are subject to removal from the rock column above <br /> the water table by two processes: <br /> 1. Neutralization reaction with sulfuric acid generated by sulfide oxidation above the water table.The ARD <br /> produced by natural oxidation of the sulfides present above the water table contains sulfuric acid, which <br /> is neutralized by reaction with calcite and dolomite.This reaction converts the carbonate into carbon <br /> dioxide,which either exits as a gas upward through the rock, or is dissolved and exits by dissolution in <br /> Page 4 <br /> Adrian Brown Consultants, Inc. <br /> 130 W.4th Ave., Denver CO 80223 <br /> 303-698-9080 www.abch2o.com <br />