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38 <br />. ore body. In the Georgetown-Silver Plume District, the ore <br />primarily occurs in veins stringing along the fractures in <br />the granite host rock and the contact of the water with veins <br />minerals is much less efficient. <br />The second parameter to be considered is the height <br />of the water table in relation to the ore occurrence. The <br />location of the ground water table in relation to the ore <br />body is important because the dissolution of pyrite is an <br />oxidation reaction. <br />2FeS2 + 702 + 2H20 - 2Fe2+ + 4SO4 + 4H+ <br />Oxygen is a necessary reactant, even when the reaction <br />is catalyzed by bacteria. If the ore occurs below the water <br />table, contact with air is cut off and oxidation occurs at <br />a much lower rate. If the ore occurs above the water table, <br />• dissolution of the vein minerals is promoted. The secondary <br />alteration of ore bodies which at some time have risen above <br />the water table is caused by the same chemical situation. <br />The Argo Tunnel was built to drain the Central City <br />District and it has been quite efficient. No natural springs <br />occur in the district and the water table is about at the <br />level of the Argo Tunnel with much of the disseminated ore <br />occurring above the water table. In other mining districts <br />in the Front Range, drainage tunnels are not as extensive <br />and springs can typically be found nearby. This implies <br />that the water table is higher in relation to the ore and <br />oxidation of the vein minerals is less extensive. <br />Certainly many mines are operated above the ground <br />•