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3.0 HYDROLOGY PRIOR TO MINING <br /> 3.1 Pre-Mining Ground-Water Hydrology <br /> 3.1.1 Geology Pertinent to Ground-Water Flow and Chemistry <br /> Rocks in the vicinity of the Sunnyside Mine are pyroclastics and flows which erupted from local <br /> calderas approximately 28 million years ago (Steven and Lipman, 1976). Local stratigraphy is <br /> summarized on Figure 2. The Sunnyside Mine is principally located within the Burns <br /> Formation, which generally consists of massive silica-rich latite flows which have locally been <br /> altered and mineralized. (Langston, pp. 34-39). The highest mine workings (above A level) <br /> extend into the overlying interbedded lava flows and air-fall tuffs of the Henson Formation, an <br /> alkali-rich andesite (Langston, p. 49). <br /> The Burns Formation was erupted from vents within the San Juan caldera (Steven and Lipman, <br /> 1976, p. 11-12). The degree of welding of the upper Burns formation generally increases <br /> towards the west in the direction of the vent source (Langston, 1978, p. 11). The Henson <br /> Formation also was derived from vents located within the San Juan caldera, but typically is less <br /> welded than the Burns Formation and contains more pyroclastic units. <br /> The extent of fracturing in volcanic rocks is directly related to the degree of welding if other <br /> factors are equal. Hence, the Burns Formation tends to be more fractured than the Henson <br /> Formation. The fact that many of the major veins in the Burns Formation pinch out <br /> approximately at the contact with the Henson (p. 68, Sunnyside Gold Corporation, 1985) may <br /> ssnjuan\sunny\110361\M91_Rpt 7 <br /> e�� simonHYRAO-HERUCH <br />