GEOCHEMICAL INVESTIGATIONS OF THE YELLOW PINE, IDAHO AND REPUBLIC, WASHINGTON MINING DISTRICTS

Two Eocene epithermal ore deposits of contrasting style located in the northwestern United States are demonstrated to have a geothermally driven meteoric water source. In both districts the early higher temperature mineralization precipitated from a fluid containing relatively heavy oxygen isotopic values indicating a large exchange with the felsic igneous wallrock. These fluid oxygen isotopic values became increasingly lighter with decreasing fluid temperature and mineral paragenesis until values very close to Eocene meteoric waters were achieved. The Yellow Pine District contains a wallrock alteration sequence of Na-metasomatism, K-metasomatism (Au), Early sericitization (Au), Silicification (Sb), and Later sericitization (Hg). The earlier gold mineralizing fluid varied from +400(DEGREES) to 180(DEGREES)C with a decreasing (delta)O(\u2718) of +3 to -7(\u27o)/oo. The later antimony-silver mineralization, which overlaps with the tungsten mineralization, had a 180(DEGREES) to 50(DEGREES)C formation temperature and precipitated from a fluid with a decreasing (delta)O(\u2718) from -7 to -14(\u27o)/oo. The altered quartz monzonite wallrocks within the district display O(\u2718) values which increase from 6-10(\u27o)/oo for Na, K metasomatism to 11-17(\u27o)/oo for final silicification and sericitization. The As content in pyrite decreases from 9-4 wt% for early Au mineralization to 4-1.5 wt% with later Au mineralization finally to 1.5-0 wt% for W, Sb-Ag mineralization. The water/rock ratio of the hydrothermal fluid based upon oxygen isotope exchange was near zero. K-Ar age determinations of 57 m.y. indicate the mineralization occurred at least 10 m.y. before eruption of the Challis Volcanics. This age and the low water/rock ratio indicate the mineralization in the Yellow Pine District was earlier and not related to the Thunder Mountain Caldera. From the oxygen isotopic composition of calcite-quartz pairs the Republic District had fluid temperatures from 480(DEGREES) to 190(DEGREES)C. At the Knob Hill Mine ore formation occurred between 480(DEGREES) and 360(DEGREES)C. Similarly textured but non-ore specimens had temperatures from 360(DEGREES) to 340(DEGREES)C. Later non-ore specimens exhibited temperatures of 340(DEGREES) to 240(DEGREES)C. Samples with isotopic temperatures of 340(DEGREES)C had average fluid inclusion homogenization temperatures of 328(DEGREES) and 320(DEGREES)C. Fluid salinities at these temperatures averaged 0.3 wt% equivalent NaCl. The (delta)O(\u2718) value of the hydrothermal fluid decreased linearly with temperature from -0.5 to -8.5(\u27o)/oo. Rhyodacite wallrock displays a pervasive intermediate argillic alteration with an average (delta)O(\u2718) whole rock value of -0.5(\u27o)/oo. The calculated water/rock ratio of the hydrothermal fluid was near 0.5