A. H. Hofstra
P. Emsbo
2003
<p><span>The final event in a complicated hydrothermal history </span>at<span> the </span>Meikle<span> </span>gold<span> </span>deposit<span> was </span>gold<span> deficient but caused extensive </span>postore<span> </span>dissolution<span> of carbonate, </span>collapse<span> brecciation, and precipitation of </span>calcite<span> and </span>barite<span> crystals in the resulting cavities. Although previously interpreted to be part of the </span>Carlin<span>-type hydrothermal system, crosscutting relationships and U-Th-Pb geochronology constrain this hydrothermal event to late Pliocene time (ca. 2 Ma), nearly 36 Ma after ore formation. Mineralogic, fluid inclusion, and stable isotope data indicate that </span>postore<span> hydrothermal fluids were reduced, H </span><sub>2</sub><span>S-rich, unevolved meteoric waters ((δ </span><sup>18</sup><span>O = -17‰) of low temperature (ca. 65°C). The δ </span><sup>18</sup><span>O values of </span>barite<span> and </span>calcite<span> indicate that these minerals were in isotopic equilibrium, requiring that </span>barite<span> SO </span><sub>4</sub><span> was derived from the oxidation of reduced sulfur; however, preexisting sulfides in </span>breccia<span> cavities were not oxidized. The δ </span><sup>34</sup><span>S (15‰) values of </span>barite<span> are higher than those of local bulk sulfide and supergene alunite indicating that SO </span><sub>4</sub><span> was not derived from supergene oxidation of local sulfide minerals. The 15 per mil δ </span><sup>34</sup><span>S value suggests that the H </span><sub>2</sub><span>S in the fluids may have been leached from sulfur-rich organic matter in the local carbonaceous sedimentary rocks. A reduced H </span><sub>2</sub><span>S-rich fluid is also supported by the bright cathodoluminescence of </span>calcite<span> which indicates that it is Mn rich and Fe poor. </span>Calcite<span> has a narrow range of δ </span><sup>13</sup><span>C values (0.3-1.8‰) that are indistinguishable from those of the host Bootstrap limestone, indicating that CO </span><sub>2</sub><span> in the fluid was from </span>dissolution<span> of the local limestone. These data suggest that </span>dissolution<span> and brecciation of the Bootstrap limestone occurred where H </span><sub>2</sub><span>S-rich fluids encountered more oxidizing fluids and formed sulfuric acid (H </span><sub>2</sub><span>SO </span><sub>4</sub><span>). Intense fracturing in the mine area by previous structural and hydrothermal events probably provided conduits for the descent of oxidized surface water which mixed with the underlying H </span><sub>2</sub><span>S-rich waters to form the dissolving acid. The surface-derived fluid apparently contained sufficient oxygen to produce H </span><sub>2</sub><span>SO </span><sub>4</sub><span> from H </span><sub>2</sub><span>S but not enough to alter pyrite to Fe oxide. Although H </span><sub>2</sub><span>S is an important </span>gold<span>-transporting ligand, the temperature was too low to transport a significant amount of </span>gold<span>. The presence of analogous </span>calcite<span>- and </span>barite<span>-lined cavities in other </span>Carlin<span>-type deposits suggests that the generation (and oxidation) of H </span><sub>2</sub><span>S-rich meteoric waters was a common phenomenon in north-central </span>Nevada<span>. Previous sulfur isotope studies have also shown that the Paleozoic sedimentary rocks were the principal source of H </span><sub>2</sub><span>S in Devonian sedimentary exhalative-type, Jurassic intrusion-related, Eocene </span>Carlin<span>-type, and Miocene low-sulfidation </span>gold<span> deposits in the region. The similar sulfur source in all of these systems suggests that basin brines, magmatic fluids, and meteoric waters all evolved to be H </span><sub>2</sub><span>S-rich ore fluids by circulation through Paleozoic sedimentary rocks. Thus, although not directly related to </span>gold<span> mineralization, the recent hydrologic history of the </span>deposit<span> provides important clues to earlier ore-forming processes that were responsible for </span>gold<span> mineralization.</span></p>
application/pdf
10.2113/gsecongeo.98.6.1243
en
Society of Economic Geologists
Origin and significance of postore dissolution collapse breccias cemented with calcite and barite at the Meikle gold deposit, Northern Carlin trend, Nevada
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