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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>R. O. Rye</dc:contributor>
  <dc:creator>Shane W. Ebert</dc:creator>
  <dc:date>1997</dc:date>
  <dc:description>&lt;p&gt;&lt;span&gt;The Crofoot-Lewis deposit is an adularia-sericite-type (low-sulfidation) epithermal Au-Ag deposit, whose well-preserved paleosurface includes abundant opaline sinters, widespread and intense silicification, bedded hydrothermal eruption breccias, and a large zone of acid sulfate alteration. Radiogenic isotope ages indicate that the system was relatively long-lived, with hydrothermal activity starting around 4 Ma and extending, at least intermittently, for the next 3 m.y.Field evidence indicates that the surficial zone of acid sulfate alteration formed in a steam-heated environment within an active geothermal system. A drop in the water table enabled descending acid sulfate waters to leach Au and Ag from zones of low-grade disseminated mineralization, resulting in the redistribution and concentration of Au and Ag into ore-grade concentrations. These zones of secondary Au-Ag enrichment are associated with opal + alunite + kaolinite + montmorillonite + or - hematite and were deposited in open space fractures at, and within a few tens of meters below, the paleowater table.The stable isotope systematics of alunite and kaolinite in the steam-heated environment are relatively complex, due to variations in the residence time of aqueous SO&amp;nbsp;&lt;/span&gt;&lt;sub&gt;4&lt;/sub&gt;&lt;span&gt;&amp;nbsp;that formed from the oxidation of H&amp;nbsp;&lt;/span&gt;&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;&amp;nbsp;S prior to precipitation of alunite, and the susceptibility of fine-grained kaolinites to hydrogen isotope exchange with later waters. Most of the alunites are enriched in&amp;nbsp;&lt;/span&gt;&lt;sup&gt;34&lt;/sup&gt;&lt;span&gt;&amp;nbsp;S relative to early sulfide minerals, reflecting partial S isotope exchange between aqueous SO&amp;nbsp;&lt;/span&gt;&lt;sub&gt;4&lt;/sub&gt;&lt;span&gt;&amp;nbsp;and H&amp;nbsp;&lt;/span&gt;&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;&amp;nbsp;S. About half of the alunites give reasonable calculated delta&amp;nbsp;&lt;/span&gt;&lt;sup&gt;18&lt;/sup&gt;&lt;span&gt;&amp;nbsp;O (sub SO&amp;nbsp;&lt;/span&gt;&lt;sub&gt;4&lt;/sub&gt;&lt;span&gt;&amp;nbsp;-OH) temperatures for a steam-heated environment indicating O isotope equilibrium between aqueous SO&amp;nbsp;&lt;/span&gt;&lt;sub&gt;4&lt;/sub&gt;&lt;span&gt;&amp;nbsp;and water. The delta D (sub H&amp;nbsp;&lt;/span&gt;&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;&amp;nbsp;O) values of the hydrothermal fluids varied by almost 60 per rail over the life of the meteoric water-dominated system, suggesting significant climate changes.Mineralization is believed to have resulted from large-scale convection of meteoric water controlled largely by basin and range fractures and a high geothermal gradient with H&amp;nbsp;&lt;/span&gt;&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;&amp;nbsp;S for Au complexing derived from organic matter in basin sediments. A wet climate resulted in the formation of a large inland lake which provided abundant recharge water for the hydrothermal system. A fluctuating water table controlled by changing climatic conditions enabled steam-heated acid sulfate fluids to overprint lower grade mineralization resulting in ore-grade precious metal enrichment.&lt;/span&gt;&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.2113/gsecongeo.92.5.578</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Society of Economic Geologists</dc:publisher>
  <dc:title>Secondary precious metal enrichment by steam-heated fluids in the Crofoot-Lewis hot spring gold-silver deposit and relation to paleoclimate</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>