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<p>The Valeriano porphyry Cu-Au deposit at the northern end of the El Indio metallogenic belt of Chile is hosted in Miocene-age granodiorite intrusions, below an outcropping lithocap with advanced argillic alteration. At the base of the lithocap a zone of nodular pyrophyllite alteration is extensive over a 2 × 1&#xa0;km NW elongate zone. The nodular replacement occurs 600–1000&#xa0;m above the porphyry intrusions, and ~ 0–500&#xa0;m above the 0.3% Cu grade shell. The nodules consist of pyrophyllite, diaspore, dickite/kaolinite and locally alunite with pyrite that replace quartz and muscovite. Pyrophyllite (Si: Al ratio of 2:1) replacement of muscovite (1:1 ratio) during cooling requires silica, which causes quartz dissolution, as evidenced by cuspate and embayment replacement textures of quartz. Sulfur isotopic compositions of pyrite-alunite pairs (<i>n</i> = 18) from the nodules yield equilibrium temperatures of 375 –275&#xa0;°C, consistent with established muscovite-pyrophyllite coexistence at ~ 350 –280&#xa0;°C at quartz saturation. The zone of nodular alteration represents a paleo-isotherm over the causative intrusions and can be used as a vector for exploration of concealed porphyry Cu deposits elsewhere.</p>

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Nodular pyrophyllite replacement of muscovite–quartz reflects paleo-isotherms over the Valeriano porphyry intrusions and Cu-Au-Mo deposit, Chile

  • 期刊论文

  • 发表日期：

    2026年4月10日

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  • Jack Halloran,
  • Jeffrey W. Hedenquist,
  • Keiko Hattori,
  • Tomás Godoy

摘要

The Valeriano porphyry Cu-Au deposit at the northern end of the El Indio metallogenic belt of Chile is hosted in Miocene-age granodiorite intrusions, below an outcropping lithocap with advanced argillic alteration. At the base of the lithocap a zone of nodular pyrophyllite alteration is extensive over a 2 × 1 km NW elongate zone. The nodular replacement occurs 600–1000 m above the porphyry intrusions, and ~ 0–500 m above the 0.3% Cu grade shell. The nodules consist of pyrophyllite, diaspore, dickite/kaolinite and locally alunite with pyrite that replace quartz and muscovite. Pyrophyllite (Si: Al ratio of 2:1) replacement of muscovite (1:1 ratio) during cooling requires silica, which causes quartz dissolution, as evidenced by cuspate and embayment replacement textures of quartz. Sulfur isotopic compositions of pyrite-alunite pairs (n = 18) from the nodules yield equilibrium temperatures of 375 –275 °C, consistent with established muscovite-pyrophyllite coexistence at ~ 350 –280 °C at quartz saturation. The zone of nodular alteration represents a paleo-isotherm over the causative intrusions and can be used as a vector for exploration of concealed porphyry Cu deposits elsewhere.