Geology and geochronology of the Tiegelongnan porphyry-epithermal Cu (Au) deposit, Tibet, China: Formation, exhumation and preservation history

Abstract

The Tiegelongnan (Rongna) deposit is a giant porphyry-epithermal Cu (Au) deposit within the Duolong porphyry district, in the south Qiangtang terrane, central-western Tibet, China. The deposit is centered on multiphase granodiorite porphyry (GP) intrusions intruded in Jurassic sandstone. Phase 1 and 2 GP are associated with biotite and sericite alteration, whereas GP3 and 4 occur in middle and shallow parts of the deposit affected by sericite and advanced argillic alteration (alunite-kaolinite-dickite-pyrophyllite). Sericite alteration is pervasive in the deep part of the deposit replacing biotite alteration and hosting porphyry stage chalcopyrite-bornite-pyrite ± molybdenite mineralization. Advanced argillic alterationis associated with epithermal stage high sulfidation (tennantite, enargite, bornite, and digenite) mineralization, overprints the sericite and biotite alteration from the shallow part of the deposit. The top of the Tiegelongnan hydrothermal system was weathered with a layer of supergene oxidation and enrichment zone, exhumated, and covered by andesite and gravel.Muscovite sampled from deep sericite alteration yields a 40Ar-39Ar age of 120.9 ± 0.8 Ma, which is consistent with previously reported hydrothermal biotite 40Ar-39Ar age of 121.1 ± 0.6 Ma, molybdenite Re-Os ages of 121.2 ± 0.6 and 119.0 ± 1.4 Ma, and GP 1 and 2 zircon U-Pb ages of 121.5 ± 1.5, 120.2 ± 1.0 Ma (LA-ICP-MS results), and 118.7 ± 0.9 Ma (SIMS result). Phase 3 GP 3 yields a CA-ID-TIMS zircon U-Pb age of 119.9 ± 0.2 Ma, which constrains the age of porphyry magmatic-hydrothermal events. The porphyry system was then unroofed, weathered, and superimposed by discrete stages of advanced argillic alteration and high sulfidation mineralization. Two pulses of alunite events yield 40Ar-39Ar ages of 116.3 ± 0.8 Ma and 111.7 ± 1.0 Ma, which represent the principal epithermal alteration and mineralization ages at Tiegelongnan. The first alunite pulse age is consistent with younger porphyry (GP4) age of 116.2 ± 0.4 Ma (zircon LA-ICP-MS age). Weathering and exhumation of the porphyry-epithermal deposit were continuing until the cover of andesite at ~110 Ma and by post-andesite gravel. The andesite was affected by a younger weak hydrothermal alteration at 108.7 ± 0.7 Ma, constrained by a muscovite 40Ar-39Ar age. Another barren alunite yields a 40Ar-39Ar age of 100.6 ± 2.0 Ma, which may represent the youngest hydrothermal event. The long life-span discrete hydrothermal activities from ~120 to ~100 Ma in the Tiegelongnan deposit is consistent with the long-lasting tectonic-magmatic events of the Bangong-Nujiang suture zone.The discrete epithermal mineralization events at Tiegelongan are younger than the porphyry mineralization events, which is analogous to several other epithermal deposits worldwide. The ~10 m.y exhumation history at Tiegelongan is slower than the typical and rapid erosion history of ~1–2 m.y porphyry deposits in low-altitude tropical climates regions, and during a specific abrupt uplift pulse in the Andean region. The slow exhumation at Tiegelongnan is interpreted to be an integrated result of an arid climate environment, relatively flat topography, and a slow uplift condition during Cretaceous in central-western Tibet.