Eocene Metallogeny In Eastwestern Changning-Menglian Tectonic Belt-Insights From Laochang And Nanla Deposits

Relatively isolated Laochang concealed porphyry Cu-Mo deposits in Lancang and the Nanla granite-hydrothermal Pb-Zn deposits cluster in Cangyuan was linked by geology,geochemistry,geochronology and mineral in-situ elements and isotope analysis techniques.This thesis present detailed process of magma origin,evolution,emplacement and mineralization of two deposits,The ages of Laochang,Nanla Yongriwai and Menglinshan granite porphyry are42.6±1.5Ma⁵4.3±2.0Ma,46.4±0.2 Ma and 47.8±0.2 Ma,respectively.The?Hf?t?value of magmatic zircons?⁴5Ma?in Laochang porphyry are widely distributed between-13^-20 and-25^-30,show interfusion of massive upper crust material.Captured zircon?245²30Ma??Hf?t?value concentrate between+6⁰ and-20^-25,consistent with the volcanic-sedimentary rock,arc granite and sync-collision S-type granite which were all formed during the evolution of Changning-Menglian ocean from middle Triassic to late Triassic,indicating that the Eocene I-type magma was fully interacted with ophiolite suite when emplacing.The?Hf?t?value of zircon?⁴5Ma?in Nanla concentrate between-5^-10 with minor mixture of upper crust material.Elements eochemistry indicted adakite-like highly differentiated I-type granites.Plagioclase and amphibole cumulates residue,corrosion surface of quartz,jagged erosion line between oscillatory zoning of feldspar.Crystallization temperature?Laochang,900?700°C;Nanla,700?650°C?of zircon and oxygen fugacity?Laochang,f?O₂?-5?-15;Nanla,f?O₂?¹2?indicated that the magma may undergone multi-stages during process,and evolution process in Laochang are more complex than Nanla.An content of plagioclase and melt temperature decreasing gradually,biotite converted from magmatic to epithermal?Laochang,680?500°C;Nanla,750?650°C?,Fe²⁺ or Mg²⁺ in melanocratic minerals reduced,oxygen fugacity decreasing due to Fe²⁺oxidizing and temperature declining.Most of lead,zinc,copper and molybdenum elements entered fluid phase,some ionic molybdenum in Laochang porphyry entered into feldspathic facies.In the early stage of evolution,metallogenic fluid show relativaly neutral pH while temperature,pressure and oxygen fugacity decreasing.Metallogenic fluid in Laochang precipitated pyrrhotite,scheelite and few molybdenite.In contrast,mineralizing fluid in Nanla precipitated pyrrhotite and pyrite.In mid-stage o,two deposits show large differences in physicochemical environment of metallogenic fluid.Metallogenic fluid of Lancang evoled into skarn-stage with continuously decreasing temperature,oxygen fugacity and pH value;chalcopyrite and molybdenite starting precipitated.Metallogenic fluid of Nanla invaded in Nanting-River Faults,and a favorable environment were created for extractting Pb⁺and Zn⁺from proterozoic metamorphic and sedimentary rocks,boiling phenomenon caused by sharp pressure declining,and oxygen fugacity falling back after intensely increasing.In later period,mineralizing fluid in Laochang blend with CO₂ and K⁺of surrounding rocks,cause pH value decreasing,plagioclase potassic alteration,micro-grained molybdenite releasing,at same time,oxygen fugacity was restrained by CH₄ of contained in basic surrounding rock,and the released Fe²⁺from biotite alteration.With pH increasing and oxygen fugacity declining in Nanla deposit,metal continued to be precipitated,and with the precipitation of calcite and fluorite,metallization completed.Eocene magma in Laochang,contaminated by reducing substances from ophiolite suite,formed“pseudo-reductive”low-grade molybdenum deposit.Eocene magma in Nanla,intruded into Proterozoic base rock which is rich in lead and zinc,metallogenic potential as Cu-Mo deposits was suppressed.The Cenozoic collisional orogeny intensively deformed the Changning-Menglian region,contrariwise corresponding magmatic activities are constrained by proto-,paleo to meso-Tethyan tectonic framework.Superimposing metallization reformed magma components,metallogenetic specialization and metallogenic potential.