Alteration zonation and metallogenic mechanism of porphyry copper deposits:A case study of thermodynamic equilibrium simulation of fluid-rock interactions in Yulong deposit
Accurate delineation of alteration zones and ore-forming mechanisms is crucial for establishing porphyry metallogenic models and ore prospecting prediction.Taking the Yulong porphyry copper deposit in the Cenozoic Jinshajiang-Ailaoshan metallogenic belt as a study case,this paper employs the law of mass action(LMA)and the Gibbs free energy minimization(GEM)model to construct the pH-fo2 phase diagram and dynamic transport model for ore-bearing hydrothermal fluid and porphyry intrusion to reveal the genesis of alteration zoning and metal mineralization of the deposit.The modeled results indicate an initial ore-forming fluid pH of 4.7,logfo2=-23.0(△FMQ=+2.7),with dissolved Cu and Mo contents of 1138 × 10-6 and 1.2 × 10-6.When the acidic and oxidized fluid infiltrates the monzonite granite porphyry within the temperature range of 450℃ to 360℃,minerals associated with such potassic alterations as K-feldspar,biotite,anhydrite,hematite and magnetite gradually precipitate;and the fluids related to potassic alteration exhibit higher pH values(5.0～7.0)and oxygen fugacity(△FMQ=+2.9～+3.6)during this process.While at temperatures ranging from 360℃ to 320℃,typical minerals representing the propylitic alteration stage,such as actinolite,chlorite and epidote,gradually form;and both of the fluid pH values(5.0 to 6.4)and oxygen fugacity(△FMQ=+1.1)decrease during this stage.As the temperature further decreases from 320℃ to 200℃,the fluid pH values(5.0～5.7)experience a slight additional decline,while the oxygen fugacity shows a slight increase(△FMQ=+1.7).During this period,minerals such as sericite and calcite begin to precipitate,marking the onset of typical phyllic alteration.Additionally,molybdenum(Mo),carried by HMoO4 and MoO4-,precipitates in a narrow high-temperature range(450～370℃);while copper(Cu),primarily carried by CuCl(CuCl43-,CuCl2-,CuCl),precipitates in the mid to high-temperature range(450～300℃).By employing the LMA inverse modeling and GEM forward modeling,this study quantitatively elucidates the hydrothermal alteration processes in the Yulong porphyry copper deposit.The findings reveal that the alteration zones in the porphyry deposit result from gradual cooling of a single magma-hydrothermal system continuously interacting with the porphyry intrusion.Different temperature ranges correspond to the formation of distinct alteration minerals,namely,potassic alteration(450～360℃),propylitic alteration(360～320℃),and phyllic alteration(320～200℃).Consequently,the rapid decline in fluid temperature may play a crucial role in the superimposed alteration processes at the Yulong copper deposit.Furthermore,the differential response of the solubility of Cu and Mo complex ions to temperature changes leads to the preferential development of Mo mineralization in the high-temperature zones near the porphyry intrusion,while Cu is distributed extensively in the high to medium-temperature zones,occurring in a veinlet-disseminated patterns superimposed on the Mo mineralization.
Fluid-rock interactionThermodynamic equilibrium simulationYulong porphyry copper depositpH-fo2 phase diagramReactive transport model
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维普
