Fluid Evolution And Mineralization In Porphyry Copper(Molybdenum) Deposits

The purpose of this Ph.D thesis serves as a preliminary study of the controls on the porphyry copper and molybdenum enrichment.The entire thesis consists of a comparison of the uniformities and variations on the ore forming processes across the wide spectrum of Chinese porphyry deposits,a detailed investigation of characteristics of ore-forming fluids in Bangpu and Jiama deposits under the framework of fluid inclusion assemblages?FIA?,and a synthetic fluid inclusion study of the halite saturation and metals precipitation in porphyry deposits.Compared to porphyry molybdenum deposits?PMD?,the porphyry intrusions in porphyry copper deposits?PCD?have high Mg#,TiO₂,Al₂O₃,CaO,P₂O₅ content,and relatively high Sr and low to moderate Rb content.The sulfidation state remains at intermediate values during the main period of ore minerals deposition at some PCD.Several PCD are reported to be hosted in adakite-like porphyries.The potassic assemblages are commonly formed at highest temperature,containing biotite,quartz,with minor magnetite and anhydrite.The associated veins often contain low grade Cu-Fu sulfide.The Cu-Fe sulfides commonly precipitate as a result of temperature and pressure decrease,which evidenced by the “boiling” FI assemblages in most PCD and the common presence of the chalcopyrite daughter minerals in brine fluid inclusions..The PMD-related intrusions are highly differentiated and characterized by low Mg# and high Rb/Sr ratios and K₂O contents.They also strongly depleted in Ba and Eu.At PMD,porphyry intrusions are emplaced at greater depths than PCD.Most of the ore-related porphyries in Chinese PMD,are quite different from adakites.During the main ore forming stage,PMD have secondary K-feldspar instead of biotite as the predominant potassic alteration phase,which appear to be precipitated by single-phase magmatic fluids at temperature greater than 550°C.The single-phase fluids from PMD can contain significant concentration of CO₂.The brine fluid inclusions in the phyllic zones generally only contain halite as daughter mineral.The high initial oxygen fugacity of the magma source may eliminate sulfide in the source region,keep the melt sulfide undersaturated,and increase initial chalcophile element concentrations and thereby promoting porphyry copper mineralization.Under less oxidized condition,Mo selectively partition into the silicate melt and its concentration in the melts do not seem to be affected by the magmatic sulfide precipitation.Consequently,these melts will yield a magmatic fluid with a high Mo/Cu ratio,which is favorable for the formation of porphyry Mo deposits.During the exsolution of volatile phases in the magma chambers,the different partitioning of volatile,Cu and Mo among silicate melts and aqueous solutions could be an important factor separating Cu and Mo from the magmas.The spatial association between molybdenite,Q2 quartz and halite-bearing fluid inclusion assemblages?FIA?in the quartz crystal from the Qtz-Mol veins indicated the precipitation of molybdenites in Bangpu deposit may be a direct consequence of halite saturation and heterogenous entrapment along the liquid-vapor-halite curve.Pressure quench and cooling together would more likely to bring saline fluid into the field of vapor+liquid+halite and result in precipitation of salt.At temperature of 373 °C and pressure of 128 bars,phase separation produces a brine with ³8 wt% NaCleq,which later move into the field of vapor+liquid+halite as cooling continues to 320 °C,causing 5-folds increasing of Mo concentrations in the brine.As the cooling continues to 300 °C,96% of Mo was precipitated from the brine to form molybdenite.The spatial coexisting between chalcopyrite inclusions and liquid-rich FIA on the growth zones or along the micro-fractures in the quartz crystals from the Qtz-Cpy veins and the?monzonite?granite porphyries indicates that chalcopyrite precipitates mainly from the single-phase aqueous fluid at Jiama deposit.At 382 °C,the ore-forming fluid with ⁵ wt% NaCleq can contain up to1900 ppm Cu.At 350 °C,the ore-forming fluid with ² wt% NaCleq have an average Cu concentration of 75 ppm.This demonstrates that cooling and depression together most likely cause chalcopyrite precipitation from the ore forming fluid,precipitating 96% of Cu over a small temperature?30 °C?and pressure?80 bars?interval.The source of the molybdenum-mineralizing fluids at Bangpu and Jiama probably were particularly large magma chamber that crystallized and fractionated in the lower crust or at mid crustal level.FIs that homogenize by halite disappearance in porphyry copper deposits may not have trapped a single-phase fluid at high pressure.The widespread occurrence of FIs that homogenize by halite dissolution in porphyry copper deposits could be a direct consequence of halite saturation and heterogeneous entrapment along the liquid-vapor-halite curve at very low pressure.Salt saturation of magmatic-hydrothermal fluid may thus act as a direct precipitation mechanism of ore minerals in porphyry copper deposits.We may not reconstruct the trapping condition for FIs homogenize by halite disappearance if they originated by heterogeneous entrapment.