Magmatic and Hydrothermal Controls on the Formation of Giant Porphyry Cu Deposits

Author:Chang Jia

Supervisor:li jian wei andreas audtat


Degree Year:2019





Globally,porphyry Cu deposits(PCDs)provide ca.70%Cu,50%of the Mo,and25%of the Au.Giant PCDs are the priority targets not only for economic geology study but also for mineral exploration,because they contain anomalously large quantities of metals and thus can be mined for the greatest profit.Understanding the mechanisms that form giant PCDs is of great significance for exploration strategies.Though the characteristics and general processes relevant to the formation of PCDs are well understood,whether the ore-forming magmas and fluids responsible for giant PCDs are unusually metal-rich remains hotly debated.This thesis aims to quantitatively constrain the behavior of ore metals(particularly Cu)during the evolution of magmatic-and hydrothermal systems responsible for giant PCDs.Here I mainly focused on the hypogene porphyry ores of the giant Yulong Cu-Mo deposit in eastern Tibet,China,containing proven reserves of 6.5 million metric tons(Mt)Cu and 0.4 Mt Mo.However,the lack of mafic to intermediate igneous rocks directly related to the Yulong deposit in and surrounding the mine disabled to investigate the metal budget and evolution of fertile magmas associated with giant PCDs.In this case,clinopyroxene-to hornblende-rich xenolith from the magmatic systems associated with the giant Santa Rita porphyry Cu-Mo±Au deposit(New Mexico,USA),which contains proven reserves of 14 Mt Cu,0.2 Mt Mo and 182 t Au,were used as an alternative to constrain the magmatic evolution and its control on metal fertility and ore formation.In addition,a set of very similar clinopyroxene-to hornblende-rich xenolith samples from the magmatic systems at Cerrillos was also investigated.Specifically,this thesis aims(i)to constrain the petrogenesis and metal contents of clinopyroxene-and hornblende-rich xenoliths at Santa Rita and Cerrillos,(ii)to provide a detailed description of the hypogene porphyry ore at Yulong,(iii)to constrain the timing and duration of the porphyry Cu-Mo ore formation at Yulong,and(iv)to unravel the formation and evolution history of multistage magmatic-hydrothermal fluids at Yulong.In chapter 2,the petrogenesis and metal contents of hornblende-rich xenoliths hosted in Laramide-age magmas at Santa Rita and Cerrillos was reconstructed based on detailed petrographic studies and laser-ablation inductively-coupled-plasma mass-spectrometry(LA-ICP-MS)analysis of minerals,melt inclusions and sulfide inclusions.The xenoliths from both locations record an evolutionary path from clinopyroxene+olivine+phlogopite cumulates(1000-1100°C),through nearly pure hornblendites(920-1000°C),to hornblende+plagioclase cumulates(830-950°C)and hornblende gabbros(850-910°C)that crystallized at a depth of 10-20 km.In contrast to proper cumulates formed by gravitational settling of phenocrysts from magmas,the abundant hornblendites seem to have formed by reaction of residual melts with previously crystallized clinopyroxene+olivine+phlogopite assemblages in a crystal mush environment.The mafic melts(≤47-53 wt%SiO2)that produced the clinopyroxene+olivine+phlogopite cumulates were sulfide-undersaturated,whereas more evolved melts(>53 to 65 wt%SiO2)were generally saturated in sulfides±anhydrite.The hornblendites of reaction-replacement origin contained up to 0.8 wt%magmatic sulfides,which occur mostly in the form of monosulfide solid solution containing 1-8 wt%Cu,0.1-2.2 wt%Ni,1-15 ppm Ag and 0.1-1.3 ppm Au,with minor intermediate solid solutions containing up to 37 wt%Cu,0.5 wt%Ni,1100 ppm Ag and 190 ppm Au.Quantitative modeling is used to reconstruct the Cu evolution recorded in sulfide inclusions and melt inclusions in xenoliths and porphyry dikes at Santa Rita,with the Cu contents of magmatic sulfides and silicate melts decreasing dramatically within a narrow melt SiO2 interval of 55-58 wt%once sulfide saturation was reached,and then declining more gradually to1.5 log units lower values.These results suggest that the formation of hornblende-rich lithologies during the ascent of primitive arc magmas through the crust should have a negative influence on the mineralization potential of the residual liquids(≥55-60 wt%SiO2),even if the magmas are relatively oxidized(2 log units above the fayalite-magnetite-quartz fO2 buffer).Therefore,fertile upper crustal magma reservoirs may be favored by mafic magma recharge.In chapter 3,I document the multistage porphyry intrusions,hydrothermal vein and alteration assemblages of the Yulong deposit by drill core logging and petrography,and then present high-precision molybdenite Re-Os dates of 14 quartz-molybdenite±chalcopyrite veins to constrain the timing and duration of the Cu-Mo ore formation.The ore-hosting Yulong composite stock consists of three successive porphyry intrusions:(1)monzonitic granite porphyry,(2)K-feldspar granite porphyry,and(3)quartz albite porphyry.The vein formation,Cu-Mo mineralization and hydrothermal alteration are grouped into early,transitional and late stages with respect to the intrusive history.The first two porphyry intrusions are followed by cyclical sequences of veining that are mainly associated with potassic alteration and have formed(1)ME vein/USTT,(2)EBE/T veins,(3)A1E/T veins,(4)A2E/BT veins,and(5)A3E/T veins.A2E/BT and A3E/T veins of the early and transitional stages are dominated by quartz and chalcopyrite±pyrite,respectively,and represent the main Cu-Mo mineralization events.More than 80%of Cu and Mo at Yulong were deposited in the early stage with the remainder being formed in the transitional stage.The late stage pyrite-quartz veins(DL)that are characterized by sericitic alteration halos have no economic significance.Molybdenite Re-Os ages of A2E and BT veins indicate that the bulk Cu-Mo ores formed in a time interval of 1.36±0.24 m.y.(1σ)with most Cu precipitated in a more restricted timespan of 0.82±0.24 m.y.(1σ)in the early stage.These results,combined with geochronologic data from PCDs elsewhere,confirm that multiple magmatic-hydrothermal pulses with a lifespan of tens to hundreds of thousands of years are sufficient to form a giant PCD.In chapter 4,fluid inclusion microthermometry and LA-ICP-MS microanalysis are combined with textural observations on magmatic apatite and various hydrothermal quartz veins to constrain the formation and evolution of the multistage magmatic-hydrothermal fluids at Yulong.Fluid inclusions hosted in quartz veins of the early and transitional stages have very similar compositions,indicating that the initial single-phase intermediate-density(ID-type)ore fluids of these two mineralizing stages were exsolved from similarly evolved magma reservoirs.During their ascent,decompression and cooling,the ID-type fluids(9 wt%NaClequiv.with1000 ppm Cu and20 ppm Mo)entered the two-phase field and condensed into a small amount of metal-rich brines(42wt%NaClequiv.with9300 ppm Cu and330 ppm Mo)coexisting with a large amount of vapors(vapor/brine mass ratio of4).The condensation of brines and their accumulation at shallow level could be an efficient mechanism to concentrate and then precipitate Cu±Mo in a small rock volume as represented by the mineralized porphyries.The sequential deposition of Mo and Cu in the early stage from the condensed brine phase caused the local Cu-Mo decoupling in shallow parts of the deposit,whereas the early precipitation of molybdenite from the ID-type fluids in the transitional stage produced the deeply located Mo-rich mineralization.The high Cs+Rb±B concentrations of melt and fluid inclusions in magmatic apatite suggest the fluids were saturated in a pegmatitic environment.The fact that liquid-rich inclusions in DL veins that are associated with sericitic alteration are similar in element/K ratios with the apatite-hosted fluid inclusions suggests that the late-stage fluids were also likely derived from highly evolved melt fractions.In chapter 5,I present Mo isotopes of molybdenite from the early and transitional stages of the Yulong deposit.The relatively uniform Mo isotopic composition(δ98Mo=0.03-0.18‰)of the early stage is consistent with molybdenite precipitation from the relatively stagnant,high-density brines.In contrast,the large spread of Mo isotopes(δ98Mo=-0.16-0.60‰)of the transitional stage requires a process similar to Rayleigh fractionation,and suggests molybdenite precipitation during rapid ascent of the single-phase,intermediate-density fluids.The Mo isotopic evidence contributes to a better understanding of the deposit-scale Cu-Mo separation at Yulong.Results presented in this thesis suggest that unusually metal-rich ore-forming magmas and fluids are not necessarily required to form giant PCDs.It is thus tentatively concluded that the formation of giant PCDs may be favored by the accumulation of a large magma reservoir at a depth of 5-15 km possibly with large-volume mafic magma recharge,and by the development of vent structures that facilitate fluid transport and localization.Therefore,the exploration strategy for giant PCDs in a prospective region is to focus on structural settings favorable for accumulation of large upper crustal magma reservoirs.The areas that are potential to discover new large to giant PCDs in the Yulong porphyry Cu belt is evaluated preliminarily.