Metallogenic Mechanism Of The Daliangzi MVT Pb-Zn(Cd) Deposit In Sichuan-Yunnan-Guizhou Metallogenic Province,SW China

The Sichuan-Yunnan-Guizhou metallogenic province（SYG）,located in the southwest margin of the Yangtze block,contains more than 400 Pb-Zn deposits and is one the most significant Pb-Zn metallogenic regions in China.In the past two decades,the epigenetic Mississippi Valley type（MVT）origin of these deposits has gradually been accepted by most researchers.However,the ore-forming ages and the origin of hydrothermal fluids are still controversial.Importantly,the ore-forming processes research and the metallogenic model construction are lacking.In this thesis,the Daliangzi Pb-Zn（Cd）deposit（45 Mt at Pb+Zn of 11.25 wt.%）,the second largest lead-zinc deposit in the SYG region,is selected as the main research object.Based on detailed geological survey and petrographic observation,we have used a variety of advanced methods,such as sphalerite Rb-Sr dating,LA-ICP-MS individual fluid inclusion composition analysis,S-Pb-Zn-Cd multivariate isotope analysis and LA-ICP-MS sulfide trace element analysis,to reveal the metallogenic age and dynamic background,the origin and evolution of ore-forming fluids,the ore-forming materials sources and the precipitation mechanism of sulfide minerals of this deposit.Furthermore,detailed ore-forming processes occurred in different mineralization stages and spatial locations was restored and the systematic metallogenic model of Daliangzi was constructed.The main conclusions of this thesis are listed below.（1）Spatially,two main mineralization styles have been identified in the Daliangzi deposit,which are the brecciated and massive mineralization occurring in the“black breccia zones”in the NE part of the No.1 ore body;and the stockwork and veined mineralization hosted in the normal fault（-fracture）system in the SW part.Temporally,the hydrothermal mineralization occurred in Daliangzi can be divided into three stages（I—III）,namely,the Stage I characterized by colloform galena（Gn I）and sphalerite（Sp I）,the Stage II composed of crystalline sphalerite（Sp II）and galena（Gn II）,and the Stage III consisting of light sphalerite（Sp III）and associated galena（Gn III）.In general,the Stage I is found only in the organic-rich part of the NE ore body.The Stage II,the main mineralization stage,occurred in the organic-poor part of the NE ore body and the fault fracture system in the SW part.And the Stage III was commonly replaced or cut through the sulfides formed earlier.（2）The Rb-Sr dating of SpⅡ shows that the Daliangzi was formed at 397±8 Ma.Combined with the previously reported age of 411-349 Ma in the Wusihe and Tianbaoshan deposits,the～400 Ma age here confirms that in addition to the Late Triassic-Early Jurassic Pb-Zn mineralization represented by the Huize deposit,an earlier Devonian-Early Carboniferous metallogenic event also existed in the SYG region.Different from the Indosinian compressional orogenic setting of the former,the latter mainly occurred in the foreland basin environment after the uplift of West Sichuan Central Yunnan（Kangdian）oldland in the Caledonian Period.（3）There are two types of fluid inclusions have been identified in the Daliangzi sphalerite:brine inclusions and organic inclusions,respectively.The former has moderate homogeneous temperature,salinity,K/Na,Ca/Na,Ba/Na ratios and low Rb/Na ratios,indicating that it represents a basin brine affected by metamorphic fluids.The detection of CH₄ and bitumen in the latter and the widespread organic matter in the deposit indicate that the organic fluids could be derived from a preexisting fossil oil-gas pool in the deposit.The homogeneous temperature and salinity of the hot ore-forming basin brine decreased systematically from the Stage I to the Stage II and from the center of mineralization to the margin of the ore body.The higher temperature and salinity of the brine fluid in the Stage III may imply the significant metamorphic fluid influence.On the other hand,the compositions of brine fluid also varied at different stages and recorded some details in the mineralization processes.For example,the high Ca/Na and low K/Na ratios of the ore-forming brine fluids in the Stage I and the end of Stage II may reflect the significant fluid-rock reaction;and the high K/Na ratio in the Stage III fluid may be related to the metamorphic fluid.In addition,this study revealed for the first time the distinct behavior of Ga and Ge in the two kinds of fluids.Gallium was commonly concentrated in the brine inclusions,while Ge is enriched in the organic inclusions.This may be attributed to the different geochemical affinities of the two critical metals:Ga is lithophilic and probably derived from the metamorphic basement;Ge,on the other hand,is organophilic,and might be originated from the organic fluid.（4）The results of in situ S isotope analysis of sulfides and barite indicate that the reduced S of Daliangzi was mainly from the thermochemical reduction（TSR）of marine sulfate and/or evaporite in the Dengying Formation with the participation of organic matter.The TSR process is likely to have occurred prior to the Pb-Zn mineralization and the reduced S may be preserved in the organic fluid.The galena formed in different stages has relatively consistent in situ Pb isotopic compositions,indicating the lead in Daliangzi may originate from the metamorphic basement.The Zn-Cd isotopic compositions of the Daliangzi sphalerite increase gradually from Stage I to III,which may be explained by Rayleigh fractionation.By comparing Zn-Cd isotopes with the potential sources,it was found that Zn and Cd in this deposit may come from the dolomite in the Dengying Formation.Considering that the initial Sr ratios of Sp II were higher than that of the Dengying dolomite and lower than that the of metamorphic basement,the metallogenic metals such as Pb,Zn and Cd in Daliangzi may mainly originate from the mixing source of the Dengying dolomite stratum and the underlying metamorphic basement.（5）The SpⅠ is enriched in Cd,Pb,Ag,Sb,As,and Tl.The SpⅡ is relatively concentrated in Fe,Cu,Ga,Ge,and Mn.However,the Sp III is depleted in most of the analysed trace elements.These trace elements were incorporated into sphalerite in various ways.Cadmium,Fe,and Mn commonly replaced Zn²⁺directly.Lead may generally exist as micro-inclusions but also can enter the sphalerite lattice.Gallium,Sb,and As generally substituted Zn²⁺as trivalent cations coupled with Cu⁺,Ag^+,and Tl⁺.Germanium may be incorporated in the Sp I by direct substitution but in the Sp II and Sp III via coupled substitution(Ge⁴⁺+2Cu⁺（?）3Zn²⁺),and also can enter the Sp III as Ge-bearing micro-inclusions.The temperature and sulfur fugacity estimations based on the sphalerite trace element contents show that the Sp I was precipitated in a changing environment with high temperature and fluctuating sulfur fugacity.The Sp II crystallized during a dramatic decrease in temperature and sulfur fugacity.However,the low content of trace elements in the Sp III resulted in significantly lower estimated temperature values than the results of microthermometry.In addition,the typical colloform texture of Sp I was formed by two pulses of metal-rich hydrothermal injection and subsequent fluid mixing with the reduced sulfur-bearing fluid in open space,while the rhythmic bands and sectors of the euhedral SpⅡ reflect a crystallographically controlled non-equilibrium self-organizing process occurred in a relatively closed environment.Galena from the Daliangzi Zn-Pb deposit is enriched in Sb and Ag,with minor Cd,Cu,and As,but extremely depleted in Bi.Antimony was generally incorporated via Sb³⁺+Ag⁺（?）2Pb²⁺substitution,and the substitution of 2Sb³⁺+□（?）3Pb²⁺and even Sb-bearing micro-inclusions also occurred at high Sb concentration.Generally,Sb,Ag,Tl,Bi,and Se are incorporated into galena;and Cd,Fe,Mn,Cu,Ga,and Ge are concentrated in coexisting sphalerite.Normally,the preferred host sulfide of trace elements is controlled by intrinsic factors（e.g.oxidation state and ion radius）,rather than external factors（e.g.,metal sources,pressure,temperature,and redox conditions）.However,external factors,such as temperature,may play important roles in the elemental partitioning between preferred and subordinate hosts.（6）The coexistence of brine inclusions and organic inclusions in sphalerite provides direct evidence for the hypothesis that the Daliangzi deposit was formed from the fluid mixing processes of hot metal-rich basin brine and cold rorganic fluid containing reduced S.In the Stage I,rapid fluid mixing caused by pulsed injection of hot metal-rich basin brine was occurred in the organic-rich part of the“black breccia zones”in the NE part of the ore body.In the Stage II,however,slow fluid mixing related to the significant decrease of temperature and sulfur fugacity took placed in the organic-poor parts of the NE ore body and the fault-fracture system in the SW part.In the last stage,recrystallization of late fluid that had replaced the early sulfides was occurred in the fissues and cavities.（7）A systematic metallogenic model of the Daliangzi deposit was established as follow.（i）Before mineralization:a SO₄^2--bearing fluid was formed from the seawater sulfate/evaporite hosted in the Dengying Formation under the leaching of meteoric water.The fluid entered the fossil oil-gas reservoir in the Daliangzi deposit along the faults and was reduced to form S^2-during the TSR process with the participation of organic matter.The resulting reduced S may be preserved in the organic fluid for a long time in the reduced environment.（ii）During the mineralization:under the background of post-Caledonian extension,the basin brine in the rifted basin formed by the uplift of the Kangdian oldland was cycled repeatedly and extracted the ore-forming metals from the dolomite strata and basement metamorphic rocks to form the hot metal-rich brine.The metal-rich hydrothermal fluid eventually migrated to the deposit site and mixed with the reducted S-bearing organic fluid in different locations and stages and precipitated into the ore bodies with different mineralization styles.And（iii）post mineralization:the pyrite with magmatic S characteristics may suggest that the magmatic hydrothermal fluid related to the E’meishan basalt had a certain influence on the Daliangzi deposit.but this is only a speculation.In addition,some implications on the metallogenetic research and Pb-Zn prospecting in SYG region were also been explored.