Mineralized Architecture And Pyrite Mineral Geochemistry Of The Super-large Duddar Pb-Zn-Ba Deposit In South-central Pakistan

The Khuzdar-Lasbela metallogenic belt in south-central Pakistan is a giant lead-zinc polymetallic metallogenic belt on the Tethys tectonic mineralization area,which has mainly experienced the formation and closure of the Paleo-Tethys,the formation and closure of the Neo-Tethys,and the continent-continent collisional process of the Arab-Indian Plate and the Eurasian Plate.More than 20 large and medium-sized Zn-Pb deposits have been developed on this metallogenic belt.Amongst them,the Duddar Pb-Zn-Ba deposit is the largest one,with a proven Zn and Pb metal resource of 3.859 Mts,accompanied by a large amount of Ag and barite.And the proved ore bodies are mainly hosted in the Duddar member(J_1-2ad)of the Lower Middle Jurassic Anjira Formation in stratiform,lamellar,and lenticular shape.Due to the tectonic influence of compressive deformation under the setting of the Cenozoic continental collision,the large-scale development of regional thrust structure strongly damage the integrity of main lead-zinc deposit（orebodies）.Up to now,there are still dispute between sedimentary exhalative（SEDEX）type and Mississippi Valley-type（MVT）for the ore genesis.Based on the systematic collection and summary of the data of previous research results,the basic geological characteristics of the Duddar Pb-Zn-Ba deposit were identified through a large number of geological surveys,data arrangement and microscopic observation,according to which the geochemical characteristics and sulfur isotopic composition of pyrite in different ore-hosting strata were systematically determined,and the source of ore-forming fluid and the origin of ore deposit were discussed,in order to provide a new basis for in-depth understanding the regional metallogeney and the future prospecting exploration.The main results and insights from this study are as follows:（1）The Duddar Pb-Zn-Ba deposit is a sedimentary exhalative（SEDEX）type Pb-Zn-Ba deposit controlled by growth faults.The occurrence and mineralisation is controlled by the Kharrari syn-sedimentary fault,with both early sy-sedimentary depositional mineralisation and later metasomatic mineralisation.（2）The Pb-Zn-Ba mineralisation exhibits both regular variations of vertical stratification and lateral zoning in spatial.The ore bodies cane be divided into three sedimentary mineralisation cycles,and each cycle can be divided into three zones from the proximal to the distal of the vent,namely the proximal brecciated Pb-Zn zone,the stratified Pb-Zn zone and the distal stratified pyrite-barite rich Pb-Zn zone,respectively.The ore minerals are mainly pyrite,albite,galena,sphalerite and minor chalcopyrite,whilst the gangue minerals include quartz,calcite,barite,and minor clay minerals.In each of the sedimentary cycles,the lateral zones from the proximal to the distal of the vent are characterised by varying colloidal pyrites,except for the general crystalline pyrites.（3）In the ore bodies（layers）of the first sedimentary cycle,the major elemental compositions of pyrites in the three different ore zones（named as Py-ⅠA,Py-ⅠB and Py-ⅠC type in order）all exhibit deficient sulphur and iron,while the trace elemental compositions are highly variable.Among them,Py-ⅠA pyrite is relatively deficient in Pb,Zn,As,Sb,etc.,and relatively enriched in Se,In,Ge and other rare elements;Py-ⅠB pyrite is relatively enriched in Pb,Zn,Mn,Tl,and relatively deficient in Ge,In while Py-ⅠC pyrite is relatively deficient in Se.（4）For pyrite in ore bodies（layers）in different ore belts（named as Py-IA,Py-IB and Py-IC type in order）,its Pb,Zn,Sb,Cu,Cd,Ge,In,Bi are in the form of mineral inclusions in the mineral lattice,and Co,Ni,As,Mn,Se,Ag,Au,Tl,etc.exist in the form of similar homogeneity.Among them,both Ge and Cu can replace Fe in pyrite,and the replacement method may be Cu²⁺+Ge²⁺（?）2Fe²⁺;Co and Ni replace Fe in a homogeneous image to form CoS₂ and NiS₂;The Co and Ni contents of pyrite in different ore belts were very low,and the Co/Ni ratio was generally less than 1 and became larger to the distal end,indicating that the metallogenic fluid temperature was low and had a trend of gradual increase.This further indicates that the deposit is of origin as a vent-proximal style sedimentary exhalative（SEDEX）type deposit.（5）The S isotopic composition of the pyrite shows that the precipitation of Py-ⅠA pyrite is dominated by TSR action,while the precipitation of Py-ⅠB and Py-ⅠC pyrite is controlled by a combination of TSR and BSR action,with seawater or sulphate minerals providing S^2-.（6）The Kharrari fault in the mine area is a syngenetic fault controlling the development of the Duddar SEDEX type Pb-Zn-Ba deposit,and the Cenozoic activation of the Kharrari and Duddar faults and related secondary faults undermine the integrity and continuity of the ore body（layer）,which also has a large potential for the future mineral recovery at depth.