Study On The Pyrite And The Implication For Genesis Of Xinqiao Deposit, Anhui Province

There are a series of stratiform and stratoid copper-gold-sulfide deposits inJiuRui ore concentration area and TongLing ore concentration area in themetallogenic belt along the middle and lower reaches of Yangtze River. Becausethese ore bodies are controlled by the Carboniferous strata, stratiform ore bodieshave certain characteristics of symphitic deposit, so whether Submarinehydrothermal-sedimentary mineralization exists in the forming of these deposits isa huge controversy. XinQiao deposit is the typical representation of these deposits,and there are three different viewpoints on the forming process of XinQiao deposit:submarine hydrothermal-sedimentary metallogenesis, superposition transformationmetallogenesis and hydrothermal metallogenesis. The key content of the argumentis about the genesis of the bedding-output colloidal pyrite in the deposit. Therefore,a systematic geological and geochemical research is conducted on this especialcolloidal pyrite and then the genesis of the colloidal pyrite is determined, whichwill help us to understand the mineralization process of this deposit in the middleand lower reaches of the Yangtze River area and guide us to explore the deposits ofthe same kind in the area. So it has important scientific research value and practicalproduction significance.In this thesis, the different kinds of pyrite in XinQiao deposit are the researchobject. Through the systematic field geological observation and the laboratorystudy of pyrite mineralography, morphology, geochemistry and isotopegeochemistry, the geological and geochemical characteristics of the different kindsof pyrite in XinQiao deposit is ascertained, the minerogenetic condition and thegenesis of these different kinds of pyrite is analyzed, the ore genesis of XinQiaodeposit is clarified and the Metallogenic Model is preliminarily established.Through detailed field geological observation and indoor mineralographystudy, the pyrite in XinQiao deposit is divided into two types by degree ofcrystallinity, one is colloidal pyrite and another one is euhedral or subhedral pyrite.These two types of pyrite have obvious difference in morphology， mineralgeochemistry and isotope geochemistry.The particle size range of colloidal pyrite is0.3μm~2.0μm, the euhedral orsubhedral pyrite is about0.1cm~1cm, and the giant one even can be 2cm~5cm.When the degree of crystallinity of pyrite is different, its unit cellparameter will change a lot. The unit cell parameter a0value is bigger than theeuhedral or subhedral pyrite,and both of them are bigger than standard pyrite. Thediversity of Co and Ni content in pyrite is the main reason for the situation thatdifferent degrees of crystallinity of pyrite have different a0values.The conduction types of colloidal pyrite are all P type, and the euhedral orsubhedral pyrite are almost P+N type. According to the Gorbachev’s temperatureruler calculation formula, it can be obtained that the colloidal pyrite is formed inlow temperature and the euhedral or subhedral pyrite pyrite is formed inmesothermal temperature.Both the major elements and the trace elements of the different types ofpyrite in XinQiao deposit have obvious differences. In the aspect of major element,the content of Fe and S which are the two main elements in pyrite are increasingwith the increasing degree of crystallinity. In the aspect of trace element, thecolloidal pyrite is rich in Bi, Cu, Pb, Au, Ag, Mn, Mg, Al, Sb and some other metalore-forming elements. But the euhedral or subhedral pyrite is rich in chalcophileand siderophile elements, such as: Co, Ni, As, Te, Se and so on. Such features as Co,Ni, Se,Se/S in different types of pyrite indicate that the ore-forming fluid ofcolloidal pyrite are derived from seawater,while the euhedral or subhedral pyritebelong to the type of hydrothermal origin.The characteristics of Organic matter biological markers in the massive pyriteore in XinQiao deposit indicate that the organic matter in pyrite ore is marinesource and the colloidal pyrite is formed by high salinity and reducing ore formingfluids. The study on Sulfur isotope indicates that the source of the sulfur inchalcopyrite and euhedral or subhedral pyrite is yanshan magmatic hydrotherm,and the sulfur value of the colloidal pyrite is different from the sulfur value of thethe euhedral or subhedral pyrite.Through the study on the geological and geochemical characteristics of pyritein XinQiao deposit, it can be confirmed that the colloidal pyrite is submarinehydrothermal sedimentary origin and the euhedral or subhedral pyrite is theyanshan magmatic hydrothermal genesis. So two periods of metallogenic model ofXinQiao deposit are established: the submarine hydrothermal sedimentary period inlate Paleozoic era formed the colloidal pyrite, and abundantCu,Pb,Au,Ag,Mn,Mg,Al,Sb and other metal elements formed the bismuth mineralinclusions, and then combining with native gold and silver, these inclusionsprecipitated and occurred in the colloidal pyrite; the yanshan magmatic hydrothermal system superposed early forming colloidal pyrite and made it occurcrystallization and metasomatism. Then extracting the Cu,Pb,Au,Ag,Mn,M,g,Al,Sband some other metal elements from the bismuth mineral inclusions in the colloidalpyrite formed chalcopyrite, bearing-copper pyrite, galena, sphalerite and so on.Meanwhile, the yanshan magmatic hydrotherm had a metasomatism reaction withthe carbonate strata, and then forming Cu-Au-S deposit.