Abstract:

As a newlydeveloped microanalytic technique, Laser ablation inductively coupled plasma mass spectrometry (LAICPMS) of sulfide minerals can provide us important informations regarding the nature of oreforming fluids, ore genesis and their implications on mineral exploration. Pyrites from Xinqiao CuAuS deposit in the Tongling mineralization belt, Anhui Province, China, have been selected for detailed study in this paper. Three types of pyrite were recognized through detailed field work and paragenetic studies, among which the earliest colloform pyrite, termed PyI, is interpreted as sedimentary in origin, whereas PyII is mainly composed of fine grained, anhedral crystals with some characteristics of deformation and recrystallization. Mediumcoarse grained and euhedral PyIII is inferred to be the result of hydrothermal activities and associated with mineralization. The LAICPMS analyses of three types of pyrite indicate that the sedimentary PyI contains the highest values of Ti, Co, Ni, As, Se, Te. PyII pyrite also is rich in Ti, Co, Ni, As, Se, Te，with variable content of oreforming elements such as Cu, Pb, Zn, Au and Ag. PyIII shows distinguished composition compared to other two types of pyrite. Cu, Pb, Zn, Au, Ag and Bi give much higher ranges in PyIII but Co, Ni and As are relatively lower. Evidences show that Co, Ni, As, Se and Te are contained in pyrite as isomorphous solid solution. Bi occurs as Bicontaining minerals which are commonly small inclusions in PyII pyrite, however, in PyIII, Bi partially replaced Fe in lattice. The oreforming elements, e.g. Cu, Pb, Zn, Au and Ag in Xinqiao deposit mainly occurred in PyIII. Cu and Zn are mainly represented by chalcopyrite and sphalerite inclusions enclosed in pyrite. Au and Ag in PyII, albeit with low content, probably occur as inclusions of freegold and freesilver. However, in PyIII, electrum is the major mineral containing Au and Ag, which is also possibly in bismuth minerals. Pb occurs in both galena and bismuth minerals inclusions in pyrite. Textures, paragenesis and trace element geochemistry of pyrite indicate that PyI was formed in the process of the Late Paleozoic submarineexhalative sedimentation, which was supported by other studies, whereas PyII and PyIII pyrite may occur during the Mesozoic tectonic transitiondeformation and subsequent hydrothermal superimposition, respectively. Therefore, the formation of Xinqiao CuAuS deposit may have recorded both the late Paleozoic submarine or exhalation sedimentation and hydrothermal telescoping in the Mesozoic. Colloform pyrite with rare Cu and Au mineralization deposited during sedimentation, while most of economic metals, e.g., Cu, Pb, Zn, Au and Ag, were introduced through hydrothermal fluids during emplacement of the Yanshanian dioritic intrusions. 

Key words: LA-ICP-MS, pyrite, trace element, Xinqiao, Tongling