Abstract
The Qinxi silver deposit is located in eastern Fujian Province, South China. It is hosted in the late Jurassic Nanyuan volcanic unit and controlled spatially by structural alteration zone. The origins of the deposit are understood poorly. In this study, systematic field geology, precise sulfide geochronology, and H–O–S–Pb isotope analyses were carried out. The symbiotic minerals and ore assemblages are interpreted as evidence that Qinxi ore is a typical middle to low temperature hydrothermal deposit. Galena and sphalerite from the ore yield a Rb–Sr isochronal age of 130 ± 2.0 Ma (MSWD = 1.7), and single galena samples yield a Rb–Sr isochronal age of 130 ± 7.1 Ma (MSWD = 2) and are evidence that the ore was formed in the early Cretaceous period. The silver-bearing quartz vein has O:H and 18O:16O ratios intermediately between magmatic and meteoric water (δ18 Owater-smow from −2.50‰ to −0.1‰ and δDv-smow from −73.6‰ to −60.7‰). The δ34 S values of metal sulfide are close to the primitive mantle sulfur values (δ34S = −1.7 to 7.1‰, avg. = 0.92‰). The Pb isotopic compositions from the ore are similar to those of the hosted volcanic unit, with a value between the upper crust and mantle isotope composition (206Pb/204Pb = 18.421~18.605, 207Pb/204Pb = 15.580~15.772, 208Pb/204Pb = 38.591~39.183). Lead–zinc sulfides have mature crustal-derived Sr isotopic features ((87Sr/86Sr)i = 0.7103). Thus, the metallogenic source materials mainly came from the crust. Moreover, regional geological studies suggest that the Qinxi ore deposit was likely formed in an extensional terrane.
Highlights
The volcanic fault-depression belt in Eastern Fujian Province is one of the most important prospective areas for silver polymetallic mineralization [1]
The samples analyzed in this study were all large Pb–Zn samples from the main metallogenic stages at different locations of the Qinxi ores with high purity
Our dating results suggest the Qinxi silver ore was formed at 130 ± 2 Ma (MSWD = 1.70) (Figure 8a)
Summary
The volcanic fault-depression belt in Eastern Fujian Province is one of the most important prospective areas for silver polymetallic mineralization [1]. The main metallogenic stage belongs to the magmatic hydrothermal period and can be divided into four stages (Table 1) due to the character of the ore and the mineral compositions: (1) the phase of pyrite and quartz (stage I): Metal minerals in this stage comprise largely pyrite, with a small amount of sphalerite, galena, and magnetite This is the early stage before significant silver mineralization happened; (2) the sulfide rich phase (stage II): In this stage, large numbers of metal sulfides, such as pyrite, galena, sphalerite, and chalcopyrite, appeared and are represented as different types of veins. These sulfides are distributed in tectonic fracture zones; (3) the silver sulfide mineral phase (stage III): Large numbers of silver bearing minerals like pyrite, sphalerite, galena, and chalcopyrite were formed in this stage by replacing early stage sulphides These minerals are represented as fine veins and distributed in the rock fractures; (4) the quartz–carbonate phase (stage IV): This is the last stage of hydrothermal alterations.
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