Cobalt enrichment mechanism in the Hengdong cobalt deposit,Northeast Hunan Province:Evidence from texture,chemical composition and sulfur isotopic composition of pyrite

The typical hydrothermal Co deposits in the world,as represented by the five-element (Ag-Bi-Co-Ni-As +/- U)vein type Co deposit,have great economic potential and research significance on account of their high metal grades (up to 8%). Native elements and arsenides are common in these high-grade hydrothermal Co deposits. The medium-scale Hengdong Co deposit (with an average Co grade of 0.04%)in Hunan Province has an ore mineral assembalge of Co-rich pyrite,chalcopyrite,sphalerite,galena and pyrrhotite. In order to characterize the Co-bearing minerals and examine the Co enrichment mechanism of the deposit,a detailed study on the texture,chemical composition and sulfur isotopic composition of pyrite in its ores was carried out by using combined EPMA,EBSD, LA.ICPMS and LA-MC-ICPMS methods. The pyrites in this deposit can be divided into three generations,i. e.,PyI,PyII and PyIII, among which the PyII is the most important Co-bearing mineral,and it always occurs as ribbon-like,oscillatory or uneven zones replacing PyI or as fine grains. The Co contents of PyII are up to 52141 x 10(-6),which is remarkably higher than those of PyI and PyIII,indicating that PyII is the major Co carrier of the deposit. The negative correlation between Co and Fe of the pyrite indicates that Co should substitute for Fe in the lattice of the pyrite. The high Co/ Ni ratios (1.05 similar to 393)and Se contents (10. 69 x 10(-6) similar to 129 x 10(-6))of the pyrite indicate that it is of a hydrothermal origin. The sharp interface (sometimes with a bay shape)between PyII and PyI,togther with the contrast chemical variation and similar crystal orientation of PyII and PyI,disclosed that the dissolution. precipitation mechanism during the rapid interaction of the Co-bearing fluid with the wall rocks and early-formed pyrite was responsible for the formation of the Co-rich PyII. The three generations of pyrite have delta(34) SV-CDT values varying from - 13.12 parts per thousand to - 8.70 parts per thousand,which is consistent with that of the host rocks of the Lengjiaxi Group. Based on the high Co concentrations (30.4 x10(-6) similar to 72.3 x 10(-6))of the basement rocks in the Northeast Hunan Province,together with previous microthermometric measurements of fluid inclusions,we proposed a new model of the Co enrichment in the deposit as follows:during the strike-slip faulting of the Changsha-Pingjiang fault zone in the period from the Late Jurassic to Early Cretaceous,the deep-sourced fluids that leached Co from the basement (i. e.,the Lianyunshan Group)progressively migrated upward along the NE-ENE fault zone;then the cyclic pressure release happened during the fault-zone movements may have led to the fluid immiscibility and vapor phases separation from the liquid phases,thus the Co should subsequently precipitate from the liquids due to the destability of Co complex. Compared with the typical hydrothermal vein-type Co deposits elsewhere in the world,the features of the ore-forming fluids with low salinity and low As but high S compositions may explain the mineral assemblage of the Hengdong Co deposit.

Automated summary

Typical hydrothermal cobalt deposits, such as the Hengdong deposit in Hunan Province, have significant economic potential and research value. Detailed studies on the texture, chemical composition, and sulfur isotopic composition of pyrite in the ores of the Hengdong deposit revealed that the second generation of pyrite is the major cobalt-bearing mineral, and its formation is attributed to the dissolution-precipitation mechanism during the rapid interaction of the cobalt-bearing fluid with the wall rocks and early-formed pyrite. A new model for cobalt enrichment in the deposit is proposed based on high cobalt concentrations in basement rocks and previous microthermometric measurements of fluid inclusions.