Study On The Ore Deposit Geochemistry Of The Dachang Polymetallic Tin Orefield Dispersed Elements In Guangxi

The well-known Dachang tin-polymetallic ore field in Guangxi province is particularly rare in the world due to abundant dispersed elements and huge reserve. In this paper, based on the field study, a comprehensive investigation was carried out on the element geochemistry, stable isotopes, and distribution and occurrence state of dispersed elements in ores, ore-bearing wall rocks and mineral separates in this area, using chemical analysis, inductively coupled plasma-mass spectrometry (ICP-MS) and electron microprobe. In addition, a systematic summarization was made on the occurrence state and enrichment regulation of dispersed elements (e.g. In, Cd, Ga and Ge), and the genesis of Tin-polymetallic deposits.The analysis on concentrations of dispersed elements and trace elements in sphalerite and ores was also conducted by using ICP-MS, revealing that the dispersed elements (i.e. In, Cd, Ga and Ge) in the Dachang tin-polymetallic ore field mainly exist in sphalerite as isomorphism, and Ga possibly enters the crystal lattice by micro-absorption.The amount of Fe in sphalerite simply takes an important effect on the color of the deposits. However, by the accurate ICP-MS testing on the amount of trace elements in sphalerite with different colors, we ensure that the amount of Fe is not the only determinative factor for the color of sphalerite and Ge can be extraordinarily enriched in dark sphalerite.A preliminary summary on the enrichment regulation for In, Ge and Ga in the Dachang tin-polymetallic ore field is given and potentially favorable locations for the enrichment of dispersed elements are pointed out as well, on the basis of the study of geochemical behaviour of In, Ge and Ga and the distribution features of such elements in different ores and ore bodies.The mineral deposits in the Dachang tin-polymetallic ore field are considered to represent products of the same tectonic-magmatic thermal event but at different stages. In combination with the close spatial association of the Longxianggai biotite granite and Dachang tin-polymetallic ore field, the formation ages of rocks and mineral deposits, and the characteristics of trace and rare-earth elements in the Longxianggai granite and ores from the mineral deposits, the sulfur and lead isotopic analyses on sulfides in the rocks and ores of different mineralization types reveal that sulfur in those ore bodies of different types comes from the same source as well as lead, and that the isotopic variation trend of those mineral deposits is intimately related to the ore-forming process, similar to that of the ore-forming fluid migrating from the bottom to the top and from the center to the sides.Based on the polygenetic compound theory, in combination with the study of regional geology, the geological characteristics of the ore field and mineral deposit geochemistry, this study discusses the constraint from geological structures and ore-bearing wall rocks on the mineralization and presents that the Dachang tin-polymetallic ore field genetically belongs to polygenetic compound type relevant to magmatic-hydrothermal superimposition and modification.