Pyrite geochemistry in a porphyry-skarn Cu (Au) system and implications for ore formation and prospecting: Perspective from Xinqiao deposit, Eastern China

Abstract

Stratabound ore has been recognized as an end member of porphyry copper systems, but pyrite chemistry has not been widely applied to linking stratabound ore with the related porphyry and skarn system. Stratabound ore is commonly developed around porphyry-skarn systems in eastern China, and is characterized by abundant colloform pyrite; however the origin of the colloform pyrite has been contentious. Xinqiao deposit is ideal for study of pyrite geology and geochemistry with the aim of elucidating formation of the stratabound ore and to decipher the evolution of pyrite compositions in a porphyry-skarn environment. The colloform pyrite paragenesis and S isotopes indicate that it formed during early skarn mineralization, based on its occurrence in stockwork veins cutting skarn minerals, and that it is replaced by later hypogene sulfides; the δ 34 S of colloform pyrite (average 6.12‰) is close to the δ 34 S value of both porphyry-(average 5.06‰) and skarn-type pyrite (average 4.65‰).The colloform texture formed as an aggregate of nanometer-or micrometer-sized (< 0.2 µm) pyrite cubes produced by rapid crystallization from a high fS 2 , low temperature, and supersaturated fluid. Supersaturation of the fluid was probably produced by rapid decompression that triggered fluid boiling and cooling when the magmatic-hydrothermal fluid (derived from Cretaceous magma) flowed along the Devonian-Carboniferous unconformity.Subsequently, the colloform pyrite was replaced by later stage pyrite with distinctive trace elements (Co, Ni and Se), indicating that the stratabound ore at Xinqiao formed from multiple pulses of magmatic-hydrothermal fluids derived from an Early Cretaceous stock.Cobalt, Ni and Se enrichment in porphyry-and proximal skarn-type pyrite suggests they formed at relatively high temperature, whereas the colloform pyrite shows trace element This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Enrichment of Se-Te-Au in the Jilongshan Au-Cu Skarn Deposit, Hubei Province: Insight from Pyrite Texture and Composition

“…The distribution of trace elements in pyrite is affected by temperature, pressure, pH, redox conditions, and other factors. Variations in these physicochemical conditions can lead to different enrichment degrees of ore-forming elements in fluids [13][14][15]41]. According to the textures and compositions of different generations of pyrites, we can retrospectively analyze the evolution of pyrite, and the variations in the concentrations of trace elements in pyrite at different stages, especially Se, Te, and Au, further indicate the enrichment process of these elements.…”

“…Trace elements occur in pyrite in two different forms: (a) substitution into the crystal lattice; and (b) nano-to-micro-scale mineral inclusions [6][7][8]. Based on the composition, concentrations, and correlation of trace elements in pyrite, some significant insights are proposed to define the possible enrichment mechanism and ore-forming processes (e.g., magma degassing, fluid boiling, and water-rock interaction) of Se, Te, and Au [6][7][8][9][10][13][14][15].…”