Metallogeny of the Yi’nan Tongjing Au–Cu skarn deposit, Luxi district, North China Craton: Perspective from in-suit trace elements, sulfur and lead isotopes of sulfides

Abstract

Gold–Cu skarn deposits are characterized by a diverse mineral assemblage, whose in-situ major/trace elements and isotope compositions can provide key constraints to the migration and enrichment of Au during hydrothermal processes. The Yi’nan Tongjing Au–Cu deposit is located in the central part of the Luxi district, and both skarn and Au–Cu ore bodies occur at the contact between the Early Cretaceous diorite porphyry and the Neoproterozoic to Cambrian carbonate rocks. Five stages of mineralization were identified: 1) early skarn (garnet–diopside–wollastonite); 2) late skarn (magnetite–epidote–actinolite±tremolite); 3) oxide (specularite–hematite); 4) sulfide (pyrite–chalcopyrite–sphalerite–quartz–chlorite); and 5) late quartz–calcite. The mineralization process in the Tongjing Au-Cu deposit was revealed by detailed scanning electron microscope-backscattered electron imaging, electron probe microanalysis, in-situ trace element, sulfur and lead isotope analysis. Magnetite is enriched in chalcophile elements (Cu, Zn, Pb), Co and Ni, probably due to hydrothermal overprint. The substitution of As and other elements in the formation of pyrite is conducive to the entry of Au into pyrite. The increase of Se and As contents in pyrite from stage IVa to IVb indicates that the temperature, salinity and oxygen fugacity of the ore-forming fluid decrease while the pH rises, resulting in the unloading of Au. The temperature of Au mineralization based on the Se content in pyrite does not exceed 300°C. Furthermore, V positively correlated with Ti and Ni/Cr ratios ≥1 in magnetite and most Co/Ni ratios in pyrite >10 all confirm their hydrothermal origins. The restricted sulfur (δ34SV-CDT = −0.5–1.2‰; mean = 0.4‰) and lead (206Pb/204Pb = 17.323–17.383; 207Pb/204Pb = 15.424–15.452; 208Pb/204Pb = 37.367–37.454) isotopic compositions suggest that the deep magma provided the primary mineralized material, accompanied by a relatively small amount of shallow crustal material. The Yi’nan Tongjing Au–Cu skarn deposit was formed in the Early Cretaceous, which is an important metallogenic response to the strong decratonization of the North China Craton induced by the paleo-Pacific Plate roll-back. This study shows that there is a large potential of Early Cretaceous skarn mineralization in the Luxi district.

Genesis of the Yi’nan Tongjing Gold–Copper Skarn Deposit, Luxi District, North China Craton: Evidence from Fluid Inclusions and H–O Isotopes

“…To the east and west of the Luxi Block lie the Tan-Lu and Liaokao faults, respectively (Figure 1b) [31][32][33]. It is composed of a Precambrian crystalline basement, Phanerozoic sedimentary cover, and Mesozoic magmatic rocks (Figure 1b) [5,33]. The Precambrian crystalline basement mainly comprises the Mesoarchean Yishui group, Neoarchaean Taishan group (amphibolite and biotite granulite with minor TTG gneisses [34,35]), and Paleoproterozoic granitoids [36,37].…”

“…The Changqing group (Zhushadong and Mantou formations) and Jiulong group (Zhangxia and Gushan formations) consist mainly of sandstone and shale intercalated with thick-bedded dolomite and (dolomitic) limestone (Figure 3a; [15]). These strata extend in a NW-SE (300 • ) direction and dip 10-30 • SW [5]. Major structures include NNE-, NW-, and SN-trending faults (Figure 2) affected by the Tan-Lu fault.…”

“…These deposits demonstrate a widespread global distribution and a close association with magmatism. The latter aspect provides essential materials and heat for mineralization [3][4][5]. Skarn Au-Cu deposits, despite being less frequent than porphyry Cu-Au deposits, often exhibit elevated grades of gold and copper [6], thus garnering considerable attention from economic geologists [4,[6][7][8].…”

“…The Luxi district, situated in the eastern part of the North China Craton (NCC) (Figure 1a), is well-known for hosting several skarn deposits containing Fe and Au-Cu ores (Figure 1b). These deposits demonstrate a close association with early cretaceous intermediate magmatic rocks such as diorite and diorite porphyry [5,[11][12][13]. However, the factors regulating the precipitation of Au-Cu by magmatic fluids in this area are not comprehensively understood.…”

“…The Yi'nan Tongjing Au-Cu deposit, located in the central-southern part of the Luxi district (Figures 1b and 2), constitutes a typical skarn deposit that provides a promising avenue to explore the interplay between magmatic/hydrothermal fluid evolution and Au-Cu mineralization. It has attracted significant scholarly attention and represents an exemplary Au-Cu deposit in the Luxi district [5,[14][15][16][17][18][19][20][21]. Despite past research efforts, a comprehensive understanding of the evolution path of ore-forming fluids and crucial information pertaining to the physicochemical conditions of mineralization is still lacking.…”