Geochemical constraints on Cu-Fe and Fe skarn deposits in the Edong district, Middle-Lower Yangtze River metallogenic belt, China

摘要

Copper and iron skarn deposits are economically important types of skarn deposits throughout the world, especially in China, but the differences between Cu and Fe skarn deposits are poorly constrained. The Edong ore district in southeastern Hubei Province, Middle-Lower Yangtze River metallogenic belt, China, contains numerous Fe and Cu-Fe skarn deposits. In this contribution, variations in skarn mineralogy, mineralization-related intrusions and sulfur isotope values between these Cu-Fe and Fe skarn deposits are discussed. The garnets and pyroxenes of the Cu-Fe and Fe skarn deposits in the Edong ore district share similar compositions, i.e., dominantly andradite (Ad_(29-100)Gr_(0-68)) and diopside (Di_(54-1000)Hd_(0-38)), respectively. This feature indicates that the mineral compositions of skarn silicate mineral assemblages were not the critical controlling factors for variations between the Cu-Fe and Fe skarn deposits. Intrusions associated with skarn Fe deposits in the Edong ore district differ from those Cu-Fe skarn deposits in petrology, geochemistry and Sr-Nd isotope. Intrusions associated with Fe deposits have large variations in their (La/Yb)_N ratios (3.84-24.6) and Eu anomalies (8Eu = 0.32-1.65), and have relatively low Sr/Y ratios (4.2-44.0) and high Yb contents (1.20-11.8 ppm), as well as radiogenic Sr-Nd isotopes (εNd(t) = -12.5 to -9.2) and (~(87)Sr/~(86)Sr)_i = 0.7067 to 0.7086. In contrast, intrusions associated with Cu-Fe deposits are characterized by relatively high Sr/Y (35.0-81.3) and (La/Yb)_N (15.0-31.6) ratios, low Yb contents (1.00-1.62 ppm) without obvious Eu anomalies (8Eu = 0.67-0.97), as well as (~(87)Sr/~(86)Sr)j = 0.7055 to 0.7068 and sNd(t) = - 7.9 to - 3.4. Geochemical evidence indicates a greater contribution from the crust in intrusions associated with Fe skarn deposits than in intrusions associated with Cu-Fe skarn deposits. In the Edong ore district, the sulfides and sulfates in the Cu-Fe skarn deposits have sulfur isotope signatures that differ from those of Fe skarn deposits. The Cu-Fe skarn deposits have a narrow range of δ~(34)S values from - 6.2‰ to + 8.7‰ in sulfides, and + 13.2‰ to +15.2%.. in anhydrite, while the Fe skarn deposits have a wide range of δ~(34)S values from +10.3‰ to + 20.0‰ in pyrite and +18.9‰ to + 30.8‰ in anhydrite. Sulfur isotope data for anhydrite and sedimentary country rocks suggest that the formation of skarns in the Edong district involved the interaction between magmatic fluids and variable amounts of evaporites in host rocks.