Search

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.

Several large high-grade iron skarn deposits in the Laiwu district of Shandong Province (North China Craton) are developed along the contact zone between the Early Cretaceous Kuangshan diorite pluton and Middle Ordovician dolomitic limestone. In sharp contrast, no iron skarn mineralization has been recognized in association with other contemporaneous dioritic intrusions (Jiaoyu, Jinniushan, Tietonggou) in this district. The reason for this contrast remains elusive. Here we present a comparative study of the orerelated Kuangshan pluton and its barren equivalents using whole-rock and mineral (zircon, apatite) geochemistry combined with existing Sr-Nd isotope data to provide new insights into the critical controls on iron skarn mineralization and shed light on future mineral exploration. Geochemical and Sr-Nd isotope data suggest that both the ore-related and barren intrusions were derived from a common enriched lithospheric mantle source following variable degrees of fractional crystallization and crustal assimilation. Modeling based on whole-rock Sr-Nd isotopes implies that the magmas responsible for the Kuangshan and Jinniushan diorite plutons assimilated significant quantities of the Ordovician evaporite-bearing carbonates. Both the fertile and barren diorites show high wholerock Sr/Y ratios, high zircon Eu/Eu* and Eu/Eu*/Y ratios, and low zircon Dy/Yb ratios. These data, combined with high magmatic water contents (4.4-8.0 wt%) calculated from amphibole compositions, indicate that those four plutons crystallized from hydrous magmas. High whole-rock Fe2O3/FeO, and high ΔFMQ values (mostly >1; fayalitemagnetite-quartz oxygen buffers) estimated from zircon, apatite, and amphibole suggest that the magmas from which the diorite intrusions crystallized were all oxidized. Using available partitioning coefficients for Cl between apatite and magma, the parental melts of the Kuangshan and Jinniushan diorite are estimated to have had higher Cl contents (0.15-0.77 wt%) than the Jiaoyu and Tietonggou diorites (0.06-0.34 wt%). Two types of apatite from the intrusions studied are recognized based on their paragenetic relations: Type 1 apatite (Ap1) is an early-formed phase enclosed in euhedral amphibole, plagioclase, and biotite, whereas type 2 (Ap2) is a late-formed variety hosted in, or intergrown with, anhedral K-feldspar (± anhedral amphibole ± subhedral to anhedral quartz). In the fertile Kuangshan diorite, Cl and OH contents of apatite decrease, but Sr contents increase from Ap1 to Ap2. However, those parameters show reverse trends or remain constant from Ap1 to Ap2 in barren diorites. Such distinctive apatite geochemical signatures are best interpreted as reflecting more efficient fluid exsolution from the parental magma of the Kuangshan diorite than from the barren intrusions. This view is supported by the presence of abundant fluid inclusions in apatite and quartz exclusively from the mineralized intrusion, indicating its crystallization from volatile oversaturated melts. The results presented here indicate that elevated magma Cl contents and efficient separation of magmatic hydrothermal fluid from cooling magmas have been the critical factors controlling iron skarn mineralization in the Laiwu district. Mass balance constraints on Cl budget indicate that a minimum of 44-60 km3 of magma was required to form the Fe deposits associated with the Kuangshan pluton. Our preliminary results suggest that decreasing OH and Cl contents from early to late apatite in intermediate rocks can be used as an indicator for efficient fluid exsolution and consequently metal fertility of those rocks. [ABSTRACT FROM AUTHOR]