Mineralization Of Skarn Fe-Cu Deposits In Nixiong Orefield, Coqen, Tibet

The Nixiong ore field, located in the central part of western Gangdise, was researched in detail in this paper. Base on the previous research and detail field investigation the writer focusing on the analysis of the ore-forming geological background, the deposit characteristics, ore types, ore characteristics, ore texture and structure, minerals sequences of the Gunjiu Fe deposit and Ri'a Cu deposit. The typical skarn mineral, geochemical characteristics and the cause of the intrusion related, characters and sources of the ore fluids of the two deposits was also analyzed. Finally, the writer discussed the metallogenic mechanism and gave a model of the two deposits.The iron ore of Gunjiu Fe deposit occurred in the contact belt which is between monzogranite, granodiorite and Xiala, Dibucuo Formation of Permian, some were also located in the interlayer fracture zone of Xiala Formation and Dibucuo Formation. The orebody was controlled by the NW-NWW fracture structure, including extrusion pushover structural belt, interlayer sliding surface, inner sliding surface and intrusive interface. The main skarn minerals such as grossular, andratite, diopside, sahlite, ferrosalite, serpentine, phlogopite, actinolite, epidote, chlorite were developed in Gunjiu iron deposits, were attributed to calc-Magnesium skarn.The copper deposits in Ri'a mainly occurred in the skarn and skarnization marble, which were laid between the biotite granite, monzogranite and Xiala Formation of Permian. The most important structure is intrusive interface. The main skarn minerals in Ri'a copper deposit were grossular, andratite, diopside, serpentine, phlogopite, actinolite, humite, epidote, chlorite, sericite and K-feldspar etc, which pertained to magnesium skarn.The monzogranite and granodiorite related to the mineralization of Gunjiu Fe deposit are enriched in silica and alkali (the average of SiO2 are 70.98% and 66.76%, the average of Alk are 6.28% and 7.7%), aluminum saturation index are less than 1.1.Both of them belong to medium-high K calc-alkaline granite. There show similar chondrite-normalized REE patterns and primitive mantle normalized trace element spectrum, low ?REE (134.28 and 130.61) and high ?Eu (0.88 and 0.84), with (La/Yb)N value of 6.72 and 8.07. There also enriched in LILEs(Rb?K?Pb?Th) and LREE, depleted in Ba and HFSEs (U?Nb?Ta? P?Ti), and have characteristic of arc volcanic rocks. Ziron LA-ICP-MS U-Pb weighted mean age of 112.6±1.6 Ma and 113.16±1.2 Ma. The characteristic of geochemistry indicated that both rocks were formed from a mixed magma between the partial melting materials of the upper crust and asthenospheric mantle in extensional environment.The biotite granite and diabase-porphyrite in Ri'a Cu deposit was outputted in the form of bimodal pulse. The biotite granite is high in SiO2(72.91%?79.47%) and Alk (the average of Alk is 8.57%),but poor in MgO (the average value is 0.35%). The SiO2 in diabase-porphyrite is 49.56%?54.9%. Both of the rocks are belong to high K calc-alkaline granite and/or shoshonite. There also showed similar chondrite-normalized REE patterns. There are enriched in LREE and poor in HREE, but different in ?Eu. The 8Eu of biotite granite is 0.6, and 0.93 of diabase-porphyrite. The biotite granite is enriched in LILEs (Rb?K?P?Sr?Th), but depleted in HFSEs (Nb?Ta?P?Ti). The diabase-porphyrite is enriched in LILEs (Rb?K?P?Th) depleted in HFSEs (Nb?Ta). There are have the characteristic with arc volcanic rocks. Compared to biotite granite, the diabase-porphyrite show no loss of Ti and P, but the content of Th?U is low. It may indicate that both of them are from different origin. Zircon LA-ICP-MS U-Pb dating showing that the rocks formed in 87.2±1.6 Ma and 89.9±1.6 Ma. The characteristic of geochemistry also indicated that the rocks were formed from a mixed magma between the partial melting materials of the crust and fulids in extensional environment.The ore-forming fluids are pertained to the NaCl-H2O of Gunjiu Fe deposit and Ri'a Cu deposit. In the early stage of skarnization, the ore-forming fluids was in high temperature (488?558?) and high salinity(39.8?44.3 wt% NaCl) of Gunjiu Fe deposit. When it came to quartz sulfide stage(middle to late of the mineralization stage), the temperature dropped to 256?328? and the salinity reduced to 35.3?39.8 wt% NaCl. At last the temperature and the salinity down to 188?251?and 4.0?20.0 wt% NaCl in the carbonate stage after mineralization. In dry skarn stage of Ri'a Cu deposit, the ore-forming fluids was in supercritical state(>570?) and with high salinity(40.6?48.5 wt% NaCl). To quartz sulfide stage (the mineralization stage), the temperature dropped to 238?316? and the salinity reduced to 0.9?19.7 wt% NaCl. Calcite was mainly formed in the carbonate stage, which has relatively low temperature (184? 236?) and salinity (0.9?6.7 wt% NaCl). According to skarn minerals, fluid inclusions and laser Raman analysis of both deposits we can infer that ore-forming fluids contain Na+?Ca2+?K+?CO32-?SO42+?Cl-? F- and so on.The H, O isotope indicated that the the water of the ore fluids came from magmatic water and mixed with some meteoric water. The value of ?DV-SMOW‰ is -121‰ and ?18OV-SMOW=8.5‰ in garnet The DV-SMOW and ?18OV-SMOW are -129‰?-107‰ and 4.7‰?6.7‰ in magnetite. The epidote is with the DV-SMOW value of -90‰ and ?18OV-SMOW of 5.7‰. The DV-SMOW and ?18OV-SMOW are -124‰?-105‰ and 15.9-17.1‰ in quartz. In Ri'a Cu deposit, the author yielded the value of DV-SMOW-128‰?-97‰ and ?18OV-SMOW 6.3?8.4‰ in garnet, DV-SMOW=-107‰ and ?18OV-SMOW=1.0‰ in magnetite, DV-SMOW=-84‰ and ?18OV-SMOW=7.4‰ in phlogopite, DV-SMOW -181‰?-172‰ and ?18V-SMOW 5.7‰?6.5‰ in serpentine, DV-SMOW=-110‰ and ?18OV-SMOW=6.4‰ in epidote, DV-SMOW -82‰?-62‰ and ?18OV-SMOW 12.0‰?12.9‰ in quartz. The C of ore-forming fluids mainly came from magmatic hydrothermal. The value of ?13CV-PDB is range from -2.4‰?-0.5‰ with average of -1.47‰ in Gunjiu Fe deposit, In Ri'a Cu deposit the ?13Cv-PDB is -4.1‰.?34SV-CDT have an average of 8.36‰ (range from 4.2‰ to 11.1‰)in Gunjiu Fe deposit and 4.58‰ (range from 2.1‰ to 5.8‰) in Ri'a Cu deposit, also show the characteristic of granitic magma. So, we can summarize that the Gunjiu Fe deposit and Ri'a Cu deposit have similar stable isotope composition; both are derived from the granitic magma. But there are some differences, compared with the Gunjiu Fe deposit, mantle material has larger influence on the ore-forming fluids and minerals of Ri'a Cu deposit.The mixed magma of crust and mantle which control the formation of Gunjiu Fe deposit was intrusion in late Early Cretaceous (113Ma). Simultaneity, a large number of high temperature and high salinity fluids were differentiated from the mixed magma, which were enriched in mineral (Fe and Cu) element. Because of the contact of ore-forming fluids and limestone, the acidic components in the ore-forming fluids and reaction with the limestone. Meanwhile meteoric water continuing joined in and fluids boiling lead to SO2, H2S, HCl, HF separated from the liquid into the gas, which led to the pH value of fluids increased. At last, the minerals (Fe) which migrated in the form of halides and complexes decomposed and precipitation into deposit. The mixed magma of crust and mantle which control the formation of Ri'a Cu deposit was intrusion in the early Late Cretaceous (90Ma). It was experienced similar early evolution to the Gunjiu Fe deposit. The temperature and pressure reduced rapidly, the pH value increased and the redox changed of the ore-forming fluids after the precipitation of Fe. At last, the Cu became precipitation.