Metallogeny of the Ergu Fe-Zn polymetallic deposit, central Lesser Xing'an Range, NE China: Evidence from skarn mineralogy, fluid inclusions and H-O-S-Pb isotopes

The Lesser Xing'an Range is an important skarn Fe-Zn metallogenic area in Northeast China. To reveal the metallogenic characteristics and attributes of the skarn deposits and establish geological model for deposit formation and prospecting, we selected the Ergu skarn Fe-Zn polymetallic deposit for comprehensive study, including detailed field observation and petrographic, mineralogy, fluid inclusions (FIs), and H-O-S-Pb isotope analyses. The ore bodies can be divided into magnetite, magnetite-sphalerite, pyrrhotite-pyrite-arsenopyrite, copper polymetallic and galena-sphalerite ore bodies. These ore bodies hosted in the contacts between the granodiorite and carbonate rocks of the lower Cambrian Qianshan Formation. The ore-forming processes involved two metallogenic periods-the skarn period (I) and quartz-sulfide period (II), including six stages. The prograde skarn stage (I-1) is characterized by garnet and pyroxene. The retrograde skarn stage (I-2) is dominated by epidote, actinolite, chlorite, hornblende and a few magnetite. The oxide stage (I-3) is typified by abundant magnetite, with lesser amounts of epidote, quartz, molybdenite and pyrrhotite. The quartz-Fe-Cu sulfide stage (II1) is enriched in pyrite, pyrrhotite, arsenopyrite, chalcopyrite and quartz. The quartz-Pb-Zn sulfide stage (II2) is enriched in galena, sphalerite and quartz. The quartz-carbonate stage (II3) is typified by the formation of quartz, calcite and fluorite. Electron probe microanalysis (EPMA) showed that the garnets are composed of reddish-brown to yellowish-green andradite (And(63.7 similar to 939)Gro(1.15 similar to 34.6)Ura + Pyr + Spe(1.55 similar to 5.00)) and brown grossular (And(22.4 similar to 48.1)Gro(46.0 similar to 74.9)Ura + Pyr + Spe(1.82 similar to 3.22)) (Ura-Uvarovite, And-Andradite, Pyr-Pyrope, Spe-Spessartine, Gro-Grossular). The pyroxene is composed of light green diopside (Di(81.2 similar to 93.7)Hd(5.30 similar to 17.2)Jo(1.03 similar to 1.98)) (Di-Diopside, Hd-Hedenbergite, Jo-Johannsenite); the epidote has a high pistacite component (Ps value) (Fe3+/(Fe3++Al) = 0.09-0.18) and Fe3+/Fe2+ value (1.17-4.67). These skarn mineral assemblages record the transition from weakly acidic to alkaline, and an oxidized condition for the skarnization process. The FIs mainly include daughter mineral-bearing three-phase (S-type), liquid-rich (W1-type) and vapor-rich (W2-type) FIs. Laser Raman analysis suggest that the gas phase composition of FIs are dominated by H2O and CO2, and a few CH4. Based on fluid inclusions (FIs) and H-O isotopes studies, the skarn period was formed at high temperature (341-467 degrees C), low to high salinity (1.22-40.2 wt%) and originated from magmatic-hydrothermal fluids (delta O-18(H2O) = 2.58 parts per thousand to 10.2 parts per thousand, delta D-H2O = -70.7 parts per thousand to -63.8 parts per thousand). Fe-Cu sulfide-forming stage (stage II1) shows clear meteoric water input, as constrained from the delta O-18(H2O) (8.25 parts per thousand to 11.7 parts per thousand) and delta D-H2O (-114 parts per thousand to -106 parts per thousand) values. In this stage, the ore-forming fluids belong to a H2O-NaCl-CO2 +/- CH4 system with moderate to high temperatures (247-360 degrees C) and low salinities (1.22-7.15 wt%). This system lasted until stage II2 with moderate-high temperatures (193-306 degrees C) and low salinities (1.56-4.17 wt%). Finally, in stage II3, the fluid transitioned into a homogeneous H2O-NaCl system with low to moderate temperature (169-263 degrees C) and low salinity (1.39-4.48 wt%). The sulfur isotope (delta S-34 = 2.2-4.6%) and lead isotopic (Pb-206/Pb-204 = 18.0-18.6, Pb-207/Pb-204 = 15.5-15.6, Pb-208/Pb-204 = 37.8-38.2, similar to granodiorite) compositions of sulfides indicate that the ore-forming materials were mainly derived from granodioritic magma. Based on the above research results, we propose that the Ergu deposit is a typical Fe-Zn skarn deposit and the main mechanisms of mineral precipitation were fluid boiling and cooling.