CARBONIC FLUID OVERPRINTS IN VOLCANOGENIC MASSIVE SULFIDE DEPOSITS: EXAMPLES FROM THE KELAN VOLCANOSEDIMENTARY BASIN, ALTAIDES, CHINA

Abstract

The Tiemurte and the Dadonggou Zn-Pb-(Cu) volcanogenic massive sulfide (VMS) deposits, situated in the Devonian Kelan volcanosedmentary basin of the south margin of the Chinese Altaides, were metamorphosed and overprinted by metamorphic sulfide-quartz veins during Early Carboniferous to Early Permian. Two mineralizing periods of ore mineral growth can be identified: (1) disseminated, banded, and massive sulfide ores related to a primary depositional sea-floor volcanic-hydrothermal activity; and (2) the foliated sulfide-quartz veins (Q1) related to synorogenic metamorphism and late chalcopyrite-bearing quartz veins (Q2) cutting the schist related to a younger metamorphic overprinting event. Carbonic (CO 2 -CH 4 -N 2 ) fluid inclusions are ubiquitous in Q1 and Q2 veins. A few carbonic fluid inclusions may be primary and some may be pseudosecondary, whereas the vast number of carbonic fluid inclusions are secondary, representing later events. A microthermometry study shows that primary carbonic fluid inclusions in Q1 and Q2 have Tm CO2 ranging from −64.5° to about −59.4°C with Th CO2 = −13.4° to about +18.6°C. The secondary carbonic fluid inclusions exhibit two behaviors when cooling and heating: the Tm CO2 of the first group (L CO2 ) ranges from −63.3° to −57.7°C, and that of the second group (L CO2-CH4-N2 ) ranges from −83.4° to −61°C. The second group of carbonic fluids has much higher CH 4 and/or N 2 proportions than the first group. The trapping temperatures for the carbonic inclusions have been estimated to be 243.1° to 412.1°C (for Tiemurte) and 216° to ~430°C (for Dadonggou) on the basis of some L CO2 -L H2O inclusions associated with carbonic inclusions, and the trapping pressures have been estimated to be 120 to ~340 MPa, which are consistent with deformation P-T conditions of quartz, and slightly less than the P-T conditions of the biotite and garnet metamorphic zones. These abundant carbonic inclusions at the Tiemurte and the Dadonggou deposits were not a part of a volcanogenic ore-producing system but represent a much younger event, possibly having originated from a synorogenic metamorphism which may have contributed to orogenic gold.