矿床地球化学国家重点实验室知识库

The Lanping basin is a significant Pb-Zn-Cu-Ag mineralization belt in the Sanjiang Tethyan metallogenic province. A series of sediment-hosted Himalayan Cu-Ag-Pb-Zn polymetallic deposits have been discovered in the western part of the basin, controlled by a thrust-nappe system. In the thrust-nappe system, the Cu orebodies mainly occur in the western and relatively deep part of the mineralization system (the root zone), whereas the Pb-Zn-Ag (+/- Cu) orebodies occur in the eastern and relatively shallow part of the system (the front zone), both as vein-type mineralization.

In this paper we present new data, combined with existing data on fluid inclusions, isotopes and geologic characteristics of representative deposits, to provide the first study that contrasts mineralizing fluids in the Cu-Ag (Mo) and Pb-Zn-Ag (Cu) polymetallic deposits.

Fluid inclusion and isotope studies show that the Cu-Ag (Mo) mineralization in the root zone formed predominantly from deep crustal fluids, with the participation of basinal brines. The deep crustal fluids are marked by high CO2 content, relatively high temperatures (280 to 340 degrees C) and low salinities (1 to 4 wt.% NaCl equivalent), whereas the basinal brine shows relatively low temperatures (160 degrees C to 220 degrees C) and high salinities (12 to 22 wt.% NaCl equivalent), containing almost no CO2. In comparison, hydrothermal activity associated with the Pb-Zn-Ag ( Cu) deposits in the front zone is characterized by basinal brine, with relatively low temperatures (130 degrees C to 180 degrees C), high salinities (9 to 24 wt.% NaCl equivalent), and low CO2 concentrations. Although evolved meteoric waters have predominantly been proposed as the source for deep crustal fluids, magmatic and metamorphic components cannot be completely excluded. The basinal brine was predominantly derived from meteoric water. The delta S-34 values of sulfides from the Cu-Ag (Mo) deposits and Pb-Zn-Ag (+/- Cu) deposits range from -17.9 to 16.3 parts per thousand and from 25 to 11.2 parts per thousand, respectively. These ranges may relate to variations in physicochemical conditions or compositional variation of the sources. Lead isotope compositions indicate that the ore-forming metals were predominantly derived from sedimentary rocks of the Lanping basin.