| 其他摘要 | The Lala copper deposit is located in the Kangdian Rift, western margin of the Yangtze Block, and its copper mineralization occurs in the marine metamorphic volcanic-sedimentary rocks of the Paleoproterozoic Hekou Group. This deposit is the most economically significant copper deposit in Sichuan Province, and is also one of the typical volcanic-hosted massive sulfide (VHMS) deposits which was affected or reconstructed by metamorphic fluid in China. In combination with previous work, systematic studies on field investigation and sampling, mineral chemistry by electron microprobe, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, trace element (including REE) and isotope geochemistry (Rb-Sr, Sm-Nd, S) were conducted to demonstrate the metamorphic conditions of regional metamorphism, the protoliths of the metamorphic rocks in the LaLa copper deposit, the geochemical characters of mafic dyke, the characters of wall rocks and ores, sources of sulpher element, mineralization epoch, as well as the mineralization mechanism of this deposit. Some important conclusions are obtained in this study as follows:
(1) The emplacement age of the metavolcanic rocks of the Hekou Group is about 1695±20Ma, and the parental magmas were contaminated by the older Paleoproterozoic crust. During 700-800Ma, the Hekou Group experienced regional metamorphism and deformation.
(2) On the basis of the analytical results of widespread garnet-biotite schist in the Lala deposit by electron microprobe, the calculated metamorphic temperature is 530-580℃ and the upper limit of pressure is 0.66-0.76GPa, using garnet-biotite geothermometer and geobarometry of Si in phengite. This metamorphic event mainly corresponds to high greenschist facies metamorphism.
(3) The wall rocks and ores are mianly mica schist and albite rocks in the Lala copper deposit, which are the products of regional metamorphism during Neoproterozoic. The protoliths of these metamorphic rocks were reconstructed by element geochemistry and isotope geochemistry of the representative rocks in this deposit, using DF fuction, (Al/3-K)-(Al/3-Na) diagram and La/Yb-TR diagram. The protoliths of mica schist of the Hekou Group were mainly shale rocks, which came from the weathering of the continental arc and upper crust; whereas the protoliths of albite rocks were volcanic rocks, including “right-slope type albitite” and “flat type albitite”, both of which were derived from an Enriched Mantle and contaminated by crust.
(4) Tectonic setting discrimination of the metasedimentary and metavolcanic rocks suggests that they were formed in a continental back-arc environment.
(5) Based on the characteristics and petrogenesis of the metavolcanic rocks, and sulfur isotope compositions, the possible concentrated mechanism of the ore-forming elements in this deposit was discussed. The chalcophile elements and sulfur from the mantle entered into the parent magmas which subsequently ascended into the crust and formed extensive volcanic rocks in this area.
(6) Mafic dykes in this deposit, exhibiting similar compositions to the intra-plate alkaline basalt, were generated by a high-temperature mantle plume. During emplacement into the crust, the magmas parental to these dykes were mainly contaminated by the upper-crustal material. The characters of mafic dykes indicate that the enrichment of ore-forming elements in the Lala area was strongly affected by the breakup of the Rodinia super-continent.
(7) According to previous studies and our research, we conclude that the Lala copper deposit was formed during the two main ore-forming stages: volcanic exhalative-sedimentary stage (1700Ma) and metamorphic-hydrothermal stage (700- 800Ma). Two main ore types were discovered in this deposit, namely, the “volcanic ore type” and “sedimentary ore type”. The “volcanic ore type”, mainly in the volcanic rocks, occurs as banded and striped in structure, which were formed during the former stage; the “sedimentary ore type” dominantly in the sedimentary rocks, exhibiting massive and disseminated structures, were generated during the latter stage. |
修改评论