Evidence of Hydrothermal Fluid, Escaping CO2 & Black Shale Interaction From Rhyacian PaleoProterozoic China

Fluid evolution of the Paleoproterozoic Hujiayu copper deposit in the Zhongtiaoshan region: Evidence from fluid inclusions and carbon-oxygen isotopes

Authors:

Jiang et al

Abstract:

The Zhongtiaoshan region, located in the southern margin of the North China Craton, is the host to a number of metamorphosed sediment-hosted stratiform copper deposits. These deposits are hosted by dolomitic marble and silicic albitite of the mid-Paleoproterozoic Zhongtiao Group and contain economically significant copper and cobalt. The formation of these deposits is considered to be closely associated with the evolution of the Paleoproterozoic “Zhongtiao” rift. The Hujiayu deposit, the second largest Cu deposit of this type, is mainly hosted in silicic albitite and dolomitic marble in the upper part of the Bizigou Formation but locally extends into carbonaceous shales at the bottom of the Yujiashan Formation. Mineralization of the Hujiayu Cu deposit can be divided into an early stage (diagenetic stage) with disseminated to veinlet sulfides and a late stage (metamorphism stage) with coarse-vein sulfides. Mineral assemblages are similar for the two stages, with major minerals as chalcopyrite, pyrite, and pyrrhotite and main gangue minerals as dolomite and quartz. Sulfide veinlets formed in the early stage are thin and discrete, and have irregular boundaries with the host rocks; whereas the ore-bearing veins of the late stage are controlled by fractures.

Five types of fluid inclusions are recognized in the Hujiayu Cu deposit and they are: (1) pure vapor and vapor-rich inclusion (V-type), (2) pure CO2 inclusion (PC-type), (3) CO2-H2O inclusion (C-type), (4) liquid-rich inclusion (L-type), and (5) daughter mineral-bearing inclusion (S-type). Microthermometric analysis shows that the ore-forming fluids of the early mineralization stage are characterized by high salinity (22-40 wt.% NaCl equiv.) and moderate temperature (120-280 °C). The ore-forming fluids of the late mineralization stage are characterized by CO2 enrichment, high salinity, high temperature and underwent significant unmixing at a temperature interval of 240 °C to 480 °C. Compositions of the ore-forming fluids in the early and late stages are interpreted to be mainly basinal brine and metamorphic hydrothermal solution, respectively. Carbon and oxygen isotope compositions suggest possible carbon isotope exchange between the ore-forming fluids and organic-rich carbonaceous shales during the early stage. In the late mineralization stage, both degassing of CO2 and isotopic exchange with organic carbon may have contributed to the formation of the more negative δ13CV-PDB values of mineralized carbonates.

The early stage mineralization of the Hujiayu Cu deposit may have occurred via interaction of oxidized Cu-bearing brines from the red-bed in the lower segment of the Bizigou Formation with the overlying reductive carbonaceous shales. Late stage mineralization at Hujiayu was mainly triggered by CO2 escaping from metamorphic hydrothermal solutions.