Metallogeny of the Handagai skarn Fe-Cu deposit, northern Great Xing'an Range, NE China: Constraints on fluid inclusions and skarn genesis

- Skarn mineralogy shows the Handagai is a typical calcic- skarn Fe-Cu deposit.
- The deposit belongs to medium-high temperature and medium-low salinity type.
- The ore-forming fluid experienced reduced pressure boiling, cooling and mixing.

The newly discovered Handagai skarn Fe-Cu deposit is located in the northern Great Xing'an Range of NE China and is hosted by the Ordovician Luohe Formation. The orebodies that form the deposit are generally concordant with the bedding within these sediments, and are spatially related to areas of skarn development. The Fe-Cu mineralization in this area records four stages of paragenesis, namely prograde skarn, retrograde skarn, quartz-sulfide, and quartz-carbonate stages. The Handagai deposit is a calcic skarn that is dominated by an andradite-diopside-epidote-actinolite assemblage. The mineralogy and geochemistry of the skarn indicate that it formed from a hydrothermal fluid that altered the carbonate units in this area to a garnet (And42-95Grs4-53) and pyroxene (Di71-78Hd22-29Jo0-2) bearing skarn. The epidote within the skarn has an epidote end-member composition, with the chlorite in the skarn dominantly Fe-rich, indicating that these minerals formed in an Fe-rich environment. The petrographic, microthermometric, and Raman spectroscopic analysis of fluid inclusions within garnet, epidote, actinolite, quartz, and calcite precipitated at different stages of formation of the Handagai deposit indicate that mineralization-related fluid inclusions are either liquid-rich two-phase H2O-NaCl (type I), gas-rich two-phase H2O-NaCl (type II), three-phase (liquid + vapor + solid) H2O-NaCl (type III), or CO2-H2O-NaCl inclusions (type IV). The early stages of mineralization are associated with all four types of inclusion, whereas the later stages of mineralization are only associated with type I and II inclusions. Inclusion homogenization temperatures vary between the four stages of mineralization (370 °C-530 °C and > 600 °C, 210 °C-290 °C, 190 °C-270 °C, and 150 °C-230 °C, from early to late, respectively), with salinities also varying between the earlier and later stages of mineralization (11-18 and > 45, 7-15, 6-9, and 3-7 wt.% NaCl equivalent (equiv.), respectively). The majority of the inclusions within the Handagai deposit have homogenization temperatures and salinities of 200 °C-350 °C and 4-14 wt.% NaCl equiv., respectively, indicating that this is a medium-high temperature and medium-low salinity type deposit. The fluid inclusions were trapped at pressures of 11 to 72 MPa, corresponding to depths of 0.4 to 2.9 km. The geology, mineralogy, geochemistry, and fluid inclusions microthermometry indicate that the Handagai deposit formed as a result of contact infiltration metasomatism, with the deposition of ore minerals resulting from a combination of factors that include boiling as a result of reduced pressure, cooling, and fluid mixing.