Jun-Yi Pan

Spatial distribution and variation of ore body, alteration and ore-forming fluid of the giant Zijinshan epithermal Cu-Au deposit, SE China: implication for mineral exploration

The Zijinshan high-sulphidation Cu-Au deposit located in the west Fujian Province is the largest active gold mine in China with total production of gold over 300 tonnes to date. The high-sulphidation ore bodies are characterized by the upper supergene Au and lower hypogene Cu zones bounded roughly by the paleowater table near 600 m elevation. Au-bearing goethite and limonite are the main ore minerals in the supergene zone whereas the hypogene Cu mineralization is dominated by covellite and anilite (or digenite) with minor enargite. The expected porphyry mineralization that coupled with Zijinshan is so far not exposed even to a depth of 1500 m from surface. However, the Luoboling Cu-Mo deposit which occurs only 3 km NE to Zijinshan is identified as a typical porphyry deposit. Current debate centered on whether the Zijinshan high-sulphidation epithermal deposit and the adjacent Luoboling porphyry deposit are genetically linked or discrete hydrothermal systems. Recent deep exploration at Zijinshan revealed an extension trend of Cu ore bodies and quartz-alunite alteration zone toward SE below 0 m elevation. This contradicts previous considered NE trend above 0 m elevation. In this study, fluid inclusion mapping based on rigorous strategy and sample selection was carried out on 20 sample suites scattered over the whole high-sulphidation Cu-Au ore bodies from −450 to 800 m elevation. Aqueous fluid inclusions of unambiguous primary origin in syn-ore miarolitic quartz, coarse-grained alunite crystals and quartz overgrowth zones were measured. Microthermometry results show the average Th and salinity of each sample suite ranging from 205 to 304°C and 2.4 to 5.5 wt% NaCl equiv., respectively. Both fluid temperature and salinity exhibit negative correlation with increasing elevation, indicating convective cooling by mixing and circulating of meteoric water has involved. The good consistence of mineralization distribution pattern and fluid evolution trend implies that the southeastward extending ore bodies probably reflect the fluid pathway which is connected to the deeper porphyry mineralization and their magmatic source. Considering the temperature condition (370–400°C) required for brittle-plastic transition which commonly characterizes porphyry deposits, the potential porphyry mineralization coupled to Zijinshan should have occurred at greater depth under higher temperature. Combined with recent exploration progress and the fluid evolution pattern, we therefore proposed that the Luoboling porphyry Cu-Mo deposit and the Zijinshan high-sulphidation Cu-Au deposit are likely discrete hydrothermal systems, and future porphyry deposit exploration at the deep SE segment of Zijinshan is recommended.

Geology, mineral paragenesis and fluid inclusion studies of the Yueyang Ag-Au-Cu deposit, South China: implications for ore genesis and exploration

The Yueyang epithermal Ag-Au-Cu deposit is located in the Zijinshan ore district in western Fujian Province, South China. Orebodies are predominately hosted by the Jurassic Zijinshan granite batholith and controlled by a group of NW-trending faults. Roughly all minerals were deposited in a single extended period, and three stages can be further defined: (1) the early sphalerite stage; (2) the base metal-silver stage; and (3) the post-mineralization stage. Microthermometric measurements on quartz- and sphalerite-hosted, liquid-rich fluid inclusions indicate that mixing and dilution of the ore forming fluids took place from early to late stages. However, fluid boiling may locally occur and lead to silver participation, as suggested by the presence of fluid inclusion assemblage consisting of coexisting liquid-rich and vapour-rich inclusions, and also indicated by adularia in Ag-bearing veins and the breccia type ore of bonanza grade. Compared with typical low-sulphidation epithermal deposits, the higher Ag/Au ratios and fluid salinities, Fe-poor sphalerite and large proportion of sulphide minerals suggest that the Yueyang deposit is best classified as an intermediate-sulphidation epithermal deposit. Since intermediate-sulphidation epithermal deposits favour a magmatic affinity and usually exhibit spatial and temporal association with high-sulphidation and porphyry deposits, mineral exploration at the adjacent area of the Yueyang deposit is promising.

Mapping of fluid, alteration and soil geochemical anomaly as a guide to regional mineral exploration for the Dehua gold orefield of Fujian Province, southeast China

The Dehua gold ore-field, located in the central-eastern Fujian Province, SE China, is an important part of the renowned Dehua-Youxi-Yongtai gold ore production area. The ore-field contains several low-intermediate sulphidation epithermal gold deposits and numerous gold-bearing mineralized bodies. The extensive presence of intensely altered rocks, suitable structural environment, acidic intrusions and volcanic-subvolcanic rocks indicates an important potential for epithermal gold mineralization. In this work, fluid inclusion homogenization temperature and alteration mapping was carried out by fluid inclusion microthermometry in quartz and identification of alteration minerals closely related to the known mineralization. The result shows the trend of temperature contour curves as well as the shape of low argillic alteration zones implying that the area north of Qiucun might be profitable for future mineral exploration. The fluid and alteration mapping, in combination with soil geochemical mapping, defines the architecture of the epithermal-porphyry ore-forming system in the Dehua ore-field. Finally, the presence of intensely altered areas, high value zones of hydrothermal fluid flows and combined anomaly areas of multi-element association indicate that the region north of Qiucun has an important potential as a preferred target area for gold prospecting in the Dehua ore-field.

Constraints on the genesis of the Jiande polymetallic copper deposit in South China using fluid inclusion and O-H-Pb isotopes

The Jiande copper deposit is located in the Qin-Hang metallogenic belt, South China. The deposit is dominated by “stratiform-like” ores, which are hosted in the dolomite of the Upper Carboniferous Huanglong Formation. These ore bodies were previously proposed to be Carboniferous sedimentary exhalative (Sedex) style mineralization, but they also appear to be related to the Late Mesozoic granodiorite porphyry at Jiande. Three stages of mineralization can be observed. The prograde skarn minerals garnet and diopside were formed in the pre-ore stage. The “stratiform-like” ores, with minor quartz-polymetallic veins, were formed in the main mineralization stage. The post-ore stage is characterized by quartz-calcite ± pyrite veins. Fluid inclusions in quartz from the pre-ore skarn, the main stage of mineralization, and post-ore quartz–calcite ± pyrite veins were studied. Fluid inclusion petrography shows that two-phase liquid-rich (Type I), two-phase vapor-rich (Type II), and halite-bearing (Type III) fluid inclusions can be identified in the studied hydrothermal quartz samples. Primary Type II and Type III fluid inclusions only occur in quartz from the the main stage, whereas Type I fluid inclusions are present in all three stages of hydrothermal quartz.Type I fluid inclusions in pre-ore stage have homogenization temperatures of 290-368°C and salinities of 2.6-8.8 wt.% NaCl equiv. Type II and coexisting Type III fluid inclusions in the main stage share similar homogenization temperatures of 293 to 334 °C and 290 to 326 °C,but have two contrasting salinity ranges of 1.2 to 2.2 wt.% and 31.87 to 38.16 wt.% NaCl equiv, respectively. The coexistence of Type II and Type III fluid inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred. Type I fluid inclusions in the post-ore quartz–calcite veins have homogenization temperatures of 202-278 °C and salinities of 0.2-6.5 wt.% NaCl equiv. The hydrogen and oxygen isotopes (δD ranges from -78 ‰ to -61 ‰, δ18OH2O calculated from quartz are from 8.1 ‰ to 10.6 ‰) of fluid inclusions in quartz imply that ore fluids were principally derived from magmatic water. The lead isotopes of sulfide from the ores are close to the orogenic Pb evolution curve and are also similar to those of the Jiande Jurassic igneous rocks, suggesting that the metals were sourced mostly from the Jurassic igneous rocks. The skarn alteration, inferred boiling fluid inclusion assemblages, stable isotopic features of magmatic fluids, and metal source imply that the main stages of mineralization in the Jiande polymetallic copper deposit is of skarn-type, related to Jurassic granodiorites.