Abstract:

The Mingshan gold deposit is related to NWW-trending faults that parallel to the regional Youjiang fault and hosted within the Middle Triassic calcareous argillaceous siltstone. Field work, petrography, cathodoluminescence, back scatter electron imaging and electron microprobe data indicate that hydrothermal minerals in Mingshan gold deposit can be classified into three forming stages. Most of these minerals have multiple generations. Hydrothermal minerals are featured by such as ductile intragranular characteristics as undulatory extinction, banded extinction, Behm lamellae, pressure solution of cleavage and dislocation glide. In the adit, pyrite-bearing quartz veins are characterized by ductile reformation and cut by later quartz veins, which indicates the deposit are influenced by multiphase brittle-ductile reformation. Au and As contents in the pyrite formed in different stage have regular patterns, i.e., there are lower Au and As content in the sedimentary pyrite. In contrary, there is higher Au content and lower As content in diagenesis pyrite. All three generation hydrothermal pyrites in the ore-forming period have abundant Au and As. The fact that the As content in relict pyrite in the core of banded pyrite is higher than those in diagenesis pyrites indicates that the relict pyrite is different in origin from diagenesis pyrite. The relict pyrite maybe belongs to the metamorphic genesis. As and Au contents in the diagenetic pyrite from country rocks near orebody decrease from center to edge, which suggests that elements of As and Au in the ore may have been derived from the country rocks. Based on these data, it can be deduced that there developed gold pre-enrichment in diagenetic stage, and enrichment of As element is caused by organic metamorphic fluid during the early reformation and metamorphism. During the major metamorphic period, wide infiltration metasomatism of the fluid caused the activation of Au and As from the gold-bearing and arsenic rich pyrite, which resulted in the formation of ore-forming fluid. When the ore-forming fluid met carbon calcareous argillaceous siltstone bearing the active iron, gold precipitated accompanying with the pyrite. These suggest that the metamorphic fluids may have played very important role in the formation of the Mingshan gold deposit.

Key words: Carlin gold deposit, hydrothermal minerals, microfabric, metamorphism, ore-forming process, northwestern Guangxi