Characteristics of the Bumo Gold Field, Hainan Island, China and Gold Exploration Through Soil Geochemical Surveys

Abstract

The Bumo gold deposit is a low‐sulfide quartz vein type deposit, situated in the west of Hainan Island, approximately 25 km southeast of Dongfang City (termed Basuo in alias). Tectonically, the SW‐NE‐oriented Gezhen Ductile‐Shearing Zone was largely attenuated in Bumo. It is hosted by gneisses and schists of Proterozoic age (locally called the Baoban Group), but the radiometric age of ores mainly is Jurassic–Cretaceous, being coeval to the regional metallogenesis along the Eurasian margin. The affinity with the Phanerozoic orogenic gold deposits is indicated by low salinity (2.4 wt% NaCl–10.3 wt% NaCl), intermediate temperature (150–300°C) and appropriate δ18O value (approximately 7–11‰). The gold metal tonnage, proved previously, within an area smaller than 1.0 km2 in the gold belt II amounted to 11.59 t, with the average gold grades for the major Au quartz veins (7 and 8) in this belt of 52.7 and 71.1 g/t, respectively. This shows significant enrichment of gold and an unusual “nugget effect”. A ring structure, mainly represented by ridges, valleys, dikes, big bull quartz veins and lithological borders or pertinent faults, marks the gold field, and its origin was linked to a possible magmatic diapir. It was viewed as macroscopic evidence that the dip angles of strata tend to decline gradually from deeper ones in the center area to nearly horizontal in the outskirts, and certain intrusion‐affiliated elements such as Bi, As and W are considerably abundant in soil samples. Two areas, namely the area A (3.06 km2) in the outskirts of the ring structure or in northeastern Bumo and the area B (1.32 km2) in the field center or to the west of the gold belts I and II, were selected for the geochemical survey in a 50 × 50‐m sampling grid. The analytical results of the samples from horizon C and in a −80 mesh size showed that the average contents of Bi, As and W amounted to 0.55, 19.3 and 3.67 ppm in the area A (1225 samples) and to 0.79, 68.3 and 4.14 ppm in the area B (529 samples), respectively. This geochemical exploration led to the discovery of several Au quartz veins in both areas A and B, which also manifested an Au nugget effect (the average Au grades between 50 and 100 g/t) in spite of their limited scales (from a couple of centimeters to 50 cm wide and from several meters to 15 m long). A mylonite zone in area A was also found by this study. An intimate spatial correlation between a lithological border (gneiss and schist or pegmatite) or the mylonite zone and Au anomalies indicates an important prospect for further gold exploration.

Mesozoic tectonic regime transformation in Fujian Province (Southeast China): Evidence from palaeostress field restoration and lithofacies palaeogeography evolution

“…The Anren Formation shows an angular unconformity contact with the overlying Dakeng Formation (T 3 d), characterized by thick gravel‐bearing coarse sandstone (Figure 2). During the Late Permian to Middle Triassic, granitoids were primarily distributed in Hainan Island and central Hunan Province, with almost no contemporaneous volcanic rocks in Southeast China (Huang & Depaolo, 1989; Li, Li, et al, 2006; Xie et al, 2006; Zhou et al, 2011; He et al, 2020). However, during the Late Triassic, more felsic magmatic rocks, such as monzogranite, formed in Southeast China, while coeval volcanic rocks were rare (Liu et al, 2020).…”

Genesis of Two Types of Carbonaceous Material Associated with Gold Mineralization in the Bumo Deposit, Hainan Province, South China

“…The gangue minerals mainly include quartz, CM, calcite, and sericite (Figure 3C,D). Previous research indicates that the hydrothermal event can be divided into three stages [35], i.e., quartz (Q1)-pyrite, quartz (Q2)-sulfides, and quartz (Q3)-calcite stages, with the second stage as the main mineralization stage. Different from the white quartz (Q1) with euhedral structures (Figure 3E), Q2 grains are generally anhedral (Figure 3F) and its aggregates are commonly grey.…”