One of the special features of Cao Bang Geopark is that, in addition to the outstanding karst landforms, spectacular cave systems and distinctive tectonic evolution and structural features, the composition and types of rocks and minerals are highly diverse. Some of these latter features distinguish this geopark markedly from the Dong Van Karst Plateau UNESCO Global Geopark. The following paragraphs introduce some rock and mineral heritage sites that are of major scientific, educational and conservation importance.
– Ban O tungsten mine, Thanh Cong commune. Coordinates X=589059; Y=2497741; Z=999m asl. The granitic massif of Pia Oac Complex (γKpo) intruded into the host terrigenous-carbonate rocks of Song Hien Formation. Tungsten-bearing quartz veins penetrated the granitic rocks causing hydrothermal alteration. Mining of tungsten ore in Ban O mine was begun by the French in 1908, leaving behind a more than 10m high pile of waste rock which is now re-vegetated.
– Lung Muoi tungsten mine, Quang Thanh commune. Coordinates X=589162; Y=2500481; Z=1604m asl. Similarly, the landscape features an intrusion by the Pia Oac Complex granite into the terrigenous-carbonate rocks of Song Hien Formation. The granite, in turn, is strongly cut by faults, fractures and brecciated zones of different, most popularly the NW-SE and NE-SW, directions. The site was exploited by the French from 1910 to mine the dark grey either disseminated or clustered tungsten ores in the quartz veins that penetrate the granite.
– Tinh Tuc tin mine, Tinh Tuc Town. Coordinates X=586020; Y=2502874; Z=1.037m asl. Here the same granite intruded into the crinoid fossils-bearing limestone of Bac Son Formation (C-Pbs) with subsequent tin-containing greisenization by high-temperature hydrothermal quartz veins. The Tinh Tuc placer tin mine is located in a karst valley covering an area of more than 2km2. Evidence of faulting is obvious in both the quartz veins and the present day landforms e.g. the karst valleys, the white-coloured stepped fault scarps on limestone etc. The site was mined by the French for tin from 1906. It was then mapped, prospected and explored by Vietnamese geologists before
becoming one of the main tin producers of Vietnam until recently.
– Binh Duong fluorite mine, Quang Thanh commune, about 8km to the southwest of Tinh Tuc mine. Coordinates X=586020; Y=2502874; Z=1.037m asl. The mine consists of two parts: Primary ore located on the southern slopes of Pia Oac Mountain; and placer ore (together with tin, tungsten etc.) located in the karst valley at the mountain foot. The primary ore is formed at the brecciated contact between the Pia Oac Complex granite and marbleized limestone of Mia Le Formation (D1ml). Fluorite is in the form of breccia but sometimes gathers into druses.
Considerable percentage of radioactive minerals e.g. Polonium, Radium and even Uranium was found in fluorite which explains the high interest in this mine. The placer ore was mined from the French time. It was subject to exploration by Vietnamese geologists during 1970-1990 and some small scale mining continued until recently but has been halted currently. The primary ore, on the other hand, remains subject to extensive prospecting.
All the four sites mentioned above offer useful information on how granitic magma, under fault control, intruded into and interacted with different host rocks, in the first two cases being terrigenous rocks of Song Hien formation (T1sh), and in the third and fourth cases being limestone of Bac Son (C-Pbs) and Mia Le (D1ml) formations. It is interesting to note post-magmatic activities in the form of hydrothermal quartz veins, cutting through these different environments, resulted in different alterations and mineralizations, in the first two cases being tungsten; in the third case being tin; and in the fourth case being radioactive elements-bearing fluorite, decreasing respectively from high
to low temperature. They all together have been subject to the present day denudation-erosion process which exposes the dome-like granitic massif at high altitude and the peak-cluster depression karst landform and denudation-erosion terrigenous landform at a much lower altitude. Knowledge on placer mining within the karst sinkholes and valleys, as well as adit mining into the primary ore bodies – common extraction techniques in the 1900s – can also be learned along with enjoying other values e.g. beautiful landscape, rich biodiversity and unique ethnic cultures. Remnants of the system of villas and temples built by the French and the local people respectively are of additional historic and cultural values.