The gold river of India: Is it myth, history, or real geology

In India, many river names tend towards the mythical, the colour-based, or the regional, but geology-based nomenclature is not very common. However, the Subarnarekha River is an exception in this regard. Passing through mineral-rich areas of Jharkhand, West Bengal, and Odisha, and named as such because it literally means "streak of gold," the name remains from ancient times and is as much evidence of the geology underlying the river basin as anything else. As the domains of mineral systems and ancient continental geology continue to attract the interests of researchers and scholars, the name Subarnarekha is back on the modern map once again due to its significance that is not only descriptive but also scientifically explicable, combining the process linked with the crust and the behaviour of gold moving through river sediments.The Subarnarekha River traverses the eastern part of the Indian Shield, which existed over two and a half billion years ago.The Indian Shield is estimated to hold some of the oldest continental rocks on earth, with significant geological deformation due to tectonic pressure over time.Inside this geological setting is the North Singhbhum fold and thrust belt, where there is significant structural complexity and hydrothermal activity.Such environments are well-established zones where gold prefers to accumulate, especially in environments where it is found in quartz veinings with sulphide minerals.A scientific paper published in Ore Geology Reviews presented detailed information about invisible gold in pyrite, which existed in quartz reefs in Babaikundi, with detailed microscopic and chemical analysis.Thus, there appeared to be a scientific basis to explain why the river is auriferous in nature.Contrary to traditional goldfields, where gold grains can easily be distinguished by the naked eye, gold within the Subarnarekha goldfield is dominantly present on a microscopic scale. Invisible gold, then, is gold that is dispersed among the mineral lattice structures of rocks like pyrite, rather than being present within grains. Invisible gold is characteristic of metamorphic regions that have existed for a long period of time and have given gold-carrying streams access to host rocks through gentle heat and pressure. At the Subarnarekha goldfield, gold-carrying streams occur within conduits called quartz reefs, and the host rock is mainly composed of pyrite. Invisible gold cannot be extracted through mere visual examination, but tests can identify the existence of gold within the surrounding rocks despite their non-descript nature.The rock formations exposed at the surface do not remain unchanged over time. They are subject to physical and chemical breakdown, and hydrological cycles, and will be eroded even if they are very hard rocks. In the Subarnarekha drainage, monsoon rains, which can be quite intense, are very effective at accelerating this process of breakdown. The quartz reefs, where pyrite ore is present, get cracked, oxidised, and transported down the slope as sediments. The gold present in the decomposed form of pyrite, now broken down, remains very fine in nature. The sediments have a high density and do not undergo further chemical breakdown, and are transported from the slope to the rivers because of their high density.After gold is liberated from the bedrock, its behaviour is different from that of the lighter mineral grains. The heavy gold tends to settle at sites where the flow of water is slower, like river bends and confluence areas. After a while, gold tends to accumulate at particular sediment sites, even where there is little gold. For example, the reason why gold deposits have always been found in river sand, and not on the rocky side, is that the river flows, and natural processes have naturally sorted gold from other minerals. The flooding of the river, on the other hand, tends to distribute gold dust throughout the region.Well before any formal geological theory or paradigm was established, there was recognition in the Subarnarekha of the distinctive character of its sands. Travels and accounts from the past speak of local gold washings, especially when the water was low in dry seasons. Typically, using wooden trays or small pans to separate heavier material from beach sands along riverfronts. The colonial authorities and geologists noted these activities and gold extractions from the river, when in reality it was never a source of major gold rushes. Nonetheless, these activities only served to confirm local knowledge systems long attuned to identifying the river with gold deposits. The importance of the Subarnarekha has transcended the limited value of gold deposits. The river watershed is significant in understanding ancient gold mineralisation and invisible gold deposits. Knowledge about gold deposits and mobilisation is valuable in modern gold mining exploration. This is with regard to topographical regions where visual mineralisation cannot be identified. Environmental aspects are also encompassed in understanding sulphide minerals and gold deposits. Weathered sulphide minerals affect water and sediment. In this case, the river represents an actual record of deep Earth processes in surface environments.The name of this river has retained significance in relation to geological aspects identifiable in nature.Also Read | This island country makes visitors sign a promise at the airport; here’s why