Gossan-rich monument a forgotten age-old site

Udaipur: Rajpura-Dariba, situated about 77 km from Udaipur city, is a multi-metal deposit that is presently under production. The area is also a National Geological Monument (NGM) as it has the unique and rich 'gossan' found nowhere else. However, the NGM for its location in a remote, isolated area having no other attraction for the tourists, is a least known geo-heritage site.
"None other zinc-lead or copper deposits of southern Rajasthan are capped with such impressive gossan zone as they are hosted in acid neutralizing carbonate rocks.This area has a 4.5 km long and 2-40 m thick gossan zone - an indicator of a rich base metal deposit under it - which was prospected and explored by the Geological Survey of India during 1963-1973," informs Dr P S Ranawat, an earth scientist and retired professor of Sukhadia University. After exploration, the area was handed over to Hindustan Zinc Ltd for its utilization which is still going on which includes its underground mining, milling, and smelting. Large heaps of slag and numerous underground old-workings indicate a thriving ancient mining activity here too. Carbon dating indicates an age of approx 2040 years for Rajpura-Dariba.

Status and remedies -
The area falls in the lease of Hindustan Zinc Ltd which is employing underground mining by cut-and-fill method. An explanatory engraved marble slab is installed at the site that has geologic map, name and composition of primary and secondary minerals that are present in the area. "A printed leaflet was distributed at the time of its dedication with a befitting function in 1977 that was attended by a large number of eminent earth scientists. But they were never reprinted after their stock ran out. A better upkeep of the NGM stone inscription is expected of the revenue earning lease holder, who can also develop an Interpretation Center explaining oxidation of sulfide ores, details of the primary and secondary mineral and boxwork structures of the area and details of ancient and present-day mining methods," Ranawat said.What is Gossan?
Gossan is an oxidized cap formed due to extensive chemical weathering and pervasive oxidation of the sulfide ore body through prolonged geological periods under favorable climatic and non-reactive host rock conditions. In fact, the Rajpura-Dariba multi-metal deposit was discovered because of this gossan, which is a favorable guide to search an underground ore deposit. The chief primary ore minerals at this deposit are sphalerite, galena and chalcopyrite with minor quantities of several ore minerals.