The Effects of Metamorphism on the Lead-Zinc Deposits of Rajasthan

Abstract

Metamorphism of any mineral deposit easily changes the grades of the ores at that deposit. Metamorphism of an ore deposit can even produce a newer deposit of a proportional grade. Here we try to use the examples of the lead and zinc deposits of Rajasthan, to explain enrichment of elements other than lead and zinc, caused because of metamorphism. Most of the major mineral deposits in the Earth’s crust have undergone some degree for metamorphism. It is essential that we study the effect of that metamorphism on the given orebody so that it may be mined sustainably. This understanding of zones in an orebody of distinct grades can lead to their easier extraction and will lead to sustainable mining practices which are safer as well as economic.

Introduction

Mineral Wealth and Geology of Rajasthan:

• Rajasthan is rich in mineral resources, especially lead (Pb), zinc (Zn), copper, tungsten, and silver, with over 89% of India’s Pb-Zn reserves.

• Key Pb-Zn mines: Kayad (Ajmer), Rampura-Agucha (Bhilwara), Sindesar-Khurd & Rajpura-Dariba (Rajsamand), and Zawar (Udaipur).

• These deposits lie within different geological supergroups (Delhi, Aravalli, and Bhilwara).

• Most are SEDEX-type (sedimentary exhalative) except Zawar, which is Mississippi-Valley-type.

II. Ore Mineralogy and Metamorphism:

• Main ore minerals: Sphalerite (ZnS), Galena (PbS), with other sulphide/sulfosalt minerals like pyrite, chalcopyrite, etc.

• Metamorphism varies:

  • Zawar: Greenschist facies

  • Others: Amphibolite facies; Rampura-Agucha may have reached granulite facies.

III. Sulphide Partial Melting and Remobilization:

• Partial melting can occur at relatively low temperatures in sulphide-rich rocks (~300°C).

• Melting leads to remobilization of metals, which can alter ore grades or form daughter deposits.

• Evidence of partial melting is hard to detect due to the behavior of sulphide melts.

• Rajpura-Dariba is a key example where metamorphic conditions (~580–600°C, 6.1 kbar) suggest possible sulphide partial melting.

IV. Ore Grade Comparisons and Effects:

• Comparison of Pb, Zn, and Ag grades shows that:

  • Deposits that have undergone sulphide partial melting often show higher silver content.

  • This is due to silver substituting for lead in the ore mineral structures.

  • Cadmium is another common by-product, substituting for zinc.

Key Takeaways:

• Metamorphism and partial melting play a major role in shaping grade and distribution of Pb-Zn-Ag ores in Rajasthan.

• Understanding these geological processes helps improve mineral exploration and extraction strategies.

• Rajasthan remains India’s leading state for Pb-Zn mining, supported by its diverse geology and mineralizing processes.

Conclusion

India stands sixth in the world reserves of zinc and seventh in lead. These are base metals which are of essential mineral importance. The process of partial melting of ore deposits can alter, enhance and /or reduce the grade of an ore deposit. Considering all other lead-zinc deposits of Rajasthan, apart from the Zawar Deposit, all have undergone metamorphism above or till the mid-amphibolite facies there for should have undergone sulphide partial melting. The enrichment of silver at Sindesar-Khurd is a testament to ore enrichment during sulphide partial melting. Sulphide partial melting is also characterized by the presence of low melting-point chalcophile elements. These elements consist of silver, arsenic, bismuth, mercury, selenium, antimony, tin, thallium and tellurium. Of these elements, bismuth, selenium, antimony, tin, and tellurium are listed as minerals of critical importance for India. Cadmium a by-product of smelting is also a critical mineral. [12]. It is important the sulphide partial melting is studied for an ore deposit for if it is enriched in any critical mineral a strategic way for further extraction and mining of the deposit can be carried out sustainably.

References

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Copyright

Copyright © 2025 Rachit Parihar, Vijay Pal Meena. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.