Mineralogical and geochemical characteristics of scheelite-bearing skarns, and genetic relations between skarn mineralization and petrogenesis of the associated granitoid pluton at Sargipali, Sundergarh District, Eastern India

Abstract

Skarn rocks occur at the contact between calcite-bearing dolomitic marbles and granitoids (massive varieties with pegmatites) in close spatial association with the mica schist-hosted Proterozoic Pb–Cu–Ag sulfide deposits at Sargipali, Sundergarh District, Eastern India. The exoskarn (pyroxene–garnet) of variable width (1 to 30 m) occurs in marble proximal to the granitic intrusion, and endoskarn (pyroxene–epidote) is variably developed (< 1 to 10 m). Molybdenum-free scheelite with minor pyrrhotite (0.2%) is found only in late garnet–clinopyroxene exoskarn assemblages. In the Sargipali area early regional and contact metamorphism converted impure carbonate lithologies to calc-silicate hornfels. Subsequent contact metasomatism formed the skarn rocks, which are well zoned geochemically, mineralogically, and texturally in the sequence pyroxene–garnet–amphibole from the calcite-bearing dolomitic marble to granitoid contact. The presence of zoning relative to igneous contacts indicates that skarn-forming fluids originated from the crystallizing magma. The skarns are composed of clinopyroxene, garnet, calcic amphiboles (K-rich ferropargasite, ferrohornblende, hastingsite, tschermakite, magnesiohornblende, and actinolite), wollastonite, plagioclase, potash feldspar, epidote, titanite, and quartz. The skarns are notably enriched in Al, Mg, and Fe. Garnets are grossularite–almandine with 9 to 10 mol% spessartine, whereas pyroxenes are hedenbergitic to diopsidic in composition. The variable Mg:Mn:Fe proportions in the skarn clinopyroxene suggest the formation of clinopyroxene compositions from relatively homogeneous fluids, which experienced local variations in their Mg:Mn:Fe proportion instead of from successively different compositions. The earliest hornfels assemblage (Stage I) formed initially above 500 °C. This was overprinted by prograde anhydrous skarn (Stage II) at about 500 °C–600 °C and of 3–4 kbar pressure in a mildly reducing environment under X(CO 2) = ~ 0.18. With increasing fluid/rock interaction, epidote, green amphibole ± quartz-bearing retrograde skarn (Stage III) formed as temperature decreased to approximately 480 °C at X(CO 2) = 0.05. Late hydrothermal alteration (Stage IV) caused the formation of actinolite. There is a correlation between intrusion composition and the metal contents of associated skarns. Calc-silicate mineral compositions in the Sargipali skarns are similar to those in other W skarn systems. This granitic complex is comprised of reduced, highly evolved, and metallogenically specialized S-type leucogranites, comparable to those commonly associated with Mo-poor W skarns. A syn-collisional tectonic setting is proposed, based on field evidence, the relative timing of the intrusions with respect to metasedimentary and carbonate rocks, and empirical trace-element geochemical evidence. Based on field evidence and geochemistry, two main intrusive phases have been recognized in the Sargipali granitoid pluton: (1) an undeformed massive granite in the west, and (2) a foliated granite along the eastern margin. Porphyritic granites are also recognized locally, which are older than the other units. A genetic link exists between granite magmatism, and the formation of pegmatites in the region. The granite–pegmatite system is highly peraluminous (Al-saturation index ranges from 1.2 to 1.8). The peraluminous character increases from the foliated and porphyritic granite through massive granite to pegmatite. The foliated granite has higher FeO t, TiO 2, MgO, Ba, Sr, Zr, Th, ∑ REE (~ 200 ppm), and lower SiO 2 contents than massive granite (∑ REE = ~ 22 ppm). Both of these granite phases are highly evolved, while the massive variety is more evolved, and is mostly dominated by variable source rock composition. The minimum crystallization temperature of granite magma is at 628 °C–695 °C and greater than the 3 kbar pressure. The granites may have been generated by partial melting of metasedimentary rocks of the Gangpur Group that might have been enriched in W. The exsolving W-rich magmatic fluids interacted extensively with carbonate rocks and formed disseminated scheelite mineralization. New CHIME U–Th–Pb monazite dates for foliated granites of Sargipali are 960 ± 10 Ma, while massive granite shows two ages; the U-poor population and U-rich population yielded 955 ± 15 Ma and 997 ± 13 Ma, respectively. These ages indicate that the deformed foliated granite is slightly older than the undeformed massive granite, while there exists a still older intrusion (porphyritic granite) that is syn- to post-kinematic to the major phase of deformation (D 1) of the enveloping country rock at ~ 1 Ga. Overall, the Sargipali granitoid pluton post-dates the syngenetic Pb–Cu–Ag sulfide mineralization in the region at ~ 1.69 Ga.