Abstract
Geochemical and petrological characteristics of a lamprophyre dyke at Kalagalla intruded into the auriferous schistose rocks of the Ramagiri-Penakacherla Schist Belt, Anantapur district, Andhra Pradesh, India are presented here. The Kalagalla lamprophyre (KGL) is a melanocratic rock exhibiting typical knobbly or pustular texture on the rock surface. The micro-textures and mineralogy typical of lamprophyres are obscured by metamorphism; however, it exhibits porphyritic, nemato-granoblastic texture representating greenschist facies metamorphism. The rock is sheared and carbonatised possessing several globules formed by polycrystalline aggregates of calcite rimmed by coronitic subhedral plagioclase and biotite-phlogopite, evidencing its magmatic origin. The mineral assemblages noticed in thin-sections include amphibole, plagioclase, biotite-phlogopite with calcite ocelli as essential while apatite, zircon, magnetite, ilmenite and several opaque minerals as accessory phases. The SEM-EDS investigation on the opaque minerals revealed accessory sulphide and silicate phases like As-free pyrite, haematitised Ni-bearing chalcopyrite and Ni-As-Co- minerals indicative of sulphidation associated with greenstonevauriferous lodes; along with oxide phases like LREE-bearing titanite partially transformed into leucoxene and goethite. Presence of amphibole and dominance of plagioclase, it is presumably a spessartite variety. Based on its geochemistry, the rock is classified as a shoshonitic calc-alkaline lamprophyre. The LREE-bearing titanite appears to be the contributor of LREE enrichment. The high Mg# (77-79), Ni (153-162 ppm) and Cr (380-470 ppm) support a mantle source. The trace and REE patterns (ƩREE:326-343 ppm, LREE>HREE) indicate ivolvement of residual garnet at the source enriched in phlogopite in a ‘subduction-related’ environment. The absence of Eu anomaly reflects lack of plagioclase fractionation. The high Zr/Hf ratio (163-202) indicates carbonatite metasomatism and lack of crustal contamination. Further investigations are in progress.
Highlights
Geochemical and petrological characteristics of lamprophyre dykes at Kalagalla intruded into the auriferous schistose rocks of the RamagiriPenakacherla Schist Belt, Anantapur district, Andhra Pradesh, India are presented here
The scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS) investigation on the accessory minerals revealed accessory sulphide and silicate phases like As-free pyrite, haematitised Ni-bearing chalcopyrite and Ni-As-Co- minerals indicative of sulphidation associated with greenstone auriferous lodes, along with silicates like LREE-bearing titanite partially transformed into leucoxene and oxide phases like magnetite altered to goethite at places
Feldspars occurring in the fine-grained average composition of groundmass feldspars groundmass (n=6) and those occurring at the peripheries of calcite ocelli (n=7) have been is Or1.84Ab86.26An11.90 while that of feldspars associated with calcite ocelli in the Kalagalla lamprophyre (KGL) is analysed
Summary
ABSTRACT Geochemical and petrological characteristics of lamprophyre dykes at Kalagalla intruded into the auriferous schistose rocks of the RamagiriPenakacherla Schist Belt, Anantapur district, Andhra Pradesh, India are presented here. Several lamprophyres have been reported that an extremely close genetic relation exists worldwide as occurring in association with between calc-alkaline (shoshonitic) gold-bearing greenstone belts Geochimica Brasiliensis 34(1): 1 - 27, 2020 sediments and volcanic to sub-volcanic rocks lamprophyre magma during crystallization has such as tholeiitic basalt, komatiite, arc-related been postulated as a possible source of gold in basalt, boninite, adakite, siliceous high Mg orogenic lode deposits Owing to the close spatial association continental masses and significant from between lamprophyre dykes and gold the economic perspective (e.g. SCARROW et mineralization, devolatilisation of shoshonitic al. 2011)
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