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Ravikant, Vadlamani
- Rb-Sr Direct Dating of Pyrite from the Pipela VMS Zn-Cu Prospect, Rajasthan, NW India
Authors
1 Institute Instrumentation Center, Indian Institute of Technology, Roorkee – 247 667, IN
2 Geological Survey of India Training Institute, Jaipur – 302 004, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 77, No 2 (2011), Pagination: 149-159Abstract
In unaltered volcanogenic massive sulfide (VMS) ore deposits, variable Rb/Sr ratios in the ore mineral permits application of the Rb-Sr isotopic method to directly date the time of ore formation. In contrast, post-crystallization deformation and metamorphism would open the system to metamorphic fluids that would alter elemental ratios. To test whether the Rb-Sr isotopic systematics in the ore minerals had preserved the formation time in the ∼800 Ma metamorphosed VMS ores within the ∼1 Ga Ambaji-Sendra arc terrain, Rajasthan, NW India, common sulfides, pyrite and sphalerite from the Pipela Cu-Zn prospect, were analyzed for their geochemistry and Rb-Sr isotopic systematics. Trace and rare earth elements in these minerals are resident probably at crystal defects, whereas all inclusions (including those from metamorphic fluids) were removed by a simple crush leach method. Results of direct dating by the Rb-Sr method to the hydrothermal pyrite yielded an isochron age of 1025 ± 76 Ma with an initial Sr ratio of 0.7051 ± 0.0006, similar to previously determined zircon U-Pb age of 987 Ma from associated rhyolites. This suggests the applicability of the crush leach method to date formation time of metamorphosed pyrite ores.Keywords
Hydrothermal Pyrite, Rb-Sr Dating, Pipela VMS Cu-Zn Prospect, Rajasthan.
- Petrology and Geochemistry of the Grubergebirge Anorthosite and Marginal Rocks, Central Dronning Maud Land: Further Characterization of the Late Neoproterozoic Magmatic Event in East Antarctica
Authors
1 Geochronology and Isotope Geology Division, Geological Survey of India, 15 A&B Kyd Street, Kolkata – 700 016, IN
2 Antarctica Division, Geological Survey of India, NH-5P, NIT, Faridabad - 121001, IN
3 Airborne Mineral and Surveys and Exploration Wing, WR, Geological Survey of India, Jhalana Dungri, Jaipur, IN
4 National Centre for Antarctic and Ocean Research, Headland Sada, Vasco-da-Gama, Goa - 403 004, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 78, No 1 (2011), Pagination: 7-18Abstract
The Grubergebirge anorthosite, a Late Neoproterozoic massif-type anorthosite, was emplaced into Late Mesoproterozoic-aged metamorphosed orthogneisses and supracrustal rocks in the Wohlthat Mountains. Mineralogically and chemically, the marginal rocks to the anorthosite massif classify as ferromonzodiorite and ferromonzonite.
Variations in trace and rare earth element abundances and normalized patterns between the anorthosite and associated marginal ferromonzodiorite (and minor ferromonzonite) and distinct differences between the ferromonzodiorite and ferromonzonite have been observed. Whereas the magmas from which the anorthosite crystallized have been slightly contaminated by incorporating crustal material (now occurring as enclaves), the marginal ferromonzodiorite represent rocks that originated due to mixing (hybridization) of injected primitive ferrodiorite magma(s) with preexisting crustal material, resulting in the hybrid ferromonzodiorite.
These events at the margin of the massif anorthosite represent a widespread Late Neoproterozoic magmatic event with accompanying crustal mixing; all these rocks were thereafter metamorphosed under amphibolite- to granulite-facies conditions during the Early Cambrian Period. In contrast, unmetamorphosed dykes with a ferrodiorite mineralogy, of Late Cambro-Ordovician age, have been found that probably represent a part of the late magmatic suites of the south Petermannketten (Zwiesel).