Geology


https://sites.google.com/a/gauhati.ac.in/research/geology/#ph27052018-1 Trace-element geochemistry of minerals

https://sites.google.com/a/gauhati.ac.in/research/geology/#ph27052018 Retrieving age information from rocks

https://sites.google.com/a/gauhati.ac.in/research/geology/#bkg24052018 Quantitative climate reconstruction
Ore genetic implications of refractory sulfides

Dr Pranjit Hazarika and his fellow researchers study trace-element geochemistry of pyrite and arsenopyrite minerals and report about ore genetic implications for late Archean orogenic gold deposits in southern India. This study is published in Mineralogical Magazine.



Authors
Pranjit Hazarika, Biswajit Mishra, and Kamal Lochan Pruseth
Abstract
This study demonstrates coupled behavior of Au and chalcophile elements (Ag, Cu, Te, Sb, Bi and Pb) in the Hutti and Hira-Buddini late Archean orogenic lode gold deposits. The incorporation of Au into pyrite in such deposits is most likely a function of mineral fluid interactions without any effect of arsenic content. Distribution of Au and associated trace elements in pyrite and arsenopyrite from late Archean Hutti and Hira-Buddini orogenic gold deposits, eastern Dharwar Craton, southern India was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). X-ray element maps acquired by electron probe micro analyzer (EPMA) reveal oscillatory zoning of Co and As indicating the crystallization of pyrite and arsenopyrite in an episodic fluid flow regime in which fluid salinity fluctuated due to fault-valve actions. The absence of any relationship between Au and As in pyrite obviate the role of As in the incorporation of Au into pyrite particularly here and may be generally the case in orogenic gold deposits. On the other hand, positive correlations of Au with Cu, Ag and Te suggest possible influence of these chalcophile elements in the enhanced gold concentration in sulphides. Pb-Bi-Te-Au-Ag bearing micro-particles (< 2 micron) are exclusively observed in micro-fractures and pores in arsenopyrite. The absence of replacement features and element gradient suggests direct precipitation of Pb, Bi, Te, Au and Ag from a fluid that was unreactive towards arsenopyrite. Intermittent fall in fluid pressure caused by the fault-valve action would have resulted in the sporadic precipitation of Au, Pb, Ag, Bi and Te.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/geology/#top 
 



Retrieving age information from rocks

Dr Pranjit Hazarika and his fellow researchers report about a new and low-cost method to retrieve age information from detrital, diagenetic and low to high-T metamorphic, as well as magmatic rocks. This study is published in the journal Chemie der Erde.



Authors
Pranjit Hazarika, Biswajit Mishra, Manoj Kumar Ozha, and Kamal Lochan Pruseth
Abstract
EPMA U-Th-Pbtotal dating in U- and Th bearing minerals (e.g., monazite, zircon, and xenotime) is a low-cost and reliable technique used for retrieving age information from detrital, diagenetic and low to high-T metamorphic, as well as magmatic rocks. Although, the accuracy on measured ages obtained using EPMA is considered to be poor compared to isotopic ages, the superior spatial resolution, ability to integrate textural and age information by in-situ measurement, lack of sample damage and easier and cheaper data generation in EPMA makes chemical dating a very valuable tool to decipher diverse petrological processes. This contribution presents an improved analytical protocol to obtain precise estimates of U, Th and Pb concentrations in xenotime. Results were tested on monazite standard (Moacyr pegmatite, Brazil; TIMS age: 486 to 488 Ma) as the reference material. The proposed analytical protocol has been successfully applied to achieve an analytical uncertainty of less than 10% in U, Th and Pb measurements in xenotime. The protocol was further used to resolve polygenetic xenotime ages (ca. 1.82, 1.28 and 0.93 Ga) in metapelite samples from the Mangalwar Complex, Northwestern India. Monazites in the same samples were also analyzed and found to preserve the two younger ages (i.e., ca. 1.28 and 1.0 Ga). The obtained ages from the xenotime and monazite very well corroborate with the earlier published ages from the area validating the proposed analytical protocol. This improved analytical protocol underscores the application of xenotime chemical dating in metamorphic rocks using readily available electron micro-probes.

Journal Reference
https://sites.google.com/a/gauhati.ac.in/research/geology/#top 
 



Quantitative climate reconstruction across Paleocene-Eocene from low latitude

Dr Bikash Gogoi and his collaborators present a multi-proxy study of an upper Paleocene - Lower Eocene succession from the paleo-equatorial region. The study is carried out on a coal-bearing, shallow-marine succession exposed at Jathang, East Khasi hills of Meghalaya. This research article is published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.



Authors
V Prasad, T Utescher, A Sharma, I B Singh, R Garga, B Gogoi, J Srivastava, P R Uddandam, and M M Joachimski
Abstract
The authors present a multi-proxy study of an upper Paleocene-lower Eocene succession from the paleo-equatorial region. The study is carried out on a coal-bearing, shallow-marine succession exposed at Jathang, East Khasi Hills, Meghalaya, northeastern India. The succession was deposited in a low-energy, coastal marsh-bay complex. Dinoflagellate cyst biostratigraphy yields a late Paleocene to early Eocene age for the section. The deposits of the lower part of the succession represent a transgressive systems tract (TST) defined by seven parasequences, each starting with bay sediments deposited during transgression, followed by a shallowing-upward bay fill-marsh deposit.

The main highlights of the work are 
(a) Quantitative climate reconstruction across Paleocene-Eocene from low latitude
(b) Distinct vegetation turnover in tropical region across Paleocene Eocene transition
(c) Lower plant diversity during Paleocene and significantly high during early Eocene
(d) Duration of rainfall pattern mainly determines the climate of tropical rain forest.