Geology Gold mineralisation in Bhukia deposit Ore fluid composition on Dharwar Craton Xenotime dating of Meghalayan rocks Trace-element geochemistry of minerals Retrieving age information from rocks Quantitative climate reconstruction
Looking for Bhukia's gold!

Pranjit Hazarika and his colleagues try to explain the origin of the gold mineralisation in the newly discovered Bhukia deposit in the Aravalli-Delhi belt. This work is published in the journal Lithos.

Pranjit Hazarika, Niraj Bhuyan, Dewashish Upadhyay1, Kumar Abhinay1, and N N Singh1
from other institution

Gold mineralisation in the newly discovered Bhukia deposit in northwestern India is hosted in Proterozoic metamorphosed volcano-sedimentary rocks of the Aravalli-Delhi Belt. Three generations of tourmaline occurring in different textural settings are recognised in the host rocks of the deposit. Fine-grained texturally early tourmalines (Tur-I) precipitated during the gold-sulfide mineralization stage. They are dravitic in composition and occur parallel to the S1 tectonic foliation as fine-grained crystals in tourmaline-albite layers within quartz-albite rock and as scattered grains in calc-silicate rocks. Coarse-grained texturally later schorls (Tur-II) are also restricted to calc-silicate and quartz-albite rocks and characterised by sector zoning. Late-stage type III tourmaline (Tur-III), also of schorl composition, replaces Tur-II along fractures and margins. They are interpreted to have formed during a phase of ore remobilisation. All tourmalines, especially the schorls, are strongly enriched in Li, Ga, Mn and Zn with Ga concentrations being the highest ever reported in natural tourmalines (up to 1380 ppm). The boron isotope composition is similar in all three tourmaline types with the consistently light δ 11Β (-10.4‰ to -7.2‰) indicative of a continental source for B. The chemical and B-isotopic composition of tourmaline is suggestive of the involvement of two fluids, a granitic-derived hydrothermal fluid and metapelite-derived metamorphic fluid. The δ 11Β variations in the tourmalines can be explained by mixing between 11B-poor granitic-derived hydrothermal fluid and 11B-rich metamorphic-hydrothermal fluid. While high V (2110-4247, avg. 3339 ppm) in the early dravitic tourmalines indicate mixing of granitic-derived hydrothermal fluid with pelite-derived metamorphic hydrothermal fluids during gold-sulfide mineraliσation, the enrichment of Li, Mn, Zn and Ta, and depletion of V in the later schorlitic tourmalines suggest increasing influence of granitic-derived hydrothermal fluids during ore remobiliσation.

Journal References 

A geological uniformity in Dharwar Craton

Pranjit Hazarika and coworkers report about an amazing uniformity in ore fluid composition of  host rocks of the gold mine area of Dharwar Craton. This research work is published in Geoscience Frontiers.

Biswajit Mishra1, Kamal Lochan Pruseth1, Pranjit Hazarika, and Sakthi Saravanan Chinnasamy1
1from other institutions

Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include (1) the famous Kolar mine and the world class Hutti deposit; (2) small mines at Hira-Buddini, Uti, Ajjanahalli, and Guddadarangavanahalli; (3) prospects at Jonnagiri; and (4) old mining camps in the Gadag and Ramagiri-Penakacherla belts. The existing diametric views on the source of ore fluid for formation of these deposits include fluids exsolved from granitic melts and extracted by metamorphic devolatilization of the greenstone sequences. Lode gold mineralization occurs in struc- turally controlled higher order splays in variety of host rocks such as mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. Estimated metamorphic conditions of the green- stones vary from lower greenschist facies to mid-amphibolite facies and mineralizations in all the camps are associated with distinct hydrothermal alterations. Fluid inclusion microthermometric and Raman spectroscopic studies document low salinity aqueous-gaseous (H2O þ CO2  ± CH4 + NaCl) ore fluids, which precipitated gold and altered the host rocks in a narrow P-T window of 0.7 - 2.5 kbar and 215 - 320 °C. While the calculated fluid O- and C-isotopic values are ambiguous, S-isotopic compositions of pyrite-precipitating fluid show distinct craton-scale uniformity in terms of its reduced nature and a suggested crustal sulfur source.
     Available ages on greenstone metamorphism, granitoid plutonism and mineralization in the Hutti Belt are tantamount, making a geochronology-based resolution of the existing debate on the metamorphic vs. magmatic fluid source impossible. In contrast, tourmaline geochemistry suggests involvement of single fluid in formation of gold mineralization, primarily derived by metamorphic devolatilization of mafic greenstones and interlayered sedimentary rocks, with minor magmatic contributions. Similarly, com- positions of scheelite, pyrite and arsenopyrite point toward operation of fault-valves that caused pressure fluctuation-induced fluid phase separation, which acted as the dominant process of gold precipitation, apart from fluid-rock sulfidation reactions. Therefore, results from geochemistry of hydrothermal min- erals and those from fluid inclusion microthermometry corroborate in constraining source of ore fluid, nature of gold transport (by Au-bisulfide complex) and mechanism of gold ore formation in the Dharwar Craton.

Journal Reference 

Xenotime chemical dating of Meghalayan metamorphic rocks

Pranjit Hazarika and his collaborators validate xenotime (a rare-earth phosphate mineral) chemical dating for metamorphic rocks in the Meghalaya-region. This research work is published in Journal of Earth System Science.

Pritom Borah, Pranjit Hazarika, Amulya Chandra Mazumdar, and Mridul Rabha1
1from other institution


Monazite and xenotime are the two most useful and commonly used geochronometers for deciphering ages from metamorphic rocks. The low analytical cost involved in electron probe micro-analyser chemical dating, ease of sample preparation and abundance in metamorphic rocks of wide P–T conditions make monazite and xenotime dating most widely used technique for age determination amongst metamorphic petrologists. This contribution presents age comparisons between coexisting monazite and xenotime in the basement metapelitic rocks of the central part of the Shillong–Meghalaya Gneissic Complex (SMGC). Thermobarometric estimates in the studied samples indicate granulite facies conditions of metamorphism with peak P–T conditions of ~ 6.5 kbar and -750 ºC. Results indicate that xenotime in the basement rocks in the central SMGC formed in four discrete geological events while monazite either formed only in the latest Pan-African granulite grade metamorphic event or recrystallised during this event. Monazite in the studied samples yielded a single ubiquitous age of ca. 500 Ma. Xenotime in the study area, although found in only one sample, preserves four distinct ages at 1153±29, 930±36, 823±41, and 490±11 Ma. Preservation of Grenvillian ages in xenotime from central SMGC marks the eastward extension of Rodinia amalgamation front in the Indian Shield. The Neoproterozoic ages in xenotime from central SMGC suggest that the ca. 820 Ma high-grade metamorphism in the Eastern Indian Tectonic Zone had a wider impact in the SMGC than perceived previously. 

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.

Pranjit Hazarika, Biswajit Mishra, and Kamal Lochan Pruseth
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 

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.

Pranjit Hazarika, Biswajit Mishra, Manoj Kumar Ozha, and Kamal Lochan Pruseth
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 

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.

V Prasad, T Utescher, A Sharma, I B Singh, R Garga, B Gogoi, J Srivastava, P R Uddandam, and M M Joachimski
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.