scholarly journals Pyrite and pyrrhotite in a prograde metamorphic sequence, Hyland River region, SE Yukon: implications for orogenic gold

2021 ◽  
Author(s):  
C D W Padget ◽  
D R M Pattison ◽  
D P Moynihan ◽  
O Beyssac
Keyword(s):  
Raman Spectroscopy ◽  
Phase Equilibrium ◽  
Source Region ◽  
Carbonaceous Material ◽  
Orogenic Gold ◽  
Metamorphic Rocks ◽  
Small Interval ◽  
River Region ◽  
Gradual Release ◽  
Fluid Release

The distribution of pyrite and pyrrhotite is documented within an andalusite-sillimanite type (high-temperature, low-pressure) metasedimentary succession exposed in the Hyland River region of southeastern Yukon, Canada. The following metamorphic zones are recognized: chlorite, biotite, cordierite/staurolite (porphyroblast-in), andalusite, sillimanite, and K-feldspar + sillimanite. Pyrite occurs in the chlorite zone through the biotite zone, while pyrrhotite occurs from the chlorite zone to K-feldspar + sillimanite zone. The pyrite-pyrrhotite transition, therefore, occupies an interval in the chlorite and lower biotite zones that is terminated upgrade by a pyrite-out isograd in the upper part of the biotite zone or lowest grade part of the cordierite/staurolite zone. Pressure and temperature conditions of the rocks were estimated from phase equilibrium modelling and from Raman spectroscopy of carbonaceous material (RSCM) thermometry. Modelling indicates pressures of 3.7-4.1 kbar with temperatures of ~425 °C at the biotite isograd, 560-570 °C for chlorite-out/porphyroblast-in, ~575 °C for andalusite-in, 575-600 °C for the sillimanite isograd, and 645-660 °C at the K-feldspar + sillimanite isograd. RSCM temperatures are greater than or equal to 420 °C in the Chl zone, 500 °C at the Bt isograd, 525-550 °C for porphyroblast-in isograd, ~550 °C at the And isograd, and 580 °C at the Sil isograd. These results suggest the pyrite-pyrrhotite transition occurs from less than or equal to 420°C to ~560 °C. Thermodynamic modelling shows 0.6 wt. % H2O is released during metamorphism over the ~140 °C interval of the pyrite-pyrrhotite transition. The gradual release of fluid in the biotite zone is interpreted to have broadened the pyrite-pyrrhotite transition compared to other studies that predict a small interval of vigorous fluid release associated with volumetric chlorite consumption. Samples from the pyrite-pyrrhotite transition zone contain lower whole rock and pyrite Au values than samples from unmetamorphosed/lower rocks, suggesting that Au was removed from the rock at conditions below the pyrite-pyrrhotite transition (<420 °C). The chlorite zone and higher-grade metamorphic rocks of the Hyland River area do not appear to be a plausible source region for orogenic gold.

Applications of Raman Spectroscopy in Metamorphic Petrology and Tectonics

Elements ◽  
2020 ◽  
Vol 16 (2) ◽  
pp. 105-110 ◽  
Author(s):  
Andrey V. Korsakov ◽  
Matthew J. Kohn ◽  
Maria Perraki
Keyword(s):  
Raman Spectroscopy ◽  
Sample Preparation ◽  
Chemical Equilibrium ◽  
Carbonaceous Material ◽  
Ultrahigh Pressure ◽  
Metamorphic Rocks ◽  
Temperature Sensitive ◽  
Metamorphic Petrology ◽  
Minimal Sample ◽  
Tectonic Processes

Raman spectroscopy is widely applied in metamorphic petrology and offers many opportunities for geological and tectonic research. Minimal sample preparation preserves sample integrity and microtextural information, while use with confocal microscopes allows spatial resolution down to the micrometer level. Raman spectroscopy clearly distinguishes mineral polymorphs, providing crucial constraints on metamorphic conditions, particularly ultrahigh-pressure conditions. Raman spectroscopy can also be used to monitor the structure of carbonaceous material in metamorphic rocks. Changes in structure are temperature-sensitive, so Raman spectroscopy of carbonaceous material is widely used for thermometry. Raman spectroscopy can also detect and quantify strain in micro-inclusions, offering new barometers that can be applied to understand metamorphic and tectonic processes without any assumptions about chemical equilibrium.

The Potential Applications of Raman Spectroscopy in Unravelling Complex Palaeowildfire Ecosystems

2021 ◽  
Author(s):  
Thomas Theurer ◽  
David Muirhead ◽  
David Jolley ◽  
Dmitri Mauquoy
Keyword(s):  
Raman Spectroscopy ◽  
Carbonaceous Material ◽  
Complex Interactions ◽  
Intensity Changes ◽  
Potential Applications ◽  
Ecosystem Studies ◽  
History Of ◽  
The Impact ◽  
Unique Relationship

<p>Raman spectroscopy represents a novel methodology of characterising plant-fire interactions through geological history, with enormous potential. Applications of Raman spectroscopy to charcoal have shown that this is an effective method of understanding intensity changes across palaeofire regimes. Such analyses have relied on the determination of appropriate Raman parameters, given their relationship with temperature of formation and microstructural changes in reference charcoals. Quantitative assessments of charcoal microstructure have also been successfully applied to the assessment of carbonaceous maturation under alternate thermal regimes, such as pyroclastic volcanism. Palaeowildfire systems in association with volcanism may present a complex history of thermal maturation, given interactions between detrital charcoals and volcanogenic deposition. However, whilst palaeofire and volcanic maturation of carbonaceous material are well understood individually, their interaction has yet to be characterised. Here we present the first analysis of palaeofire charcoals derived from volcanic ignition utilising Raman spectroscopy. Our results indicate that complex interactions between volcanism and palaeofire systems may be better understood by the characterisation of charcoal microstructure, alongside palaeobotanical and ecosystem studies. Understanding the unique relationship between wildfires and volcanism, and the impact that this has on the fossil record, may better assist our understanding of wildfire systems in deep history. Further still, this highlights the potential for better understanding the socioecological impacts of modern and future wildfire systems closely associated with volcanic centres. </p>

Towards a Higher Comparability of Geothermometric Data Obtained by Raman Spectroscopy of Carbonaceous Material. Part 2: A Revised Geothermometer

2017 ◽  
Vol 41 (4) ◽  
pp. 593-612 ◽  
Author(s):  
N. Keno Lünsdorf ◽  
István Dunkl ◽  
Burkhard C. Schmidt ◽  
Gerd Rantitsch ◽  
Hilmar von Eynatten
Keyword(s):  
Raman Spectroscopy ◽  
Carbonaceous Material ◽  
Material Part

Carbon ordering in an aseismic shear zone: implications for crustal weakening and Raman spectroscopy

2020 ◽  
Author(s):  
Lauren Kedar ◽  
Clare Bond ◽  
David Muirhead
Keyword(s):  
Raman Spectroscopy ◽  
Organic Carbon ◽  
Shear Zone ◽  
Experimental Studies ◽  
Carbonaceous Material ◽  
Shear Zones ◽  
Lower Limbs ◽  
Strain Partitioning ◽  
Increasing Temperature ◽  
Two Peaks

<p>Multi-layered stratigraphic sequences present ample opportunity for the study of strain localization and its complexities. By constraining mechanisms of crustal weakening, it is possible to gain a sounder understanding of the dynamic evolution of the Earth’s crust, especially when applied to realistic, field-based scenarios. One such mechanism is that of strain-related carbon ordering. This is the process whereby the amorphous nanostructure of fossilized organic matter contained within the rock is progressively organized towards a more sheet-like structure, similar to that of graphite. One common method of studying this process is through Raman spectroscopy. This is a non-destructive tool which makes use of the relative positions and intensities of two key spectral peaks, where one peak represents graphitic carbon and the other disordered (or amorphous) carbon. The intensity ratio between these two peaks suggests the degree to which the carbon has progressed from its original kerogen-like structure towards that of graphite. This progression can be due to increasing temperature or increasing strain, and until now, these two contributory factors have been difficult to separate, particularly in field examples.</p><p>Previous field-based studies have focused on carbon ordering on fault planes, while experimental studies have monitored the effects of strain-related ordering in organic carbon on both fault surfaces and more distributed shear zones. These studies confirmed the occurrence of strain-related ordering at seismic rates, particularly in the form of graphitization of carbon. However, these experiments showed the effects of strain-related ordering at aseismic rates to be limited when distributed shear zones were considered, in part due to the geological timescales required to emulate true conditions.</p><p>In this study, Raman spectroscopy is used to compare the relative nanostructural order of organic carbon within a recumbent isoclinal fold formed of interbedded limestones and marls. The central, overturned fold limb forms a 170m wide, 1km long aseismic shear zone, with evidence of increased strain recorded in calcite grains relative to the upper and lower limbs. Raman spectroscopy intensity ratios (I[d]/I[g]) are compared across the fold, showing a marked 23% decrease in the overturned limb. Such a decrease in I[d]/I[g] suggests increased carbon ordering within the overturned limb, which in combination with evidence for increased strain in calcite, suggests that the carbon ordering here is derived directly from strain-related ordering. This has important implications. We infer, from previous studies, that strain-related carbon ordering encourages further strain partitioning in carbonaceous material, and may enhance zones of weakness in the rock. This ordering in aseismic shear zones has so far been unreported in nature, and so our field-based results are significant in supporting previous experimental evidence for this phenomenon. Our results also have implications for understanding dynamic crustal evolution, and will play an important role in the development of Raman thermobarometry, especially since current methods do not distinguish between strain-related and temperature-related ordering.</p>

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