Evidence and origin of different types of sedimentary organic matter from a Paleoproterozoic orogenic Au deposit

Aileen Mirasol-Robert, Hendrik Grotheer, Julien Bourdet, Alexandra Suvorova, Kliti Grice, T. Campbell McCuaig, Paul F. Greenwood

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Carbonaceous material (CM) is thought to be a key reductant contributing to the formation of large Au deposits, but there has been much speculation about its source, molecular composition and reactivity. The first successful analytical retrieval of organic compounds from a thermally over-mature (>550 °C) Paleoproterozoic Cosmo-Howley Orogenic Au deposit was recently achieved by Robert et al. (2016). Here, we have evaluated the nature of the CM associated with this high temperature Au mineralisation via an integrated analytical approach which combined high-resolution in situ laser Raman spectroscopy, micro to nano-scale imaging (e.g., EELS, HAADF-STEM, and HRTEM) and molecular and isotopic geochemistry. We identified two distinct CM types: CMker – an ubiquitous highly graphitic kerogen typical of high-grade metamorphic conditions formed by regional metamorphism; and CMfd – small sub-microscopic inclusion-like nodules of highly disordered carbon rich in polycyclic aromatic hydrocarbons (PAHs), coincident within the Au-bearing sulfide minerals in hydrothermal vein regions. The paragenetic emplacement and molecular characteristics of CMfd suggests a formation by metasomatic processes and introduction by a hydrothermal fluid which might also have co-transported Au. CMker and CMfd gave different Raman spectra indicative of their contrasting origin and structural response to regional and contact metamorphic history and subsequent metasomatism of the Cosmo-Howley deposit. Raman signals indicated CMker had a graphitic like structure whereas CMfd comprised high concentrations or clusters of PAHs. The broad range of Raman spectra detected here (and by others in similar studies) was likely due to the mixed signals of these two types of CM. The δ13C values of PAH products released via the HyPy treatment of the parent and sequentially demineralised kerogen fractions were measured to be in the range of −20 to −30‰, indicative of an organic biopolymeric origin. The δ13C values of PAHs products decreased with demineralisation, concomitant with an increase in their concentrations and affinity to the sulfide-minerals (and associated CMfd) suggesting a close relationship. The localised (within 20 mm) co-occurrence of different CM types and apparent abundance correlation of CMfd with Au and sulfides suggests Au mineralisation might be supported by specific CM types, and these relationships should be evaluated further including on a wider Au deposit scale.

Original languageEnglish
Pages (from-to)319-338
Number of pages20
JournalPrecambrian Research
Volume299
DOIs
Publication statusPublished - 1 Sep 2017

Fingerprint

Biological materials
Deposits
Polycyclic Aromatic Hydrocarbons
organic matter
PAH
Kerogen
Sulfide minerals
sulfide
kerogen
Raman scattering
Bearings (structural)
mineralization
Laser spectroscopy
structural response
Geochemistry
Electron energy loss spectroscopy
Reducing Agents
regional metamorphism
Raman spectroscopy
Sulfides

Cite this

@article{57b93a4431224abab6bbbff343070d89,
title = "Evidence and origin of different types of sedimentary organic matter from a Paleoproterozoic orogenic Au deposit",
abstract = "Carbonaceous material (CM) is thought to be a key reductant contributing to the formation of large Au deposits, but there has been much speculation about its source, molecular composition and reactivity. The first successful analytical retrieval of organic compounds from a thermally over-mature (>550 °C) Paleoproterozoic Cosmo-Howley Orogenic Au deposit was recently achieved by Robert et al. (2016). Here, we have evaluated the nature of the CM associated with this high temperature Au mineralisation via an integrated analytical approach which combined high-resolution in situ laser Raman spectroscopy, micro to nano-scale imaging (e.g., EELS, HAADF-STEM, and HRTEM) and molecular and isotopic geochemistry. We identified two distinct CM types: CMker – an ubiquitous highly graphitic kerogen typical of high-grade metamorphic conditions formed by regional metamorphism; and CMfd – small sub-microscopic inclusion-like nodules of highly disordered carbon rich in polycyclic aromatic hydrocarbons (PAHs), coincident within the Au-bearing sulfide minerals in hydrothermal vein regions. The paragenetic emplacement and molecular characteristics of CMfd suggests a formation by metasomatic processes and introduction by a hydrothermal fluid which might also have co-transported Au. CMker and CMfd gave different Raman spectra indicative of their contrasting origin and structural response to regional and contact metamorphic history and subsequent metasomatism of the Cosmo-Howley deposit. Raman signals indicated CMker had a graphitic like structure whereas CMfd comprised high concentrations or clusters of PAHs. The broad range of Raman spectra detected here (and by others in similar studies) was likely due to the mixed signals of these two types of CM. The δ13C values of PAH products released via the HyPy treatment of the parent and sequentially demineralised kerogen fractions were measured to be in the range of −20 to −30‰, indicative of an organic biopolymeric origin. The δ13C values of PAHs products decreased with demineralisation, concomitant with an increase in their concentrations and affinity to the sulfide-minerals (and associated CMfd) suggesting a close relationship. The localised (within 20 mm) co-occurrence of different CM types and apparent abundance correlation of CMfd with Au and sulfides suggests Au mineralisation might be supported by specific CM types, and these relationships should be evaluated further including on a wider Au deposit scale.",
keywords = "HRTEM, Hydropyrolysis, Hydrothermal fluid, Laser Raman spectroscopy, Organic geochemistry, Orogenic gold",
author = "Aileen Mirasol-Robert and Hendrik Grotheer and Julien Bourdet and Alexandra Suvorova and Kliti Grice and McCuaig, {T. Campbell} and Greenwood, {Paul F.}",
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Evidence and origin of different types of sedimentary organic matter from a Paleoproterozoic orogenic Au deposit. / Mirasol-Robert, Aileen; Grotheer, Hendrik; Bourdet, Julien; Suvorova, Alexandra; Grice, Kliti; McCuaig, T. Campbell; Greenwood, Paul F.

In: Precambrian Research, Vol. 299, 01.09.2017, p. 319-338.

Research output: Contribution to journalArticle

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T1 - Evidence and origin of different types of sedimentary organic matter from a Paleoproterozoic orogenic Au deposit

AU - Mirasol-Robert, Aileen

AU - Grotheer, Hendrik

AU - Bourdet, Julien

AU - Suvorova, Alexandra

AU - Grice, Kliti

AU - McCuaig, T. Campbell

AU - Greenwood, Paul F.

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N2 - Carbonaceous material (CM) is thought to be a key reductant contributing to the formation of large Au deposits, but there has been much speculation about its source, molecular composition and reactivity. The first successful analytical retrieval of organic compounds from a thermally over-mature (>550 °C) Paleoproterozoic Cosmo-Howley Orogenic Au deposit was recently achieved by Robert et al. (2016). Here, we have evaluated the nature of the CM associated with this high temperature Au mineralisation via an integrated analytical approach which combined high-resolution in situ laser Raman spectroscopy, micro to nano-scale imaging (e.g., EELS, HAADF-STEM, and HRTEM) and molecular and isotopic geochemistry. We identified two distinct CM types: CMker – an ubiquitous highly graphitic kerogen typical of high-grade metamorphic conditions formed by regional metamorphism; and CMfd – small sub-microscopic inclusion-like nodules of highly disordered carbon rich in polycyclic aromatic hydrocarbons (PAHs), coincident within the Au-bearing sulfide minerals in hydrothermal vein regions. The paragenetic emplacement and molecular characteristics of CMfd suggests a formation by metasomatic processes and introduction by a hydrothermal fluid which might also have co-transported Au. CMker and CMfd gave different Raman spectra indicative of their contrasting origin and structural response to regional and contact metamorphic history and subsequent metasomatism of the Cosmo-Howley deposit. Raman signals indicated CMker had a graphitic like structure whereas CMfd comprised high concentrations or clusters of PAHs. The broad range of Raman spectra detected here (and by others in similar studies) was likely due to the mixed signals of these two types of CM. The δ13C values of PAH products released via the HyPy treatment of the parent and sequentially demineralised kerogen fractions were measured to be in the range of −20 to −30‰, indicative of an organic biopolymeric origin. The δ13C values of PAHs products decreased with demineralisation, concomitant with an increase in their concentrations and affinity to the sulfide-minerals (and associated CMfd) suggesting a close relationship. The localised (within 20 mm) co-occurrence of different CM types and apparent abundance correlation of CMfd with Au and sulfides suggests Au mineralisation might be supported by specific CM types, and these relationships should be evaluated further including on a wider Au deposit scale.

AB - Carbonaceous material (CM) is thought to be a key reductant contributing to the formation of large Au deposits, but there has been much speculation about its source, molecular composition and reactivity. The first successful analytical retrieval of organic compounds from a thermally over-mature (>550 °C) Paleoproterozoic Cosmo-Howley Orogenic Au deposit was recently achieved by Robert et al. (2016). Here, we have evaluated the nature of the CM associated with this high temperature Au mineralisation via an integrated analytical approach which combined high-resolution in situ laser Raman spectroscopy, micro to nano-scale imaging (e.g., EELS, HAADF-STEM, and HRTEM) and molecular and isotopic geochemistry. We identified two distinct CM types: CMker – an ubiquitous highly graphitic kerogen typical of high-grade metamorphic conditions formed by regional metamorphism; and CMfd – small sub-microscopic inclusion-like nodules of highly disordered carbon rich in polycyclic aromatic hydrocarbons (PAHs), coincident within the Au-bearing sulfide minerals in hydrothermal vein regions. The paragenetic emplacement and molecular characteristics of CMfd suggests a formation by metasomatic processes and introduction by a hydrothermal fluid which might also have co-transported Au. CMker and CMfd gave different Raman spectra indicative of their contrasting origin and structural response to regional and contact metamorphic history and subsequent metasomatism of the Cosmo-Howley deposit. Raman signals indicated CMker had a graphitic like structure whereas CMfd comprised high concentrations or clusters of PAHs. The broad range of Raman spectra detected here (and by others in similar studies) was likely due to the mixed signals of these two types of CM. The δ13C values of PAH products released via the HyPy treatment of the parent and sequentially demineralised kerogen fractions were measured to be in the range of −20 to −30‰, indicative of an organic biopolymeric origin. The δ13C values of PAHs products decreased with demineralisation, concomitant with an increase in their concentrations and affinity to the sulfide-minerals (and associated CMfd) suggesting a close relationship. The localised (within 20 mm) co-occurrence of different CM types and apparent abundance correlation of CMfd with Au and sulfides suggests Au mineralisation might be supported by specific CM types, and these relationships should be evaluated further including on a wider Au deposit scale.

KW - HRTEM

KW - Hydropyrolysis

KW - Hydrothermal fluid

KW - Laser Raman spectroscopy

KW - Organic geochemistry

KW - Orogenic gold

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JF - Precambrian Research

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