Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for... An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over‐thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low‐P high‐T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper‐earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper‐later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid‐crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Metamorphic Geology Wiley

Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

Loading next page...
 
/lp/wiley/metamorphism-in-archaean-greenstone-belts-calculated-fluid-fn5o9BDAgE
Publisher
Wiley
Copyright
Copyright © 1991 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0263-4929
eISSN
1525-1314
D.O.I.
10.1111/j.1525-1314.1991.tb00510.x
Publisher site
See Article on Publisher Site

Abstract

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over‐thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low‐P high‐T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper‐earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper‐later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid‐crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.

Journal

Journal of Metamorphic GeologyWiley

Published: Mar 1, 1991

References

  • An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K 2 O–Na 2 O–CaO–MgO–MnO–FeO–Fe 2 O 3 –Al 2 O 3 –TiO 2 –SiO 2 –C–H 2 –O 2
    Holland, Holland; Powell, Powell
  • A model for low‐pressure facies metamorphism during crustal thickening
    Loosveld, Loosveld; Etheridge, Etheridge
  • High‐grade metamorphic processes which influence Archaean gold deposits, with particular reference to Big Bell, Australia
    Phillips, Phillips; Nooy, Nooy

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off