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Late-kinematic gold mineralisation during regional uplift and the role of nitrogen: an example from the La Codosera area, W. Spain

Published online by Cambridge University Press:  05 July 2018

S. J. Dee  and
S. Roberts
S. J. Dee
Affiliation:
Department of Geology, The University, Highfield, Southampton SO9 5NH, U.K.
S. Roberts
Affiliation:
Department of Geology, The University, Highfield, Southampton SO9 5NH, U.K.
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Abstract

Vein formation occurred throughout a deformation sequence which involved early transpressive ductile deformation through to late-kinematic transpressive brittle structures which host a series of gold prospects. Fluid inclusion data from (S1) fabric parallel veins associated with early deformation suggest that a low-salinity aqueous fluid, with a mean salinity of 6.4 wt.%, was present during peak metamorphism, Pelite mineralogy and isochores constrain peak metamorphism to the lowermost part of the upper greenschist facies at 325 to 425°C and 1.4 to 3.4 kbar.

Fluid inclusion data from auriferous and barren late-kinematic quartz veins, both containing unmixing assemblages of aqueo-carbonic inclusions with low salinities of ≈2.7 wt.% NaCl equiv., indicate unmixing occurred at 300°C and 1.5 kbar.

Volatiles (CO2, N2, CH4) are observed in all the late-kinematic veins. The N2 contents of veins with elevated gold grades are typically higher than those with low gold grades. N2 reaches 8.7 mole% in a vein with 0.49−4.6 p.p.m. Au compared to <1 mole% in a vein with <0.05 p.p.m. Au. The CH4 content of late kinematic veins is generally less than 1 mole% and shows no relative enrichment in mineralised veins. The generation of N2 in the mineralising fluid most likely results from interaction of fluid with the ammonium ion, NH4+, in micas and feldspars. This interaction could take place either at source, due to metamorphic devolatisation reactions, or along those structures which acted as fluid conduits due to fluid-rock interaction.

Keywords

gold mineralisationnitrogenSpainfluid inclusions
Type
Research Article
Information
Mineralogical Magazine , Volume 57 , Issue 388 , September 1993 , pp. 437 - 450
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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