Iron isotope fractionation during hydrothermal ore deposition and alteration

G. Markl, F. von Blanckenburg, Thomas Wagner

Forskningsoutput: TidskriftsbidragArtikelVetenskapligPeer review

Sammanfattning

Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources, or fluid histories. While the former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, the latter allows the reconstruction of precipitation and redox processes related to ore formation or alteration. To investigate the suitability of this new isotope method as a proxy for ore-related processes, 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, these ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300°C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100°C). Measured iron isotope compositions are highly variable and cover a range in δ56Fe between –2.3 and +1.3‰. Primary hematite (δ56Fe: –0.5 to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: –0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: –1.4 to –0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitated from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethite from oolithic sedimentary iron ores were also analyzed. Their comparatively narrow compositional range appears to indicate oxidative precipitation from a relatively uniform seawater Fe reservoir. This comprehensive iron isotope study illustrates the potential of this new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.

Originalspråkengelska
TidskriftGeochimica et Cosmochimica Acta
Volym70
Sidor (från-till)3011-3030
ISSN0016-7037
DOI
StatusPublicerad - 2006
Externt publiceradJa
MoE-publikationstypA1 Tidskriftsartikel-refererad

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@article{994f899309fc447e92ff0e94ba26290f,
title = "Iron isotope fractionation during hydrothermal ore deposition and alteration",
abstract = "Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources, or fluid histories. While the former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, the latter allows the reconstruction of precipitation and redox processes related to ore formation or alteration. To investigate the suitability of this new isotope method as a proxy for ore-related processes, 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, these ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300°C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100°C). Measured iron isotope compositions are highly variable and cover a range in δ56Fe between –2.3 and +1.3‰. Primary hematite (δ56Fe: –0.5 to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: –0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: –1.4 to –0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitated from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethite from oolithic sedimentary iron ores were also analyzed. Their comparatively narrow compositional range appears to indicate oxidative precipitation from a relatively uniform seawater Fe reservoir. This comprehensive iron isotope study illustrates the potential of this new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.",
author = "G. Markl and {von Blanckenburg}, F. and Thomas Wagner",
year = "2006",
doi = "10.1016/j.gca.2006.02.028",
language = "English",
volume = "70",
pages = "3011--3030",
journal = "Geochimica et Cosmochimica Acta",
issn = "0016-7037",
publisher = "Elsevier Scientific Publ. Co",

}

Iron isotope fractionation during hydrothermal ore deposition and alteration. / Markl, G.; von Blanckenburg, F.; Wagner, Thomas.

I: Geochimica et Cosmochimica Acta, Vol. 70, 2006, s. 3011-3030.

Forskningsoutput: TidskriftsbidragArtikelVetenskapligPeer review

TY - JOUR

T1 - Iron isotope fractionation during hydrothermal ore deposition and alteration

AU - Markl, G.

AU - von Blanckenburg, F.

AU - Wagner, Thomas

PY - 2006

Y1 - 2006

N2 - Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources, or fluid histories. While the former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, the latter allows the reconstruction of precipitation and redox processes related to ore formation or alteration. To investigate the suitability of this new isotope method as a proxy for ore-related processes, 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, these ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300°C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100°C). Measured iron isotope compositions are highly variable and cover a range in δ56Fe between –2.3 and +1.3‰. Primary hematite (δ56Fe: –0.5 to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: –0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: –1.4 to –0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitated from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethite from oolithic sedimentary iron ores were also analyzed. Their comparatively narrow compositional range appears to indicate oxidative precipitation from a relatively uniform seawater Fe reservoir. This comprehensive iron isotope study illustrates the potential of this new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.

AB - Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources, or fluid histories. While the former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, the latter allows the reconstruction of precipitation and redox processes related to ore formation or alteration. To investigate the suitability of this new isotope method as a proxy for ore-related processes, 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, these ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300°C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100°C). Measured iron isotope compositions are highly variable and cover a range in δ56Fe between –2.3 and +1.3‰. Primary hematite (δ56Fe: –0.5 to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: –0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: –1.4 to –0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitated from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethite from oolithic sedimentary iron ores were also analyzed. Their comparatively narrow compositional range appears to indicate oxidative precipitation from a relatively uniform seawater Fe reservoir. This comprehensive iron isotope study illustrates the potential of this new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.

U2 - 10.1016/j.gca.2006.02.028

DO - 10.1016/j.gca.2006.02.028

M3 - Article

VL - 70

SP - 3011

EP - 3030

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -