TY - JOUR
T1 - A shear zone related greenstone belt hosted gold mineralization in the Archean of West Greenland. A petrographic and combined Pb-Pb and Rb-Sr geochronological study
AU - Juul-Pedersen, Astrid
AU - Frei, Robert
AU - Appel, Peter W.U.
AU - Persson, Mac Fjellerad
AU - Konnerup-Madsen, Jens
N1 - Funding Information:
This paper is based on the M.Sc. thesis written by the first author. The authors wish to thank GEUS for field assistance and use of their laboratories. The authors also wish to thank the laboratory staff at the Geological Institute, University of Copenhagen, for help in sample preparation and mass spectrometric analyses of the samples. R.F. is thankful for financial support by FNU-grant (Forskningsrådet for Natur og Univers) no. 21-01-0492. Constructive and helpful comments were provided by Peter Neumayr and an anonymous reviewer.
PY - 2007/9
Y1 - 2007/9
N2 - Widespread gold showings have been found in an Archean greenstone belt on Storø in Godthåbsfjord, West Greenland. The Storø Greenstone Belt (SGB) comprises metasedimentary and metavolcanic rocks with minor magnetite-rich bands. Gold mineralization is hosted by amphibolites, mica schists and garnetite rocks. Gold occurs in various mineral associations, but its presence in löllingite-arsenopyrite-pyrrhotite parageneses is prevalent. To a lesser extent, native gold occurs within seriticized plagioclase, in sheeted quartz veins, and as inclusions in garnet. Based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold-bearing löllingite-arsenopyrite-pyrrhotite assemblage on Storø: In the first stage, arsenopyrite likely formed with gold in solid solution. In the second stage, gold-bearing arsenopyrite was replaced by löllingite and pyrrhotite. This took place during regional prograde metamorphism. Gold accumulated within löllingite as discrete grains. During subsequent retrograde metamorphic conditions, löllingite and pyrrhotite reacted and formed arsenopyrite, and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite-löllingite grain boundaries. During a later event a garnet overgrowth partly affected the gold-bearing löllingite-arsenopyrite-pyrrhotite assemblage. The isotopic composition of Pb released from these garnets by Pb stepwise leaching experiments (PbSL) and the bulk Pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of Pb in the fluid from which first the sulfides and subsequently garnet were formed. Bulk Pb analyses and PbSL experiments of arsenopyrite and PbSL experiments of garnet yield ages of 2863 ± 24 Ma (2σ; MSWD = 0.2) for arsenopyrite and 2748 ± 62 Ma (2σ; MSWD = 220) for garnet. The 2863 ± 24 Ma age is thus a minimum age for the mineralization. Rb-Sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal Middle Proterozoic (ca. 1.65 Ga) mineral ages, well in accordance with a regional reheating event known to have affected the area, and are thus interpreted as cooling ages thereof. This regional reheating event has apparently not affected the Pb-Pb isotopic system of arsenopyrite, implying that the blocking temperature of the U-Pb system in arsenopyrite is likely to be above ∼ 300 °C. The Late Archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time. The ore textures preserved today are interpreted as those formed under this regional metamorphic event.
AB - Widespread gold showings have been found in an Archean greenstone belt on Storø in Godthåbsfjord, West Greenland. The Storø Greenstone Belt (SGB) comprises metasedimentary and metavolcanic rocks with minor magnetite-rich bands. Gold mineralization is hosted by amphibolites, mica schists and garnetite rocks. Gold occurs in various mineral associations, but its presence in löllingite-arsenopyrite-pyrrhotite parageneses is prevalent. To a lesser extent, native gold occurs within seriticized plagioclase, in sheeted quartz veins, and as inclusions in garnet. Based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold-bearing löllingite-arsenopyrite-pyrrhotite assemblage on Storø: In the first stage, arsenopyrite likely formed with gold in solid solution. In the second stage, gold-bearing arsenopyrite was replaced by löllingite and pyrrhotite. This took place during regional prograde metamorphism. Gold accumulated within löllingite as discrete grains. During subsequent retrograde metamorphic conditions, löllingite and pyrrhotite reacted and formed arsenopyrite, and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite-löllingite grain boundaries. During a later event a garnet overgrowth partly affected the gold-bearing löllingite-arsenopyrite-pyrrhotite assemblage. The isotopic composition of Pb released from these garnets by Pb stepwise leaching experiments (PbSL) and the bulk Pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of Pb in the fluid from which first the sulfides and subsequently garnet were formed. Bulk Pb analyses and PbSL experiments of arsenopyrite and PbSL experiments of garnet yield ages of 2863 ± 24 Ma (2σ; MSWD = 0.2) for arsenopyrite and 2748 ± 62 Ma (2σ; MSWD = 220) for garnet. The 2863 ± 24 Ma age is thus a minimum age for the mineralization. Rb-Sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal Middle Proterozoic (ca. 1.65 Ga) mineral ages, well in accordance with a regional reheating event known to have affected the area, and are thus interpreted as cooling ages thereof. This regional reheating event has apparently not affected the Pb-Pb isotopic system of arsenopyrite, implying that the blocking temperature of the U-Pb system in arsenopyrite is likely to be above ∼ 300 °C. The Late Archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time. The ore textures preserved today are interpreted as those formed under this regional metamorphic event.
KW - Geochronology
KW - Gold mineralization
KW - Lead isotopes
KW - Rubidium-strontium isotopes
KW - Storø
KW - Western Greenland
UR - http://www.scopus.com/inward/record.url?scp=34548086556&partnerID=8YFLogxK
U2 - 10.1016/j.oregeorev.2007.01.002
DO - 10.1016/j.oregeorev.2007.01.002
M3 - Article
SN - 0169-1368
VL - 32
SP - 20
EP - 36
JO - Ore Geology Reviews
JF - Ore Geology Reviews
IS - 1-2
ER -