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

摘要

Widespread gold showings have been found in an Archean greenstone belt on Storo in Godthabsfjord, West Greenland. The Storo 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 lollingite-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 lollingite-arsenopyrite-pyrrhotite assemblage on Storo: In the first stage, arsenopyrite likely formed with gold in solid solution. In the second stage, gold-bearing arsenopyrite was replaced by lollingite and pyrrhotite. This took place during regional prograde metaorphism. Gold accumulated within lollingite as discrete grains. During subsequent retrograde metamorphic conditions, lollingite and pyrrhotite reacted and formed arsenopyrite, and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite-lollingite grain boundaries. During a later event a garnet overgrowth partly affected the gold-bearing lollingite-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 (2sigma; MSWD=0.2) for arsenopyrite and 2748 + - 62 Ma (2sigma; 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 approx 300 deg C. The Late Archean ages for arsenopyrite and garnet are consistent with the regional amphibolite fades 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.