Geology and genesis of Nalunaq Palaeoproterozoic shear zone-hosted gold deposit, South Greenland

Abstract

The Nalunaq deposit is a shear zone-hosted gold deposit characterized by very high grades (up to 5240 g Au/t over 0.8 m). The host rocks are Palaeoproterozoic metabasic rocks of the Nanortalik Nappe, which is part of the Ketilidian Mobile Belt. The Nanortalik Nappe consists of bedded massive pyrrhotite/graphitic chert horizons overlain by fine-grained metabasalts with pillow structures and metadolerites followed by metatuffs and meta-agglomerate. This sequence was intruded by a porphyritic biotite granite (ca 1755-1725 m.y.) and subsequently by rapakivi granites.The gold mineralization occurs in a 0.5-2.0 m wide shear zone consisting of sheeted quartz veins with associated calc-silicate alteration. The mineralized shear zone is exposed for 800 m on the east face and 1200 m on the north face of Nalunaq Mountain. Surface mapping, diamond drilling (78 holes totalling 11 452 m) and adit development (288 m) have shown that the shear zone that hosts the gold-bearing Main Vein is a continuous structure. This structure was subsequently displaced by slightly rotating dextral strike-slip faults.Gold is present mainly as native gold and as minor maldonite. Free gold appears mainly as inclusions in quartz, diopside, plagioclase and as minor inclusions in löllingite and arsenopyrite. Gold also occurs in fractures and at grain boundaries of quartz, löllingite and arsenopyrite. These occurrences of gold indicate that several influxes of gold and/or stages of remobilization took place. The gold grains vary in size from 1 µm to 4 mm and their distribution is irregular, which makes it difficult to estimate the grade of the deposit from 1.5-to 5-kg samples.Fluid inclusions associated with gold in quartz and diopside contain three types of fluids: aqueous, carbonic and mixed carbonic-aqueous. Moderate to highly saline (14-26 wt% NaCl+CaCl2 equivalent) aqueous fluids with the inferred composition H2O-NaCl-CaCl/MgCl±LiCl predominate. The average homogenization temperature is 201°C. The carbonic fluid contains less than 5 mol% CH4 and has densities in the range 45-60 cm3/mole.The proximal alteration minerals (e.g. diopside, Ca-rich amphibole and Ca-rich plagioclase) and opaque assemblages (e.g., löllingite, arsenopyrite and pyrrhotite) show that the deposit was formed under upper greenschist- to amphibolite-facies conditions. The texture of löllingite rimmed by arsenopyrite suggests that the deposit was formed at high temperatures and the gold-mineralized structure was subsequently also active at lower temperatures.The shear zone was formed under ductile conditions and the major emplacement of quartz veins occurred in the subsequent brittle-ductile regime during multiple influxes of fluids. The shear zone clearly postdates the amphibolite-facies regional metamorphism as it is discordant to the metamorphic foliation.This mineralization, together with the similar hypothermal shear zone-hosted gold mineralization at Xutseq on the southeast coast and stream sediments that are generally highly anomalous in gold, indicates that a widespread gold-mineralizing system may have been active in South Greenland in the late stages of evolution of the Ketilidian Mobile Belt.