Geochemical investigation of the lower crustal rocks in Bamble Shear Belt, southern Norway: Implications for the source of gold in lode gold deposits

The source of gold in lode gold deposits in metamorphic terrains is debatable. As a test of the “deep source model”, the Bamble Shear Belt of southern Norway was studied. A transition from amphibolite to granulite facies takes place over the 30 km width of the Shear Belt. Some 400 samples from various rock units across the Belt were analyzed for a wide range of elements. The results, combined with petrographic studies and data from elemental and isotopic composition of sulfides, are interpreted with emphasis on the processes that have affected the distribution of gold.; The oldest exposed rocks in Bamble are a sequence of sedimentary and volcanic materials of early- to mid-Proterozoic age. During the Kongsbergian, or Gothian, orogeny (1600–1450 Ma), all the supracrustal rocks were intruded by a suite of acid-intermediate magmas, and a series of basic dykes and sills known as metabasites.; The highest grade portion of Bamble is dominated by tonalitic-trondhjemitic charnockites and subordinate metabasites and metasediments. All rocks in this zone are strongly depleted in LILE, LREE, and HFSE relative to their equivalent rocks from amphibolite and transition zones. Depletion of the elements can be explained by the presence of a CO₂-rich fluid during magmatic crystallization, or metamorphic recrystallization.; Both metabasites and hyperites are of low-Mg tholefitic nature (MgO < 10%), and bear features typical of destructive margin settings. On MORB-normalized plots, they exhibit marked enrichments in LILE and LREE. The absence of Mg-rich cumulates argues against a functional crystallization model. Both series are the products of low degrees of partial melting.; Supracrustal rocks of varied type and origin in Bamble are strongly depleted in Au, As, Sb, and Se relative to the crustal abundance of these elements. The average Au in the Bamble rocks, 0.35 ppb, is only 25% of the crustal abundance of this element. Charnockites are also depleted in Au, As, Sb, and Se. The mean value of Au in these rocks (0.2 ppb) is 20% of the general abundance of this element in felsic rocks elsewhere.; Lode gold deposits are commonly associated with alkaline and light REE metasomatism, and a significant enrichment in As, Sb, and Se. Neither special Au-enriched rocks, nor unreasonably large volumes of source rocks are required to produce a giant gold deposit. Shear zones serve as major source areas and principal conduits for extraction and transportation of Au. (Abstract shortened by UMI.)...