ALTERATION ASSOCIATED WITH THE GOLD-BEARING QUARTZ VEINS AT MIDDAGSBERGET, NORTHERN SWEDEN

The mesothermal Au deposit at Middagsberget in northern Sweden is asso ciated with irregular stock-works of quartz veins occurring in shear z ones across a dioritic intrusion. Alteration in the shear zones is cha racterized by sericitization, chloritization, the presence of sulphide s and several generations of veins and small-scale fractures. Small fr actures which are filled with variable amounts of quartz, carbonate, s ericite, chlorite, albite and sulphides, are particularly abundant in zones having a high Au content. In general, these fractures are younge r than the major quartz veins and were apparently important for strong Au-enrichment. Au is associated with arsenopyrite and it occurs as dr oplets or interfillings in the arsenopyrite or at the crystal surface; together with pyrrhotite as inclusions in arsenopyrite; as free grain s in silicates but close to arsenopyrite; or in very small fractures i n carbonates. The Au-mineralization is associated with elevated conten ts of As, S, Sb and W and depletion of Cu, Zn, Ge and Bi. The entire a ltered areas are enriched in Au compared with the < 1 to 15 ppb found in the host intrusion. During alteration an initial increase in Na or K was accompanied by a weak enrichment of Au and associated elements t ogether with a depletion of elements such as Ca and Mg. In Au-rich sam ples this alkali enrichment was overprinted by the carbonate-and sulph ide-bearing fractures, which often led to an increased Ca-content. An clear zonation of alteration types has not been found. The host rock h as been altered by several generations of fluids: the shear zones were repeatedly ruptured and new small-scale fractures were opened. The di fferent fluids did not, therefore, flow through identical paths althou gh they penetrated the same major zones. This resulted in a complex pa ttern of variously superimposed alterations.