STRATIGRAPHY, AGE, AND SM-ND ISOTOPE SYSTEMATICS OF THE COUNTRY ROCKSTO ZN-PB SULFIDE DEPOSITS, AMMEBERG DISTRICT, SWEDEN

The Ammeberg Zn-Pb ore district constitutes the southern margin of the Bergslagen mining district in south-central Sweden. It is hosted by a succession of Paleoproterozoic supracrustal units belonging to the Sv ecofennian domain and was considered as a genetically distinct ore dis trict within Bergslagen already at the beginning of this century. The most prominent ore deposit belonging to this district is Zinkgruvan wh ich has been a major zinc producer for more than 100 years. In this pa per, the results from an investigation of the stratigraphy, geochemist ry, and U-Pb and Sm-Nd isotope systematics are reported from the ca. 1 0-km-thick volcanosedimentary succession which forms the country rocks in the ore district, informally named the Emme group. The Emme group may be subdivided stratigraphically into eight lithological units and generally into four parts. The lowermost part (the Mosjon and Gokberge t units and the Igelfors formation) is dominated by felsic volcanic ro cks and contains the subeconomic Zn-Pb-(Cu) deposit at Marketorp. The lowermost part of the group also contains intrusions of subvolcanic rh yolitic porphyries (U/Pb age 1901 +/- 18 Ma). The lowermost volcanite- dominated part grades upward into homogeneous, medium-grained arkoses (the Narkesberg formation) of continental derivation. Two selected sin gle zircons from these arkoses yielded Pb-207/Pb-205 ages of 1959 +/- 52 and 2703 +/- 178 Ma, respectively, indicating derivation from both Paleoproterozoic and Archean sources. This arkosic unit is overlain by an upper part containing felsic volcanic rocks (Mariedamm volcanic un it and its lateral correlatives, the Godegard volcanic unit; U/Pb ape 1889(-24)(+35) Ma) which locally display strong hydrothermal alteratio n and which are associated with deposition of the Zinkgruvan Zn-Pb-Ag ores and related carbonate rocks (the Hoksjon limestone). Tholeiitic s ubvolcanic intrusions are common in the upper part of tile Emme group. The uppermost part of the Emme group is composed of a succession of s anely to muddy turbidites (the Vintergolen formation). The epsilon(Nd) (1.9 Ga) values from the volcanogenic units range from 0.1 to 1.1 and are comparable with other felsic volcanic rocks elsewhere in Bergslag en. The epsilon(Nd) values for the arkosic units range from -2.8 to -1 .9 and for the turbidites from -2.6 to +0.9. This demonstrates a rapid dilution of Archean detritus with material from younger sources in di e uppermost part of the Emme group. The epsilon(Nd)(1.9 Ga) values for the mafic subvolcanic intrusions range from 1.4 to 4.6 and are among the highest epsilon(Nd)(1.9 Ga) values reported from Bergslagen. These data indicate that a primitive rift component was introduced in the u pper part of the Emme group before tile formation of this volcanosedim entary succession was completed. The occurrences of these mafic rift-r elated intrusions are considered to be genetically related to the depo sition of the Zinkgruvan ore, partly by providing a separate metal sou rce and partly by providing more heat to the convecting hydrothermal c ells. The deposition of the Emme group demonstrates an interplay betwe en voluminous, felsic volcanism and the input of large quantities of c ontinentally derived arkosic sediments. The probable depositional site was along an active continental margin, where subsidence of the basin varied from slow and regular to very fast. All sulfide ores in the Am meberg district were previously interpreted to constitute a characteri stic ore type in Bergslagen. The present investigation demonstrates th at at least two different ore-bearing levels (Zinkgruvan and Marketorp ), constituting two distinct volcanosedimentary environments, can be d istinguished in the Ammeberg area. It is suggested that the sulfide or es at the Zinkgruvan level represent a distinct ore-forming environmen t for Bergslagen, located along an active continental margin and depos ited in a marine environment. The particular environment characterizin g the Zinkgruvan ores appears to extend all along the southern margin of Bergslagen.