Evolution of the Paleoproterozoic Volcanic-Limestone-Hydrothermal Sediment Succession and Zn-Pb-Ag and Iron Oxide Deposits at Stollberg, Bergslagen Region, Sweden

摘要

The Stollberg Zn-Pb-Ag and magnetite mining field is located in the Bergslagen region of the Fennoscandian Shield. The main Stollberg ore deposits comprise a chain of orebodies that occur discontinuously for 5 km along a prominent marble and skarn horizon. Orebodies mainly contain magnetite and combinations of sphalerite, galena, pyrrhotite, and lesser pyrite and chalcopyrite within marble and skarn. Previously, the two main limestone (marble) units in the Stollberg area were regarded as structural repetitions of one single horizon. Based on sedimentary and volcanic facies and structural analysis, the mineralized Stollberg limestone is now shown to be the uppermost of two different limestone units within a ca. 3-km-thick Paleoproterozoic (1.9 Ga) volcanosedimentary succession. Approximately 2 km of preserved footwall stratigraphy is recognized below the Stollberg limestone, as opposed to ca. 500 m in previous structural models. This new interpretation has allowed the stratigraphic evolution prior to the mineralizing event and extent of the Stollberg hydrothermal system to be investigated in detail. After formation of the Staren limestone ca. 1 km below Stollberg, the depositional basin subsided to below wave base, while adjacent areas were uplifted and eroded. This led to the deposition of a ca. 600-m-thick, shallowing-upward sedimentary sequence in which normal-graded subaqueous mass flow deposits pass upward to polymict limestone-volcanic breccia-conglomerates. This sequence is attributed to progradation of a fan delta depositional system. The breccia-conglomerates are overlain by ca. 500 m of juvenile rhyolitic pumice breccia that is interpreted as a major pyroclastic deposit. Conformably above is the Stollberg ore host, which comprises planar-stratified, rhyolitic ash-siltstone interbedded with Fe-Mn-rich hydrothermal sedimentary rocks and limestone, all deposited below wave base. This ore host package is extensively altered to skarn and mica schist. The thickness, extent, and homogeneous composition of the rhyolitic pumice breccia below the ore host suggest that volcanism was accompanied by caldera subsidence and that the Stollberg ore deposits formed within the caldera structure. The ore host is overlain by planar-stratified, rhyolitic ash-siltstone and subordinate sedimentary breccias deposited below wave base from turbidity currents and suspension. Skarns in the Stollberg ore host unit are interpreted as metamorphosed mixtures of variably altered rhyolite, limestone, and hydrothermal sediments. Whole-rock contents of Al, Ti, Zr, Hf, Nb, Sc, Th, Ta, U, and heavy rare-earth elements are highly correlated in skarns, limestone, magnetite mineralization, and variably altered rhyolites in the Stollberg succession, suggesting that these elements were supplied by a felsic volcaniclastic component and were immobile during alteration. The felsic volcaniclastic component is calc-alkaline and characterized by negative Eu anomalies and light rare-earth element enrichment. Strong positive Eu anomalies are only observed in limestone, skarn, and iron ore in the Stollberg ore host, i.e., in samples rich in Mn, Ca, and Fe. The Stollberg ore deposits are interpreted as metamorphosed, hydrothermal-exhalative and carbonate replacement-type mineralization. The hydrothermal-exhalative component formed first by accumulation of sediments rich in Mn and Fe, coeval with limestone formation during waning volcanism. Burial of the hydrothermal system by sediments of the stratigraphic hanging wall led to a gradual shift to more reducing conditions. At this stage, the Stollberg limestone interacted with more sulfur rich hydrothermal fluids below the sea floor, producing strata-bound, replacement-type Zn-Pb-Ag sulfide and additional iron oxide mineralization.