Using iron formations during exploration for c. 1.9 Ga Zn-Pb-Ag sulphide deposits, Jugansbo area, Bergslagen, Sweden
Authors: Jansson Nils , Allen Rodney 
Published: March 2020

&lt;p&gt;Oxide- and silicate-dominated, stratiform iron formations are abundant in the northern part of the Sala inlier, Bergslagen, Sweden. The iron formations are commonly laminated and are associated with fine-grained siliciclastic and felsic volcaniclastic rocks in a 1.91-1.89 Ga succession dominated by pumiceous and lithic-bearing rhyolitic volcaniclastic rocks. Depositional features are consistent with a volcanically active, submarine environment, in which the iron formations and fine-grained host strata to sulphide mineralization accumulated during pauses in volcanism. At c. 1.87-1.81 Ga, the succession underwent polyphase folding and shearing under lower amphibolite facies conditions, followed by polyphase faulting under more brittle conditions.&lt;/p&gt;&lt;p&gt;The iron formations are locally directly stratigraphically overlain by &amp;#160;stratiform Zn-Pb-Ag sulphide mineralization. Detailed geological mapping has demonstrated that sulphide-bearing (proximal) iron formation is gradational into sulphide-poor (distal) iron formation along a strike extent of more than 7 km. Proximal iron formation is dominated by magnetite, grunerite, tremolite, quartz, almandine-rich garnet (Alm&lt;sub&gt;54&lt;/sub&gt;Sps&lt;sub&gt;35&lt;/sub&gt;Grs&lt;sub&gt;8&lt;/sub&gt;), muscovite, and chlorite, whereas distal iron formation is characterized by hematite, magnetite, epidote, actinolite, spessartine-rich garnet (Sps&lt;sub&gt;53&lt;/sub&gt;Adr&lt;sub&gt;29&lt;/sub&gt;Grs&lt;sub&gt;15&lt;/sub&gt;) and locally calcite.&amp;#160;&lt;/p&gt;&lt;p&gt;Elevated contents of Mn, Zn and Co are observed in both distal and proximal iron formation, whereby these elements help pinpoint the favorable horizon, but are of less use for vectoring along strike. Whole-rock lithogeochemistry samples of proximal iron formation differ from distal iron formation in: (1) Eu/Eu*&gt;1, (2) Ce/Ce*&lt;1, (3) suprachondritic Y/Ho, (4) elevated Tl, Cs, Cd, Sn, S, Cu, Pb, Sb and Au (5) lower volcaniclastic/siliciclastic content based on lower Al, Ti and Zr. Collectively, these features are indicative of Fe mineralization following interaction of a hot, acid and reduced hydrothermal fluid with oxidized seawater in a vent proximal position which was deprived of clastic or volcaniclastic input.&lt;/p&gt;&lt;p&gt;Sulphide mineralization, ranging from banded, to disseminated and fracture-hosted, is associated with chlorite-rich, locally graphitic mudstone immediately overlying proximal iron formation. Multi-grain &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;V-CDT&lt;/sub&gt; of sphalerite, pyrite and pyrrhotite are exclusively negative, ranging from -10.6 to -0.25 with no clear mode. The &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;V-CDT&lt;/sub&gt; distribution is unusual for Bergslagen deposits, and is indicative of a significant contribution of sulphur via bacteriogenic or thermochemical reduction of seawater SO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;Stratigraphic analysis suggest that proximally, the mineralizing event followed a sudden deepening of the basin, and progressed from Fe oxide to polymetallic sulphide mineralization. The temporal zonation probably reflect a decrease in the redox potential of the basin, possibly due to venting and ponding of reduced hydrothermal fluids. Ore textures and host facies are consistent with of an exhalative mode of formation for both deposit types, albeit an importance of subseafloor mineralization processes is implied by lateral variability in both sulphide and chlorite content. In relation to the local stratigraphic evolution in the area, the mineralizing event can be directly linked to an event of basin deepening following a caldera-forming volcanic eruption. The results from stratigraphic analysis along with aforementioned proxies for redox and vent-proximity present first order vectors to stratiform Zn-Pb-Ag mineralization in the Jugansbo area, Bergslagen.&lt;/p&gt;