Distribution and zoning of silver and associated elements in the complex sulfide deposit at Saxberget, central Sweden

Abstract

The ore at Saxberget is a strata-bound massive to semimassive zinc-lead-copper-silver mineralization of Proterozoic age. It has the form of a slab extending 400 m in the horizontal (E-W) and more than 1,500 m in the pitch direction (35 degrees -45 degrees to the SE). The thickness varies between 1 and 20 m and generally increases upward. Although of complex character the ore can be divided into a more or less brecciated zinc- and lead-rich ore, mostly in altered Ca-Mg-Fe-rich sedimentary rocks (skarn), and a subordinate copper-rich ore confined to quartzitic rocks. Based on metal content the zinc- and lead-rich ore can further be divided into a massive ore and a disseminated ore. The transition between the two subtypes is often gradual. The geochemistry of main ore elements as well as of silver and associated minor elements suggests that the lithostratigraphy is inverted.The distribution of Zn/Pb ratios is bimodal with a strong maximum at about 2.5. This maximum corresponds to the massive ore whereas a weaker maximum at about Zn/Pb = 6.3 corresponds to the disseminated zinc-lead ore (+ copper quartzitic ore). Zinc and lead are strongly intercorrelated only in the massive ore ([rmacron] = 0.89). 1 Silver is strongly correlated with both lead and copper (R = 0.93) with mean Pb/Ag and Cu/Ag ratios of about 700 and 1,000, respectively. In galena, silver is associated with bismuth and to a lesser degree with antimony suggesting a coupled substitution (Ag:Bi + Sb [asymp] 1:1, in mole percent), at least in the disseminated ore. A lack of bismuth and antimony associated with chalcopyrite suggests a direct (Cu,Ag)FeS 2 solid solution in this mineral. Overall, the Pb/Ag ratio increases with increasing lead content.It is proposed that:1. There is a depositional change from early copper quartzitic ore in the east, to disseminated zinc and lead-rich ore in the west. The latter represents the culmination of the ore-forming pulse(s). Thus the ore discharges mainly took place laterally, i.e., perpendicular to the longest dimension. The distribution of copper quartzitic ore in the pitch direction thus suggests deposition along a zone of weakness rather than through a single conduit. The massive ore was deposited very near the lithospheric-hydrospheric boundary whereas the disseminated ore and especially the copper quartzitic ore were deposited somewhat below the sea floor.2. An increase in the Pb/Ag ratio and a decrease in the Bi + Sb/Ag ratio in galena, from the disseminated ore to the massive ore, can be attributed to a decreasing solubility of AgBi compounds in galena with a decrease in deposition temperature. Alternatively, bismuth and silver were depleted more rapidly than galena in the formation of massive ore. Thus, it is considered that the depositional milieu and the distribution of galena and chalcopyrite, as well as the contents of bismuth and antimony, have been of importance for the distribution of silver at Saxberget.