REGIONAL AND LOCAL HYDROTHERMAL ALTERATION OF PROTEROZOIC METAVOLCANIC FOOTWALL ROCKS OF THE ZINKGRUVAN ZN-PB-AG ORE DEPOSIT, CENTRAL SWEDEN

The Zinkgruvan Zn-Pb-Ag deposit in Central Sweden is a Proterozoic str atiform blanket-like ore body, hosted by a series of chemical-clastic metatuffites, calc-silicate rocks, metacherts and marbles. This series is underlain by a pile of felsic to intermediate metavolcanic rocks, and covered by a sequence of quartz- feldspar- biotite sediments. The metavolcanic footwall rocks have been affected by a number of hydrothe rmal alterations: 1) regional sub-seafloor, low temperature K-feldspar alteration in the upper parts of the volcanic column, overprinted by higher temperature albitization in the lower parts; 2) local scale bor on alteration (tourmalinization) in the footwall rocks and sedimentary -exhalative sequences with mm-thick tourmalinites within and dm-thick tourmalinites away from the main ore zone; 3) local scale silicificati on overprinting the K- and Na-altered volcanites in restricted zones, occasionally accompanying tourmalinization; 4) local scale ferromagnes ian alteration (''magnesium metasomatism'') in semi-conformable zones below a minor sulphide deposit; 5) local scale, infiltration and vein controlled calc-silicate alteration, clearly overprinting older silici fication; and 6) local scale phyllic alteration of various types and a ges. Field observations and microscopic studies indicate that tourmali nization, silicification and ferromagnesian alteration occurred prior to regional compressional deformation. Most of the tourmaline, however , has moved out from its original host rock into tourmaline-rich pegma titic veins during regional high-grade metamorphism, and into very lat e tourmaline-quartz joint fillings during post-tectonic block movement . Since the anomalous concentrations of tourmaline occur both in parts of the footwall rocks as well as in the exhalative, ore-bearing zone, this type of alteration is likely to be part of the productive hydrot hermal system. In contrast, calc-silicate alteration, occurring in def ormed veinlets and flattened network veins is probably of later, syn-m etamorphic origin. The primary driving force behind the circulation of the hydrothermal fluids is presumably the high geothermal gradient, a s suggested for so many other sediment-hosted exhalative zinc-lead dep osits. A combination of the macroscopically identifiable local alterat ions, that left their traces as now secondary infillings with tourmali ne and calc-silicates, should rouse enough suspicion for the presence of a Zinkgruvan-type deposit. Once the target areas have been identifi ed, the sulphide mineralizations can be traced by following the uneven ly distributed geochemical anomalies: very high K2O contents (> 9 wt % ), B, Ba, CaO, MnO, TiO2 and Cs for the Zinkgruvan-type of ores, and F e2O3 + MgO, Ce and Cs for the Lovdalen-type of sulphide mineralization related to ferro-magnesian alteration.