Origin of siderite mineralisation in Petrova and Trgovska Gora Mts., NW Dinarides

The Petrova and Trgovska Gora Mts. (Gora=Mountain) are Variscan basement units incorporated into the northwestern Dinarides during the Alpine orogeny. They host numerous siderite-quartz-polysulphide, siderite-chalcopyrite, siderite-galena and barite veins, as well as stratabound hydrothermal-replacement ankerite bodies within carbonates in non-metamorphosed, flysch-like Permo-Carboniferous sequences. The deposits have been mined for Cu, Pb, Ag and Fe ores since Medieval times. Fluid inclusion studies of quartz from siderite-polysulphide-quartz and barite veins of both regions have shown the presence of primary aqueous NaCl-CaCl(2)+/- MgCl(2)-H(2)O +/- CO(2) inclusions. The quartz-sulphide stage of both regions show variable salinities; 2.7-26.2 wt% NaCl eq. for the Trgovska Gora region and 3.4-23.4 wt% NaCl eq. for the Petrova gora region, and similar homogenisation temperatures (100-230A degrees C). Finally, barite is precipitated from low salinity-low temperature solutions (3.7-15.8 wt % NaCl equ. and 115-145A degrees C). P-t conditions estimated via isochore construction yield formation temperatures between 180-250A degrees C for the quartz-sulphide stage and 160-180A degrees C for the barite stage, using a maximum lithostatic pressure of 1 kbar (cc. 3 km of overburden). The sulphur isotope composition of barite from both deposits indicates the involvement of Permian seawater in ore fluids. This is supported by the elevated bromium content of the fluid inclusion leachates (120-660 ppm in quartz, 420-960 ppm in barite) with respect to the seawater, indicating evaporated seawater as the major portion of the ore-forming fluids. Variable sulphur isotope compositions of galena, pyrite and chalcopyrite, between -3.2 and +2.7aEuro degrees, are interpreted as a product of incomplete thermal reduction of the Permian marine sulphate mixed with organically- and pyrite-bound sulphur from the host sedimentary rocks. Ore-forming fluids are interpreted as deep-circulating fluids derived primarily from evaporated Permian seawater and later modified by interaction with the Variscan basement rocks. (40)Ar/(39)Ar data of the detrital mica from the host rocks yielded the Variscan age overprinted by an Early Permian tectonothermal event dated at 266-274 Ma. These ages are interpreted as those reflecting hydrothermal activity correlated with an incipient intracontinental rifting in the Tethyan domain. Nevertheless, 75 Ma recorded at a fine-grained sericite sample from the alteration zone is interpreted as a result of later resetting of white mica during Campanian opening/closure of the Sava back arc in the neighbouring Sava suture zone (Ustaszewski et al. 2008)

Metamorphic and basin fluids in quartz-carbonate-sulphide veins in the SW Scottish Highlands: a stable isotope and fluid inclusion study

Metalliferous (Fe-Cu-Pb-Zn) quartz-carbonate-sulphide veins cut greenschist to epidote-amphibolite facies metamorphic rocks of the Dalradian, SW Scottish Highlands, with NE-SW to NW-SE trends, approximately parallel or perpendicular to regional structures. Early quartz was followed by pyrite, chalcopyrite, sphalerite, galena, barite, late dolomite-ankerite and clays. Both quartz-sulphide and carbonate vein mineralisation is associated with brecciation, indicating rapid release of fluid overpressure and hydraulic fracturing. Two distinct mineralising fluids were identified from fluid inclusion and stable isotope studies. High temperature (>350°C) quartz-precipitating fluids were moderately saline (4.0-12.7 wt.% NaCl equivalent) with low (approximately 0.05). Quartz 18O (+11.7 to +16.5) and sulphide 34S (13.6 to 1.1) indicate isotopic equilibrium with host metasediments (rock buffering) and a local metasedimentary source of sulphur. Later, low-temperature (TH = 120-200°C) fluids, probably associated with secondary carbonate, barite and clay formation, were also moderately saline (3.8-9.1 wt.% NaCl equivalent), but were strongly enriched in 18O relative to host Dalradian lithologies, as indicated by secondary dolomite-ankerite (18O = +17.0 to +29.0, 13C = 1.0 to 3.0). Compositions of carbonate-forming fluids were externally buffered. The veins record the fluid-rock interaction history of metamorphic host rocks during cooling, uplift and later extension. Early vein quartz precipitated under retrograde greenschist facies conditions from fluids probably derived by syn-metamorphic dehydration of deeper, higher-grade rocks during uplift and cooling of the Caledonian metamorphic complex. Veins are similar to those of mesothermal veins in younger Phanerozoic metamorphic belts, but are rare in the Scottish Dalradian. Early quartz veins were reactivated by deep penetration of low-temperature basin fluids that precipitated carbonate and clays in veins and adjacent Dalradian metasediments throughout the SW Highlands, probably in the Permo-Carboniferous. This event is consistent with paragenetically ambiguous barite with 34S characteristic of late Palaeozoic basinal brines