The Mari Rosa late Hercynian Sb-Au deposit, western Spain Geology and geochemistry of the mineralizing processes L. Ortega \ R. Oyarzun 1.2, M. Gallego3 � partamento de Cristalografay Mineraogia, Facultad de c.c. Geolgicas, Universidad Computense, E-28040 Madrid, Spain epartament of Earth Sences, The Unversy o eeds, Leeds LS2 9T, U K   Av. Ajalvir 8, E-28806 Madrid, Spain Abstract. The central Iberian zone of the Hesperian M assifhosts a seriesoflateHercynian vein-type Sb de posits. One of them is the Mari Rosa mineralization, hostedbymetagreywackesandslatesoftheSchist-Grey wackeComplex(UpperPrecambrian).Themineralization is characterized by a complex paragenesis comprising three hydrothermalstages stage H1 - arsenopyrite-(py rite);stageH 2  - stibnite-gold;andstageH3 - pyrite-pyr rhotite-galena-sphaleri te-cha1copyri te-tetrahedrite-boul angerite-stibnite.Oftheseonlythesecondepisodewasof importanceandgaverise tothemainmineralizedbodies ofthedeposit.Hydrothermalalterationconsistsofamild sericitization, chloritization and carbonatization of the metasedimentaryrocks aroundthe veins. Chemical cha nges in the hydrothermal halos include a remarkable increase in the ratio KzO/Na20, and a decrease in the ratio S iOz/volatiles, together with a sharp increase in Sb,Mo,AuandN. Fluidsassociatedwithoredeposition lie intheHzO-NaC-COz -CH4-Nzcompositionalsystem. These fuids evolved, progressively cool ing,from initial circulaiontemperaturescloseto600C intheearlystage ( H 1 ) to temperaturesofapproximately 1 50ÍC inthelate one(H3).Fluidcompositionevolution wascharacterized byaprogressive increaseinthebulkwater contentof the fuidsandwithan increase intherelative proportion of N2withrespect toCH 4 andCOzinthevolatilefraction. Massive stibnite deposition resulted from a boil ingpro cess developed at 300ÝC and 0.9-1Kb at a depth of 4-5 km.Geological,geochemicalandfuid inclusionevid ence suggest that the intrusion of the Alburquerque batholith( lateHercynianS-typegranitoids)triggeredhy drothermal activityleadingto thetransport anddepos itionofSb andAuinMari Rosa. Thelate Hercynianextensional episode (Carboniferous Permian) was accompaniedinthe Iberian Peninsula by widespread plutonic activity. Th is magmatic activity, characterized by the intrusion of granitoids, triggered hydrothermal activity leading to one ofthe most out standing metallogenic episodes of this part of îurope. Antimony veins, some of them gold bearing, represent part of the deposits formed during this episode. These antimonydeposi tscropoutwithinthecoresofHercynian anticl ines and arehosted by pper Precambrianmeta sedimentaryrocks.Theyallhavestibniteas themainore mineral. Genetic relationships have been suggested be tween some ofthese mineral izations and the granitoids (Gumiel1983).However,preliminaryfuid inclusionstud ies intheareaofMariRosa(Fig.1)(Ortegaetal.199 1 a,b) showedfuidcompositionswithinthesystemHzO-NaC CO-CH4-Nz, thus suggesting a possible metamorphic signature (e.g. Swanenberg 1 980; Kreulen and Schui ling 1982;vandenKerkhof 1988Guillot 1 989 Boiron et al. 1990).Inthissense, the aimofthe work wastodevelop a better understandingof theore formingprocessfrom onepreviouslyselectedcasei.e .theMariRosaveins.Mari Rosawaselectedasastudysitebecause thisdeposit has already received some attention from a fuid inclusion studies viewpoint and moreover was considered to be representative enough ofa wide number ofSb veins in westerncentralSpain.Theworkrequiredacomprehens ivestudy,involvingareappraisaloftheregionalgeologi cal background,the structural control ofthe mineraliz ation, the geochemistryofaltered rocks, a characteriza tionof fuidcompositionandevolutionandanestimation ofconditionsfororedeposition.Anintegratedinterpreta tion of these data has allowed defnition of a complex metallogenical process involving fuid-rock interactions under medium PT conditions for the Sb precipitation (Ortega 1 993). Geology The Mari Rosa deposit (Fig. 1) crops out in Upper Pecambrian metasedimentaryrocksafectedbylow-grademetamorphism. Some 8km to the south, the Alburquerque batholith (Carboniferous) definesoneofthemostoutstanding regionalfeatures.Theintrusion of these peralminous cal-alkaline granitoids (S-type) induced acontactmetamorphismaureolesomekilometerswide (Perez Cer d et al. 1991). Early Hercynian compression defned the main deformational pattern resulting in a generalized pervasive olding andthe dvelopment ofa highly penetrative cleavage.