Fluid and melt inclusion study of magmatic-hydrothermal mineralization in the Ossa Morena Zone (SW Spain)

Abstract

The Ossa Morena Zone (OMZ) in the southwest of Spain is one of the most complex and best studied areas in the Variscan Belt, and records a heterogeneous tectonic evolution from the Cadomian (Neoproterozoic Early - Cambrian) to the Variscan orogeny (Late Paleozoic). The OMZ host a wide variety of ore deposits related to both orogenies and a rifting and stable platform event that occurred in the Early Cambrian-Mid Ordovician. Particularly important are the Fe oxide, Cu-Au and Ni-Cu orebodies situated along the Olivenza-Monesterio Belt (OMB), a major Variscan anticlinal structure located in the central part of the OMZ. This study is focused on three areas of the OMB that represent well defined metallogenic problems: the intrusionȬrelated Sultana Cu-Au vein deposit, the La Berrona magnetite deposit and some uncommon albititic rocks from the Valuengo area which are related to the La Berrona deposit. The Sultana Cu-Au deposit is located in the southeast of the OMB and consists of quartz-ankerite veins that are hosted by the Variscan Sultana Stock and the Precambrian black shale and metagreywacke of the Serie Negra Unit. The ore is mostly composed of chalcopyrite, bismuthinite and maldonite with sericitic alteration of the host rock. The La Berrona deposit is a cylindrical epigenetic magnetite orebody hosted by the La Berrona albitite granite and Early to Mid Cambrian limestone of the Malcocinado Formation. The mineralization is replacive with development of breccias and a stockwork system in the upper parts the albitite intrusion. This albitite forms part of the Cambrian albitite granites from the Valuengo area in the central OMB, some of them associated to magnetite deposits. The albitite shows a porphyritic texture with predominance of albite plagioclase and intrudes the Early to Mid Cambrian limestone and related shale and calcȬ silicate hornfels as well as the Precambrian black shale of the Serie Negra Unit. The main goals of this thesis were (1) to characterize the fluids and investigate the causes for the precipitation of Cu and Au at the Sultana deposit, (2) to investigate how the La Berrona albitite stock formed and if it was of magmatic or metasomatic origin, and (3) to characterize the magmatic-hydrothermal evolution of the La Berrona magnetite deposit, from the albitite host rock formation to the deposition of the ore. To achieve these goals fluid and melt inclusions were characterized by detailed petrography and scanning electron microscope-cathodoluminescence (SEMȬCL) imaging, and analyzed by combining analytical techniques including microthermometry, laser ablation inductively coupled plasma mass spectroscopy (LAȬICPMS) analyses, and Raman spectroscopy analyses. In addition, S stable and Sr-Nd-Pb radiogenic isotope studies were undertaken to support hypotheses established on the basis of obtained fluid and melt inclusions data. The SEM-CL images of quartz in the Sultana vein show three generations of quartz implying a multistage vein filling with dissolution and precipitation into open spaces and fractures. The detailed fluid inclusion study developed in correlation with the quartz textures suggest that a low saline (<4 to 15 wt % NaCl eq; average of 5 wt % NaCl eq.), CO2-bearing (10.4 mol %) intermediate density fluid with high ore-metal concentration (> 537 ug/g Cu, >539 S ug/g) was trapped at estimated fluid pressures of -800 bar and >420 ºC of temperature. Coinciding with the dissolution of the early generation of quartz (Q1), a phase separation occurred by condensation of minor brine (~ 40 wt % NaCl eq.) and a low saline vapor phase (~ 2.4 wt % NaCl eq.) at ~ 350 ºC and 100-300 bar, both carrying most of the copper and possibly gold (335 ± 123 ug/g Cu, 9.5 ± 6.6 ug/g Au) into the system. Copper sulfides were precipitated into open spaces before or together with the precipitation of a late generation of quartz (Q2). Consequently, Cu concentrations drop from 700 ug/g to less than 0.1ȱ ug/g in the petrographically distinguished secondary fluid inclusions. Gold was later deposited in fractures cutting chalcopyrite, probably due to the excess of sulfur in the vapor phase that transported gold after Cu precipitation. The recognition of silicate melt inclusions (SMI) in the quartz phenocrysts of the La Berrona albitite demonstrate that this rock is magmatic in origin, and not formed by metasomatic processes as it has been suggested previously in the literature. The SMI as well as few coexisting iron oxide blebs (IOB) were petrographically characterized and analyzed by LA-ICPMS. Silicate melt inclusions show an albititic composition with SiO2 (73.2±1.9 wt%), Na2O (6.8±1.1 wt%), K2O (0.1±0.1 wt%) and CaO (0.6±0.4 wt%) as major elements. In agreement with the compatible/incompatible behavior of trace elements in magmatic systems, elements such Ba, Sr, V, Zr, Ni and P are compatible with the bulk-mineralogy of the albitite whereas incompatible elements as well as ore metals such as Cu, Pb, Zn, As and Mo stay in the residual melt represented by the SMI. Comparison with experimental studies and isotopic data suggest partial melting of crustal rocks as plausible mechanism to form this albititic melt, with additional fluxing of H2O and possible presence of F that deviate the eutectic of the haplogranite system to the albite component. The study of the IOB of Fe-Ti-P-rich composition suggests the existence of an iron-titanium-phosphorous oxide melt phase that segregated from the silicate melt. The La Berrona magnetite mineralization is undoubtedly of hydrothermal origin with development of breccia, a stockwork system and magnetite-albite-actinolite replacement of the host carbonate and the albitite rock. The magnetite in the mineralization is low in Ti and P, suggesting that the fluids did not interact with the immiscible iron-oxide phase rich in these elements. The results presented in this thesis represent case studies that have the following important implications: (1) the intrusion-related copper-gold vein systems can show similar ore precipitation mechanisms as the porphyry copper deposits (i.e. phase separation, cooling, retrograde solubility of quartz), and also have characteristics of mesothermal and metamorphic ore deposits in terms of geological setting and fluid composition; (2) the albitite related to the magnetite deposits is of magmatic origin and it plays an important role in the epigenetic magnetite deposit formation. This is perhaps the most innovative contribution of this Ph.D. and opens a new research line for future studies in the magmatic origin of plagioclase rich felsic rocks as well as in studies related to iron oxide fluid/melt exolution as major mechanism for the hydrothermal/magmatic Fe oxide - (Cu – Au) deposit formation respectively; and, (3) the careful treatment of fluid and melt inclusions as coevally existing inclusion assemblages is of critical importance in order to obtain reliable and realistic geochemical data. In addition, the combination of fluid/melt inclusions with other techniques such as LA-ICPMS, SEM-CL imaging and Raman spectroscopy has shown to be a powerful tool for the study of ore deposits.