Rare aluminium phosphates and sulphates (APS) and clay mineral assemblages in silicified hydraulic breccia hosted by a Permian granite (Velence Mts., Hungary) as indicators of a high sulfidation type epithermal system (original) (raw)
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Geologica Carpathica
The Weinsberg granite, a coarse-grained biotite granite with abundant K-feldspar megacrystals, is the volumetrically dominant and most characteristic granite type of the late-Variscan Moldanubian Batholith in the Moldanubian zone of the Bohemian Massif. In the western batholith area, a local orthopyroxene-bearing variant (charnockite) of the Weinsberg granite has been identified and given the name of the Sarleinsbach quartz-monzodiorite in previous studies. Whole rock analysis of the charnockite and the relatively mafic Weinsberg granite in the immediate neighborhood show no significant geochemical differences with respect to either the major or trace elements. The mineralogy and petrology of the charnockite and surrounding granite are the same except for the presence of orthopyroxene ± clinopyroxene in the charnockite. In addition, the charnockite is characterized by the presence of dark grey, glassy orthoclase megacrysts with only some partial conversion to microcline, whereas in the granite the K-feldspar megacrysts consist of white microcline. The Fe-Mg silicates in the charnockite (orthopyroxene, clinopyroxene, amphibole, and biotite) are relatively Fe-rich (X Fe = 0.6-0.7) whereas the plagioclase is more albitic (X Ab = 0.6) than anorthitic. Fluid inclusions from the granite and associated charnockite are investigated and the results compared. The basic conclusion is that the magma responsible for the granite was dominated by an H 2 O-rich fluid with a CaCl 2 component. The magma responsible for the charnockite was dominated by a CO 2-rich fluid with a minor NaCl component, which lowered the H 2 O activity sufficiently below 1 such that orthopyroxene ± clinopyroxene was the stable Fe-Mg silicate phase during crystallization as opposed to the biotite in the granite. Taking into account that CO 2-rich and H 2 O-rich fluids are immiscible in the presence of NaCl and CaCl 2 over the P-T range of the overall crust, the implication is that in granitoid melts, if CO 2 is present, there will be regions dominated by CO 2 and regions dominated by H 2 O. The extent of either region will be determined by the overall CO 2 /H 2 O ratio in the melt. In the CO 2-dominated regions, the H 2 O activity could be sufficiently lowered such that orthopyroxene is the stable Fe-Mg silicate phase during crystallization, though this will also be dependent on the Fe/Mg ratio of these phases and the overall whole rock chemistry of the melt. In addition to incipient solid state charnockitization, commonly seen in the Archean terranes of southern India and elsewhere, this suggests that a certain subset of granites and granitoids worldwide should have patches and/or limited areas of charnockite if the amount of CO 2 present in the original magma goes above a certain fraction.
Journal of South American Earth Sciences, 2017
In the Barker-Villa Cacique area (Tandilia belt), remarkable megabreccias, limestone breccias and phosphate-bearing breccias hosted in black limestone and along the contact with the upper section of the sedimentary succession are exposed. These rocks are the result of extensive hydrothermal alteration of the original micritic limestone and other fine-grained clastic sediments. Typical alteration minerals are sericite, chlorite, interstratified chlorite/Kwhite mica, kaolinite, dickite, pyrite, chalcopyrite, goethite, quartz, calcite, Fe-calcite, dolomite, ankerite, fluor-apatite, barite and aluminium-phosphate-sulfate (APS) minerals. Quartz and calcite cements from hydraulic breccias in the limestone contain low-salinity aqueous fluid inclusions. Corresponding homogenization temperatures display 200-220ºC and 110-140 °C in hydrothermal quartz, and 130-150ºC in late calcite cement. Carbon and oxygen stable isotope analyses of carbonates from the Loma Negra quarry (LNQ) support the major role of hydrothermal activity. A significant difference was found between δ 18 O car values from unaltered micritic limestone (ca. 23.8 ‰ SMOW) and secondary calcite (ca. 18.5 ‰ SMOW). The lower δ 18 O car values are interpreted as a result of calcite precipitation from hot hydrothermal fluids. At a late stage, the hydrothermal fluid containing H 2 S mixed with descending and oxidizing meteoric waters. Circulation of the ensuing acid fluids resulted in the partly dissolution and collapse brecciation of the Loma Negra Formation. The hydrothermal stage can be tentatively dated ca. 590-620 Ma corresponding to the Brasiliano orogeny.
Fault-bound hydrothermally mineralized breccias with a multistage deformation history occur in crystalline rocks of the Aar massif at Grimsel Pass, Central Swiss Alps. The breccias crop out over 4.5 km E-W along strike and over 900 m in vertical extent between Trübtensee and Gletsch, and are up to 2 m wide. A characterization of these "Grimsel Breccias" was carried out to elucidate their formation with respect to alpine uplift and fluid circulation history, and to search for possible evidence of past microbial activity. Breccias vary widely in grain size and range from matrix-rich to clast-supported with high porosity in the youngest equivalents. Centimeter-sized voids typically contain stratified geopetal infills of fine-grained hydrothermal minerals. The hydrothermal mineral assemblage is dominated by quartz (including chalcedony), adularia, illite, celadonitic clay minerals, pyrite (As-rich), marcasite, and fine-grained Mo-sulfide. Analyses of bulk rocks (kg) and small subsamples (grams) show significant enrichments of Mo, As, Sb, Au, Cs, Hg, Tl and in some samples of U.
Magmatic Fluids in the Breccia-Hosted Epithermal Au-Ag Deposit of Rosia Montana, Romania
Economic Geology, 2006
The breccia-hosted epithermal Au-Ag deposit of Roşia Montană is located 7 km northeast of Abrud, in the northern part of the South Apuseni Mountains, Romania. Estimated total reserves of 214.91 Mt of ore at 1.46 g/t Au and 6.9 g/t Ag (10.1 Moz of Au and 47.6 Moz of Ag) make Roşia Montană one of the largest gold deposits in Europe. At this location, Miocene calc-alkaline magmatic and hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to the indentation of the Adriatic microplate into the European plate during the Carpathian orogenesis. The host rocks of the magmatic complex consist of pre-Mesozoic metamorphosed continental crust covered by Cretaceous turbiditic sediment (flysch). Magmatic activity at Roşia Montană and its surroundings occurred in several pulses and lasted about 7 m.y. Roşia Montană is a breccia-hosted epithermal system related to strong phreatomagmatic activity due to the shallow emplacement of the Montana dacite. The Montana dacite intruded Miocene volcaniclastic material (volcaniclastic breccias) and crops out at Cetate and Cârnic Hills. Current mining is focused primarily on the Cetate open pit, which was mapped in detail, leading to the recognition of three distinct breccia bodies: the dacite breccia with a dominantly hydrothermal matrix, the gray polymict breccia with a greater proportion of sand-sized matrix support, and the black polymict breccia, which reached to the surface, contains carbonized tree trunks and has a dominantly barren clastic matrix. The hydrothermal alteration is pervasive. Adularia alteration with a phyllic overprint is ubiquitous; silicification and argillic alteration occur locally. Mineralization consists of quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, and native gold and occurs as disseminations, as well as in veins and filling vugs within the Montana dacite and the different breccias. The age of mineralization (12.85 ± 0.07 Ma) was determined by 40 Ar-39 Ar dating on hydrothermal adularia crystals from vugs in the dacite breccia in the Cetate open pit. Microthermometric measurements of fluid inclusions in quartz phenocrysts from the Montana dacite revealed two fluid types that are absent from the hydrothermal breccia and must have been trapped at depth prior to dacite dome emplacement: brine inclusions (32-55 wt% NaCl eq., homogenizing at Th > 460°C) and intermediate density fluids (4.9-15.6 wt% NaCl eq., Th between 345° and 430°C). Secondary aqueous fluid inclusion assemblages in the phenocrysts have salinities of 0.2-2.2 wt% NaCl eq. and Th of 200°-280°C. Fluid inclusion assemblages in hydrothermal quartz from breccias and veins have salinities of 0.2 to 3.4 wt% NaCl eq. and Th from 200° to 270°C. The oxygen isotopic composition of several zones of an ore-related epithermal quartz crystal indicate a very constant 18 O of +4.5 to +5.0 ‰ for the mineralizing fluid, despite significant salinity and temperature variation over time. Following microthermometry, selected fluid inclusion assemblages were analyzed by laser-ablation inductivelycoupled-plasma mass spectrometry (LA-ICPMS). Despite systematic differences in salinity between phenocryst-hosted fluids trapped at depth and fluids from quartz in the epithermal breccias, all fluids have overlapping major and trace cation ratios, including identical Na : K : Rb : Sr : Cs : Ba. Consistent with the constant near-magmatic oxygen isotope composition of the hydrothermal fluids, these data strongly indicate a common magmatic component of these chemically conservative solutes in all fluids. Cu, Pb, Zn and Mn show variations in concentration relative to the relatively nonreactive alkalis, reflecting the precipitation of sulfide minerals together with Au in the epithermal breccia, and possibly of Cu in an inferred subjacent porphyry environment. The magmatic-hydrothermal processes responsible for epithermal Au-Ag mineralization at Roşia Montană are, however, not directly related to the formation of the spatially associated porphyry Cu-Au deposit of Roşia Poieni, which occurred about 3 m.y. later.
Journal of South American Earth Sciences, 2000
The Neoproterozoic of northeastern Brazil was marked by the development of collisional fold belts, mainly surrounding the São Francisco Craton, and an associated widespread granitic magmatism. The Casa Nova 555^10 Ma; Sr i 0:7068 and Engraçadinha syenites and granites, intrusive in the Riacho do Pontal Fold Belt, are related to the late stages of this collisional event. Melanocratic syenites, probably generated by magmatic-flow cumulate processes, and mesocratic and leucocratic syenites, representing magmatic liquids, are associated with granites, pegmatites, and syenite-granite dykes. Homogeneous or perthitic alkali feldspar, quartz, aegirine-augite, diopside, titanite, apatite, magnesian biotite, winchite-richterite, and magnetite are the dominant mineral phases. It is suggested that these magmas belong to an ultrapotassic silica-saturated alkaline series, defined on the basis of its alkaline, silica-saturated character and by a K 2 O/Na 2 O ratio of Ͼ3.0 -that is, intermediate. Major-and trace-element evolution is consistent with mineral fractionation processes, controlled by magmatic flow, and dominated by apatite-titanite-pyroxene in the less differentiated terms and by alkali feldspar in the more evolved. The source of primary magmas is a previously subduction-metasomatised mantle, probably with anomalous enrichment in LREE and LILE elements. Bariteilmenite mineralisations are related to the more differentiated Engraçadinha granites. ᭧
Journal of …, 2011
Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na 2 O þK 2 O)5Al 2 O 3 ] and peralkaline [(Na 2 O þK 2 O)4Al 2 O 3 ] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe^Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (AE ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite þ aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na^K^Ca( Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na^K^Ca^(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (mNa 2 O, mCaO, mK 2 O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution.
Detailed geologic mapping has been used to show that Agua Rica is a porphyry-style Cu-Mo-Au deposit that was first overprinted by polystage brecciation associated with a high sulfidation epithermal event and then by a barren surface-venting phreatomagmatic diatreme, prior to a final stage of supergene enrichment. It was emplaced in the Miocene (~8–5 Ma) as an outlier of the Farallón Negro Volcanic Complex in northwestern Argentina. The Agua Rica deposit lies next to the contact between Precambrian or lower Paleozoic metasedimentary rocks and coarse-grained Ordovician granites. In a first pulse of Miocene magmatism, equigranular to porphyritic intrusions were emplaced, with minor potassic alteration and weak Cu-Mo mineralization. Subsequent intrusion of feldspar porphyries was associated with intense porphyry-style stockwork veining, potassic and propylitic alteration, and disseminated Cu-Mo-Au mineralization (molybdenite, chalcopyrite ± bornite ± pyrite). The present alteration and mineralization pattern is dominated by an almost pervasive overprint of high sulfidation epithermal assemblages (phyllic and advanced argillic alteration and Cu-Au-Ag-As-Pb-Zn mineralization) in breccia cements and as void fillings. Covellite is the dominant copper mineral in the ore and seems to have partly or completely replaced chalcopyrite and bornite of the earlier porphyry events. The high sulfidation epithermal assemblages are closely related to the emplacement of a largely clast-supported hydrothermal breccia. Three major bodies of this breccia have been mapped on the basis of clast lithology, clast shape and size, degree of alteration, and composition of breccia matrix. Igneous breccia with a fine-grained porphyritic matrix is intimately associated and interfingers with the base of the hydrothermal breccia columns. A final phase of magmatic hydrothermal activity formed a matrix-supported and commonly bedded crater infill breccia. It formed by a surface-venting phreatomagmatic eruption, as shown by a continuous downward transition from bedded breccias to clast-supported breccias with sandy or pumiceous matrix to a solid igneous breccia with a fine-grained porphyritic matrix in the lower core of the conical crater infill breccia body. Graded, matrix-rich epiclastic sediments subsequently filled the crater. Magmatic activity was terminated by a dike of unmineralized biotite porphyry, which intruded the crater infill breccia. Talus breccia was shed into the crater from the rim. Supergene leaching and enrichment, which replaced covellite, pyrite, chalcopyrite, and bornite by chalcocite and secondary covellite, formed an enrichment blanket that was dissected by the present-day, steeply incised topography. The distinctive feature of the Agua Rica hydrothermal system is the occurrence of early, weakly mineralized intrusions, later feldspar porphyries with stockwork-hosted chalcopyrite-bornite-molybdenite mineralization, hydrothermal breccias with an epithermal pyrite-covellite overprint, and barren surface-venting breccias—all exposed at one location within 1,000 m of vertical exposure. Reconstruction of the time sequence of these geologic elements indicates that Agua Rica is the result of a protracted history of magmatic hydrothermal activity with superposition of several intrusion events that probably extended over several million years during progressive regional uplift, erosion, and explosive unroofing.