Subvolcanic Bodies Research Articles

Rosia Poieni copper deposit, Apuseni Mountains, Romania: advanced argillic overprint of a porphyry system

The Rosia Poieni deposit is the largest porphyry copper deposit in the Apuseni Mountains, Romania. Hydrothermal alteration and mineralization are related to the Middle Miocene emplacement of a subvolcanic body, the Fundoaia microdiorite. Zonation of the alteration associated with the porphyry copper deposit is recognized from the deep and central part of the porphyritic intrusion towards shallower and outer portions. Four alteration types have been distinguished: potassic, phyllic, advanced argillic, and propylitic. Potassic alteration affects mainly the Fundoaia subvolcanic body. The andesitic host rocks are altered only in the immediate contact zone with the Fundoaia intrusion. Mg-biotite and K-feldspar are the main alteration minerals of the potassic assemblage, accompanied by ubiquitous quartz; chlorite, and anhydrite are also present. Magnetite, pyrite, chalcopyrite and minor bornite, are associated with this alteration. Phyllic alteration has overprinted the margin of the potassic zone, and formed peripheral to it. It is characterized by the replacement of almost all early minerals by abundant quartz, phengite, illite, variable amounts of illite-smectite mixed-layer minerals, minor smectite, and kaolinite. Pyrite is abundant and represents the main sulfide in this alteration zone. Advanced argillic alteration affects the upper part of the volcanic structure. The mineral assemblage comprises alunite, kaolinite, dickite, pyrophyllite, diaspore, aluminium-phosphate-sulphate minerals (woodhouseite-svanbergite series), zunyite, minamyite, pyrite, and enargite (luzonite). Alunite forms well-developed crystals. Veins with enargite (luzonite) and pyrite in a gangue of quartz, pyrophyllite and diaspore, are present within and around the subvolcanic intrusion. This alteration type is partially controlled by fractures. A zonal distribution of alteration minerals is observed from the centre of fractures outwards with: (1) vuggy quartz; (2) quartz + alunite; (3) quartz + kaolinite ± alunite and, in the deeper part of the argillic zone, quartz + pyrophyllite + diaspore; (4) illite + illite-smectite mixed-layer minerals ± kaolinite ± alunite, and e) chlorite + albite + epidote. Propylitic alteration is present distal to all other alteration types and consists of chlorite, epidote, albite, and carbonates. Mineral parageneses, mineral stability fields, and alteration mineral geothermometers indicate that the different alteration assemblages are the result of changes in both fluid composition and temperature of the system. The alteration minerals reflect cooling of the hydrothermal system from >400 °C (biotite), to 300–200 °C (chlorite and illite in veinlets) and to lower temperatures of kaolinite, illite-smectite mixed layers, and smectite crystallization. Hydrothermal alteration started with an extensive potassic zone in the central part of the system that passed laterally to the propylitic zone. It was followed by phyllic overprint of the early-altered rocks. Nearly barren advanced argillic alteration subsequently superimposed the upper levels of the porphyry copper alteration zones. The close spatial association between porphyry mineralization and advanced argillic alteration suggests that they are genetically part of the same magmatic-hydrothermal system that includes a porphyry intrusion at depth and an epithermal environment of the advanced argillic type near the surface.

Anomalous Reverse Zoning of Saponite and Corrensite Caused by Contact Metamorphism and Hydrothermal Alteration of Marly Rocks Associated with Subvolcanic Bodies

AbstractThe effects of a subvolcanic intrusion on its host rocks in the Priego de Córdoba area (SE Spain) was studied by optical microscopy, X-ray diffraction and electron microscopy. The intrusion of a laccolith of stratiform dolerite in partially consolidated marly sediments at quite shallow depths below the ocean floor took place during the intracontinental rifting phase of the Subbetic zone of the Betic Cordilleras. In the first stage, contact metamorphism caused the crystallization of calc-silicate consisting of grossular andradite garnet, diopside, vesuvianite and titanite for which the upper temperature limit was 500°C. Phyllosilicates are found in a network of veins cross-cutting the carbonate and calc-silicate matrix, indicating their formation during a process of hydrothermal alteration superimposed on the contact aureole. In the area closest to the subvolcanic rocks, saponite is the main phyllosilicate although some dispersed chlorite is found. In the zone furthest from the contact, corrensite together with saponite and beidellitic smectites were observed. The presence of low-temperature phases (saponite) in the area closest to the laccolith reveals the evolution of the hydrothermal system toward cooler conditions. In the early stage, the circulation of a hot hydrothermal fluid caused the crystallization of chlorite in the area surrounding the subvolcanic body and corrensite in the more distal area, which might have begun even during the contact metamorphism stage. A cooling phase followed, resulting in the crystallization of saponite in the host rocks, and the crystallization of dioctahedral and trioctahedral smectites inside the subvolcanic body.

Anomalous Reverse Zoning of Saponite and Corrensite Caused by Contact Metamorphism And Hydrothermal Alteration of Marly Rocks Associated with Subvolcanic Bodies

Anomalous Reverse Zoning of Saponite and Corrensite Caused by Contact Metamorphism And Hydrothermal Alteration of Marly Rocks Associated with Subvolcanic Bodies

Shrimp U-Pb age and Sr-Nd isotopes of the Morro do Baú mafic intrusion: implications for the evolution of the Arenópolis volcano-sedimentary sequence, Goiás Magmatic Arc

The Arenópolis volcano-sedimentary sequence is located in the southern part of the Goiás Magmatic Arc and includes a ca. 900 Ma calc-alkaline arc sequence made of volcanic rocks ranging in composition from basalts to rhyolites, metamorphosed under greenschist to amphibolite facies. Small calc-alkaline gabbro to granite sub-volcanic bodies are also recognized. The Morro do Baú intrusion is the largest of these intrusions, and is made of gabbros and diorites. Zircon grains separated from one gabbro sample and analyzed by SHRIMP I yielded the mean 206Pb/238U age of 890 +/- 8 Ma, indicating that the intrusion is roughly coeval or only slightly younger than the Arenópolis volcanics. Contrary to the metavolcanics, which are juvenile, the Nd isotopic composition of the Morro do Baú gabbro indicates strong contamination with archean sialic material (T DM of 2.8 Ga and EpsilonNd(T) of -9.7), represented in the area by an allochthonous sliver of archean/paleoproterozoic gneisses (Ribeirão gneiss) which are the country-rocks for the gabbro/dioritic intrusion. The emplacement age of ca. 890 Ma represents a minimum age limit for the tectonic accretion of the gneiss sliver to the younger rocks of the Arenópolis sequence. The data suggest that this happened early in the evolution of the Goiás Magmatic Arc, between ca. 920 and 890 Ma.

Fluid origin of the Ixtacamaxtitlán hydrothermal deposits, Puebla State, Mexico

The hydrothermal deposit of Ixtacamaxtitlán (Puebla, Mexico) is made up by a succession, from bottom to top, of quartzveins and stockwork enclosed in a porphyritic subvolcanic body, a kaolinitized rhyolitic tuff and a layered opal deposit. This vertical arrangement coupled with the distribution of the different alteration assemblages lead to the interpretation of the whole as a low-sulfidation epithermal deposit. The fluid inclusion study carried on the veins and the stockwork along with the stable isotopic analyses performed on the kaolinitized bodies helped us to propose two major hydrothermal events that occurred in the area: an early event, characterized by hot, hypersaline fluids (up to 280 °C and 36 wt.% NaCl eq.) closely associated with a potassic alteration episode; and a late event, distinguished by cooler and dilute fluids (up to 150 °C and 4 wt.% NaCI eq.), associated with propylitic and quartz-sericite alterations at depth and acid-sulfate alteration close to the paleosurface due to steam-heated phreatic waters. Post-trapping changes found affecting the primary fluid inclusions (hook-shaped morphologies) suggest that there was an uplifting period between the two stages.

Geological and Petrographical study of Montecampione Triassic subvolcanic bodies (Southern Alps, Italy): preliminary geodynamic results

Geological and Petrographical study of Montecampione Triassic subvolcanic bodies (Southern Alps, Italy): preliminary geodynamic results

A Província Alcalina de Goiás e a extensão do seu vulcanismo kamafugítico

The Late Cretaceous Goias Alkaline Province (GAP) is located along a 250 x 70 Km, NW-trending region, and consists of mafic ultramafic alkaline plutonic complexes in the north, sub-volcanic bodies in the central portion and kamafugitic lava flows in the south. This provice contains some of the largest known kamafugite exposures, including the Santo Antonio da Barra flows. The lava volume in these flows reaches at least 23 Km 3 , spread over an area of 371 Km 2 , and are the world’s largest known kamafugite lava accumulation. The calculated lava volume is consistent with magma chambers equivalent in size to the plutonic complexes in the north of the Province, or with the kamafugite-carbonatite intrusion in the nearby Alto Paranaiba Igneous Province (APIP).

Porphyry-copper mineralisation in the central Srednogorie zone, Bulgaria

The porphyry-copper systems in the central part of the Srednogorie zone (Bulgaria) are represented by three major deposits (Elatsite, Medet and Assarel) and several smaller deposits and occurrences, all of them within the Panagyurishte ore district. The hydrothermal systems are related to Late Cretaceous calc-alkaline igneous complexes. Ore mineralisation is developed predominantly in the apical parts of subvolcanic and intrusive bodies as well as within the volcanic and basement metamorphic rocks. Several of the porphyry systems are spatially associated with shallow-level intermediate and high-sulphidation volcanic-hosted epithermal deposits of economic importance, such as the major gold-copper mine at Chelopech located 10 km from the Elatsite porphyry-copper deposit. Mineralisation processes in the porphyry deposits start with intensive hydrothermal alteration of the wall rocks. K-silicate alteration is characteristic for pre-ore hydrothermal activity in all of them, and it is located mostly in their central parts. Propylitic alteration is prominent in the Medet and Assarel deposits. The Assarel deposit is located in the central part of a palaeovolcanic structure and shows a large spectrum of pre-ore alterations, including propylitic, sericitic, and advanced argillic assemblages. The initial stages of the hydrothermal systems are characterised by high temperatures (>550–500 °C) and highly saline (50–20 wt% NaCl equiv.) and vapour-rich fluids of likely magmatic origin. The composition of the fluids gradually changes from H2O–NaCl±FeCl2 to H2O–NaCl–KCl and H2O–NaCl-dominated as the fluids cool, react with wall rocks, and may become diluted with meteoric water. Fe-Ti-oxide mineral associations were formed early in all deposits, later followed, in the Elatsite deposit, by an assemblage of bornite, chalcopyrite, platinum group element (PGE) phases, Co-Ni thiospinels, Ag- and Bi-tellurides, and selenides. The main ore stage in all deposits is dominated by chalcopyrite ± pyrite, associated with propylitic alteration at Medet, K-silcate-sericite alteration at Elatsite, and chlorite-sericite alteration at Assarel. Specific geochemical features of the ore systems are marked by precipitation of numerous fine-grained mineral inclusions in chalcopyrite and pyrite (palladoarsenide, Pd-rammelsbergite, Cu-Ni sulpharsenides in Elatsite; Co-and Ni-pyrite, carrolite, vaesite, sulvanite and colusite in Medet; enargite, As-sulvanite, colusite, goldfieldite, wittichenite, calaverite and aikinite in Assarel). Native gold and electrum are observed as small grains, except in Elatsite where larger aggregates up to several millimetres are found. Molybdenite is present as small flakes within chalcopyrite, or forms felted aggregates within thin quartz veinlets in the Elatsite and Medet deposits, and very rarely at Assarel. Thin veinlets of quartz-galena-sphalerite are found at the upper parts of the deposits. Supergene alteration of the primary sulphide minerals is developed in the Assarel deposit where a thick blanket of secondary copper mineralisation (chalcocite, covellite) has formed. In the other two deposits, secondary mineral associations (Cu and Fe oxides and hydroxides) are limited to near the surface. Differences in the deposits studied suggest that Elatsite and Medet are formed probably at deeper levels of the porphyry systems, whereas Assarel is developed at rather shallower and lower-temperature conditions typical of the apical part of a subvolcanic body intruding its volcanic superstructure.

THE BAN PHUC Ni Cu PGE DEPOSIT RELATED TO THE PHANEROZOIC KOMATIITE BASALT ASSOCIATION IN THE SONG DA RIFT, NORTWESTERN VIETNAM

The Permian–Triassic komatiite–basalt complex in the Song Da rift, northwestern Vietnam, occurs in the axial part of this structure and includes komatiites, komatiitic basalts, olivine basalts, and subvolcanic bodies of dunite and plagioclase-bearin g wehrlite hosting Ni–Cu–PGE sulfide ores. Volcanic rocks contain olivine- and pyroxene-spinifex textures. The Ban Phuc deposit consists of two major orebodies. The first one is of vein type, zonal, with average Ni/Cu of 2.5 and Ni/Co of 33. In the centra l part, it is composed of massive chalcopyrite – pentlandite – (violarite) – pyrrhotite ores with associated sulfarsenides of Ni and Co , heazlewoodite, tsumoite, parkerite, sperrylite, and michenerite. Marginal parts of (pentlandite) – pyrrhotite – chalcopyrite, a nd violarite – chalcopyrite composition contain nickeline, maucherite, sperrylite, and michenerite. The second orebody is composed of disseminated ores with an average Ni content of 1.2%. The plots of mantle-normalized contents of noble metals, Ni and Cu reflect their non-fractionated distribution, typical of komatiites. Ore formation proceeded in two stages: 1) magmatic stage, including sulfide–silicate immiscibility and fractional crystallization of the sulfide melt, and 2) postmagmatic hydrothermal processes. The saturation of the komatiitic melt with respect to a sulfide at an early stage of formation of the Ban Phuc suite is suggested by a steeper trend of olivine composition on the Ni in olivine – Fo diagram compared with olivine from ore-free volcanic rocks of the komatiite–basalt series. Later on, fractional crystallization of a fluid-bearing sulfide melt was the mos t important factor in the formation of ore. Enrichment of Cu-bearing ores from zonal orebodies in Pd and Au corresponds to their concentration in the residual sulfide melt during fractionation of Mss. The unusual Pt concentrations of the Fe–Ni ores are due to the presence of sperrylite, which contains between 0.9 and 3.4 wt.% Sb at the high-temperature stage (900–1000 °C) of the volatile-saturated sulfide. At the lower-temperature stage, sulfarsenides of Ni, Co, and Fe appeared, together with michenerite at 500–400°C. At 300°C and below, tsumoite and the Cu–Pb–Ag–Bi sulfosalts made their appearance.

Rhenium - rich molybdenites in Thracian porphyry Mo±Cu occurrences, NE - Greece

The present study correlates both the mineralogy of the hydrothermal alteration and the mineral chemistry of molybdenites from three porphyry Mo ± Cu occurrences in Thrace: Melitena, Pagoni Rachi/Kirki and Ktismata/ Maronia. The mineralisations are genetically related to calcalkaline, subvolcanic bodies of Tertiary age. According to their mineralogical and chemical composition the host rocks are characterized as dacite (Melitena), dacitic andésite (Pagoni Rachi) and porphyry microgranite (Ktismata/Maronia). The molybdenites occur in disseminated form, as fracture fillings, as well within quartz stockworks crosscuting the central alteration zones of the intrusives. They are accompanied by the following mineral assemblages: quartz, sericite, pyrophyllite, diaspore, Ca-Ba-rich alunite, pyrite (Melitena); quartz, albite/K-feldspar, biotite, actinolite, magnetite (Pagoni Rachi); and sericite, kaolinite, pyrophyllite, chlorite (Ktismata). Preliminary microthermometric results showed homogenisation temperatures from 352° to 390 °C for Pagoni Rachi area and from 295° to 363 °C for Melitena area. The salinities range between 4.5 and 6.1 wt% eq. NaCl and between 2.7 and 3.4 wt% eq. NaCl, respectively. Detailed study on over 400 fluid inclusions from the porphyry Cu-Mo deposit in Maronia area revealed formation temperatures from 300° to 420 °C, whereas salinities are distincted in two different groups from 6 to 16 wt% eq. NaCl and from 28 to 55 wt% eq. NaCl. The chemical composition of the molybdenites from the three porphyry Mo±Cu deposits in Thrace was studied with 155 microprobe analyses. The results revealed unusual high and variable Re concentrations in the studied molybdenites. Re content in molybdenite from Melitena area vary from 0.21 to 1.74 wt%, 0.79 wt% on average. The highest values were measured in samples from Pagoni Rachi (0.45-4.21 wt%, 1.98 wt% on average). Finally, microprobe analyses from molybdenite in Ktismata/Maronia showed Re content between 0.12 and 2.88 wt% (0.76 wt% on average). Rhenium is a very rare element with many definite uses, and is mainly associated with molybdenite in porphyry type deposits. According to the data published so far the Re content in molybdenite reaches up to 0.42 wt%. It is obvious therefore that such high Re concentrations (0.12 to 4.22 wt%) from the studied molybdenites in Thrace, are very ineresting for a possible future exploitation.

Fluid evolution and chemical controls in the Fazenda Maria Preta (FMP) gold deposit, Rio Itapicuru Greenstone Belt, Bahia, Brazil

The Fazenda Maria Preta (FMP) gold deposit is confined to two regional-scale, sinistral-oblique brittle–ductile shear zones which are located in a greenschist facies metamorphosed volcano-sedimentary sequence in the northern sector of the lower Proterozic Rio Itapicuru greenstone belt, NE Brazil. Alteration is pervasive throughout the shear zones and characterized by carbonatization and sericitization of intermediate volcanic, volcaniclastic, and sub-volcanic rocks. The orebodies are mainly shear veins hosted by, or directly associated with, highly deformed carbonaceous volcaniclastic rocks, breccia and stockwork systems or disseminations in felsic and mafic sub-volcanic bodies. Native gold appears as free grains finely disseminated in the quartz veins or closely related to arsenopyrite, pyrite, pyrrhotite, sphalerite, and Fe-chlorite in the carbonaceous wallrocks. Fluid inclusion studies by microthermometry and laser Raman microspectroscopy revealed that the vein quartz is largely dominated by primary, pseudosecondary and rarely secondary, populations of CO 2–(±CH 4±N 2) inclusions (type I), whereas primary groups of low salinity (<6 wt.% eq. NaCl) H 2O–CO 2–(±CH 4±N 2) inclusions (type II) comprise the dominant inclusion type in only a few veins. The ThCO 2 (L–V→L) data indicated a variation in the CO 2 density of 1.05 to 0.60 g/cm 3 for the type I inclusions and, conversely, a narrower range of 0.76–0.73 g/cm 3 for the type II inclusions. The broader variation of the CO 2 density for the CO 2-rich fluid could have been the result of trapping of the vein fluid under a variable pressure regime, or by its re-equilibration during continuous deformation within the shear zone domains and reduction of the overburden pressure during uplift. The CO 2-rich fluid inclusions provide no evidence that they have been the result of H 2O loss either during infiltration (e.g., hydration reactions) or after trapping of an original H 2O–CO 2 fluid. Accordingly, the CO 2±(CH 4+N 2) and the H 2O–CO 2–(±CH 4±N 2) fluids are considered as representatives of two distinct fluid regimes, which were active during the gold mineralizing events within the shear zones. Both types of ore fluids are interpreted as part of a deep metamorphic–magmatic hydrothermal system in which (i) a CO 2-rich fluid, probably originated in the mantle, was transported to, and released in, higher structural levels mainly by tonalitic–granodioritic and alkaline magmas, whereas (ii) a H 2O–CO 2 fluid, generated by devolatilization reactions during the regional metamorphism, was either directly channeled into favorable structural sites (i.e., metamorphic fluid), or absorbed and later exsolved by the crystallization of felsic magmas (i.e., magmatic fluid sensu lato). In this context, gold deposition occurred between 320°C and 420°C and 2.1 to 4.4 kb and calculated values of fO 2 between 10 −26.5 and 10 −32.4 bar reveal the relatively reducing nature of the mineralizing fluids. The ore paragenesis constrains fS 2 to the range of 10 −10.1–10 −6.5 bar for these fluids. Under such conditions, the gold was transported mainly as bisulfide complexes by the H 2O–CO 2 fluid, but in the case of the CO 2-rich fluid, the role of thiocomplexes or some other types of ligands is obscured by the lack of high temperature experimental data. Deposition of the metal occurred in response to redox changes, which accompanied fluid–carbon interaction, particularly a decrease in fO 2 and fS 2 of the fluid. On a microscopic scale, phisisorption and chemisorption processes at the fluid–iron sulfide interface may have further enhanced the deposition of the gold, particularly on the surfaces of precipitating pyrite and arsenopyrite.

Adularia-sericite gold deposits of Marmato (Caldas, Colombia): field and petrographical data

Abstract At Marmato (Caldas, Colombia), epithermal gold mineralization is associated with volcanic to subvolcanic (dacite to andesite) bodies intruding a late Miocene sedimentary sequence and older basement rocks. Mineralization occurs in dilational veins related to a post-magmatic brittle deformation event, possibly connected with reactivation of the Cauca-Romeral fault system. Within the vein system, a vertical zonation is observed. In the lower part the veins are mainly composed of adularia + pyrite ± quartz ± calcite. In the upper part the dominant assemblage is calcite + quartz + pyrite + sphalerite + arsenopyrite + galena + chalcopyrite, and minor argentite, pyrrhotite, polybasite and freibergite. Data from drill cores show that gold is principally related to the hydrothermal veins and dissemination in the wall rock is of minor importance. Wall-rock alteration around the veins is mainly represented by a strong, often complete, sericitization, with minor silicification. The sericitization post-dates an earlier propylitic alteration event, unrelated to gold deposition. The hydrothermal circulation was characterized by ascent of low salinity fluids with a low CO 2 content focused along a structurally controlled fracture system. Boiling was a major controlling factor in deposition in the lower part of the system, while in the upper part mixing also occurred.

Bismuth and bismuth–antimony sulphosalts from Neogene vein mineralisation, Baia Borşa area, Maramureş, Romania

AbstractSeveral complex Cu-Pb-Bi, Cu-Pb-Bi-Sb and Ag-Pb-Bi sulphosalt minerals have been identified in samples from hydrothermal vein mineralisation associated with the Toroiaga sub-volcanic body in the Baia Borşa area of Maramureş County, northwest Romania. This is the first chemically-documented report of Bi-sulphosalts in the Neogene metallogenic province around Baia Mare. The investigated samples contain abundant amounts of matildite solid solution within galena, the Cu-Pb/Bi sulphosalts aikinite, friedrichite, krupkaite, hammarite, lindströmite and gladite as well as nuffieldite and berryite. Within the Ag-Pb/Bi group, the majority of analysed grains can be regarded as members of the lillianite homologous series. Three distinct lillianite homologues were identified, which correspond to (i) phases along the lillianite-gustavite solid solution join (Pb3Bi2S6-AgPbBi3S6), (ii) phases within solid solution field of heyrovskyite, and (iii) compositions which best correspond to ‘schirmerite’,sensuMakovicky and Karup-Møller (1977b), but may represent disordered gustavite, vikingite or eskimoite. Some of the analysed lillianite homologues contain excess Cu, which may occupy interstitial sites. Furthermore, a large proportion of the lillianite homologues display significant substitution of Sb for Bi within the limits predicted by experimental investigations. Cosalite, also showing a range of compositions including Sb-rich varieties is recognised. Izoklakeite, Cu2Pb22(Sb,Bi)22S57, is an abundant phase throughout the analysed samples, its composition is in good agreement with previously published analyses, except for excess Cu and Fe beyond the limits previously reported. The description of several minerals from this new occurrence and compositional data on them, including the Sb-bearing varieties, provides valuable additional information on compositional limits in natural samples.

The Pantelleria caldera geothermal system: Data from the hydrothermal minerals

This paper proposes, on the basis of petrographic and mineralogic data on cutting and cores from two deep wells (“Pantelleria 1” and “Pantelleria 2”), the first model of the active hydrothermal system of the island of Pantelleria. Phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes. The results of these studies emphasize differences between the intracaldera and pericaldera areas of the island. Within the 45 ka caldera there is a high-temperature (240–260 °C at 600–800 m depth) active hydrothermal system with five zones of characteristic alteration minerals with increasing depth. Rocks are unaltered to a depth of 200 m, contain smectite and mixed-layer chlorite-smectite (C/S) between 200 and 380 m, chlorite, illite, chalcedony and quartz from 380 to 500 m, albite, adularia and saponite from 500 to 680 m, mixed-layer biotite-vermiculite from 680 m to the depth drilled (1100). Outside the caldera, but near the rim, a low-temperature and low-permeability (< 140 °C) hydrothermal system is characterized by smectite, dolomite and ankerite at depths of 390 to 650 m, chlorite and calcite at 650–900 m, and mixed layers of chlorite-smectite, illite-smectite and iron carbonates (ankerite, siderite) from 900 m of the well at 1203 m. The superimposition of hydrothermal mineral assemblages is evidence for cooling in the hydrothermal system both inside and outside the caldera. We propose that a high-temperature hydrothermal system developed inside the caldera. In an early stage in the area surrounding the subvolcanic body, biotite isograd is reached and an alkali-metasomatism zone develops inside the body itself. This phase may also account for the development of a chlorite-albite-adularia zone extending to 400 m. A cooling phase (nearly 50 °C) followed, resulting in the substitution of biotite by mixed-layer biotite-vermiculite and by the crystallization of Fe-rich saponite instead of chlorite, within the currently active reservoir. A cooling phase has also been identified in the well outside the caldera.

Geological Structure and Ore Mineralogy of the Juliette Gold-Silver Deposit, Northeast Russia

Julietta is a rich epithermal gold-silver deposit of the low-sulfidation, adularia-sericite type, located in the Cretaceous Okhotsk-Chukchi volcanic-plutonic belt 250 km northeast of Magadan. The deposit was discovered in 1989 by a regional soil geochemical survey in an area previously considered barren on the basis of a regional stream-sediment survey. The deposit has not been completely explored, but presently is in the feasibility stage; proven reserves are 26 metric tons of Au (grades averaging 23 g/t) and the Au/Ag ratio is about 1:10. The deposit occurs on the periphery of a large volcanic-tectonic depression. Host andesite, andesite-basalt lava, corresponding subvolcanic bodies, and tuff are cut by Early Cretaceous quartz diorite stocks. Six vein zones occur in tensional and compression fissures. Ore shoots and smaller bonanzas comprising most of the gold reserves are located in flexures of the ore-host fissures. Ore mineralization was preceded by intense voluminous propylitization and linear seric...

Duration of strike-slip movements in large shear zones: The Red River belt, China

Sixty four size fractions of zircon, monazite, xenotime and titanite were dated by the UPb method to temporally and spatially constrain left-lateral movements in the Red River shear zone. Between the depths of 15 and 20 km, the shearing led to the formation of the high-grade metamorphic gneisses that form the bulk of the ∼ 4200 m high Diancang Shan and ∼ 3100 m high Ailao Shan mountain ranges. The omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust, and deformation patterns are very homogeneous over the entire length of the belt. North of the gneisses, subvolcanic bodies intrude unmetamorphosed cover rocks—in particular Eocene red beds—along the projected trend of the Red River shear zone. For a leucogranitic layer in the Diancang Shan gneisses, an age of 24.2 ± 0.2 Ma (2σ) is obtained for monazite and 22.4 ± 0.2 Ma is obtained for xenotime, whereas zircon formed through two growth stages at 24–23 Ma. These ages record solidification of the anatectic melts over a period of at least 1.8 ± 0.4 Ma, during which left-lateral shear was continuously active. In the Ailao Shan gneisses, zircon and titanite ages are 26.3 ± 0.3 and 26.1 ± 0.2 Ma for two monzonitic bodies, and 24.1 ± 0.2 and 22.4 ± 0.2 Ma for two pegmatitic layers that occur within the monzonitic intrusions. These results show that zircon, monazite, titanite and probably also xenotime are closed systems for U and Pb under melting conditions ( > 650°C). Therefore, the ages date mineral formation in the melt, rather than cooling below certain closure temperatures. A significantly older age of 35.0 ± 0.1 Ma is obtained for a monzonitic subvolcanic intrusion north of the Diancang Shan gneisses. Because these intrusions follow the trend of the shear zone, they most likely represent the surface expression of early activity along that zone, indicating that strike-slip movement had already occurred in latest Eocene time. The total set of UPb ages shows that magmatic and metamorphic activity occurred from 35 to 20 Ma, strengthening the idea that the Red River shear zone absorbed most of the intraplate movements required to compensate the opening of the South China Sea between 32 and 16 Ma. The existence of monzonitic-syenitic intrusions along the Red River zone implies that melting affected source rocks other than the Diancang Shan and Ailao Shan gneiss protoliths. Potential sources to produce such rocks lie in the deep crust, possibly including melts from the mantle. Consequently, the Red River shear zone must be a deep-rooted structure, along which melts migrated to the surface. Moreover, the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike-slip activity.

Types of epithermal silver deposits, northeastern Russia

Three main epithermal silver provinces occur in northeastern Russia: (1) the Cretaceous Okhotsk-Chukotka volcanic plutonic belt (Russia's part of the Circum-Pacific belt), (2) adjacent Mesozoic orogenic belts, and (3) Paleozoic complexes formed of intermediate massifs. Within the most studied Okhotsk-Chukotka province, three types of deposits are distinguished by structural position and geologic environments: (1) those in volcanic intrusive domes, (2) those in siliceous subvolcanic bodies, and (3) those in contact halos around granodioritic intrusions. Gold-silver, silver-lead-zinc, and tin-silver ore deposits are distinguished, according to the ore composition. The silver deposits of the Ducat ore district are examples of these three types.Genetically, ore mineralization of the three types differs. Gold-silver mineralization was formed in near-surface, open hydrothermal systems during mixing of metalliferous solutions with meteoric waters, as indicated by low salinities (2.5-3.5% NaCl equiv) of the fluid inclusions. Subvolcanic and intrusive bodies were the local heat sources, which caused a field with high thermal gradients (avg 200 degrees -370 degrees C). Boiling of the fluids was the most effective mechanism of ore deposition with which ore shoot formation was connected as evidenced by texture and fluid inclusion data. In contrast, silver-lead-zinc and tin-silver mineralizations formed in closed hydrothermal systems in which boiling did not occur. Their relatively low uniform temperatures (avg 240 degrees -320 degrees C) and low thermal gradients were associated with a more steady regime of Ag-Pb-Zn-Sn ore deposition compared to the gold-silver deposits. The relatively high salinity of fluids (up to 7.5% NaCl equiv) resulted from the absence of mixing of metalliferous solutions and meteoric waters.

The structure of Mesta volcanic massif. Part I. Structural features of the volcanics in Mesta graben

Mesta Paleogene volcanic massif (SW Bulgaria) is a complex synsedimentary structure. Its lava rocks form numerous endogenous domes and subvolcanic bodies that are components of two polygonal cauldrons and two linear volcano-tectonic zones. The related pyro- and epiclastics interager laterally with a 1500-2000 m thick terrigenous Upper Eocene-Oligocene section. The volcanics are silicic-rhyodacites and dacites (with very rare transitions to latites). According to Na2O+K2O/SiO2 they are transitional and normal, and according to K2O/SiO2 the rocks belong to the HKCA and SH series. Their structural anisotropy is defined by planar and linear flow structures of diverse morphology and petrography. Flow layers of different colour, texture and composition are widespread. Part of the structures (magmatic boudinage, magmatic folds and magmatic cleavage) resulted from synmagmatic deformations of the already layered but still not solidified melts. Magmatic folds are represented by mesoscopic folds and by crenulations on flow layers surfaces. Magmatic cleavage corresponds to parallel and convergent axial-plane cleavage, the cleavage planes being cemented during the cooling of the melts. Jointing in the magmatic bodies is controlled by their facies. Block, prismatic and platy jointing as well as various transitional types have been observed. A vertical zoning in the distribution of flow structures and jointing types was inferred. The air fall tuffs show parallel bedding while base surge and pyroclastics surge deposits are characterized by cross-bedding. The jointing in pyroclastics is a combination of regional jointing and jointing due to local syn- and post magmatic faulting.

Petrochemical and geodynamic correlation of the porphyry-copper systems in the Eurasian copper belt: the segment Romania – Pakistan

The porphyry-copper systems are represented by ore-bearing magmatic complexes which to a greater or lesser extent are the host rocks of mineralization, and the processes involved in the differential formation of intrusive and subvolcanic bodies are similar to those leading to the release and concentration of useful components during some of the subsequent postmagmatic processes. Producers of porphyry-copper ore are mostly the intermediate and slightly acid rocks, the majority showing some tendency towards increased potassic alkalinity. The magmatic systems involved usually manifest several pulses of different compositions and/or fabric grouped into larger and more complex intrusive centres, sometimes called "batholiths". Copper, however, concentrates in separate, small and relatively late magmatic differentiates of increased acidity. Porphy.ry-copper mineralizations are characteristic of magmatic systems of homodromous evolution during subduction. Porphyry-copper deposits associated with the bimodal basalt-rhyolite magmatism of the epicontinental riftogenesis or with MORB are not known. In the Eurasian copper belt, the porphyry-copper systems are typically associated with massive and disseminated copper (polymetallic) - pyrite (±Au) mineralizations in propylites and secondary quartzites and with copper-gold (±Ag) base-metal veins and skarns. All three types are closely associated in space forming, along with the productive volcano-intrusive centres, an integral polygenic and polyfacial ore-magmatic system. The segment discussed in this study includes 65 deposits located exclusively within the Eurasian Plate active margin and associated with the formation of mature magmatic arcs during the period from the Upper Cretaceous till the Neogene inclusive. The most typical feature of the geodynamic setting is that a number of small continental blocks (micro-continents) have been also involved in the process of ocean (Neotethys) closing accompanying the collision of the major lithospheric plates (the Afro-Arabian and Eurasian ones). These blocks probably broke off the Afro-Arabian Plate and migrated to the north where they stuck to the Eurasian Plate southern margin forming an accretionary collage. The formation of porphyry-copper systems in the magmatic arcs is exclusively a post-accretion process at advanced subduction and collision. An especially characteristic feature is the localization of the largest and best defined deposits inside the accreted exotic blocks. Deposits formed in the back-arc thrust zone of the Eurasian Plate active margin are comparatively less widespread. In Romania, the porphyry-copper systems formed during the Upper Cretaceous and the Neogene are located in a block collage traced back, rather guestionably, to the Gondwana. The porpltyry-copper systems in East Serbia occur in a similar setting. As to the Tertiary deposits in North Macedonia, Krusha Mountain and Halkidiki Peninsula, they definitely belong to the Serbo-Macedonian accreted terrane. On the territory of Bulgaria, the majority of the Upper Cretaceous porphyry-copper deposits are lo;atd in the Central Srednogorie, Sakar Strandja and East-Thracia terranes. Separate deposits are situated (possibly as parts of allochthons) in the Balkaoid's thrust-nappe – back-arc belt. In Turkey (Strandja Mt.), a group of complex copper-molybdenum-tungsten porphyry deposits are located not far away from the Bulgarian ones, again in the East Thracia terrane. Anotller, less conspicuous group of Upper Cretaceous (and m1y be even younger) age is located in the active southern margin of the Eurasian Plate. Further east, all porphyry-copper deposits of the USSR, Iran and Pakistan occur in accreted exotic terranes including those of Lesser Caucasus, Northwest Iran, Central Iran and Hilmend. Local petrochemical correlation is based on the K2O+Na2O/SiO2 diagram on which fields of porphyrycopper deposits from the Circum-Pacific metallogenic belt and from the Caribbean have been plotted prior to analysis. Compared to these plots, the porphyry-copper systems in the Eurasian belt show analogous acidity of magmatism. Alkalinity is variable but generally the Eurasian (Alpine, Mediterranean) belt as compared to the Pacific one is characterized by a pronounced potassic alkilinity (high·potassic calc-alkaline series). This is consistent with the overall Mediterranean petrochemistry. Outside it, e.g. further east in Pakistan, the ore-bearing magmatic complexes are of normal calc-alkaline character. A general petrochemical correlation shows that compared to the Pacific belt the porphyry-copper systems in the Romania-Pakistan segment of the Eurasian copper belt are formed in a mixed geodynamic setting involving both continental and island arcs. It is suggested that the present-day mixing of geodynamic settings is a secondary phenomenon caused by Late Alpine collision processes which have deformed the primary tectonic features and have pushed to the north many of the magmatic arcs involved.

Low-grade metamorphism in granitoids and volcanic rocks, Cordillera Neuquina, Argentina

Granitoid and volcanic rocks of the Cordillera Neuquina between 38°45′ and 39°30′ south latitude have been metamorphosed under zeolite, prehnite-pumpellyite, and transitional greenschist facies conditions. This low-grade metamorphism is related to the circulation of hydrothermal solutions associated with Tertiary(?) subvolcanic bodies and/or shallow intrusives through a pre-existing fracture system. Three progressive metamorphic zones have been established, with temperatures ranging from 100° to 300°C and pressures less than 4 kb. The pressures are similar to those estimated for metamorphic rocks at Taringatura, New Zealand, and in the Tanzawa Mountains of Japan. The mineral assemblages are influenced by host-rock composition. In the granitoids, the sequence is: Zone I — heulandite, stilbite, K-feldspar, albite, quartz, calcite; Zone II — chlorite, prehnite, pumpellyite, albite, K-feldspar, titanite, quartz; and Zone III — chlorite, pistacite, piedmontite, titanite, actinolite, K-feldspar, albite, quartz, calcite. In the basalts, the sequence is: Zone I — heulandite, stilbite, calcite; Zone II — analcime, thomsonite, laumontite, natrolite, celadonite, albite; and Zone III — chlorite, pistacite, actinolite, pumpellyite, laumontite, albite, quartz. In the andesites, the sequence is: Zone I — stilbite, calcite, quartz; Zone II — pumpellyite, chlorite, titanite, celadonite, albite, quartz, calcite; and in Zone III — pistacite, quartz, chlorite, albite. Pyrite, chalcopyrite, arsenopyrite, and copper are always found in association with quartz and epidote in Zone III.