Vein Textures Research Articles

Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behaviour hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneities within shallow conduits, their relationship with tuffisite vein propagation, and the implications for fragmentation and degassing processes. Similarly, the magmatic vesiculation response to upper conduit pressure perturbations, such as those related to the slip of dense magma plugs, remains largely undefined. Here we provide a detailed characterization of an exceptionally large tuffisite vein within a rhyolitic obsidian bomb ejected during transitional explosive-effusive activity at Chaiten, Chile in May 2008. Vein textures and chemistry provide a time-integrated record of the invasion of a dense upper conduit plug by deeper fragmented magma. Quantitative textural analysis reveals diverse vesiculation histories of various juvenile clast types. Using vesicle size distributions, bubble number densities, zones of diffusive water depletion, and glass H2O concentrations, we propose a multi-step degassing/fragmentation history, spanning deep degassing to explosive bomb ejection. Rapid decompression events of ~3-4 MPa are associated with fragmentation of foam and dense magma at ~200-350 metres depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of metres. Permeable pathway occlusion in the dense conduit plug by pyroclast accumulation and sintering preceded ultimate bomb ejection, which then triggered a final bubble nucleation event. Our results highlight how the vesiculation response of magma to decompression events is highly sensitive to the local melt volatile concentration, which is strongly spatially heterogeneous. Repeated opening of pervasive tuffisite vein networks promotes this heterogeneity, allowing juxtaposition of variably volatile-rich magma fragments that are derived from a wide range of depths in the conduit. This process enables efficient but explosive removal of gas from rhyolitic

Epithermal and arc-related layered mafic platinum-group element mineralisation in the mafic–ultramafic rocks of eastern Papua

ABSTRACTThis paper describes contrasting platinum-group element (PGE) mineralisation in the mafic–ultramafic rocks of eastern Papua, discovered nearly 100 years after the first record of detrital ‘osmiridium’ in alluvial gold production from the region. The two mineralisation types include: (1) the Doriri Pd–Pt–Ni lode, a new type of epithermal shear zone-hosted PGE mineralisation from the Ada'u Valley; and (2) stratiform Pt–Pd ± Au mineralisation hosted in an arc-related layered mafic intrusion at the Goroa prospect in the Waria Valley. Doriri is the first description of the transport and deposition of PGE in epithermal conditions, confirming the case long made by geochemists for the solubility, transport and deposition of significant concentrations of Pd and Pt in hydrothermal fluids up to 300°C. Doriri's temperature of formation (100–220°C) is based on chlorite thermometry, clearly placing the deposit in the epithermal field. Other epithermal-hydrothermal features of the deposit include formation at a shallow depth, episodic mineral deposition and the development of rhythmically layered chlorite and magnetite, hydrothermal brecciation, cross-cutting vein textures, the presence of low-temperature marcasite in the lode's pyrite–pentlandite–pyrrhotite sulfide mineralisation and a Pd:Pt ratio >7. The stratiform Goroa Pt–Pd ± Au mineralisation contains concentrations up to 311 ppb Pt + Pd over a 3 m interval in a plagioclase-rich gabbro. Grey–black micrograbbo commonly forms alternating thin layers within plagioclase-rich gabbro and many of these mafic layers contain visible sulfides. Sulfide content of the plagioclase-rich gabbro is ≪1 vol%. A Pt:Pd ratio of ∼1 is typical of the cumulate layered mineralisation. The association of Goroa mineralisation with (1) plagioclase-rich gabbroic rocks, rather than ultramafic rocks, (2) trace sulfides, rather than chromite, and (3) the occurrence with Pt–Pd detrital alloys, suggests a genetic relationship with arc-related layered mafic intrusions. The Goroa diorite–gabbro–ultramafic sequence may be genetically linked to late Paleocene–middle Eocene tonalite, quartz diorite and porphyry arc-related stocks present in the lower Waria River district.

Process of serpentinization in the ultramafic massif of Beni Bousera (internal Rif, Morocco)

The Beni Bousera massif forms part of the Sebtide units in the internal Rif Mountain (Morocco). It is mainly composed of mantle peridotites surrounded by crustal metamorphic rocks (kinzigites, micaschists, and schists). The serpentinization affects all of peridotite massif to various degrees. Serpentinization is concentrated at the top of the peridotites, along the mylonitized zone, and in the NE part of the massif. It is manifested by the formation of mesh and hourglass textures along the tectonic foliation in the highly serpentinized peridotites; and brecciated texture in the least serpentinized peridotites. Pyroxene minerals are still intact hosting few serpentine veins. These petrographic features are consistent with the geochemical data, marked by the increasing of LOI and decreasing of MgO and FeO toward the top of the massif and Aaraben fault. The Raman characterization of serpentine with the brecciated mesh and hourglass textures correspond to lizardite type whereas the serpentine with the vein texture is formed by lizardite + chrysotile.

Serpentinization, element transfer, and the progressive development of zoning in veins: evidence from a partially serpentinized harzburgite

Serpentinization is an important geochemical process that affects the chemistry and petrophysical properties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen. Here, we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water (H2O) and silica (SiO2) activities on mineral assemblages produced during progressive serpentinization of a harzburgite. We describe a harzburgite from the Santa Elena Ophiolite in Costa Rica that is ~30 % serpentinized. Serpentine + brucite ± magnetite veins occur in olivine, Al-rich serpentine + talc veins occur in orthopyroxene, and Al-rich serpentine ± talc ± brucite veins occur at the boundary of orthopyroxene and olivine. Bulk vein chemistry and element distribution maps demonstrate distinct chemical zonations within veins and chemical gradients between orthopyroxene- and olivine-dominated areas. Specifically, the sample records (1) varying brucite composition depending on whether or not it is associated with magnetite, (2) formation of magnetite from Fe-rich brucite (±Fe-rich serpentine) during olivine hydration, where magnetite coexists with brucite Mg#96 and serpentine Mg#99, (3) chemical gradients in Si, Al, Cr, and Ca within and between orthopyroxene- and olivine-hosted veins, and 4) local (different) equilibrium assemblages within different zones of veins. The studied sample preserves rarely observed textures documenting continuous replacement of olivine, rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites. Furthermore, the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral textures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization.

Local and Regional Mass Transfer During Thrusting, Veining, and Boudinage in the Genesis of the Giant Shale-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

Multiscale structural and geochemical studies have been applied to understand the genesis of the Paracatu deposit in Brazil, a shallow-dipping, bulk-tonnage, vein-style low-grade Au(-Ag-Pb-Zn) orebody hosted in ~1000 Ma black phyllites of the Paracatu Formation, associated with intense shearing accompanying thrusting during the ~680 Ma Brasiliano orogeny. Massive to laminated quartz-sulfide-carbonate veins and associated alteration and Au mineralization formed early in the deformation history, and these veins were boudinaged during subsequent progressive increase in shear strain. Geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100-m scales with respect to Ti, Al, Zr, V, and rare earth elements, including a footwall with protolith attributes similar to those of the ore. Oxygen, hydrogen, and sulfur isotopes do not reveal a distinctive external fluid source; rather, they reflect fluid-rock equilibration with the host phyllites during greenschist facies regional metamorphism. Detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that Si, Ca, and Sr were removed and Al, K, and Ti were residually concentrated during formation of synboudinage dark selvages. This process of mass transfer occurred at scales more local than the processes responsible for ore genesis. Despite the local processes of chemical equilibration and redistribution, many of the main ore components appear externally derived at scales broader than the orebody. Gold, As, Ag, Sb, Bi and, to some extent, Pb and Zn show a strong spatial and statistical correlation with boudinaged quartz-sulfide ± carbonate veins, and these are surrounded by a broader (10–50-m vertical scale) halo of enriched K, Ba, and volatiles (loss on ignition), with depleted Na and Sr. The majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins, and gold-bearing pyrite shows mineral chemistry consistent with a hydrothermal origin. The main population of large veins locally preserve preboudinage, complex internal vein textures, including sulfide-rich wall-rock laminae similar to laminated fault-fill veins in many orogenic-style vein deposits. These larger veins contain most of the gold in the Paracatu resource. Metals were most likely sourced distal to the Paracatu Formation and were precipitated in sheeted shear or tensile veins during the early stages of the Brasiliano thrusting. The oblate strain recorded by the boudinaged, mineralized veins is anomalous and distinctive to Paracatu at km scales, even though the thrusting is regional. Although the orebody may have been physically dislocated by thrusting from the place where the early veins formed, the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or, perhaps, a spatially restricted competency contrast within the stratigraphy, which contributed to the localization of the present mineralization during the thrusting.

GEOLOGY AND ORE MINERALISATION OF NEOGENE SEDIMENTARY ROCK HOSTED LS EPITHERMAL GOLD DEPOSIT AT PANINGKABAN, BANYUMAS REGENCY, CENTRAL JAVA, INDONESIA

Low suphidation (LS) epithermal gold deposits were recently found in the Paningkaban area, Central Java province, Indonesia, with more than five hundred artisanal gold miners currently operating in the area. This study is aimed to understand the geological factors controlling the gold mineralization and to characterize the alteration and ore mineralogy of the deposit. Several epithermal veins/veinlets trending N–S, NW–SE, and NE–SW are hosted by Tertiary turbiditic volcanoclastic sedimentary rocks of the Halang formation. This formation is composed of looping gradation of sandstone and siltstone units. Pre- and syn-mineralization structures such as extension joints, normal sinitral fault and sinitral fault control the gold mineralization. Fault movements formed dilational jogs manifested by NW-SE-trending en-echelon tension gash veins. Four main alteration zones are identified: (a) phyllic, (b) argillic, (c) sub propylitic and (d) weak subpropylitic. Ore minerals consist of native gold, electrum, native silver, pyrite, chalcopyrite, sphalerite, galena, arsenopyrite, cubanite, marcasite, covellite and tennantite, which are commonly associated withargillic alteration. Vein structures such as massive, swarm and low angle veins, stockwork and veins dispersed in diatreme breccia are present. Normal banded, cockade, crustiform, bladed carbonates as well as, comb and saccharoidal features are the typical vein textures. It is noteworthy that the veins are basically composed of carbonate with minor quartz at gold grades of up to 83 g/t Au. Based on the vein structures and textures, four stages of ore mineralization were developed consisting of (a) early stage (fluidized breccia and quartz vein), (b) middle stage (carbonate base metal), (c) late stage (late carbonate), and supergene stage. Gold mineralization originated mainly during middle and late stages, particularly in association with cockade, crustiform, bladed carbonate base metal veins. Based on those various features, the LS epithermal deposit in the study area is categorized as carbonate-base metalgold mineralization type.

An effective hand vein feature extraction method

As a new authentication method developed years ago, vein recognition technology features the unique advantage of bioassay. This paper studies the specific procedure for the extraction of hand back vein characteristics. There are different positions used in the collecting process, so that a suitable intravenous regional orientation method is put forward, allowing the positioning area to be the same for all hand positions. In addition, to eliminate the pseudo vein area, the valley regional shape extraction operator can be improved and combined with multiple segmentation algorithms. The images should be segmented step by step, making the vein texture to appear clear and accurate. Lastly, the segmented images should be filtered, eroded, and refined. This process helps to filter the most of the pseudo vein information. Finally, a clear vein skeleton diagram is obtained, demonstrating the effectiveness of the algorithm. This paper presents a hand back vein region location method. This makes it possible to rotate and correct the image by working out the inclination degree of contour at the side of hand back.

Copper Ores in Mawat Ophiolite Complex (Part of ZSZ) NE Iraq

Ore microscopy study for some samples from Mawat complex NE Iraq (part of Zagros Suture Zone) showed the presence of copper ore minerals was irregularly distributed, mainly throughout the basic rocks of Mawat Ophiolite Complex. The main sulfides minerals identified were: chalcopyrite, pyrite, bornite, chalcocite and covellite. Iron hydroxides/oxides including goethite and magnetite were also observed associated with sulfide ore. Chalcopyrite and pyrite were the main primary minerals from which other ores were formed. They were originated at the orthomagmatic stage, and also deposited from aqueous solution of final stage in the consolidation of a magma forming vein textures which was observed in many samples. Two types of pyrite generations were recognized. They were pre-tectonic pyrite and post-tectonic pyrite. The secondary recognized ore textures were rim, pseudo morph, zoning, relicts, atoll and vein replacement textures. Secondary textures resulting from exsolution were also presented. Three stages of mineralization could be concluded: orthomagmatic stage (Earliest), the hydrothermal activations stage (Hypogene) and the weathering stage (Supergene). Paragenaticaly, the primary copper sulfides including chalcopyrite and bornite were responsible for other copper ores formation. These minerals were observed disseminated throughout the host mafic rocks which were considered as endogenic deposits (magmatic and hypogene origin) and they were modified as small patches of chalcocite and covellite.

Contact-free palm-vein recognition based on local invariant features.

Contact-free palm-vein recognition is one of the most challenging and promising areas in hand biometrics. In view of the existing problems in contact-free palm-vein imaging, including projection transformation, uneven illumination and difficulty in extracting exact ROIs, this paper presents a novel recognition approach for contact-free palm-vein recognition that performs feature extraction and matching on all vein textures distributed over the palm surface, including finger veins and palm veins, to minimize the loss of feature information. First, a hierarchical enhancement algorithm, which combines a DOG filter and histogram equalization, is adopted to alleviate uneven illumination and to highlight vein textures. Second, RootSIFT, a more stable local invariant feature extraction method in comparison to SIFT, is adopted to overcome the projection transformation in contact-free mode. Subsequently, a novel hierarchical mismatching removal algorithm based on neighborhood searching and LBP histograms is adopted to improve the accuracy of feature matching. Finally, we rigorously evaluated the proposed approach using two different databases and obtained 0.996% and 3.112% Equal Error Rates (EERs), respectively, which demonstrate the effectiveness of the proposed approach.

The role of silica redistribution in the evolution of slip instabilities along subduction interfaces: Constraints from the Kodiak accretionary complex, Alaska

Quartz veins from a subhorizontal shear zone in the Kodiak accretionary complex in southwest Alaska record cycles of cracking and sealing contemporaneous with simple shear, and analysis of the vein array in the context of silica dissolution, diffusion, and precipitation leads to the conclusion that the time needed to seal after a fracture event overlaps with the time scales associated with the recurrence of slip instabilities along active megathrusts. The central belt of the Kodiak accretionary complex is interpreted as a shear zone developed adjacent to a paleo-decollement that was active in the Late Cretaceous based on large strain magnitudes, recumbent isoclinal folds, and regional cleavage patterns that depict rotation of the steep cleavage that characterizes most of the Kodiak Formation into the horizontal fabrics of the central belt. Within this regional shear zone, crack-seal veins composed of quartz with phyllosilicate inclusion bands are spaced at less than a cm but en echelon vein arrays are spaced at 500 cm. These arrays indicate top-to-the southeast shear on southeast dipping brittle-ductile shear zones. Vein textures indicate cracking and sealing of thin veins in the matrix, but thicker veins in en echelon sets record initial cracking and sealing followed by periodic collapse of fracture porosity at a frequency that is lower than the frequency of crack-seal cycles. Silica depletion zones in the wall rock adjacent to veins indicate local transport of silica by diffusion. By combining the maximum time in which diffusion could dominate over advection with the time needed to precipitate the quartz associated with one crack-seal band, we show that a reasonable range of ΔP, or pressure difference between matrix and crack, can produce crack seal bands in less than 10 days. This time frame is less than the recurrence of earthquakes, and similar to the duration of some slow earthquakes in nature. Therefore, silica redistribution could play a role in modulating the frequency of plate boundary slip instabilities along convergent plate boundaries.

Extraction of the Palm Vein Texture Features Based on Gabor Wavelet Transforms

A texture feature extraction method based on Gabor wavelet transform is presented to avoid the limitations of the shape structure characteristics extraction in the palm vein image. First acquire the palm vein images by independent structures of vein acquire device then extracts ROI area of vein images by determining the centerline of the middle finger and ring finger interphalangeal. Correcting the vein image by Gamma operator will improve the contrast of the processing image. Finally, transform the image based on Gabor wavelet to extract texture feature.

鉱物脈組織から読み解く地殻流体流動

Fluids in the Earth's crust play crucial roles in various geological phenomena including earthquakes, volcanism and formation of ore deposits. Mineral-filling veins are fossils of fluid-filled fractures, but it is not easy to extract information on nature of fluid flow during vein formation. Mineral veins in the metamorphic rocks show systematic variation in internal textures (grain shape, mineral distribution, crystallographic orientation, relation to the host rock) as a function of aperture size and host rock. Hydrothermal experiments for silica precipitation have revealed that the occurrences and mineralogy of silica minerals are strongly controlled by solution chemistry (supersaturation, minor elements), rock substrates (quartz distribution, grain size), fracture geometry (aperture distribution, surface roughness) as well as temperature, which results in a variation in vein texture. Combining modeling and analyses of natural and synthetic veins, vein texture is a potentially useful indicator of hydrological properties (flow velocity, flow direction, degree of supersaturation, duration of sealing etc.) in the crust.

Vein Texture Extraction Using the Multiscale Second-Order Differential Model

In order to analyze the back of hand vein pattern rapidly and effectively, a novel approach based on multi-scale second-order differential model is proposed to extract the vein texture from vein samples directly, which is made up of two section: one is the foundation of local second-order differential model of vein texture(VLSDM), the other is texture extraction based on the multi-scale VLSDM. This paper analyzes the vein extraction using the multi-scale VLSDM and handles the filter response using the method of multi-scale analyzed noise filtered. This new algorithm has achieved good results for the vein texture, which is fuzzy, uneven distributed and cross-adhesion. Additionally this method keeps the original form of local shape and achieves orientation and scale information of the vein texture. The experiment result getting from this new method has also compared with another method and shown its outstanding performance. DOI: http://dx.doi.org/10.11591/telkomnika.v11i12.3658 Full Text: PDF

Palm-Dorsal Vein Recognition Method Based on Histogram of Local Gabor Phase XOR Pattern with Second Identification

This paper proposes a palm-dorsal vein recognition method with local Gabor phase features, which includes a second identification for more accuracy. First, we extract quadrant-bit codes from the 2D Gabor transformation of a vein image. Then using the Histogram of the Local Gabor Phase XOR Pattern (HLGPXP) obtains the vein texture features, which combines the global information and the local information. Finally, the chi-square distance is adopted for recognition. Using the texture features based on the local Gabor codes above, the Second Identification (SI) segments the vein images and regards the non-overlap degree between images as a matching criterion. The experimental results show the Error Equation Rate (EER) of our method (HLGPXP-SI) decreases by 11.7 %, 4.8 % respectively than Modified Local Binary Pattern (MLBP) [1], Local Gabor Binary Pattern (LGBP) [2] on Database A (204 high-quality palm-dorsal vein images from 68 hands), and on Database B (400 low-quality palm-dorsal vein images from 100 hands), it decreases by 18.94 %, 15.51 % respectively than Selected Gabor Phase and Amplitude Features (SGPAF) [3], Direct Gabor Phase Coding (DGPC) [4].

Spinel-peridotites of the Frido Unit ophiolites (Southern Apennine-Italy): evidence for oceanic evolution

The ophiolitic rocks of the Frido Unit include serpentinites derived from a lherzolitic to cpx-rich harzburgitic mantle, as suggested by microstructural and petrographical features. The serpentinites are englobed in tectonic slices where they are associated with mafic dykes and medium to high-grade metamorphic rocks such as amphibolites, gneiss, granofels as well as gabbros and basalts with pillow structures. The matrix of tectonic slices is mainly composed of calchschists and phyllites. The serpentinites of the Frido Unit show pseudomorphic and vein textures. Primary mantle minerals are represented by olivine, orthopyroxene, clinopyroxene and spinel and are identifiable either for the occurrence of mineral relics or pseudomorphs maintaining the mineral shape. Pseudomorphic minerals are serpentine, magnetite, chlorite, and tremolite. Olivine is replaced by serpentine forming a mesh texture; orthopyroxene is mostly altered to bastite; in some cases it shows exsolution lamellae of clinopyroxene and kink bands; fresh orthopyroxene is preserved as exsolution inside clinopyroxene porphyroclasts. Clinopyroxene is armoured by tremolite rim. Spinel shows a holly-leaf habit and is often armoured by a corona of Cr-chlorite. The core of the analyzed spinel has a Cr-Al spinel composition corresponding to chromite (Al2O3=29-31 wt %; Cr2O3= 28-37 wt%), whereas the rim has a Fe-Cr spinel composition corresponding to ferrit-chromite (Al2O3= 1-2% wt; Cr2O3=28-30 wt %). The Cr-Al spinel/ferritchromite ratio may be various in different spinel porphyroclasts. Magnetite replaces spinel or occurs within the mesh textured serpentine. The metamorphic assemblages in the Frido Unit serpentinites allow to infer qualitatively the physical conditions operating during serpentinization. The metamorphic mineral assemblages are typical of the greenschist-amphibolite transition and greenschist facies conditions, as suggested by the occurrence of tremolite replacing clinopyroxene and of Cr-chlorite and ferrit-chromite after Al-rich chromite. These minerals were produced by interactions between hydrothermal fluids and mantle peridotites. Serpentinites are cut by veins filled with mineralogical assemblages typical of the prehnite-pumpellyite facies likely related to later orogenic Apennine evolution. HP/LT metamorphic conditions (blueschist facies) have been documented in mafic dykes enclosed in serpentinites, but similar conditions are not recorded in serpentinites. This suggests that P-T conditions during the orogenic event were not able to produce a HP/LT mineral assemblage in serpentinites.

Textures of syntaxial quartz veins synthesized by hydrothermal experiments

Syntaxial quartz veins were synthesized by hydrothermal flow-through experiments using rock blocks (metachert, sandstone, and granite) containing slits. Based on analyses of vein textures using birefringence imaging microscopy, we identified two stages of crystal growth. During stage 1, quartz grain growth occurs without an increase in grain width. During stage 2, quartz grains develop facets and grow preferentially parallel to the c-axis orientation, and the aspect ratio of quartz grains shifts toward ∼2.9. Competitive growth occurs significantly at stage 2, and the transition from stage 1–2 occurs at a critical distance from the vein wall, being approximately equal to the host-rock grain size. Crystal growth in the slits produce various textures controlled by the ratio of slit aperture ( H) to host-rock grain size ( d). In high H/d cracks, elongate–blocky texture develops by grain impingement during stage 2, whereas in low H/d cracks, crystals that bridge the crack form without competitive growth by grain impingement at stage 1. Heterogeneous structures of fracture porosity are produced during syntaxial vein formation, due to the anisotropy in the growth rate of quartz. Such “incompletely sealed” cracks may act as important fluid pathways and as weak planes in the upper crust.

The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone, south Hungary

This study reconstructs the palaeohydrogeologic evolution of the shallow-to-moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid-flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable-isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture-controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress-dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock-dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid–rock interaction. Cathodoluminescence microscopy and fluid-inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Moragy Block (including the MZ).

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

The Bohaun gold deposit in County Galway, western Ireland, is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks that have only reached the late diagenetic zone of low-grade metamorphism. The mineralization consists of undeformed veins, stockworks, and breccias. Quartz dominates all the veins and commonly shows multiple growth stages and a variety of classic epithermal textures, including comb, plumose, and banded, indicative of low confining pressures. The veins have a simple mineralogy made up of quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite. Vein quartz is associated with two fluids compositions: type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126° to 257°C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175° and 245°C. The geodynamic setting of Bohaun, which is located in a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition, and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine-type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic-grade gold mineralization, independent of other fluids.

Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java

DOI: 10.17014/ijog.v5i4.108The Bunikasih vein system in the Pangalengan district of West Java is a low-sulfidation, adularia sericite epithermal gold deposit. It is hosted by Late Miocene andesitic volcanic and volcanoclastic rocks occurring in the south western margin of Malabar Volcano complex. Gold ore and alteration minerals related to deposition of gold in Bunikasih deposits superimposed on Late Tertiary-Quaternary andesitic formation that were altered and mineralized by some hydrothermal events. The veins consist almost entirely of quartz, with small amounts of adularia, bladed calcite, pyrite, and gold. Gold ore shoots are vertically restricted and are more continuous horizontally. The veins display complex and multi episodic filling with texture characteristics of open space precipitation such us colloform, lattice bladed, crustiform banding, vugs, breccia, and cockade and comb texture. The presence of bladed calcite and silica pseudomorph after bladed calcite suggests that the hydrothermal fluids boiled. In the Cibaliung section of the area, anomalous gold is related to veins trending northeast - southwest, milky quartz with dark grey to black manganese staining is found intermittently for a length of about 800m. The mineralized andesite ore bodies exhibit broad alteration patterns adjacent to mineralization, passing from fresh rock into anargillic, chlorite zone, and then sericite-silica close to mineralization. An argillic assemblage composed of kaolinite with fine-grained pyrite bulb is present in the upper portions and surrounding of the quartz vein system. The veins range from centimeter to meter in size. Of 24 vein samples collected, gold averages up to 0.3 grams per tone ("g/t"), to a high of 24.6 g/t. The Bunikasih epithermal gold deposit was mined by people for more than 10 years, mainly for the gold ore.

Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java

DOI: 10.17014/ijog.v5i4.108 The Bunikasih vein system in the Pangalengan district of West Java is a low-sulfidation, adularia sericite epithermal gold deposit. It is hosted by Late Miocene andesitic volcanic and volcanoclastic rocks occurring in the south western margin of Malabar Volcano complex. Gold ore and alteration minerals related to deposition of gold in Bunikasih deposits superimposed on Late Tertiary-Quaternary andesitic formation that were altered and mineralized by some hydrothermal events. The veins consist almost entirely of quartz, with small amounts of adularia, bladed calcite, pyrite, and gold. Gold ore shoots are vertically restricted and are more continuous horizontally. The veins display complex and multi episodic filling with texture characteristics of open space precipitation such us colloform, lattice bladed, crustiform banding, vugs, breccia, and cockade and comb texture. The presence of bladed calcite and silica pseudomorph after bladed calcite suggests that the hydrothermal fluids boiled. In the Cibaliung section of the area, anomalous gold is related to veins trending northeast - southwest, milky quartz with dark grey to black manganese staining is found intermittently for a length of about 800m. The mineralized andesite ore bodies exhibit broad alteration patterns adjacent to mineralization, passing from fresh rock into anargillic, chlorite zone, and then sericite-silica close to mineralization. An argillic assemblage composed of kaolinite with fine-grained pyrite bulb is present in the upper portions and surrounding of the quartz vein system. The veins range from centimeter to meter in size. Of 24 vein samples collected, gold averages up to 0.3 grams per tone ("g/t"), to a high of 24.6 g/t. The Bunikasih epithermal gold deposit was mined by people for more than 10 years, mainly for the gold ore.