Fracture system characterisation and implications for fluid flow in volcanic and metamorphic rocks

<p>Fluid flow pathways in volcanic and metamorphic rocks are dominantly controlled by fracture systems. Although these fracture systems are critical for developing reservoirs in an economical and sustainable way, and for understanding processes that cause earthquakes, they are often poorly constrained. This thesis studies the geometry of fracture systems, the factors influencing their geometries, and their possible impacts on permeability in three contrasting settings: an outcropping andesite lava flow of the Ruapehu volcano; the andesite-hosted Rotokawa geothermal reservoir; and the Alpine Fault hangingwall metamorphosed schists. We use datasets from a combination of cores, acoustic borehole televiewer (BHTV) logs, outcrop scanlines, and terrestrial laser scanner (TLS) point clouds, which span multiple scales of observation. Fracture geometries are studied in a young (~6 ka-old) blocky andesitic lava flow on the Ruapehu volcano, as a representative example of weakly-altered andesitic lava flows emplaced over gentle topography in the absence of glaciers. Fractures were formed during cooling and emplacement of the lava flow. Fractures are automatically detected from the 3-D TLS point cloud of an outcrop area of ~3090 m2 using a plane detection algorithm, and calibrated with manual scanlines and high-resolution panoramic photographs. Column-forming fractures dominate the fracture system, are either sub-horizontal or sub-vertical (i.e., sub-parallel or sub-perpendicular to the brecciated margins) without mean strike orientation, and have an exponential length distribution. Sub-horizontal, clustered platy fractures sub-parallel to the flow direction arrest or deflect column-forming fractures. Areal and volumetric fracture intensity analyses reveal a ~0.5 % connected fracture volume which, although seemingly small, promotes fluid flow due to the planarity and connectivity of the system. Autobreccias are partially connected to column-forming fractures, and may promote lateral flow or form barriers depending on the extent of post-cooling alteration and mineralisation. Discrete fracture network models generated with the measured geometrical parameters are in agreement with the observed highly connected fracture system. Fractures in the andesite-hosted Rotokawa Geothermal Field are described in cores and BHTV logs. Fractures interpreted on BHTV logs are separated into sets of similar orientation using quantifiable clustering algorithms. Fracture thickness and spacing probability distributions are estimated from maximum likelihood estimations applied to truncated distributions, taking sampling biases into consideration. Spacing of the predominant sub-vertical NE-SW-striking fracture set, and subordinate NW-SE-striking fracture set, are best approximated by log-normal distributions and interpreted to be controlled by stratifications within the lava flow sequence. By contrast, spacing of other subordinate fracture sets, either dipping 60° and striking NE-SW, or steeply dipping and striking N-S, are best approximated by power-law distributions and interpreted to be fault-controlled. Fracture thicknesses in both cores and BHTV logs are approximated by a single power-law distribution, which reflects heterogeneous pathways observed at reservoir scale. Previously reported ~5 µm-thick fractures studied in thin section do not follow this power-law distribution and have an isotropic orientation, which suggests a change of controls on fracture density and orientation from thermal stresses at thin-section scale, to tectonic and lithological at core and BHTV log scales. However, fractures occupy ~5 % of the rock mass at the three scales of observations, suggesting a self-similar behaviour of fracture volumes in 3-D. In contrast to the Ruapehu and Rotokawa reservoir studies, scientific drilling in 2014 of the DFDP-2B borehole offered a unique opportunity to investigate the foliation and fractures along a 630 m-long borehole section in metamorphic rocks in the hangingwall of the Alpine Fault. BHTV log interpretation reveals a constant foliation and foliation-parallel fracture orientation (60°/145°; dip magnitude/dip direction) similar to nearby outcrops and parallel to the regional strike of the Alpine Fault. This foliation orientation may reflect the orientation of the Alpine Fault at ~1 km depth. In addition, sub-vertical fractures striking NW-SE above ~500 m, and sub-horizontal fractures between ~ 500-820 m below ground, are interpreted as exhumation-related joints and inherited hydrofractures respectively. Finally, we recognise metre-thick fault zones similar to those identified from BHTV logs and cores in the nearby DFDP-1B borehole. The three fracture set orientations, and observed fault zones, promote high hydraulic connectivity in the Alpine Fault hangingwall, which fosters fluid flow. This thesis helps quantify the geometrical parameters of fractures and their associated uncertainties in non-sedimentary settings, which are required to constrain numerical models and unravel fluid flow pathways in heterogeneous rocks. We identified lithological, tectonic and thermal controls on fracture geometries, which can constrain conditions and processes by which these fractures formed, and improve the prediction of fracture system architecture away from sparse borehole observations. The results of this thesis are relevant to similar lithological and tectonic settings elsewhere where observations are scarce. This study has also yielded an essential fracture dataset for better understanding of the structural and hydrological conditions at depth near the Alpine Fault prior to a large earthquake.</p>

Fracture system characterisation and implications for fluid flow in volcanic and metamorphic rocks

<p>Fluid flow pathways in volcanic and metamorphic rocks are dominantly controlled by fracture systems. Although these fracture systems are critical for developing reservoirs in an economical and sustainable way, and for understanding processes that cause earthquakes, they are often poorly constrained. This thesis studies the geometry of fracture systems, the factors influencing their geometries, and their possible impacts on permeability in three contrasting settings: an outcropping andesite lava flow of the Ruapehu volcano; the andesite-hosted Rotokawa geothermal reservoir; and the Alpine Fault hangingwall metamorphosed schists. We use datasets from a combination of cores, acoustic borehole televiewer (BHTV) logs, outcrop scanlines, and terrestrial laser scanner (TLS) point clouds, which span multiple scales of observation. Fracture geometries are studied in a young (~6 ka-old) blocky andesitic lava flow on the Ruapehu volcano, as a representative example of weakly-altered andesitic lava flows emplaced over gentle topography in the absence of glaciers. Fractures were formed during cooling and emplacement of the lava flow. Fractures are automatically detected from the 3-D TLS point cloud of an outcrop area of ~3090 m2 using a plane detection algorithm, and calibrated with manual scanlines and high-resolution panoramic photographs. Column-forming fractures dominate the fracture system, are either sub-horizontal or sub-vertical (i.e., sub-parallel or sub-perpendicular to the brecciated margins) without mean strike orientation, and have an exponential length distribution. Sub-horizontal, clustered platy fractures sub-parallel to the flow direction arrest or deflect column-forming fractures. Areal and volumetric fracture intensity analyses reveal a ~0.5 % connected fracture volume which, although seemingly small, promotes fluid flow due to the planarity and connectivity of the system. Autobreccias are partially connected to column-forming fractures, and may promote lateral flow or form barriers depending on the extent of post-cooling alteration and mineralisation. Discrete fracture network models generated with the measured geometrical parameters are in agreement with the observed highly connected fracture system. Fractures in the andesite-hosted Rotokawa Geothermal Field are described in cores and BHTV logs. Fractures interpreted on BHTV logs are separated into sets of similar orientation using quantifiable clustering algorithms. Fracture thickness and spacing probability distributions are estimated from maximum likelihood estimations applied to truncated distributions, taking sampling biases into consideration. Spacing of the predominant sub-vertical NE-SW-striking fracture set, and subordinate NW-SE-striking fracture set, are best approximated by log-normal distributions and interpreted to be controlled by stratifications within the lava flow sequence. By contrast, spacing of other subordinate fracture sets, either dipping 60° and striking NE-SW, or steeply dipping and striking N-S, are best approximated by power-law distributions and interpreted to be fault-controlled. Fracture thicknesses in both cores and BHTV logs are approximated by a single power-law distribution, which reflects heterogeneous pathways observed at reservoir scale. Previously reported ~5 µm-thick fractures studied in thin section do not follow this power-law distribution and have an isotropic orientation, which suggests a change of controls on fracture density and orientation from thermal stresses at thin-section scale, to tectonic and lithological at core and BHTV log scales. However, fractures occupy ~5 % of the rock mass at the three scales of observations, suggesting a self-similar behaviour of fracture volumes in 3-D. In contrast to the Ruapehu and Rotokawa reservoir studies, scientific drilling in 2014 of the DFDP-2B borehole offered a unique opportunity to investigate the foliation and fractures along a 630 m-long borehole section in metamorphic rocks in the hangingwall of the Alpine Fault. BHTV log interpretation reveals a constant foliation and foliation-parallel fracture orientation (60°/145°; dip magnitude/dip direction) similar to nearby outcrops and parallel to the regional strike of the Alpine Fault. This foliation orientation may reflect the orientation of the Alpine Fault at ~1 km depth. In addition, sub-vertical fractures striking NW-SE above ~500 m, and sub-horizontal fractures between ~ 500-820 m below ground, are interpreted as exhumation-related joints and inherited hydrofractures respectively. Finally, we recognise metre-thick fault zones similar to those identified from BHTV logs and cores in the nearby DFDP-1B borehole. The three fracture set orientations, and observed fault zones, promote high hydraulic connectivity in the Alpine Fault hangingwall, which fosters fluid flow. This thesis helps quantify the geometrical parameters of fractures and their associated uncertainties in non-sedimentary settings, which are required to constrain numerical models and unravel fluid flow pathways in heterogeneous rocks. We identified lithological, tectonic and thermal controls on fracture geometries, which can constrain conditions and processes by which these fractures formed, and improve the prediction of fracture system architecture away from sparse borehole observations. The results of this thesis are relevant to similar lithological and tectonic settings elsewhere where observations are scarce. This study has also yielded an essential fracture dataset for better understanding of the structural and hydrological conditions at depth near the Alpine Fault prior to a large earthquake.</p>

Mineralization in the andesitic lava from Kildyam volcanic complex, central Yakutia, Russia

This contribution presents the first detailed analysis of a new volcanic succession of olivine-pyroxenites, andesite, and dacite discovered in the Kildyam Late Jurassic complex in Central Yakutia. Petrographic and microprobe studies confirmed the liquid immiscibility in silicate melts during crystallization. Immiscible liquids are preserved as globules of one glass in another in andesites and as melted inclusions of native iron in matrix, clinopyroxene and plagioclase phenocrysts. Our analyses reveal the complex textural relationships between silicates and Fe-oxides, native iron and (Cu, Pb, Ag and Au)-rich phases, and provide unequivocal textural evidences, not observed previously. Purpose of this research is to preserve a very important data on IO (Iron Oxide) or IOCG (Iron Oxide Copper Gold) mineralization. Obtained results support occurrence and diverse of gold, silver, copper and lead minerals in magnetite lavas. During the early stage of fine-grained subvolcanic olivine-clinopyroxenite end pyrrhotite, globular igneous sulfides is a first proposed style of economic deposit formation. The second proposed style of economic mineralization in Kildyam is to be a magnetite-bearing lava; iron enrichment of the melilitic melt phase, followed by iron depletion and silica enrichment. The vesicle-hosted alloys and sulfides provide significant new data on metal transport and precipitation from high-temperature magmatic vapors. During syneruptive vapor phase exsolution, volatile metals (Cu-Zn, Fe-Al-Cu, Ni-Fe-Cu-Sn) and Ag-Cu-sulfides contribute to the formation of economic concentrations. Major conclusions contribute to 3-step genetic model. (1) Early-formed magmatic minerals led to partial dissolution of olivine-clinopyroxenite and their enrichment in Cu, Co and Ni relative to other metals, while troilite globules droplets grew.(2) First stage of division into two immiscible silicate and sulfide melt liquids (a) K-rich dacitic and rhyolithic glass, and (b) vesicles of heavy sulfide minerals with a large segregations and drops of native iron. (3) Lava of fused magnetite crystals and voids enriched in silver and gold, and (b) globular disseminated chalcopyrite in mineralized melilitic rocks.

The importance of overbank deposits and paleosol analyses for comprehensive volcanic hazard evaluation: The case of Holocene volcanism at Miravalles Volcano, Costa Rica

On the flanks of the dormant Miravalles volcano, systematic fieldwork and radiocarbon dating of buried humus-rich soils (paleosols) and wood fragments, augmented by mineralogical and geochemical analysis, reveal extensive and previously undocumented Holocene activity. Phase 1 consisted of 8.3 ka (~6300 BCE) volcanic debris avalanche and thick lapilli blast and fallout deposit that appear coeval. Hiatus 1 marks 2600 years of inactivity ensued followed by Phase 2 lapilli interbedded with ~5.5 ka lahars below a 5.3 ka basaltic lava flow (~3400 BCE). Hiatus 2 lasted 1800 years from 5.3 ka to 3.5 ka (3300-1500 BCE), after which a very active Phase 3 ensued (3.5 to 0.5 ka; 1600 BCE to 1500 CE) with > four lapilli eruptions, > 4 lahars, > 6 layers of ash and pumice, and small andesitic lava flows. The most recent evidence for eruption is an 880-year-old (0.9 ka; 1070 CE) lapilli overlain by gravels that may represent distal lahar sediments. Evidence indicates the occurrence of at least two, if not three, destructive lahars on the southwest flank of Miravalles in the past 500 years. The overbank sedimentary record indicates much more activity of Miravalles volcano over the past 3500 years (since 1500 BCE) than previously known, with a minimum of 24 events in that span. Overbank floodplain deposits are likely to contain the most compete record of recent activity in active and dormant volcanoes, and in the absence of dateable vegetation fragments, radiocarbon dating of paleosol A-horizons is very useful, with a precision of ~ 10%, i.e. 800 + 80 ybp.

Eksplorasi Geokimia untuk Menentukan Daerah Prospek Mineralisasi Emas Tipe Urat Epitermal: Studi Kasus di Daerah Tompaso, Kabupaten Minahasa Selatan, Sulawesi Utara

ABSTRAK Sulawesi Utara termasuk daerah Tompaso, Kabupaten Minahasa Selatan merupakan jalur magmatik yang potensial menghasilkan endapan bijih hidrotermal. Penelitian ini bertujuan menentukan daerah prospek mineralisasi emas khususnya tipe urat epitermal berdasarkan eksplorasi geokimia meliputi geokimia batuan dan sedimen sungai, khususnya metode BLEG (Bulk Leach Extractable Gold). Metode penelitian meliputi pemetaan geologi, alterasi dan jalur urat, percontoan (bijih/batuan dan sedimen sungai), dan analisis geokimia. Sampel bijih dianalisis dengan metode FA/AAS untuk emas dan metode AAS untuk unsur lain, sedangkan analisis sampel sedimen sungai dilakukan dengan metode cyanide leach dan AAS. Hasil penelitian menunjukkan batuan induk mineralisasi berupa lava andesit dan intrusi diorit. Batuan ini mengalami alterasi silica-clay, argilik, dan propilitik. Perpaduan antara pemetaan geologi, zona alterasi, dan jalur urat dengan eksplorasigeokimia batuan dapat menentukan daerah prospek mineralisasi yaitu Prospek Asam dan Polangkok. Pada Prospek Asam, sampel bijih memiliki kandungan emas sampai 0,03 ppm dengan anomali emas pada sampel BLEG menunjukkan nilai threshold 13,52 ppb Au. Pada Prospek Polangkok ditemukan 2 jalur urat (P1 dan P2) berarah baratlaut-tenggara dengan lebar sampai 5 m. Urat P1 memiliki kadar Au mencapai 0,31 ppm dan pada urat P2 mencapai 0,16 ppm Au. Mineralisasi pada Prospek Polangkok didukung oleh anomali Ag pada sampel BLEG dengan nilai threshold 67,18 ppb. Kedua daerah prospek tersebut direkomendasikan untuk eksplorasi lanjut (follow-up exploration). ABSTRACT North Sulawesi, including the Tompaso region, South Minahasa district, is a part of the magmatic belts hosting potential hydrothermal ore deposits. This study is aimed to determine the prospect area for epithermal vein-type gold mineralization, based on geochemical exploration including rock and BLEG (Bulk Leach Extractable Gold) stream sediment geochemistry. Mapping of geology, alteration and vein direction, sampling (ore/rock and stream sediment), and geochemical analysis were performed. Gold in rock samples was analyzed by FA/AAS, and other elements were detected by AAS, while BLEG samples were analyzed using the cyanide leach and AAS methods. The study area is occupied by andesitic lava and diorite, which are suffered by silica-clay, argillic and propylitic alteration. An integration of geological mapping, alteration zones and vein direction with lithogeochemical survey enables to determine the prospect areas, which consist of Asam and Polangkok prospects. At the Asam prospect, the ore sample contains gold up to 0.03 ppm, where the gold anomaly in BLEG samples show a threshold of 13.52 ppb Au. At the Polangkok prospect, two NW-SE trending veins (P1 and P2 Veins) were discovered with a width of up to 5 m. Vein P1 and P2 contains of up to 0.31 and 0.16 ppm Au, respectively. Mineralization at the Polangkok prospect coincides with Ag anomaly of BLEG samples with a threshold of 67.18 ppb. The two prospect areas are recommended for follow-up exploration.

Controls on andesitic glaciovolcanism at ice-capped volcanoes from field and experimental studies

Glaciovolcanic deposits at Tongariro and Ruapehu volcanoes, New Zealand, represent diverse styles of interaction between wet-based glaciers and andesitic lava. There are ice-confined lavas, and also hydroclastic breccia and subaqueous pyroclastic deposits that formed during effusive and explosive eruptions into meltwater beneath the glacier; they are rare among globally reported products of andesitic glaciovolcanism. The apparent lack of hydrovolcanically fragmented andesite at ice-capped volcanoes has been attributed to a lack of meltwater at the interaction sites because either the thermal characteristics of andesite limit meltwater production or meltwater drains out through leaky glaciers and down steep volcano slopes. We used published field evidence and novel, dynamic andesite-ice experiments to show that, in some cases, meltwater accumulates under glaciers on andesitic volcanoes and that meltwater production rates increase as andesite pushes against an ice wall. We concur with models for eruptions beneath ice sheets showing that the glacial conditions and pre-eruption edifice morphology are more important controls on the style of glaciovolcanism and its products than magma composition and the thermal properties of magmas. Glaciovolcanic products can be useful proxies for paleoenvironment, and the range of andesitic products and the hydrological environments in which andesite erupts are greater than hitherto appreciated.

Hydrothermal Alteration and Ore Metal Mineralisation at Temon, Pacitan, East Jawa, Indonesia.

Pacitan area is known as Tertiary volcanic arc in Java, as the result of subduction zone of the Indian-Australian Plate beneath the Eurasian Plate since Oligocene. It was superimposed volcanism which formed a wide area of hydrothermal alteration zone, resulting potential ore metals mineralization, such at Temon and its vicinities, Pacitan Regency, East Java Province, Indonesia. The aim of study was to analyze hydrothermal alteration and ore metal mineralization zones. Method was surface mapping, thin section analyses, mineragraphic analyses and X-Ray Diffraction (XRD) analyses. Field study observed denuded and deformed volcanic crater geomorphology. There are ore placer deposits within the sand dunes of Grindulu River, which it consists of andesitic lava and breccia of Early Oligocene Mandalika Formation; Early Miocene lithic and vitric tuffs; and dacitic intrusion. The dikes of dacite as the last of volcanism was the host rock controlling the zonation of alteration and mineralization stages. Oblique normal faults and shear faults were cross over dilating formed fractures, which were as bodies to depositing the ore metals. There are (zone 1st) the argillic clay consists of quartz+alunite+dickite+kaolinite±illite with vuggy structures, (zone 2nd) the argillic clay consists of quartz+montmorillonite±illite zone with quartz vents, brecciated and sulfide massive, and (zone 3rd) as the chloritized zone with low grade and supergene on the edge of hydrothermal alteration. It was fluid overprinted that very acid to the core of zone 1st (pH2-4) into more netral pH 4-6 (zone 2nd) and (pH5-6) in the edge zone 3rd. The potentials ore metal mineralization are Fe and Cu by pyrite, chalcopyrite, hematite, and covellite. Other potential ore metal mineralization was also from enargite by the supergene alteration.