Result Of Hydrothermal Alteration Research Articles

Interpretation of Subsurface Structure Based on the Magnetic Data at Semurup Geothermal Area Kerinci

The interpretation of subsurface structures in Semurup geothermal area has been carried out using the geomagnetic method. Data were collected in an area of 1500 m × 1400 m consisting of 160 points. The magnetic anomaly value obtained was derived from the total magnetic induction value that has been corrected by IGRF and diurnal variation, then transformed by reduction to equator and upward continuation to remove noise and separate local and regional anomalies. The results of data processing showed the total magnetic field values in the study area ranged from -1730.4 nT to 1909.0 nT. Magnetic anomalies in this study area are dominated by negative values that may be caused by demagnetised rocks (a result of hydrothermal alteration). The results of 2D modeling, it has 5 rock layers that can be classified into 3 main parts of the geothermal system The first and second layers are caprock with a depth of up to 850 meters consisting of sedimentary rock, clay, and sandstone. The third layer is indicated as a reservoir with a depth from 850 to 1450 m and is dominated by sandstone and clay alteration Hot rock in the fourth and fifth layers is dominated by basalt igneous rock and the presence of dacitic lava intrusion from the northeast of the study area at depths below 1450 m, and the Siulak fault as a outflow zone for geothermal fluid. The presence of the caprock, reservoir, hot rock, and fault zones indicates that the Semurup area has geothermal potential and is suitable for further exploration.

Differentiating between hydrothermal and diagenetic carbonate using rare earth element and yttrium (REE+Y) geochemistry: a case study from the Paleoproterozoic George Fisher massive sulfide Zn deposit, Mount Isa, Australia

Carbonate minerals are ubiquitous in most sediment-hosted mineral deposits. These deposits can contain a variety of carbonate types with complex paragenetic relationships. When normalized to chondritic values (CN), rare-earth elements and yttrium (REE+YCN) can be used to constrain fluid chemistry and fluid-rock interaction processes in both low- and high-temperature settings. Unlike other phases (e.g., pyrite), the application of in situ laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) data to the differentiation of pre-ore and hydrothermal carbonates remains relatively untested. To assess the potential applicability of carbonate in situ REE+Y data, we combined transmitted light and cathodoluminescence (CL) petrography with LA-ICP-MS analysis of carbonate mineral phases from (1) the Proterozoic George Fisher clastic dominated (CD-type) massive sulfide deposit and from (2) correlative, barren host rock lithologies (Urquhart Shale Formation). The REE+YCN composition of pre-ore calcite suggests it formed during diagenesis from diagenetic pore fluids derived from ferruginous, anoxic seawater. Hydrothermal and hydrothermally altered calcite and dolomite from George Fisher is generally more LREE depleted than the pre-ore calcite, whole-rock REE concentrations, and shale reference values. We suggest this is the result of hydrothermal alteration by saline Cl--rich mineralizing fluids. Furthermore, the presence of both positive and negative Eu/Eu* values in calcite and dolomite indicates that the mineralizing fluids were relatively hot (>250°C) and cooled below 200–250°C during ore formation. This study confirms the hypothesis that in situ REE+Y data can be used to differentiate between pre-ore and hydrothermal carbonate and provide important constraints on the conditions of ore formation.

Highly siderophile and chalcogen element constraints on the origin of components of the Allende and Murchison meteorites

187Re-187Os systematics, abundances of highly siderophile elements (HSE: Re, PGE, and Au), chalcogen elements (Te, Se, and S), and some major and minor elements were determined in physically separated components of the Allende (CV3) and Murchison (CM2) carbonaceous chondrites. Substantial differences exist in the absolute and relative abundances of elements in the components, but the similarity of calculated and literature bulk rock abundances of HSE and chalcogens indicate that chemical complementarity exists among the components, with CI chondrite-like ratios for many elements. Despite subsequent alteration and oxidation, the overall cosmochemical behavior of most moderately to highly siderophile elements during high-temperature processing has been preserved in components of Allende at the sampling scale of the present study. The 187Re-187Os systematics and element variations of Allende are less disturbed compared with Murchison, which reflects different degrees of oxidation and alteration of these meteorites. The HSE systematics (with the exception of Au) is controlled by two types of materials: Pd-depleted condensates and CI chondrite-like material. Enrichment and heterogeneous distribution of Au among the components is likely the result of hydrothermal alteration. Chalcogen elements are depleted compared with HSE in all components, presumably due to their higher volatility. Small systematic variations of S, Se, and Te in components bear the signature of fractional condensation/partial evaporation and metal–sulfide–silicate partitioning.

Hydrothermal alteration and melting of the crust during the Columbia River Basalt–Snake River Plain transition and the origin of low-δ18O rhyolites of the central Snake River Plain

We present compelling isotopic evidence from ~15 Ma rhyolites that erupted coeval with the Columbia River Basalts in southwest Idaho's J-P Desert and the Jarbidge Mountains of northern Nevada at that suggests that the Yellowstone mantle plume caused hydrothermal alteration and remelting of diverse compositions of shallow crust in the area where they erupted. These rhyolites also constitute the earliest known Miocene volcanism in the vicinity of the Bruneau–Jarbidge and Twin Falls (BJTF) volcanic complexes, a major center of voluminous (103–104km3) low-δ18O rhyolitic volcanism that was previously defined as being active from 13 to 6Ma. The Jarbidge Rhyolite has above-mantle δ18O (δ18O of+7.9‰ SMOW) and extremely unradiogenic εHf (−34.7) and εNd (−24.0). By contrast, the J-P Desert units are lower in δ18O (+4.5 to 5.8‰), and have more moderately unradiogenic whole-rock εHf (−20.3 to −8.9) and εNd (−13.4 to −7.7). The J-P Desert rhyolites are geochemically and petrologically similar to the younger rhyolites of the BJTF center (the one exception being their high δ18O values), suggesting a common origin for J-P Desert and BJTF rhyolites. The presence of low-δ18O values and unradiogenic Nd and Hf isotopic compositions, both of which differ greatly from the composition of a mantle differentiate, indicate that some of these melts may be 50% or more melted crust by volume. Individual J-P Desert units have isotopically diverse zircons, with one lava containing zircons ranging from −0.6‰ to +6.5‰ in δ18O and from −29.5 to −2.8 in εHf. Despite this diversity, zircons all have Miocene U/Pb ages. The range of zircon compositions fingerprints the diversity of their source melts, which in turn allow us to determine the compositions of two crustal end-members which melted to form these rhyolites. These end-members are: 1) Archean basement with normal to high-δ18O and unradiogenic εHf and 2) hydrothermally altered, shallow, young crust with low-δ18O (0–1‰) and more radiogenic εHf. We suggest that the shallow crust's low-δ18O composition is the result of hydrothermal alteration which was driven by a combination of normal faulting and high heat fluxes from intruding Yellowstone plume-derived basalts shortly prior to the onset of silicic magmatism. Furthermore, zircon diversity in the J-P Desert units suggests rapid assembly of zircon-bearing melts of varying isotopic composition prior to eruption, creating well-mixed magmas with heterogeneous zircons. We suggest that this hydrothermal priming of the crust followed by remelting upon further heating may be a common feature of intraplate mantle plume volcanism worldwide.

The porosity origin of dolostone reservoirs in the Tarim, Sichuan and Ordos basins and its implication to reservoir prediction

Origin of dolostone remained a controversial subject, although numerous dolomitization models had been proposed to date. Because of the dolomitization’s potential to be hydrocarbon reservoirs, one debatable issue was the role of dolomitization in porosity construction or destruction. Based upon case studies of dolostone reservoirs in various geological settings such as evaporative tidal flat (Ordos Basin, NW China), evaporative platform (Sichuan Basin, SW China), and burial and hydrothermal diagenesis (Tarim Basin, NW China), here we systematically discuss the origin of porosity in dolostone reservoirs. Contrary to traditional concepts, which regarded dolomitization as a significant mechanism for porosity creation, we found two dominant factors controlling reservoir development in dolostones, i.e., porosity inherited from precursor carbonates and porosity resulted from post-dolomitization dissolution. Actually, dolomitization rarely had a notable effect on porosity creation but rather in many cases destroyed pre-existing porosity such as saddle dolostone precipitation in vugs and fractures. Porosity in dolostones associated with evaporative tidal flat or evaporative platform was generally created by subaerial dissolution of evaporites and/or undolomitized components. Porosity in burial dolostones was inherited mostly from precursor carbonates, which could be enlarged due to subsequent dissolution. Intercrystalline porosity in hydrothermal dolostones was either formed during dolo- mitization or inherited from precursor carbonates, whereas dissolution-enlarged intercrystalline pores and/or vugs were usually interpreted to be the result of hydrothermal alteration. These understandings on dolostone porosity shed light on reservoir prediction. Dolostone reservoirs associated with evaporative tidal flat were laterally distributed as banded or quasi-stratified shapes in evaporite-bearing dolostones, and vertically presented as multi-interval patterns on tops of shallowing-upward cycles. Dolostone reservoirs associated with evaporative platform commonly occurred along epiplatforms or beneath evaporite beds, and vertically presented as multi-interval patterns in dolostones and/or evaporite-bearing dolostones of reef/shoal facies. Constrained by primary sedimentary facies, burial dolostone reservoirs were distributed in dolomitized, porous sediments of reef/shoal facies, and occurred vertically as multi-interval patterns in crystalline dolostones on tops of shallowing-upward cycles. Hydrothermal dolomitization was obviously controlled by conduits (e.g., faults, unconformities), along which lenticular reservoirs could develop.

The hydrothermal vent system of Mount Ruapehu, New Zealand — a high frequency MT survey of the summit plateau

High frequency magnetotelluric (MT) measurements made on the summit plateau of Mount Ruapehu, some 1 km to the north of the presently active vent beneath Crater Lake, have been used to derive the electrical resistivity structure associated with the volcanic hydrothermal vent system. The entire summit plateau area is underlain at shallow depth by low resistivity which is inferred to be the result of hydrothermal alteration caused by rising volcanic gases mixing with local groundwater. Two areas of localised higher resistivity, one between 200 and 500 m depth beneath the central part of the plateau, and one at a depth of 1000 m below the northern part of the plateau, are interpreted as being the result of hydrothermal alteration at higher temperature forming chlorite dominated alteration products. These regions are believed to represent the locations of further heat pipes within the volcanic system. Both correlate with the locations of eruption centres on Ruapehu active within the last 10 ka.

A-type magmatism in the Western Lachlan Fold Belt? A study of granites and rhyolites from the Grampians region, Western Victoria

The Victoria Valley Batholith, a granite–rhyolite complex of unusual composition, was emplaced in the Grampians–Stavely Zone of western Victoria at 396 Ma. Petrographic and geochemical features of the suite share important similarities with A-type magmas and we propose that they be classified as such. The close association between metaluminous to weakly peraluminous A-types and peralkaline granites within the Victoria Valley suite requires that, at least in some cases, these lithologies should be grouped together as part of a single A-type suite. This relationship is not necessarily a magmatic one as at least some of the peralkaline rocks in this suite appear to be the result of hydrothermal alteration. The Sr, Nd and Hf isotopic compositions of the magmas at the time of their emplacement (0.70459, 0.51220 and 0.28274) are remarkably similar to those of Cambrian mafic to intermediate lithologies calculated at 396 Ma. We suggest that the Victoria Valley suite was produced via the partial melting of basement Cambrian rocks (perhaps similar to diorites associated with the ∼ 495 Ma Bushy Creek pluton to the southeast) and that the magmas subsequently underwent variable fractional crystallisation dominated by hornblende and plagioclase. The oxidation state of the Victoria Valley magmas remains unclear, but is likely to have been higher than that observed in ferroan A-types suggesting that low oxygen fugacity is not an essential pre-requisite for A-type magmatism.

Magnetic anomalies associated with high temperature reservoirs in the taupo volcanic zone (New Zealand)

Distinct magnetic total force anomalies are associated with many, but not all, liquid-dominated geothermal reservoirs in the Taupo Volcanic Zone, which stand in thick, young (<0.7 Ma) volcanic host rocks. The anomalies are caused by demagnetised rocks (a result of hydrothermal alteration) in the upper 0.5–1 km section of the reservoirs; the phenomenon has been confirmed by core studies. Four different patterns of magnetic anomalies have been recognised. The most common type can be found over high-temperature systems where demagnetisation is confined to the main reservoir. Different anomaly patterns are associated with prospects characterised by subsurface outflows and reservoirs hosted in both normally and reversely magnetised rocks. A rather subdued and almost featureless pattern occurs over a few prospects where rocks lying outside the reservoir have also lost thei magnetisation by interaction with acidic, steam-heated water.

Extinct spreading center in the Labrador Sea: Crustal structure from a two‐dimensional seismic refraction velocity model

The Labrador Sea contains a rare example of an abandoned mid‐ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates. Seismic refraction data were collected along a line which transversely crosses the extinct spreading center. Two‐dimensional analyses of the refraction data, using ray‐tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center. In the extinct spreading center, a crustal thickness of approximately 4 km is determined, compared with 5.5 km for the flanks. Substantial lateral variations in P wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center. Low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration. The anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction.

Gold abundance and an aluminosilicate-rich alteration zone in the Val D'Or mining district, abitibi, Quebec: Examples of the fossil geothermal fields

Investigation of gold abundance in a gold-rich mining district by accurate analytical techniques (e.g., Instrumental Neutron Activation and/or Radiochemical Neutron Activation) shows that gold is anomalously distributed in some parts of the Val d'Or mining district of Quebec. It is possible to distinguish: (1) background values (1.4–3.5 ppb Au); (2) zones of primarily anomalous gold values around the Lamaque-Sigma mines (median: 15 ppb Au); (3) enrichment halos around gold orebodies (median: 70 ppb Au); (4) secondary gold enrichment in shear zones; and (5) high background values for the East Sullivan Monzonitic Stock. The Bourlamaque batholith is considered to be a gold-enriched intrusion because of areas (shear zones) that are anomalously enriched in gold (median: 10 ppb Au). It is concluded that there are auriferous geological units in the Val d'Or mining district, such as the Bourlamaque batholith and Val d'Or Formation. The latter contains the Lamaque-Sigma gold mines, representing 49.6% of the total gold production in the district. It is interpreted that the Val d'Or Formation is part of a central volcanic complex within an island arc system, that the centre of this complex is located in the main Lamaque plug, and that this environment may be compared to high-temperature active geothermal systems which are commonly responsible for the formation of epithermal gold deposits. The Lamaque-Sigma area is considered to be a fossil geothermal field, potentially explaining the anomalous gold values. In addition, a dumortierite-bearing, aluminosilicate-rich zone of hydrothermal alteration in the Val d'Or mining district is considered, as for other similar occurrences, to be the result of hydrothermal alteration within a geothermal field producing an Al- and Si-rich leached protholith which has subsequently been metamorphosed and deformed. A model is proposed for the formation of the gold deposits within the primary gold-rich environment, by mobilisation of gold in the volcanic rocks due to metamorphism and deformation. Therefore, two distinct and successive events are postulated: (1) gold-rich synvolcanic geothermal activity; and (2) late remobilisation from the host rocks, followed by deposition of gold ore within favourable structures.

An oxygen isotope study of the Kidd Creek, Ontario, volcanogenic massive sulfide deposits; evidence for a high 18 O ore fluid

The hydrothermally altered rhyolites (delta 18 O = 10-16ppm) at the Kidd Creek Cu-Zn-Ag-Pb volcanogenic massive sulfide deposit (Archean, Abitibi greenstone belt) are all markedly enriched in 18 O compared to almost all other massive sulfide deposits. A chalcopyrite stockwork zone, forming a concordant keel stratigraphically below the massive sulfide, typically has whole-rock delta 18 O = 10 to 12 per mil. Beneath the stockwork keel, rhyolite delta 18 O values systematically increase stratigraphically downward for about 50 m. Below this level, all of the footwall rhyolites in the Kidd Creek area display exceptionally high delta 18 O values of 13 to 16 per mil. The groundmass quartz in the rhyolites ranges from 13.6 to 15.5 per mil, and crosscutting quartz veins associated with the chalcopyrite stockwork mineralization have delta 18 O = 11.9 per mil. By contrast, the postore basalts of the hanging wall are weakly altered, show only modest 18 O enrichments (delta 18 O = 8-10ppm), and are isotopically similar to the volcanic rocks in other parts of the Abitibi greenstone belt.Petrographically, all of the Kidd Creek footwall rhyolites are intensely altered (typically containing 76-90 wt % SiO 2 ), indicating that the high 18 O characteristics are not primary magmatic features but are the result of hydrothermal alteration. Several disequilibrium oxygen isotope relationships are present. For example, volcaniclastic rhyolites consist of extremely 18 O-rich, chlorite-rich, and sericite-rich clasts enclosed within a lower 18 O, sericitically altered rhyolite tuff matrix. This implies at least a two-stage hydrothermal history, involving either different temperatures of alteration or different fluid sources, or both. Using a temperature of 300 degrees to 350 degrees C, the main ore-forming fluid must have had delta 18 O = 6 to 9 per mil. Inasmuch as the associated felsic volcanic rocks were apparently erupted in a shallow subaqueous (locally subaerial?) environment, the most plausible origin of the ore fluid is evaporation and enrichment in 18 O of normal seawater in a closed basin at the western edge of the Abitibi belt. This would produce a strongly 18 O-shifted, high-salinity fluid that would have been capable of transporting large amounts of metals. The high 18 O signature of the Kidd Creek-Timmins volcanic rocks may constitute an exploration target that warrants further study and evaluation in other areas.

Age of the Austurhorn intrusion; a net-veined complex in southeastern Iceland

Abstract Previous attempts to date the rocks of eastern Iceland by radiometric means have resulted in a considerable scatter in ages. This was probably the result of hydrothermal alteration, affecting the investigated material, particularly lavas, to a varying degree. This study reports the results of a combined Rb-Sr and K-Ar whole-rock investigation of granophyric material from the Austurhorn complex, southeastern Iceland. The Rb-Sr isochron age obtained for four samples is 12.8 ± 5.3 Ma, consistent with the K-Ar ages which average 11.8 Ma.