Hydrothermal Fluid Activity Research Articles

As jazidas de talco no contexto da história metamórfica dos metadolomitos do grupo Itaiacoca, PR

Four main types of talc occurrences have been recognized in the Proterozoic metadolomites of the Itaiacoca Group, Paraná State, southern Brazil: a) talc schists along transcurrent shear zones; b) incipient talc formation along fractures and bedding planes; c) irregular pockets of fine-grained, massive talc; d) retrometamorphic talc associated with lenses of olivine, diopside, tremolite, calcite and quartz in the contact zone with the neighbouring granites. Of these, only types (a) and (c) form ore deposits. The Itaiacoca Group underwent four metamorphic episodes: 1) greenschist facies regional metamorphism during the Neoproterozoic Brasiliano orogeny; 2) thermal metamorphism along the contact with the Neoproterozoic Cunhaporanga granite batholith; 3) hydrothermalism related to transcurrent shear zones, that formed large volumes of talc schists and irregular pockets of fine-grained massive talc; 4) small-scale thermal metamorphism related to Mesozoic diabase dikes. The Itapirapuã transcurrent shear zone marks the contact between the Itaiacoca Group and the Proterozoic Três Córregos granite batholith, and the main talc deposits are aligned along its branches. Enhanced permeability due to shearing provided channels for percolation of the silica-rich aqueous solutions that reacted with the metadolomite to form the talc deposits. Minor amounts of talc occur along fractures and sedimentary layering. The massive pockets of talc (type c) that replaced the metadolomite probably formed statically at lower temperatures by the activity of hydrothermal fluids introduced through a network of microfractures.

05/00036 Activity of high-temperature hydrothermal fluids and the formation of the hydrocarbon reservoirs in the Wen'an slope of the Bohaiwan Basin, China: Meng, Y. et al. Yanshi Xuebao, 2003, 19, (2), 337–394. (In Chinese)

05/00036 Activity of high-temperature hydrothermal fluids and the formation of the hydrocarbon reservoirs in the Wen'an slope of the Bohaiwan Basin, China: Meng, Y. et al. Yanshi Xuebao, 2003, 19, (2), 337–394. (In Chinese)

Characteristics of mantle degassing and deep-seated geological structures in different typical fault zones of China

n this paper a comprehensive tracing study is conducted on mantle degassing and deep-seated geological structures in different types of fault zones in the continent of China based on the helium isotope data, coupled with some indices such as CO 2 / 3 He, CH 4 / 3 He and 40 Ar/ 36 Ar, and geological tectonics data. There are four representative types of fault zones: (1) Lithospheric fault zones in the extensional tectonic environment are characterized by a small Earth’s crust thickness, a lower CH 4 / 3 He-high R and lower CO 2 / 3 He-high R system, the strongest mantle de-gassing, and the dominance of mantle fluid, as is represented by the Tancheng-Lujiang fault zone. (2) The lithospheric fault zones or the subduction zone in the strongly compresso-tectonic envi-ronment, for instance, the Bangonghu-Nujiang fault zone, are characterized by a huge thick Earth’s crust, with the R/Ra values within the range of 0.43―1.13, and weak mantle degassing with mantle-source helium accounting for 5%―14% of the total. (3) The deep-seated fault zones at the basinal margins of an orogenic belt are characterized by R values being on order of magnitude of 10 -7 , and the CH4/3He values, 10 9 ―10 10 , CO 2 / 3 He values, 10 6 ―10 8 ; as well as much weak mantle degassing. (4) The crustal fault zones in the orogenic belt, such as the Yaojie fault zone (F 19 ), possess a high CH 4 / 3 He-low R (10 -8 ) and high CO 2 / 3 He-low R system, with no obvi-ous sign of mantle degassing. Studies have shown that the deep-seated huge fault zones are the major channel ways for mantle degassing, the main factors controlling the intensity of mantle degassing are fault depth, tectonic environment and crust thickness; the intensity of mantle de-gassing can reflect the depth and the status of deep-seated tectonic environment of fault, while the geochemical tracing studies of gases can open up a new research approach; upwelling ac-tivity of hydrothermal fluids from the deep interior of the Earth may be one of the driving forces for the formation and evolution of the huge deep fault zones. Piedmont fault zones are the locations where deep-seated tectonic activity and crust/mantle structure are transformed, which are of great significance in understanding the mechanisms of formation of orogenic belts and basins.

Cambrian granitoids in pre-Alpine basement of Crete (Greece): evidence from U-Pb dating of zircon

The pre-Alpine basement of eastern Crete consists of at least three sub complexes which show differences in age and grade of pre-Alpine metamorphism. U-Pb analyses of zircons of two orthogneisses yielded upper intercepts in the concordia diagram at 511±16 Ma (Chamezi crystalline complex) and 514±14 Ma (Myrsini crystalline complex). These values are interpreted as protolith ages of the gneisses, suggesting that parts of the basement underwent Cadomian (Pan-African) imprints at the northern margin of Gondwana. The lower intercept ages are attributed to Alpine subduction which led to high-pressure/low-temperature metamorphism (T=ca. 300 °C), brittle-ductile deformation, and hydrothermal fluid activity. Due to pervasive Alpine fluid migration, the zircons of the Chamezi crystalline complex display disturbed zonation and solution, resulting in 87% radiogenic Pb loss, Ca- and Al-gain, and Zr depletion.

Reconciling fluid inclusion types, fluid processes, and fluid sources in skarns: an example from the Bismark Deposit, Mexico

The Bismark deposit (8.5 Mt at 8% Zn, 0.5% Pb, 0.2% Cu, and 50 g/t Ag) located in northern Mexico is an example of a stock-contact skarn end member of a continuum of deposit types collectively called high-temperature, carbonate-replacement deposits. The deposit is hosted by massive sulfide within altered limestone adjacent to the Bismark quartz monzonite stock (~42 Ma) and the Bismark fault. Alteration concurrently developed in both the intrusion and limestone. The former contains early potassic alteration comprising K-feldspar and biotite, which was overprinted by kaolinite-rich veins and alteration and later quartz, sericite, and pyrite with minor sphalerite and chalcopyrite. Prograde exoskarn alteration in the limestone consists of green andradite and diopside, and transitional skarn comprising red-brown andradite, green hedenbergite and minor vesuvinite, calcite, fluorite, and quartz. The main ore stage post-dates calc-silicate minerals and comprises sphalerite and galena with gangue pyrite, pyrrhotite, calcite, fluorite, and quartz. The entire hydrothermal system developed synchronously with faulting. Fluid inclusion studies reveal several distinct temporal, compositional, and thermal populations in pre-, syn- and post-ore quartz, fluorite, and calcite. The earliest primary fluid inclusions are coexisting vapor-rich (type 2A) and halite-bearing (type 3A; type 3B contain sylvite) brine inclusions (32 to >60 total wt% salts) that occur in pre-ore fluorite. Trapping temperatures are estimated to have been in excess of 400 °C under lithostatic pressures of ~450 bar (~1.5 km depth). Primary fluid inclusions trapped in syn-ore quartz display critical to near critical behavior (type 1C), have moderate salinity (8.4 to 10.9 wt% NaCl equiv.) and homogenization temperatures (Th) ranging from 351 to 438 °C. Liquid-rich type 1A and 1B (calcite-bearing) inclusions occur as primary to secondary inclusions predominantly in fluorite and show a range in Th (104–336 °C) and salinity (2.7–11.8 wt% NaCl equiv.), which at the higher Th and salinity ranges overlap with type 1C inclusions. Oxygen isotope analysis was carried out on garnet, quartz, and calcite (plus carbon isotopes) in pre-, syn-, post-ore, and peripheral veins. Pre-ore skarn related garnets have a δ18Omineral range between 3.9 and 8.4‰. Quartz from the main ore stage range between 13.6 and 16.0‰. Calcite from the main ore stage has δ13C values of –2.9 to –5.1‰ and δ18O values of 12.3 to 14.1‰, which are clearly distinct from post-ore veins and peripheral prospects that have much higher δ18O (16.6–27.3‰) and δ13C (1.3–3.1‰) values. Despite the numerous fluid inclusion types, only two fluid sources can be inferred, namely a magmatic fluid and an external fluid that equilibrated with limestone. Furthermore, isotopic data does not indicate any significant mixing between the two fluids, although fluid inclusion data may be interpreted otherwise. Thus, the various fluid types were likely to have formed from varying pressure–temperature conditions through faulting during exsolution of magmatic fluids. Late-stage hydrothermal fluid activity was dominated by the non-magmatic fluids and was post-ore.

Three–Dimensional Isotopic Characteristics of Crystalline Limestone around the Sakonishi Zn Ore Bodies in the Kamioka Mining District, Japan

Abstract: Predominant Zn‐rich ore bodies were found to a deep part of the Sakonishi area in the Kamioka mining district, Japan. The ore mineralization was recognized at 230 to 300m above sea level in the Sakonishi area. Since crystalline limestone is broadly distributed over the area, and the variation in isotopic composition is easily detected, the isotopic prospecting should be powerful in surveying of the extent of the ore bodies and the related hydrothermal system. Although isotopic anomalies have been extracted two‐dimensionally so far, three‐dimensional information is possibly more powerful.In this paper, Zn‐rich ore bodies in the Sakonishi area are treated as hydrothermal ore deposits, and the importance of the activity of hydrothermal fluids during mineralization is emphasized. Oxygen and carbon isotopic ‘iso–surfaces’ are three‐dimensionally calculated for the Sakonishi area. The δ18O values of crystalline limestone from the surface and from the drill holes range from +8. 1 to +21. 1% and from –2. 7 to +20. 4%, respectively. The δ13C values of crystalline limestone from the surface and from the drill holes range from –1. 0 to +5. 3% and from –7. 7 to +4. 6%, respectively. The oxygen and carbon iso‐topic ratios at the mineralization level are extremely low, but there are exceptions as to carbon isotopes. The oxygen isotopic ratios of crystalline limestone may decrease by isotopic exchange reaction with a hydrothermal fluid, while the carbon isotopic ratios slightly change. Since the precipitated calcite from a hydrothermal fluid has low carbon isotopic ratio and various oxygen isotopic ratio depending on the formation temperature, the bulk sample of crystalline limestone containing the precipitated calcite has oxygen and carbon isotopic ratios of relatively low values accordingly. Thus the decrease mechanism for carbon isotopic ratio of crystalline limestone is different from that for oxygen isotopic ratio. Samples with the carbon isotopic ratio of –4 to –8% are considered to be crystallized from hydrothermal fluids.Since the oxygen and carbon isotopic ratios of crystalline limestone at the ore mineralization level are low, the ore bodies are considered to have formed by a prominent hydrothermal activity. Thus oxygen and carbon isotopic ratios of crystalline limestone can be used as an indicator of the related hydrothermal activity. The alteration such as chloritization is intense near fractures in the Sakonishi area, showing that the hydrothermal system is controlled by a fracture system. It is assumed that the decreased isotopic ratios indicate the high degree of reactivity with hydrothermal fluids, and the depleted zone in oxygen and carbon isotopes may correspond to the conduit of the hydrothermal fluids.

Sulfide-poor platinum-group mineralization in komatiitic systems; Boston Creek flow, layered basaltic komatiite, Abitibi Belt, Ontario

Geochemical studies of platinum-group elements (PGE), Au, Ag, and other metals (S, Se, As, Sb, Cu, Ni, Cr, Co, V, Ti) and electron microprobe studies of them have provided new constraints on the character and origin of the recently discovered sulfide-poor PGE mineralization in the Boston Creek flow layered basaltic komatiite of the Abitibi belt. High PGE levels characterize a 2-m-thick interval overlapping the contact of the clinopyroxenite and gabbro layers (50 ppb Pt, 75 ppb Pd, 10 ppb Au), the basal part of the clinopyroxenite layer (20 ppb Pt, 33 ppb Pd, 49 ppb Au), and the basal part of the peridotite layer (3 ppb Ir, 15 ppb Pd). The PGE-rich clinopyroxenite and gabbro layers contain grains of Pd, Pt, and Rh bismuthotellurides, antimonides and arsenides, and gold and silver minerals, which occur in spatial association with sparsely disseminated interstitial chalcopyrite, skeletal magnetite-ilmenite, and secondary amphibole alteration of chalcopyrite and magnetite-ilmenite. These rocks are spatially associated with zones of sheared amphibolite that are relatively poor in PGE and rich in Ni, Co, S, and Se. Minor anorthosite patches, dikes, and discontinuous layers immediately up section of the mineralized gabbro are relatively poor in PGE, especially Ir (< or =0.02 ppb), and Ni, Cu, Co, and S. The PGE-enriched peridotite is slightly richer in Cr and Ni but poorer in Co, Ag, S, and Se compared to the overlying peridotite.The textural and geochemical characteristics indicate a sulfide control in the origin of the PGE mineralization, but they contradict an important role for settling of segregated sulfide, Cl-rich late-stage magmatic volatiles, or hydrothermal fluid activity during metamorphism. Alternatively, they suggest PGE mineralization by in situ segregation of sulfide accompanying the crystallization of Fe-Ti oxide from upwardly mobile, volatile-charged interstitial melt that evolved in the clinopyroxenite and basal gabbro and subsequently formed the anorthosite. Nickel, Co, Se, S, Ag, and Cu were selectively mobilized relative to the PGE in the mineralized rocks by hydrothermal fluids and reconcentrated in zones of shear deformation and brittle failure during greenschist facies contact metamorphism. Iridium and Pd probably concentrated in the basal peridotite by coprecipitation of an alloy-sulfide phase during the fractionation of olivine + or - chromite. Exploration criteria potentially useful in the search for economic concentrations of PGE in geologically similar environments of the Abitibi belt and elsewhere may include magnetite-ilmenite-rich clinopyroxenite and gabbro within thick layered mafic-ultramafic flows and especially intrusions; and Cu, V, Fe, and Ti fractionation trends and anomalies.

Halogen chemistry as an indicator of metamorphic fluid interaction with the Ponder pluton, Coast Plutonic Complex, British Columbia, Canada

During hydrothermal fluid activity associated with the intrusion of the Ponder pluton, Coast Plutonic Complex, British Columbia, Canada, the halogen composition of biotite, amphibole and apatite is controlled by the composition of the metamorphic fluid. The core of the pluton was not affected by infiltrating fluids, and the fluorine composition of biotite was controlled by the biotite Fe-Mg ratio. The edge of the cooling, crystallized pluton interacted with a chlorine-rich, fluorine-poor fluid which changed the halogen composition of biotite and amphibole. The composition of the metamorphic fluid derived from biotite and apatite compositions was relatively high in chlorine and low in fluorine. The concentration of chlorine in the metamorphic fluid increases towards the pluton, whereas fluorine remains constant. This suggests that the metamorphic fluid near the pluton had the greatest potential for masstransfer of metals.

Notes on the petrology of some ophiolites, othris mountains, Greece

Igneous rocks typical of ophiolite complexes are preserved in a stack of thrust sheets in the Othris Mountains of eastern Greece. Mineralogical studies of mafic rocks indicate that the majority have tholeiitic affinities, but there is evidence of a subordinate, mildly alkaline trend. Petrographic and chemical evidence suggest that the present mineral assemblage in the mafic rocks may be the result primarily of the activity of deuteric or secondary hydrothermal fluids. The “amphibolites” so common in dredge-hauls from the oceans may have originated in the same way. These metamorphic rocks, with perfectly preserved igneous textures, must be distinguished from schistose amphibolites, which also occur on the ocean floors and in ophiolite complexes.

Titaniferous iron ore

The Abagong apatite-rich iron deposit is located in the Kelang volcano-sedimentary basin in the southern margin of the Chinese Altay Orogenic Belt (AOB), which is an important polymetallic belt in China. The ore bodies are interbedded with the upper Silurian to Lower Devonian metamorphosed calc-alkaline felsic volcanic rocks that show well-preserved porphyritic and volcaniclastic textures. The ore mineralization is aligned along faults that controlled their emplacement and occurs as lenses, veins, and stratoids bodies that generally cut the host rocks at a high angle and show sharp contacts. The ore bodies and host rocks have undergone greenschist facies metamorphism, deformation and folding. The iron ores have massive, densely disseminated, banded, and brecciated structures.Magnetite from the Abagong deposit is low in Ti (TiO2 = 0.002–0.129 wt.%) and the apatites from different ore types (P-poor and P-rich ores) are typical igneous fluorapatites with minor Cl contents. Apatites of different forms and from different ore types show similar rare earth element (REE) and trace element-normalized patterns with weak-to-moderate enrichment in light REE, Th, U, and Pb, depletion in Sr, Ba, Nb, Ta, and Ti, and negative Eu anomalies, indicating a common source and genesis. The similar REE patterns for the magnetite, apatite, and volcanic host rocks suggest their close genetic linkage and support a magmatic origin for the deposit. The Abagong deposit shows the typical characteristics of Kiruna-type deposits with regard to the mineral assemblages, ore texture and structure, and the apatite and magnetite geochemistry. We propose that the Kiruna-type Abagong apatite-rich iron deposit was derived from Fe–P-rich melt through liquid immiscibility and the activity of hydrothermal fluids.