Formation Of Ankerite Research Articles

Elemental Mass Balance Calculation of the Hydrothermal Alteration in Awak Mas Gold Deposit, Sulawesi Island, Indonesia

Abstract Primary gold mineralization is associated with the hydrothermal processes. Interaction between the hydrothermal fluids and the wall rock resulting an alteration which characterized and indicated a mineralization process. The understanding of the mechanisms of alteration requires quantification of elemental changes in proximal to distal alteration zones. This study was conducted on an orogenic gold deposit at Awak Mas determine the lithogeochemical in each alteration zone caused by the hydrothermal processes. A total of eight schist rock samples geochemical data were applied in this study representing the least altered, quartz-albite zone, and albite-ankerite-pyrite zones. Major elements geochemical data was resulted by XRF analysis, and trace elements obtained by ICP-MS analytical method. The calculation method for mass balance during alteration is carried out using Microsoft Excel spreadsheet software by applying the graphical isocon method. The calculation results show that there are gains and losses of elements that cause changes in rock volume and mass due to the reaction of hydrothermal solutions with wallrocks. From the least altered schist to quartz-albite zone, the alteration process increases 19,9% SiO2, 4.5% Na2O, 3.4% Fe2O3 and decrease of some elements, while Al and Ti were relatively immobile. This alteration process causes a gain in total rock volume of 20.7%. Furthermore, the alteration from quartz-albite to albite-ankerite-pyrite showed the gains of 0.16% SiO2, 0.43% Na2O, 1.3% SO3, 1.55% CaO, this reaction resulted in an additional volume to 9.7%. These calculations indicate that the hydrothermal solution is dominated by silica and sodium elements which form the secondary minerals of quartz and albite in distal zone. In the proximal alteration zone, which carries gold mineralization besides silica and sodium there is also characterized by enrichment of calcium, iron and sulfur which results in the formation of ankerite and pyrite. Gold is associated with pyrite which are formed in the albite-ankerite-pyrite zone.

Diagenetic Impact on High-Pressure High-Temperature Reservoirs in Deep-Water Submarine Fan Sandstone of Qiongdongnan Basin, South China Sea

The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial.

Observations of CO2 Corrosion-Induced Carbonate Scale Formation and Inhibition on Mild Steel

Summary Aqueous CO2-containing environment is ubiquitous in oil and gas production. Carbonate scales (e.g., calcite) tend to form in such an environment. Meanwhile, the CO2 corrosion of mild steel infrastructure may result in corrosion-induced scales including siderite (FeCO3). Previously, siderite was generally treated as a corrosion problem rather than a scale problem. However, the relationship between the corrosion-induced scale and other metal carbonate scales on the steel surface is unclear. For example, how does siderite influence calcite deposition on the mild steel? In this study, the mild steel corrosion and mineral carbonate scaling behaviors were investigated simultaneously in the presence of various cations such as Ca2+ and Mg2+. We observed a two-layer scale structure on the mild steel surface under simulated oilfield conditions. The inner layer is an iron-containing carbonate scale such as ankerite or siderite, while the outer layer is calcite. In addition, calcite deposition at a very low saturation index was observed when the inner layer was present. Furthermore, a common scale inhibitor [diethylenetriaminepentakis(methylenephosphonic acid) or DTPMP] can effectively mitigate calcite, siderite, and ankerite formation on the steel surface, but meanwhile, aggravate the steel corrosion because of the absence of protective scale layers.

Dawsonite and ankerite formation in the LDX-1 structure, Yinggehai basin, South China sea: An analogy for carbon mineralization in subsurface sandstone aquifers

The geochemistry and petrology of the LDX-1 structure of the Yinggehai basin, a natural analog of a sedimentary carbon storage site, was investigated to understand the consequences of the charging of CO2 gas in this system. The rocks in this structure are dominated by subarkose and sublitharenite sandstones. The authigenic minerals formed after CO2 injection are dawsonite, microcrystalline quartz, kaolinite and ankerite. Dawsonite and ankerite are formed just beneath a CO2 bearing anticlinal structure due to the reactions between silicate minerals (feldspars and clay minerals) and the fluid phase. Carbon and oxygen isotopic analyses indicate that the main carbon source for dawsonite and ankerite formation was mantle magmatic CO2. The aqueous activities of sodium and calcium, the partial pressure of CO2, pH and temperature are the key factors influencing the stability of the dawsonite and ankerite. The presence of the anticlinal structure, maintaining a locally high CO2 partial pressure in the waters beneath this structure, is likely responsible for the observed long-term persistence of dawsonite and ankerite in this system.

Case study of the igneous intrusion effect on the mineralogical composition of the Carboniferous coal from Jingxi Coalfield, North China

Igneous intrusions significantly affect the mineralogical composition of coal. The Jingxi Coalfield, North China was subjected to the intrusion of a Mesozoic aplite; however, the resulting coal mineralogy has not been well investigated. This paper reports on a study of the mineralogical composition of five coal bench samples collected from the No. M3 coal in the Jingxi Coalfield, which was intruded by an aplite sill along the roof. The minerals present in the highest proportions in the No. M3 coal are margarite–paragonite group mineral (21.4%) and ammonian illite (72.7%), followed by ankerite (4.2%), anatase (0.7%), and rutile (0.9%). The formation of margarite, paragonite and ammonian illite is attributed to the intrusion of the aplite sill. The margarite and paragonite were formed prior to the formation of the ammonian illite. The proportion of the margarite–paragonite group mineral decreases, and the proportion of ammonian illite increases from the sill to the roof. The Ca2+ and K+ are more easily incorporated into the aluminosilicate mineral lattices than Na+ and NH4+, respectively. Thus, the Ca2+/Na+ ratio in the margarite–paragonite group mineral decreases, and the NH4+/K+ ratio in the ammonian illite increases from the sill to the roof. The formation of ankerite is also attributed to the igneous intrusion, but this mineral was formed later than the aluminosilicate minerals. The proportion of ankerite in the coal samples increases from the sill to the roof.

Hydrothermal experiments reveal the influence of organic matter on smectite illitization

Smectite illitization is an important diagenetic phenomenon of mudstones, but only rarely has the influence of organic matter (OM) on this process been examined. In the present study, hydrothermal experiments were conducted with smectite (M1, total organic carbon (TOC) 1%). X-ray diffraction (XRD), infrared, X-ray fluorescence (XRF), and organic carbon analyses were employed to characterize the mineralogy and OM of the samples and the effect of OM on smectite illitization. The XRD patterns showed changes in clay mineral parameters with increased temperature. These changes varied in both M1 and M2 and indicated a difference in the degree of smectite illitization. Moreover, the OM in M2 was mainly adsorbed in smectite interlayers, the OM was largely desorbed/decomposed at temperatures above 350°C, and the OM was the main reason for differences in the degree of smectite illitization between M1 and M2. Bulk mineral composition, elemental content, and infrared absorption band intensities were changed with increased temperature (especially above 350°C). This indicated the formation of new minerals (e.g., ankerite). Overall, OM entered the interlayer space of smectite in M2 and delayed the exchange of K+ by interlayer cations, and thus, suppressed the transformation of smectite to illite and resulted in differences in smectite illitization of M1 and M2. In particular, the formation of CO2 after the decomposition of OM at temperatures above 300°C led to the formation of ankerite in M2. This demonstrated the effect of organic-inorganic interactions on smectite illitization and mineral formation. The disparities in smectite illitization between M1 andM2, therefore, were linked to differences in the mineral formation mechanisms of a water-rock system (M1) and a water-rock-OM system (M2) in natural environments. The insights obtained in the present study should be of high importance in understanding organic-mineral interactions, hydrocarbon generation, and the carbon cycle.

Evaporite minerals of the lower 538.5 m sediments in a long core from the Western Qaidam Basin, Tibet

Qaidam Basin is a tectonically controlled Mesozoic–Cenozoic depression on the northern margin of the Tibetan Plateau. A 938.5 m-long core was drilled in the Qahansilatu sub-basin in the western Qaidam Basin, with an average core recovery of 95%. It revealed alternating salt layers and carbonate clay layers. Samples were collected at 10–40 cm intervals for mineralogical analysis by XRD and chemical analysis by ICP-OES. The lower 538.5 m sediments are composed of halite, gypsum, anhydrite, gaylussite, calcite, aragonite, ankerite, dolomite, and an unnamed mineral (Mg0.92Ca0.08CO3·3H2O), with trace eugsterite. The mother brines could be Na-type, Na–Ca-type, Na–Ca–(Mg)-type, Ca–(Na)-type, Ca–(Mg)–(Na)-type, and trace Ca–Mg–(Na). Reflux and bacterial activity could be suitable for the formation of dolomite. Deep burial diagenesis could have played an active role in the formation of ankerite and anhydrate. The continuous presence of halite suggested the paleo-lake water was highly brackish or saline, with high evaporation since 2.77 Ma. Salt layers in the lower 538.5 m-long sediments were present from 2221 ka to 2052 ka, corresponding to Pleistocene salt formation in the Qaidam Basin. As hydrated carbonate minerals, both gaylussite and the unnamed mineral are deposited under high precipitation rates with microbial activity. Gaylussite was deposited from Na–Ca-enriched solutions with molar ratios of Na/Ca more than 2 from 691 m (2226 ka) to 413.6 m (1222 ka). The un-named mineral (Mg0.92Ca0.08CO3·3H2O) was found from 523.4 m (1728 ka) to 724.2 m (2308 ka). Anhydrite could be transformed from gypsum under deep burial from 657.42 m (2052 ka) to 867 m (2556 ka). Alternating salt and clay layers in the lower part of the core recorded arid and relatively wet climatic oscillations and the evolution of brine. As a tectonic sub-basin, tectonic activities could change the local climate during episodes of uplift and subsidence. The dominant minerals in the Chahansilatu sub-basin are similar to those of the other sub-basins in the western Qaidam Basin, but have asynchronous evolutionary stages of brine.

Geology, petrography, and mineral chemistry of the Vazante non-sulfide and Ambrósia and Fagundes sulfide-rich carbonate-hosted Zn–(Pb) deposits, Minas Gerais, Brazil

The Vazante–Paracatu region represents the most important Zn district known in Brazil and includes the Vazante hypogene non-sulfide Zn deposit composed mainly of willemite (Zn 2SiO 4) and sphalerite-rich carbonate-hosted Zn–(Pb) deposits. Fagundes is a stratabound deposit, characterized by strong silicification, dolomitization and a Fe-rich carbonate alteration halo. The main ore is represented by rhythmically banded, colloform, and zoned pyrite, sphalerite and galena, related to wall rock dissolution and sulfide infilling, which probably occurs late during the burial diagenesis. Later veins and breccia ore types reflect epigenetic mobilization, related to brittle–ductile structures. The Ambrósia mineralization is mainly fault-controlled and related to brecciated dolomite, which is tectonically imbricated with black shales and slates. Typical features include host rock recrystallization, minor silicification, baroque dolomite and ankerite formation. Pyrite, marcasite, sphalerite and minor galena occur in brecciated comb-veins and veinlets, which overprint stylolites and tectonic fractures, suggesting an epigenetic origin for the ore. The Vazante deposit differs from all other deposits of the district due to the presence of willemitic ore, which is composed of willemite, dolomite, siderite, quartz, hematite, Zn-rich chlorite, barite, franklinite and zincite. The willemitic ore occurs tectonically imbricate to small sulfide ore bodies, which comprises sphalerite and galena, metabasites and hydrothermalized dolomites within the Vazante Shear Zone. The relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the Vazante Shear Zone. Low Zn/Cd ratios in sphalerite from Vazante (64 to 98) and Fagundes colloform (96 to 244) and zoned sphalerite (89 to 305) could reflect the regional role of mineralizing fluids with similar low Zn/Cd ratios and low contents of reduced sulfur (∑S red). High Ge (up to 2200 ppm) and the low Fe, Cu, Mn and Ag contents in late-diagenetic Fagundes sphalerite might suggest that this metal-bearing fluid could be derived from the underlying basin fill, which comprise clastic sediments and organic matter-rich pelitic sequences. Systematic relationships among sphalerite composition and paragenetic evolution of the Fagundes and Ambrósia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide-rich deposits in the district. These mixing processes possibly involved the hot metal-bearing fluid with low contents of reduced sulfur and moderate-temperature, highly saline fluids, which could represent an important sulfur supply. The predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the Brasiliano compressive events. The small variation in chemical and sulfur isotopic composition of the Vazante sphalerite could imply that the high-temperature metal-bearing fluid with low Zn/Cd ratios could represent a minor reservoir of reduced sulfur, which permitted only subordinate sphalerite precipitation in the Vazante deposit. The lack of reduced shale sequences above the Vazante deposit could represent a limiting factor for H 2S supply. Additionally, overall mixture between this hot sulfur-deficient metal-bearing fluid and meteoric fluids channeled to the Vazante Shear Zone favor the high fO 2/S 2 conditions responsible for the stability of the Vazante willemitic assemblage.

Mineralogy and geochemistry of a Late Permian coal in the Dafang Coalfield, Guizhou, China: influence from siliceous and iron-rich calcic hydrothermal fluids

This paper describes the influence of siliceous and iron-rich calcic low-temperature hydrothermal fluids (LTHF) on the mineralogy and geochemistry of the Late Permian No. 11 Coal (anthracitic, R r=2.85%) in the Dafang Coalfield in northwestern Guizhou Province, China. The No. 11 Coal has high contents of vein ankerite (10.2 vol.%) and vein quartz (11.4 vol.%), with formation temperatures of 85 and 180 °C, respectively, indicating that vein ankerite and vein quartz were derived from low-temperature calcic and siliceous hydrothermal fluids in two epigenetic episodes. The vein quartz appears to have formed earlier than vein ankerite did, and at least three distinct stages of ankerite formation with different Ca/Sr and Fe/Mn ratios were observed. The two types of mineral veins are sources of different suites of major and trace metals. Scanning electron microscope and sequential extraction studies show that, in addition to Fe, Mg, and Ca, vein ankerite is the dominant source of Mn, Cu, Ni, Pb, and Zn in the coal, and the contents of these five elements are as high as 0.09% and 74.0, 33.6, 185, and 289 μg/g, respectively. In contrast, vein quartz is the main carrier mineral for platinum-group elements (PGEs) Pd, Pt, and Ir in the coal, and the contents of Pd, Pt, and Ir are 1.57, 0.15, and 0.007 μg/g, respectively. Sequential extraction showed a high PGE content in the silicate fraction, up to 10.4 μg/g Pd, 1.23 μg/g Pt, and 0.05 μg/g Ir, respectively. It is concluded that the formation of ankerite and quartz and the anomalous enrichment of trace elements in the No. 11 Coal in the Dafang Coalfield, Guizhou, result from the influx of calcic and siliceous low-temperature hydrothermal fluids.

The origin of authigenic ankerite from the Ninian Field, UK North Sea

The carbonate minerals ankerite and ferroan dolomite are widely reported in studies of concretions1,2 and of sandstone hydrocarbon reservoirs3–8. These minerals are often thought to have formed relatively late in the sandstones' diagenetic history and in the Gulf Coast their occurrence has been linked to clay mineral diagenesis in the surrounding shales9. I describe here authigenic ankerite from the Ninian Field10. Ankerite formation was probably synchronous with hydrocarbon emplacement and is related to clay mineral diagenesis in the hydrocarbon source rocks of the Viking Graben. It occurs throughout the reservoir sandstones but is particularly abundant below the oil–water contact. This distribution reflects hydrocarbon emplacement arresting diagenesis at progressively deeper levels of the reservoir. The resulting preferential cementation may adversely affect transmissibility between the aquifer and the oil column and consequently could reduce hydrocarbon recovery.