scholarly journals Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

Minerals ◽  
2019 ◽  
Vol 9 (10) ◽  
pp. 601 ◽  
Author(s):  
Amores-Casals ◽  
Melgarejo ◽  
Bambi ◽  
Gonçalves ◽  
Morais ◽  
...  
Keyword(s):  
Element Concentration ◽  
Hydrothermal Fluids ◽  
Critical Element ◽  
High Grade ◽  
Magma Mingling ◽  
Hydrothermal Processes ◽  
Crystal Accumulation ◽  
Carbonatite Magma

The Bonga complex is composed of a central carbonatite plug (with a ferrocarbonatite core) surrounded by carbonatite cone sheets and igneous breccias of carbonatitic, fenitic, phoscoritic and lamprophyric xenoliths set in a carbonatitic, lamprophyric or mingled mesostase. To reconstruct the dynamics of the complex, the pyrochlore composition and distribution have been used as a proxy of magmatic-hydrothermal evolution of the complex. An early Na-, F-rich pyrochlore is disseminated throughout the carbonatite plug and in some concentric dykes. Crystal accumulation led to enrichment of pyrochlore crystals in the plug margins, phoscoritic units producing high-grade concentric dykes. Degassing of the carbonatite magma and fenitization reduced F and Na activity, leading to the crystallization of magmatic Na-, F- poor pyrochlore but progressively enriched in LILE and HFSE. Mingling of lamprophyric and carbonatite magmas produced explosive processes and the formation of carbonatite breccia. Pyrochlore is the main Nb carrier in mingled carbonatites and phoscorites, whereas Nb is concentrated in perovskite within mingled lamprophyres. During subsolidus processes, hydrothermal fluids produced dolomitization, ankeritization and silicification. At least three pyrochlore generations are associated with late processes, progressively enriched in HFSE, LILE and REE. In the lamprophyric units, perovskite is replaced by secondary Nb-rich perovskite and Nb-rich rutile. REE-bearing carbonates and phosphates formed only in subsolidus stages, along with late quartz; they may have been deposited due to the release of the REE from magmatic carbonates during the hydrothermal processes.

scholarly journals Orange opals from Buriti dos Montes, Piauí: solid inclusions as genetic guides

2015 ◽  
Vol 68 (1) ◽  
pp. 53-59 ◽  
Author(s):  
Gisele Tavares Marques ◽  
Marcondes Lima da Costa ◽  
Érico Rodrigues Gomes
Keyword(s):  
Chemical Composition ◽  
Aqueous Solutions ◽  
High Stability ◽  
Hydrothermal Fluids ◽  
Northeastern Brazil ◽  
Quartz Crystals ◽  
Hydrothermal Processes ◽  
Hydrothermal Environment

Orange opals from Buriti dos Montes (Piauí, northeastern Brazil) have gemological properties that favor their use as jewelry; these characteristics include their colors, transparency, relatively high stability and hardness. The exotic content of solid inclusions provides greater beauty to the opals of this region. These opals originated from hydrothermal processes and are found mainly as veinlets and veins in the sandstones of the Serra Grande Group, sectioned by diabase dikes and sills of the Sardinha Formation. Solid inclusions, such as bubbles, botryoidal aggregates, dendrites, and nodules, among others, consist mainly of kaolinite, hematite/goethite and quartz and influence the chemical composition of opals. Intense zoning of quartz crystals and high values of Ba and Fe suggest that opal deposits were formed in a hydrothermal environment. Diabase dykes could have been responsible for heating the hydrothermal fluids. Sandstones, rich in aqueous solutions, also contributed to the available silica for the saturation of these solutions, and fractures enabled the migration and entrapment of hydrothermal fluids, resulting in the mineralized veins.

scholarly journals A study on Chinese ancient jades with mercury alteration unearthed from Lizhou’ao Tomb

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Yi Bao ◽  
Changqing Xu ◽  
Qinwen Zhu ◽  
Yuesheng Li
Keyword(s):  
Chemical Composition ◽  
Surface Water ◽  
Human Activity ◽  
Environmental Conditions ◽  
Structural Changes ◽  
Hydrothermal Fluids ◽  
Jiangxi Province ◽  
High Grade ◽  
Structural Hierarchy ◽  
Spring And Autumn Period

Abstract“Alteration” geologically refers to chemical composition and/or structural changes of minerals under the influences of hydrothermal fluids, surface water, seawater, or other environmental conditions. In this paper, we use the word “alteration” to refer to chemical component and structural changes in jade artifacts caused by human activity and natural weathering, which is different from the term in geology. “Mercury alteration”, a kind of black alteration related to Hg, is unique among the several types of alteration that occur in Chinese ancient jades. Mercury alteration often appears on ancient jade artifacts unearthed from high-grade tombs of the pre-Qin period (before 221 B.C.). Therefore, ancient jades with mercury alteration have attracted substantial attention from Chinese archaeologists. This paper reports the use of materials analytic techniques to study such ancient jade fragments. The studied jade samples date to the middle and late periods of the Spring and Autumn Period (~500 B.C.) and were unearthed from Lizhou’ao Tomb in Jiangxi Province, China. Structural analyses revealed the internal microstructure of the ancient jade fragments and the microdistribution of the mercury alteration. The jade fragments exhibit typical characteristics of round holes and structural hierarchy, which imply that the jades were heated before burial. The black alteration on these jade samples was found to be rich in Hg. The results of this study will be widely useful in the study of ancient jade artifacts and jade culture in Chinese archeology.

scholarly journals Rare Earth Elements in Mineral Deposits: Speciation in Hydrothermal Fluids and Partitioning in Calcite

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-19 ◽  
Author(s):  
Emily P. Perry ◽  
Alexander P. Gysi
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Hydrothermal Fluids ◽  
Mineral Deposits ◽  
Chloride Complexes ◽  
Natural Systems ◽  
Low Salinity ◽  
Numerical Predictions ◽  
Hydrothermal Processes ◽  
Bayan Obo

Studying the speciation and mineral-fluid partitioning of the rare earth elements (REE) allows us to delineate the key processes responsible for the formation of economic REE mineral deposits in natural systems. Hydrothermal REE-bearing calcite is typically hosted in carbonatites and alkaline rocks, such as the giant Bayan Obo REE deposit in China and potential REE deposits such as Bear Lodge, WY. The compositions of these hydrothermal veins yield valuable information regarding pressure (P), temperature (T), salinity, and other physicochemical conditions under which the REE can be fractionated and concentrated in crustal fluids. This study presents numerical simulation results of the speciation of REE in aqueous NaCl-H2O-CO2-bearing hydrothermal fluids and a new partitioning model between calcite and fluids at different P-T-x conditions. Results show that, in a high CO2 and low salinity system, bicarbonate/carbonate are the main transporting ligands for the REE, but predominance shifts to chloride complexes in systems with high CO2 and high salinity. Hydroxyl REE complexes may be important for the solubility and transport of the REE in alkaline fluids. These numerical predictions allow us to make quantitative interpretations of hydrothermal processes in REE mineral deposits, particularly in carbonatites, and show where future experimental work will be essential in improving our modeling capabilities for these ore-forming processes.

A Distal, High-grade Irish-type Orebody: Petrographic, Sulfur Isotope, and Sulfide Chemistry of the Island Pod Zn-Pb Orebody, Lisheen, Ireland

2021 ◽  
Author(s):  
Aileen L. Doran ◽  
Steven P. Hollis ◽  
Julian F. Menuge ◽  
Stephen J. Piercey ◽  
Adrian J. Boyce ◽  
...  
Keyword(s):  
Sulfur Isotope ◽  
Isotope Composition ◽  
Spatial Scales ◽  
Isotope Analysis ◽  
Mineral Chemistry ◽  
Hydrothermal Activity ◽  
Hydrothermal Fluids ◽  
High Grade ◽  
Complex Deformation ◽  
Chemical Zoning

Abstract Irish-type Zn-Pb deposits are important global sources of zinc, but despite a fundamental understanding of ore genesis within the Irish orefield, a detailed understanding of fluid migration and chemical evolution pathways related to sulfide and carbonate precipitation is lacking. We present the first petrographic, paragenetically constrained sulfur isotope and mineral chemistry study of mineralization at the Island Pod orebody, Lisheen deposit. The Island Pod orebody comprises high-grade mineralization that is less deformed than elsewhere in the Irish orefield. Consequently, studies of the Island Pod orebody and its mineralization provide information on the evolving nature of hydrothermal fluids involved in ore deposition. The Island Pod orebody consists almost exclusively of pyrite, sphalerite, and galena, with several stages of calcite and dolomite precipitation. Pre-ore, diagenetic pyrite is commonly overgrown by early main ore-stage pyrite, with both phases frequently replaced by main ore-stage sphalerite. In many cases, early main ore-stage pyrite is texturally zoned and exhibits chemical zoning patterns, reflecting that episodic influxes of hydrothermal fluids contained variable concentrations of As, Co, Ni, and Tl. The main ore stage was dominated by the formation of sphalerite and galena from mineralizing fluids that were depleted in these trace elements (e.g., As, Co, Tl) compared to the early main ore stage. Sulfur isotope analysis reveals four distinctive but slightly overlapping isotopic groupings, corresponding to different mineral and paragenetic stages: (1) δ34S values range from –47.7 to –30.7‰, associated with diagenetic pyrite; (2) δ34S values range from –34.3 to –14.7‰, related to early main ore-stage pyrite; (3) δ34S values range from –15.5 to + 1.7‰, corresponding to main ore-stage sphalerite; and (4) δ34S values range from –11.1 to + 17.4‰, associated with galena. Large variations in S isotope composition are common at intragrain and at other small spatial scales. The textures, paragenetic sequence, and ranges in δ34S values are consistent with hydrothermal sulfide deposition where the fluids containing bacteriogenic sulfide mixed with metal-bearing fluids. Replacement and remobilization from other Lisheen orebodies may have contributed to some of the higher sulfur isotope ratios observed in the Island Pod orebody. The excellent preservation of sulfide textures in the Island Pod orebody observed during this study demonstrates that it is an ideal location to study hydrothermal fluid evolution, including episodic fluid flow, mixing, precipitation, and compositional variations during the early main ore stage. In other Irish Zn-Pb orebodies, these early-ore textures are often obscured due to more complex dissolution and replacement processes, making interpretation of the early hydrothermal activity challenging. Consequently, the petrographic, mineral chemistry, and sulfur isotope studies of the Island Pod orebody presented here contribute to an enhanced understanding of ore-forming processes in similar deposits, where mineralization is often associated with more complex deformation or repeated pulses of hydrothermal activity.

A multivariate approach to geochemical distinction between tin-specialized and uranium-specialized granites of southern Nova Scotia

1983 ◽  
Vol 20 (3) ◽  
pp. 420-430 ◽  
Author(s):  
A. K. Chatterjee ◽  
D. F. Strong ◽  
G. K. Muecke
Keyword(s):  
Factor Analysis ◽  
Nova Scotia ◽  
Discriminant Analysis ◽  
Hydrothermal Fluids ◽  
Multivariate Approach ◽  
The South ◽  
South Mountain Batholith ◽  
Hydrothermal Processes ◽  
Magmatic Processes ◽  
One Step

Like most granitoid bodies that host significant granophile deposits, the South Mountain batholith (SMB) of Nova Scotia consists of an earlier suite intruded by a later suite of smaller plutons that are more silicic and peraluminous. The latter generally show a range of other geochemical features that readily identify them as "specialized" and allow for their separation from the earlier "non-specialized" granites. In the case of mineralization associated with the SMB we can go one step further and clearly separate the specialized granites into stanniferous and uraniferous series, which can be identified even where no Sn or U enrichment is present. Although this can be done successfully using simple histograms of a number of elements (Li, P, Sn, and F), we show that combining all elements in discriminant analysis guarantees 100% success in separating the two groups. R-mode factor analysis aids in accounting for the variance within each group, the most important effect being a "biotite factor" because of both its magmatic role and its early breakdown by hydrothermal fluids. The stanniferous series is marked by what we call the "lithophile factor" (heavily loaded by K2O, P2O5, Rb, F, Li, and W) and the uraniferous series by a "metallization factor" (loaded by Cu, Sn, Mn, and Zn). These can be interpreted in terms of dominantly magmatic processes for the former and dominantly hydrothermal processes for the latter. Furthermore, we emphasize that the separate suites do exhibit these independent characteristics even without mineralization being present.

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