Nanoparticles of noble metals in the supergene zone

Formation of noble metal nanoparticles is related to various geological processes in the supergene zone. Dispersed mineral phases appear during weathering of rocks with active participation of microorganisms, formation of soil, in aqueous medium and atmosphere. Invisible gold and other noble metals are incorporated into oxides, hydroxides, and sulfides, as well as in dispersed organic and inorganic carbonic matter. Sulfide minerals that occur in bedrocks and ores unaltered by exogenic processes and in cementation zone are among the main concentrators of noble metal nanoparticles. The ability of gold particles to disaggregate is well-known and creates problems in technological and analytical practice. When Au and PGE nanoparticles and clusters occur, these problems are augmented because of their unusual reactions and physicochemical properties. The studied gold, magnetite, titanomagnetite and pyrite microspherules from cementation zone and clay minerals of laterites in Republic of Guinea widen the knowledge of their abundance and inferred formation conditions, in particular, in the contemporary supergene zone. Morphology and composition of micrometer-sized Au mineral spherules were studied with SEM and laser microprobe. The newly formed segregations of secondary gold on the surface of its residual grains were also an object of investigation. The character of such overgrowths is the most indicative for nanoparticles. The newly formed Au particles provide evidence for redistribution of ultradispersed gold during weathering. There are serious prerequisites to state that microorganisms substantially control unusual nano-sized microspherical morphology of gold particles in the supergene zone. This is supported by experiments indicating active absorption of gold by microorganisms and direct evidence for participation of Ralstonia metallidurans bacteria in the formation of peculiar corroded bacteriomorphic surface of gold grains. In addition, the areas enriched in carbon and nitrogen have been detected with SEM on the surface of gold spherules from Guinea. Such organic compounds as serine, alanine, and glycine are identified on their surface with Raman spectroscopy. The experiments have been carried out and new data have been obtained indicating the role of micromycetes in concentration and distribution of noble metals in ferromanganese nodules of the World Ocean. Au and Pt were detected in the system with radioisotopes. It has been established that two forms of gold distribution develop within pseudomorphs of fungi colonies: (1) as pseudomorphic concentrates and (2) dispersed form unrelated to the colony structure. Inhomogeneities in distribution of dispersed platinum are manifested in the form of linear anomalies with elevated concentrations at the margins of the colonies.     

Speciation of noble metals and conditions of their concentration in massive sulfide ores of the Urals

The distribution of noble metals has been studied in ores and sulfide concentrates from the Gai, Uchaly, Uzel’ga, Aleksandrinsky, Degtyarsk, and Saf’yanovka deposits. The ores, technological products, and hand-picked monofractions were analyzed with INAA; PGE were determined with kinetic and chromatographic methods after their preliminary chemical separation. The ultraheavy fractions from Au-rich samples were used for examining minerals of noble metals. Phase relations and compositions of ore minerals were studied with an X-ray microprobe and electron microscope equipped with an energy dispersive X-ray analyzer. Gold is associated largely with Fe and Cu minerals (pyrite, chalcopyrite, fahlore) and has been detected as an admixture in Pb, Bi, and Ag tellurides. Pyrite—the major mineral of massive sulfide ores—is the main gold concentrator (up to 20 ppm, ～1 ppm on average). As follows from the results of rational analysis, the concentration of finely dispersed gold in sulfide ores from the studied deposits ranges from 0.8 to 5.0 ppm, i.e., is less than the bulk Au content in the respective samples (0.93–21.2 ppm). Formation conditions of Au-enriched massive sulfide ores were estimated from the homogenization temperature of fluid inclusions in minerals and on the basis of the electrum-argentite-pyrite-sphalerite and electrum-hessite geothermometers, taking into account the sulfur and tellurium fugacities. The appearance of visible gold and tellurides in ores is caused by recrystallization of their fine-grained intergrowths with ore-forming minerals and, likely, by release of isomorphic admixtures contained in sulfides during epigenetic hydrothermal alteration.     

On the formation conditions of porphyry Cu-Mo polymetallic deposits in the Xiaosigou district

The host mass consists of quartz diorite-porphyry (marginal facies), granodiorite porphry (intermediate facies) and granite porphry (central facies). The petrographical facies are asymmetrically spread. The formation temperature of the rock mass ranges from 890 to 800°C, the pressure from 330 to 380 bars and the depth from about 1.5 to 2 km. Mineralization is controlled by the N-E contact zone and adjacent fault structures. Mo-mineralization occurs in the granite porphyry with strong potash metasomatism. Cu-Mo-mineralization is distributed in the inner contact zone of the rock mass, and the wallrock is K-silicified granite porphyry. Cu-pyrite-mineralization is recognized in skarn and serpentinite. Pb-Zn ore veins occur in marbleized limestones. Rock and mineral analyses, fluid inclusion studies and highT-P experiments indicate that extensive precipitation of Cu-bearing pyrite took place atT = 290–250°C, andP = 330–380 bars and that of copper atT = 400–310°C andP = 330–380 bars. Precipitation of Pb and Zn was followed by the transformation of hydrothermal solutions from alkaline to intermediate with decreasing temperature.     

New mechanism of the formation of the uranium-titanium association in ores of paleovalley deposits

Doklady Earth Sciences -     

Isotopic and fluid-inclusion constraints on the formation of polymetallic vein deposits in the central Argentinian Patagonia

The lead isotope compositions of galena and the fluid-inclusion systematics of nine barite-bearing polymetallic (Au, Ag, Pb, Zn) deposits of the central Argentinian Patagonia (Chubut and Rio Negro provinces) have been investigated to constrain the compositions and sources of the mineralizing fluids. Most of the deposits occur as veins, with less common wall-rock disseminations and/or stockworks, and are low-sulfidation epithermal deposits hosted in Jurassic volcanic rocks. Fluid-inclusion homogenization temperatures (Th) from quartz and sphalerite from the deposits fall within the range of 100-300 °C, with the highest measured average temperatures for the most eastern deposits (Mina Angela - 298 °C; Cañadón Bagual - 343 °C). The salinities of the hydrothermal fluids at all deposits were low to moderate (⢪.4 equiv. wt% NaCl). Three groups of ore deposits can be defined on the basis of ²⁰⁶Pb/²⁰⁴Pb ratios for galena and these show a general decrease from west to east (from 18.506 to 18.000). The central Argentinian Patagonia deposits have distinctly less radiogenic lead isotope compositions than similar deposits from Peru and Chile, except for the porphyry copper deposits of central and southern Peru. Galena from the Mina Angela deposit is characterized by very low radiogenic lead isotope compositions (18.000<²⁰⁶Pb/²⁰⁴Pb<18.037 and 38.03<²⁰⁸Pb/²⁰⁴Pb<38.09) and reflects interaction with Precambrian basement. The geographic trend in lead isotope compositions of both galena and whole rocks indicates a crustal contribution which increases eastwards, also reflected in the strontium-neodymium isotope systematics of the host lavas. Finally, due to the lack of precise age determinations for the central Patagonian polymetallic deposits, a potential link with Andean porphyry copper systems remains an open question.     

Indium in cassiterite and ores of tin deposits

The results obtained with LA-ICP-MS by less abundant lighter ¹¹³In isotope and EPMA show that in cassiterite of cassiterite–quartz veins the indium contents do not exceed 160 ppm, while cassiterite from Sn–sulfide veins is characterized by higher indium contents from 40 to 485 ppm; sulfides of Sn–sulfide veins unlike sulfides of cassiterite–quartz veins also have the highest indium contents: Fe-sphalerite (100–25,000 ppm), chalcopyrite (up to 1000 ppm), and stannite (up to 60,000 ppm). Indium contents in the Sn–sulfide ore of the Tigrinoe and Pravourmiiskoe deposits obtained using SR-XRF, ICP-MS and atomic absorption methods range from 10 to 433 ppm with average values of 56–65 ppm. Indium-rich Sn–sulfide mineralization in five large Sn–Ag ore districts of the Far East Russia (Khingansky, Badzhalsky, Komsomolsky, Arminsky, Kavalerovsky) provides the impetus for further exploration of deposits with Sn–sulfide mineralization as the most promising indium resources in Russia. Empirical observations from geology and geochronology of cassiterite–quartz and Sn–sulfide mineralization show that the combined contribution from granite and alkaline–subalkaline mafic sources and multistage ore-forming processes doubled indium resources of deposits being the main factors in the formation of high grade indium mineralization.     

Uranium anomalies in paleo-aquifers near sandstone-type uranium deposits in the devonian catskill formation of Pennsylvania

Background values of U and Th in 371 clastic sedimentary rocks from the Catskill Formation correlate negatively with Si and positively with Al and most other major elements because of the low content of U and Th in quartz and the relative enrichment in clays, Fe oxides, and other fine-grained components. Background U also correlates closely with Th. Similar results are obtained from 100 samples from western Colorado near the Uravan mineral belt. Weak anomalies in U are much more easily recognized by study of residuals or deviations from regressions of U against Al or Th than in the raw data.At Penn Haven Junction near Jim Thorpe, Pa., roll-type uranium deposits similar to those in Wyoming are localized around an iron-stained paleo-aquifer of conglomeratic sandstone. Twenty-four rock samples from this paleo-aquifer contain 1–12 ppm U; eight of the samples contain less than 4 ppm U, the background level for shales of the area. Uranium residuals from regressions against Al or Th are clearly anomalous for all samples. Experiments on another group of samples suggest that readily extractable U (H₂O₂-acetic acid leach) would also show the paleo-aquifer to be anomalous.Anomalies of this type in paleo-aquifers should be useful in evaluation of drill holes, outcrops, and radiometric data in prospective districts, especially when paleo-aquifers are difficult to recognize visually. The anomalies also indicate that U is added to sandstones of the paleo-aquifer rather than leached out to form the ore bodies in reduced rocks at the margins of a geochemical cell.     

Behavior of mercury in the supergene zone of geothermal deposits,southern Kamchatka

Mercury distribution was determined in all types of solid materials from the supergene zone of geothermal deposits in southern Kamchatka: rocks, hydrothermally altered rocks (metasomatic rocks), soils, soil—pyroclastic cover, bottom sediments of perennial and intermittent streams, hydrothermal clays, artificial siliceous precipitates, and iron sulfides formed owing to thermal water discharge from a well. The mercury content varies from background values for the Kurile-Kamchatka region in fresh rocks to high and extremely high values in hydrothermal clays and monomineralic pyrite samples. The sources, migration conditions, and concentration mechanisms of mercury were evaluated. Mercury is supplied to the surface of geothermal deposits and thermal fields by a deep hydrothermal flow and is concentrated on thermodynamic barriers in hydrothermal clays, siliceous sinters (silica gel), and soils showing high salinity owing to the deposition in them of silica, sulfates, and other compounds from a vapor-water mixture. Newly formed clay minerals, iron sulfides (pyrite), silica gel, and biological materials (peat) can probably efficiently sorb mercury under geothermal conditions at atmospheric pressure and temperatures from 20°C to 120°C.     

Primary and secondary structures in the polymetallic ores of Rajpura-Dariba,Rajasthan, India

The Precambrian Rajpura-Dariba polymetallic deposits of Rajasthan, associated with black schists, quartzite and calcsilicate rocks, exhibit several primary structures, suggesting a syn-sedimentary origin. The secondary features of the sulphides are attributed to the effects of metamorphism.     

Supergene sulphides and related minerals in the supergene profiles of VHMS deposits from the South Urals

The mineralogy and structure of the supergene profile in recently-exploited volcaniс hosted massive sulphide (VHMS) deposits of Cyprus, Uralian and Kuroko type in the South Urals, Russia, have been studied. Specific subzones enriched in secondary sulphides and associated minerals have been distinguished in residual pyrite and quartz–pyrite sands at the Gayskoye, Zapadno-Ozernoye, Dzhusinskoye and Alexandrinskoye deposits. Besides minerals which are common to the cementation subzones (covellite, chalcocite and acanthite), non-stoichiometric colloform and framboidal pyrite, pyrite–dzharkenite, pyrrhotite-like and jordanite-like minerals, metacinnabar, sphalerite, selenium-enriched tetrahedrite and unidentified As-, Sb sulphosalts of Pb or Hg and Ag, sulphur-bearing clausthalite, naumannite and tiemannite were also found. Secondary sulphide minerals in VHMS deposits of the South Urals region are characterized by light sulphur isotope compositions (− 8.1 to − 17.2‰). Superposition of the advanced oxidation of colloform pyrite, an enrichment in impurities (sphalerite, galena, and tennantite) from the primary ores, stagnant water conditions, an elevation of the water table during oxidation, and bacterial activity led to supergene concentrations of the base metals as sulphide, selenides or sulphosalts.     

The copper polymetallic deposits and resource potential in the Tibet Plateau

Many large and super-large copper deposits have been discovered and explored in the Tibet Plateau, which makes it the most important copper resource reserve and development base in China. Based on the work of the research team, the paper summarizes the geological characteristics of the main copper deposits in Tibet and puts forward a further prospecting direction. A series of large accumulated metal deposits or ore districts from subduction of Tethys oceanic crust to India-Asia collisionhave been discovered, such as Duolong Cu (Au) ore district and Jiama copper polymetallic deposit. The ore deposits in the Duolong ore district are located in the lowstand domain, the top of lowstand domain, and the highstand domain of the same magmatic-hydrothermal metallogenic system, and their relative positions are the indicators for related deposits in the Bangong Co-Nujiang metallogenic belt. The polycentric metallogenic model of the Jiama copper polymetallic deposit is an important inspiration for the exploration of the porphyry mineralization related to collision orogeny. Further mineral exploration in the Tibet Plateau should be focused on the continental volcanic rocks related to porphyry-epithermal deposits, orogenic gold deposits, hydrothermal Pb-Zn deposits related to nappe structures, skarn Cu (Au) and polymetallic deposits, and the Miocene W-Sn polymetallic deposits.©2021 China Geology Editorial Office.     

还原性围岩在斑岩钼矿成矿中的作用

斑岩钼(铜)矿是重要的钼矿资源类型,高氧逸度岩浆是公认的评价斑岩成矿的有效指标。但前人关注的焦点是成矿母岩浆的起源与演化,还原性围岩在斑岩成矿中的作用长期被忽视,是什么触发了高氧化性含矿岩浆热液的还原与成矿还存在不同的认识。文章在前人工作基础上,以中国秦岭-大别、华北克拉通北缘的南泥湖-三道庄-上房沟、沙坪沟、曹四夭等斑岩型钼矿和美国Climax-Henderson巨型斑岩钼矿带中Mt. Emmons等斑岩钼矿为例,重点研究了斑岩钼矿的区域分布与还原性围岩之间的空间关系及成因联系,探讨了斑岩成矿系统氧化还原状态在成矿过程中的变化及触发机制。笔者发现斑岩钼矿的区域分布明显受黑色含碳质地层和中基性火山岩控制,在成矿过程中黑色含碳质围岩普遍发生褪色蚀变;主成矿期矿物流体包裹体中普遍含有甲烷,蚀变围岩和矿床中热液方解石的δ¹³C值普遍较低。因此,笔者提出含碳质地层和中基性火山岩等围岩中还原性组分的加入是引发斑岩钼(铜)矿成矿系统氧化-还原状态转变和成矿金属沉淀的关键。碳质围岩中有机质热解/碳-水反应产生的甲烷是重要还原剂,CH₄沿构造裂隙扩散进入斑岩...     

Timing of supergene enrichment of low-grade sedimentary manganese ores in the Kalahari Manganese Field,South Africa

Low-grade carbonate-rich manganese ore of sedimentary origin in the giant Kalahari Manganese Field, South Africa, is upgraded to high-grade todorokite–manganomelane manganese ore by supergene alteration below the unconformity at the base of the Cenozoic Kalahari Formation. Incremental laser-heating ⁴⁰Ar/³⁹Ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the Kalahari unconformity peaked at around 27.8 Ma, 10.1 Ma and 5.2 Ma ago. Older ages are dominant in the upper part of the weathering profile, while younger ages are characteristic of the deeper part of the profile. Younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years. The oldest age obtained in the weathering profile, namely 42 Ma, is considered a minimum estimate for the onset of the post African I cycle of weathering and erosion that followed the break up of Gondwanaland and formation of the Cretaceous to early Cenozoic African land surface. The youngest ages, recorded at around 5 Ma, in turn, correspond well to the Pliocene transition from humid to arid climatic conditions in Southern Africa.     

五台—恒山地区多金属矿远景区预测及找矿方向

利用移动平均法对五台—恒山地区7幅1∶20万水系沉积物数据进行重新处理,并结合本区1∶20万重力数据和1∶5万航磁数据,发现所获得的综合异常与燕山期岩体相关矿床具有很好的吻合性。研究选取Au、Ag、Cu、Pb、Zn等综合异常以及高磁、低重异常作为预测准则,在本区共预测出13处与岩体成矿作用有关的多金属矿远景区。已有资料显示,本区共存在9条NW向断裂构造带,岩体侵位受NW向断裂和NE向断裂交叉部位控制,已知出露岩体与本次预测的11处远景区绝大部分分布在断裂构造带上,构成多条构造岩浆岩带,多金属矿床与燕山期岩体有着密不可分的联系。     

大厂锡多金属矿床热液喷气沉积的证据——含矿建造及热液沉积岩

通过对大厂锡多金属矿床含矿建造特征及容矿岩石的岩石学、地球化学研究,探讨了矿床形成的地质环境,证明了主要容矿岩石是从海底热卤水系统中以化学沉淀方式形成的热液沉积岩。