Vein Mineralization Research Articles

Fluid boiling during high-grade gold deposition in an epithermal gold-telluride deposit, Guilaizhuang, China

Boiling is a well-established mechanism for precious metal mineralization in low-sulfidation epithermal veins, and is considered a key process in the formation of high-grade bonanza zones in those systems. In contrast, the potential role of fluid boiling in the formation of epithermal gold-telluride veins associated with alkalic porphyries has rarely been discussed. The Guilaizhuang gold-telluride deposit, Shandong Province, China, is an ideal location to investigate the possible role of fluid boiling because the deposit comprises several distinct styles of orebodies, ranging from finely-disseminated gold mineralization hosted in carbonate rocks at depth, through to very high-grade mineralized breccia zones at shallower levels. Here, we examine the relationships between ore-forming fluids and mineralization styles in this deposit, based on paragenesis, vein textures and fluid inclusions. In the deeper carbonate-replacement style orebodies, we found no evidence of fluid boiling and, as such, alternative mechanisms such as fluid mixing or fluid-rock reactions were likely occurring. In contrast, the shallower, high-grade breccias show clear evidence of fluid boiling, which we infer contributed to the formation of exceptional bonanza zones. Our results thus indicate that fluid boiling can play a key role in precious metal mineralization in gold-telluride deposits, analogous to the low-sulfidation epithermal deposits. Furthermore, our results illuminate how contrasting mechanisms of gold mineralization in these deposits (i.e., boiling versus non-boiling) can give rise to the diversity and distribution of gold grades and reserves within an epithermal gold-telluride system.

Assessment of radarsat-1, ALOS PALSAR and sentinel-1 SAR satellite images for geological lineament mapping

Lineament mapping is a very important step in geological studies and mineral exploration, with the evolution of remote sensing, processing methods as well as the availability of several optical and radar satellite data, lineaments can be detected without using traditional methods. The objective of this work is to recommend the most suitable data for lineaments to be used in the field of geological science, by comparing the performance of three different RADAR data, namely ALOS PALSAR (The Advanced Land Observing Satellite Phased Array type L-band Synthetic Aperture Radar), Radarsat-1 and Sentinel-1 for automatic lineament extraction using a combination of edge detection and line linking algorithms. The methodology consists to link between the length, number, orientation and density of lineaments with surface characteristics such as the slope, faults, lithology, discontinuities and mineral veins. The results obtained show that the extracted lineaments from Sentinel-1 VH polarization have a better correlation with geological units, tectonic system direction, as well as shading and slope maps, which is due to the effectiveness of VH polarization, which is independent of soil properties compared to the other polarizations that have shown an overestimation of the lineaments.

Implications of Ore Textures for Gold Recovery at Esaase Deposit, Asanko Gold Mine

The Esaase Deposit of the Asanko Gold Mine Ltd (AGM) contains a system of epigenetic gold-related quartz veins hosted by tightly folded and foliated metasedimentary rocks. The gold mineralisation is associated with disseminated sulphide minerals in the quartz-carbonate veins and within the alteration zones of the host metasedimentary rocks. AGM employs a combination of Gravity Concentration and Carbon-In-Leach (CIL) methods to recover its gold and this has resulted in a recovery rate that ranges from 81.3% to 94.1%. The high recovery rate (>90%) suggests that the gold is free and therefore not locked up in the sulphides that are associated with the mineralisation. However, the recovery rate drops from 94.1 to 81.3% even when the samples contain high amounts of sulphides. Since the presence of sulphides in altered rock is one major indicator of gold mineralisation, this research was conducted to determine the ore textural relationship between gold and sulphides and the implication of the ore textures on the current gold recovery methods employed on the deposit. Polished block sections were prepared from samples from the ore zones: Transition and Cobra. Ore petrographic studies were conducted on the samples using the Leica DM2700P petrographic microscope at the University of Mines and Technology, Tarkwa, Ghana. These studies revealed that the low gold recoveries may be attributed to the gold occurrence as inclusions in the sulphides, which is typical for refractory ores, thus, making it difficult to be extracted by the current method employed at Asanko. Therefore, AGM may have to consider the economics of employing a combination of other methods to liberate the refractory gold and possibly, result in optimum recovery.

Analysis of geochemical patterns in a soil profile over mineralized bedrock

Transported soils cause difficulties in the identification of geochemical anomalies. It has been demonstrated that the joint application of local singularity analysis (LSA) and principal component analysis (PCA) can identify geochemical anomalies effectively, especially in regolith-covered areas. However, more convincing evidence is needed to explain the reasons for this. In this study, a soil profile overlying several mineralized veins cutting through bedrock was analysed in situ using a portable X-ray fluorescence spectrometer. The patterns of two mineralization-related elements, Cu and Mo, were analysed. The results revealed that the element concentrations of the soil sharply decreased as the distance from the bedrock increased, and this relationship can be described by a power-law model. LSA enhanced several vein-like anomalies corresponding to the mineralization veins in the bedrock, and the presence of vertically elongated weak anomalies in the soil indicates the migration of ore elements originating from the underlying bedrock through the soil. The statistics show that the patterns of the local singularity index (LSI) are stable at different depths and in different media, whereas the concentration patterns are not. In addition, the mineralization-related elements have a higher correlation coefficient for the LSI than for the concentration. Since a previous simulation study determined that a mineralization-indicative first principal component prefers that the variables have a close relationship and that the variables have similar patterns in different geological objects, the patterns discovered in this study explain why LSA is effective in identifying geochemical anomalies, especially when combined with PCA. Supplementary material: the high-resolution photo, the element concentration data and the lithologic data of the profile are available at https://doi.org/10.6084/m9.figshare.c.5957122 Thematic collection: This article is part of the Applications of Innovations in Geochemical Data Analysis collection available at: https://www.lyellcollection.org/cc/applications-of-innovations-in-geochemical-data-analysis

HIGH-PRESSURE METAMORPHIC ROCKS OF THE CHARA OPHIOLITE BELT (CAOB): AGE AND EXHUMATION CONDITIONS

A critical element in the construction of tectonic models of exhumation of basalts that have undergone high-pressure metamorphism is the diagnosis of synchronicity and the genetic relationship between deformations and high-pressure metamorphic changes. In this report, these issues are discussed on the example of the metabasalts of the Chara ophiolite belt. In the bottom of the basalt block with the separation of pillow lavas near the Burshabulak farm, crack-vein systems with high-pressure mineral associations were discovered and analyzed, which directly indicates their tectonic origin. It is shown that the conditions for the formation of such vein mineral associations correspond to eclogite-like rocks (P=18.5 kbar temperatures of no more than 520 °C). The age of the high-pressure metamorphism basalts of the synchronous type with deformations according to 40Ar/39Ar isotope dating of amphiboles cannot be younger than the boundary of 452±14 million years.

Deformed Vein Sets as a Record of Synmetamorphic Volume Change: Quantifying Solution Mass Transfer in Subduction‐Type Metasediments of the Del Puerto Canyon Region, Franciscan Belt, USA

AbstractThe volume change component of deformation is often ignored or assumed to be zero in tectonic studies of metamorphic belts. However, when estimating the original geometries of deformed regions, the volume change is just as important as the other two components of deformation, finite strain, and rotation. Major permanent volume change in metamorphic rocks is accomplished by solution transfer facilitated by H2O fluids and estimates of volume change can be combined with solubilities to provide required time‐integrated volumes of fluid flow. Previously applied methods for estimating rock volume change are based on absolute stretch or changes in whole‐rock chemical compositions. Estimates based on absolute stretches generally indicates large volume reductions whereas geochemical measurements generally imply limited volume change. These contrasting results exist even when the two methods are applied to the same region. In this study, we develop a largely unexplored method for estimating volume change using the direction and deformation type of deformed mineral veins. The assumptions in this method are few and appropriate uncertainties can be estimated. Application of the new method to the metagreywacke in the Del Puerto Canyon of the Franciscan belt constrains the syn‐metamorphic volume change to be greater than 7 vol.%, contrasting with previous proposals for large volume‐loss in the same region. The results of previous studies can be modified by taking into account grain rigid body rotation and grain boundary sliding. The final result of our approach yields a volume change of 7–21vol.% and implies large amounts of water‐rich fluid must have passed through the rock.

Sulphur and metal sources of polymetallic vein-type, sedimentary exhalative-type and Mississippi Valley-type Zn–Pb deposits along the southeast margin of the Yangtze Block

Three types of zinc-lead deposits, including polymetallic vein-type, sedimentary-exhalative (SEDEX), and Mississippi valley-type (MVT) Zn–Pb deposits, have been discovered along the southeast margin of the Yangtze Block. A systematically comparative study on the sulphur and metal sources for these deposits, is absent, which significantly limits the understanding of the metallogenic model and geological exploration. Thus, in order to reveal the S and Pb sources of Pb–Zn deposits, an investigation of trace elements of sphalerite and of S–Pb isotopic compositions of sulphides in these three types of Zn–Pb deposits was conducted in this study. We found that, in the vein-type, SEDEX, and MVT Zn–Pb deposits, (1) the sulphur was derived respectively from granitic magmatism related to the Guangxi Orogeny, early Cambrian stratified seawater and connate water; (2) the lead isotope variation indicates that the ore-forming metals were mainly derived from the granitic hydrothermal fluids for vein-type deposits, from basement rocks for SEDEX deposits, and mainly from early Cambrian sedimentary rocks for MVT mineralization. The Nanhua rifting lead to the syngenetic Zn-Pb mineralization, while the Guangxi Orogeny resulted in magmatic-hydrothermal vein mineralization at depth and in MVT mineralization at the shallow crustal levels.

Late Triassic Au-Mo mineralization in the Xiaoqinling region and a genetic connection to carbonatitic magmatism

The southern margin of the North China craton hosts a number of quartz vein-style deposits of the unusual Au-Mo ore spectrum, including the Dahu deposit in the Xiaoqinling region. Pristine euhedral monazite and rutile crystals from the Dahu Au-Mo mineralized veins yield LA-ICP-MS weighted mean 206Pb/238U ages of 212.1 ± 1.7 Ma (1 s, MSWD = 1.13) and 209.7 ± 2.7 Ma (2 s, MSWD = 1.7), respectively. These ages point to a Late Triassic gold mineralization event in the Xiaoqinling region, aside from the regionally well-recognized Early Cretaceous gold mineralization event. Tectonic setting, mineral association, elemental and isotopic signatures, and fluid characteristics of the Dahu deposit suggest that the Late Triassic Au-Mo mineralization represents a carbonatite-associated hydrothermal system, with ore fluids exsolved from a late-stage carbonatite system. Mineralogically, the Dahu deposit is enriched in hydrothermal minerals common to carbonatite systems, such as carbonates, sulfates, and REE minerals. Trace element and isotope compositions of vein minerals also show similarities to those of carbonatites. Monazite displays a pronounced LREE-enriched and smooth pattern (no Eu anomaly), with εNd(t) values ranging from −14.9 to −13.3. Rutile is characterized by high Nb concentrations (6 to 18 wt%) and extremely high Nb/Ta ratios of 236 to 2044. Fluid inclusions in the Dahu Au-Mo mineralized veins and the regionally distributed coeval carbonatites are all dominated by low to moderate salinity H2O-CO2 fluids that contain sulfates as trapped solids/daughter minerals. Our study suggests that the southern margin of the North China craton has potential for the exploration of polymetallic hydrothermal mineralization related to hidden carbonatites.

Geochemistry of Syntectonic Carbonate Veins Within Late Cretaceous Turbidites, Hikurangi Margin (New Zealand): Implications for a Mid‐Oligocene Age of Subduction Initiation

AbstractWe document the geochemistry of calcite veins in the Late Cretaceous Tikihore Formation (Raukumara Peninsula, New Zealand) to characterize their fluid composition and source and to help establish the age of subduction initiation at the Hikurangi margin of the Australia‐Pacific plate boundary. The calcite veins occur within normal faults offsetting turbidites that accumulated in a lower slope basin. Vein calcite trace metal content and rare earth element patterns are consistent with a seawater‐derived brine composition. Oxygen isotope (δ18O) values range from −6.1 to +8.4‰ and are −0.2‰ VPDB on average; positive δ13C values of up to +28‰ VDPB reflect methanogenesis. Oxygen isotope temperature data indicate that calcite vein mineralization occurred at temperatures in the range of 29°C–48°C. This is markedly less than the maximum burial temperature experienced by the host rocks, which we estimate to be 104 ± 10°C at 30–27 Ma from the inverse modeling of apatite fission track data. The vein calcite has a 28.5 ± 4.9 Ma U‐Pb age. From these data, we infer that the succession above Tikihore Formation was removed by slumping, thereby resulting in fluid overpressure in the reservoir, followed by hydraulic fracturing and the precipitation of the vein calcite. Ultimately, the data presented here from the Tikihore veins are consistent with subduction initiation at 30–27 Ma, based on the U‐Pb age of the vein calcite and modeling of apatite fission track data for the host sandstone, corroborated by the 30–27 Ma timing of back thrusting on the Taranaki Fault and related foredeep development in eastern Taranaki Basin.

Implicit modeling of narrow vein type ore bodies based on Boolean combination constraints

In this paper, we implement an automatic modeling method for narrow vein type ore bodies based on Boolean combination constraints. Different from the direct interpolation approach, we construct the implicit functions of the hanging wall and foot wall surfaces, respectively. And then the combined implicit function is formed to represent the complete ore body model using the Boolean combination constraints. Finally, the complete ore body is obtained by Boolean operation of the hanging wall and foot wall surfaces. To model complex vein surfaces, some modeling rules are developed to allow the geological engineers to specify vein thickness constraints and vein boundary constraints. The method works for narrow vein type ore bodies (e.g., vein gold deposits and mineral sand deposits) which are large in two dimensions and narrow in the third. Taking the implicit function of radial basis functions interpolation as an example, several experiments are carried out by using the real geological sampling data of the mines. The experimental results show that the method is suitable for the modeling of narrow vein type ore bodies.

Geochemical Evidence and Geological Prerequisites for Isochemical Metamorphism in the Kochumdek Contact Aureole (East Siberia)

Abstract —Marly limestone, marble, and gabbro–dolerite samples from the Kochumdek contact aureole are studied in order to estimate to which degree the spurrite–merwinite (T = 750–900 °C) metamorphism at the site was isochemical. The 3 m wide zoned Kochumdek aureole is located at the contact between the Early Triassic trap intrusion of the Kuzmovka complex (νβT1kz) and the overlying Llandovery marine marly limestones (S1ln) of the lower Kochumdek subformation in the right side of the Kochumdek River (tributary of the Podkamennaya Tunguska, East Siberia). The Kochumdek rocks are remarkable by the lack of prominent skarn or vein mineralization in marbles and autometasomatism in gabbro. The samples have been analyzed for major, chalcophile, rare-earth, and high-field strength elements. The Kochumdek marbles and marly limestones share similarity in major-element patterns that record mixing of fine siliciclastic material with biogenic and chemogenic CaCO3 components. The major-element chemistry of marly limestones has been controlled by the compositions of fine clastic material transported from the provenance (Trans-Angara part of the Yenisei Ridge) and by the deposition environment of the early Silurian marine basin. The contents of Cu, Ni, Co, V, Mo, Se, Cr, and U in marly limestones and marbles correspond to marine sediments deposited in a low-productive oxigenated environment. The Ni/Co, V/Cr, Cu/Zn, V/(V+Ni), and U/Th ratios in spurrite marbles at the intrusive contact remain within typical limestone ranges but differ from those in gabbro, which have higher V and Cu enrichment and moderate contents of Zn, Ni, and Co. Marly limestones and spurrite marbles show identical REE+Y patterns and average ΣREE contents. The major- and trace-element similarity between marbles and the protolith limestones, along with the absence of gabbro geochemical signatures in the metamorphic rocks, indicate that the thermal metamorphism was nearly isochemical. The reactions occurred without any large-scale transport of major and trace elements across the sill/sediment boundary. The conditions were unfavorable for metasomatic alteration due to low fluid saturation of the Kochumdek intrusion and its structural setting, as well as to the lithological, mechanical, and thermal properties of the sediments.

Tracing the formation and modification of the Keketale VMS-type Pb-Zn deposit, Altai Mountains: Insights from ore deposit geology, geochronology, and magnetite geochemistry

The Keketale Pb-Zn deposit (KKTL deposit) is one of the largest VMS-type Pb-Zn deposits in the Altai Mountains, Central Asian Orogenic Belt. Field investigations and microscopic observations suggest that the evolution of the deposit can be divided into two major stages: syngenetic massive sulfide mineralization related to coeval volcanism and epigenetic polymetallic vein mineralization linked to metamorphism and deformation. Supported by the U-Pb dating of zircon and garnet of immediate host rocks (i.e. garnet biotite schist), the former occurred in ∼400 Ma during the Early Devonian, and the latter developed in ∼265 Ma during the Middle Permian. The original VMS hydrothermal system is varying in the physicochemical conditions (e.g., temperature, oxygen fugacity, and sulfur fugacity) in space, together with fluid-rock interactions, causing two types of magnetites with different compositions and textures in different sites. Furthermore, it is revealed that the spatial variations of temperature-sensitive elements (e.g., Ti and Sn) of magnetite can be used to find the location of the hydrothermal vent, which can serve to guide the regional prospecting. Modifications of the KKTL deposit are indicated by deformed sulfides, annealed sulfides, remobilized sulfides, and epigenetic polymetallic veins. We draw a conclusion that the KKTL deposit witnessed the evolution of the Chinese Altai and the Paleo-Asian Ocean, the syngenetic volcanic-associated VMS mineralization was linked with arc/arc-back tectonic setting caused by the subduction of the Paleo-Asian Ocean toward the Chinese Altai, and the modification of the KKTL deposit was related to the collision between the Chinese Altai and the Junggar Block, along with the closing of the back-arc basin.

Ore-formation at the Halasheng Ag-Pb-Zn deposit, northeast Inner Mongolia as revealed by trace-element and sulfur isotope compositions of ore-related sulfides

The Early Cretaceous Halasheng Ag-Pb-Zn deposit in NE China, which is mainly hosted in Jurassic rhyolitic tuff, andesite, and volcanoclastic rocks, comprises both proximal skarns and more distal hydrothermal discordant ore veins and stockworks with associated Fe-Mn-carbonate, illite, and silica alteration in NNW-trending extensional faults. Suggestions of a magmatic-hydrothermal relationship are tested by LA-ICP-MS trace-element and sulfur isotope studies of ore minerals, particularly sphalerite which is a major ore mineral.Sphalerite has at least three types: SpS, anhedral, mainly dark red sphalerite in skarns; Sp1, euhedral, normally pale yellow coarse-grained, and rarely oscillatory zoned sphalerite formed in Stage 2 of hydrothermal vein mineralization; and Sp2, fine-grained, red, non-zoned sphalerite formed in Stage 3 of hydrothermal mineralization. Sphalerite with different textures has As, Ag, Cd, Sb, and Pb contents that increase from proximal skarn to distal hydrothermal mineralization consistent with the increase in sulfosalt minerals in the late hydrothermal stage. In situ time-resolved depth profiles, together with tangential profiles, and LA-ICP-MS element mapping of sphalerite indicate that trace elements are largely in solid solution in Halasheng sphalerite with substitution mechanisms including: Zn2+ ↔ Fe2+, Cd2+, Mn2+, and 3Zn2+↔ (As3+, Sb3+) + Ag++Pb2+. Enrichment of In and Sn, and depletion of Ga and Ge, together with In/Sn ratios indicate the high-temperature ore-forming fluids at Halasheng that are most likely closely related to intrusions. In situ mean δ34S values of SpS, Sp1, and Sp2 sphalerites are 5.3‰, 7.6‰, and 8.3‰, respectively, indicating that sulfur was derived both from magmatic-hydrothermal fluids and from wall rocks with higher (>10‰) values.These data help confirm that the Halasheng Ag-Pb-Zn deposit is an intrusion-related deposit with local proximal skarn mineralization at the contact of Tamulangou Formation andesite with Ergunahe Formation crystalline limestone, and more distal vein-type volcanic-hosted hydrothermal mineralization, with both ore and gangue mineral assemblages and available fluid inclusion data suggesting an intermediate-sulfidation epithermal association. As ore-forming hydrothermal fluids cooled and evolved they became enriched in silver, with silver-bearing sulfosalt minerals and silver-rich sphalerite precipitated in the latter stages of deposition.In terms of future exploration, there is potential for the concealed Lower Cretaceous Pb-Zn skarns and intermediate sulfidation Ag-Pb-Zn epithermal deposits at Halasheng to be part of a larger porphyry-related mineral system with porphyry Cu-Au-Mo deposits, similar to those of Middle Triassic to Early Jurassic age in the same metallogenic belt, at deeper crustal levels than the known deposits.

MİNERALOGİCAL COMPOSİTİON, TEXTURE-STRUCTURAL PROPERTİES AND MİNERAL FORMATİON SEQUENCE OF THE ORE OF GOSHA GOLD- PYRİTE FİELD

As a result of microscopic researches it has been determined that mineralogical composition of the ore field consists of hypogenic,vein and hypergen minerals. A number of factors in formation of ore texture types and structure, including morphological properties of ore minerals, characters of ore solutions, affect of tectonic processes after ore formation are not excluded. Due to the influence of the above mentioned factors stratum, massive, nested textures replace each other in sequence in Gosha ore field. Stratum texture consists of fine, medium and big grain pyrite, quartz, carbonate sequence. Mass texture consists mainly of hardenied pyrite which gradually passes into the granular ore. Nested texture is formed in initval stage of ore formation as a result of pyrite cry stallation and is commonly found in non-ore area. Breccia - like texture is usually found on the edges of ore. Such a texture type mainly consists of pyrite and it is formed by the incorporation of ore solutions into derivative quartz. In the article, along with the texture properties of ore, its structure has already been studied. The structures found in the ore of the field are as follows: rhythmic zoning, porphyry-shaped, residual, cataclysmic. Ore formation process in Gosha field begins from quartz-pyrite. Then it proceeded with quatrz-gold-telluride, quartz-semisulfide, quatrz-carbonate and quartz-goldhelite-hydrochetite stages. This, mineralogical composition, texture-structure properties, mineral formation sequence of Gosha¬pyrite field ore give reason to say that it did not undergo post-metamorphism after the ore formation, and that the field was formed at a low depth, where metacolloid processes were widespread. Keywords: mineralformation, texture, structure, metacolloid, pyrite.

Breccia and vein mineralization of the Balatoc Diatreme, Acupan gold deposit, Baguio Mineral District: An example of a diatreme-hosted epithermal deposit in the Philippines

The Acupan epithermal gold deposit is one of the Philippines’ largest gold camps, having produced over 200 t Au in the last century with average grades of ∼ 6 g/t Au mostly from well-studied vein orebodies hosted by the Virac Granodiorite. However, vein and breccia mineralization (90/99 veins and GW orebodies, respectively) currently being mined in the northeastern portion of the Acupan deposit is hosted by the less-studied Balatoc Diatreme. This study demonstrates the role of the diatreme as a precursor to epithermal mineralization at Acupan, providing structural control in focusing the fluid flow along the diatreme margin. Mineralization in the Balatoc Diatreme formed across five mineralization stages. Stage I, the main gold mineralization stage, is characterized by gray quartz with pyrite + marcasite + arsenopyrite + electrum + sphalerite ± chalcopyrite. Stage II is typified by white quartz associated with pyrite + electrum + chalcopyrite. Stage III, a newly recognized stage for the Acupan gold deposit, is composed of clear quartz hosting pyrite + stibnite + chalcopyrite + galena + sphalerite + electrum (±pyrite + marcasite + arsenopyrite). Stage IV and Stage V are associated with calcite and gypsum, respectively, hosting trace amounts of pyrite and sphalerite.Fluid inclusions hosted by Stage I and Stage II vein quartz revealed homogenization temperatures (Th) ranging from 220 to 230 °C and 280 to 290 °C, respectively. The fluid inclusions in Stage III quartz breccia cement recorded bimodal homogenization temperatures, 230 to 240 °C and 280 to 290 °C. Fluid inclusion and textural evidence from Stage I quartz and Stage II quartz suggest boiling conditions during ore formation of veins transecting the Balatoc Diatreme. Fluid mixing, on the other hand, is proposed for the formation of the base-metal rich Stage III mineralization of the GW orebodies quartz cement. Pyrite and sphalerite δ34S values (0.8 to 1.5 ‰) from Stage I to Stage IV veins and breccias indicate reduced ore-forming conditions in an H2S-dominated system. Meanwhile, the negative δ34S values (−1.6 to −1.5‰) measured from the pyrite of Stage V breccia, first reported in this study, imply partitioning of the heavier isotopes to the gypsum sulfate in an oxidizing environment and suggest possible spatial variations of sulfur isotope signatures across the Acupan epithermal vein system. This study emphasizes the significance of fluid mixing in diatreme-hosted epithermal deposits such as Acupan. The diatreme possibly provided pathways between the contrasting magmatic-hydrothermal and meteoric environments, resulting in ore precipitation.

A geochemical study of the Sweet Home mine, Colorado Mineral Belt, USA: formation of deep hydrothermal vein–type molybdenum greisen and base metal mineralization

AbstractDeep hydrothermal Mo, W, and base metal mineralization at the Sweet Home mine (Detroit City portal) formed in response to magmatic activity during the Oligocene. Microthermometric data of fluid inclusions trapped in greisen quartz and fluorite suggest that the early-stage mineralization at the Sweet Home mine precipitated from low- to medium-salinity (1.5–11.5 wt% equiv. NaCl), CO2-bearing fluids at temperatures between 360 and 415 °C and at depths of at least 3.5 km. Stable isotope and noble gas isotope data indicate that greisen formation and base metal mineralization at the Sweet Home mine was related to fluids of different origins. Early magmatic fluids were the principal source for mantle-derived volatiles (CO2, H2S/SO2, noble gases), which subsequently mixed with significant amounts of heated meteoric water. Mixing of magmatic fluids with meteoric water is constrained by δ2Hw–δ18Ow relationships of fluid inclusions. The deep hydrothermal mineralization at the Sweet Home mine shows features similar to deep hydrothermal vein mineralization at Climax-type Mo deposits or on their periphery. This suggests that fluid migration and the deposition of ore and gangue minerals in the Sweet Home mine was triggered by a deep-seated magmatic intrusion. The findings of this study are in good agreement with the results of previous fluid inclusion studies of the mineralization of the Sweet Home mine and from Climax-type Mo porphyry deposits in the Colorado Mineral Belt.

Timing and ore formation of the Xiaokele porphyry Cu (–Mo) deposit in the northern Great Xing’an Range, NE China: Constraints from geochronology, fluid inclusions, and H–O–S–Pb isotopes

The Xiaokele Cu (–Mo) deposit is a newly discovered porphyry deposit in the northern Great Xing’an Range (GXR) of northeast China. The multiphase hydrothermal superposition on the Xiaokele Cu (–Mo) deposit obscures temporal relationships between quartz veins/veinlets in different mineralization stages, and consequently limits the understanding of the origin and evolution of the ore-forming fluids and metal precipitation mechanism. In this study, we document detailed descriptions of alteration and vein mineral paragenesis in the Xiaokele Cu (–Mo) deposit, present LA–ICP–MS zircon U–Pb and molybdenite Re–Os geochronology, the microthermometry and laser Raman spectroscopy of the fluid inclusions (FIs), and H–O–S–Pb isotope compositions. Four mineralization stages are identified, with stage II being the main Cu mineralization stage. The molybdenite Re–Os dating yielded a weighted average age of 148.7 ± 0.9 Ma (MSWD = 0.52), which is similar to the LA–ICP–MS zircon U–Pb age of 149.3 ± 0.9 Ma (MSWD = 0.15) yielded by the granodiorite porphyry. The δ34SV–CDT values of the sulfides (–1.2 to 2.4‰) conform the recognized magmatic sulfur values, indicating that the sulfur may have a magmatic origin. The Pb isotope compositions of the sulfides (206Pb/204Pb = 18.280–18.386, 207Pb/204Pb = 15.549–15.588, and 208Pb/204Pb = 38.153–38.359) coincide well with those of the Xiaokele granodiorite porphyry. These results indicate that the Xiaokele Cu (–Mo) deposit has a close genetic relationship with the granodiorite porphyry, and the ore-forming materials likely originated from the granodiorite porphyry. Petrographic/compositional characteristics of FI assemblages suggest that the ore-forming fluids belong to an H2O–NaCl–CO2 system in stage I and an H2O–NaCl system from stages II to IV, and fluid boiling occurred from stages I to III. The FI homogenization temperatures show a decreasing trend from stages I to IV. The δ18OH2O values (6.4 to 7.7‰) of quartz in stages I and II are similar to typical magmatic water values, but the δD (–142.5 to –125‰) and δ18OH2O (–1.1 to 3.4‰) of quartz in stages III and IV are significantly lower than magmatic water values, indicating that the ore-forming fluids were magmatic in origin and gradually mixed with meteoric water from stages III to IV. Microthermometric data show that Cu-bearing sulfides in stage II were mainly deposited at a temperature below 400 °C, indicating that fluid cooling was the primary factor that controlled Cu precipitation in the Xiaokele Cu (–Mo) deposit. Our research on the Xiaokele porphyry Cu (–Mo) deposit will help to provide a scientific basis for future prospecting of this deposit and even the Late Jurassic porphyry deposits in the northern GXR.

The Age of Hubi Copper (Cobalt) Ore Mineralization in the Zhongtiao Mountain Area, Southern Margin of the Trans-North China Orogen: New Constraints from U-Pb Dating of Rutile and Monazite

The Hubi copper (cobalt) ore district, one of the largest typical examples of the sediment-hosted stratiform type in the Zhongtiao Mountain area, is located on the southern margin of the Trans-North China Orogen within the North China Craton (NCC) and has a copper reserve of 0.79 Mt. Mineralization is mainly hosted by the Zhongtiao Group, a sequence of metasedimentary rocks deposited from ~2168 Ma to ~2059 Ma. Subsequently, a collisional orogeny (Trans-North China Orogen) occurred at ~1.85 Ga. The absolute age of mineralization has not been well constrained due to the lack of suitable minerals for dating. Rutile and monazite are common accessory minerals and are intergrown with Cu mineralization in Cu-bearing veins in the Hubi-type copper (cobalt) deposits. This study presents the first new LA-ICP-MS U-Pb ages of hydrothermal rutile and monazite for the Tongmugou and Laobaotan copper (cobalt) deposits in the ore district, which yield lower intercept rutile U-Pb ages of 1815 ± 30 Ma (Mean Squared Weighted Deviation, MSWD = 5.0) and 1858 ± 27 Ma (MSWD = 5.2) for Tongmugou and 1876 ± 30 Ma (MSWD = 5.9) for Laobaotan. Monazite crystals separated from Cu-bearing carbonate veins within the orebody of Tongmugou yield a weighted mean 207Pb/206Pb age of 1856 ± 14 Ma (MSWD = 1.9), which is close to that of rutile within error. Mineralogical observations and geochemical characteristics suggest that both monazite and rutile crystallized in the hydrothermal fluid system and are closely related to Cu sulfide mineralization. Therefore, their nearly identical U-Pb isotope age of ca. 1850 Ma directly reflects the timing of metamorphic hydrothermal Cu mineralization. This age is indistinguishable from that of metamorphism during the collisional orogeny (Trans-North China Orogen) that led to the final amalgamation of the Eastern and Western Blocks. According to previous studies, the primary sedimentary mineralization of the Hubi-type copper (cobalt) deposits was synchronous with the deposition of the Zhongtiao Group. From the perspective of mineralization age, both the Congolese–Zambian Copperbelt and the Hubi copper (cobalt) ore district experienced early preorogenic sedimentary diagenetic mineralization and late metamorphic hydrothermal mineralization related to orogenesis, and the Hubi-type copper (cobalt) deposits may also be some of the oldest sediment-hosted stratiform-type deposits in the world. Moreover, this metamorphic hydrothermal Cu mineralization spread throughout the Zhongtiao Mountain area.

Metal recycling tracked by mercury and helium isotopes in platinum–palladium nuggets from Córrego Bom Sucesso, Brazil

The enigmatic botryoidal nuggets of platinum (Pt) and palladium (Pd) from Córrego Bom Sucesso in the southern Serra do Espinhaço, Minas Gerais, Brazil, are considered to have formed during supergene alteration of placer deposits. This requires hitherto unidentified precious-metal enrichment processes and is inconsistent with Pt–Os age of 180 Ma that entails formation at depth, as recently proposed. Here we report the first mercury (Hg) and helium (He) isotopic determinations of Pt–Pd nuggets. Mercury isotopic compositions have a mass-independent fractionation (MIF) signature with an odd-mass deficit (Δ199Hg = − 0.22 ± 0.04; 1SD, n = 15), which requires aqueous photochemical reduction of Hg (II). Extremely low 3He/4He (<0.001 Ra) and extremely high concentrations of He (up to 1.9 × 1017 at/g) are indicative of nugget formation from He-enriched fluids within the quartzite sequence of the Espinhaço basin, not from meteoric surface water. The data are consistent with a nugget-forming setting in the deep biosphere, as a result of groundwater interaction with Pt–Pd–Hg minerals in Pan-African-Brasiliano post-orogenic veins. We propose that the negative Hg-MIF signature was inherited from the vein minerals that originally acquired their Hg from Earth's surface during the intracratonic sedimentation of the Proterozoic Espinhaço basin.

Preservation of Terrestrial Microorganisms and Organics Within Alteration Products of Chondritic Meteorites from the Nullarbor Plain, Australia.

Meteorites that fall to Earth quickly become contaminated with terrestrial microorganisms. These meteorites are out of chemical equilibrium in the environments where they fall, and equilibration promotes formation of low-temperature alteration minerals that can entomb contaminant microorganisms and thus preserve them as microfossils. Given the well-understood chemistry of meteorites and their recent discovery on Mars by rovers, a similarly weathered meteorite on Mars could preserve organic and fossil evidence of a putative past biosphere at the martian surface. Here, we used several techniques to assess the potential of alteration minerals to preserve microfossils and biogenic organics in terrestrially weathered ordinary chondrites from the Nullarbor Plain, Australia. We used acid etching of ordinary chondrites to reveal entombed fungal hyphae, modern biofilms, and diatoms within alteration minerals. We employed synchrotron X-ray fluorescence microscopy of alteration mineral veins to map the distribution of redox-sensitive elements of relevance to chemolithotrophic organisms, such as Mn-cycling bacteria. We assessed the biogenicity of fungal hyphae within alteration veins using a combination of Fourier-transform infrared spectroscopy and pyrolysis gas chromatography-mass spectrometry, which showed that alteration minerals sequester and preserve organic molecules at various levels of decomposition. Our combined analyses results show that fossil microorganisms and the organic molecules they produce are preserved within calcite-gypsum admixtures in meteorites. Furthermore, the distributions of redox-sensitive elements (e.g., Mn) within alteration minerals are localized, which qualitatively suggests that climatically or microbially facilitated element mobilization occurred during the meteorite's residency on Earth. If returned as part of a sample suite from the martian surface, ordinary chondrites could preserve similar, recognizable evidence of putative past life and/or environmental change.