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Editorial for Special Issue “Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits”

by Theodore J. Bornhorst

Minerals 2018, 8(6), 251; https://doi.org/10.3390/min8060251 - 14 Jun 2018

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Abstract

The Special Issue of Minerals on Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits presents the results of diverse geochemical and mineralogical research from across the globe[…] Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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16 pages, 3194 KiB

Open AccessArticle

A Fluid Inclusion Study of the Brookbank Deposit, Northwestern Ontario: A Transition from Mesothermal to Epithermal Gold Deposition

by Barbara Kowalski and Stephen Kissin

Minerals 2018, 8(3), 92; https://doi.org/10.3390/min8030092 - 1 Mar 2018

Cited by 1 | Viewed by 4035

Abstract

The Brookbank and Cherbourg Zone gold deposits are located in the Beardmore–Geraldton greenstone belt of the Wabigoon Subprovince of the Archean Superior Province of the Canadian Shield. Brookbank is a shear zone deposit, whereas the Cherbourg Zone, joined by a shear zone to [...] Read more.

The Brookbank and Cherbourg Zone gold deposits are located in the Beardmore–Geraldton greenstone belt of the Wabigoon Subprovince of the Archean Superior Province of the Canadian Shield. Brookbank is a shear zone deposit, whereas the Cherbourg Zone, joined by a shear zone to the Brookbank, is a fissure vein deposit, presumably formed by the same ore-depositing fluid. A total of 375 fluid inclusions, all with low salinity (generally <1 °C freezing point depression) and presence of CO₂, from the two deposits were studied. Cross-cutting relationships delineated three stages of vein formation: (1) pre-ore, (2) ore stage and (3) post-ore. Stage 1 inclusions homogenized at ca. 230 °C with low salinity, although at Brookbank a high temperature episode at ca. 360 °C was noted. Stage 2 homogenized at ca. 266 °C at the Cherbourg Zone and over a range 258–269 °C with somewhat higher salinity. Eutectic temperatures indicate that the fluid composition was within the MgCl₂-NaCl-H₂O system. Fluid immiscibility was prominent in that inclusions homogenized to liquid and vapor at the same temperature were observed. The majority of inclusions from Brookbank homogenized to vapor, whereas those from the Cherbourg Zone homogenized to liquid, marking the transition from mesothermal to epithermal lode gold deposition. Stage 3 fluid inclusions from the Cherbourg Zone homogenized at slightly elevated temperatures and contained fluid possibly representing metastable hydrates of KCl. These post-ore fluids may have been the source of potassium feldspar alteration that overprinted earlier alteration assemblages. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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1276 KiB

Open AccessArticle

Constraints on Conceptual and Quantitative Modeling of Early Diagenetic Sediment-Hosted Stratiform Copper Mineralization

by Alex C. Brown

Minerals 2017, 7(10), 192; https://doi.org/10.3390/min7100192 - 13 Oct 2017

Cited by 5 | Viewed by 6386

Abstract

Early diagenetic sediment-hosted stratiform copper (eSSC) mineralization results from low-temperature cuprous chloride complexes carried by saline aqueous solution circulating through footwall aquifers. Favorable copper solubilities are attained in moderately oxidizing, near-neutral pH solutions. That specific oxidation level is not determined by co-existence with [...] Read more.

Early diagenetic sediment-hosted stratiform copper (eSSC) mineralization results from low-temperature cuprous chloride complexes carried by saline aqueous solution circulating through footwall aquifers. Favorable copper solubilities are attained in moderately oxidizing, near-neutral pH solutions. That specific oxidation level is not determined by co-existence with hematite, with its near-indiscriminant control over Eh. Instead, redbed footwall aquifers are signatures of diagenetic oxidation. Relentless in-situ oxidation of ferrous minerals in redbeds produces pore waters too reduced to transport copper, thus eliminating compaction waters as ore solutions. Continuous early influxes of descending oxygen-rich meteoric waters which have assimilated evaporitic salts may redden aquifers and still retain oxidation levels capable of carrying copper to form downstream eSSCs. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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6091 KiB

Open AccessArticle

Copper Isotope Constraints on the Genesis of the Keweenaw Peninsula Native Copper District, Michigan, USA

by Theodore J. Bornhorst and Ryan Mathur

Minerals 2017, 7(10), 185; https://doi.org/10.3390/min7100185 - 30 Sep 2017

Cited by 30 | Viewed by 11899

Abstract

The Keweenaw Peninsula native copper district of Michigan, USA is the largest concentration of native copper in the world. The copper isotopic composition of native copper was measured from stratabound and vein deposits, hosted by multiple rift-filling basalt-dominated stratigraphic horizons over 110 km [...] Read more.

The Keweenaw Peninsula native copper district of Michigan, USA is the largest concentration of native copper in the world. The copper isotopic composition of native copper was measured from stratabound and vein deposits, hosted by multiple rift-filling basalt-dominated stratigraphic horizons over 110 km of strike length. The δ⁶⁵Cu of the native copper has an overall mean of +0.28‰ and a range of −0.32‰ to +0.80‰ (excluding one anomalous value). The data appear to be normally distributed and unimodal with no substantial differences between the native copper isotopic composition from the wide spread of deposits studied here. This suggests a common regional and relatively uniform process of derivation and precipitation of the copper in these deposits. Several published studies indicate that the ore-forming hydrothermal fluids carried copper as Cu¹⁺, which is reduced to Cu⁰ during the precipitation of native copper. The δ⁶⁵Cu of copper in the ore-forming fluids is thereby constrained to +0.80‰ or higher in order to yield the measured native copper values by reductive precipitation. The currently accepted hypothesis for the genesis of native copper relies on the leaching of copper from the rift-filling basalt-dominated stratigraphic section at a depth below the deposits during burial metamorphism. Oxidative dissolution of copper from magmatic source rocks with magmatic δ⁶⁵Cu of 0‰ ± 0.3‰ is needed to obtain the copper isotopic composition of the metamorphogenic ore-forming hydrothermal fluids. In order to accommodate oxidative dissolution of copper from the rift-filling basalt source rocks, the copper needs to have been sited in native copper. Magmatic native copper in basalt is likely stable when the magma is low in sulfur. Low sulfur is predicted by the lack of sulfide minerals in the ore deposits and in the rift-filling basalt-dominated section, which are source rocks, the same rocks through which the ore fluids moved upwards, and the host rocks for the native copper ores. When combined with geologic evidence and inferences, the copper isotopic composition of native copper helps to further constrain the genetic model for this unique mining district. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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15568 KiB

Open AccessArticle

Geochronology of Hydrothermal Processes Leading to the Formation of the Au–U Mineralization at the Rompas Prospect, Peräpohja Belt, Northern Finland: Application of Paired U–Pb Dating of Uraninite and Re–Os Dating of Molybdenite to the Identification of Multiple Hydrothermal Events in a Metamorphic Terrane

by Ferenc Molnár, Hugh O’Brien, Holly Stein and Nick D. J. Cook

Minerals 2017, 7(9), 171; https://doi.org/10.3390/min7090171 - 15 Sep 2017

Cited by 18 | Viewed by 6081

Abstract

The Peräpohja belt comprises a greenschist to amphibolite facies; multiply-folded supracrustal sequence of quartzites; mafic volcanics; carbonate rocks; black shales; mica schists and greywackes deposited from ca. 2.44 Ga to 1.92 Ga; during protracted rifting of the Archaean basement. Metamorphism and multiple folding [...] Read more.

The Peräpohja belt comprises a greenschist to amphibolite facies; multiply-folded supracrustal sequence of quartzites; mafic volcanics; carbonate rocks; black shales; mica schists and greywackes deposited from ca. 2.44 Ga to 1.92 Ga; during protracted rifting of the Archaean basement. Metamorphism and multiple folding of the basin fill occurred during the Svecofennian orogeny (1.92–1.80 Ga). The Rompas Au–U mineralization is hosted within deformed and metamorphosed calcsilicate veins in mafic volcanics. Textural evidence suggests that deposition and periods of uraninite re-mobilization were followed by localized hydrocarbon-bearing fluid flow which produced pyrobitumen crusts around grains of uraninite. Gold precipitated during the latest hydrothermal event at around 1.75 Ga. In situ U–Pb dating of uraninite by laser ablation inductively coupled mass spectroscopy (LA-ICP-MS), and Re–Os dating of molybdenite, indicate that primary hydrothermal uranium mineralization forms two age clusters; about 2.03–2.01 and 1.95–1.94 Ga. Resetting of the U–Pb system and precipitation of new generations of uraninite are associated with major deformation and metamorphic stages of the Svecofennian orogeny at 1.91–1.89 Ga, 1.85 Ga, and 1.80 Ga. Gold deposition was synchronous with the emplacement of the 1.75–1.78 Ga late/post-orogenic granitoids. The gold-producing hydrothermal event is also recorded by Re–Os dating of molybdenite from the gold-bearing Mg-metasomatized metasedimentary and metavolcanic units at the Palokas prospect; a few kilometres from Rompas. Results of this study confirm that some domains in the structure of uraninite may preserve the original crystallization age, despite an overprinting amphibolite facies metamorphic and other hydrothermal events. The study supports the utility of in situ U–Pb dating of uraninite and the ability of Re–Os dating to assist in sorting out different hydrothermal events in areas with complex tectonic; magmatic and metamorphic histories. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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Open AccessArticle

Muscovite ⁴⁰Ar/³⁹Ar Age and H-O-S Isotopes of the Shimensi Tungsten Deposit (Northern Jiangxi Province, South China) and Their Metallogenic Implications

by Wen-Feng Wei, Bing Yan, Neng-Ping Shen, Lei Liu, Yong Zhang and Xin-Kui Xiang

Minerals 2017, 7(9), 162; https://doi.org/10.3390/min7090162 - 11 Sep 2017

Cited by 12 | Viewed by 5010

Abstract

The Shimensi deposit (Northern Jiangxi, South China) is a recently discovered super-large tungsten deposit. Muscovite ⁴⁰Ar/³⁹Ar dating yielded a plateau age of 145.7 ± 0.9 Ma, with normal and inverse isochronal ages being 145.4 ± 1.4 Ma and 145.3 ± [...] Read more.

The Shimensi deposit (Northern Jiangxi, South China) is a recently discovered super-large tungsten deposit. Muscovite ⁴⁰Ar/³⁹Ar dating yielded a plateau age of 145.7 ± 0.9 Ma, with normal and inverse isochronal ages being 145.4 ± 1.4 Ma and 145.3 ± 1.4 Ma, respectively. The muscovite ⁴⁰Ar/³⁹Ar age, which can represent the mineralization age, coincides well with the published zircon U–Pb ages (143–148 Ma) of the ore-hosting granites, which indicates that the tungsten mineralization was syn-magmatic. The new age reported here confirms that the Shimensi tungsten deposit is part of a large Early Cretaceous (147–136 Ma) tungsten-polymetallic belt in South China. Measured and calculated sulfur isotopic compositions (δ³⁴S_minerals = −3.0‰ to 1.1‰, average −1.3‰;

δ^{34} S_{H_{2} S}

= −4.5‰ to +1.2‰, average −1.8‰) of the Shimensi ore-forming fluids indicate that the sulfur was mainly magmatic-derived. The calculated and measured oxygen and hydrogen isotopic compositions (

δ^{18} O_{H_{2} O}

= 4.1‰ to 6.7‰, δD = −62.7‰ to −68‰) of the ore-forming fluids indicate a dominantly magmatic source with a meteoric water input. Oxygen isotopic modelling of the boiling/mixing processes indicates that the Shimensi tungsten mineralization was caused mainly by fluid mixing of magmatic hydrothermal fluid with meteoric water. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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2793 KiB

Open AccessArticle

Contrasting Surficial Composition of Native Gold from Two Different Types of Gold Ore Deposits

by Vladimir L. Tauson, Raisa G. Kravtsova, Artem S. Makshakov, Sergey V. Lipko and Kirill Yu. Arsent’ev

Minerals 2017, 7(8), 142; https://doi.org/10.3390/min7080142 - 10 Aug 2017

Cited by 4 | Viewed by 5091

Abstract

Native gold grains sampled at two different gold ore deposits in Eastern Russia have been studied by the techniques of electron spectroscopy (X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)), electron microprobe analysis (EMPA), and scanning electron microscopy with energy dispersive X-ray [...] Read more.

Native gold grains sampled at two different gold ore deposits in Eastern Russia have been studied by the techniques of electron spectroscopy (X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)), electron microprobe analysis (EMPA), and scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX). The high-fineness gold of the deposit hosted by relatively high temperature gold-quartz-sulfide mesothermal ore formation contains no oxidized Au constituents on grain surfaces, whereas the less fine gold of the epithermal Au-Ag deposit contains gold oxidized to the Au (I) state, or higher, in half of the cases. At this deposit the surface of native Au consists of a thin layer (~15 nm) with elevated Ag and S contents and an underlying SiO₂–containing layer ~30–60 nm thick. Such a composite coating can perform a protective function and prevent the gold-silver sulfides in surficial parts of AuAg grains from oxidation. The sulfur-enriched marginal parts of native gold particles do not always correlate with the stoichiometry of well-known binary AuAg-sulfides and have a variable composition. This may be due to the existence of solid solutions, Ag_2−xAu_xS, if there is enough S or S adsorption-stabilized cluster agglomerates, Ag_nAu_mS, under sulfur deficit. The effect of the formation of nano-scale surficial zonality on the surface of native gold is quite common in nature and applicable to geological exploration and technology of gold-ore processing. It can facilitate establishing the geochemical environment and genetic type of Au mineralization. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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31899 KiB

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Hydrothermal Alteration in the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Evidence from Petrography and Silicates Mineral Chemistry

by Jeff B. Chaumba

Minerals 2017, 7(7), 127; https://doi.org/10.3390/min7070127 - 22 Jul 2017

Cited by 13 | Viewed by 13701

Abstract

The main platinum-group element (PGE) occurrence in the Great Dyke of Zimbabwe, the Main Sulfide Zone (MSZ), is a tabular stratabound layer hosted in pyroxenites. A petrographic and silicate composition study across the MSZ at Unki Mine in the Shurugwi Subchamber was conducted [...] Read more.

The main platinum-group element (PGE) occurrence in the Great Dyke of Zimbabwe, the Main Sulfide Zone (MSZ), is a tabular stratabound layer hosted in pyroxenites. A petrographic and silicate composition study across the MSZ at Unki Mine in the Shurugwi Subchamber was conducted to help place some constrains on the origin of the mineralization. The PGE-enriched zone at Unki Mine is a ~10 m thick package of rocks ranging from gabbronorites, a chromitite stringer, plagioclase websterite, plagioclase pyroxenite (pegmatitic in one narrow zone), a base metal sulfide zone and it is largely located below the contact of the Mafic and Ultramafic Sequences. Pyroxenes have been partially hydrothermally altered to amphibole and chlorite in most lithologies. In addition, sulfides tend to occur as cumulus phases or as inclusions in all the silicate phases. Two generations of sulfide mineralization likely occurred at Unki Mine with primary sulfides occurring in association with cumulus phases, and the relatively finer-grained, often lath-like, sulfides that occur in association with alteration phases of chlorite and amphibole that were likely formed later during hydrothermal alteration. Chlorite thermometry yields temperatures ranging from 241 to 390 °C, and from 491 to 640 °C, and they are interpreted to be temperatures recording the hydrothermal event(s) of magmatic origin which affected the mineralization at Unki Mine. Two-pyroxene thermometry yields temperatures that range from 850 to 981 °C, and these temperatures are interpreted to indicate a hydrothermal imprint on the minerals that constitute the MSZ. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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Review

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Formation and Aggregation of Gold (Electrum) Nanoparticles in Epithermal Ores

by James A. Saunders and Michelle Burke

Minerals 2017, 7(9), 163; https://doi.org/10.3390/min7090163 - 8 Sep 2017

Cited by 49 | Viewed by 11627

Abstract

Here, we review the concept that nanoparticles and colloids may have played a significant role in forming some types of hydrothermal ores deposits, particularly epithermal. This concept was first proposed almost a century ago but the development of new analytical technologies, lab experiments, [...] Read more.

Here, we review the concept that nanoparticles and colloids may have played a significant role in forming some types of hydrothermal ores deposits, particularly epithermal. This concept was first proposed almost a century ago but the development of new analytical technologies, lab experiments, and the discovery of new epithermal deposits where nanoparticles are evident have added credence to the “gold colloid theory”. Nanoparticles are defined to have at least one dimension <10⁻⁷ m, and may have different chemical and physical properties than the bulk solids. Colloids are typically <10⁻⁶ m in diameter and have the added characteristic that they are dispersed in another medium. In epithermal ore-forming solutions, gold or electrum nanoparticles nucleate from supersaturated hydrothermal solutions, and thus this is a “far-from-equilibrium” process. In some cases, gold nanoparticles may simply play a transitory role of aggregating to form much coarser-grained crystals, where all of the evidence of nanoparticles precursor phases is not preserved. However, in some epithermal ores, silica nanoparticles also formed, and their co-deposition with gold (electrum) nanoparticles preserved the gold aggregation features as self-organized “fractal” dendrites. Here, we review existing the data on gold and electrum nanoparticles in epithermal ores, present images of electrum nanoparticles and their aggregates, and discuss the significance of gold nanoparticles formation and aggregation in helping to produce some of the highest-grade gold ores in the world. Full article

(This article belongs to the Special Issue Geochemistry and Mineralogy of Hydrothermal Metallic Mineral Deposits)

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