Disseminated Sulfide Mineralization Research Articles

Multiple stages of Au mobilization in the Changshagou Au deposit, Eastern Tianshan, NW China: Insights from mineral chemistry and fluid inclusions

Gold deposits involving multiple stages of mineralization have received considerable attention, but the relationship between these Au enrichment episodes remains enigmatic. The Changshagou Au deposit in the Chinese Tianshan Orogenic Belt exhibits two contrasting styles of mineralization. Early stockwork disseminated sulfide mineralization is associated with potassic alteration and acid magmatic fluids (log fO2 = -30 to -25, pH = 3 to 6), whereas late auriferous vein mineralization is associated with silicification and weakly alkaline hydrothermal fluids (log fO2 = -35 to -32, pH = 4 to 8). Early mineralization (average Au grade = 1.8 g/t) is dominated by invisible gold in sulfides, whereas late mineralization (average grade = 3.7 g/t) contains abundant native gold (fineness = 799 to 872). Fluids with high Au solubilities (∼104 ppb) from the late-stage mineralization remobilized pre-existing Au and formed As-enriched overgrowth bands on pyrite, with the core of these pyrite containing up to 1800 ppm Au, but little As, and their rim containing up to 4300 ppm As, but little Au. Subsequent fluid boiling accompanied with the escape of CO2 and H2S elevated the pH and redox state of the fluid (log fO2 = -32 to -28, pH >8), which caused a decrease in Au solubilities and precipitation of native gold (average grade = 28.6 g/t). In summary, we suggest the Changshagou Au deposit formed as a result of three key processes — pre-enrichment, re-mobilization, and re-enrichment — all of which are important to the formation of Au deposits characterized by multiple stages of mineralization.

Deposit-scale controls of hypozonal gold mineralization in amphibolite facies metaturbidites: 3D modelling of the Twin Hills deposit, Damara Belt, Namibia

The recently discovered Twin Hills deposit is a new resource in the Pan-African Damara Belt of central Namibia, described by a finely dispersed, low-grade and high tonnage disseminated and vein-style gold mineralization. The underlying assumption of the 3D implicit modelling followed in this study is that the integration of gold grades and ore shells with structural and lithological data from oriented core logging reflects the underlying controls of hydrothermal fluid flow and mineralization at Twin Hills. The deposit comprises distinct clusters of economic-grade gold mineralization, measuring several hundred meters along strike, separated by several kilometer-long intervals of only sporadic and sub-economic gold grades. Mineralization is developed on the subvertical southern limb of a regional-scale syncline hosted in highly-strained compositionally heterogeneous, amphibolite facies metaturbidites. Additionally, the stretch of the main mineralization in the central parts of the ∼12 km long east-northeast-trending mineralized Twin Hills corridor coincides with the change of the fold vergence direction. We suggest that the steep orientation of the wall-rock strata generated high-permeability fluid conduits for focusing regional fluid flow during flexural-slip folding and associated local dextral strike-slip component of D2/3 deformation. Disseminated sulfide mineralization is mostly developed in pelitic units, whereas networks of quartz-sulfide veins and veinlets with subordinate disseminated sulfides are present in rigid psammitic beds in interbedded pelite-psammite horizons. While the wide distribution of finely disseminated sulfides point to the rather pervasive fluid flow along grain-scale permeabilities during recrystallization of mineral fabrics, the formation of fractures and resultant dilatancy in psammitic units created hydraulic gradients and focused fluid flow into the psammite horizons. High-grade ore shoots are controlled by the combination and interplay of two main factors, namely lithological contacts and subtle deflections of bedding of the highly-strained, subvertical metaturbidites. The mineralized corridor of the Twin Hills deposit is largely confined to imbricated and/or tightly folded packages of interbedded pelites-psammites within the otherwise pelite-dominated sequence. It is these units that promoted fracturing, fluid flow and the development of fine-scale vein networks as a result of pronounced strain partitioning between pelitic units (ductile) and interbedded psammites (brittle) during regional deformation. Ore shoots of higher-grade mineralization (>1.5 g/t Au) follow these main lithological contacts, but are laterally confined to subtle (5–15°) clockwise deflections of bedding (dilational jogs) from its regional east-northeast trends. Outside these deflections, gold mineralization is patchy and only sub-economic. The structural and lithological controls of the Twin Hills mineralization closely resemble those of orogenic gold deposits, whereas the fine-grained and dispersed texture of gold and associated sulfide mineralization are more reminiscent of disseminated gold deposits. We suggest that the Twin Hills deposit is an example of a hypozonal orogenic gold deposit formed under peak metamorphic amphibolite conditions.

Remote sensing and geochemical investigations of sulfide-bearing metavolcanic and gabbroic rocks (Egypt): Constraints on host-rock petrogenesis and sulfide genesis

Field geology, geochemical analyses and multi-sensor remotely sensed data of Landsat-8, ASTER and Sentinel-2 were used to detect various types of hydrothermal alteration and sulfide-gold mineralization in Neoproterozoic metavolcanic and gabbroic rocks from Darhib and El-Beida areas in the South Eastern Desert (SED) of Egypt. We also used RADAR data of Sentinel-1 using PCI Geomatica software to decipher the structural lineaments that control this mineralization. Different styles of disseminated to massive sulfide deposits include: 1) undeformed magmatic disseminated ores in Darhib gabbros, metavolcanic rocks and metabasic dykes; 2) deformed (remobilized) massive ores mostly of a magmatic-hydrothermal origin in Darhib tremolite talc rocks along E-W shear zones; 3) hydrothermal sulfide ores along El-Beida NW–SE shear zones. Cu-Zn massive ores represent the main sulfides in the Darhib silicified tremolite-talc rocks and meta-andesites, and occur as bands, veins and patches along Darhib E-W shear zones. Discontinuous occurrences of sulfide ores-bearing tremolite-talc rocks indicate that major shear zones act as structural controls on mineralization. Sulfide ores consist chiefly of chalcopyrite, sphalerite, pyrite and galena as well as minor covellite and bornite. These ores are highly concentrated along E-W striking Darhib shear zones and resulted from granite-derived hydrothermal fluids that form/and or accumulate sulfide minerals from the sulfide-bearing metavolcanic rocks around shear zones. Two stages of hydrothermal alterations are recorded in the Darhib talc mine including pre- (talcification, amphibolitization and carbonatization) and syn- (silicification and chloritization) sulfide ore alterations. Disseminated sulfide mineralization hosted in Darhib gabbros, metavolcanic rocks and metabasic dykes is of a magmatic origin, similar to Cu-Ni sulfide deposits hosted in gabbroic varieties in worldwide mafic–ultramafic intrusions. On the other hand, El-Beida gold-sulfide-bearing quartz and quartz carbonate veins are hosted in ferruginated and silicified metavolcanics/ metavolcaniclastic rocks along NW–SE shear zones (Najd Fault System trend). These are spatially associated with regional convergent structures (high-angle convergent wrench structures), suggesting orogenic gold type mineralization. The E-W with minor WNW-ESE shear zones in Darhib and NW-SE trending shear zones in El- Beida structurally controlled sulfides and gold mineralization in the SED of Egypt. El-Beida sulfide mineralization occurs as disseminated ores that consist mainly of pyrite, chalcopyrite, covelite and chalcocite with occasional occurrence of gold. The spatial occurrence of native gold in ferrigenous alteration zones indicates the role of iron in gold precipitation through buffering of gold-bearing complexes. Mineralogical and geochemical data of Darhib and El-Beida sulfide-bearing rocks indicate that they crystallized from depleted mantle sources in a back-arc basin setting, starting with the generation of arc-like volcanic rocks in a nascent BAB stage followed by the emplacement of plutonic equivalent of MORB/OIB-like rocks such as gabbroic intrusions in a mature BAB.

Characterization of potential Cu-Au mineralization using electrical resistivity and induced polarization methods: a case study in Eastern Labo, Bicol, Philippines

Abstract Electrical resistivity and induced polarization (ER-IP) surveys were carried out in Eastern Labo, Bicol, Philippines, to assess the potential Cu-Au mineralization in pyroclastic-covered areas. An electrical resistivity tomography method with a dipole–dipole array was used, and the L1 norm (robust) inversion approach was employed to generate the 16 2D ER-IP models. The analysis of the resistivity and chargeability classified the underlying lithology into eight zones. Zones from C to F are the potential mineralized zones, while Zones A and B (Labo Volcanic Complex) and Zones G and H (Tumbaga Formation and Tamisan Diorite) are the non-mineralized zones. Zone C, represented by low chargeability (<15 mV V−1) and intermediate resistivity (15–100 Ω-m) values, is characterized by a high concentration of disseminated sulfide minerals (e.g. chalcopyrite). Zone D has a chargeability range comparable to Zone C but higher resistivity (>100 Ω-m), which might be due to silicification. Zone E corresponds to the oxide zone with concentrated sulfide minerals along clayey, weathered, fractured areas; it has moderate to high chargeability (>15 mV V−1) and low resistivity (<15 Ω-m) signatures. Zone F has a similar resistivity range to Zone E but has lower chargeability (<15 mV V−1) values. The interpreted underlying lithological units were confirmed using borehole data. Because of the extensive occurrence of high chargeability zones with moderate resistivity anomalies, potentially mineralized areas for further investigation (e.g. drilling) were identified in the north-eastern portion of the study area.

Geochronological, mineralogical and geochemical studies of sulfide mineralization in the Yueyawan mafic intrusion in the East Tianshan orogenic Belt, NW China

The Yueyawan Cu-Ni sulfide deposit, located in the northern part of Dananhu island arc, is a new case of Cu-Ni mineralization related to Early Permian mafic magmatism in the East Tianshan orogenic belt, NW China. Olivine gabbro, hornblende gabbro, and gabbro are the major rock types of this complex. Disseminated sulfide mineralization with a pentlandite + pyrrhotite + chalcopyrite assemblage occurs in olivine gabbro. Zircon crystals from olivine gabbro yielded a U-Pb age of 275 ± 2 Ma, almost contemporaneous with the postcollisional extension-related Cu-Ni deposits in the East Tianshan nickel belt (e.g., Tulaergen, Xiangshan, Huangshan, Tudun, and Baixintan deposits). The Yueyawan olivine gabbro is characterized by light rare earth element enrichment, pronounced negative Nb-Ta anomalies, low initial 87Sr/86Sr ratios from 0.703282 to 0.703420, positive εNd(t) values from 3.4 to 7.2, zircon εHf(t) values from + 15.5 to + 18.5, indicating that the parental magma of the Yueyawan intrusion was derived from a metasomatized mantle source, similar to other contemporaneous mafic–ultramafic intrusions in the region. The δ34S values of sulfide of the Yueyawan deposit are similar and vary from 0.60 to 1.85 ‰, which are within the range of typical mantle values as well as that of the country rocks, providing no direct evidence for addition of external sulfur. However, the negative correlation between Se/S (54.01 to 77.56 × 10−6) ratios and sulfur contents in the disseminated sulfides indicate crustal contamination played a critical role for sulfide segregation of the magma. The olivine Fo and Ni contents from sulphide-bearing mafic rocks show a negative correlation. Olivine chemistry and Sr-Nd-Hf-S isotope data indicate that fractional crystallization and crustal contamination played a role in triggering sulfide saturation in the Yueyawan mafic magma.

Geochemical and Isotopic Characteristics of Disseminated Sulfide Mineralization of Orogenic Gold Deposits of the Yana–Kolyma Metallogenic Belt, Northeast Russia

Geochemical and Isotopic Characteristics of Disseminated Sulfide Mineralization of Orogenic Gold Deposits of the Yana–Kolyma Metallogenic Belt, Northeast Russia

Lithological and structural controls of disseminated-type orogenic gold mineralization in high-grade metamorphic turbidites from the Central Zone of the Damara Belt, Namibia

We report the characteristics and controls of low-grade gold-sulfide disseminations in the Twin Hills prospects, the Central Zone of the Damara Belt in Namibia, that combines to a large (>12 km strike length) zone of mineralization. The host rocks are subvertical, tightly folded and transposed, amphibolite facies metaturbidites (cordierite-biotite metapelites and quartz-biotite metapsammites) that form the subvertical limb of a regional-scale syncline. Sulfide and gold grains are finely dispersed in the host rocks or form networks of quartz-sulfide veinlets. Zones of economic-grade gold mineralization are associated with fine-scale quartz-sulfide (pyrrhotite > arsenopyrite ≫ pyrite) vein stockworks. Morphology and deformational textures of the mineralized vein networks suggest their formation during flexural-slip folding and the progressive transposition of the metaturbidites into the regional, ENE trending steep foliation. The more competent metapsammites promoted fracturing while ductile strain during flexural-slip was localized into the schistose metapelites. As a result, economic-grade mineralization is best developed in well-bedded, compositionally heterogeneous parts of the metaturbidites that experienced pronounced strain partitioning. Metapelite-dominated packages, in contrast, show mainly disseminated sulfide mineralization, fewer and variably transposed vein networks and only sub-economic gold grades. The association of disseminated and vein-type mineralization suggests that pervasive fluid flow was channelized within the lithologically heterogeneous packages that provided fracture permeabilities and hydraulic gradients for more focused fluid flow. Higher gold grades correspond to subtle (5-15°) deflections of bedding and the regional foliation that may relate to dilational jog geometries. On a prospect-scale, occurrence of economic-grade gold mineralization coincides with the inflection and change of the vergence of regional-scale first-order folds. The style of mineralization shares similarities with turbidite-hosted orogenic gold deposits. The differences in gold grades and styles of mineralization can be explained by syn-tectonic (D2) fluid flow, the different metamorphic grade (amphibolite versus mid- to lower-greenschist) and ductility of wall rocks that prevented the formation of larger, brittle, high-permeability structures.

Mineralogical, magnetic and geochemical data constrain the pathways and extent of weathering of mineralized sedimentary rocks

The oxidative weathering of sulfidic rock can profoundly impact watersheds through the resulting export of acidity and metals. Weathering leaves a record of mineral transformation, particularly involving minor redox-sensitive phases, that can inform the development of conceptual and quantitative models. In sulfidic sedimentary rocks, however, variations in depositional history, diagenesis and mineralization can change or overprint the distributions of these trace minerals, complicating the interpretation of weathering signatures. Here we show that a combination of bulk mineralogical and geochemical techniques, micrometer-resolution X-ray fluorescence microprobe analysis and rock magnetic measurements, applied to drill core samples and single weathered fractures, can provide data that enable the development of a geochemically consistent weathering model.This work focused on one watershed in the Upper Colorado River Basin sitting within the Mesaverde Formation, a sedimentary sandstone bedrock with disseminated sulfide minerals, including pyrite and sphalerite, that were introduced during diagenesis and subsequent magmatic-hydrothermal mineralization. Combined analytical methods revealed the pathways of iron (Fe), carbonate and silicate mineral weathering and showed how pH controls element retention or release from the actively weathering fractured sandstone. Drill core logging, whole rock X-ray diffraction, and geochemical measurements document the progression from unweathered rock at depth to weathered rock at the surface. X-ray microprobe analyses of a 1-cm size weathering profile along a fracture surface are consistent with the mobilization of Fe(II) and Fe(III) into acidic pore water from the dissolution of primary pyrite, Fe-sphalerite, chlorite, and minor siderite and pyrrhotite. These reactions are followed by the precipitation of secondary minerals such as of goethite and jarosite, a Fe-(oxyhydr)oxide and hydrous Fe(III) sulfate, respectively. Microscale analyses also helped explain the weathering reactions responsible for the mineralogical transformations observed in the top and most weathered section of the drill core. For example, dissolution of feldspar and chlorite neutralizes the acidity generated by Fe and sulfide mineral oxidation, oversaturating the solution in both Fe-oxides. The combination of X-ray spectromicroscopy and magnetic measurements show that the Fe(III) product is goethite, mainly present either as a coatings on fracture surfaces in the actively weathering region of the core or more homogeneously contained within the unconsolidated regolith at the top of the core. Low-temperature magnetic data reveal the presence of ferromagnetic Fe-sulfide pyrrhotite that, although it occurs at trace concentrations, could provide a qualitative proxy for unweathered sulfide minerals because the loss of pyrrhotite is associated with the onset of oxidative weathering. Pyrrhotite loss and goethite formation are detectable through room-temperature magnetic coercivity changes, suggesting that rock magnetic measurements can determine weathering intensity in rock samples at many scales. This work contributes evidence that the weathering of sulfidic sedimentary rocks follows a geochemical pattern in which the abundance of sulfide minerals controls the generation of acidity and dissolved elements, and the pH-dependent mobility of these elements controls their export to the ground- and surface-water.

New insights into the petrogenesis of rhyolites and evolution of volcanic‐hosted massive sulphide mineralization from shallow borehole core, Betul Belt, Central India

Shallow borehole drilling has been conducted at the Sarni area of Betul Belt, Central India to understand the subsurface geology and nature of mineralization. The predominant lithounits are ferruginous quartzites, felsic volcanic rocks, and with minor intrusions of amphibolites. The encountered felsic volcanic rocks are rhyolites varying from pink (rich in K‐feldspar) to grey (rich in Na‐feldspar and amphiboles). The rhyolitic rocks have undergone intense hydrothermal alteration. The alteration zones are defined by the presence of biotite‐chlorite‐garnet‐gahnite; biotite‐sericite‐chlorite‐anthophyllite and anthophyllite‐actinolite‐tremolite‐carbonate mineral assemblages. Sulphide pits, highly leached zones with occasional bluish tinges, limonitic patches are present in the felsic volcanic rocks, followed by semi‐massive to disseminated sulphide mineralization indicative of Kuroku‐ type Zn–Cu–Pb volcanic‐hosted Massive Sulphide (VHMS) mineralization. Scanning electron microscope and energy‐dispersive x‐ray spectroscopy studies documented sphalerite, pyrite, chalcophyrite, arsenopyrite, pyrrhotite, and minor Au–Ag phases. The Sarni rhyolites exhibit chalcopyrite disease in sphalerite, replacement, and overgrowth ore‐textures. The high HFSE, low Zr/Y ratio, pronounced negative Eu anomalies characterize the felsic volcanic rocks as FIIIb type with high zircon saturation temperatures of 856–931°C. These rhyolites are formed at a back‐arc rift setting in an ensialic island‐arc tectonic regime. The synvolcanic structures facilitated the magma ascent to the surface. The hydrothermal circulation due to adequate heat flow in the back‐arc rift environment, in an episodic multicyclic process provided sufficient time for accumulation of sulphides from the seafloor, which was later deposited to form a VHMS mineralization. We also report tungsten (W) mineralization in the quartz veins of these felsic volcanic rocks. It is suggested that the felsic magma has interacted with metasedimentary sequences and assimilated W‐ore bearing fluids, and during remobilization in the later stages of tectonic activity enabled the deposition of tungsten in these quartz veins.

Пирит зон сульфидизации терригенных комплексов Яно-Колымского орогенного пояса (Северо-Восток России): генерации, типохимизм, минеральные ассоциации

Finely disseminated sulfide mineralization in terrigenous carbon-bearing strata is known to have a certain metal-bearing potential and to be an additional source of ores for deposits. In the zones of finely disseminated sulfide mineralization in the Debinsky, Chay-Yuryinsky and Orotukan-Srednekansky faults of the Yana-Kolyma orogenic belt, three generations of pyrite have been identified and studied: sedimentogenic, hydrothermal-metamorphogenic, and tentatively ore-hydrothermal with different sets of mineral associations. The studies were carried out in the Late Triassic and Early Jurassic sediments adjacent to the faults in the basins of the Spokoyny-Matrosova Creek, the Chay-Yurya River, the Ozyorny Creek, the Khara-Yuryakh Creek, the Nera River, and the Pravy Srednekan River. It was found that the sulfide mineralization had been formed in several stages. Morphological and genetic varieties of pyrite permit to distinguish sedimentary and metamorphogenic pyrite from ore pyrite. According to the quantitative analysis performed on a Camebax microanalyzer with an EDS detector X-Max Oxford, the main trace impurities in pyrite are As, Co, and Ni.

Petrogenesis and Ni-Cu exploration potential of Devonian mafic–ultramafic intrusions in the southern part of the Central Asian Orogenic Belt, NW China: constraints from zircon O isotopes and whole-rock Sr-Nd isotopes

ABSTRACT Weakly disseminated sulphide mineralization is present in the outcrops of several mafic–ultramafic intrusions in the Beishan region of the Central Asian Orogenic Belt in western China, but the potential of these intrusions to host economic sulphide ores at depths is not known. A better understanding of the genetic control on the mineralization may help to answer this important question. With this in mind, we have selected two of them that were emplaced during the Devonian, a period when subduction-related mafic magmatism undoubtedly occurred in the region, for a geochemical study. They are the Miaomiaojing and Guaishishan mafic–ultramafic intrusions. Our new zircon U-Pb dating indicates that they were emplaced at 377.5 ± 5.3 and 358.6 ± 3.9 Ma, respectively. Lherzolite, olivine websterite, troctolite and gabbro are the main rock types for both intrusions. Disseminated sulphide mineralization is concentrated in the ultramafic units of both intrusions. The whole-rock samples are characterized by light REE enrichments, pronounced negative Nb-Ta anomalies, and variable εNd(t) values from −3.54 to +2.17. Zircons from the intrusions have elevated δ18O values (average value: 7.48 ± 0.09 ‰) compared to that of the mantle. The trace element and isotope data support the interpretation that the parental magmas for the Devonian mafic–ultramafic intrusions are contaminated arc mafic magmas. Petrological modelling reveals that fractional crystallization together with crustal assimilation may have played an important role in triggering the sulphide saturation in the magmas. The Ni tenors (in recalculated 100% sulphide) for the sulphide-mineralized samples from both intrusions are similar and up to 6.7 wt% Ni and 2.8 wt% Cu, comparable to the metal tenor of most economic Ni-Cu deposits in the CAOB, implying that samples containing higher amounts of sulphides would become Ni-Cu ores. The olivine Fo and Ni contents from sulphide-bearing ultramafic rocks show a negative correlation, consistent with sub-solid Fe-Ni exchange between olivine and sulphide melt. Together with the similar mineralization characteristics between these sulphide-bearing intrusions and the coeval Heishan Ni-Cu deposit, we suggest that these intrusions have the potential to host Ni-Cu sulphide ores at depths, which could be further tested by geophysical survey followed by drilling.

Disseminated Gold–Sulfide Mineralization in Metasomatites of the Khangalas Deposit, Yana–Kolyma Metallogenic Belt (Northeast Russia): Analysis of the Texture, Geochemistry, and S Isotopic Composition of Pyrite and Arsenopyrite

At the orogenic gold deposits of the Yana–Kolyma metallogenic belt (northeast Russia) both Au–quartz-sulfide mineralization with native gold and disseminated sulfide mineralization with invisible Au developed. The textural and mineralogical-geochemical features, isotope-geochemical characteristics of gold-bearing sulfides from proximal metasomatites, and possible forms of Au occurrence in pyrite and arsenopyrite have been studied using electron microprobe, atomic absorption, LA-ICP-MS trace element, isotope analysis, and computed microtomography. Four generations of pyrite (Py1, diagenetic; Py2, metamorphic; Py3, metasomatic; Py4, veined) and two generations of arsenopyrite (Apy1, metasomatic; Apy2, veined) have been identified at the Khangalas deposit. In the proximal metasomatites, the most common are Py3 and Apy1. Studying their chemical composition makes it possible to identify the features of the distribution patterns of typochemical trace elements in pyrite and arsenopyrite, and to establish the nature of the relationship between Au and these elements. In Py3 and Apy1, structurally bound (solid solution) Au+ prevails, isomorphically entering the crystal lattice or its defects. Isotope characteristics of hydrothermal sulfides (δ34S = −2.0 to −0.6‰) indicate that mantle/magmatic sulfur was involved in the formation of the deposit, though the participation of sulfur from the host rocks of the Verkhoyansk clastic complex cannot be ruled out. The Khangalas deposit has much in common with other gold deposits of the Yana–Kolyma metallogenic belt, and from this point of view, the results obtained will help to better reveal their gold potential and understand their origin.

Early Permian subduction-related Ni-Cu sulfide mineralization in the Central Asian Orogenic Belt: A case of the Halatumiao deposit

The recently discovered Halatumiao magmatic Ni-Cu sulfide deposit is located in the western part of the Baolidao terrane, which represents the middle-eastern segment of the Central Asian Orogenic Belt (CAOB), Inner Mongolia, NE China. This deposit is an important case of a magmatic sulfide deposit related to Early Permian subduction magmatism in an arc setting. In this contribution, we report in-situ zircon U-Pb ages, whole-rock major and trace element and PGE data, and Os and S isotope compositions from the Halatumiao ore-bearing intrusion to constrain its genesis and relationship to the Early Permian subduction-related magmatism. The Halatumiao mafic-ultramafic intrusion is composed of wehrlite, olivine websterite and gabbro. Semi-massive and disseminated sulfide mineralization with a pentlandite + pyrrhotite + chalcopyrite assemblage occurs in both mafic and ultramafic rocks of the intrusion. In-situ dating of magmatic zircon grains separated from an ultramafic rock sample yielded a U-Pb age of 292.3 ± 1.5 Ma. This age is very close to the age of the Erbutu Ni-Cu sulfide deposit (294.2 ± 2.7 Ma) located in the western part of the northern margin of the North China craton. The high Ni/Cu ratios (6.7 in average, except one sample) of ore samples indicate that the parental magma to the Halatumiao deposit was a primitive basalt. The good positive correlation between IPGEs (Ir vs. Os, Ru and Rh) indicates that fractional crystallization of MSS is the key factor for the PGE tenors. The positive γOs(t) values (from +46 to +68) and mantle-like S isotopic compositions (δ34S mostly between −1.5 and 0.5‰), The trace element characteristics show that a minor to moderate degree of crustal contamination (5–10%) has occurred in the magma. Fractional crystallization and crustal contamination were likely the key factors for sulfide saturation in the Halatumiao mafic magma. The relatively high Ni tenors of sulfides (avg. 6.4 wt%) indicate that the Late Paleozoic mafic-ultramafic intrusions abounding in the Xing’an-Inner Mongolia region have a good potential for hosting economic Ni-Cu sulfide mineralization.

Origin and Nature of Parental Magma and Sulfide Segregation of the Baixintan Magmatic Ni–Cu Sulfide Deposit, Southern Central Asian Orogenic Belt (CAOB), NW China: Insights from Mineral Chemistry of Chromite and Silicate Minerals

The mineral chemistry of chromite and silicate minerals in the Baixintan magmatic Ni-Cu sulfide deposit in the Northern Tianshan, southern Central Asian Orogenic Belt (CAOB) are reported here. Two types of chromite were identified in mafic-ultramafic rocks. Type I chromite occurs as inclusions encased in olivine and has a primary and magmatic origin and homogeneous composition with Cr# values (49–66). It is characterized by high Ti contents (0.33–1.52 wt%) and small quantities of ZnO (0–0.21 wt%), MnO (0.28–0.45 wt%), and NiO (0.06–0.24 wt%) contents. In contrast, type II chromite with interstitial phase and larger compositional variations has significantly higher TiO2 (up to 6.2 wt%) and FeOt contents (up to 69.3 wt%) and slightly lower Al2O3 (minimum 3.0 wt%) and MgO contents (minimum 0.53 wt%). It is considered to crystallize from a more evolved and fractionated melt and suffers from post-magmatic alteration, such as serpentinization and chloritization. The olivine has forsterite values (Fo) varying from 76.8 to 85.6. The parental magma is characterized by high temperature (1389 °C), high pressure (3.8 Gpa), and high Mg content (11.4 wt%) with oxidized (FMQ + 1.6) and hydrous nature based on compositions of primary chromite and olivine–chromite pairs. The intrusion originated from high-degree partial melting of depleted mantle that had been modified by crustal components and metasomatized by subduction fluid in a post-orogenic extensional setting. Two stages of sulfide segregation have been recognized. Early segregation led to the depletion of platinum group elements (PGE), and disseminated sulfide mineralization was the product of later segregation. The assimilation of crustal Si and S components played more important roles on sulfide segregation rather than fractional crystallization.

Multistage fluid sources and evolution of Qinglong Sb-(Au) deposit in northern margin of Youjiang basin, SW China: REE geochemistry and Sr-H-O isotopes of ore-related jasperoid, quartz and fluorite

Silicic and fluorite alterations are widespread and spatially linked to Au and Sb mineralization in the Youjiang basin, SW China. However, metallogenic studies on the hydrothermal quartz and fluorite, and their implications on the fluid source(s) and evolution are very limited, leading to ambiguity about the Au-Sb ore formation. The large Qinglong Sb-(Au) deposit in the basin is characterized by an early-ore jasperoid + stibnite + fluorite assemblage, and a late-ore stibnite + Au-bearing pyrite + veined-quartz assemblage. Early-ore jasperoid-fluorite-stibnite precipitated mainly in open-space wallrock breccias. The relatively wide fluorite 87Sr/86Sr range (0.70776–0.70931) suggests the mixing of two 87Sr isotopically distinct end-members. The highly-radiogenic Sr (87Sr/86Sr > 0.70800) of the Qinglong fluorite suggests that the fluids were likely originated from the Lower Paleozoic basement rocks of the basin. MREE-enriched patterns of the fluorite samples suggest that MREEs were leached from these Lower Paleozoic sulfate-rich basement rocks by the fluids (which contain mainly metal sulfate complexes), as also supported by the positive Y anomalies (Y/Y* = 2.89 – 4.50) and elevated Y/Ho ratios (86.76 – 134.73) of the fluorite. Moreover, the depleted radiogenic Sr, absence of Eu anomalies, negative Ce anomalies (Ce/Ce* = 0.72 – 0.87), and the presence of hydrocarbon-rich fluid inclusions indicate basinal fluid (paleo-seawater/formation water) incursion into the ore fluids. Hydrogen and oxygen isotope compositions of the fluid inclusions (in fluorite) also indicate that the ore fluids have had basinal carbonic fluid contribution. A compilation of the published H-O-S isotope compositions of coeval Carlin-like gold deposits in the Youjiang basin suggests that the initial fluid may have had a magmatic origin. A general fluid δ18O drop may have occurred during the ore formation, and was likely attributed to the varying degree of fluid dilution. Taken together, the ore fluids may have mixed with the circulating basinal fluids at shallow depths, causing significant temperature and fO2 drop, which further lowered the stibnite solubility and triggered its precipitation (together with jasperoid and fluorite). At the late-ore stage, the lack of fluorite suggests that the basinal fluid incursion had waned, and the hydrothermal fluid was again dominantly magmatic-hydrothermal. Quartz veins/stockworks were developed in non-brecciated tuffaceous wallrocks and have disseminated sulfide mineralization. This, and the presence of CO2-rich fluid inclusions, could be caused by the dissolution of wallrock carbonate minerals by the magmatic-hydrothermal fluids. Therefore, we proposed that the mixing between magmatic-hydrothermal and basinal fluids had formed the early-ore Sb mineralization, whilst fluid-rock interactions had formed the varying fluid compositions and late-ore Au mineralization.

Multiple S Isotopes and S Isotope Heterogeneity at the East Eagle Ni-Cu-Platinum Group Element Deposit, Northern Michigan

Abstract The East Eagle Ni-Cu-platinum group element deposit is a conduit-type deposit located in northern Michigan, in close spatial proximity to the currently producing Eagle deposit. Massive and semimassive (net-textured) sulfide mineralization at East Eagle occurs approximately 800 m lower in the stratigraphic sequence than that at Eagle and only ~200 m above the contact between Proterozoic and Archean rocks. Although sulfide mineralogy and textural types are similar at the two occurrences, there are important differences in their S isotope systematics. Massive sulfide mineralization at East Eagle is characterized by a relatively narrow range of δ34S values from 1.5 to 3.2‰. Semimassive sulfides show a similar range from 2.1 to 3.8‰. In strong contrast to these values, those from disseminated sulfides that border the massive and semimassive mineralization define a much larger range from –4.3 to 22.8‰. The much more restricted range in δ34S values recorded in the massive and semimassive sulfide mineralization compared to that of the disseminated mineralization is thought to reflect isotopic exchange reactions in the conduit involving accumulated sulfide and pulses of magma containing S of mantle origin. The ∆33S values of all three major types of sulfide mineralization at East Eagle are near 0‰, with most values between –0.03 and 0.03‰. Unlike ∆33S values from semimassive sulfide mineralization at Eagle, the ∆33S values at East Eagle show no, or very limited, evidence for the involvement of S derived from Archean sedimentary rocks. The wide range in δ34S values recorded in the disseminated mineralization provides strong evidence that S from Proterozoic sedimentary host rocks was involved in the mineralization; in some cases, as much as 85% of the S may have been of external origin. In addition to the wide range in δ34S values, the disseminated mineralization is characterized by spatially heterogeneous δ34S values. Meter-scale S isotope variations, as well as variations in Pt and Pd tenor, are consistent with multiple inputs of magma, each characterized by distinct S isotope ratios. Heterogeneity of several per mill at the centimeter scale indicates that the degree of supercooling exceeded the S diffusivity, preserving small-scale S isotope variability inherited from the sedimentary country-rock source. Elongate, branching plagioclase grains in many of the gabbroic rocks that host the disseminated sulfide mineralization are consistent with a rapid second stage of cooling.

Sulfide segregation mechanism of magmatic Ni mineralization in western Beishan region, Xinjiang, NW China: Case study of the Hongshishan mafic–ultramafic complex

A series of early Permian mafic–ultramafic complexes associated with Ni-Cu mineralization are distributed in western Beishan region, Xinjiang, NW China. The Ni mineralization in this area consists of two sub-belts, which have the same magma conduit at depth, similar rock types and similar emplacement ages, but different mineralization types. The Ni mineralization in the north (Hongshishan) sub-belt consists of low-tenor disseminated sulfide, whereas that in the south (Pobei) sub-belt consists of disseminated sulfide and vein-type sulfide with high tenor, suggesting different sulfide segregation mechanisms. The Hongshishan complex consists of dunite, wehrlite, olivine clinopyroxenite, troctolite, olivine gabbro, and gabbro (from earlier to later). The mafic rocks occur along margins of the complex, and the mineralized ultramafic rocks are confined to the center. The dunite and wehrlite host significant amounts of disseminated sulfide. The parental magma of the Hongshishan complex may have originated from 17.0% partial melting of the depleted mantle. It is characterized by high temperature (1368 ℃), high Mg (picritic affinity, 14.52 wt% MgO), oxidation and platinum group element (PGE) depletion, as constrained by the major and trace elements, olivine and chromite mineralogy, and PGE contents. Two stages of sulfide segregation have been recognized. Early sulfide segregation at depth, which is indicated by elevated Cu/Pd ratios and sulfide inclusions in chromite, produces the PGE-depleted parental magma. Later in situ segregation results in disseminated sulfide mineralization. Model of the olivine crystallization and PGE content shows that 0.01%−0.015% of the sulfide is segregated in situ, with R = 100–500. Significant crustal contamination is suggested by xenoliths of wall rock as well as negative Nb, Ta, and Ti anomalies and positive Pb and Sr anomalies in intrusive rocks. The addition of crustal Si and decrease in FeO by fractional crystallization are the main factors triggering S-oversaturation and later in situ segregation. The early segregation at depth is more likely caused by the addition of crustal S, although the δ34S values of sulfides (-0.75‰ to 1.15‰) indicate the isotopic signature of mantle sulfur. For comparison, the development of vein-type sulfide mineralization in the Poyi deposit is ascribed to assimilation of S-bearing Archean rocks. Finally, a model of “early segregation at depth + later in situ segregation + multistage pulsation” is proposed for the Ni mineralization in this area.

Predicting Missing Seismic Velocity Values Using Self-Organizing Maps to Aid the Interpretation of Seismic Reflection Data from the Kevitsa Ni-Cu-PGE Deposit in Northern Finland

We use self-organizing map (SOM) analysis to predict missing seismic velocity values from other available borehole data. The site of this study is the Kevitsa Ni-Cu-PGE deposit within the mafic-ultramafic Kevitsa intrusion in northern Finland. The site has been the target of extensive seismic reflection surveys, which have revealed a series of reflections beneath the Kevitsa resource area. The interpretation of these reflections has been complicated by disparate borehole data, particularly because of the scarce amount of available sonic borehole logs and the varying practices in logging of borehole lithologies. SOM is an unsupervised data mining method based on vector quantization. In this study, SOM is used to predict missing seismic velocities from other geophysical, geochemical, geological, and geotechnical data. For test boreholes, for which measured seismic velocity logs are also available, the correlation between actual measured and predicted velocities is strong to moderate, depending on the parameters included in the SOM analysis. Predicted reflectivity logs, based on measured densities and predicted velocities, show that some contacts between olivine pyroxenite/olivine websterite-dominant host rocks of the Kevitsa disseminated sulfide mineralization—and metaperidotite—earlier extensively used “lithology” label that essentially describes various degrees of alteration of different olivine pyroxenite variants—are reflective, and thus, alteration can potentially cause reflectivity within the Kevitsa intrusion.

Magmatic-Hydrothermal Gold Mineralization at the Lone Tree Mine, Battle Mountain District, Nevada

Abstract The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry. The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 40Ar/39Ar age of 40.14 ± 0.74 Ma. Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave δ18O values of 1.6 to 9.5‰ and δD values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded δ34S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite δ34S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average δ34S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H2S/SO2 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS δ34S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.

Gualcamayo mining District, Argentina: An example of Carlin-like Au deposits

In the mid-1990′s Minas Argentina S.A. (MASA), subsidiary of Viceroy Exploration, began a reevaulation of the Gualcamayo district that included a drilling program. Low-grade gold was intercepted in breccias and in the San Juan Formation limestones. MASA estimated an inferred resource of 37.7 million tonnes @1.16 g/t gold in 2000, and Mineral Resources Development, Inc (MRDI) estimated resources were 37.2 million tonnes grading 1.13 g/t gold. The mineralized zones were superimposed on skarn, marble, breccias, and veins that appear related to a dacitic intrusive complex. Viceroy was subsequently purchased by Yamana Gold in 2003 who continued to explore the project area. Their 2006 National Instrument (NI) 43–101 report for the QDD portion of the Gualcamayo system estimates a measured and indicated resource of 74 million tones (Mt) averaging 0.88 g/t Au (cut-off of 0.3 g/t Au) with an additional 13.9 Mt grading 1.17 g/t Au inferred, yielding 2.6 million ounces of Au. Annual production of gold by Yamana Gold has ranged from 180,412 (2014) to 180,674 (2015) to 154,052 (2017) ounces of gold.This study was initiated to determine the origin of the gold deposits. Field and laboratory studies were conducted by the senior author as part of a doctoral research program comparing ores from Gualcamayo with those of the Great Basin, Nevada, including polished and thin section studies of ore and gangue minerals, followed by fluid inclusion, SEM, stable isotopic, and 40Ar/39Ar analyses to support the interpretations on ore genesis. The area under study corresponds to the Quebrada Del Diablo (QDD) target, which is located in the eastern part of the property.The disseminated gold mineralization in Gualcamayo is localized mainly within collapse breccias with minor values in decarbonatized limestone; all ore intervals are closely associated with elevated arsenic intervals. The San Juan Formation, principal ore host, with 50 m of dolomitic öoid packstone represents the stratigraphic unit that reacted with hydrothermal fluids. Intensely decarbonatized zones have collapsed, forming extensive zones of breccia. Hydrothermal fluid percolated through E-, W- and ENE-trending fault zones and along some bedding planes. The gold grades are maximized where those faults cut an earlier-formed NNW-trending antiform.Alteration of wallrocks in Gualcamayo (decarbonatization, carbon enrichment, local silicification and argillization) associated with disseminated gold includes many of the assemblages associated with Carlin-type (CTD) and distal-disseminated gold (DDD) deposits (Theodore et al., 2002). Additionally, the associated realgar, orpiment, cinnabar, pyrite, arsenian pyrite, marcasite, and minor base metal sulfides are common to CTD and DDD deposits.The Gualcamayo Igneous Complex evolved from an early tholeiitic to calc-alkaline series of magmas. The first stage of igneous activity occurred near 13 ± 2 Ma, and is represented by the Filon Capa Las Vacas dacite. At 8.99 ± 0.01 Ma the Varela Dacite Porphyry was emplaced. The last episode of magmatism corresponds to intrusion of El Rodado Dacite Porphyry at 5.6 ± 0.2 Ma. Field and petrographic relationships observed, together with the δ34S 0 permil values from disseminated sulfide minerals and skarn, suggest that gold mineralization stages were related to the emplacement of the Varela Dacite Porphyry. The inferred age for the disseminated-gold mineralization is <8.99 ± 0.01 Ma.The stable isotope values obtained for the host San Juan Formation limestone (δ18O values: 21.7 to 24‰ and δ13C values: −1.9 to 1.7‰), dolostone (δ18O values: 20.7–22.2‰ and δ13C values: −0.1 to 1.5‰), marbles and breccias at Gualcamayo are interpreted to have been developed as a result of exchange with hydrothermal fluids. The more altered and mineralized rocks exhibit a greater decrease in δ18O values. Some samples show an increase in their δ13C due to the presence of organic carbon in the system.Many characteristics common to Carlin-type (CTD) and Distal-disseminated (DDD) deposits are found in the Gualcamayo disseminated gold deposits. The similarities include the carbonate host rock, the disseminated minerals and textures, sulfide and gangue paragenesis, pyrite with arsenian pyrite overgrowths and associated gold, the alteration types, and comparable isotopic ratios. These similarities allow classifying the Gualcamayo deposits as Carlin-like deposits. Based on the relationship between the mineralization and the igneous activity in the area, the deposit could be interpreted as an intrusion-centered mineral deposit (Sillitoe and Bonham, 1990), characterized by a porphyry complex in the center, which grades outward through Fe-(Cu) skarn with minor Mo, Au, Ag, Bi and W to Pb-Zn-Ag vein, and to the disseminated carbonate- and breccia-hosted Au deposits similar to the McCoy-Cove system along the Battle Mountain-Eureka trend of Nevada.