Massive Skarn Research Articles

Geology and Geochronology of Magmatic–Hydrothermal Breccia Pipes in the Yixingzhai Gold Deposit: Implications for Ore Genesis and Regional Exploration

Magmatic–hydrothermal breccia pipes are widespread in numerous major porphyry and epithermal gold deposits globally, representing significant repositories of metal resources and serving as potential indicators for exploration targeting. More than ten breccia pipes occur in the Central Taihangshan District (CTD) of the North China Craton. Some of these breccia pipes host gold mineralization and are proposed to be related to the adjacent lode gold mineralization. However, the lack of detailed geological constraints make this hypothesis ambiguous. To address this, the present study conducted comprehensive field observations, drill core logging, an in situ sulfur isotope analysis of pyrite, and the 40Ar/39Ar dating of adularia along a 1400 m section of the Tietangdong breccia pipe at Yixingzhai. Three distinct breccia facies were identified at Tietangdong, exhibiting variable proportions across the entire section, including a massive skarn breccia; polymictic, skarn matrix-supported breccia; and polymictic, intrusive rock cement chaotic breccia. Furthermore, adularia 40Ar/39Ar dating indicates a syn-/post-gold mineralization age of 136 ± 1.5 Ma, coinciding with the age of post-breccia felsite dike. The deepest sampled pyrite displays δ34S values of ~2.7‰, strongly indicating a magmatic–hydrothermal signature. These results, when combined with the geological, geochronological, and isotopic studies on the adjacent lode gold mineralization, further suggest a close genetic relationship between the breccia pipes and the lode Au mineralization, paving the way for their utilization as effective indicators for gold targeting within the CTD.

Multiple stages of skarnization in the world-class Zhuxi scheelite skarn deposit, South China

Although previous studies consistently revealed that the world’s largest W deposit, Zhuxi (3.44 Mt. WO3 @0.54 %) was formed by a single metallogenic stage in the Late Jurassic (∼150 Ma), this contribution identifies an unrecognized, postdate metallogenic event. After detailed field investigation, we noted that the early disseminated scheelite-bearing garnet–pyroxene massive skarn ores are crosscut by vein-type skarn ores, mainly composed of garnet, coarse-grained scheelite, and apatite. The mineral assemblage of the vein-type skarn ore is different from the scheelite-bearing veins that formed during the retrograde alteration stage. Moreover, some massive skarn ores crosscut by vein-type skarn ores have undergone obvious retrograde alteration. Given that a complete metallogenic event in skarn deposits is characterized by retrograde alteration following the prograde skarn stage, the component of the vein-type skarn ores shows affinity to that of the prograde massive skarn ores and the vein-type skarn is superimposed on both previous skarns and their retrograde altered ores, indicating another skarnization event occurred in conjunction with a new metallogenic event in the Zhuxi deposit.The studied massive and vein-type skarn scheelites are characterized by low Sr contents (usually less than 40 ppm), positive Eu anomalies (δEu = 2–105), and low Mo contents (usually less than 300 ppm); thus, the massive and vein-type skarns were both formed by reduced magmatic hydrothermal fluids, with no oxidized meteoric water that is commonly prevalent in the retrograde alteration stage. Compared to the massive skarn garnets, the vein-type skarn garnets are richer in Al and poorer in Fe, indicating lower oxygen fugacity conditions during their crystallization and arguing against the involvement of oxidized meteoric water in their formation. The vein-type skarn garnets have higher Sn contents, lower U contents, and similar W contents compared to the massive skarn garnets; thus, the massive and vein-type skarns did not form from the same hydrothermal fluid system at different evolution stages. This is because garnets crystallized from an evolved hydrothermal fluid system under lower oxygen fugacity conditions are expected to have lower Sn and W contents with higher U contents than their predecessors. The in situ U–Pb age (142.8 ± 2.4 Ma) of apatite from the vein-type skarn reveals that apart from the widely known Late Jurassic metallogenic event (∼150 Ma), within the Zhuxi deposit, an Early Cretaceous metallogenic event also occurred, and the latter has been widely identified in the other districts of the Jiangnan tungsten belt. From this multistage metallogenic event superposition, early disseminated scheelite could leach and recrystallize as larger scheelite grains, forming high-grade, world-class scheelite skarn deposits, such as the Zhuxi (3.44MtWO3) deposit and similar deposits worldwide.

Trace elements and textures of scheelite in porphyry-skarn Cu-Au systems: The example of Dongguashan deposit, eastern China

Porphyry-skarn deposits are the major sources of Cu and Au in eastern China, and W is a common byproduct of these mines. Scheelite is the W-bearing mineral in these deposits, and is widespread in both porphyry and skarn ore, however, systematic variations in the textures and geochemistry of the scheelite are not well constrained. Here, multiple generations of scheelite in Dongguashan deposit provide an opportunity for systematic study of the evolution of scheelite textures and chemistry, and their implications for ore-forming processes in a porphyry-skarn Cu-Au system. Scheelite is widespread in the porphyry and skarn zones at Dongguashan, and five types of scheelite are identified. Type1 scheelite occurs in quartz-sulfide veins from the porphyry potassic alteration stage; it displays oscillatory zoning and patchy cathodoluminescent (CL) zones. Type2 and 3 scheelite show relatively homogeneous CL image and occur interstitial to garnet and pyroxene. Type4 scheelite from the skarn ore has patchy CL zones; type5 scheelite shows patchy (core-rim) CL texture and occurs in quartz-sulfide veins that cut massive skarn or skarn sulfide ore. We conclude that the oscillatory banding of type1 scheelite was controlled by crystal kinetics, whereas the patchy, chaotic and sector CL textures in other types were mainly produced by extrinsic fluids of variable composition. The trace elements Mo, Na, Nb, V and REE strongly affect the CL intensity of scheelite at Dongguashan, and trace elements Na, Nb, V and ∑REE decrease in the order type1 and 5 → type4 → type2 and 3. All scheelite types show Y/Ho ratios (12.5–30) similar to igneous rocks (20–27.5), indicating the tungsten mineralization is genetically related to the associated quartz monzodiorite. High Mo, Nb, V and the negative Eu anomaly in type1 scheelite indicate that the fluid related to the porphyry ore was oxidized and F-rich. The homogeneous compositions of type2, 3 and 4 scheelite, together with their weakly negative Eu anomalies show that they formed in a stable high T oxidized environment. The hydrothermal vein features with enrichment of REE (1 wt%) in the late type5 scheelite indicate it formed by injection of a late magmatic-hydrothermal pulse, and the variations in Eu anomalies in type5 suggest the fO2 gradually decreased in late fluids. Our study highlights that scheelite CL textures and geochemistry are effective for tracing multiple ore-forming processes in porphyry-skarn Cu-Au systems.

LA-ICP-MS Analysis of Minerals from the Shizhuyuan W-Polymetallic Deposit, South China: Implications for Mineralization of Pb, W, Mo and Bi

The South China Block (SCB) is a globally important metallogenic district containing numerous W-Sn deposits. Extensive studies of W-polymetallic deposits in this region have greatly improved our understanding of the petrogenesis, geochronology and metallogenesis of these systems. However, studies on the mobilization of ore-forming elements between mineralization- and alteration-related minerals using in situ analyses are rare. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we analyzed W, Pb, Mo and Bi concentrations in silicate minerals and scheelite from granites and skarns associated with the Shizhuyuan W-polymetallic deposit in the Nanling Range in the SCB. Data show that muscovitized biotite in granites contains high W contents. Pb mainly occurs in K-feldspar and plagioclase in granites and epidote and scheelite in skarns. Bi mainly occurs in epidote in skarns. Scheelite in skarns contains high W and Mo contents. Pb isomorphously substitutes K or Ca in silicate minerals and scheelite. W isomorphously substitutes Ti in biotite. Mo isomorphously substitutes W in scheelite and occurs as W-bearing submicroscopic inclusions in minerals with low contents. Bi isomorphously substitutes Pb when the Pb content is relatively high and occurs as Bi-bearing micro or submicroscopic inclusions in minerals when the Pb content is low. Biotite and feldspar are altered in a magmatic-hydrothermal process, W enriched in biotite, Pb enriched in feldspar and the W-, Pb-, Mo- and Bi-bearing submicroscopic inclusions are excluded from minerals and released to the magmatic-hydrothermal fluids. Large amounts of W are precipitated in scheelite when the fluids come in contact with carbonate rocks to form skarn, while a few contents of Pb, Mo and Bi are distributed in skarn minerals. Thus, large amounts of Pb, Mo, Bi and residual W remain in the fluids, which results in the formation of a W-Sn-Mo-Bi massive skarn ore.

Multistage W-mineralization and magmatic-hydrothermal fluid evolution: Microtextural and geochemical footprints in scheelite from the Weondong W-skarn deposit, South Korea

Weondong deposit is one of the W-skarn deposits in South Korea, and main ore mineral is scheelite (CaWO4). Scheelite occurs in both a massive skarn zone (MSZ; Type I) and a quartz porphyry fracture zone (QPFZ; Type II), forming solid solution with powellite (CaMoO4). We examined the relationship between the evolution of ore-forming fluids and formation of multistage scheelites based on microtextural and geochemical features. Type I scheelites can be divided into Type Ia and Type Ib. Type Ia scheelites are characterized by complex internal textures with oscillatory zoning. The texturally associated clinopyroxene (diopside), garnet (andradite), and magnetite strongly suggest an oxidized prograde skarn origin. They are also characterized by enriched light rare earth elements (LREEs) and a negative Eu anomaly. Type Ib scheelites are W-enriched (74–81 wt% of WO3) with no internal zoning. Relict grossular garnet, calcite, pyrrhotite, serpentine, and vesuvianite indicate its relatively reduced retrograde skarn origin. Their rare earth element (REE) pattern is flat with a negative Eu anomaly. Type II scheelites can further be divided into Types IIa and IIb based on textural and geochemical differences. The former largely exhibit patchy zoning with minor oscillatory zoning that normally occurs along microfractures in association with fluorite. Type IIa scheelites show REE fractionation between W- and Mo-enriched zones (i.e., those containing 60–71 wt% of WO3 and 15–25 wt% of MoO3, respectively). Tungsten enriched scheelites show middle rare earth element (MREE)-enriched patterns with a strong negative Eu anomaly; Mo-rich zones are MREE-depleted and show a weak negative Eu anomaly. Type IIb scheelites are W-rich and rarely have zoned texture, similar to Type Ib scheelites. Type IIb scheelites generally coexists with sulfide minerals, indicating a relatively high sulfur fugacity (fS2) retrograde conditions. The REE pattern is flat with a negative Eu anomaly. These results indicate that Type Ia and Type IIa scheelites were formed during prograde conditions at each stage, followed by Type Ib and Type IIb in retrograde conditions. Type Ia scheelites have LREE-enriched patterns, which mimic the REE profile of juvenile magmatic fluids under open systems. Type IIa scheelites have MREE-enriched fractionation patterns that likely reflect a highly evolved magmatic fluid origin in a closed system. The flat REE pattern in Type Ib and IIb scheelites is due to meteoric water in retrograde conditions. This causes changes in the trace element composition of scheelites that reflect differences in the fluid origin and stages of evolution. The geochemical characteristics of both scheelite types match well with their microtextural features and provide crucial evidence for understanding the multistage formation of the Weondong W-skarn.

Mineralogy, geochemistry, and evolution of the Mivehrood skarn and the associated pluton, northwest Iran

An Upper Miocene (9.12 ± 0.19 Ma; biotite 40Ar/39Ar) shallow pluton and numerous dykes and sills of felsic–intermediate compositions intruded the Upper Cretaceous, flysch-type sediments in the Mivehrood area, northwest Iran. The intrusions caused extensive thermal metamorphism and metasomatism, leading to the formation of hornfels and skarn. A massive skarn, 1–10 m thick, immediate to the intrusive contact, is bordered by a banded skarn, 100–400 m thick, that grades outward into hornfels and original sediments. The Mivehrood pluton is characterized by steep REE pattern, high Al2O3 (14.64–16.4 wt%) and Sr (380–786 ppm), and low MgO (1.3–3.4 wt%), Y (4.8–10.7 ppm), and Yb (0.35–0.95 ppm), characteristics typical of high-silica adakites. Skarn formation started with thermal metamorphism, followed by anhydrous prograde and hydrous retrograde stages. Prograde and retrograde mineral assemblages are developed in both skarns, represented by garnet–clinopyroxene–wollastonite and epidote–actinolite–scapolite–chlorite, respectively. Granditic F-bearing garnet dominates clinopyroxene in both skarns. The banded skarn contains minor scapolite of marialite composition. The calc-silicate mineral assemblages and the mineral chemistry allow the Mivehrood skarn to be classified as a calcic, oxidized skarn. Mass balance assessments suggest that Fe, Si, and S were significantly enriched, and Na, LILEs, and LREEs were strongly depleted, in the massive skarn. In the banded skarn, Na, K, Si, and S were enriched. Significant dehydration and carbon degassing occurred in both skarns. Stockworks, veins, and replacement bodies of pyrite ± chalcopyrite locally occur in the pluton and the dykes and in the skarns. The δ34SCDT values for the sulfides fall in a narrow range around 0.0 ‰, suggesting a magmatic source for sulfur and possibly the hydrothermal water and solutes involved in the skarn formation.

Mineralogical and Physiochemical Evolution of the Los Santos Scheelite Skarn, Salamanca, NW Spain

The Los Santos scheelite skarn deposit, located in the province of Salamanca, northwest Spain, occurs in meter-thick strata-bound but irregular lenses of ≤1 km in length and contains a current total resource of 3.09 million metric tons (Mt) with an average grade of 0.54 percent WO3. It was produced by the metasomatic alteration of Upper Vendian-Lower Cambrian metasedimentary rocks and granitic rocks located in the western part of the Avila batholith plutonic complex. Magmatic activity in the Los Santos zone comprises two intrusions, including biotite monzogranite (281–280 ± 6 Ma, granite I) and porphyritic biotite granodiorite-monzogranite (270-269 ± 6 Ma, granite II). The mineralized skarn distributed close to the intrusions occurred in two main stages: (I) prograde skarn, and (II) retrograde alteration of the stage I skarn. The prograde exoskarn is made up of a thick, almost monomineral mass of hedenbergite (Hd82–87), accompanied by grossular (Grs53–73) and scheelite rich in Mo (Sch I). Early mineralization is associated with granite I; thus, it is possible to recognize an endoskarn of calcic plagioclase and clinopyroxene developed over aplitic granite. The stage I skarn formed from H2O-rich fluids (XCO2 <0.1) with an upper temperature limit of 630°C at 0.18 to 0.2 GPa, a log f O2 of −21, and log f S2 ranging from −9.7 to −5.0. After the emplacement of granite II there was a late event characterized by pegmatites, dike swarms, breccia pipes, and intense fracturing associated with the retrograde alteration, which was superimposed on massive skarn. The retrograde skarn has a complex paragenesis that can be divided into two substages. In the first, subcalcic garnet formed, associated with the leaching of early scheelite and the precipitation of late scheelite poor in Mo (Sch II). There is also ferroactinolite, together with important amounts of anorthite, zoisite, apatite, titanite, quartz, and sulfides, mainly pyrrhotite. The first substage skarn formed from a complex H2O-(CO2 ± CH4 ± N2 ± C3H8)-(NaCl ± CaCl2) mixture with f O2 values of −23 and log f S2 ranging from −10 to −7. The data on fluid inclusions indicate that fluid boiling and immiscibility, at temperatures between 439° and 405°C and pressures ranging from 0.1 to 0.042 GPa, was a possible mechanism in the Sch II and sulfide deposition during the first substage of the retrograde alteration of the skarn. The second substage is characterized by the formation of clinozoisite, prehnite, quartz, calcite, chlorite, white mica, zeolite, and mineral phases belonging to the Bi-Te-Ag-S-Au system. The mineral assemblages indicate that the f O2 values are between −25.9 and −33.4 while log f S2 ranges from −10.6 to −6.3 for T <300°C. All the data suggest that the development of the Los Santos skarn represents the evolution of a magmatic-hydrothermal system. In this model, carbon-aqueous fluids would have been exsolved from the plutonic complex during its emplacement and crystallization. The gradual interaction of magmatic fluids at high temperatures with the graphitic rocks of the Schist-Greywacke Complex resulted in a reduction in ore fluids and higher CH4/CO2 ratios. The fluid associated with the last substage of the retrograde alteration of the deposit belongs to the H2O-NaCl system and suggests an influx of a surface-derived fluid.

Re-Os Dating of Pyrrhotite from the Chaoshan Gold Skarn, Eastern Yangtze Craton, Eastern China

The Chaoshan gold skarn, located in the central part of the Tongling Cu-Au-Mo district in eastern China, consists of 10 ore bodies occurring along an intrusive contact between the Early Cretaceous Baimangshan pyroxene diorite (BPD) and limestones of the Middle Triassic Nanlinghu Formation. Less commonly, ores occur in interlayered fractures within the calcareous wall rocks. Endoskarn alteration is well developed and manifested by the presence of massive skarn bodies consisting of garnet, scapolite, vesuvianite, epidote, actinolite, plagioclase, and lesser amounts of diopside. Toward the intrusion, the endoskarn alteration grades into potassic alteration composed of K-feldspar and biotite, suggesting that skarn formation, and thus gold mineralization, is directly related to the BPD. Metallic minerals are mainly pyrite, pyrrhotite, arsenopyrite, chalcopyrite, galena, and sphalerite, of which pyrrhotite, pyrite, and arsenopyrite are the dominant Au-bearing phases. Re-Os dating of auriferous pyrrhotite was performed to determine the age of mineralization of this deposit. Fourteen duplicate analyses from seven samples define a rough isochron on the 187Re/188Os versus 187Os/188Os diagram, yielding an age of 146 ± 47 Ma (2σ) and an initial 187Os/188Os of 0.97 ± 0.25 (MSWD = 17). A greatly improved Re-Os isochron was obtained by plotting seven analyses from four samples that have similar initial 187Os/188Os and thus are assumed to have been derived from the same source, yielding an isochron age of 141.7 ± 9.9 Ma (2σ) and an initial 187Os/188Os of 1.027 ± 0.058 (MSWD = 1.15). This age is consistent with the SHRIMP zircon U-Pb age (142.9 ± 1.1 Ma) of the BPD and molybdenite Re-Os ages (141.7 ± 2.5 to 136.9 ± 2.2 Ma) of Cu-Mo deposits throughout the Tongling district, and therefore is interpreted as the timing of gold mineralization. The extremely radiogenic initial 187Os/188Os implies a significant crustal component in the Os budget of the pyrrhotite samples, likely derived from an enriched mantle source and crustal contamination of the BPD magma during its ascent through the crust.

Mineral zoning and gold occurrence in the Fortuna skarn mine, Nambija district, Ecuador

The Fortuna oxidized gold skarn deposit is located in the northern part of the Nambija gold district, southern Ecuador. It has been subdivided into four mineralized sites, covering a distance of 1 km, which are named from north to south: Cuerpo 3, Mine 1, Mine 2, and Southern Sector. Massive skarn bodies occur in K–Na metasomatized volcanic and volcaniclastic rocks of the Triassic Piuntza unit. They appear to result from selective replacement of volcaniclastic rocks. Very minor presence of bioclast relicts suggests the presence of subordinate limestone. Endoskarn type alteration with development of Na-rich plagioclase, K-feldspar, epidote, actinolite, anhedral pyroxene, and titanite affects a quartz–diorite porphyritic intrusion which crops out below the skarn bodies in Mine 2 and the Southern Sector. Endoskarn alteration in the intrusion grades into a K-feldspar ± biotite ± magnetite assemblage (K-alteration), suggesting that skarn formation is directly related to the quartz–diorite porphyritic intrusion, the latter being probably emplaced between 141 and 146 Ma. The massive skarn bodies were subdivided into a dominant brown garnet skarn, a distal green pyroxene–epidote skarn, and two quartz-rich varieties, a blue-green garnet skarn and light green pyroxene–garnet skarn, which occur as patches and small bodies within the former skarn types. The proximal massive brown garnet skarn zone is centered on two 060° trending faults in Mine 2, where the highest gold grades (5–10 g/t) were observed. It grades into a distal green pyroxene–epidote skarn zone to the North (Cuerpo 3). Granditic garnet shows iron enrichment from the proximal to the distal zone. Diopsidic pyroxene exhibits iron and manganese enrichment from proximal to distal zones. The retrograde stage is weakly developed and consists mainly of mineral phases filling centimeter-wide veins, vugs, and interstices between garnet and pyroxene grains. The main filling mineral is quartz, followed by K-feldspar, epidote, calcite, and chlorite, with minor sericite, apatite, titanite, hematite, pyrite, chalcopyrite, and gold. Metal and sulfur contents are low at Fortuna, and the highest gold grades coincide with high hematite abundance, which suggests that retrograde stage and gold deposition took place under oxidizing conditions. Fluid inclusions from pyroxene indicate precipitation from high temperature—high to moderate salinity fluids (400 to 460°C and 54- to 13-wt% eq. NaCl), which result probably from boiling of a moderately saline (∼8-wt% eq. NaCl) magmatic fluid. Later cooler (180 to 475°C) and moderate to low saline fluids (1- to 20-wt% eq. NaCl) were trapped in garnet, epidote, and quartz, and are interpreted to be responsible for gold deposition. Chlorite analysis indicates temperature of formation between 300 and 340°C in accordance with fluid inclusion data. It appears, thus, that gold was transported as chloride complexes under oxidizing conditions and was deposited at temperatures around 300°C when transport of chloride complexes as gold carriers is not efficient.

Mineralization and Fluid Inclusion Study of the Shizhuyuan W-Sn-Bi-Mo-F Skarn Deposit, Hunan Province, China

The Shizhuyuan deposit, China, is a world-class W-Sn-Bi-Mo-F skarn deposit hosted by Devonian limestone in the thermal aureole of the Qianlishan granite complex. The Qianlishan complex comprises five separate intrusions, including fine-grained porphyritic biotite granite (182–187 Ma, granite 1), medium-grained biotite-K feldspar granite (158–163 Ma, granite 2), fine-grained biotite and K feldspar granite (granite 3), granitic porphyry (144–146 Ma, granite 4), and diabase (142 Ma). The upper part of the granite complex is characterized by extensive greisen alteration. Skarn zones distributed around the intrusions are mainly calcic, consisting of garnet, garnet-pyroxene, vesuvianite-garnet, and wollastonite-vesuvianite, progressively outward from the intrusion. Following the primary skarn formation, some of the skarn zones underwent retrograde alteration. The high garnet/pyroxene ratio and the diopside-rich and andradite-rich compositions of the pyroxene and garnet indicate that the skarn belongs to the oxidized type. Mineralization consists of Sn-Be veinlet ore (type I) in marble and porphyry, massive W-Bi-Mo-Sn skarn ore (type II), stockwork W-Sn-Bi-Mo-F ore (type III), and W-Sn-Mo-Bi greisen ore (type IV), mainly associated with granite 2. Emplacement of granite 2 was accompanied by late, intense fracturing characterized by stockwork mineralization (type III), which was superimposed on massive skarn and greisen zones. The stockwork ore consists mainly of greisen and skarn veins and veinlets with scheelite, wolframite, molybdenite, cassiterite, bismuthinite, and fluorite. The Sm-Nd method has been used to analyze the Sm-Nd concentration, and data for pyroxene and garnet in the massive skarn (type II ore) associated with granite 2 yields an isochron age of 157 Ma. This age corresponds closely to the age of granite 2. Fluid inclusion studies of four types of ore samples reveal four types of inclusions: aqueous two-phase inclusions, H2O-CO2 inclusions, gas-rich inclusions, and daughter mineral-bearing inclusions containing halite or calcite. Homogenization temperatures of fluid inclusions in skarn minerals range from 350° to 535°C. The homogenization temperatures of fluid inclusions in greisen and stockwork are lower than that in skarn, and range from 200° to 360°C. The fluid inclusion data indicate that there are two types of fluids associated with the massive skarn and greisen, having salinities from 26 to 41 wt percent NaCl equiv, and from 1 to 21 wt percent NaCl equiv, respectively. The high-salinity inclusions are daughter mineral bearing, whereas the low-salinity inclusions are dominantly aqueous with a few gas-rich inclusions. The results suggest the evolution of the ore-forming fluids either by immiscibility or by mixing between a high-temperature, high-salinity magmatic water and a low-temperature, low-salinity fluid such as meteoric water, consistent with previously published isotopic studies. The Shizhuyuan deposit is a multi-element skarn, highly enriched in W, Sn, Bi, Mo, and F. This is interpreted to reflect multiple sources and stages of mineralization, which, in turn, may account for the large size of the Shizhuyuan deposit.

Geochemical Behaviors of REE and Other Trace Elements during the Formation of Strata‐bound Skarns and Related Deposits: A Case Study of the Dongguashan Cu (Au) Deposit, Anhui Province, China

Abstract REE and other trace elements in the altered marbles, massive skarns and ores, as well as garnet and quartz were determined in order to examine the behaviors of trace elements during hydrothermal alteration. It is demonstrated that the high‐field‐strength (HFS) elements Zr, Hf, Th and Nb were immobile while other trace elements were mobile during the formation of skarns and related deposits. REE and ore‐forming elements such as Cu and Ag in hydrothermally‐altered marbles and skarns were provided primarily by hydrothermal fluids. In the direction transverse of the strata, the more deeply the marbles were altered, the higher the total REE abundance and the larger the negative Eu anomalies would be. The chondrite‐normalized REE patterns of skarns are similar to those of the marbles, but the former are distinguished by much higher REE contents and more remarkable negative Eu anomalies. Those patterns were apparently not inherited from the marble protolith, but were controlled by garnets, which were determined by the hydrothermal solutions involved in the infiltration metasomatism. The hydrothermal fluids are characterized by having slightly right‐hand dipping REE patterns, being enriched in LREE and having strongly negative Eu anomalies. The uptake of REE from hydrothermal fluids during the growth of garnet crystals resulted in the garnets having similar REE patterns to the hydrothermal fluids. Based on the spatial variation of REE in skarns and the structures of the hosting strata, we can get a better understanding of the transport path and pattern of the fluids involved in the formation of skarns and their related deposits. REE geochemistry investigation can help elucidate the genesis of skarns and skarn‐related deposits.

Geochemistry and origin of Proterozoic skarns at the Mount Elliott Cu-Au(-Co-Ni) deposit, Cloncurry district, NW Queensland, Australia

Diopside-rich, skarn-hosted, copper–gold ore derived primarily from carbonaceous metapelites at Mount Elliott forms a distinctive member of the spectrum of Cu–Au–(Fe oxide) deposit styles in the Cloncurry district of the Paleoproterozoic to Mesoproterozoic Mount Isa Block. The mine sequence is a package of carbonaceous metapelites and metagreywackes containing amphibolites derived from tholeiitic basic rocks. A 40Ar–39Ar age spectrum with an extensive plateau-like segment at 1,510 ± 3 Ma from an actinolite associated with sulfides is taken to represent the age of mineralization and is identical within error to the ages of most of the nearby batholithic granitoids. The mine sequence is locally intruded by 1- to 10-m-thick late- to post-tectonic trachyandesite dykes, which were emplaced during the hydrothermal activity that created the orebodies and have affinities with the regional high potassium “Eureka” supersuite granitoids. Stable isotope data are consistent with dominantly magmatic fluids during mineralization and the regionally distinctive skarn (Ca–Mg) and Cu–Au–Ni–Co–Te–Se (low Pb–Zn–Ag–Sb) chalcophile element associations may reflect a primitive magmatic fluid source and/or leaching of these elements from country rocks. Mount Elliott is an unusual skarn deposit characterized by pronounced early albitization (K–Fe–Mg depletion) of the host rocks succeeded by predominantly open-space deposition of sodic diopside ± actinolite ± scapolite ± andradite ± magnetite ± sulfides ± apatite ± allanite ± tourmaline ± calcite. The Ca–Fe–Mg(–Na)-rich (manganese-poor) chemistry was imposed from the fluid phase in the absence of carbonate-rich protoliths. Immobile trace element (Ti, Zr, Nb) geochemistry shows that Mount Elliott skarns formed in both metasedimentary and mafic metavolcanic host rocks, but the former are the main hosts of ore in upper and lower ore zones that represent most of the resource. Banded skarns derived from a distinct calc-silicate/marble package at the nearby SWAN prospect have higher Nb/TiO2 and Zr/TiO2 ratios than the Mount Elliott metasediment-derived skarns, consistent with different provenance of the detrital components in the two sequences. Medium- to coarse-grained massive skarn and skarn breccia in the Mount Elliott lower ore zone formed in pelites and the trachyandesite dykes are the only intrusive rocks that could be genetically related to the mineralization in the immediate vicinity of the orebodies.

Mineralogy and geochemistry of the Dongmyeong tungsten mine, Korea

Skarn of the Dongmyeong tungsten mine developed at the contact between a Paleozoic limestone and a Jurassic granodiorite. Early, massive skarn is marked by diopsidic clinopyroxene and wollastonite within recrystallized limestone at the skarn front and salitic clinopyroxene within granodiorite. Later massive skarn has clinopyroxenes with salite to ferrosalite compositions and garnets with grossular-rich compositions. Most scheelite deposition occurred at the end of the early skarn stage. Late skarn formed narrow areas of hedenbergitic clinopyroxene, anisotropic garnet, wollastonite and/or quartz along fractures in limestone. Retrograde hydrous alteration is represented by breakdown of clinopyroxene and formation of calcic amphibole, calcite and quartz, accompanied by minor scheelite deposition. In the waning portions of the late skarn stage, base-metal sulfides were superimposed upon previous tungsten mineralization. Whole-rock and mineral compositions indicate that the major element chemistry of the metasomatic skarn-forming fluids was controlled largely by the granodiorite intrusion. Phase equilibria and fluid inclusions indicate that skarn evolution was the result of interaction of water-rich fluids (X CO 2 ≈ 0.1) with original lithologies at ≈ kb with declining temperature (endoskarn, ≈630°; massive skarn, ≈580°–490°C; vein skarn, ≈500°–420°C; hydrothermal sulfides, ≈300°C.

On the Yangtze-type copper deposits and their polygenetic origin

The so-called “Yangtze-type” copper deposits include: (1) Cu-bearing massive pyrite bed, (2) Cu-bearing skarn and magnetite-type ore deposits, with replacement Cu-vein-type deposits near the metasomatic zone, and (3) mineralized intrusive bodies and breccia pipes, some of which are known as porphyry copper ores (e. g. Chengmenshan). This type of ore deposits is a typical example to verify the polygenesis of most of the deposits in China, as has been promoted by Prof. Tu Guangchi in view of the polycyclic development of the geological history in China. This paper is concerned with one sub-type of such deposits.

Sm-Nd isotope systematics in uranium rare-earth element mineralization at the Mary Kathleen uranium mine, Queensland

Sm-Nd isotope and rare earth element geochemical data for U-rare earth element ore of the Mary Kathleen mine and surrounding country rocks reveal a complex history of metasomatic alteration and multistage U-rare mineralization in metasedimentary rocks of the host Corella Formation. Widespread high-temperature contact metasomatism and skarn formation related to the emplacement of the U-rich, 1,730-m.y. A-type Burstall Granite produced large volumes of banded calc-silicate skarn. An Sm-Nd age of 1,766 + or - 80 m.y. (2Sigma ) was obtained on skarn produced at the time of granite emplacement. Sm-Nd data on other skarn types, including massive garnet-diopside skarn which hosts the Mary Kathleen mineralization, suggest that most of the skarn formed during this contact alteration event.The combined Sm-Nd isotope data for whole rocks and minerals of two high-grade ores yield a significantly younger age of 1,472 + or - 40 m.y. (2Sigma ) which is similar to an earlier reported 1,550 + or - 15 m.y. U-Pb age for uraninite. The age discrepancy is considered the result of minor postcrystallization disturbance of the Sm-Nd isotope systems of ore minerals. These ages indicate that the Mary Kathleen mineralization formed long after emplacement of the 1,730-m.y. U-rich Burstall Granite and its related metasomatism. Differences between initial 143 Nd/ 144 Nd ratios of the ore (epsilon Nd = -9.5) and the contemporaneous country rocks including the Burstall Granite and the host skarn (epsilon Nd = -5 to -7.5) at 1,470 m.y. preclude a simple genetic relationship between them.The data suggest a regional metamorphic origin for the present mineralization, probably related to structurally controlled hydrothermal fluid flow around and within massive skarn bodies. The operation of lithological-tectonic control mechanisms on metamorphic fluid circulation has recently been demonstrated for large parts of the Mary Kathleen fold belt, in particular the area affected by massive premetamorphic skarn formation and garnetization. This is supported by a 1,557 + or - 40-m.y. Sm-Nd age for complex garnet-pyroxene-feldspar skarn associated with metamorphic-hydrothermal alteration around prominent D 2 /D 3 deformation and minor rare earth element-U mineralization.The significantly lower epsilon Nd of the ore relative to the country rocks at the time of ore deposition is consistent with an origin from remobilized older, highly light rare earth element-enriched sources. Although other possibilities cannot be ruled out completely, we consider a 1,730-m.y.-old U-rare earth element skarn protore the most likely source for both light rare earth elements and U. Isotopic and mass balance constraints require this protore to be very similar in element contents and ratios to the present mineralization, i.e., it was highly enriched in light rare earth elements and had a long history of a very low Sm/Nd ratio. The preservation of this protore isotopic signature during formation of the present mineralization involved essentially in situ hydrothermal reworking of the protore without significant additions of externally derived rare earth elements.The results of this study demonstrate that combined rare earth element geochemical and Sm-Nd isotope studies provide a powerful tool to date U mineralizing events and to trace the origin of rare earth elements and U in the ore solutions.

Geological and fluid inclusion studies of the Dongpo tungsten skarn ore deposit, China

The Dongpo tungsten ore deposit, the largest scheelite skarn deposit in China, is located at the contact of a 172-m. y. biotite granite with a Devonian marble. The mineralization associated with the granite includes W, Bi-Mo, Cu-Sn and Pb-Zn ores. Several W mineralization stages are shown by the occurrence of ore in massive skarn deposits and in later cross-cutting veins. The high garnet/pyroxene ratio, the hedenbergite and diopside-rich pyroxene and the andradite-rich garnet show the deposit belongs to the oxidized skarn type. Detailed fluid inclusion studies of granite, greisen, skarn and vein samples reveal three types of fluid inclusion: (1) liquid-rich, (2) gas-rich and (3) inclusions with several daughter minerals. Type (3) is by far the most common in both skarn and vein samples. The dominant daughter mineral in fluid inclusions is rhembic, highly birefringent, and does not dissolve on heating even at 530°C. We assume that this mineral is calcite. The liquid phase in most of the fluid inclusions has low to moderate salinities: 0–15 wt. %; in a few has higher salinities (30–40 wt. % NaCl equivalent). The homogenization temperatures of inclusions in the skarn stage range from 350°C to 530°C, later tungsten mineralization-stage inclusions homogenize between 200°C and 300°C, as do inclusions in veins. Fluid inclusions in granite and greisen resemble those of the late tungsten mineralization stage, with low salinity and homogenization temperatures of 200°–360°C. The tungsten-forming fluids are probably a mixture that came from biotite granite and the surrounding country rocks.

Contact skarn formation at Elkhorn, Montana; I, P-T-component activity conditions of early skarn formation

Black Butte Stock. The initial stage of skarn development (I) is characterized by a variety of three-phase and less often four-phase assemblages which usually include garnet and pyroxene and occasionally epidote, vesuvianite, or clintonite. The second stage of skarn development (II) was marked by local replacement of garnet and pyroxene upon cooling by one or more of epidote, amphibole, clintonite, chlorite, calcite, and quartz. Electron microprobe analyses indicate wide variations in pyroxene and garnet compositions in stage I. Application of experimental and calculated phase equilibria to observed skarn assemblages has allowed estimations of general physical-chemical conditions of formation for stage I skarns: P = 1.0 + or - 0.3 kb, T = 525 + or - 35 degrees C, 0.04 <X(CO 2 ) <0.30, -22.7 <log f(O 2 ) <-19.0, log f(S 2 ) <-3.3, -1.65 <log a(SiO 2 ) <0.0, -1.85 <log a(Al 2 O 3 ) <+0.1. The hydration and carbonation reactions that characterize stage II occurred at lower temperature and in a water-rich fluid. Phase equilibria applications indicate the entire zoning sequence of the massive skarns could have equilibrated within a continuous gradient in X(CO 2 ) from 0.30 + or - 0.05 at the skarn-marble interface to 0.03 to 0.07 at the intrusive contact.--Modified journal abstract.

Skarn and porphyry copper mineralization at Mines Gaspe, Murdochville, Quebec

Mines Gaspe extracts copper and molybdenum from skarn and porphyry copper orebodies at Murdochville in the Gaspe Peninsula, Quebec. Both types of orebodies occur in the zoned Copper Brook aureole, which formed in calcareous Lower Devonian strata around the Copper Mountain plug, one of a group of small rhyodacite porphyry sills, dikes, and plugs of Upper Devonian to Mississippian age. Progressive metamorphism is marked by the alteration of diagenetic pyrite to pyrrhotite and the successive appearance of phlogopite, tremolite, diopside, and grossularitic garnet toward the plug. Diopside formation was accompanied by destruction of calcite, pyrrhotite, and carbonaceous material to form a bleached inner aureole. Similar metamorphic zones surround the nearby Porphyry Mountain plug and occur (without garnet) in the Porphyry Brook aureole. The three aureoles appear to coalesce some 1,200 to 1,500 m below surface.Crackle veinlets in the strata domed by eraplacement of the Copper Mountain plug were healed by quartz, feldspars, diopside, and grossularitic garnet, the same minerals that were formed in the metamorphosed wall rocks. Wollastonite partially replaced diopside in a ring around the plug. Iron metasomatism near the plug then replaced diopside with hedenbergitic pyroxene and replaced pale grossularitic garnet and wollastonite with darker andraditic garnet in veinlets and concretions. Massive skarn formed in some metamorphosed limestone units. The Porphyry Mountain plug also domed the surrounding sedimentary rocks. No crackle veinlets or wollastonite formed but andraditic garnet and hedenbergitic pyroxene were deposited in adjacent joints and bedding fractures. Metamorphism and metasomatism were followed, in some parts of the Copper Brook aureole, by formation of scapolite and a second generation of tremolite. Two major periods of mineralization followed.Early mineralization is characterized by pyrrhotite, minor chalcopyrite, and traces of sphalerite throughout the three aureoles. It is associated with alteration of Ca-Mg-Fe silicates to tremolite or actinolite, chlorite, and epidote or clinozoisite and with deposition of albite, calcite, and quartz. More intense mineralization formed replacement orebodies at Needle Mountain and Needle East, some distance from the Copper Mountain plug. These orebodies were cut by late 1 quartz veins containing chalcopyrite, pyrrhotite, fluorite, and traces of scheelite.Porphyry copper mineralization occurs as disseminations and in four sets of veins centered on the Copper Mountain plug. Late 2 veins, which contain quartz + anhydrite + K-feldspar with traces of chalcopyrite and magnetite, may be coeval with biotization and K-feldspar alteration in the plug. Late 3 veins, which account for most of the copper and molybdenum mineralization, contain quartz + or - anhydrite with chalcopyrite, pyrite, and molybdenite. They are variously associated with kaolinization, sericitization, and minor anhydrite alteration in the plug and adjacent intrusions and with formation of tremolite, epidote, chlorite, and minor anhydrite in the metasedimentary rocks. Late 4 veins contain calcite or dolomite, quartz + or - anhydrite, and pyrite, with traces of K-feldspar, chalcopyrite, sphalerite, and galena. They are associated with calcite + sericite + pyrite alteration of the plug and calcite, dolomite, and phlogopite alteration of the metasedimentary rocks. Late 3 and late 4 veins often occupy almost vertical fractures parallel to the local north-northwest joint trend. Late 5 veins contain calcite, quartz, laumontite, and apophyllite, with traces of K-feldspar, chlorite, molybdenite, chalcopyrite, and pyrite or pyrrhotite. Traces of scheelite occur in late 1, late 3, and late 5 veins and in quartz + garnet veins which predate all sulfide mineralization.A model is proposed whereby emplacement of a pluton generated a convective flow of water which metamorphosed the overlying strata to form the aureoles. The Copper Mountain and Porphyry Mountain plugs, apophyses of the pluton, intruded the metasedimentary rocks and superimposed further metamorphic and metasomatic effects on them. Early mineralizaotion, like the prograde metamorphism, is widespread throughout the aureoles (although very weak in most areas). Late mineralization, by contrast, is closely associated with the Copper Mountain plug. The succession of widespread and spatially restricted events suggests that the postulated pluton and the visible plugs alternately controlled the sequence of metamorphism, iron metasomatism, and early and late mineralization. The difference in zoning patterns between Mines Gaspe and other porphyry copper-skarn complexes was probably due to the fact that the Copper Mountain orebody formed in previously decarbonated wall rocks.

Stable isotope studies of metasomatic Ca-Fe-Al-Si skarns and associated metamorphic and igneous rocks, Osgood Mountains, Nevada

Garnet-pyroxene skarns were formed 90 m.y. B.P. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation. The metasomatic replacement followed contact metamorphic recrystallization of the Preble. The sources, temperature, and variation in H2O/CO2 ratios of the metasomatic fluid are interpreted from 269 analyses of oxygen, carbon, hydrogen, and sulfur isotopes in whole rocks, minerals and inclusion fluids. Skarns formed in three mineralogical stages. Oxygen isotope data indicate that temperatures during the crystallization of garnet, pyroxene and wollastonite (Stage I) were least 550 ° C, and that the metasomatic fluid had an $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035 in the massive skarns, and ≦ 0.12 in vein skarns up to 3 cm thick. Pore fluids in isotopic equilibrium with garnet in calc-silicate metamorphic rocks, on the other hand, had $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.15. The metasomatic fluids of Stage I were derived primarily from the crystallizing magma. The isotopic composition of magmatic water was δ18O =+9.0, δD= −30 to −45. Oxygen isotope temperatures of greater than 620 ° C were determined for the granodiorite. Isotopic and chemical equilibria between mineral surfaces and the metasomatic fluid were approached simultaneously in parts of the skarn several meters or more apart, while isotopic and chemical disequilibria (i.e. zoning) have been preserved between 20 to 40 μm-thick zones in grandite garnet. More Fe-, or andradite-rich garnet crystallized in more H2O-rich C-O-H fluids ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.01) than present with grossularite-rich garnet ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035). Stage II was marked by the replacement of garnet and pyroxene by quartz, amphibole, plagioclase, epidote, magnetite, and calcite. Many of the replacement reactions took place over a relatively narrow range in temperature (480–550 ° C), as indicated by 18O fractionations between quartz and amphibole. Meteoric water comprised 20 to 50% of the metasomatic fluid during Stage II. Calcite was formed along with pyrite, minor pyrrhotite, and chalcopyrite during Stage III, although the crystallization of pyrite and calcite had begun earlier, during Stages I and II, respectively. Carbon and sulfur isotope compositions of calcite and pyrite indicate a magmatic source for most of the C and S in the metasomatic fluids of Stage III. By the end of Stage III, meteoric water constituted as much as 100% of the metasomatic fluid. Minerals from grandiorite and skarn do not show large depletions in 18O because the oxygen isotope composition of the metasomatic fluid was buffered by the calcareous wall rocks and the grandiorite. Meteoric water in the vicinity of the Osgood Mountains during the Late Crectaceous (δ18Ocale. ≃ −14.0, δD = − 107) was slightly enriched in 18O and D relative to present-day meteoric water (δ18O = 15.9, δD = − 117)