Distal Skarn Research Articles

Mineral Chemistry and In Situ LA-ICP-MS Titanite U-Pb Geochronology of the Changba-Lijiagou Giant Pb-Zn Deposit, Western Qinling Orogen: Implications for a Distal Skarn Ore Formation

The giant Changba-Lijiagou (Ch-L) Pb-Zn deposit is in the northeast part of the Xicheng ore cluster, Western Qinling Orogen. The ore genesis remains controversial; it could be either a sedimentary exhalative genetic type or an epigenetic hydrothermal genetic type. Here, in situ titanite U-Pb dating for the two kinds of titanite is presented, yielding ages of 212.8 ± 3.0 Ma in the mineralized skarn ore and 214.6 ± 5.1 Ma in the host rock. These ages conform to the previously reported magmatic zircon age (229–211 Ma) based on the in situ zircon U-Pb dating of plutons in this district and the time of large-scale magmatic–hydrothermal activities in Western Qinling Orogen (229–209 Ma). Titanites occurring in mineralized skarn and those that are calcite-hosted are similar to hydrothermal-origin titanites in major element characteristics. The Eu anomalies in the two types of titanite record oxidizing conditions during the mineralization process. A mineral assemblage of garnet, pyroxene, riebeckite, biotite, and potash feldspar, replacing the albite, is well-developed in the deposit. The mineralogical and geochronological characteristics indicate that the Ch-L Pb-Zn deposit is a distal skarn deposit and the result of intensive tectonomagmatic processes in the Xicheng ore cluster during the process of the Late Triassic orogeny.

Constraints of in-situ elemental compositions and U–Pb ages of cassiterite on the origin of the Cretaceous Gejiu and Dulong tin deposits, SW China: Implications for the linkage of tin belts in SE Asia

The western Yunnan tin belts in SW China and the southern Myanmar tin belt in SE Asia are traditionally thought to be part of the SE Asian tin metallogenic province. However, the tectonic setting of the Youjiang tin belt in SW China and its genetic relationship to the other tin belts are poorly documented. In this belt, the giant Gejiu and Dulong tin deposits are typical of skarn-type deposits and both contain contact and distal skarn orebodies. Tin ores of distal orebodies in these two deposits are composed of cassiterite, sulfide and calcite. In both deposits, most cassiterite grains from these ores are rich in Fe and W and depleted in Nb and Ta. They have 238U/206Pb ages between 81.4 and 84.1 Ma, similar to hosting granitic plutons. Two generations of cassiterite with distinctly different microtextures and trace elemental compositions can be recognized. The first generation (Cst-I) is compositionally and texturally homogeneous, but the second generation (Cst-II) displays distinctly oscillatory zoning. Cst-II contains Nb, W, and U lower, and Fe, Ta, Zr and chalcophile elements (Ga, Ge, In, and Sb) higher than Cst-I. These textural and compositional variations reflect the potential involvement of meteoric water in the magmatic-hydrothermal system. In this system, ore-forming fluids were evolved to more alkaline and oxidized to facilitate the precipitation of cassiterite. Our study indicates that these two deposits in the Youjiang tin belt have mineralization styles similar to those in the adjacent western Yunnan and southern Myanmar tin belts and all these tin belts belong to the SE Asian metallogenic provinces. A westward-younging trend of tin mineralization in these three belts can be explained by the rollback of the subducted Neo-Tethyan oceanic slab. This setting is different from the one related to the subduction of the Paleo-Pacific plate.

Newly discovered Early Cretaceous Au mineralization overprinting Late Jurassic Pb-Zn-Ag mineralization in the Qin-Hang metallogenic belt (South China)

Magmatic and mineralizing events in South China were traditionally thought to be limited between 150 Ma and 130 Ma despite the numerous Mesozoic-aged magmatic-hydrothermal deposits in the region. This viewpoint is being reevaluated in light of new age data. Kangjiawan is a representative Pb-Zn-Ag-Au deposit in the Hengyang Basin in the Qin-Hang metallogenic belt of South China that contains 1.96 Mt of Pb-Zn resource with 4.70% Zn, 6.02% Pb; 2280.5 t of Ag resource with 108.23 g/t of Ag; and 58 t of Au resource with 3.25 g/t of Au. Its mineralization can be divided into four stages: (I) pre-ore quartz−pyrite, (II) quartz−pyrite−galena−sphalerite, (III) quartz−galena−sphalerite, and (IV) calcite−pyrite. Multiple generations of pyrite were identified at Kangjiawan. Generation I pyrite (Py1, formed in Stage I) is coarse-grained, texturally homogeneous, and contains abundant silicate inclusions (e.g., K-feldspar and titanite). Generation II pyrite (Py2, formed in Stage II) is generally fine-grained and exhibits a core-rim texture, with the porous, sphalerite−galena inclusion-bearing core (Py2a) having replaced Py1 and the rim (Py2b) being homogeneous. Generation III pyrite (Py3, formed in Stage IV) is subdivided into Py3a and Py3b, with the homogeneous Py3a often replaced by the porous Py3b. The low Co/Ni ratio of Py1, along with the abundant silicate inclusions, is suggestive of intense fluid-rock interaction during Stage I. Py2a is As-Cu-Pb-Ag-Au−rich, Co-depleted, and has elevated Ag/Co (average 3.24) and As/Co (average 10,217.14) ratios, which are suggestive of extensive fluid boiling during the formation of Py2a. This is also supported by the ubiquitous Py2-cemented hydrothermal breccia. In contrast, Py2b is depleted in As-Cu-Pb-Ag-Au but enriched in Co, which is indicative of non-boiling conditions. Subsequent sealing of fractures by veins may have caused the formation of coarse-grained sphalerite and galena in Stage III. The similar 207Pb/206Pb values among Py1 (average 0.849), Stage II galena (average 0.849), Stage III galena (average 0.849), and the nearby Shuikoushan granodiorite stock (corrected average 0.848) suggests that mineralization in stages I−III may represent distal skarn-type Pb-Zn-Ag mineralization genetically related to the Shuikoushan stock. This interpretation is supported by the similar ages of mineralization Stage I (apatite U-Pb: 159.4 ± 1.0 Ma) and Stage II (fuchsite 40Ar-39Ar: 158.1 ± 0.4 Ma) to the age of Shuikoushan stock (zircon U-Pb age: ca. 158 Ma). As the primary Au-hosting mineral at Kangjiawan, the As- and Au-rich Py3a occurs in sharp contact with porous Co-Ni-Se-Bi-Mo-As−poor Py3b, which commonly contains abundant fine-grained galena and sphalerite inclusions. This suggests that Py3b formed via coupled dissolution−re-precipitation of Py3a. This coupled dissolution−re-precipitation reaction favors the remobilization of Au from within the Py3a structure and its re-enrichment as Au inclusions in Py3b; this is supported by the anomalous Au peaks in time-resolved signal profiles from laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and the abundant outliers for Au concentrations in Py3b. The markedly distinct trace-element geochemistry of Py3 compared to those of Py1 and Py2, and the U-Pb age of Stage IV calcite (ca. 138 Ma), are indicative of an Early Cretaceous Au mineralization event. Published calcite C-O isotope data, coupled with the narrow range of δ34S values (0.6‰−2.2‰) of Py3, suggest that the Kangjiawan Au mineralization may be associated with a concealed Early Cretaceous pluton. We propose that the Kangjiawan deposit formed via the overprinting of Late Jurassic distal skarn Pb-Zn-Ag mineralization by Early Cretaceous magmatic-hydrothermal Au mineralization, which demonstrates that 150−135 Ma may also be an important period for the formation of the magmatic-hydrothermal mineralization belt.

Settings Trace elements and garnet formation in a distal skarn zone: a case study of the Rudnik deposit, Central Serbia

The Rudnik Pb-Zn deposit is hosted in skarns and hornfels formed in the late Oligocene by contact metamorphism of limestones, sandstones and shales. Garnets, together with epidote, represent the main non-metallic minerals in the Rudnik skarn. In the distal skarn zone, the garnets are rare and their occurrence is related to the flow path of hydrothermal fluids. To constrain the hydrothermal and physicochemical conditions, in situ elemental SEM-WDS and LA-ICP-MS analyses, and fluid inclusion microthermometric measurements, were made. The Rudnik garnets from the distal skarn zone are predominantly of andradite-grossular composition (Adr39.3–88.9Grs2.9–53.9Alm0.5–10.0), with a small amount of spessartine. Generally, the Fe-rich garnets show a positive Eu anomaly with LREE enrichment and a HREE flat pattern, with homogenization temperatures and salinities of fluid inclusions ranging from 373 to 392°C and from 14.25 to 15.27% NaCl equivalent, respectively. The trace elements and microthermometric properties indicate that the garnets formed at moderately high temperatures, mildly acidic pH levels and increased oxygen fugacity

Garnet and Zircon U‐Pb Geochronology and Geochemistry Reveal Genesis of the Dafang Au‐Pb‐Zn‐Ag Deposit, Southern Hunan

AbstractGarnet is a primary mineral in skarn deposits and plays a significant role in recording copious mineralization and metallogenic information. This study systematically investigates the geochemistry and geochronology of garnet and zircon in the Dafang Au‐Pb‐Zn‐Ag deposit, which represents prominent gold mineralization in southern Hunan, China. Garnet samples with distinct zoning patterns and compositional variations were identified using various analytical techniques, including Backscattered Electron (BSE) imaging, Cathodoluminescence (CL) response, textural characterization, and analysis of rare‐earth elements (REE), major contents, and trace element compositions. The garnet was dated U‐Pb dating, which yielded a lower intercept age of 161.06 ± 1.93 Ma. This age is older than the underlying granodiorite porphyry, which has a concordia age of 155.13 ± 0.95 Ma determined by zircon U‐Pb dating. These results suggest that the gold mineralization may be related to the concealed granite. Two groups of garnet changed from depleted Al garnet to enriched Al garnet, and the rare earth element (REE) patterns of these groups were converted from light REE (LREE)‐enriched and heavy REE (HREE)‐depleted with positive europium (Eu) anomalies to medium REE (MREE)‐enriched from core to rim zoning. The different REE patterns of garnet in various zones may be attributed to changes in the fluid environment and late superposition alteration. The development of distal skarn in the southern Hunan could be a significant indicator for identifying gold mineralization.

Garnet Geochemistry of Pertek Skarns (Tunceli, Turkey) and U-Pb Age Findings

The Fe skarn and vein-type Cu mineralization types are common in the Eastern Taurus Mountains. This study aims to determine the U-Pb geochronology of garnets of varying sizes within the skarn zone developed at the quartz diorite–marble contact zone in Ayazpınar, Pertek District, Tunceli Province, Turkey. Additionally, this study aims to determine the age of the skarnization and the types of inclusion minerals in the garnets. Faulting and magma emplacement along the thrust plane caused mineralization in the Eastern Taurus Mountains, especially at the marble and quartz diorite contact zone between the cities of Elazığ and Tunceli. The greenish garnets found in the Ayazpınar deposit are characteristic of distal skarns, while red or brown Pertek garnets are observed in the proximal skarns. The garnets typically feature a core–rim texture. The cores of the garnet crystals are large, reddish in color, and have a high REE (Ce, Pr) content. Moreover, the cores have higher Fe and lower Al ratios, alongside higher La, Ce, and Pr contents, than the rims. We propose that the compositional differences between the rims and the cores reflect the transition from oxidized REE- and Fe3+-rich liquids to liquids with lower REE and Fe3+ contents, producing the differences in the garnet colors. Green garnets show lower REE contents than brown garnets with Fe-rich cores. The skarn under study includes the following successively formed zones: diorite → epidote skarn → garnet–magnetite → pyrite–garnet–magnetite → calcite carbonate. Diopside, magnetite, and hematite, including small grains, are surrounded or enveloped by garnets. The U-Pb age of the mineralization is 74.1 ± 5 Ma, indicating that the mineralization occurred concurrently with the intrusion settlement.

Constraints on Ore Genesis from Trace Ore Mineralogy: A New Occurrence of Kupčíkite and Paděraite from the Zhibula Cu Skarn Deposit, Southern Tibet

Mineral assemblages containing Cu-Bi sulfosalts, Bi chalcogenides, and Ag-(Au) tellurides have been identified in the mid-Miocene Zhibula Cu skarn deposit, Gangdese Belt, southern Tibet. Different mineral assemblages from three locations in the deposit, including proximal massive garnet skarn, proximal retrogressed pyroxene-dominant skarn in contact with marble, and distal banded garnet–pyroxene skarn hosted in marble, are studied to constrain the evolution of the mineralization. Hypogene bornite contains elevated Bi (mean 6.73 wt.%) and co-exists in proximal andradite skarn with a second bornite with far lower Bi content, carrollite, Au-Ag tellurides (hessite, petzite), and wittichenite. This assemblage indicates formation at relatively high temperatures (>400 °C) and high fS2 and fTe2 during prograde-stage mineralization. Assemblages of Bi sulfosalts (wittichenite, aikinite, kupčíkite, and paděraite) and bismuth chalcogenides (e.g., tetradymite) in proximal pyroxene skarn are also indicative of formation at relatively high temperatures, but at relatively lower fTe2 and fS2 conditions. Within the reduced distal skarn (chalcopyrite–pyrrhotite-bearing) in marble, cobalt, and nickel occur as discrete minerals: cobaltite, melonite and cobaltic pentlandite. The trace ore mineral signature of the Zhibula skarn and the distributions of precious and critical trace elements such as Ag, Au, Co, Te, Se, and Bi support an evolving magmatic–hydrothermal system in which different parts of the deposit each define ore formation at distinct local physicochemical conditions. This is the first report of kupčíkite and paděraite from a Chinese location. Their compositions are comparable to other occurrences, but conspicuously, they do not form nanoscale intergrowths with one another.

Ore genesis of the Mengya'a distal Pb–Zn deposit in the Lhasa terrane, Tibet, China: Constraints from in situ garnet U–Pb dating and C–H–O–Sr isotopes

A large amount of distal skarn Pb–Zn deposits without any causative intrusions exposed have been discovered along the eastern Nyainqêntanglha metallogenic belt (NMB) in the Gangdese region, Tibet, China. As a typical distal skarn Pb–Zn deposit in the east of NMB, the Mengya'a deposit is adjacent to the Longmala Pb–Zn ± Cu ± Fe deposit. However, the linkage between these two deposits is not unclear due to the chaotic dating results. In situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) U–Pb dating of reddish‐brown and yellow‐green garnet from Mengya'a yielded lower intersect ages of 53.6 ± 3.1 and 52.4 ± 4.4 Ma, respectively, indicating that Pb–Zn mineralization took place during the main collision stage of the India–Asia plates. The δDSMOW values of garnet, quartz and calcite vary from −148.4‰ to −140.4‰, −123.5‰ to −115.3‰ and −127.9‰ to −121.3‰, with the δ18OH2O values ranging from 5.8‰ to 6.6‰, 0.4‰ to 3.7‰ and 1.0‰ to 1.7‰, respectively. The δ13CV‐PDB and δ18OSMOW values of calcite range from −8.2‰ to −4.7‰ and 9.8‰ to 18.6‰, respectively. These C–H–O isotopic compositions of hydrothermal minerals indicate that the ore‐forming fluids were initially derived from the magmatic water with progressive input of meteoric water from the prograde stage to the retrograde stage. The 87Sr/86Sr(53 Ma) ratios of sphalerite (0.72321–0.73474) are consistent with those of Ando gneiss in the Lhasa‐Golmud area, indicating that the basement gneiss makes a great contribution for the source of ore‐forming materials. The newly obtained garnet U–Pb ages at Mengya'a are consistent with those of mineralization and causative intrusion in the Longmala Pb–Zn ± Fe ± Cu deposit, suggesting that the Mengya'a and Longmala deposits constitute an integrated distal skarn‐type system. This proposed hypothesis suggests that the Fe–Cu ± Pb ± Zn mineralization within the contact zone should focus on the locations beneath the identified Pb–Zn orebodies at Longmala and Pb‐21 orebody at Mengya'a.

Petrographic and geochemical evidence of calc-silicate xenolith – magma interaction in the western Bushveld Complex, South Africa

Abstract Underground workings at Rowland Shaft, Lonmin Platinum, near Marikana have exposed a large (>7 x 5 m) calc-silicate xenolith in mottled anorthosite ~20 m below the UG 1 chromitite of the upper Critical Zone. The xenolith comprises a monticellite + forsterite + spinel assemblage that is consistent with a metamorphic Tmax of ≥975°C being reached at P ~1 kbar and XCO2 ~1. Internal variations in the modal proportions of these phases are interpreted as an artefact of sedimentary layering in the xenolith. The xenolith is flanked by a 20 to 30 cm wide magmatic skarn that contains several small, cm- to dm-scale, calc-silicate fragments displaying the same peak assemblage as the main xenolith, as well as a dm-scale chromitite autolith and mm- to dm-scale lenses and stringer-like masses of fine-grained uvarovite. The skarn matrix is mineralogically zoned, with an up to 10 cm wide clinopyroxenite layer proximal to the xenolith contact and calc-silicate fragments grading outwards into a gabbroic, plagioclase-clinopyroxene assemblage in which plagioclase becomes progressively more dominant distally. Notwithstanding its pseudo-ophitic texture, the distal skarn matrix displays a range of features consistent with crystallisation of magma contaminated by Ca from the decomposing xenolith under elevated fO2 conditions, including: (a) the unusual mineral compositions (zoned aluminian-ferrian diopside and anorthite, An>99), (b) the presence of calcite as inclusions in plagioclase and as a minor interstitial phase, (c) corroded wollastonite inclusions in clinopyroxene, and (d) thin grossular reaction rims between clinopyroxene and plagioclase. This chemical evolution is further substantiated by zoned spinel grains comprising Cr-rich cores and Al-enriched rims, which are, in turn, enclosed in garnet aggregates that grade outwards from uvarovite to grossular. Local development of grossular-vesuvianite symplectite indicates limited retrograde hydrous (XCO2 <0.1) fluid influx under subsolidus conditions (T <750°C). Bedding in the xenolith displays a steep westerly dip, approximately orthogonal to the magmatic layering. This, together with evidence in the skarn of both plastic shear strain and out-of-sequence magmatic autoliths, suggests that the xenolith underwent density-driven foundering within the RLS magma from an initial location above the UG 1 chromitite layer.

Thermal infrared spectroscopy studies on skarn minerals for exploration of the Jiama Cu-Mo deposit, Tibet, China

The zonation patterns of skarn minerals, including garnet, diopside, and wollastonite, are important for exploration of skarn deposits. Thermal infrared (TIR) spectroscopy, which covers the wavelength range of 6–14 μm, is a new eco-friendly technology for skarn mineral identification. In this study, the TIR spectra data were collected, using an Agilent 4300 Handheld FTIR Spectrometer from nine drill holes within Jiama Cu-Mo deposit in Tibet, China. This study focuses on the TIR characteristics of grandite-series garnets (i.e., grossular to andradite), which have twin peaks in the 10–13 μm range and a diagnostic absorption valley (T absorption valley) at around 11.5 μm shifting based on the contents of Al2O3 and Fe2O3. The T absorption valley in the zoned garnet sample showed an undulating pattern, shifting towards longer wavelengths from the core to the rim, indicating the increase in Fe2O3 content and a decrease in Al2O3, MnO, and TiO2 content. The mixed spectral features of three significant skarn minerals including garnet, diopside, and wollastonite were used to classify the skarn zonation into garnet skarns, diopside-garnet skarns, and wollastonite-garnet skarns. The results from nine drill holes of the No. 24 exploration line in the cross section showed that the garnet T absorption valley shifted towards shorter wavelengths from the proximal to the distal skarn, as well as from the lower to the upper skarn. Furthermore, intensive copper mineralization occurred in the skarn when the garnet T absorption valley exceeded 11.50 μm. In comparison, the garnet T absorption valley at 11.35–11.50 μm is related to more intense molybdenum mineralization. This study suggests that TIR spectroscopy has the potential to rapidly identify skarn zonation in the field, and providing a suitable vector tool for skarn ore exploration.

Origin of the Amensif Zn-Cu (Pb-Ag-Au) distal skarn deposit (Western High Atlas, Morocco): Constraints from C-O-S isotopes

The Amensif Zn-Cu (Pb-Ag-Au) ore deposit is located in the northern part of the western High Atlas. The deposit hosts 1 million tons (Mt) of Zn-Cu-Pb metal reserves at 3% Zn, 2% Cu, and 1% Pb. The lenticular sulfide ores are structurally controlled open-space fillings (fractures, faults) in the Lower Cambrian carbonates. The mineralogical paragenetic sequence consists of andradite-quartz-epidote-dolomite-calcite-sphalerite-chalcopyrite-pyrite-galena with subordinate amounts of barite. The pre-ore stage and ore stage I dolomite samples have δ13CPDB and δ18OSMOW values in the range of −10.2 to −2.6‰ and 17.4–26.8‰, respectively. The lower δ18O value of these dolomites suggests oxygen isotope exchange between the ore-forming fluids and the carbonate host rock whereas negative δ13CPDB values reveal the interaction between these hydrothermal fluids and black shales. The δ34S values of sulfides ore (sphalerite, chalcopyrite, and pyrite) are high, ranging from 6.4 to 14.4‰. These S isotopic values reveal that sulfur derived from the mixture of magmatic sulfur and lower Cambrian carbonate-derived sulfur in almost equal proportions. Sulfide ores precipitated from the mixing between the magmatic-hydrothermal fluids exsolved from a hidden magma or the nearby Azegour granite of Permian age and an external cooler fluid of unknown origin. Fluid-rock interactions with the carbonate host rock contributed in the precipitation of the ore. Mineralogical and geological features allow us to ascribe the Amensif Zn-Cu (Pb-Ag-Au) polymetallic ore as a distal skarn system which is spatially associated with the lower Permian granite intrusion.

Garnet geochemistry of the giant Beiya gold–polymetallic deposit in SW China: Insights into fluid evolution during skarn formation

The Beiya deposit in the central Jinshajiang–Ailaoshan metallogenic belt of SW China is the country’s largest gold skarn and third largest gold deposit. In this study of the Beiya deposit, we use garnet mineralogy and geochemistry to delineate three skarn alteration zones (i.e., contact, proximal and distal skarn). We investigate the possible controls on the rare earth element (REE) incorporation into skarn-type garnet, the ore-fluid evolution, and metasomatic dynamics. Garnet in the Beiya deposit belongs to the grossular–andradite solid solution series, and comprises three types: (1) Grt1 andradite (Adr67–88Grs9–32) in the contact skarn, exhibiting light REE (LREE) enrichments, heavy REE (HREE) depletions, and positive Eu anomalies; (2) Oscillatory-zoned Grt2 andradite (Adr55–98Grs1–44) in proximal skarn, which has moderate to low LREE/HREE enrichments and varying Eu anomalies; (3) Grt3 andradite (Adr29–76Grs22–69) in distal skarn with low LREE/HREE enrichments and minor negative Eu anomalies. Grt1, Grt2 and Grt3 yielded lower intercept U–Pb ages of 36.4 ± 1.8 Ma (MSWD = 0.87, n = 67), 34.8 ± 0.7 Ma (MSWD = 1.8, n = 31) and 33.4 ± 1.0 Ma (MSWD = 1.2, n = 25). Garnet shows compositional changes from Grt1 to Grt3 due to the fluid-wallrock reactions. With decreasing andradite and increasing grossular contents, the (La/Yb)N ratio decreases. Ore-forming fluids changed from diffusive metasomatism to infiltration metasomatism, and then again to diffusive metasomatism. Meanwhile, the ore-fluid physicochemical conditions changed from acidic to near-neutral (both oxidised), whilst the water/rock ratio first increased and then decreased. The proximal skarn recorded high water/rock ratio with intermittent boiling events, leading to fluctuating oxygen fugacity, which is conducive to ore deposition. Therefore, the skarn-type garnet composition from the various alteration zones can reveal the fluid activity and physicochemical fluid evolution path, preserving rich information about the mineralisation process.

Discrete late Jurassic Sn mineralizing events in the Xianghualing Ore District, South China: Constraints from cassiterite and garnet U-Pb geochronology

Abstract Numerous skarn-type Sn deposits have been identified in the Nanling Range (South China), of which the Shizhuyuan W-Sn-Bi-Mo, Xianghualing Sn, Jinchuantang Sn-Bi, and Hehuaping Sn deposits are the largest. The Xianghualing deposit, which is the focus of this study, hosts a resource of 0.17 Mt Sn grading 0.93–1.39 wt% SnO2. Whether the distal skarn-type mineralization and the cassiteritesulfide vein-type orebody in the Xianghualing district are genetically related to the Laiziling granitic pluton, which produced the proximal skarn-type Sn mineralization, however, is still unknown. The Xianghualing Sn mineralization occurs exclusively as cassiterite and has been subdivided into four ore-types: (1) lenticular proximal skarn ore (Cst I) containing the mineral assemblage cassiteritepyrrhotite-chalcopyrite-actinolite-wollastonite; (2) layered distal skarn ore (Cst II) containing the mineral assemblage cassiterite-pyrrhotite-chalcopyrite-actinolite; (3) vein cassiterite-sulfide ore (Cst III) distal from the skarn and associated granite containing the mineral assemblage cassiterite-arsenopyrite-pyrrhotite-muscovite-fluorite; and (4) veinlet Sn-Pb-Zn ore (Cst IV) distal from the skarn and associated granite containing the mineral assemblage cassiterite-galena-sphalerite-topaz-quartz. Here, we report the results of in situ laser ablation inductively coupled plasma mass spectrometric (LA-ICPMS) U-Pb age determinations for garnet from the Xianghualing skarn and the above four types of cassiterite. Our age determinations indicate that there were two independent magmatic-hydrothermal events at ~160 and 156~150 Ma, both of which led to Sn mineralization. The first Sn mineralization event at ~160 Ma (Cst IV U-Pb ages of 159.6 ± 1.4 to 158.5 ± 0.8 Ma) is interpreted to have been associated with a speculative unexposed granitic pluton, which is coeval with the nearby Jianfengling granite intrusion. The second Sn mineralization event at 156~150 Ma (Cst I to Cst III U-Pb ages of 155.9 ± 0.7 to 152.3 ± 1.1 Ma and garnet U-Pb ages of 153.6 ± 7.6 to 151.5 ± 3.5 Ma) is genetically related to the adjacent Laiziling granitic intrusion (152.8 ± 1.2 Ma, zircon U-Pb age). This event was responsible for the bulk of the Sn resource (>95%). Our age determinations provide convincing evidence for superimposed Jurassic Sn mineralizing systems at Xianghualing. They also show the value of combining garnet and cassiterite U-Pb age determinations to constrain the timing of skarn and Sn mineralization and distinguish discrete Sn mineralizing events in a protracted metallogenic history.

Metallogenic model of the Fengyan stratabound Zn-Pb deposit in Fujian of southeastern China: Constraints from fluid inclusions, C[sbnd]O[sbnd]S-Pb isotopes, and pyrite chemistry

The Fengyan stratabound Zn-Pb deposit (total resources: 24.7 Mt of ore @ 4.09 % Zn and 1.92 % Pb) is hosted by Neoproterozoic marble and metavolcanic rocks in the Meixian ore field of southeastern China. Hitherto, the genesis of this deposit has been a matter of debate, including a Neoproterozoic volcanogenic massive sulfide genesis (VMS) or an Early Cretaceous magmatic-hydrothermal origin. Based on fluid inclusion data from various hydrothermal stages, stable isotope data of hydrothermal minerals, and in-situ trace element analyses of poly-stage pyrite (Py1-3), the deposit can be classified as a distal skarn-type. A complex paragenesis reflects four hydrothermal stages: stage I (pyroxene + garnet + Py1), stage II (epidote + hematite + magnetite + Py2), stage III (quartz + sphalerite + galena + Py3a + Py3b), and stage IV (calcite). The prograde skarn comprises andradite-rich garnet (on average Ad57Gr34Py9) and pyroxene (on average Hd58Di10Jo32) with high Mn contents. Fluid inclusion data reveal salinity and temperature ranges for the prograde and retrograde skarn fluids as 11.5 to 14.6 wt% NaCl equiv. at 278 to 338℃ and 7.5 to 11.2 wt% NaCl equiv. at 235 to 275℃, respectively. The entrapment pressure is calculated as 130 bars. The sulfide ore minerals of stage III were precipitated at lower temperatures (186 to 259℃) from aqueous fluids with lower salinities (<6.6 wt% NaCl equiv.). Late-stage calcite was deposited from a more dilute (<2.2 wt% NaCl equiv.) and cooler (<200℃) fluid with extremely depleted δ18Ofluid values of −10.1 to −2.0 ‰ (V-SMOW), consistent with a meteoric origin. A mean δ34SV-CDT of 1.7 ± 0.9 ‰ of sulfide minerals points to a magmatic sulfur source. Lead isotopic compositions of sphalerite separates (206Pb/204Pb = 18.295–18.381, 207Pb/204Pb = 15.644–15.662, and 208Pb/204Pb = 38.494–38.724) are similar to those of the Yanshanian granitic bodies in the region but different from those of the ore-hosting marble and metavolcanic rocks. Calcite in the late stage has δ13CV-PDB values of −5.4 to −1.5 ‰, consistent with a predominant magmatic origin. Variations of Co, Ni, Se, and As in pyrite of different generations are interpreted to reflect changes in temperature during hydrothermal evolution. The Co/Ni ratios in all generations of pyrite, mainly between 1 and 10, indicate a magmatic-hydrothermal source. Overall, our new data indicate progressive mixing of a granite-derived magmatic fluid with meteoric waters, as is expected in a distal skarn deposit.

An Efficient and Economical Combination of Exploration Methods for Pb-Zn Polymetallic Skarn Deposits: A Case Study of the Periphery of Hetaoping Deposit, Yunnan Province, China

The Hetaoping ore district in Baoshan City, Yunnan Province, is one of the major localities of Pb-Zn polymetallic skarn deposits in China, where geophysical and geochemical surveys play an important role in exploring Pb-Zn polymetallic mineral resources. Based on the exploration and prospecting carried out at the periphery of the Hetaoping Pb-Zn polymetallic deposit, this study proposed an aero-ground joint exploration method to determine the metallogenic model of distal skarns in the Hetaoping ore district, achieving ideal prospecting results. The steps of this method are as follows. First, the locations of ore-induced anomalies were determined using high-amplitude aeromagnetic anomalies. Then, the ore-induced anomalies were determined to be anomalies of Pb-Zn polymetallic deposits through geochemical surveys of soil samples and ground geophysical surveys. Based on these data, a quantitative analysis and metallogenic potential assessment of ore bodies and their surrounding rocks were conducted using the interactive 2.5D magnetic inversion. In addition, the 3D inversion of regional gravity data was also performed in order to determine the spatial location of the deep magma chamber. Accordingly, the metallogenic geological process in this area was analyzed by determining the spatial morphology of the deep magma chamber, and a prospecting model of the Pb-Zn polymetallic deposits was finally built. The results show that the aero-ground joint exploration method, which first conducts a rapid scanning survey using the aeromagnetic method and then locates, distinguishes, and assesses significant aeromagnetic anomalies by combining comprehensive verification means such as ground geophysical, geochemical, and geological surveys, is efficient and economical. This study will guide regional metallogenic research and the exploration and prospecting of Pb-Zn polymetallic deposits.

Metal mobilization and precipitation in a Sn-W skarn system, Gejiu Sn district, China

In the Gejiu Sn-Cu polymetallic district, skarns with or without mineralization developed both at the contact zones between a granitic intrusion and carbonate-rich wall rocks and within carbonate-rich wall rocks. We analyzed the chemical and Sr, Nd, Pb, Li, and B isotopic compositions of mineralogically different zones in two hand specimens, i.e., a banded carbonate and a distal skarn with distinct reaction zones, to interpret chemical and isotopic variation in terms of (i) temporal compositional changes of skarn-forming fluids and (ii) reaction-induced exchange between fluids and carbonate wall rocks. The banded carbonates have seen little fluid-mediated exchange and therefore the compositions of the various bands reflect a two-component system of carbonates and detrital silicates. The compositional range of the banded carbonates represents the compositional heterogeneity of the protoliths of the distal skarn that experienced a pronounced impact by metasomatic fluids. The studied distal skarn is characterized by the vein-like aquifer (Zone I), reaction zones (Zone II and Zone III), and the carbonate aquitard (Zone IV). As evident from the distal skarn, the chemical and isotopic compositions of skarn-forming fluids change over time due to (i) processes in the fluid sources, (ii) variable contributions of different fluid sources, and (iii) reaction-induced exchange with the rocks bordering the aquifer, leading to selective addition or removal of elements from the fluid. During early interaction between aquifer fluids and its wall-rocks, the aquitard is partially consumed and the chemical and isotopic signature of the newly-formed minerals in the reaction zone represent a mixture of material derived from either fluids or wall rock. Once formed, the reaction zones largely protected the aquitard from further interaction with fluids. The Sr-budget of zones I to IV is dominated by Sr derived from the carbonate aquitard and redistributed during the consumption of older reaction zones. The Pb, Li, and B budgets of zones I to III are dominated by materials derived from the fluids. In contrast, the Nd isotopic compositions in zones II to III fall between the values of the carbonate aquitard and the aquifer. The ore elements occur in different zones, i.e., (i) scheelite, sphalerite, pyrite, and chalcopyrite precipitate in the aquifer and (ii) Sn substitutes in calc-silicates at the reaction front between zones II and III and is present in late fractures in Zone III. Thus, ore elements may have been transported by different skarn-forming fluids. The chemical and isotopic signatures of reaction zones do neither strictly reflect the nature of the reactive fluid nor reflect simple bulk mixing between fluid and wall rock, as (i) reaction-induced volume decrease may allow later fluids to infiltrate into earlier reaction zones, (ii) the fluid may be depleted in elements that have been scavenged from the fluid before, (iii) some elements having low compatibility in minerals of the reaction zones may have strongly partitioned into the fluid and are lost from the system. Although reaction zoning provides information about the nature of the reactive fluids, the mineralogically-controlled selective behavior of trace elements, i.e., most ore elements, does not allow to use the ore element content in skarn as a vector to mineralization.

A newly identified Permian distal skarn deposit in the Western Tianshan, China: New evidence from geology, garnet U[sbnd]Pb geochronology and S[sbnd]Pb[sbnd]C[sbnd]H[sbnd]O isotopes of the Arqiale Pb[sbnd]Zn[sbnd]Cu deposit

The Arqiale PbZnCu deposit is located in the Wusun Range, Western Tianshan, China. Prograde and retrograde skarn minerals are abundant and associated with the PbZnCu mineralization. Given that the orebodies are stratabound and hosted in limestone and causative intrusions were not identified, the genetic relationship between mineralization and magmatic hydrothermal activity remains controversial. Geology, in-situ garnet LA-ICP-MS UPb dating and SPbCHO isotopes were carried out in this study, aiming to constrain the mineralization age, source of ore-forming components, and examine the link between mineralization and magmatism. In-situ garnet LA-ICP-MS UPb dating yielded a well-defined lower-intercept age of 268.6 ± 5.9 Ma, reflecting that the mineralization occurred much younger than that of ore hosting strata and formed in a post-collisional setting. In-situ δ34S values of sulfides (chalcopyrite, sphalerite and pyrite) range from −5.81‰ to −7.57‰ with an average of −6.37‰. The δ34S values of sulfide separates published previously vary from −7.1‰ to 1.3‰ (average −1.5‰). The in-situ206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of galena vary from 18.262 to 18.269, 15.561 to 15.567 and 38.049 to 38.065, respectively. The aforementioned S-Pb isotopic compositions indicate that the metals and sulfur were sourced partially from causative intrusion and partially from hosing sedimentary rocks. The δ13C (-2.6 – −0.9‰), δ18OV-SMOW (3.2 – 7.7‰), δ18OH2O (-5.2 – −0.7‰) and δDH2O (-128.0 – −109.1‰) values of ore-bearing calcite suggest that the ore-forming fluids were initially sourced from magmatic water exsolved from the hidden causative intrusions, with increasing input of meteoric water in later time. These geological, geochemical and isotopic data confirm that Arqiale is a typical distal skarn PbZnCu deposit. Considering the chalcopyrite-garnet-ilvaite contents increasing and Zn/Pb rations decreasing from the shallow part in the south to the deeper part in the north, it is inferred that the hidden causative intrusions and Cu mineralization within contact zone may occur at depth in the northern part of Arqiale.

The fractionation of iron isotope and its constraints on the sources of ore-forming materials in the Jinchanghe skarn polymetallic deposit in Sanjiang region, Southwest China

金厂河矿床是西南三江成矿省保山地块最具代表性的远端矽卡岩型多金属矿床之一，查明其成矿金属来源对理解该类矿床成因以及区域成矿规律具有重要意义。本文通过分析不同成矿阶段代表性含铁矿物的铁同位素组成，探讨其在成矿过程中的分馏机制，从而示踪成矿金属的源区特征。金厂河矽卡岩型矿床中成矿前阶段未蚀变的石榴子石和氧化物成矿阶段的磁铁矿均相对富集铁的重同位素，其δ⁵⁶Fe值分别为0.05‰~0.16‰和0.07‰~0.18‰，而硫化物成矿阶段的黄铁矿和黄铜矿则相对富集铁的轻同位素，其δ⁵⁶Fe值分别为-0.12‰~0.17‰和-0.54‰~-0.38‰，整体显示出从高氧逸度的成矿前阶段向低氧逸度的硫化物成矿阶段演化过程中矿物δ⁵⁶Fe值逐渐降低的趋势，指示Fe³⁺富集铁的重同位素，Fe²⁺富集铁的轻同位素。同时，金厂河矿床各阶段矿物的δ⁵⁶Fe值均显著低于碳酸盐围岩，而接近全球花岗岩的δ⁵⁶Fe值，表明成矿的铁不是由围岩贡献，而是来自于隐伏的中酸性岩体。;Jinchanghe is a representative distal skarn polymetallic deposit in Baoshan block in the Sanjiang metallogenic province, finding out the sources of ore-forming metals is of great significance to understand the genesis of deposit and regional metallogeny. In this study, we analyzed the iron isotopic compositions of Fe-bearing minerals from different stages, to investigate the fractionation mechanism of iron in ore-forming process, and to trace the sources of ore-forming metals. The unaltered garnet from pre-ore stage and magnetite from oxide-ore stage in Jinchanghe skarn deposit are relatively rich in heavy isotopes of iron, with δ⁵⁶Fe values of 0.05‰~0.16‰ and 0.07‰~0.18‰, respectively. Whereas, the pyrite and chalcopyrite from sulfide-ore stage are relatively rich in light isotope, with δ⁵⁶Fe values of -0.12‰~0.17‰ and -0.54‰~-0.38‰, respectively. All results show that, from pre-ore stage of high oxygen fugacity to sulfide-ore stage of low oxygen fugacity, the δ⁵⁶Fe values of minerals gradually decrease, indicating that the heavy isotope of iron is enriched in Fe³⁺, but the light isotope of iron is enriched in Fe²⁺. Furthermore, the iron isotopes of minerals from different stages in Jinchanghe are significantly lower than that of carbonate wall rock, and close to that of global granite, implying that the ore-forming iron are not contributed by wall rock, but from the concealed granitic rocks.

Composition of garnet from the Yongping Cu skarn deposit, South China: Implication for ore-fluid evolution

Garnet typically occurs as a major constituent in skarn deposits. It commonly displays variations in texture and composition due to changing physicochemical conditions. In this study, we integrate textural and geochemical studies of garnet from the Yongping Cu skarn deposit (South China) to constrain the nature and evolution of the ore-forming fluids. In the Yongping deposit, garnet shows red or brown colors in proximal skarns, and shows green color in distal skarns. All types of garnet grains show a core-rim texture. The cores generally have higher proportions of Fe and lower proportions of Al than the rims. In red or grown garnet grains, the Fe-rich cores have relatively high contents of REE, typically with an upward-facing parabola pattern with a peak at Ce or Pr. The rim domains have lower contents of REE, in particular La and Ce, than the cores. The compositional differences between the cores and rims reflect a transition from oxidized, Fe3+- and REE-rich fluids to fluids with lower contents of Fe3+ and REE. In green garnet, the Fe-rich cores have obviously lower contents of REE than the red/brown garnet cores, and their REE patterns resemble those of ore-hosting marbles. It indicates that the distal fluids were relatively poor in REE, and partial REE in garnet may be sourced from the ore-hosting marbles. The rims of green garnet usually have obviously higher contents of REE than the cores, which may imply ingress of new pulses of REE-rich, magmatic-hydrothermal fluids during the garnet growth process. In both proximal and distal skarns, the prograde garnet grains may be partially replaced by Al-rich domains in the retrograde skarn alteration stage. The replaced domains generally have higher contents of F, Ti, HREE and lower contents of LREE than the original domains. It indicates that the metasomatic fluids were relatively reducing, and have higher contents of F, Ti and HREE than the earlier-stage fluids. In general, the composition of garnets can record multiple pluses of fluid activity and the complicated physic-chemical evolution paths of the fluids. The influence of wall-rocks on the compositions of minerals is also indicated. Thus, garnet from skarns can preserve a wealth of information regarding mineralization processes in their crystal growth stratigraphy.

Geology, mineralogy, and cassiterite geochronology of the Ayawilca Zn-Pb-Ag-In-Sn-Cu deposit, Pasco, Peru

The Ayawilca deposit in Pasco, Peru, represents the most significant recent base-metal discovery in the central Andes and one of the largest undeveloped In resources globally. As of 2018, it hosts an 11.7 Mt indicated resource grading 6.9% Zn, 0.16% Pb, 15 g/t Ag, and 84 g/t In, an additional 45.0 Mt inferred resource grading 5.6% Zn, 0.23% Pb, 17 g/t Ag, and 67 g/t In, and a separate Sn-Cu-Ag inferred resource of 14.5 Mt grading 0.63% Sn, 0.21% Cu, and 18 g/t Ag. Newly obtained U–Pb dates for cassiterite by LA-ICP-MS (22.77 ± 0.41 and 23.05 ± 2.06 Ma) assign the Ayawilca deposit to the Miocene polymetallic belt of central Peru. The polymetallic mineralization occurs as up to 70-m-thick mantos hosted by carbonate rocks of the Late Triassic to Early Jurassic Pucará Group, and subordinately, as steeply dipping veins hosted by rocks of the Pucará Group and overlying Cretaceous sandstones-siltstones of the Goyllarisquizga Group. Relicts of a distal retrograde magnesian skarn and cassiterite (stage pre-A) were identified in the deepest mantos. The volumetrically most important mineralization at Ayawilca comprises a low-sulfidation assemblage (stage A) with quartz, pyrrhotite, arsenopyrite, chalcopyrite, Fe-rich sphalerite, and traces of stannite and herzenbergite. Stage A sphalerite records progressive Fe depletion, from 33 to 10 mol% FeS, which is compatible with the observed transition from low- to a subsequent intermediate-sulfidation stage (B) marked by the crystallization of abundant pyrite and marcasite. Finally, during a later intermediate-sulfidation stage (C) sphalerite (up to 11 mol% FeS), galena, native bismuth, Cu-Pb-Ag sulfosalts, siderite, Mn-Fe carbonates, kaolinite, dickite, and sericite were deposited. This paragenetic evolution shows striking similarities with that at the Cerro de Pasco Cordilleran-type polymetallic deposit, even if at Ayawilca stage C did not reach high-sulfidation conditions. The occurrence of an early retrograde skarn assemblage suggests that the manto bodies at Ayawilca formed at the transition between distal skarn and skarn-free (Cordilleran-type) carbonate-replacement mineralization. Mineral assemblages define a T-fS2 evolutionary path close to the pyrrhotite-pyrite boundary. Buffering of hydrothermal fluids by underlying Devonian carbonaceous phyllites of the Excelsior Group imposed highly reduced conditions during stage A mineralization (logfO2 < − 30 atm). The low fO2 favored efficient Sn mobility during stages pre-A and A, in contrast to other known ore deposits in the polymetallic belt of central Peru, in which the occurrence of Sn minerals is minor. Subsequent cooling, progressive sealing of vein walls, and decreasing buffering potential of the host rocks promoted the shift from low- (stage A) to intermediate-sulfidation (stages B and C) states. LA-ICP-MS analyses reveal significant In contents in Fe-rich sphalerite (up to 1.7 wt%), stannite (up to 1908 ppm), and chalcopyrite (up to 1185 ppm). The highest In content was found in stage A sphalerite that precipitated along with chalcopyrite and stannite, thus pointing to the early, low-sulfidation assemblage as prospective for this high-tech metal in similar mineral systems. Indium was likely incorporated into the sphalerite crystal lattice via Cu+ + In3+ ↔ 2 Zn2+ and (Sn, Ge)4+ + (Ga, In)3+ + (Cu + Ag)+ ↔ 4 Zn2+ coupled substitutions. Indium incorporation mechanisms into the stannite and chalcopyrite crystal lattices remain unclear.