Fe Skarn Research Articles

Geochemistry of magnetite from Proterozoic Fe-Cu deposits in the Kangdian metallogenic province, SW China

Fe-Cu deposits in the Kangdian Fe-Cu metallogenic province, SW China, are hosted in Paleoproterozoic meta-volcanic-sedimentary sequences and are spatially associated with coeval mafic intrusions. Several well-known examples are the giant Lala, Dahongshan, and Yinachang deposits. They have a common paragenetic sequence of an early Fe-oxide stage associated with sodic alteration and a late Cu-sulfide stage associated with potassic-carbonate alteration. Magnetite dominates the Fe-oxide stage of these deposits but is also present in the Cu-sulfide stage of the Lala deposit. This study uses trace element compositions of magnetite to examine the nature and origin of the ore-forming fluids. The magnetite has variable concentrations of Ti, Al, Mg, Mn, Si, V, Cr, Ca, Co, Ni, Sc, Zn, Cu, Mo, Sn, and Ga, which are thought to have been controlled mainly by fluid compositions and/or intensive parameters (e.g., temperature and oxygen fugacity (fO2)). Fluid-rock interaction and coprecipitating mineral phases appear to be less important in controlling the magnetite compositions. Magnetite grains in the Fe-oxide stage of the Lala and Dahongshan deposits have comparable trace element compositions and were likely precipitated from chemically similar fluids. High Ni contents of magnetite in both deposits, coupled with previous isotopic data and the fact that the two deposits are spatially associated with coeval mafic intrusions, strongly suggest that the ore-forming fluids were genetically related to the mafic magmas that formed the intrusions. Magnetite grains in the Fe-oxide stage of the Yinachang deposit have much lower V and Ni but higher Sn and Mo contents than those of the Lala and Dahongshan deposits and are thus thought to have precipitated from more oxidized and Mo-Sn-rich fluids that may have evolved from relatively felsic magmas. Magnetite grains from the Cu-sulfide and Fe-oxide stages of the Lala deposit are broadly similar in composition, but those in the Cu-sulfide stage have slightly higher Cu, Zn, and Mn and are thought to have crystallized from relatively low-temperature and Cu-Zn-Mn-rich fluids evolved from the fluids of the early Fe-oxide stage. Our results show that magnetite from the Fe-Cu deposits in the Kangdian Province, banded iron formation, Fe skarn deposits, diabase-hosted hydrothermal Fe deposits, and magmatic deposits has significantly different compositions. We propose that covariations of Co-Ni, Zn-Sn, and Co/Ni-Mn can be used to effectively discriminate different deposit types.

Geology, Mineralogy and Geochemistry of the Shinyemi Iron Deposit, Korea: Implications for a Genetic Model

The Shinyemi iron deposit, one of the biggest Fe deposits in South Korea, is proximal polymetallic deposits along the contact between the Cambro- Ordovician Makgol formation of the Joseon Supergroup and the Cretaceous granitoid. According to spatial location of the ore bodies, it can be divided by eastern Pb-Zn-Mo orebody and western Fe-Mo orebody. In recent boring data, these Shinyemi granitoid was intruded along gentle slope of Makgol carbonates and proximal iron orebody formed at both upper and lower part of the subvolcanic intrusives. The lower (eastern) orebody was transformed from the proximal Fe skarn at the bottom to a transitional Zn-CuMo skarn at the top. On the other hand, the upper (western) orebody exhibits predominant Fe-mineralization with brecciation. The aims of this study are to present and document characteristics of related igneous rocks and to discuss the evolution of skarn and ore formation model in the Shinyemi polymetallic mineralogical deposits.

Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS

The El Laco magnetite deposits consist of more than 98 % magnetite but show field textures remarkably similar to mafic lava flows. Therefore, it has long been suggested that they represent a rare example of an effusive Fe oxide liquid. Field and petrographic evidence, however, suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs. We determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from various settings at El Laco and compared them with magnetite from both igneous and hydrothermal environments. This new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite “lava flows” formed. The trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite, including primary magnetite phenocrysts from the unaltered andesite host rocks at El Laco. Instead, the El Laco magnetite is most similar in composition to hydrothermal magnetite from high-temperature environments (>500 °C), such as iron oxide-copper-gold (IOCG) and porphyry-Cu deposits. The magnetite trace elements from massive magnetite are characterised by (1) depletion in elements considered relatively immobile in hydrothermal fluids (e.g. Ti, Al, Cr, Zr, Hf and Sc); (2) enrichment in elements that are highly incompatible with magmatic magnetite (rare earth elements (REE), Si, Ca, Na and P) and normally present in very low abundance in magmatic magnetite; (3) high Ni/Cr ratios which are typical of magnetite from hydrothermal environments; and (4) oscillatory zoning of Si, Ca, Mg, REE and most high field strength elements, and zoning truncations indicating dissolution, similar to that formed in hydrothermal Fe skarn deposits. In addition, secondary magnetite in altered, brecciated host rock, forming disseminations and veins, has the same composition as magnetite from the massive lenses. Euhedral magnetite lining both open-spaced veins in the brecciated host rock and along the walls of large, hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and/or physio-chemical conditions of the fluid. Thus, the chemical fingerprint of magnetite from the supposed El Laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows, by dissolution and precipitation of magnetite from high-temperature fluids, rather than a magmatic origin from an effusive Fe oxide liquid.

In-situ LA–ICP-MS trace elemental analyses of magnetite: The Mesozoic Tengtie skarn Fe deposit in the Nanling Range, South China

The Nanling Range in South China hosts numerous world-class W–Sn deposits and some Fe deposits. The Mesozoic Tengtie Fe skarn deposit in the southern Nanling Range is contemporaneous with the regional Sn mineralization. The deposit is composed of numerous ore bodies along the contacts between the late Paleozoic or Mesozoic carbonate rocks and the Yanshanian Lianyang granitic complex. Interaction of the magma with hosting dolomitic limestone and limestone formed calcic (Ca-rich) and magnesian (Mg-rich) skarns, respectively. The Tengtie deposit has a paragenetic sequence of the prograde stage of anhydrous skarn minerals, followed by the retrograde stage of hydrous skarn minerals, and the final sulfide stage. Magnetite in the prograde and retrograde skarn stages is associated with diopside, garnet, chlorite, epidote, and phlogopite, whereas magnetite of the final stage is associated with chalcopyrite and pyrite. Massive magnetite ores crosscut by quartz and calcite veins are present mainly in the retrograde skarn stage. Laser ablation ICP-MS was used to determine trace elements of magnetite from different stages. Some magnetite grains have unusually high Ca, Na, K, and Si, possibly due to the presence of silicate mineral inclusions. Magnetite of the prograde stage has the highest Co contents, but that of the sulfide stage is extremely poor in Co which partitions in sulfides. Magnetite of magnesian skarns contains more Mg, Mn, and Al than that of calcic skarns, attributed to the interaction of the magma with compositionally different host rocks. Magnetite from calcic and magnesian skarns contains 6–185ppm Sn and 61–1246ppm Sn, respectively. The high Sn contents are not due to the presence of cassiterite inclusions which are not identified in magnetite. Instead, we believe that Sn resides in the magnetite structure. Regionally, intensive Mesozoic Sn mineralization in South China indicates that concurrent magmatic–hydrothermal fluids may be rich in Sn and contribute to the formation of high-Sn magnetite. Our study demonstrates that trace elements of magnetite can be a sensitive indicator for the skarn stages and wall-rock compositions, and as such, trace elemental chemistry of magnetite can be a potentially powerful fingerprint for sediment provenance and regional mineralization.

Erratum to: Re–Os isotope evidence for mixed source components in carbonate-replacement Pb–Zn–Ag deposits in the Lavrion district, Attica, Greece

The Lavrion ore district contains carbonate-replacement and vein-type Pb–Zn–Ag deposits as well as low-grade porphyry Mo, Cu–Fe skarn, and minor breccia-hosted Pb–Zn–Cu sulfide mineralization. These ore types are spatially related to a Late Miocene granodiorite intrusion (7 to 10 Ma), and various sills and dikes of mafic to felsic composition. Samples of sphalerite and pyrite from the Ilarion carbonate replacement deposit, and galena from Vein 80 (vein-type mineralization) in the Adami deposit show heterogeneous Re–Os values. These values were partially disturbed by hydrothermal activity associated with the formation of hydrothermal veins (e.g., Vein 80). A plot of initial 187Os/188Os versus 1/Oscommon ratios for pyrite and sphalerite from the Ilarion deposit form a mixing line (r2 = 0.78) between high concentration crustal-like and low concentration mantle-like end-members, or two crustal end-members one of which was more radiogenic than the other. Based on the Re–Os systematics and previously published geological and geochemical evidence, the most plausible explanation for the Re–Os isotope data is that ore-forming components were derived from mixed sources, one of which was a radiogenic crustal source from schists and carbonates probably near intrusion centers and the other, intrusive rocks in the district that are less radiogenic. Although the Re and Os concentrations of galena from Vein 80 are above background values they cannot be used as a chronometer. However, the results of the current study suggest that although pyrite, sphalerite, and galena are poor geochronometers in this ore deposit, due to partial open-system behavior, they still yield valuable information on the origin of the source rocks in the formation of bedded replacement and vein mineralization in the Lavrion district.

Late Cretaceous to Middle Eocene Magmatism and Metallogeny of a Portion of the Southeastern Anatolian Orogenic Belt, East-Central Turkey

Rapidly changing ore deposit types in the Late Cretaceous and early Tertiary in the Southeastern Anatolian orogenic belt in east-central Turkey, from Baskil on the south to Divrigi on the north, broadly reflect shifting tectonic environments in an evolving collisional orogen, a part of the extensive Tethyan orogen. Late Cretaceous (~83–79 Ma) subduction-related arc magmatism in the Baskil arc was followed by extension that exhumed the metamorphic basement, unroofed the obducted ophiolites, and filled basins with Maastrichtian to Paleocene subaerial and submarine sedimentary rocks. Calc-alkaline (Divrigi pluton, ~76–73 Ma) and alkaline igneous rocks (Hasancelebi and Keban districts, ~76–74 Ma) accompanied post-Baskil arc extension. In the early to middle Eocene, shallow and locally voluminous intrusions of intermediate composition (Copler, Kabatas, Bizmisen-Calti, Dedeyazi-Polat, Karamadazi, and Horoz), localized mainly along major strike-slip faults, formed during closure of the Neotethyan Ocean. These intrusions are dated between ~50 and 44 Ma, peaking at 48 Ma. Geochemical data from a suite of igneous rock samples across the transect reflect temporally based changes from arc-type to late orogenic environments, as demonstrated by Late Cretaceous rocks in the Baskil area on the south and the more deeply eroded Late Cretaceous plutons in the Divrigi area on the north (Divrigi). These magmatic products were followed in the Eocene by calc-alkaline magmatism related to either incipient slab-rupture or crustal-scale faults that cut from northeast (Copler, Kabatas, Bizmisen-Calti, and Polat-Cavuslu) to southwest (Karamadazi and Horoz) obliquely across the transect. The multielement patterns for the Late Cretaceous magmatic rocks resemble the patterns for magmas generated in enriched source regions compared to primitive mantle and mid-oceanic-ridge basalt. Locally, at Divrigi-Hasancelebi and Keban, the geochemical characteristics are suggestive of the influx of low field strength elements into the mantle source regions from a subducted slab, and an arc to postcollisional to late-orogenic setting for their generation. Eocene magmatic rocks display similar geochemical characteristics regardless of the geographic setting and associated hydrothermal system. Their characteristics favor a subduction zone enrichment that was inherited either from an older event, or from an older slab, or from a mantle wedge contaminated by a contemporaneous subduction component. 40Ar/39Ar and U/Pb geochronology on biotite, zircon, K-feldspar, hornblende, and sericite from a suite of magmatic rocks and alteration zones defines three magmatic-related metallogenic events in the Southeastern Anatolian orogenic belt between 78 and 44 Ma. The oldest metallogenic event between ~78 to 74 Ma is related to arc-type calc-alkaline magmatic rocks (83–79 Ma) intruded into the Bitlis-Poturge and Malatya-Keban metamorphics and overlying Komurhan-Ispendere ophiolites of Tauride platform. Uneconomic porphyry-type Cu-Au systems formed at Topalkem-Baskil (Elazig) and Ispendere-Sisman (Malatya). A second metallogenic event, between ~74 and 69 Ma, is contemporaneous with postcollisional exhumation and extension-related magmatism. During this event, calc-alkaline followed by alkaline magmatism (76–73 Ma) was accompanied by formation of iron oxide copper-gold (IOCG) systems at ~74 to 73 Ma at Divrigi (Sivas). Alkaline magmatism at ~76 to 74 Ma is associated with IOCG systems at Hasancelebi (Malatya) and porphyry Cu-Mo and Pb-Zn veins at Keban (Elazig). The youngest metallogenic event between ~50 and 40 Ma is represented by porphyry Cu-Au deposits associated with calc-alkaline intrusive complexes at Copler and Kabatas and Fe skarns at Karamadazi, Horoz, Dedeyazi, and Bizmisen-Calti.

The mineralogy and petrology of the Pahnavar Fe skarn, In the Eastern Azarbaijan, NW Iran

Abstract The Pahnavar calcic Fe-bearing skarn zone is located in the Eastern Azarbaijan (NW Iran). This skarn zone occurs along the contact between Upper Cretaceous impure carbonates and an Oligocene granodioritic batholith. The skarnification process can be categorized into two discrete stages: prograde and retrograde. The prograde stage began immediately after the initial emplacement of the granodioritic magma into the enclosing impure carbonate rocks. The effect of heat flow from the batholith caused the enclosing rocks to become isochemically marmorized in the pure limestone layers and bimetasomatized (skarnoids) in the impure clay-rich carbonates. Segregation and evolution of an aqueous phase from the magma that infiltrated to the marbles and skarnoids through fractures and micro-fractures took place during the emplacement of magma. The influx of Fe, Si and Mg from the granodiorite to the skarnoids and marbles led to the crystallization of anhydrous calc-silicates (garnet and pyroxene). The retrograde stage can be divided, in turn, into two distinct sub-stages. During earliest sub-stage, the previously formed skarn assemblages were affected by intense hydro-fracturing; in addition, Cu, Pb, Zn, along with H2S and CO2 were added. Consequently, hydrous calc-silicates (epidote and tremolite-actinolite), sulfides (pyrite, chalcopyrite, galena and sphalerite), oxides (magnetite and hematite) and carbonates (calcite) deposited the anhydrous calc-silicates. The late-retrograde sub-stage was due the incursion of colder oxidizing fluids into the skarn system, causing the alteration of the previously formed calc-silicate assemblages and the development of fine-grained aggregates of chlorite, illite, kaolinite, hematite and calcite. The lack of wollastonite in the mineral assemblage, along with the garnet-clinopyroxene paragenesis, suggests that the prograde stage formed under temperature and fO2 conditions of 430–550°C and 10−26–10−23, respectively.

Multiple Mesozoic mineralization events in South China—an introduction to the thematic issue

Mesozoic mineral deposits in South China include world-class deposits of W, Sn and Sb and those that provide the major sources of Ta, Cu, Hg, As, Tl, Pb, Zn, Au and Ag for the entire country. These deposits can be classified into polymetallic hydrothermal systems closely related to felsic intrusive rocks (Sn–W –Mo granites, Cu porphyries, polymetallic and Fe skarns, and polymetallic vein deposits) and low-temperature hydrothermal systems with no direct connection to igneous activities (MVT deposits, epithermal Au and Sb deposits). Recent studies have shown that they formed in the Triassic (Indosinian), Jurassic–Cretaceous (Early Yanshanian), and Cretaceous (Late Yanshanian) stages. Indosinian deposits include major MVT (Pb–Zn–Ag) deposits and granite-related W–Sn deposits. Early Yanshanian deposits are low-temperature Sb–Au and high-temperature W–Sn and Cu porphyry types. Many Late Yanshanian deposits are low-temperature Au–As–Sb–Hg and U deposits, and also include high-temperature W–Sn polymetallic deposits. The formation of these deposits is linked with a specific tectonothermal evolution and igneous activities. This special issue brings together some of the latest information in eight papers that deal with the origins and tectonic environments of mineral deposits formed in these stages. We anticipate that this issue will stimulate more interests in these ore deposits in South China.

Petrogenetic–metallogenetic setting and temporal–spatial framework of the Yueshan district, Anhui Province, east-central China

The Yueshan district is located in the Anqing–Guichi ore deposit area of the Middle–Lower Yangtze Metallogenic Belt. Two groups of intrusive rocks and three main types of ore mineralization occur in this district: diorite plutons (e.g. Yueshan, Zongpu, Wuheng, and Yangshan) and granite plutons (Hongzhen and Dalongshan), Cu–Au–(Fe) skarn deposits (e.g. Anqing, Tiepuling), Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits (Tongniujing), and hydrothermal uranium mineralization (Dalongshan). Detailed geological and geochemical work suggests that the Cu–Au–(Fe) skarn deposits and the Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits have a close spatial and genetic relationship with the dioritic plutons, whereas the hydrothermal uranium mineralization is associated with A-type granite plutons. Based on the highly precise dating of metal deposits and related plutons in the Yueshan district, such as the molybdenite Re–Os, Os–Os dating, 39Ar–40Ar dating of potassium-bearing minerals and quartz, several Rb–Sr isochrons, SHRIMP zircon U–Pb dating + single-grain zircon U–Pb dating, and the SHRIMP zircon U–Pb dating of Hongzhen granite pluton, we suggest that the extensive magmatism and mineralization in the Yueshan district took place in two episodes: (1) the first episode involved the mineralization of both skarn and vein-type hydrothermal deposits, c.a. 136–139 Ma, related to diorite plutons emplaced at 138.7 ± 0.5 Ma; (2) the second episode attended the hydrothermal uranium mineralization at 106.4 ± 2.9 Ma, related to granite intrusive activity at 126.8 ± 1.0 Ma. These two times of Yueshan petrogenetic–metallogenetic development appear to be consistent with a tectonic environment transition from compression to extension.

Geology, petrography and geochemistry of igneous rocks related to mineralized skarns in the NW Neuquén basin, Argentina: Implications for Cordilleran skarn exploration

Five mineralized Cu, Au, and Fe skarns and related igneous rocks of the Andes Cordillera of NW Neuquén (Paleogene Campana Mahuida, Caicayén and Cerro Nevazón) and SW Mendoza (Neogene Vegas Peladas and Hierro Indio) provinces of Argentina (34–38°S) are reviewed to demonstrate that geochemical signatures of igneous rocks can be used to predict the metal potential of skarn prospects of the Andes Cordillera of Argentina. These igneous rocks are calc-alkaline, metaluminous, and derived from a sub-arc mantle source without residual garnet. They were emplaced at shallow depths. The Vegas Peladas, Hierro Indio, and Cerro Nevazón igneous rocks have similar major and trace element contents, and are typical of primitive, I-type plutons associated with Fe skarns worldwide. The Vegas Peladas and Hierro Indio plutons are, however, less reduced and they have lower Ni concentration than the Cerro Nevazón rocks, whose skarns have higher Au concentrations. In the Caicayén district, small skarns with sub-economic Cu concentrations and abundant pyrite are associated with porphyry copper style alteration in igneous rocks that underwent amphibole fractionation. Their chemical compositions are intermediate between the most primitive plutons associated with Fe skarns and the most evolved plutons associated with Cu skarns, both with high fO 2. At Campana Mahuida a small porphyry copper deposit and associated Cu skarns are linked to andesite dikes and pluton with similar SiO 2 contents but richer in incompatible trace elements (K, Rb, Sr, Ba, La, Ce, and Th) than the Mendoza, Cerro Nevazón and Caicayén igneous rocks. This suggests that their parental magmas evolved in an open system and were contaminated with crustal material, thus resulting in the strongest fractionation of amphibole and accessories phases. These intrusions share many features with typical Cu skarns-related plutons.

Suture zones of the Urals as integral prospective ore-bearing tectonic structures

Rift-related (1.2 Ga) and collision (380–240 Ma) suture zones of the Urals are described. The riftrelated suture zones comprise an ultramafic-gabbro complex with titanomagnetite mineralization, an ultramafic complex with chromite mineralization, and a complex of alkali granitoids with rare-metal (including REE) mineralization accompanied by K-feldspathites, albitites, and calcite metasomatic rocks. The collision suture zones are distinguished by early collision granitoids specialized for tungsten (scheelite) and gold, as well as by raremetal granites and such derivatives of them as pegmatite and greisen with rare-metal and colored-stone mineralization. The suture zones are characterized by long-term (up to 80 Ma or more) continuous-discontinuous periods of ore deposition; heterogeneous sources of ore matter and ore-bearing fluids; a polyelemental composition of lithogeochemical halos and an integral mineral composition of altered wall rocks; and the occurrence of mafic, intermediate, and felsic dikes at large gold deposits, as well as wide variations in PT parameters of the ore-forming process: T = 620−150°C and P = 3.2−0.6 kbar. Collision played a dual role in ore formation. On the one hand, collision led to deformation and metamorphism of precollision massive sulfide deposits and, to a lesser degree, Au-bearing Fe and Cu skarn and porphyry copper deposits, and, on the other hand, to the formation of new gold, rare-metal, quartz, colored-stone, talc, muscovite, and noble serpentine deposits. As a rule, this polygenetic mineralization differs in age and is related to collision volcanic and plutonic complexes. This diversity can be a good basis for metallogenic analysis, forecasting, and prospecting of various metallic deposits and industrial minerals. Polygenetic mineralization of various age known in suture zones is accompanied by integral lithogeochemical and metasomatic halos characterized by a continuous-discontinuous history. The complexity of ore mineralization and often nonlinear metallogeny increase the practical value of suture zones.

Iron Skarns of the Vegas Peladas District, Mendoza, Argentina

The Andean belt southwest of Mendoza, Argentina, hosts 23 Fe, Fe-Cu, and Cu (Ag) deposits classified variously in the literature as skarn, iron oxide-copper-gold (IOCG), and manto-type Cu deposits. The Vegas Peladas deposit is one of the best exposed Fe skarns with mineral assemblages and hydrothermal features similar to many other calcic Fe skarns of the world. The plutonic rocks of Vegas Peladas consist of a series of diorite to granite stocks, dikes, and sills. The major-, trace-, and rare earth-element geochemistry analyses of these igneous rocks indicate they were derived from subarc mantle sources. The Vegas Peladas deposit formed by the overprinting of two different metamorphic and metasomatic events associated with early diorite and later granite intrusions. Alteration associated with the early diorite intrusion consists of a metamorphic halo (800 m wide) and a zoned calcic skarn with inner garnet (Ad31–89 Py0–2) + clinopyroxene + magnetite + quartz, intermediate garnet (Ad38–51 Py1–2) ± clinopyroxene, and distal veins of garnet (Ad92–100) ± clinopyroxene (Hd72–29 Jo1–4). The latest alteration consists of widespread albite (Ab96–98) ± epidote ± quartz ± calcite ± chlorite ± pyrite ± titanite. Magnetite and hematite are the main iron ore minerals and occur as massive orebodies and veins associated with retrograde epidote and amphibole. Alteration of the diorite consists of early orthoclase + quartz followed by later amphibole ± quartz ± magnetite ± epidote ± feldspar. The granite-related skarn overprints the earlier diorite-related skarn and consists of garnet + clinopyroxene + scapolite (Me28–36) ± quartz ± alkali feldspar endoskarn and a zoned exoskarn with proximal garnet ± clinopyroxene ± quartz, intermediate green garnet (Ad30–81 Py0–1) + clinopyroxene (Di82–93 Jo4–2), and distal scapolite (Me25–36) ± ferroactinolite ± pyrite veins. Based on fluid inclusion, stable isotope, and REE data, the prograde skarn formed at depths of ~ 3.5 km under lithostatic pressure of ~1 kbar, from high temperature (670°–400°C), saline and iron-rich (>50 wt % NaCl equiv, NaCl ± KCl ± FeCl2) magmatic fluids (garnet δ18OH2O = 7.2–8.5‰) with intermediate oxygen fugacity. Iron ore and retrograde exoskarn assemblages formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (T 550°C) and also generated saline (30.3 to 45.3 wt % NaCl equiv, H2O-NaCl-FeCl2) + vapor fluids by immiscibility that redistributed some of the iron from the previous skarn.

Garnetization as a ground preparation process for copper mineralization: evidence from the Mazraeh skarn deposit, Iran

The Mazraeh Cu–Fe skarn deposit, NW Iran is the result of the intrusion of an Oligocene–Miocene granitic pluton into Cretaceous calcareous rocks. The pluton ranges in composition from monzonite to quartz monzonite, monzogranite, tonalite and granodiorite with I-type, calc-alkaline, and weakly peraluminous characteristics. The Mazraeh pluton was emplaced in a volcanic arc setting in an active continental margin at a depth of ~8 km. Pyroxene skarn, garnet skarn, and epidote skarn zones were formed during the intrusive phase. The garnet skarn developed as exoskarn and endoskarn from the calcareous wall rocks and the pluton, respectively, prior to mineralization. Garnet skarn from the exoskarn zone is identified by relict layering inherited from the precursor calcareous lithologies. Mass balance calculation of garnet skarn in the endoskarn zone indicates that hydrothermal fluids originating from the cooling magma introduced Si, Fe, Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, and Y. The main mass loss in the garnet skarn was due to destruction of feldspars in the Mazraeh plutonic rocks and leaching of K2O and Na2O. Released Ca has been fixed in the andraditic garnet. Garnetization of the Mazraeh pluton was accompanied by mass and volume increase. The magnitude of these changes depends mainly on the degree of alteration and composition of the precursor. The brittle behavior of the endoskarn zone was increased due to formation of massive garnet which subsequently fractured. These fractures not only facilitated movement of hydrothermal fluids but also provided new locations for Cu mineralization. Therefore locating strongly garnetized zones may be a vector to ore in skarn deposits.

Fe Skarn, Iron Oxide Cu-Au, and Manto Cu-(Ag) Deposits in the Andes Cordillera of Southwest Mendoza Province (34 -36 S), Argentina

Several Fe, Fe-Cu, and Cu-Ag prospects hosted by Mesozoic carbonate-rich sedimentary rocks that were intruded by diorite stocks define an approximately 20 × 200 km belt along the Andes Cordillera of southwest Mendoza province, Argentina (34°–36°S). This belt includes the Hierro Indio and Vegas Peladas Fe skarns, Las Choicas, and several other Cu-(Ag) prospects. The main features of the Fe skarns include: association with mantle-derived middle Miocene (~15–11 Ma) diorite stocks and sills; widespread alteration including epidote ± amphibole ± magnetite endoskarns, and zoned garnet (Grs0–66 Adr32.5–100) ± magnetite ± pyroxene (Di24–50 Jo2–9 Hd74–41) exoskarns formed from oxidized, saline, high-temperature brines (530°–660°C; 60–70 wt.% NaCl equiv.); and magnetite-hematite orebodies associated with quartz ± epidote ± calcite ± actinolite formed at lower temperatures (290°–436°C) from saline fluids (32–50 wt.% NaCl equiv.) of magmatic origin. West of the Fe skarn belt, the Valle Hermoso district covers more than 300 km2 and hosts several poorly known Cu-(Ag) prospects (estimated 30 Mt with 1.5% Cu and 20 g/t Ag). These prospects share many characteristics with the iron oxide copper-gold-type systems and manto-type Cu deposits of northern and central Chile. One of the prospects (Las Choicas) is located at the apex of a regional 015°-trending anticline, along the contact of stratified calcareous sandstone and limestone (Neocallovian-Oxfordian) with a diorite pluton. This pluton has positive Ta and Nb anomalies (on normalized diagrams) and a high Ti content, features uncommon in Miocene diorites associated with Fe skarns and of magmas derived from sub-arc mantle sources. Alteration at Las Choicas includes early widespread chlorite + calcite ± albite ± scapolite ± ilmenite or titanite ± apatite, and local actinolite ± hematite ± biotite alteration of diorite and sedimentary rocks along contacts. Later Cu mineralization (1%–10% Cu) is associated with calcite ± albite ± quartz alteration in fractures, vein networks, and crackle breccias. Mineralization is zoned from bornite ± chalcopyrite ± millerite with Zn, As, Mo, Ag, and U anomalies in the main orebody, through chalcopyrite-rich breccias with Zn anomalies, and chalcopyrite ± tetrahedrite vein networks with Sb, As, Ag, and Zn anomalies in the uppermost zone, to a Cu-depleted pyrite-rich envelope with Co anomalies, and finally to late, barren calcite veins in distal zones.

Geology and Chemistry of the Low Ti Magnetite-bearing Heff Cu-Au Skarn and its Associated Plutonic Rocks, Heffley Lake, South-Central British Columbia

Research Article| July 01, 2007 Geology and Chemistry of the Low Ti Magnetite-bearing Heff Cu-Au Skarn and its Associated Plutonic Rocks, Heffley Lake, South-Central British Columbia G.E. Ray; G.E. Ray † 12234 McNeill Avenue, Victoria, British Columbia, V8S 2Y7. †Corresponding Author: E-mail: geray@shaw.ca Search for other works by this author on: GSW Google Scholar I.C.L. Webster I.C.L. Webster 2B.C. Geological Survey, P.O. Box 9320, Victoria,British Columbia, V8W 9N3. Search for other works by this author on: GSW Google Scholar Exploration and Mining Geology (2007) 16 (3-4): 159–186. https://doi.org/10.2113/gsemg.16.3-4.159 Article history received: 02 Nov 2006 accepted: 29 Apr 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation G.E. Ray, I.C.L. Webster; Geology and Chemistry of the Low Ti Magnetite-bearing Heff Cu-Au Skarn and its Associated Plutonic Rocks, Heffley Lake, South-Central British Columbia. Exploration and Mining Geology 2007;; 16 (3-4): 159–186. doi: https://doi.org/10.2113/gsemg.16.3-4.159 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyExploration and Mining Geology Search Advanced Search Abstract The magnetite-rich Heff Cu-Au skarn lies in the Quesnel Terrane of south-central British Columbia, approximately 26 km northeast of Kamloops. The skarn formed close to the northern margin of the Heffley Creek pluton, and is hosted by Carnian (Late Triassic) Nicola Group limestone. The Late Triassic pluton (208.1 ± 6.1 Ma, U-Pb zircon) is a composite body of alkaline affinity that includes dioritic, gabbroic, and magnetite-rich clinopyroxene ± olivine ultramafic phases.The skarn has been discontinuously traced along strike for more than 1 km, and individual skarn layers are up to 40 m thick. The most common non-opaque minerals are garnet, clinopyroxene, and carbonate, with lesser epidote, biotite, amphibole, and chlorite. Apatite locally forms more than 3 vol.% of the rock. Compositionally the garnet is a low Mn grandite averaging Pyr3–Grs41–Adr56 mol.%.Mineralization consists of lenses, up to 10 m thick, containing massive low Ti-V magnetite (avg. 0.03 wt.% TiO2 and 0.01 wt.% V2O3). Magnetite is associated with up to 20 vol.% pyrrhotite, lesser pyrite, and trace chalcopyrite and gold. There is a moderate enrichment in rare earth elements with assays up to 570 ppm La and 490 ppm Ce.The Heffley Creek pluton belongs to a suite of Late Triassic–Early Jurassic alkalic and Fe-rich intrusions that are widely developed throughout the Quesnel Terrane. Some host Cu-Au porphyry mineralization, but less commonly they are associated with magnetite-apatite veins such as at the Glen Iron mine and Magnet occurrence, or with magnetite-rich Cu-Au skarns such as the Heff occurrence.Although abundant low Ti magnetite is present in the Heff skarn, it does not appear to represent iron oxide copper-gold (IOCG) type mineralization. Microprobe analyses completed during this study show that low Ti magnetite is not a unique feature of IOCG deposits, but also characterizes some other types of hydrothermal mineralization. In British Columbia these include some alkalic porphyry-related magnetite-apatite veins in the Quesnel Terrane, as well as the well-known Wrangellia-hosted Fe skarn deposits. Thus, low Ti magnetite appears to be a common signature of many magmatic hydrothermal deposits in contrast to the high Ti primary magnetite found in most igneous rocks. You do not currently have access to this article.

Nature, diversity of deposit types and metallogenic relations of South China

The South China Region is rich in mineral resources and has a wide diversity of deposit types. The region has undergone multiple tectonic and magmatic events and related metallogenic processes throughout the earth history. These tectonic and metallogenic processes were responsible for the formation of the diverse styles of base and precious metal deposits in South China making it one of the resource-rich regions in the world. During the Proterozoic, the South China Craton was characterised by rifting of continental margin before eruption of submarine volcanics and development of platform carbonate rocks, and the formation of VHMS, stratabound copper and MVT deposits. The Phanerozoic metallogeny of South China was related to opening and closing of the Tethyan Ocean involving multiple orogenies by subduction, back-arc rifting, arc–continent collision and post-collisional extension during the Indosinian (Triassic), Yanshanian (Jurassic to Cretaceous) and Himalayan (Tertiary) Orogenies. The Late Palaeozoic was a productive metallogenic period for South China resulting from break-up and rifting of Gondwana. Significant stratabound base and precious metal deposits were formed during the Devonian and Carboniferous (e.g., Fankou and Dabaoshan deposits). These Late Palaeozoic SEDEX-style deposits have been often overprinted by skarn systems associated with Yanshanian magmatism (e.g., Chengmenshan, Dongguashan and Qixiashan). A number of Late Palaeozoic to Early Mesozoic VHMS deposits also developed in the Sanjiang fold belt in the western part of South China (e.g., Laochang and Gacun). South China has significant sedimentary rock-hosted Carlin-like deposits, which occur in the Devonian- to Triassic-aged accretionary wedge or rift basins at the margin of the South China Craton. They are present in a region at the junction of Yunnan, Guizhou, and Guangxi Provinces called the ‘Southern Golden Triangle’, and are also present in NW Sichuan, Gansu and Shaanxi, in an area known as the ‘Northern Golden Triangle’ of China. These deposits are mostly epigenetic hydrothermal micron-disseminated gold deposits with associated As, Hg, Sb + Tl mineralisation similar to Carlin-type deposits in USA. The important deposits in the Southern Golden Triangle are Jinfeng (Lannigou), Zimudang, Getang, Yata and Banqi in Guizhou Province, and the Jinya and Gaolong deposits in Guangxi District. The most important deposits in the Northern Golden Triangle are the Dongbeizhai and Qiaoqiaoshang deposits. Many porphyry-related polymetallic copper–lead–zinc and gold skarn deposits occur in South China. These deposits are related to Indosinian (Triassic) and Yanshanian (Jurassic to Cretaceous) magmatism associated with collision of the South China and North China Cratons and westward subduction of the Palaeo-Pacific Plate. Most of these deposits are distributed along the Lower to Middle Yangtze River metallogenic belt. The most significant deposits are Tonglushan, Jilongshan, Fengshandong, Shitouzui and Jiguanzui. Au–(Ag–Mo)-rich porphyry-related Cu–Fe skarn deposits are also present (Chengmenshan and Wushan in Jiangxi Province and Xinqiao, Mashan-Tianmashan, Shizishan and Huangshilaoshan in Anhui Province). The South China fold belt extending from Fujian to Zhejiang Provinces is characterised by well-developed Yanshanian intrusive to subvolcanic rocks associated with porphyry to epithermal type mineralisation and mesothermal vein deposits. The largest porphyry copper deposit in China, Dexing, occurs in Jiangxi Province and is hosted by Yanshanian granodiorite. The high-sulphidation epithermal system occurs at the Zijinshan district in Fujian Province and epithermal to mesothermal vein-type deposits are also found in the Zhejiang Province (e.g., Zhilingtou). Part of Shandong Province is located at the northern margin of the South China Craton and the province has unique world class granite-hosted orogenic gold deposits. Occurrences of Pt–Pd–Ni–Cu–Co are found in Permian-aged Emeishan continental flood basalt (ECFB) in South China (Jinbaoshan and Baimazhai in Yunnan Province and Yangliuping in Sichuan Province). South China also has major vein-type tungsten–tin–bismuth–beryllium–sulphide and REE deposits associated with Yanshanian magmatism (e.g., Shizhuyuan and Xihuashan), important world class stratabound base metal–tin deposits (Dachang deposit), and the large antimony deposits (Xikuangshan and Woxi). During the Himalayan Orogeny, many giant deposits were formed in South China including the recently emerging Yulong and Gangdese porphyry copper belts in Tibet and the Ailaoshan orogenic gold deposits in Yunnan.

Geochemical effects of endoskarn formation in the Mazraeh Cu–Fe skarn deposit in northwestern Iran

The Mazraeh Cu–Fe skarn deposit, located in the Ahar region of northwestern Iran, is currently being mined and contains reserves of about 400,000 tons grading 1.2 wt% Cu. The deposit contains both exoskarn formed within the calcareous wall rock and endoskarn formed within the associated granodiorite. Mineralogically, there are three types of skarn at Mazraeh: (1) pyroxene exoskarn; (2) garnet endoskarn and exoskarn; and (3) secondary epidote skarn. Locally, gradational alteration of the Mazraeh granodiorite to garnet endoskarn is present over a width of one metre. Mass-balance calculations show that garnetization of the Mazraeh granodiorite was the result of interaction with a hydrothermal solution enriched in Si, Fe 2+ , Fe 3+ , Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, and Y. These elements are thought to have originated from the magma. The calcareous wall rocks (limestone and marble) were the main source of Ca and Mg within the endoskarn. This hydrothermal alteration resulted in an average 157% volume gain in the affected wall rocks. The dilatent structures and open spaces were filled by paragenetically younger minerals, especially magnetite, pyrite, chalcopyrite, bornite, covellite, chalcocite, epidote, calcite and quartz, during the late stage of alteration. During the endoskarnification process, primary biotite, hornblende and titanite in the Mazraeh granodiorite have been replaced by chlorite and rutile, and plagioclase and K-feldspar have been replaced by garnet. These reactions have leached (removed) K, Na, Ba, Rb and possibly Sr and added Si, Fe, Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, W and Y. Orthogonal jointing in the Mazraeh pluton played a significant role in the channelling and migration of the hydrothermal solutions in the pluton and development of the endoskarn. Although most of the copper and iron were precipitated in the exoskarn due to reaction between hydrothermal solutions and marble and limestone, locally significant skarn mineralization is also developed in the pluton. Although significant sulphide mineralization was not precipitated during garnet endoskarn formation, porosity and permeability created by this process served as the sites of sulphide mineral precipitation during late stages of skarn formation. Thus, the exploration for, and delineation of garnet endoskarn may result in the discovery of ore. Field guides for the exploration of garnet endoskarn include: (1) its high physical relief due to abundant garnet which is highly resistant to surface weathering processes; (2) predominance of red-brown andraditic garnet species over grossular-rich ones; and (3) local development of secondary malachite and azurite after primary Cu and Fe sulphides.

Major and trace element geochemistry and 40Ar/ 39Ar geochronology of Laramide plutonic rocks associated with gold-bearing Fe skarn deposits in Guerrero state, southern Mexico

Fe–Au skarn deposits related to intrusive centers, mostly of granodioritic composition, are widespread in southern Mexico's Guerrero state. These intrusive rocks are largely associated with the NW–SE-oriented Laramide magmatic belt that extends across most of western Mexico. The geochemical composition and ages of representative rocks from the Mezcala mining district in central Guerrero are studied to evaluate the petrogenetic aspects of the ore-related magmas. Some major and trace elements display nearly linear silica variation trends, which suggest a possible comagmatic origin. However, other elements have scattered distributions, possibly due to irregular mantle-to-crust magma mixing ratios, heterogeneities in the composition of the assimilated crustal material, or modifications during the emplacement or postemplacement processes. Major element chemistry indicates calc-alkalic metaluminous compositions, whereas trace element data suggest a volcanic arc tectonic setting, confirming that these rocks evolved from magmas generated above a subduction zone. Compared with the Laramide granites from the northern part of the belt in northwestern Mexico, which intruded a crust underlain by Proterozoic North American rocks, the studied samples are similar but relatively low in Nb and high in Sr, the middle rare earth elements (REE), P, and Zr. They also display minor Ti enrichments and a moderate depletion in the heavy REE. These characteristics may indicate a source of basaltic composition. New 40Ar/ 39Ar dating of granodiorites and dacite porphyries shows a north-to-south age progression from 66.2±0.8 Ma in the northern part of the belt to 62.2±0.7 Ma in the south. Moreover, the argon dates identify a younger postorogenic igneous event 35–30 Ma ago. This event is poorly documented and may have occurred after the extinction of the Laramide arc and prior to the mid-Tertiary Sierra Madre Occidental ignimbrite flare-up. On the basis of limited geochemical data, these rocks appear to be depleted in P 2O 5 and Sr and enriched in U relative to the studied Laramide granites. A Fe skarn deposit located in Buena Vista de Cuéllar, in the north central part of Guerrero, suggests that this magmatic pulse took place after the ore development of the Mezcala district.

BROMINE IN SCAPOLITE-GROUP MINERALS AND SODALITE: XRF MICROPROBE ANALYSIS, EXCHANGE EXPERIMENTS, AND APPLICATION TO SKARN DEPOSITS

Application of an X-ray fluorescence (XRF) microprobe for the analysis of single grains (80 to 1,000 μm in diameter) of Cl-rich minerals for Br has been evaluated for fluorapatite, chlorapatite, scapolite-group minerals (marialite and meonite) and sodalite. A calibration curve based on the Br contents in four international reference materials has been confirmed by measurements on Br-bearing standard solutions and by agreement with the results of Cl-rich minerals from instrumental neutron-activation analyses. Absolute errors associated with individual XRF microprobe analyses ( i.e ., counting statistics alone) are less than 5%, and the calculated limit of detection in the analysis of single mineral grains is ~1 ppm Br. Matrix and grain-size effects are shown to be negligible. Experiments at 1 atmosphere and 800 to 1000°C yield the following distribution coefficients for Br–Cl exchanges between marialite or sodalite and hydrous NaCl–NaBr melts: KDmarialite–melt = 0.97 ± 0.08 and KDsodalite–melt = 0.9 ± 0.1. Therefore, the Cl/Br values in marialite and sodalite closely reflect the halogen proportions of their coexisting melts or fluids. The diffusivity of Br in sodalite follows an Arrhenius relation: DBr = 6.5 × 10−7 exp(−270 ± 10kJ/mol/RT) m2/s, over the temperature range from 800 to 1000°C. DBr in marialite is 1.7 ± 0.3 × 10−19 m2/s at 800°C. The Cl/Br weight ratios of marialite in the Tieshan Fe skarn deposit, China, cluster around 650 ± 40, supporting an origin involving hydrothermal brines from associated evaporites. Scapolite-group minerals in the exoskarns of the Nickel Plate Au skarn deposit, British Columbia, have Cl/Br from 560 to 570, higher than those (130 to 180) of their counterparts in the endoskarns and vuggy cavities. This variation is attributable to an increased involvement of magmatic water from distal to proximal zones. Similarly, scapolite-group minerals in the Grenville U–Th–Mo REE pegmatite–skarn–vein deposits vary widely in Cl/Br, from 80 to 380, indicative of mixed sources of hydrothermal fluids. This study shows the potential of Br analysis of Cl-rich silicate minerals for constraining the sources and evolution of hydrothermal fluids.

Tectonic setting and petrogenesis of the Çelebi granitoid, (Kırıkkale-Turkey) and comparison with world skarn granitoids

Many studies have shown systematic correlations between the composition of plutons worldwide and the metal content of associated skarns. This is the first report of similar correlations between the composition of Çelebi granitoid and skarns of the Çelebi district in Central Anatolia, Turkey. The Çelebi district is well known for its polymetallic Fe–W and Cu vein ores. These are hosted by calcic skarn zones. Both exoskarns (pyroxene–garnet) and endoskarns (epidote–pyroxene) occur in the district formed mainly along the granitoid contacts and along the fractures within the marble. Based on mineralogy, petrology and geochemistry, two different igneous rocks were recognized in the Çelebi granitoid, referred to as leucocratic (felsic) and mesocratic (intermediate) Çelebi granitoid. The leucocratic Çelebi occurs as dominant rock type, and is classified as granite. The mesocratic Çelebi is not widespread and is classified as adamellite, tonalite, quartz monzonite and quartz monzodiorite. The mesocratic Çelebi has I-type characteristics, and have subalkaline, calc-alkaline and metaluminous characteristics like most worldwide skarn granitoids. A post-collisional tectonic setting is proposed on the basis of field evidence, the relative timing of intrusions with respect to metamorphic and obducted ophiolitic rocks and trace element geochemistry. The high abundance of La and Ce and the enrichment of V in mafic components suggest that Çelebi granitoids are formed by partial melting of mantle rocks, but have been contaminated by interaction with continental crust involving possible magma mixing processes (i.e. mixing of coexisting felsic and mafic magmas). In the district, the mesocratic type and mafic microgranular enclaves (MME) mainly within leucocratic type represent a mafic underplating magma that was mixed with and/or injected into felsic magma of the leucocratic type. The present study shows that Fe mineralization is associated with mesocratic Çelebi type, whereas W mineralization is associated with leucocratic type. Mesocratic Çelebi granitoid is significantly different from the worldwide average of plutons associated with Fe skarns. In particular, MgO vs. SiO 2, FeOt+CaO+Na 2O/K 2O vs. SiO 2, Fe 2O 3/Fe 2O 3+FeO vs. SiO 2 and V vs. Ni vary from typical values (are lower than values typical for plutons associated with Fe skarns) for plutons associated with Fe skarns. Instead, it resembles the geochemical characteristics of plutons associated with worldwide Cu and possibly Au skarns. This suggests new exploration possibilities for copper and gold in the Çelebi district.