Skarn System Research Articles

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe 3+) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids. Garnets from the Crown Jewel deposit range from Adr 30Grs 70 to almost pure andradite (Adr >99). Fe-rich garnets (Adr >90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE 3+ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr >90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE 3+ following a coupled, YAG-type substitution mechanism ( [ X 2 + ] - 1 VIII [ REE 3 + ] + 1 VIII [ Si 4 + ] - 1 IV [ Z 3 + ] + 1 IV ) , whereas Eu 2+ substitutes for X 2+ cations. Thermodynamic data (e.g., H mixing) in grossular–andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions. Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/ R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/ R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/ R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.

The origin of the Tongkeng-Changpo tin deposit, Dachang metal district, Guangxi, China: clues from fluid inclusions and He isotope systematics

Tongkeng-Changpo is the largest tin deposit within the giant Dachang polymetallic tin ore field in Guangxi, southern China, which is part of a large skarn system associated with Cretaceous granitoids. The Tongkeng-Changpo mineralization consists of veins and stockworks in the upper levels and replacement stratiform orebodies (mantos) at lower levels. Based on textural relationships, three major mineralizing stages can be recognized: stage I with cassiterite, sulphides, stannite, tourmaline, and quartz; stage II with cassiterite, sulphides, sulphosalts, quartz, and calcite; and stage III with calcite as the main phase. The study of fluid inclusions has shown that there are two main fluid types: CO2 and NaCl-H2O. Homogenization temperatures are 270 to 365°C, 210 to 240°C, and 140 to 190°C for stages I, II, and III, respectively. Salinities range from 1 to 7 wt.% NaCl equiv. in the early ore stage and 3 to 10 wt.% NaCl equiv. in the late stages. Laser Raman Spectroscopy indicates that the inclusion fluids in stages I and II were of carbono-aqueous composition, with minor amounts of CH4 and H2S, whereas those in stage III were aqueous. Helium isotopic analyses of inclusion fluids indicate that the 3He/4He ratios in the ore veins are in between 1.2 to 2.9 Ra (Ra = 1.4 × 10−6, modern atmospheric ratio), and range from 1.6 to 2.5 Ra in the stratiform orebodies. This range of 3He/4He ratios is significantly higher than that of crustal fluids (0.01–0.05 Ra). The similar characteristics of fluid inclusions and their He isotopic composition, as well as age constraints, indicate that the ore veins and stratiform orebodies of the Tongkeng-Changpo deposit formed from the same hydrothermal system, likely related to granite intrusions of the Mesozoic Yanshanian tectono-thermal event. In addition, the high R/Ra ratios indicate a mantle contribution in the ore fluids.

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.

The mineralogy of copper-bearing skarn to the east of the Sungun-Chay river, East-Azarbaidjan, Iran

A calcic copper-bearing skarn zone in East-Azarbaidjan, NW of Iran is located to the east of the Sungun-Chay river. Skarn-type metasomatic alteration and mineralization occurs along the contact between Upper Cretaceous impure carbonates and an Oligo-Miocene Cu-bearing granitoid stock. Both endoskarn and exoskarn are developed along the contact. Exoskarn is the principal skarn zone enclosed by a marmorized and skarnoid–hornfelsic zone. The skarnification process occurred two stages: (1) prograde and (2) retrograde. The prograde stage is temporally and spatially divided into two sub-stages: (a) metamorphic–bimetasomatic (sub-stage I) and (b) prograde metasomatic (sub-stage II). Sub-stage I began immediately after the intrusion of the pluton into the enclosing impure carbonates. Sub-stage II commenced with segregation and evolution of a fluid phase in the pluton and its invasion into fractures and micro-fractures of the marmorized and skarnoid–hornfelsic rocks developed during sub-stage I. The introduction of considerable amounts of Fe, Si and Mg led to the development of substantial amounts of medium- to coarse-grained anhydrous calc-silicates. From texture and mineralogy the retrograde metasomatic stage can be divided into two discrete sub-stages: (a) early (sub-stage III) and (b) late (sub-stage IV). During sub-stage III, the previously formed skarn zones were affected by intense multiple hydro-fracturing phases in the Cu-bearing stock. In addition to Fe, Si and Mg, substantial amounts of Cu, Pb, Zn, along with volatile components such as H 2S and CO 2 were added to the skarn system. Consequently considerable amounts of hydrous calc-silicates (epidote, tremolite–actinolite), sulfides (pyrite, chalcopyrite, galena, sphalerite, bornite), oxides (magnetite, hematite) and carbonates (calcite, ankerite) replaced the anhydrous calc-silicates. Sub-stage IV was concurrent with the incursion of relatively low temperature, more highly oxidizing fluids into skarn system, bringing about partial alteration of the early-formed calc-silicates and developing a series of very fine-grained aggregates of chlorite, clay, hematite and calcite.

2-1: Pb–Zn deposits in the Madan ore field, South Bulgaria: Madan District: Lat. 41°30′ N, Long. 24°56′ E

The superlarge Luziyuan Zn-Pb-Fe(-Cu) polymetallic deposit is located in the southern Baoshan Block of western Yunnan (SW China). The deposit contains over 4.2 million tonnes (Mt) of contained Zn + Pb metal reserves at 5.09% Zn and 2.40% Pb, and 301 Mt of Fe ore resources at 30.02% TFe. The stratiform or vein-type orebodies are hosted in the Upper Cambrian Shahechang Formation. Similar to many major Zn skarn deposits worldwide, the Luziyuan deposit shows a distinct metal zonation pattern with depth, which correlates with the different calc-silicate mineral assemblages: From Zn-Pb (sphalerite + galena) ores in marble, through Zn-Pb-Fe-Cu (sphalerite + galena + magnetite ± pyrite + chalcopyrite) ores in pyroxenoid (± clinopyroxene)-bearing skarn, to Fe-Cu (magnetite ± pyrite + chalcopyrite) ores in garnet (± clinopyroxene ± pyroxenoid)-bearing skarn. The syn-ore calcite has δ13CPDB and δ18OSMOW values of −1.0 to +0.9‰ and +10.3 to +12.7‰, respectively, suggestive of isotopic affinities to both granite and marble. δDH2O values of the fluid inclusions in the pre-ore rhodonite and the syn-ore quartz range from −81.7 to −68.8‰ and −76.6 to −56.3‰, respectively; whilst the calculated δ18OH2O values of the rhodonite, syn-ore calcite, and quartz are of +5.4 to +6.1‰, +1.4 to +3.8‰, and −5.3 to −2.0‰, respectively. This indicates that the ore-forming fluids were sourced from both magmatic and meteoric waters. δ34S values of the syn-ore sulfides (+8.9 to +12.0‰) are slightly higher than those of typical crustal-derived granites (δ34S = −4.0 to +9.0‰), but are significantly lower than those of Cambrian seawater sulfates. This indicates that the source of S is correlated with both granite and seawater sulfates. We propose that the Luziyuan Zn-Pb-Fe(-Cu) polymetallic mineralization occurred in a distal skarn system, and was genetically linked to concealed Early Cretaceous granite intrusions.

2–2: Ada Tepe sedimentary-hosted, low-sulphidation epithermal Au deposit, SE Bulgaria: District: Lat. 41°26′ N, Long. 25°39′ E

The Tethyan Metallogenic Belt (TMB), extending from Europe through Anatolia to Iran, is one of the world's major metal producing belts, and consists of many sectors. Mineral deposits of the Biga Peninsula in northwestern Turkey exhibit, in many ways, the characteristics of mineral deposits found throughout the belt. Biga Peninsula tectonically forms the westernmost part of the Sakarya Zone and easternmost part of the Rhodope Zone at the intersection of Gondwana and Laurasia.The Biga Peninsula metallogeny research and exploration project created a GIS inventory of mineral deposits and prospects, and classified them genetically to evaluate the mineral deposit potential using genetic models based on descriptive data. The GIS database, consisting of 128 deposits or prospects, helped to generate new prospects and potential prospects. This field-based study indicated that the Biga Peninsula forms a prime target for gold–copper exploration not only in Turkey but in the world.The current economically significant mineral deposits of the Biga Peninsula were shaped by Cenozoic calc-alkaline magmatism, ranging between 52 and 18 Ma, and related to mainly collisional and post-collisional tectonic regime. Epithermal Au–Ag deposits including high-(HS), low-(LS) and intermediate-sulfidation (IS) styles, porphyry Au–Cu–Mo and base-metal skarn systems are economically the most important. Though there are no currently economic examples of some of them in the Biga Peninsula, other deposit types include Carlin-like distal disseminated Au–Ag, orogenic Au, especially listwanite hosted, volcanogenic Mn and U, lateritic (ferricrete) Fe deposits, carbonate replacement (CR) and placers.Several active metal mines, such as Balya, Arapucandere and Koru, are operating in the Biga Peninsula. Kucukdere Au–Ag deposit in Balikesir is the only gold mine in the Biga Peninsula, except for by-product gold produced from base-metal deposits. Results of the study show current total gold endowment of the Biga Peninsula including reserves and/or resources is 9.18 Moz gold [284.2 t] contained in twelve different deposits. Of these only 6 contain significant gold [> 0.3 Moz or 10 t]. Halilaga porphyry and Agi Dagi and Kirazli HS epithermal systems have an ongoing resource estimate, and Halilaga is a candidate to be one of the largest Cu–Au deposits not only in the Biga Peninsula, but in Turkey. Currently newly discovered Tepeoba is the largest porphyry Cu–Mo–Au deposit with known resources in the Biga Peninsula.Here, the first40Ar/39Ar step-heating age data conducted on some of the major HS epithermal gold deposits and causative intrusives in the Biga Peninsula are reported. Geochronological results from this project, evaluated with previous studies, indicate at least 3 phases of porphyry and 2 phases of high-sulfidation epithermal gold mineralization in the Biga Peninsula. The most important mineralizing phases and related host rocks for gold mineralization range from 38 to 22 Ma. The Oligocene is especially important for economic epithermal and porphyry systems in the Biga Peninsula, which is comparable to deposits in the Oligo-Miocene Serbomacedonian-Rhodope metallogenic belt of the Balkan Peninsula in SE Europe.

The Rare Earth Elements and Uranium in Garnets from the Beinn an Dubhaich Aureole, Skye, Scotland, UK: Constraints on Processes in a Dynamic Hydrothermal System

Garnets from skarns in the Beinn an Dubhaich granite aureole, Isle of Skye, Scotland, have a large range of concentrations of uranium (0·2–358 ppm) and the rare earth elements (REE) (23–4724 ppm). Variations in these concentrations correlate with major element zonation within the garnets, and with changes in the shape of REE patterns. Typical patterns in most garnets display light REE (LREE) enrichment, flat heavy REE (HREE) distribution and a negative Eu anomaly. These patterns are interpreted to represent equilibrium trace element exchange between pre-existing pyroxene, hydrothermal fluid and calcic garnets. Iron-rich zones are characterized by positive Eu anomalies and an increase in the abundance of the LREE relative to the HREE. These patterns are interpreted as resulting from changes in REE speciation related to the introduction of externally buffered fluid to the skarn system. Relatively Fe-poor zones show strongly HREE-enriched patterns with negative Eu anomalies and in some instances depletions in Y relative to Ho and Dy, which are interpreted as resulting from surface sorption of the REE during rapid, disequilibrium garnet growth. Strong correlations between U abundance and the REE patterns indicate that the same processes have affected U distribution. Both types of pattern can be modified by the effects of closed-system crystallization on REE abundance in the fluid, and changes in fluid major element chemistry.

Skarn textures and a case study: the Ocna de Fier-Dognecea orefield, Banat, Romania

Skarn textures and a case study: the Ocna de Fier-Dognecea orefield, Banat, Romania

Formation of Anhydrous and Hydrous Skarn in Cu-Au Ore Deposits by Magmatic Fluids

Most skarn ore deposits are characterized by two distinctly different alteration styles. An early prograde stage with anhydrous minerals, such as garnet and pyroxene, forms from relatively high-temperature, hypersaline liquid. A later retrograde stage with hydrous minerals, such as epidote, amphibole, and chlorite plus sulfide ore minerals, forms from lower temperature, lower salinity fluids. These two alteration stages commonly have been thought to reflect a dominance of magmatic and meteoric water, respectively, with relevance to the source of ore metals. We report data from two different skarn systems, one being part of the world’s largest Cu-Au resource. Stable isotope compositions of anhydrous and hydrous alteration minerals from both deposits indicate a magmatic source for both the prograde and retrograde stages: δ 18O averages 5.0 per mil for garnet (range, 3.4–7.2‰), 6.5 per mil for pyroxene (4.3–8.2‰), and 7.1 per mil for amphibole (4.3–8.7‰). The δ D values of late amphibole are more complex, with magmatic values (–77 to –78‰) for one deposit and both magmatic and lighter values for another deposit that could be explained either by magmatic degassing or by limited mixing with meteoric water. We conclude that the differences in fluid composition—prograde versus retrograde stages—resulted from a magmatic fluid that intersected its solvus during the early stage, creating vapor and hypersaline liquid, whereas in the later stage this magmatic fluid did not intersect its solvus because it followed a different cooling path. This late, low- salinity liquid only boiled once its vapor-pressure curve was reached, causing sulfide ore to precipitate during the retrograde stage.

Granite- and gabbrodiorite-associated skarn deposits of NW Iran

Field and laboratory studies show that there are two types of skarn deposits in NW Iran: granite-associated (type I) and gabbrodiorite-associated (type II). Granite-associated deposits are accompanied by Cu and Fe mineralisation, whereas Mn and Fe are the main ore metals in gabbrodiorite-associated skarn deposits. There are some differences in the mineralogy of these skarn deposits. Bixbyite, piemontite and Cr-bearing garnet are found only in gabbrodiorite-associated skarns, whereas idocrase occurs only in granite-associated deposits. Type II skarns show exoskarn features, whereas some type I skarns have developed endoskarn as well. Evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition. Gabbrodiorite-associated skarns show higher fO 2 than granite-associated deposits. Based on mineralogical and textural evidence, mineralisation in both groups has started from about 550 °C. Early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 °C. It seems that due to low fluid content in the gabbrodioritic magma, heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite-associated skarn system.

Paragenesis of Cu-Fe ores from Ocna de Fier-Dognecea (Romania), typifying fluid plume mineralization in a proximal skarn setting

AbstractTrace mineral assemblages in the bornite- and chalcopyrite-rich Cu-Fe zone of the Ocna de Fier-Dognecea skarn deposits, Banat, SW Romania provide additional constraints on the genesis of this classic zoned skarn system. Observed assemblages substantiate a model, in which the Cu-Fe zone forms the proximal fluid-plume root of the system. Observed trace mineral assemblages in the magnesian forsterite-bearing skarns crystallized at ~650°C in a volatile-rich environment, evidenced by widespread phlogopite, ludwigite, valleriite and apatite. The entire assemblage thus belongs to the initial stage of skarn formation. Prolonged cooling led to sequential exsolution of trace mineral phases from bornite and chalcopyrite during the retrograde stage, although still at temperatures in excess of 500°C. Bornite is typified by the abundance of exsolved phases along cleavage planes and along crystal margins, notably chalcopyrite and pyrrhotite, but also cobalt pentlandite, carrollite, wittichenite, galena, mawsonite, silver and electrum. Chalcopyrite hosts cobalt pentlandite, carrollite, wittichenite, galena and a sequence of Se- and Te-bearing minerals (kawazulite, bohdanowiczite, hessite, volynskite), along, although not restricted to, grain margins. The assemblage bornite-chalcopyrite-magnetite, with the trace phases, cobalt pentlandite, carrollite, wittichenite and various Se- and Te-bearing minerals represents a characteristic assemblage common to a disparate range of deposits formed at temperatures in excess of 500°C in the presence of volatiles and typified by relatively low fS2 fluids.

Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius

Research Article| November 01, 2001 Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius Paolo Fulignati; Paolo Fulignati 1Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Vadim S. Kamenetsky; Vadim S. Kamenetsky 2School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Paola Marianelli; Paola Marianelli 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Alessandro Sbrana; Alessandro Sbrana 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Terrence P. Mernagh Terrence P. Mernagh 4Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Paolo Fulignati 1Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Vadim S. Kamenetsky 2School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia Paola Marianelli 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Alessandro Sbrana 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Terrence P. Mernagh 4Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia Publisher: Geological Society of America Received: 07 Mar 2001 Revision Received: 28 Jun 2001 Accepted: 20 Jul 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2001) 29 (11): 1043–1046. https://doi.org/10.1130/0091-7613(2001)029<1043:MIROIB>2.0.CO;2 Article history Received: 07 Mar 2001 Revision Received: 28 Jun 2001 Accepted: 20 Jul 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paolo Fulignati, Vadim S. Kamenetsky, Paola Marianelli, Alessandro Sbrana, Terrence P. Mernagh; Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius. Geology 2001;; 29 (11): 1043–1046. doi: https://doi.org/10.1130/0091-7613(2001)029<1043:MIROIB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Foid-bearing syenites and endoskarn xenoliths of the A.D. 472 Vesuvius eruption represent the magma chamber–carbonate wall-rock interface. Melt inclusions hosted in crystals from these rocks offer a rare opportunity to depict the formation and the composition of metasomatic skarn-forming fluids at the peripheral part of a growing K-alkaline magma chamber disrupted by an explosive eruption. Four principal types of melt inclusions represent highly differentiated phonolite (type 1), hydrosaline melt (type 3), unmixed silicate– salt melts (type 2), and a complex chloride-carbonate melt with minor sulfates (type 4). The high-temperature (700–800 °C) magmatic-derived hydrosaline melt is considered to be the main metasomatic agent for the skarn-forming reactions. The interaction between this melt (fluid) and carbonate wall rocks produces a Na-K-Ca carbonate-chloride melt that shows immiscibility between carbonate and chloride constituents at ∼700 °C in 1 atm experiments. This unmixing can be viewed as a possible mechanism for the origin of carbonatites associated with intrusion-related skarn systems. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magnetite-rich calc-silicate alteration in relation to synvolcanic intrusion at the Ansil volcanogenic massive sulfide deposit, Rouyn-Noranda, Quebec, Canada

The Ansil Cu–Au volcanogenic massive sulfide deposit is located within an Archean-age cauldron infill sequence that contains the well-known Noranda base metal mining district. The deposit is unusual in that 17% of the massive pyrrhotite–chalcopyrite orebody is replaced by semi-massive to massive magnetite. Temporally associated with the magnetite formation are several calc-silicate mineral assemblages within the massive sulfide lens and the underlying sulfide stockwork vein system. Coarse-grained andradite–hedenbergite and ferroactinolite–ilvaite alteration facies formed in the immediate footwall to the massive magnetite–sulfide lens, whereas an epidote–albite–pyrite-rich mineral assemblage overprints the margins of the chlorite-rich stockwork zone. The epidote-rich facies is in turn overprinted by a retrograde chlorite–magnetite–calcite mineral assemblage, and the andradite–hedenbergite is overprinted first by ferroactinolite–ilvaite, followed by semi-massive to massive magnetite. The footwall sulfide- and magnetite-rich alteration facies are truncated by a late phase of the Flavrian synvolcanic tonalite–trondhjemite complex. Early phases of this intrusive complex are affected to varying degrees by calc-silicate-rich mineral assemblages that are commonly confined to miarolitic cavities, pipe vesicles and veins. The vein trends parallel the orientation of synvolcanic faults that controlled volcanism and hydrothermal fluid migration in the overlying cauldron succession. The magnetite-rich calc-silicate alteration facies are compositionally similar to those of volcanic-hosted Ca–Fe-rich skarn systems typical of oceanic arc terranes. Tonalite–trondhjemite phases of the Flavrian complex intruded to within 400 m of the base of the earlier-formed Ansil deposit. The low-Al trondhjemites generated relatively oxidized, acidic, Ca–Fe-rich magmatic–hydrothermal fluids either through interaction with convecting seawater, or by assimilation of previously altered rocks. These fluids migrated upsection along synvolcanic faults that controlled the formation of the original volcanogenic massive sulfide deposit. This is one of the few documented examples of intense metasomatism of a VMS orebody by magmatic–hydrothermal fluids exsolved from a relatively primitive composite sub-seafloor intrusion.

Estimation of Fluorine Concentrations in Fluids of Mineralized Skarn Systems

The experimentally calibrated phlogopite and biotite fluorimeters and the experimental results for mineral-fluorine buffer reactions: quartz + fluorite + H 2 O = wollastonite + 2HF, and corundum + sellaite + H 2 O = spinel + 2HF, were used to estimate the HF concentration ( M HF , mole/dm 3 ) in fluid associated with different skarn deposits, granitoids, pegmatites, and marble. The equations of the phlogopite and biotite fluorimeters are the following: log M HF (phl) = X Mg [log (X F /(1 – X F )) – 1722/T(K) – 0.2112] + log a H 2 O , and log M HF (bt) = log (X F /(1– X F )) – 1722/T(K) – 1.107 X Mg + 0.216 (Al-2) + 0.8958 + log a H 2 O , which take into account the absolute temperature (Kelvin) of mica formation and complex compositions of mica solid solution and hydrothermal fluid. The estimation of the HF concentration in fluids related to different skarn deposits, granitoids, pegmatites, and marble shows two behavioral trends. The high F trend is typical of rare metal leucogranite, Li-F granite and pegmatite, and related W, Be, U, Th, REE, Ta, Nb, Li, and Sn skarn deposits; the HF concentration in the fluids associated with these rocks is approximately 10 –1.5 to 10 –1 mole/dm 3 and remains almost constant, although formation temperatures vary from 900° to 500°C. The low F trend is characteristic of barren diorite, tonalite, granodiorite, and granite plutons and porphyry Cu and Mo deposits, as well as fluorine-poor phlogopite, scheelite, and other base metal skarn deposits. The HF concentration in the fluids related to these rocks decreases with the formation temperature. At low temperature, the HF concentration is considerably lower than in the high F trend, e.g., at 500°C, it ranges between 10 –3 to 10 –2.6 mole/dm 3 , which is close to the experimental positions of quartz-fluorite-wollastonite equilibrium on the log M HF -T diagram. The coincidence of high HF concentrations in fluid and the metallic association of W, Be, U, Th, REE, Ta, Nb, Li, and Sn in skarn deposits indicates a petrogenetic relationship that is a function of magmatic fractionation, melt-volatile partitioning, oxidation state of melt and fluid, or metal-ligand complexing behavior. Therefore, estimation of the HF concentration in fluids using biotite and phlogopite fluorimeters may aid in the exploration of plutons and related skarn ore deposits and shed light on genetic problems related to fluorine in skarn fluid ore systems.

A Low F Pegmatite-Related Mo Skarn from the SouthwesternGrenville Province, Ontario, Canada: Phase Equilibria and PetrogeneticImplications

The Hunt Mo skarn deposit is one of the best examples of a late tectonic, granitic pegmatite-related, skarn system in the Grenville province. An Re-Os age of 1069 ± 11 Ma was obtained from a molybdenite crystal (6.4 ppm Re) hosted in proximal skarn (sample 50O) consistent with a contact metasomatic origin related to the granitic body. Although less evolved than other Grenvillian pegmatites, the Condon Lake granitic pegmatite is a low-temperature, A-type intrusion, with moderate redox characteristics. In contrast to many Mo-bearing skarns in the region, this deposit has low U, Th, REE, F, and P contents. The reduced marginal magnesian skarn is well zoned geochemically, mineralogically, and texturally from the contact outward into the dominant graphite-bearing calc-silicate-calcite-dolomite marble, which is locally intercalated with clinopyroxenite, orthoamphibolite, and pyroxene-biotite quartzofeldspathic gneiss. A narrow zone of endoskarn (<1 m; scapolite-K feldspar-Ca clinopyroxene) and wider, zoned exoskarn (<10 m; scapolite-Ca clinopyroxene (proximal), Ca clinopyroxene-phlogopite, Ca clinopyroxene-tremolite-phlogopite, tremolite-phlogopite (distal), marble), which hosts the bulk of the primary molybdenite (± pyrrhotite) and minor secondary pyrite-pyrrhotite-molybdenite veins. In addition to the obvious Si, Fe, Ti, Mn, S, and Mo addition, Cu, Zn, Y, Nb, and Zr are also slightly enriched. The high f (H 2 O) / f (HF) ratios (138,000–204,000) and moderate f (H 2 O) / f (HCl) ratios (2,200–1,000) at the Hunt deposit contrast with the lower fugacity ratios from other Mo-bearing U-Th-REE skarns in the region, indicating that Mo transport and deposition is principally related to a hydroxide complex. The calc-silicate phase equilibria used to describe the zonation sequence have been reinterpreted using quartz-undersaturated (silica) activities; this shifts the calc-silicate-forming reaction boundaries to very low X (CO2 ) (<0.005), consistent with decarbonation reactions induced via dissolved silica infiltration. The modeled fluid/rock in the skarn based on dissolved molybdenum is greater than 20. The carbon and oxygen isotopes of calcite are consistent with infiltrative metasomatic skarns from the pegmatite and a dominantly igneous source for the sulfur ( δ 34 S = 2–9‰). Overall, the deposition of Mo is probably facilitated by a reaction with carbonate (increase in Ca(OH) 2 , CaCl 2 resulting in complex destabilization), a decrease in f O2 triggering an increase H 2 S/SO 2 , S in the host rocks ( f S2 ), and a slight decrease in temperature (650°–500°C).

U-Pb Ages Constraining Batholith Emplacement, Contact Metamorphism, and the Formation of Gold and W-Mo Skarns in the Southern Cross Area, Yilgarn Craton, Western Australia

Gold mines in the Archean Southern Cross greenstone belt, central Yilgarn craton, have produced more than 220 metric tons (t) of gold. The deposits are characterized by a pyroxene-rich gangue and have been interpreted as synmetamorphic amphibolite facies replacement deposits and as postmetamorphic intrusion-related skarns. We present ion microprobe U-Pb zircon data constraining the age of amphibolite facies contact metamorphism in the aureole of the Ghooli Dome, the largest granitoid batholith intruding the Southern Cross belt. Drill cores from the Copperhead gold mine provided zircons from an altered quartz porphyry sill, located 300 m southwest of the Ghooli Dome, and zircons from the monzogranite gneiss at the border of the batholith. The porphyry sill contains embayed igneous zircons dated by a concordant U-Pb age of 2912 ± 5 Ma and euhedral zircons of metamorphic crystal habitus dated by a second concordant age of 2772 ± 5 Ma. The igneous zircons are slightly younger than those recovered from another sill of quartz porphyry, sampled at the Southern Star mine, which define a single concordant U-Pb age of 2934 ± 7 Ma. The metamorphic zircons in the porphyry at Copperhead record the same age as the igneous zircons (2775 ± 10 Ma) in the monzogranite gneiss of the Ghooli Dome. This pluton extends more than 40 km to the southeast of Copperhead and also forms the batholith border at the Corinthian and Fraser’s mines. These gold deposits, too, are hosted by amphibolite facies greenstones contact metamorphosed at 2772 ± 5 Ma. The Corinthian skarn (2620 ± 6 Ma) formed 150 m.y. after high-grade metamorphism and, consequently, cannot be classified as synmetamorphic or metamorphogenic. A review of field relationships and of published geochronometric data suggests that the gold skarns in the Southern Cross area are related in space and time to a suite of magnetite series granite-pegmatite complexes. These granites, dated regionally at 2.66 to 2.60 Ga, are also associated with gold-bearing W-Mo skarn-greisen or skarn systems of up to 6-km strike length. The numerous skarns exposed in the deeply eroded (14 km paleodepth), central Yilgarn craton are members of a recently recognized group of intrusion-related gold deposits which are part of continental-margin tungsten-tin provinces.

Estimation of Fluorine Concentrations in Fluids of Mineralized Skarn Systems

Estimation of Fluorine Concentrations in Fluids of Mineralized Skarn Systems

Skarns Related to Porphyry-Style Mineralization at CaicayenHill, Neuquen, Argentina: Composition and Evolution of Hydrothermal Fluids

Located in the Main Cordillera of northwest Neuquen, Argentina, the Caicayen district shares the following features with Cu skarn–porphyry copper deposits in other parts of South, Central, and North America, as well as Asia and the South Pacific: (1) alteration and mineralization are related to I–type, magnetite series, calc–alkaline porphyritic sills of intermediate composition with stockwork veining, brittle fracturing, and brecciation; these are features indicative of a relatively shallow environment of formation (~500 bars); (2) skarns have an oxidized mineralogy dominated by garnet (Ad 45–97 ), clinopyroxene (Hd 18–60 ), epidote, and hematite–magnetite; (3) skarns contain up to 15 percent sulfides (pyrite >> pyrrhotite–chalcopyrite) and locally are associated with distal massive magnetite and hematite lodes; (4) massive silica–pyrite bodies replace prograde skarn and marble; (5) pyroxene–skarn and silica– pyrite alteration distal to the potassic core are Fe and Zn enriched, respectively; (6) mineralized igneous rocks exhibit characteristic early potassic–propylitic and late phyllic alteration that in the skarns can be correlated with prograde garnet–pyroxene and retrograde silica–pyrite, respectively; and (7) Cu–Au anomalies occur in exposures within the potassic and phyllic halos. Fluid inclusions from igneous rocks record multiple fluid events: (1) a hypersaline inclusion population (up to 67 wt % NaCl equiv for halite + sylvite–bearing inclusions, 2–36 wt % KCl equiv, and 31–45 wt % NaCl equiv) that homogenized by salt dissolution between 355° and 400°C, interpreted to have been trapped at a lithostatic pressure of approximately 500 bars; (2) a second hypersaline inclusion population that homogenized by vapor bubble disappearance at 215° to 385°C and may have been trapped at similar temperatures but lower pressures relative to the first population; and (3) a population of coexisting NaCl–saturated inclusions (average salinity of 35 wt % NaCl equiv) and vapor–rich inclusions in igneous rocks close to skarn homogenized at similar temperatures (326°–360°C) that may have formed from boiling of a low–salinity fluid (up to 8 wt % NaCl) at hydrostatic pressures of 120 to 170 bars. Skarn minerals also record hypersaline fluids with similar homogenization temperatures to those measured in nearby igneous rocks but have slightly lower salinities (23.3–26 wt % NaCl equiv). Inclusions from limestone next to skarn have similar homogenization temperatures and salinities (33 wt % NaCl equiv) to skarn and igneous rock inclusions. As evidenced by boiling fluids in the phyllic zone and supercritical fluids in silica–pyrite, hydrostatic regime dominated during retrograde alteration of skarn and porphyry–style alteration. The vapor phase formed by boiling fluids in igneous rocks nearby skarn produced a vapor plume that may be responsible for retrograde alteration and also of cavernous porosity in carbonate rocks at the skarn front. Overall, Caicayen is similar to worldwide copper skarn systems emplaced at shallow levels, but it lacks multiple intrusive phases and successive fluid pulses; thus, resulting in lower copper grades than mined in other copper skarns.

Regional oxygen isotope zonation at Broken Hill, New South Wales, Australia; large-scale fluid flow and implications for Pb-Zn-Ag mineralization

Metasedimentary and metavolcanic rocks at Broken Hill, Australia, show regional-scale lowering of delta 18 O values from as high as 16 per mil in Paragon Group metasedimentary rocks to values as low as 7 per mil within a few hundreds of meters of Pb-Zn-Ag orebodies. Such large-scale oxygen isotope resetting cannot be achieved by closed-system processes (such as partial melting or devolatization), implying that the rocks were affected by fluid flow. The preservation of peak metamorphic 18 O fractionations between coexisting minerals, and a lack of correlation between delta 18 O values and the intensity of retrogression, suggests that oxygen isotope resetting occurred at, or prior to, the peak of regional metamorphism. Fluid flow during the peak of regional metamorphism is unlikely due to widespread fluid-absent partial melting and internal buffering of volatile activities at that time. Thus, fluid flow most probably predated regional metamorphism. The association of rocks with low delta 18 O values with the Pb-Zn-Ag orebodies suggests a link with mineralization. The delta 18 O values of rocks adjacent to the orebodies are similar to those recorded in volcanic-hosted massive sulfide deposits where convective circulation of ocean water has occurred. Base metal mineralization at Broken Hill may have occurred at hydrothermal vents at, or close to, the sea floor producing localized low delta 18 O values with later kilometer-scale fluid circulation, possibly driven by the same or related intrusions, causing regional resetting of oxygen isotopes. However, mineralization in skarn systems associated with pre-regional metamorphic granites is also a plausible model that could explain the regional delta 18 O trends and much of the other geochemical data. Alternatively, the large-scale resetting of oxygen isotopes may be unrelated to mineralization. While the stable isotope data may not unambiguously constrain the origin of the base metal orebody, they indicate that mineralization most probably occurred prior to regional metamorphism.

Carbonatite genesis: A reexamination of the role of intrusion-related pneumatolytic skarn processes in limestone melting

Research Article| April 01, 1999 Carbonatite genesis: A reexamination of the role of intrusion-related pneumatolytic skarn processes in limestone melting David R. Lentz David R. Lentz 1Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (4): 335–338. https://doi.org/10.1130/0091-7613(1999)027<0335:CGAROT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David R. Lentz; Carbonatite genesis: A reexamination of the role of intrusion-related pneumatolytic skarn processes in limestone melting. Geology 1999;; 27 (4): 335–338. doi: https://doi.org/10.1130/0091-7613(1999)027<0335:CGAROT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The unusual elemental enrichment typical of many carbonatites and their stable and radiogenic isotope signatures—which are unlike those of sedimentary limestones—forced researchers to abandon limestone melting theories in the early 1960s and to support mantle-related models of carbonatite genesis. However, the fluid compositions [CO2/(H2O + CO2) = 0.05] required to melt limestone near its eutectic in the CaO-MgO-CO2-H2O system (600–675 °C) are virtually identical to those found in infiltrative magmatic-hydrothermal, skarn-forming systems; therefore, carbonates within such systems would melt via volatile fluxing. Skarn-related decarbonation reactions produce the CO2 required to form the carbonic acid (H2CO3) in the infiltrative H2O-rich fluid essential to carbonate melting. In addition to H2O, other fluxes (HF, HCl, H3PO4) and related salts derived from fluid-phase saturation of silicate intrusions could further depress the carbonate-melting eutectic temperatures, as well as enhance mass transfer of mineralizing elements into a forming skarn system and any low-viscosity carbonate melts produced within the skarn. The isotopic signatures of the resultant carbonate melts should reflect the elemental mass transfer of constituents from the intrusion, as well as Rayleigh decarbonation and elemental mixing processes typical of contact-metasomatic (pneumatolytic) processes. Many intrusions exsolving volatiles into limestone during final stages of solidification should produce some carbonate melt. Only carbonatites with enrichments in F, P, Sr, Nb, U, Th, and rare-earth-elements, have been considered intrusive melts, whereas the solidified products of other melts may have been erroneously considered hydrothermal veins. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.