Skarn Fluids Research Articles

Geochemistry and geochronology of multi-generation garnet: New insights on the genesis and fluid evolution of prograde skarn formation

Origin of garnet in skarn (magmatic vs. hydrothermal) and the prograde skarn fluid evolution are still controversial. Two generations of garnet (Grt1, Grt2) were identified at the Tongshankou deposit: Grt1 is anisotropic with oscillatory zoning and resorbed boundary, whilst Grt2 grew around Grt1 and formed oscillatory rims. In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept 206Pb/238U age of 142.4 ± 2.8 Ma (n = 57; MSWD = 1.16) and 142.3 ± 9.6 Ma (n = 60; MSWD = 1.06), respectively, coeval with the ore formation and ore-related granodiorite emplacement. Positive Eu anomaly, non-CHARAC Y/Ho value and low TiO2 content, together with the mineral assemblages indicate that both Grt1 and Grt2 have a hydrothermal origin. The existence of melt and melt-fluid inclusions in Grt1, together with similar LREE-enriched patterns to the granodiorite, further indicate that Grt1 may have formed in the magmatic-hydrothermal transition. Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO2. The inner gray Grt2 rims (Grt2A) is HREE-enriched with low U contents, indicating that fluid II has nearly neutral pH and high fO2. The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims (Grt2B) show that the evolved magmatic fluid II had mixed with an external fluid, characterized by being mildly acidic pH and with high fO2. Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou, and the mixing of meteoric water may have been prominent in the prograde skarn stage.

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

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

Skarn fluid sources as indicators of timing of Cordilleran arc emergence and paleogeography in the southwestern United States

Abstract Oxygen isotope ratios of garnet provide well-established means to investigate crustal fluid histories. Traditionally, δ18O values from skarn garnets have been used to track the hydrothermal evolution of an individual skarn body through time. We, however, use garnet from 14 skarns from the Jurassic (ca. 175 to ca. 148 Ma) Cordilleran margin arc (southwestern United States) to provide regional tectonic context to arc magmatism and hydrothermal activity. We document arc-wide garnet δ18O variability of ~19‰ (−8.9‰ to +10.3‰, n = 159), providing a record of contrasting meteoric fluid ingress between northern (Sierra Nevada) and southern (Mojave Desert) arc segments. Strongly negative garnet δ18O values (≤−3‰) are limited to the Mojave Desert arc segment and can only form in the presence of meteoric fluid, requiring shallow formation in subaerial crust. When combined with U-Pb garnet ages, the δ18O data provide a minimum radiometric age of local subaerial arc emergence and temporal constraint on the migration of the Jurassic paleoshoreline in the Mojave Desert section of the arc.

MVT-Like Fluorite Deposits and Oligocene Magmatic-Hydrothermal Fluorite–Be–U–Mo–P–V Overprints in Northern Coahuila, Mexico

The formation of most fluorite deposits in northern Coahuila (NE Mexico) is explained by MVT models, and is a part of the metallogenic province of northeastern Mexico. However, fluorite skarn deposits also occur in the same region, and there is evidence for late hydrothermal manifestations with no clear origin and evolution. The latter are the main focus of this study; in particular, F–Be–U–Mo–V–P stringers in the Aguachile-Cuatro Palmas area that overprint preexisting fluorite mantos. The region experienced the emplacement of several intrusives during the Eocene and the Oligocene that are collectively grouped into the East Mexico Alkaline Province (EMAP) and postdate MVT-like deposits. Some of these intrusives have associated skarn deposits; most of them are polymetallic, but the unusual El Pilote deposit contains fluorite mineralisation that was remobilised from MVT-like deposits. The formation of the Aguachile deposit (and, collectively, part of the Cuatro Palmas deposit) has been attributed to a shallow retrograde skarn model. The Cuatro Palmas and Las Alicias fluorite deposits consist of MVT-like deposits overprinted by late hydrothermal fluorite mineralisation rich in Be–U–Mo–V–P, and the Aguachile deposit consists entirely of the latter type. The systematic fluid inclusion study of MVT-like, skarn, and late hydrothermal fluorite deposits reveals a very different distribution of temperature and salinity data that allows the discrimination of mineralising fluids for the type of deposit. MVT-like deposits were formed by fluids with temperatures of homogenisation that range between 50 °C and 152 °C and salinities between 5 and 15.5 wt.% NaCl equivalent. The El Pilote fluorite skarn was formed by fluids with temperatures of homogenisation that range between 78 °C and 394 °C and salinities between 5 and 34 wt.% NaCl equivalent, and include CaCl2-rich brines with salinities that range between 24.5 and 29.1 wt.% CaCl2. Late shallow fluorite–Be–U–Mo–V–P hydrothermal deposits were formed by fluids with temperatures of homogenisation that range between 70 °C and 180 °C and salinities between 0.9 and 3.4 wt.% NaCl equivalent; the sole exception to the above is the La Fácil deposit, with salinities that range between 7.9 and 8.8 wt.% NaCl equivalent. While temperatures of homogenisation are similar between MVT-like and late hydrothermal deposits, and both even have hydrocarbon-rich fluid inclusion associations, the salinity of late deposits is similar to that of retrograde skarn fluids, although further diluted. However, homogenisation temperatures tend to be higher in late hydrothermal than in MVT-like deposits, thus making them more similar to retrograde skarn fluids. Although this characteristic cannot solely establish a genetic link between a retrograde skarn model and late hydrothermal deposits in the study area, the characteristics of fluids associated with the latter separate these deposits from those ascribed to an MVT-like model. Assuming that mineralising fluids for late fluorite–Be–U–Mo–V–P hydrothermal deposits may correspond to a retrograde skarn (or “epithermal”) deposit, the source for fluorine may be either from (A) the dissolution of earlier formed MVT-like deposits, (B) the entrainment of remaining F-rich basinal brines, or (C) hydrothermal fluids exsolved from highly evolved magmas. Possibilities A and B are feasible due to a hypothetical situation similar to the El Pilote skarn, and due to the occurrence of hydrocarbon-rich fluid inclusions at the La Fácil deposit. Possibility C is feasible because intrusive bodies related to highly evolved magmas would have provided other highly lithophile elements like Be, U and Mo upon the exsolution of their hydrothermal fluids. Such intrusive bodies occur in both study areas, and are particularly conspicuous at the Aguachile collapse structure.

Evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the Cretaceous Muamsa Granite, South Korea

The Muamsa Granite in the Hwanggangri Mineralized Zone, South Korea, intruded the Cambro-Ordovician Samtaesan Formation, which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite. Intrusion of the granite caused an isotopic depletion of 13C and 18O in the carbonate rocks. Calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn. It was initiated with volatilization accompanying calcsilicate-forming reactions, and then, with increased permeability, H 2O-rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid-rock interaction. For the skarn fluid infiltration yielded fluid–rock ratios as high as 4.2 in open system, and caused increased 18O depletion rates compared to the volatilization stage. More than 70% of the isotopic depletion in the calcitic marble is attributed to the infiltration process. With such strong infiltration, mineral assemblages such as phlogopite in the absence of K-feldspar, phlogopite+diopside, and wollastonite are commonly observed in the marble. The dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of X CO2 and low fluid–rock ratios (<1.0). Active decarbonation in dolomitic marble was restricted by silica-poor dolomite protolith, which caused limited fluid infiltration and resulted in high X CO2 mineral assemblages represented by dolomite+calcite+diopside+forsterite and more 13C-depleted nature compared to dolomite+calcite+tremolite+forsterite assemblage.

U-Pb ages of plutonism, wollastonite formation, and deformation in the central part of the Lac-Saint-Jean anorthosite suite

The Lac-Saint-Jean anorthosite suite (LSJAS) is the largest ensemble of plutons in the Grenville Province. Zircons from a pluton in the central part of the LSJAS yield an age of 1140+10–5 Ma, which accords with existing 1157–1142 Ma ages determined from the southern part of the LSJAS. The Du Bras granite is a linear pluton emplaced into the dominantly dextral Pipmuacan shear zone (PSZ). The U–Pb zircon age of 1148 ± 2 Ma for the Du Bras granite confirms earlier proposals that it is a granophyre, formed at greater depths by partial melting of paragneiss by the heat of the crystallizing anorthosite. Parts of the pluton are strongly deformed by the shear zone, and other parts cut the shear zone. This suggests that the pluton dates one phase of movement of the PSZ. The Canton-St-Onge wollastonite deposit was developed in paragneisses preserved within the PSZ. Titanite from this deposit gives an age of 1163 ± 18 Ma, identical within error to that of the Du Bras granite, indicating that this pluton may have provided the skarn fluids. A second plutonic cycle comprises the post-tectonic Astra granite, which stitches the PSZ, and an amazonite-bearing pegmatite dyke. Zircons gave an age of 1028 ± 2 Ma for the Astra granite. The amazonite-bearing dyke only yielded monazite, one fraction of which gave an age similar to that of the Astra granite. Other fractions record the effects of a thermal event at 1020–1017 Ma that may be related to a widespread magmatic event. The undeformed Venus de Milot syenitic intrusion yields a U–Pb age of 988 ± 2 Ma. It was emplaced along the northwest-trending Lac Rioux fault, indicating that one phase of movement occurred before 988 Ma. Another strand of the same fault system also cuts the intrusion, showing later reactivation that may have been associated with the formation of the Saguenay Graben.

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.

The Palaeoproterozoic boron deposits in eastern Liaoning, China: a metamorphosed evaporite

We have carried out a field study of the Palaeoproterozoic borate deposits of eastern Liaoning Province, China. The borates are hosted within Mg-silicate and Mg-carbonate rocks and have previously been interpreted as either skarns or metamorphosed volcanogenic exhalative deposits. The deposits contain textures that are compatible with a sedimentary origin of the borates and there is a lack of the associated mineralization that is characteristic of magnesian skarns. In addition, preliminary boron isotope data indicate that the boron within the borates was not derived from the footwall rocks that have been proposed as the source of the skarn fluids. A purely exhalative model can be dismissed on the basis of geochemical arguments and comparison with modern environments, which show that borates are too soluble to precipitate directly from hydrothermal fluids. The geology and field relationships show a number of similarities with recent non-marine evaporite settings and young continental rifts in arid environments. We conclude, therefore, that the Liaoning deposits are metamorphosed evaporites, in which the boron was originally present as primary, evaporite borate minerals. This model places the Liaoning borates in the same category as the majority of other economic borate deposits in the world.

Mineral and fluid geochemistry of the Hoosier Vein, Chloride mining district, Sierra County, New Mexico

Quartz veins in the Chloride mining district, Sierra County, New Mexico, host an unusual iron-poor calc-silicate and copper-rich, iron-poor sulfide mineral assemblage. The Hoosier vein, crosscutting Paleozoic sedimentary and Tertiary volcanic rocks, was studied in detail. Bornite, chalcocite, chalcopyrite, galena, sphalerite, and traces of pyrite and hematite are present. Silver occurs in chalcocite (less than 32 wt %) and bornite (less than 0.7 wt %). Calc-silicates include pyroxene (Jo (sub 18-91) , Di (sub 8-73) , Hd (sub 0-18) , garnet (Ad (sub 20-74) , epidote Ps (sub 30-40) , prehnite, and subcalcic amphibole. Three paragenetic stages have been recognized. Stage 1 brecciation, cemented by quartz, garnet-epidote, and sulfides, cuts pyroxene skarn replacing carbonate rocks and K feldspar alteration of volcanic rocks. Stage 2 silver-rich bornite-chalcocite overlaps the brecciation and banded vein stages. Stage 3 banded quartz veins with minor sulfides or calc-silicates cut earlier stages. Average quartz fluid inclusion T h values range from 284 degrees (200 m below the present surface) to 261 degrees C (present surface) and decrease to 241 degrees C in later, shallow veins. Stage 1 fluids were boiling, indicating a paleodepth of 500 m. Fluid salinity in early stages varied from 0.35 to 5.85 wt percent NaCl equiv. Later stage fluids had low salinities (less than 1 wt % NaCl equiv). Early skarn fluids evolved into stage 1 fluids with increasing a (sub O 2 ) and pH due to boiling. High a (sub O 2 ) and near-neutral pH suppressed iron solubility and decreased available reduced sulfur. Fluids from adjacent red beds possibly supplied metals for stage 2 mineralization.

Origin and evolution of skarn fluids, Empire zinc skarns, Central Mining District, New Mexico, U.S.A.

Garnet-pyroxene-sphalerite skarns in the Empire Mine replace Paleozoic carbonates adjacent to the Tertiary Hanover-Fierro granodiorite. Skarn geometry suggests that fluids migrated up pre-ore dikes, faults and the igneous contact, and were deflected laterally into the permeable Tierra Blanca Limestone beneath the relatively impermeable Parting Shale. Silicates associated with propylitically altered pre-ore dikes are enriched in deuterium (D), and depleted in 18O relative to the Hanover-Fierro pluton and post-ore igneous rocks. Early skarn silicates are also depleted in 18O with respect to the pluton, while later skarn minerals are depleted in both D and 18O. Variations in isotope composition of alteration and skarn minerals indicate isotope heterogeneities in mineralizing fluids, even at the small scale of centimeters. Isotope thermometry indicates that there is some degree of subsolidus re-equilibration of igneous and alteration minerals. Several possible fluid flow regimes may have operated to produce the fluids calculated to be in exchange equilibrium with the various rocks and minerals of the Empire skarn system, and mixing of end-member meteoric, formation and magmatic fluids in different proportions can produce observed δD δ 18O trends. An end-member magmatic fluid could produce the D-enrichment observed for early skarn fluids, but this would require isolating magmatic fluids from external fluid sources during cooling of the system from magmatic temperatures of 700°C to skarn temperatures of the order of ≤ 400°C. The D-enrichment may also be explained by the mixing of magmatic and formation waters. Lower δD values, however, require that a large proportion of late-stage skarn fluids must be a D-depleted Tertiary meteoric water, and magmatic water is restricted to a relatively minor component. The end-member mixing approach indicates significant changes in fluid flow systematics over a relatively narrow range in temperature. Alternatively, observed trends in both δD and δ 18O for skarn fluids can also be reproduced by interacting a D-depleted meteoric water with the Hanover-Fierro pluton at low and variable system water-rock ratios, and temperatures between 250 and 400°C. During migration along the long fluid flow paths implied by the low system water-rock ratios (≤0.1), the salinity of dilute meteoric waters could increase through interaction with minerals or leaking fluid inclusions in the country rock. Correlation of isotope depletions of the carbonate wallrocks with inferred fluid flow conduits, suggests significant amounts of fluid-rock exchange at relatively high local water-rock ratios during focusing of flow by critical structures. Although different C sources might require smaller values, it is clear that large (>1) local water-rock ratios are required to produce depletions observed in both 18O and 13C in hydrothermal calcites. Stable isotope evidence does not require the presence of a significant magmatic fluid component, and suggests that the bulk of the skarn fluids could instead be derived predominantly from a D-depleted meteoric water.

The Buckhorn Mountain (Crown Jewel) gold skarn deposit, Okanogan County, Washington

The gold skarn system at Buckhorn Mountain is related to the Cretaceous (?) Buckhorn Mountain pluton (diorite to granodiorite) and its associated dikes and sills. The host rocks are Pennsylvanian to Triassic (?) clastic and carbonate sedimentary rocks of the Thompson assemblage and the andesitic flows of the Brooklyn Formation. These have been altered to proximal garnet-magnetite-sulfide, intermediate garnet-pyroxene-epidote-sulfide, and distal pyroxene and hornfels assemblages. Typically, the pyroxene is iron rich (Hd (sub 20-94) ), averaging Hd 51 , and the garnet is intermediate granditc (Ad (sub 36-75) ). Retrograde epidote, calcite, amphibole, and zoisite are common. Skarn associations vary according to original lithologies; the carbonate-rich layers altered to a more garnet-magnetite rich assemblage, whereas the argillaceous and siliceous layers altered to a pyroxene dominant assemblage. Alteration in the intrusion takes many forms, including potassic, propylitic, argillic, albitization, and complete alteration to garnet-epidote-pyroxene skarn.Gold mineralization is spatially associated with a variety of skarn assemblages in locations ranging from proximal to distal. There are positive correlations of gold with epidote, magnetite, bismuthinite, native bismuth, pyrrhotite, and cobaltitc. Structure and original lithologies were important controls for fluid movement, and therefore, gold mineralization. The gold probably was carried in reduced fluids by bisulfide complexes and probably was deposited at temperatures less than 400 degrees C.Average pressure-corrected temperatures for skarn-forming fluids are 465 degrees C for prograde (garnet-pyroxene) skarn and 360 degrees C for retrograde (epidote) skarn. Fluids in skarn minerals, as measured from last ice-melting phenomena in fluid inclusions, are undersaturated, and fall within the 18 to 22 wt percent NaCl equiv range. No daughter minerals were found in any skarn mineral fluid inclusions. Liquid to vapor ratios are relatively constant, implying that there was no boiling. A comparison of prograde skarn and quartz vein fluid inclusion characteristics indicates that the two may have formed at roughly the same time from similar fluids. The skarn formed from pervasive fluid infiltration under dominantly lithostatic conditions, whereas quartz veins formed from fluids channeled along fault structures, largely under hydrostatic conditions.

Base metal exchange between magnetite and a chloride-rich hydrothermal fluid

Base metal exchange reactions between magnetite and a supercritical aqueous chloride solution have been experimentally calibrated at T = 600 to 800°C, P = 2 kb, and total chloride = 2.06 M. The results are expressed as the distribution coefficients Kd MFe= (X MX Fe ) fl (X MX Fe ) s , where fl and s represent the fluid and the solid solution, respectively, and X M= m M (m M + m Fe total (m M + m Fe total ) are the mole fractions of the metal of interest and of total Fe, respectively. The experimentally determined distribution coefficients are Kd MnFe = 64, 33, and 18, and Kd ZnFe=161, 87, and 53, at T=600, 700, and 800°C, respectively. Linear regression yields the best fit equations: ln Kd ZnFe = 5198/ T(K) − 0.872 ( r 2 = 0.9999) and ln Kd MnFe = 5941/ T(K) − 2.63 ( r 2 = 0.9995). Kd values were independent of composition over the range of concentrations investigated. Clearly, both Mn and Zn are strongly fractionated into the fluid with respect to Fe. A combination of Kd MnFe and Kd ZnFe yields Kd ZnMn = 2.54, 2.67, and 2.94 at 600, 700, and 800°C, where Kd = (X Zn X Mn ) fl / (X Zn X Fe ) s . The partitioning and temperature dependence described by Kd ZnMn are significantly weaker than those shown by Kd MnFe and Kd ZnFe. Results for the CdFe and CuFe exchange reactions indicate that both Cd and Cu are extremely partitioned into the fluid with respect to Fe and very strongly partitioned into the fluid with respect to Zn and Mn. The experimental results suggest that even minor to trace concentrations of these base metals in natural magnetites indicate strong enrichment of Zn, Mn, Cu, and Cd relative to Fe in chloride-rich hydrothermal fluids. In particular, Kd MnFe was applied to magnetites from skarn deposits. Calculations at 360 to 420°C suggest that skarn fluids in equilibrium with magnetites containing only minor concentrations of Mn can have Mn/Fe ratios greater than 1.

The CanTung E Zone scheelite skarn orebody, Tungsten, Northwest Territories; oxygen, hydrogen, and carbon isotope studies

Contact skarn formation at the CanTung scheelite deposit (Tungsten, Northwest Territories, Canada) involved replacement of calcitic marbles by garnet-pyroxene, pyroxene-pyrrhotite, amphibole-pyrrhotite, and bidtite-pyrrhotite skarns, adjacent to a peraluminous biotite mon-zogranite pluton (delta 18 O (sub whole rock) = 10.9ppm). Replacement of marble (delta 18 O = 19.7-20.5ppm; delta 13 C = -0.5 to +1.7ppm) by skarns resulted in significant 18 O and 13 C depletion. Fluid inclusion studies presented in Mathieson and Clark (1984) and oxygen isotope thermometry data presented here are consistent with the initial development, essentially simultaneously, of a zoned array of both anhydrous and hydrous skarns at 450 degrees to 500 degrees C, as proposed by Dick and Hodgson (1982). Development of hydrous skarn continued to lower temperatures, as indicated by the fluid inclusion studies of Mathieson and Clark (1984) and suggested by the calcite-pyroxene isotopic data presented here.The hydrogen and oxygen isotope data at CanTung suggest that early skarn-forming fluids were comprised mainly of fluid in equilibrium with the intrusion (> or = 75 mole % intrusive fluid; or = 90 mole % intrusive fluid; < or = 10 mole % meteoric water). These portions of meteoric water would be lower for any hydrous skarns formed at T < 475 degrees C. Because the calculated delta D values of the hydrous skarn fluid would increase with decreasing temperature, any retrograde development of hydrous skarn was not accompanied by a progressive influx of meteoric water. Such increases could be the result of an influx of D-enriched formation water or of progressive formation of CH 4 in the skarn fluids at lower temperatures (<350 degrees C). The hydrogen isotope data indicate that meteoric water was not a dominant component of the skarn fluids throughout most of the period of skarn development. However, the isotopic data alone cannot rule out the possibility that the skarn fluids were derived from formation waters which were isotopically similar to magmatic water.Mass balance calculations of 18 O and 13 C depletions resulting from equilibrium decarbonation at 475 degrees C indicate that the observed depletions of 18 O and 13 C in skarn calcites (delta 18 O = 12.9ppm, delta 13 C = -6.2ppm) could not have arisen simply from decarbonation of the marble host rocks. Mass balance calculations also indicate that the depletions could not have arisen from simple isothermal isotope exchange between the skarn-forming fluid and the marble wall rock at 475 degrees C. However, the oxygen isotope compositions of the skarn calcites can be interpreted to have formed over a range of falling temperature ( approximately 400 degrees -270 degrees C) from a fluid of nearly constant oxygen and carbon isotope composition, which was isotopically equivalent to that responsible for formation of the higher temperature ( approximately 475 degrees C) skarn silicates.The hydrogen and oxygen isotope data indicate that there was only a limited and nearly constant influx of meteoric water into the skarn system. The hydrothermal fluids associated with each of the skarn facies have nearly identical ranges of salinities (Mathieson and Clark, 1984) and nearly identical oxygen isotope compositions. With one exception, the calculated delta D values of the skarn fluid for both amphibole and biotite skarns are similar and display relatively little variation (7ppm). These characteristics contrast with the wide variations in scheelite and pyrrhotite contents in the skarns (Mathieson, 1982; Mathieson and Clark, 1984) and suggest that variation in the influx of meteoric water, with accompanying changes in salinity, hydrogen isotope composition, or temperature, was not the controlling factor governing scheelite deposition at CanTung. The isotope data also do not support a model of skarn petrogenesis for CanTung which invokes an early high-temperature, exclusively anhydrous skarn stage formed dominantly from magmatic water, followed by a later, distinctly lower temperature (retrograde) hydrous stage formed from fluids dominated by meteoric water.

Contact skarn formation at Elkhorn, Montana; II, Origin and evolution of C-O-H skarn fluids

Contact skarn formation at Elkhorn, Montana; II, Origin and evolution of C-O-H skarn fluids