Zonation of Sulfate and Sulfide Minerals and Isotopic Composition in the Far Southeast Porphyry and Lepanto Epithermal Cu–Au Deposits,Philippines

The world‐class Far Southeast (FSE) porphyry system, Philippines, includes the FSE Cu–Au porphyry deposit, the Lepanto Cu–Au high‐sulfidation deposit and the Victoria–Teresa Au–Ag intermediate‐sulfidation veins, centered on the intrusive complex of dioritic composition. The Lepanto and FSE deposits are genetically related and both share an evolution characterized by early stage 1 alteration (deep FSE potassic, shallow Lepanto advanced argillic‐silicic, both at ~1.4 Ma), followed by stage 2 phyllic alteration (at ~1.3 Ma); the dominant ore mineral deposition within the FSE porphyry and the Lepanto epithermal deposits occurred during stage 2. We determined the chemical and S isotopic composition of sulfate and sulfide minerals from Lepanto, including stage 1 alunite (12 to 28 permil), aluminum–phosphate–sulfate (APS) minerals (14 to 21 permil) and pyrite (?4 to 2 permil), stage 2 sulfides (mainly enargite–luzonite and some pyrite, ?10 to ?1 permil), and late stage 2 sulfates (barite and anhydrite, 21 to 27 permil). The minerals from FSE include stage 2 chalcopyrite (1.6 to 2.6 permil), pyrite (1.1 to 3.4 permil) and anhydrite (13 to 25 permil). The whole‐rock S isotopic composition of weakly altered syn‐mineral intrusions is 2.0 permil. Stage 1 quartz–alunite–pyrite of the Lepanto lithocap, above about 650 m elevation, formed from acidic condensates of magmatic vapor at the same time as hypersaline liquid formed potassic alteration (biotite) near sea level. The S isotopic composition of stage 1 alunite–pyrite record temperatures of approximately 300–400°C for the vapor condensate directly over the porphyry deposit; this cooled to <250°C as the acidic condensate flowed to the NW along the Lepanto fault where it cut the unconformity at the top of the basement. Stage 1 alunite at the base of the advanced argillic lithocap over FSE contains cores of APS minerals with Sr, Ba and Ca; based on back‐scattered electron images and ion microprobe data, these APS minerals show a large degree of chemical and S‐isotopic heterogeneity within and between samples. The variation in S isotopic values in these finely banded stage 1 alunite and APS minerals (16 permil range), as well as that of pyrite (6 permil range) was due largely to changes in temperature, and perhaps variation in redox conditions (average ~ 2:1 H₂S:SO₄). Such fluctuations could have been related to fluid pulses caused by injection of mafic melt into the diorite magma chamber, supported by mafic xenoliths hosted in diorite of an earlier intrusion. The S isotopic values of stage 2 minerals indicate temperatures as high as 400°C near sea level in the porphyry deposit, associated with a relatively reduced fluid (~10:1 H₂S:SO₄) responsible for deposition of chalcopyrite. Stage 2 fluids were relatively oxidized in the Lepanto lithocap, with an H₂S:SO₄ ratio of about 4. The oxidation resulted from cooling, which was caused by boiling during ascent and then dilution with steam‐heated meteoric water in the lithocap. This cooling also resulted in the sulfidation state of minerals increasing from chalcopyrite stability in the porphyry deposit to that of enargite in the lithocap‐hosted high‐sulfidation deposit. The temperature at the base of the lithocap during stage 2 was ≥300°C, cooling to <250°C within the main lithocap, and about 200°C towards the limit of the Lepanto orebody, approximately 2 km NW of the porphyry deposit. Approximate 300°C and 200°C isotherms, estimated from S isotopic and fluid inclusion temperatures during stage 1 and stage 2, shifted towards the core of the FSE porphyry deposit with time. This general retreat in isotherms was more than 500 m laterally within Lepanto and 500 m vertically within FSE as the magmatic–hydrothermal system evolved and collapsed over the magmatic center. During this evolution, there is also evidence recorded by large S isotopic variations in individual crystals for sharp pulses of higher temperature, relatively reduced fluid injected into the porphyry deposit.     

Mineralogy, geochemistry, and origin of hydrothermal manganese veins at Wadi Maliek, Southern Eastern Desert, Egypt

The present work deals with the geology, mineralogy, geochemistry, and origin of the metagabbroic-hosted manganese deposits at Wadi Maliek in the southern Eastern Desert of Egypt. The manganese veins are found in the shear zones and channel ways of the fault planes within the metagabbroic rocks pointing to those hydrothermal solutions carrying manganese and iron load penetrating along these fractures. These faults are striking N 80° E?CS 80° W with dipping 65°. These veins vary in thickness from 15?cm up to 125?cm wide; each vein may show difference in thickness from bottom to top. Microscopic examinations, X-ray diffraction, infrared spectral, differential thermal (DTA), thermogravimetric (TGA), and ESEM-EDAX analyses revealed that the manganese minerals consist mainly of pyrolusite, psilomelane, and ramsdellite. Goethite and hematite are the common iron minerals. Petrographically, the manganese deposits can be classified into three ore types based on the predominance of manganese and iron minerals: manganese, manganese?Ciron, and iron ore types. The geochemistry of Maliek deposits indicated that the total averages of some major oxides in manganese, manganese?Ciron, and iron ore types are respectively as follows: SiO₂ (15.64%, 11.52%, and 20.58%), MnO (39.9%, 17.81%, and 0.77%), FeO* (7.13%, 33.31%, and 37.08%), CaO (5.89%, 5.82%, and 5.32%), and Na₂O (1.04%, 1.61%, and 1.53%). With regard to trace elements, the Maliek manganese deposits are rich in Zn, Ba, Pb, Sr, and V. Based on the geological, mineralogical, and geochemical results, the studied manganese deposits are considered to be precipitated from hydrothermal solution.     

In situ iron isotope ratio determination using UV-femtosecond laser ablation with application to hydrothermal ore formation processes

The feasibility of in situ stable Fe isotope ratio measurements using UV-femtosecond laser ablation connected to a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been investigated. Different types of matrices, independently determined by solution MC-ICP-MS after chromatographic separation of Fe, have been analysed by laser ablation using the isotopically certified iron reference material IRMM-014 as the bracketing standard. The samples have been pure iron metal (JM Puratronic), Fe-meteorites (North Chile, Glenormiston and Toluca), the meteorite phases kamacite and taenite in Toluca and Fe-sulphides. Furthermore, Fe isotope ratios from hydrothermal hematite, siderite and goethite from an old mining area in the Schwarzwald, Germany, and of magnetite from the metamorphic Biwabik iron formation have been determined. The results show that a precision of better than 0.1‰ (2 sigma) can be achieved with laser ablation and that all the results obtained agree with those determined by solution ICP to better than 0.1‰. This precision and accuracy is achievable in both raster and spot ablation mode. A matrix-matched bracketing standard is not required , and all these materials can be measured accurately against a metal standard. The hydrothermal minerals show significant Fe isotope zonations. In some samples the range of δ⁵⁶Fe in a single aggregate encompasses the entire spectrum of ratios found by bulk solution analyses in multiple samples distributed over the whole mining district. For example, isotopic zonations found in secondary fibrous hematites show a continuous change in the δ⁵⁶Fe values from −0.5‰ in the core to −1.8‰ in the rim. Primary hydrothermal siderite shows the reverse pattern with lighter values in the core than in the rim. While the siderite is thought to record primary fluid histories, the hematite pattern is interpreted as a reworked isotopic signature generated by oxic dissolution of primary zoned siderite and immediate close range re-precipitation of the oxidized Fe. Abrupt changes are documented for secondary goethite showing a distinct overgrowth that is 0.4‰ lighter than the core of the grain. If indeed Fe isotopes in secondary minerals from hydrothermal ore deposits record the initial isotopic signatures of their precursor minerals, and these in turn record hydrothermal fluid histories, then the tools are in place for a detailed reconstruction of the deposit‘s genesis. We expect similar observations from other Fe-rich deposits formed at intermediate and low-temperatures (e.g. banded iron formations). Laser ablation now provides us with the spatial resolution that adds a further dimension to our interpretation of stable Fe-isotope fractionation.     

Hydrothermal alterations in the Echassières granitic cupola (Massif central,france)

Detailed petrographic and mineralogic investigations of an albite-lepidolite granite at Echassières (Massif Central, France; scientific deep drill program) shows the existence of hydrothermal stages which are closely related to the magmatic and structural history. According to fluid inclusion data, K-Ar datations and ¹⁸O/¹⁶O-D/H compositions of secondary minerals, two successive hydrothermal periods have been recognized. The early one (273–268 million years) produced a series of aluminous phyllosilicates: muscovite, pyrophyllite, donbassite, tosudite, kaolinite which are observed as vein deposits (<10 mm wide) and alteration products of primary minerals in wall-rocks. The vein system was sealed by monomineralic assemblages during a cooling period (400–150°C). This early hydrothermal alteration stage was controlled by interactions of rock with low salinity (1–10 wt% NaCl equivalent) fluids expelled from the granitic body during the cooling processes. The chemical properties of these fluids were the following: low pH, very low Mg and Fe and high Li, Na and K contents. Thermodynamic calculations show that the sequence pyrophyllite, Li-bearing donbassite, tosudite is mostly temperature dependent. From the chemical composition of secondary minerals and isotopic data it can be deduced that these fluids, which have a meteoric origin, have been expelled from the granite body during its cooling period and after interaction with it at high temperature. The late hydrothermal stage corresponds to deposits of fluorite and Fe-Mg rich illite (151 million years) in subvertical fractures. Temperature conditions did not exceed 250° C and fluids came through the surrounding metamorphic rocks into the granitic body. IIlite/smectite mixed-layer minerals have been identified in subvertical fractures which were opened during Tertiary periods. In the host micaschists, successive hydrothermal alterations took place during the cooling of the Beauvoir granite. Early magmatic fluids interacted with these micaschists. Locally, the metamorphic assemblage is replaced by a metasomatic one. Secondary topaz and (F, Li)-rich mica crystals were formed over a range of 450 of 150°C. Later hydrothermal fluids reacted with the country rocks to form phengite-biotite, chlorite-illite and kaolinite over a range of 300 to 150°C. Illite/smectite mixed-layer minerals crystallized in the roof micaschists and within the Beauvoir granite during the Tertiary alteration period. Meteoric water invaded open fractures producing supergene alteration mineral assemblages.     

Hydrogen Isotope Study of Fluid Inclusions in Vein Quartz of the Hishikari Gold Deposits, Japan

Abstract: The origin of mineralizing fluids responsible for the Hishikari vein-type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (δD) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present δ values and the oxygen isotopic ratios (δ¹⁸O) previously reported by Shikazono and Nagayama (1993). The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500°C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450°C. The δD values were determined by mass spectrometer. The δD values of inclusion fluid obtained from quartz range from –61 to –114%. These are significantly lower than the δD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water. Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous δD values of lower than –100%. Such D depletion cannot be caused by simple oxygen-shift of meteoric water or by contribution of magmatic volatiles. The δD values of water released from the shale samples of the Shimanto–Supergroup, a major host to the Hishikari veins range from –132 to –148%. Therefore, the anomalous δD values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.     

Anhydrite-bearing rocks from the Rožná district (Moldanubian zone,Czech Republic): high-grade metamorphosed exhalites?

Several types of anhydrite-bearing rocks have been found in the amphibolite-facies metamorphosed rocks at the north-eastern margin of the Moldanubian Zone. Anhydrite either forms monomineralic bands up to 40 cm thick, or occurs in the form of disseminated grains in surrounding calc-silicate gneiss together with feldspar, scapolite, amphibole, pyroxene, epidote and pyrite. The isotopic composition of sulphur ('³⁴S=30.6 to 32.3‰) and strontium (⁸⁷Sr/⁸⁶Sr=0.70797 to 0.70781) in anhydrite may indicate a marine source of sulphate. The isotopic ratio of strontium is in the same range as that of metamorphosed strata-bound barite-sulphide ores, which have been previously described in the same area. The '³⁴S values of coexisting pyrite range from 21.4 to 22.5‰, the (³⁴S_{anhydrite-pyrite} corresponding to the metamorphic temperature of 600 to 660 °C. In contrast to many submarine-exhalative deposits, the oxygen isotopic compositions of anhydrite ('¹⁸O=9.3 to 10.2‰) are lighter than that of barite ('¹⁸O=10.4 to 13.8‰). This indicates that the both minerals are not in isotopic equilibrium. Therefore, it is probable that anhydrite and barite from the Ro—ná district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks. The '¹³C values in calcite ('¹³C=-17.2 to -18.7‰) from anhydrite-bearing rock are lower than those in distant marbles. As graphite is absent in anhydrite- and calcite-bearing rocks, impoverishment in the ¹³C isotope cannot be attributed to the graphite-carbonate isotopic exchange during metamorphism. It is proposed that low '¹³C values in carbonates are caused by pre-metamorphic oxidation of organic matter in course of hydrothermal processes. Anhydrite and anhydrite-bearing calc-silicate gneiss from the north-eastern part of the Moldanubian Zone are interpreted to be the high-grade metamorphosed analogue of anhydrite-rich exhalites commonly found in submarine-exhalative hydrothermal deposits.     

Quantification of mixing processes in ore-forming hydrothermal systems by combination of stable isotope and fluid inclusion analyses

In the Schwarzwald area, southwest Germany, more than 400 hydrothermal veins hosting different gangue and ore mineral assemblages cross-cut the crystalline basement rocks. Many of the post-Variscan fluorite-barite-quartz veins are considered to have precipitated through mixing of a deep saline brine with meteoric, low salinity waters. This hypothesis was tested using carbon, sulfur, and oxygen isotope data of sulfides, sulfates and calcite, coupled with fluid inclusion studies. Primary hydrothermal calcites from the deposits show a positive correlation of their δ¹³C (V-PDB) and δ¹⁸O (V-SMOW) values, which range from −12 to −3‰ and from 12 to 18.5‰, respectively. Carbon and oxygen isotope compositions of paragenetically young, remobilized calcite types are shifted towards higher values and range from −12 to −1‰ and from 20 to 25‰, respectively. We developed an improved calculation procedure for modeling the covariation of carbon and oxygen isotopes in calcite resulting from mixing of two fluids with different isotopic compositions and total carbon concentrations. In our model, the carbon speciation in the two model fluid end-members and the fluid mixtures are calculated using a speciation and reaction path code. The carbon and oxygen isotope covariation of primary Schwarzwald calcites can effectively be modeled by a mixing trend of a deep saline brine and a meteoric, low salinity water. Sulfur isotope data of barites from 44 hydrothermal fluorite-barite-quartz veins vary from 9 to 18‰ (CDT), sulfide ore minerals show δ³⁴S values between −14.4 and 2.9‰. Calculated sulfide-sulfate equilibrium temperatures are in the range between 300 and 350 °C. These temperatures differ significantly from the formation temperatures of 150 to 200 °C of most of the deposits as estimated from fluid inclusions, and are interpreted as preserved paleotemperatures of the deep aquifer. This assumption has been carefully checked against possible contamination of an equilibrated sulfide-sulfate system from the deep aquifer with sulfate from surface-derived sources, considering also the kinetics of the sulfide-sulfate isotope exchange. A combination of the S isotopic results with microthermometric fluid inclusion data and constraints on the temperature of the meteoric water was used to calculate mixing ratios of the two fluid end-members. The results indicate that mass fractions of the deep saline brine in the mixed fluid were between 0.5 and 0.75. Considering all geologic, geochemical and isotopic information, we propose that the majority of the post-Variscan hydrothermal veins in the Schwarzwald area were precipitated by district-scale mixing of a homogeneous deep saline brine with meteoric waters.     

广西东平地区碳酸锰矿床Ga含量高异常的发现及成因初探

Ga是一种典型的稀有分散元素,主要产于铝土矿、闪锌矿及煤矿之中。最近,在广西东平地区下三叠统北泗组碳酸锰矿床中发现Ga高异常含量,w(Ga)介于5.16×10~(-6)~82.80×10~(-6)之间,平均为33.76×10~(-6),达到了Ga工业品位标准;锰矿层和围岩中w(Ga)平均分别为46.40×10~(-6)、19.31×10~(-6),高于国内外已报道的大部分锰矿床。文中根据北泗组碳酸锰矿床地球化学特征,揭示了该锰矿床为热水沉积;同时,结合现代大洋铁锰沉积有关Ga的最新报道,提出北泗组碳酸锰矿床中Ga的赋存与含锰矿物密切相关,其来源与海底热液活动有关。最后,文中还利用Mn/Fe-Ga、Co-Ga关系图判别了古代铁锰沉积的成因类型。     

Mineralogy of metamorphosed manganese deposits of the South Urals

A mineralogical investigation of metamorphosed manganese rocks was carried out at ore deposits related to the Devonian volcanic complexes of the Magnitogorsk paleovolcanic belt of the South Urals. The mineralogical appearance of these rocks is determined by three consecutively formed groups of mineral assemblages: (1) assemblages occupying the main volume of orebodies and formed during low-grade regional metamorphism (T = 200−250°C, P = 2–3 kbar); (2) assemblages of segregated and metasomatic veinlets that fill the systems of late tectonic fractures; and (3) assemblages of near-surface supergene minerals. Sixty-one minerals have been identified in orebodies and crosscutting hydrothermal veinlets. The major minerals are quartz, hematite, hausmannite, braunite, tephroite, andradite, epidote, rhodonite, caryopilite, calcite, and rhodochrosite. The mineral assemblages of metamorphosed manganese rocks (metamanganolites) are characterized. Chemical compositions of braunite, epidote-group minerals, piemontite, pyroxenes, rhodonite, pyroxmangite, and winchite are considered. The bibliography on geology and mineralogy of the South Ural manganese deposits is given.     

Lead sources in Mesozoic and Cenozoic Andean ore deposits,north-central Chile (30–34°S)

Mesozoic and Cenozoic ore deposits in the Chilean Andes between La Serena (~30°S) and Santiago (~34°S) include polymetallic vein, low- and high-sulfidation epithermal vein, skarn, porphyry copper-molybdenum and porphyry copper-gold. These deposits are associated with volcanic and plutonic complexes emplaced in eastward-migrating longitudinal arcs which formed during subduction along the continental margin of South America since the Middle Jurassic. Stratabound, but epigenetic, volcanic rock- and sedimentary rock-hosted manto deposits contain additional copper resources. Lead isotopic compositions in ore minerals from 29 deposits vary with age and geographic location, and hence with basement and host rocks. Lead in most ore deposits is derived from temporally related igneous rocks, except for the manto deposits whose lead is derived from host volcanic and sedimentary rock sequences. Lead in the ore deposits is dominated by two crustal sources. Low ²⁰⁷Pb/²⁰⁴Pb characterizes one source whereas high ²⁰⁷Pb/²⁰⁴Pb characterizes the second source. Lead isotopic compositions of Jurassic and Miocene ore minerals (²⁰⁶Pb/²⁰⁴Pb>18.50; ²⁰⁷Pb/²⁰⁴Pb>15.61) lie along the average crustal growth curve. By contrast, most Cretaceous deposits have ore minerals with lower ²⁰⁶Pb/²⁰⁴Pb (<18.39) and ²⁰⁷Pb/²⁰⁴Pb (<15.58) than Jurassic ore minerals. The shift in lead isotopic composition to lower lead isotopic values precludes derivation of lead from a source of similar composition to those in the Jurassic or Tertiary deposits. For Cretaceous deposits, polymetallic and low-sulfidation epithermal veins and a skarn have lower ²⁰⁶Pb/²⁰⁴Pb than a porphyry copper-gold system and peripheral gold veins at Andacollo (18.43-18.50). Late Cretaceous veins from the Bellavista deposit have the lowest ²⁰⁶Pb/²⁰⁴Pb (18.33) of all deposits. Ore minerals in Miocene and Pliocene porphyry copper-molybdenum deposits have higher ²⁰⁶Pb/²⁰⁴Pb (18.58-18.67) than Cretaceous deposits, consistent with their age being younger. The Miocene and Pliocene ore minerals also have higher ²⁰⁷Pb/²⁰⁴Pb (15.58-15.66) than Cretaceous ore minerals, thereby requiring an additional input from the high-²⁰⁷Pb/²⁰⁴Pb source into the younger deposits. Miocene auriferous deposits in the north have similar ²⁰⁶Pb/²⁰⁴Pb values as the Miocene and Pliocene porphyry copper-molybdenum deposits in the south, but they are distinguished by higher and variable ²⁰⁷Pb/²⁰⁴Pb (15.61-15.66) and ²⁰⁸Pb/²⁰⁴Pb (38.54-39.01), which are arrayed along steep mixing trends. These ore minerals have the largest input of high-²⁰⁷Pb/²⁰⁴Pb material in the deposits studied. By contrast, lead in the epigenetic manto deposits appears to be derived from the host volcanic or sedimentary rock-dominated sequences, and locally exhibits large-scale isotopic heterogeneity within a deposit. Overall, the lead isotopic compositions of ore minerals mimic the values and variations established in age-equivalent rock sequences. The low-²⁰⁷Pb/²⁰⁴Pb material in the deposits is derived from Cretaceous igneous rocks or their sources as they evolved with time; low ²⁰⁷Pb/²⁰⁴Pb characterizes these rocks. By contrast, high-²⁰⁷Pb/²⁰⁴Pb material is likely derived from Carboniferous to Triassic igneous rocks or their sources, as this lead isotopic characteristic dominates these rocks.     

In situ LA-(MC)-ICP-MS trace element and Nd isotopic compositions and genesis of polygenetic titanite from the Baogutu reduced porphyry Cu deposit,Western Junggar,NW China

Titanite (sphene, CaTiSiO₅) is sensitive to changes in temperature, oxygen and water fugacity, and fluid composition. In order to understand formation processes and the nature of hydrothermal fluids, various types of titanite from Cu ores at the Baogutu reduced porphyry Cu deposit were chosen for detailed study. Magmatic titanite is associated with biotite, plagioclase and K-feldspar, whereas hydrothermal titanite occurs with K-feldspar, chlorite, actinolite and calcite. The formation of hydrothermal titanite was related to hydration of igneous minerals under high fH₂O, whereas the widespread replacement of ilmenite by titanite (without magnetite) indicates a relatively low oxygen fugacity. Magmatic titanite has low Al, high Fe, Y, Sn, Zr, Nb and REE contents, relative to hydrothermal titanite. On the basis of the Zr-in-titanite and Al-in-chlorite geothermometers, formation temperatures for magmatic and hydrothermal titanite are estimated to be 687–739 °C and 250–670 °C, respectively. The gradual decrease in REE, Y and Sn contents from magmatic to late hydrothermal titanite was probably caused by precipitation of REE-bearing minerals. Magmatic and hydrothermal titanites have similar chondrite-normalized REE patterns with negative Eu anomalies and relatively flat HREE. Randomly selected titanites have Nd isotopic compositions similar to the host rocks. Thus, both magmatic and hydrothermal titanite are believed to have been predominantly derived from a mantle source.     

Association of manganese ore and phosphorite-bearing facies in sedimentary sequences: Communication 1. Parastereses and parageneses of phosphorus and manganese in Mesozoic-Cenozoic and Upper Paleozoic rocks

It is shown that cooccurrences (parastereses) of chemical elements, minerals, sedimentary rocks, facies, and formations should be distinguished from their parageneses, which represent geological formations related to a single geological process. It has been established that phosphorus is concentrated in sedimentary manganese ores and is not accumulated in volcanosedimentary and hydrothermal deposits. Parageneses of Mn and P in sedimentary deposits are characterized. Parastereses of manganese ore and phosphorite-bearing facies in Oligocene rocks of southwestern Eurasia, Mesozoic and Upper Paleozoic sections of the Urals, and Mesozoic- Cenozoic sequences of Morocco are considered.     

Manganese oxide mineralogy,petrography and genesis,Pilbara Manganese Group,Western Australia

Supergene manganese oxides, occurring in shales, breccias and dolomites of Proterozoic Age, in the Western Australian Pilbara Manganese Group, have Mn/Fe ranging from 1.9 to 254 and Mn⁴⁺ to Mn (Total) of 0.49–0.94. The manganese mineralogy is dominated by tetravalent manganese oxides, especially by cryptomelane, with lesser amounts of pyrolusite, nsutite, manjiroite, romanechite and other manganese oxide minerals. The manganese minerals are commonly associated with iron oxides, chiefly goethite, indicating incomplete separation of Mn from Fe during Tertiary Age arid climate weathering of older, manganiferous formations. These manganese oxides also contain variable amounts of braunite and very minor hausmannite and bixbyite. The braunite occurs in three generations: sedimentary-diagenetic, recrystallised sedimentary-diagenetic, and supergene. The mode of origin of the hausmannite and bixbyite is uncertain but it is possible that they resulted from diagenesis and/or low-grade regional metamorphism. The supergene manganese deposits appear to have been derived from manganiferous Lower Proterozoic banded iron formations and dolomites of the Hamersley Basin and overlying Middle Proterozoic Bangemali Basin braunite-containing sediments.     

Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach

We report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and sub-marine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). While Li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site, the δ⁷Li signatures for each of these reservoirs were significantly different. The first low temperature (25-250 °C) experiments of Li isotope exchange during seawater/basalt interaction confirmed that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranged from +19.4‰ (Δ_{solution-solid}) at 25 °C to +6.7‰ at 250 °C. These experiments demonstrated the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature: Δ_{solution-solid} = 7847/T − 8.093. Application of experimental results and literature data to the Bouillante area suggested that geothermal water was in equilibrium at 250-260 °C. It likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust, just below the transition zone between andesite volcanic flows and the basaltic dikes. The upper dike section, from which Li is extracted by hydrothermal fluids, was characterized by light Li isotopic values in the rocks, indicating retention of ⁶Li by the altered rocks. For the Lamentin and Diamant areas, the geothermal fluids appeared to be in equilibrium with reservoir volcano-sedimentary rocks at 90-120 °C and 180 °C, respectively. Further evidence for this argument is provided by the fact that only the Na/Li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids. This suggests the importance of a sedimentary signature in these reservoir rocks. Altogether, this study highlights that the use of Li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks.     

Geological setting and mineral composition of Famennian manganese ores in the Lemva zone of Pai-Khoi: Evidence from ore occurrences of the Nizhny Silov group

The results of geological-mineralogical study of stratificated manganese ores in Famennian rocks of the Lemva facies in Pai-Khoi are presented. Carbonate manganese ores make up conformable stratified and lenticular bodies (up to 0.6 m thick) in the interval between the Gromashor and Silovayakha formations that are composed of jasperoids and carbonate-siliceous rocks. Ores are characterized by fine wavy bedding and development of transverse quartz veinlets. The ores are mainly composed of kutnahorite. Secondary minerals are represented by dolomite, calcite, pyrite, ransayite, and cryptomelane (?). Mn-muscovite, micro-cline, pyrophanite, galena, barite, apatite, and monazite are accessory minerals. The average MnO content is 23.81 wt %. The interval also includes a long (up to 40 m) lens of rhodonite rocks (Silovayakha occurrence) that replace carbonate ores along the strike. The major minerals in these rocks are represented by rhodochrosite, rhodonite, pyroxmangite, and quartz. Secondary minerals are observed as tephroite, alleghanyite, friedelite, caryopilite, neotocite, sussexite, pyrite, and supergene manganese oxides. Spessartine, albandine, barite, and apatite are accessory minerals. Based on the analysis of factual material, we suggest that ore material was derived from hydrothermal paleoceanic systems associated with Devonian volcanism. Ore concentration in the sedimentation zone was related to the stagnant reduced setting of bottom water. Ore deposition was promoted by the delivery of fresh portions of ocean water. Ore matter was accumulated mainly by chemogenic mechanism and partly with the’ participation of bacteria. It is suggested that boundary between the Gromashor and Silovayakha formations should be corrected to unite rocks of the Famennian manganiferous association into a single formation.     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

Implications of rock mineralogy and texture on the feasibility of in situ leach mining of Mn-bearing iron formations of central Minnesota,U.S.A.

The U.S. Bureau of Mines is investigating the feasibility of extracting Mn using in situ leach mining methods. Among the deposits being examined are the iron formations of the Cuyuna range, Minnesota, which contain high-tonnage, low grade deposits of manganese oxides. Manganese minerals identified include pyrolusite, cryptomelane-hollandite, manganite, braunite and lithiophorite. Ore reactivities, as measured by batch leaching tests using aqueous SO₂, are compared to theoretical estimates of the leaching behavior of individual manganese minerals based on kinetic and thermodynamic evaluations. Experimental results in some cases show opposite trends to those predicted by theoretical estimates. In batch leaching tests of Cuyuna Range ores, texture was shown to be of greater importance, than thermodynamic reactivity in determining amenability to leaching. Predicting the practical potential for recovery of Mn by in situ leaching must involve identification of the effects of texture (massive, fractured-host, or interstitial) in addition to ore mineral reactivity.     

Boron isotopic composition of marine and nonmarine evaporite borates

Here we present the results of the boron isotopic analysis of borates from some well-known marine and nonmarine evaporite deposits. Combining new analyses with analyses from the literature yields a mean δ¹¹B value for nine marine evaporite borate analyses of 25 ± 4 permil (¹¹B/¹⁰B = 4.15 ± 0.02) and a mean for twenty-five nonmarine evaporite borates of −7 ± 10 permil (¹¹B/¹⁰B = 4.02 ± 0.04). Previous studies have shown that the boron isotopic composition of seawater (δ¹¹B ~ 40 permil, ¹¹B/¹⁰B ~ 4.20) is about four percent heavier than the boron isotopic composition of most crustal rocks and minerals. We suggest that our results for marine and nonmarine evaporite borates reflect this difference. Thus it should be possible to use boron isotopic composition as a tracer for boron of marine evaporitic origin. Applications include borate-bearing evaporite deposits and, perhaps, borate and/or borosilicate-bearing metamorphic rocks.     

Cu, Mn, and Ag mineralization in the Quebrada Marquesa Quadrangle, Chile: the Talcuna and Arqueros districts

The Quebrada Marquesa Quadrangle in Chile exhibits a series of mineralizations comprising manto-type manganese and copper deposits of Lower Cretaceous age, and copper and silver veins of Tertiary age. The deposits are hosted by volcanic and volcaniclastic units of the Arqueros (Hauterivian-Barremian) and Quebrada Marquesa (Barremian-Albian) Formations. Three episodes of manganese mineralization (Mn_1-3) are recognized within the study area. Hydrothermal activity leading to episodes 1 and 3 was of minor importance, while the second one (Mn₂) gave rise to major manto-type deposits of both manganese and copper in the Talcuna mining district. Extensional faulting during Tertiary time resulted in block faulting and the unroofing of the oldest andesitic volcanics and marine sediments (Arqueros Formation). This episode was accompanied by magmatic and hydrothermal activity leading to vein formation in the Arqueros (Ag) and Talcuna (Cu) districts. The latter veins cross-cut the previous manto-type copper deposits. Ore mineralogy is similar in both styles of mineralization (manto- and vein-type) and consists mainly of chalcopyrite and bornite, with variable amounts of galena, tetrahedrite (vein-related), chalcocite, sphalerite, pyrite, hematite, digenite and covellite. Alteration processes at Talcuna can be divided into two categories, those related to the Lower Cretaceous manto-type episode (LK alteration: chlorite-epidote-calcite-albite, prehnite, zeolite), and those associated with the locally mineralized normal faults of Tertiary age (Tt alteration: chlorite-calcite, sericite). The Arqueros silver veins display an ore mineralogy consisting of arquerite, argentite, native silver, polybasite, cerargyrite and pyrargyrite-proustite; associated alteration includes strong chloritization of the country rock. The manto-type deposits formed from fluids of salinity between 11 and 19 wt.% NaCl equivalent and temperatures between 120 and 205 °C. Mineralizing fluids during the vein-type stage circulated at lower temperatures, between 70 and 170 °C, with salinity values in a wide range from 3 to 27 wt.% NaCl equivalent. This distribution of salinities is interpreted as the result of the complex interplay of two different processes: boiling and fluid mixing; the former is considered to control the major mineralogical, textural and fluid inclusion features of the vein-type deposits. We suggest that the Lower Cretaceous mineralization (manto-type stage) developed in response to widespread hydrothermal activity (geothermal field-type) involving basinal brines. Received: 18 July 1997 / Accepted: 28 January 1998     

贵重和有色金属矿床钨氧化钐-钕同位素定年研究及其地质意义

本文结合国内外典型研究事例分析 ,介绍了钨氧化物钐 钕同位素直接定年技术在贵重和有色金属矿床研究中的适用性和局限性 ,探讨了利用钕同位素数据示踪成矿物质来源与流体运移轨迹的可能性 ,指出对同一金属矿床开展多种同位素分析的重要性和必要性。