Formation of the Wiesloch Mississippi Valley-type Zn-Pb-Ag deposit in the extensional setting of the Upper Rhinegraben, SW Germany

The Mississippi Valley-type (MVT) Zn–Pb–Ag deposit in the Wiesloch area, Southwest Germany, is controlled by graben-related faults of the Upper Rhinegraben. Mineralization occurs as vein fillings and irregular replacement ore bodies consisting of sphalerite, banded sphalerite, galena, pyrite, sulfosalts (jordanite and geocronite), barite, and calcite in the Middle Triassic carbonate host rock. Combining paragenetic information, fluid inclusion investigations, stable isotope and mineral chemistry with thermodynamic modeling, we have derived a model for the formation of the Wiesloch deposit. This model involves fluid mixing between ascending hot brines (originating in the crystalline basement) with sedimentary formation waters. The ascending brines originally had a near-neutral pH (around 6) and intermediate oxidation state, reflecting equilibrium with granites and gneisses in the basement. During fluid ascent and cooling, the pH of the brine shifted towards more acidic (around 4) and the oxidation state increased to conditions above the hematite-magnetite buffer. These chemical characteristics contrast strongly with those of the pore and fracture fluid residing in the limestone aquifer, which had a pH between 8 and 9 in equilibrium with calcite and was rather reduced due to the presence of organic matter in the limestone. Mixing between these two fluids resulted in a strong decrease in the solubility of silver-bearing sphalerite and galena, and calcite. Besides Wiesloch, several Pb–Zn deposits are known along the Upper Rhinegraben, including hydrothermal vein-type deposits like Badenweiler and the Michael mine near Lahr. They all share the same fluid origin and formation process and only differ in details of their host rock and fluid cooling paths. The mechanism of fluid mixing also seems to be responsible for the formation of other MVT deposits in Europe (e.g., Réocin, Northern Spain; Trèves, Southern France; and Cracow-Silesia, Poland), which show notable similarities in terms of their age, mineralogy. and mineral chemistry to the MVT deposit near Wiesloch.     

Physicochemical Conditions during the Formation of Dalingkou Ag—Pb—Zn Deposit,Zhejiang Province

On the basis of mineral paragenesis and the chemistry and homogenization temperatures of fluid inclusions, the physicochemical parameters were calculated for the formation of the Dalingkou Ag-Pb-Zn deposit in Zhejiang. From the early to the late stage of mineralization the ore-forming temperature venation was found to be 298.5 °C → 267.0 °C → 217.6 °C → 167.3 °C, with a corresponding pH change of 3.0 ∼ 5.8 → 6.1 → 6.7→ 5.0 ∼ 7.3. The pressure changed from 403.8 to 128.5 atm, andlogf S₂-9.9 → -11.2 → < -15; logf o₂< -44→ -45.6 ∼ -42.6 → > -44.2; and logf co₂ around -1.55. In conjunction with geological observations, the deposit is considered to be of meso-epithermal origin, i.e., it was formed after continental volcanic-subvolcanic activity. The major factors affecting ore precipitation are the decreasing temperature and the increasing pH of ore-forming solutions.     

Magmatism and formation conditions of the Urma helvite-bertrandite deposit,West Transbaikal berillium province

The relationships between mineralization and magmatism during the formation of the Early Mesozoic West Transbaikal beryllium province are exemplified in the Urma helvite-bertrandite deposit. The deposit is drawn toward granitoids of elevated alkalinity, which belong to the Tashir Complex. Mineralization is related to leucogranite and characterized by patched distribution controlled by localization of metasomatic alteration. The latter is identified owing to replacement of feldspar with microcline and albite followed by silicification related to fracture zones. Helvite and bertrandite are the major Be minerals at the deposit. The Be grade of the ore is nonuniform and varies from 740 to 25000 ppm. Zircon, malacon, monazite, allanite, bastnaesite, columbite, and xenotime occur in metasomatic rocks together with Be minerals. Geochemical characteristics of alkali granites and metasomatic rocks are similar in a wide range of incompatible elements. Both are characterized by lowered Ba, Sr, P, and Eu contents and enriched in Th, U, Pb, Zr, and Hf. The degree of enrichment is the highest in the ore. The Be content in the ore correlates with concentrations of a number of other rare metals typical of host granite, which form their own mineralization against the background of metasomatic alteration, including Zr and REE minerals. Similarity in geochemistry of granitic rocks and Be ore indicates that the Urma deposit was related to the evolution of magmatic melt. Regional correlation shows that the ore-magmatic system of the Urma deposit is close to that of the Orot deposit, one of the largest in the central segment of the West Transbaikal metallogenic province. Both deposits are characterized by a similar composition of granitoids and comparable localization of ore zones in the structure of plutons. This similarity supports the high ore resource potential of Early Mesozoic alkali granites in the western Transbaikal region. Taking into account that these granitoids are widespread in the West Transbaikal Rift Zone that controls the metallogenic province, one can expect the discovery of new deposits therein.     

Intense pyrite formation under low-sulfate conditions in the Achterwasser lagoon, SW Baltic Sea

In comparison to similar low-sulfate coastal environments with anoxic-sulfidic sediments, the Achterwasser lagoon, which is part of the Oder estuary in the SW Baltic Sea, reveals unexpectedly high pyrite concentrations of up to 7.5 wt%. Pyrite occurs mainly as framboidal grains variable in size with diameters between 1 and 20 μm. Pyritization is not uniform down to the investigated sediment depth of 50 cm. The consumption of reactive-Fe is most efficient in the upper 20 cm of the sediment column, leading to degrees of pyritization (DOP) as high as 80 to 95%.Sediment accumulation in the Achterwasser takes place in high productivity waters. The content of organic carbon reaches values of up to 10 wt%, indicating that pyrite formation is not limited by the availability of organic matter. Although dissolved sulfate concentration is relatively low (<2 mmol/L) in the Achterwasser, the presence of H₂S in the pore water suggests that sulfate is unlikely to limit pyrite authigenesis. The lack of free Fe(II) in the pore waters combined with the possibility of a very efficient transformation of Fe-monosulfides to pyrite near the sediment/water interface suggests that pyrite formation is rather controlled by (i) the availability of reactive-Fe, which limits the FeS formation, and by (ii) the availability of an oxidant, which limits the transformation of FeS into pyrite. The ultimate source for reactive-Fe is the river Oder, which provides a high portion of reactive-Fe (∼65% of the total-Fe) in the form of suspended particulate matter. The surficial sediments of the Achterwasser are reduced, but are subject to oxidation from the overlying water by resuspension. Oxidation of the sediments produces sulfur species with oxidation states intermediate between sulfide and sulfate (e.g., thiosulfate and polysulfides), which transform FeS to FeS₂ at a significant rate. This process of FeS-recycling is suggested to be responsible for the formation of pyrite in high concentrations near the sediment surface, with DOP values between 80 and 95% even under low sulfate conditions.A postdepositional sulfidization takes place in the deeper part of the sediment column, at ∼22 cm depth, where the downward diffusion of H₂S is balanced by the upward migration of Fe(II). The vertical fluctuation of the diffusion front intensifies the pyritization of sediments. We suggest that the processes described may occur preferentially in shallow water lagoons with average net-sedimentation rates close to zero. Such environments are prone to surficial sediment resuspension, initiating oxidation of Fe-sulfides near the sediment/water interface. Subsequent FeS₂ formation as well as postdepositional sulfidization leads to a major pyrite spike at depth within the sediment profile.     

On the conditions of formation of Gushan iron ore deposit

On the basis of its geological characteristics, the Gushan iron deposit should be assigned to volcano-hydrothermal type with hematite-quartz as its principal mineral assemblage. Iron concentration of the ore solution has been estimated from the ratio of hematite to quartz in the ore. By using experimental and thermodynamic data, the solubility of ferro-minerals at elevated temperatures and pressures have been calculated in the system FeO?Fe₂O₃?NaCl?HCl?H₂O. The effects ofT, P, pH,f _{O 2} and total Cl concentration on the solubility of ferrominerals are discussed. Thermodynamic calculations based on presumed physico-chemical conditions for the ore solution are in good agreement with geological observations. The calculation shows that ferro-minerals were deposited at logf _{O 2}=?21～?25, log(m_HCl+m_H+)=?2.5～?3,P=1?0.75 (or 0.5) kb, andT=400°?350°C. It is believed that the original solution was an acid NaCl-bearing solution of magmatic derivation. However, iron in the solution was enriched with falling temperatures by dissolving pre-existing ferro-minerals in consolidated rocks rather than extracting directly from the magma. Either temperature (below 400°C) or pressure decrease may result in the precipitation of ferro-minerals from the solution, but the Fe/Si ratio in the ore is dependent mainly upon pH. The widespread siliceous veins of later stages are a reflection of decreasing acidity of the solution. An increase inf _{O 2} will also favor the deposition of ferro-minerals. The hypabyssal occurrence and the existence of the Huangmaqing sandy shale have contributed greatly to the formation of hematite.     

Geological setting of the Wassa gold deposit,SW Ghana

Including past production, current indicated and inferred resources, Wassa is a 5 Moz poly-deformed early-orogenic gold deposit located on the eastern flank of the Ashanti Belt, in southwest Ghana. It is hosted by metamorphosed volcanic, intrusive and sedimentary rocks of the Sefwi Group (ca. 2260–2160 Ma). Early mineralization has an Eoeburnean age (2164 ± 22 Ma, Re–Os on pyrite) and is characterized by quartz veins, by a carbonate alteration of the host rocks, and by deformed gold-bearing pyrite. Remobilization of this gold occurred during the late stages of the Eburnean Orogeny (~ 2.1 Ga) and is associated with quartz-carbonate veins with visible gold and euhedral pyrites.     

Paragenesis of cobalt and nickel in the Black Butte shale-hosted copper deposit,Belt Basin,Montana, USA

The Black Butte copper deposits (formerly known as Sheep Creek) are a group of sediment hosted, laterally extensive Cu–(Co–Ag) deposits hosted in dolomitic shale of the mid-Proterozoic Newland Formation. Copper–cobalt mineralization occurs in zones of massive, laminated pyrite that were locally reworked and infiltrated by Cu-rich fluids during early diagenesis. Cobalt, along with substantial nickel and arsenic, mainly occurs as impurities within early, porous pyrite, or as minute grains of sulpharsenides (i.e., cobaltite, glaucodot, and/or alloclasite). Later thermal events remobilized the Co, Ni, and As to form intergrowths of siegenite (Co,Ni)₃S₄ and tennantite. The temperature of this later event is constrained by the mineralogical assemblage to have been relatively low, between 125 and 225 °C. Although many of the characteristics of SEDEX-type deposits are present at Black Butte (e.g., laterally extensive massive pyrite horizons, interbedded black shales, abundant barite and local phosphate horizons, and rifted continental margin setting), the lack of economic Pb and Zn mineralization in the main deposits, and the abundance of Cu with high Co, is more typical of sediment-hosted stratiform copper deposits. The Neihart Formation, a hematitic quartz sandstone resting below the base of the Belt Supergroup, may have been an important source bed for Cu–Co–Ni–Ag fluids. It is speculated that these fluids, ideal for forming Cu deposits, were expelled along growth faults near the margin of the Belt Basin and deposited metals on or just below the sea floor in a setting that is typical of SEDEX deposits. This unique mineral deposit model may have applications to other districts where Cu–Co-rich sulfides are deposited in an exhalative setting.     

Mineralogy and PT formation conditions of lead-zinc ore at the Dzhimidon deposit, north Ossetia

The mineralogy and PT formation conditions of the Dzhimidon Pb-Zn deposit in the Sadon ore district are considered. The deposit is localized in metamorphic rocks of the Buron Formation, which pertain to the pre-Jurassic basement (lower structural stage) and are cut through by Upper Paleozoic granitoids, and in the Lower Jurassic terrigenous sequence (upper structural stage). Orebodies as quartz-sulfide veins are mainly hosted in the metamorphic rocks. Galena, sphalerite, chalcopyrite, pyrite, pyrrhotite, and arsenopyrite are the most abundant sulfides, while quartz, carbonates, chlorite, sericite, and feldspar are gangue minerals. The bismuth mineralization identified at this deposit for the first time is represented by diverse phases of the Ag-Pb-Bi-S system. Five stages of the ore deposit formation are recognized: a premineral stage (quartz-feldspar), three ore-bearing stages (pyrite-arsenopyrite, pyrrhotite-chalcopyrite-sphalerite, and arsenopyrite-sphalerite-galena), and a postmineral stage (quartz-calcite); each stage comprises one or several mineral assemblages. The study of fluid inclusions in quartz, calcite, and sphalerite of the premineral, ore-forming, and postmineral stages has shown that the ore was deposited mainly from Na chloride solution with a salinity varying from >22 to <1.0 wt % NaCl equiv at a temperature from 460 to ～120°C and 430–290 bars pressure. The third stage was characterized by an abrupt increase in temperature and by the appearance of Mg(Fe,Ca) chloride solutions equally with Na chloride fluids, presumably owing to the emplacement of granite porphyry.     

Composition,formation conditions,and genesis of the Talatui gold deposit,the Eastern Transbaikal Region,Russia

The mineral composition of the Talatui gold deposit has been studied with modern methods. Previously unknown minerals (ilmenite, siegenite, glaucodot, wittichenite, matildite, hessite, pilsenite, zircon, tremolite, cummingtonite, hercynite, and goethite) have been identified in the ore. A high Re content has been detected in molybdenite. The spatiotemporal separation of Au and Ag is caused by different mineral species of these elements and their diachronous precipitation during the ore-forming process. Gold crystallized along with early mineral assemblages, beginning from virtually pure gold (the fineness is 996). Silver precipitated largely at the end of the process as hessite (Ag₂Te) and matildite (AgBiS₂). The temperature of ore deposition varied from 610 to 145°C, the pressure was 3370–110 bar, and the salt concentration ranged from 56.3 to 0.4 wt % NaCl equiv. The heterogeneous state (boiling) of fluid at the early stages has been documented. The chemical and isotopic compositions of the fluid testify to its magmatic nature and the participation of meteoric water at late stages in the ore-forming process. Thermodynamic modeling reproduces the main specific features of ore formation, including separation of Au and Ag. A physicochemical model of the gold mineralization in the Darasun ore district has been proposed. On the basis of several attributes, the Talatui deposit has been referred to the prophyry gold-copper economic type.     

Lebediny gold deposit,Central Aldan: Mineral parageneses,stages, and formation conditions

The mineral parageneses and succession of their formation are considered for the first time for the Zverevsky, Orekhovy, and Vodonosny ore lodes of the Lebediny gold deposit and the Radostny prospect in the Central Aldan ore district, which are genetically related to the epoch of Mesozoic tectonomagmatic reactivation. The orebodies, represented by two morphological varieties—ribbonlike lodes and steeply dipping veins—are hosted in lower part of the Vendian–Cambrian dolomitic sequence, which is cut through by Mesozoic subalkaline intrusive bodies. The chemistry of fahlore and rare minerals, including native gold and bismuth, altaite, aikinite, tetradymite, and sulfosalts of lillianite series, has been studied. Native gold is related to the late hydrothermal process and occurs in skarn and in quartz–tremolite–sulfide and quartz–carbonate–sulfide veins. The data on stable sulfur (δ³⁴S) isotopes of sulfides, oxygen (δ₁₈O) and carbon (δ₁₃C) isotopes of carbonates, as well as on fluid inclusions in various generations of tremolite and quartz, provide evidence for the heterogeneity of ore-bearing solutions, their relationships to magmatism, the depth of the source feeding each specific lode, and different sources of ore-forming hydrothermal solutions.     

Re-Mo-Cu-Os sulphide from the Ekojoki Ni-Cu deposit,SW Finland

Summary This short communication reports the discovery and composition of the first Os-bearing rhenium sulphide. It occurs as a single euhedral crystal in the early Proterozoic Ekojoki Ni-Cu deposit in Finland, being enclosed by interstitial magmatic Fe-Ni sulphides. Electron microprobe analyses are most consistent with a general formula of Cu(Re,Os,Mo)₅S₉. The Re-Os model age calculations indicate that approximately 2/3 osmium is "common" Os that was accommodated by the sulphide at the time of crystallisation, 1/3 being produced via post-crystallisation decay of¹⁸⁷,Re to^{18 7}Os. We propose that this sulphide crystallised from an immiscible sulphide liquid that was unusually rich in rhenium due to extensive assimilation of black schist sulphides by the magma.With 2 Figures     

德国拉梅尔斯贝格铜-锌-铅-钡矿床研究进展

拉梅尔斯贝格矿床是中欧华力西期最重要的SHMS(以沉积岩为容矿围岩的块状硫化物)类矿床之一,位于莱茵海西期地体的上哈茨地块。该矿床形成于泥盆纪,矿体赋存于艾菲尔阶的威森巴赫页岩中,经华力西造山运动发生了强烈的变形。主要有新矿体、老矿体和富含重晶石的灰色矿体,主要硫化物矿物为黄铁矿、闪锌矿、方铅矿和黄铜矿。硫同位素数据显示,拉梅尔斯贝格矿床有2个硫源,一个是热液成因;一个是生物成因,来自细菌还原的海水中的硫酸盐。铅同位素说明,它的铅主要来自均匀的地壳。与其他SHMS类矿床相比,拉梅尔斯贝格矿床明显富铜。     

The South Faizuly manganese deposit in the southern Urals: Geology,petrography, and formation conditions

The South Faizuly manganese deposit hosted in cherty rocks of the Magnitogorsk paleovolcanic belt has been studied. The geology, mineralogy, and chemistry of ores and host silicites (jasperites, jaspers, and cherty siltstones) are characterized. The deposit was formed in the following four consecutive stages: (1) sedimentation and diagenesis of ore-bearing sediments in the Middle Devonian, (2) metagenesis of Mn-bearing rocks in the Middle Devonian-Early Carboniferous, (3) hydrothermal-metasomatic stringer ore mineralization during tectonic deformation of volcanosedimentary rocks in the Middle Carboniferous-Permian, and (4) supergene alteration and partial denudation of the deposit in the Mesozoic-Quaternary. Models of Mn-bearing rock deposition in the proximal and distal zones of the hydrothermal solution discharge area are considered.Translated from Litologiya i Poleznye Iskopaemye, No. 1, 2005, pp. 35–55.Original Russian Text Copyright 2005 by Brusnitsyn, Zhukov.     

Physicochemical Conditions of the Formation of Beryl and Aquamarin in Mufushan Granopegmatite Deposit,Hunan Province,China

The formation of the Mufushan granopegmatite was closely related to the Late Yenshanian multiphase and multistage magmatic activities,More than one generation of beryl and aquamarine occur in different types of pegmatite in the granites.The presence of melt and melt-fluid inclusions strongly indicates a melt-solution character of the pegmatitic magma.Forming temperatures of the different generations of beryl in a Na^ -K^ ,Ca^2 -CO3^2--Cl^--SO4^2- solution ranges from 990℃to 200℃.Aquamarine was formed at 720-180℃.The contents of alkali metals(Na^ K^ )in th ore-formming solution of aquamarine are lower than those in the beryl,but the contents of alkali earths(Ca) and salinity are higher,The granite was generated by remelting of the basement formation(meta-sedimentary rocks of the Lengjiaxi Group)which also served as the source of ore-forming material.Beryllium in the pegmatite was transported mainly in the form of Na[Be(CO3)2],with part of it being complexed with Cl^- and SO4^2-.During the generation and evolution of the pegmatite,equilibrium might have been reached in the solid-melt-fluid or solid-fluid system.The intergranular solutions may have reacted with the early crystallized minerals,resulting in potash-feldsparization,albitization and muscovitization during which the ore-forming elements were mobilized and transported in favour of ore deposition.     

Neotectonic stage in the formation of the Khokhlovskoe uranium deposit, eastern Transural region: Structural, hydrochemical, mineralogical, and geochemical formation conditions

The structural, hydrochemical, mineralogical, and geochemical features of the Khokhlovskoe uranium deposit related to neotectonic processes are considered. The structural feature is expressed in neotectonic dislocations in the form of overall intense fragmentation of host rocks and widespread low-amplitude strike-slip faulting. The hydrochemical specificity is determined by the appearance of thermal carbonated formation water in ore-bearing aquifers. This water is similar in chemical and gas composition to hydrothermal solutions in fluid inclusions and mineral waters abundant in this district. The mineralogical and geochemical features comprise the occurrence of newly formed ferroan carbonates and late iron hydroxides in altered (bleached) pelitic rocks; the formation of silicic opal segregations in ore-bearing sand and sandstone; late sulfides, arsenides, and selenides of iron and other metals; and multiphase gel-pitchblende enriched in Zr especially typical of high-grade uranium ore. The age of high-grade ore determined by a precision uranium-ionium method coincides with the time when thermal carbonated water appeared in the host rocks. This time was estimated from a mathematical model of heat transfer and regional dynamics of underground water. This coincidence clearly indicates that the aforementioned processes are related to the late Quaternary neotectonic reactivation of the eastern Transural region.     

Mineralogy and ore formation conditions of the Bugdaya Au-Bearing W-Mo porphyry deposit, Eastern Transbaikal Region, Russia

The Bugdaya Au-bearing W-Mo porphyry deposit, Eastern Transbaikal Region, Russia, is located in the central part of volcanic dome and hosted in the large Variscan granitic pluton. In its characteristics, this is a Climax-type deposit, or an Mo porphyry deposit of rhyolitic subclass. The enrichment in gold is related to the relatively widespread vein and veinlet gold-base-metal mineralization. More than 70 minerals (native metals, sulfides, sulfosalts, tellurides, oxides, molybdates, wolframates, carbonates, and sulfates) have been identified in stockwork and vein ores, including dzhalindite, greenockite, Mo-bearing stolzite, Ag and Au amalgams, stromeyerite, cervelleite, and berryite identified here for the first time. Four stages of mineral formation are recognized. The earliest preore stage in form of potassic alteration and intense silicification developed after emplacement of subvolcanic rhyolite (granite) porphyry stock. The stockwork and vein W-Mo mineralization of the quartz-molybdenite stage was the next. Sericite alteration, pyritization, and the subsequent quartz-sulfide veins and veinlets with native gold, base-metal sulfides, and various Ag-Cu-Pb-Bi-Sb sulfosalts of the gold-base-metal stage were formed after the rearrangement of regional pattern of tectonic deformation. The hydrothermal process was completed by argillic (kaolinite-smectite) assemblage of the postore stage. The fluid inclusion study (microthermometry and Raman spectroscopy) allowed us to establish that the stockwork W-Mo mineralization was formed at 550–380°C from both the highly concentrated Mg-Na chloride solution (brine) and the low-density gas with significant N2 and H2S contents. The Pb-Zn vein ore of the gold-base-metal stage enriched in Au, Ag, Bi, and other rare metals was deposited at 360–140°C from a homogeneous Na-K chloride (hydrocarbonate, sulfate) hydrothermal solution of medium salinity.     

The Kakun igneous cumulate magnetite deposit,SW Nigeria

Summary Field and mineralogical information concerning a Precambrian iron deposit in Kakun, southwestern Nigeria is presented and a genetic model is advanced. The Kakun deposit consists of titaniferous magnetite concentrated at the base of an amphibolite sheet within a large-scale banded metasediment-orthogneiss suite of Eburnean (ca. 2000 Ma) age. The suite is truncated by concordant and discordant Pan-African (ca. 600 Ma) granitic to dioritic intrusives including pegmatites and aplites. The ore-grade zone is marked by preferential concentration of early formed heavy minerals as well as cumulate and ophitic textures, while the host amphibolite exhibits preferred mafic mineral alignment and triple junction mosaic of its felsic mineral matrix. These relationships are here explained in a two-stage model comprising an Eburnean synkinematic magmatic phase and a Pan-African deformation-metamorphic phase. The first stage involved the intrusion of a gabbroic magma as a sill into pre-existing rocks under high grade metamorphic conditions; mineral segregation of the sill into an ore-rich base by crystal settling during consolidation; followed by a post-consolidation pulse of anorthosite melt injection into the ore zone. The second stage occurred under conditions of medium grade metamorphism and heterogeneous deformation. Competence, difference and localised access of water during this stage led to preferential shearing and amphibolitisation of the ore-poor upper part of the sill.

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Die magmatische Cumulat-Magnetit Lagerstatte von Kakun, SW Nigeria

Zusammenfassung Es werden feldgeologische und mineralogische Informationen von einer präkambrischen Eisenlagerstätte in Südwest-Nigeria präsentiert, und ein genetisches Model vorgestellt. Die Kakun Lagerstätte stellt eine Ti-Magnetit Mineralisation dar, die an der Basis einer Amphibolitabfolge konzentriert ist. Die Amphibolite gehören zu einer im Groß-Maßstab gebänderten Metasediment-Orthogneis Serie von Eburnean-Alter (ca. 2000 m.y.). Diese Serie wird von konkordanten und diskordanten granitischen bis granodioritischen Intrusionen und inkludierten Pegmatiten und Apliten (ca. 600 m.y.) durchbrochen. Die vererzte Zone ist durch Konzentration von frühgebildeten Schweremineralien, Cumulat- und ophitischen Texturen gekennzeichnet; die Amphibolitabfolge zeigt Einregelung der mafischen Mineralkomponenten, und ein tripple junction Mosaik der felsischen Gemengteile. Diese Charakteristika der Erz-Zone und der Amphibolite werden mit einem zweiphasigen Model erklärt: eine Eburnean synkinematische, magmatische Phase und eine Pan-Afrikanische, deformative metamorphe Phase. Die erste Phase umfaßt die Intrusion eines gabbroischen Sills unter hochgradigen Metamorphosebedingungen, Mineralsegregation innerhalb des Sills in eine erzreiche Basis, verursacht durch Kristall-Absinken während der Konsolidierung, und schließlich die Injektion eines anorthositischen Magmenpulses in die Erzzone im post-Konsolidierungsstadium. Die zweite Phase hat unter mittelgradigen Metamorphosebedingungen und heterogener Deformation stattgefunden. Kompentenz-Unterschiede und das stellenweise Eindringen von Wasser hat in dieser Phase zu Scherung und Amphibolitisierung des erzarmen, hangenden Anteils des Sills geführt.