Mineral Chemical Analyses Research Articles

Episodic Melt Percolating and Chromite Mineralization in Yushigou Peridotite, North Qilian Suture Zone

AbstractHarzburgite‐dunite associations are the dominant type of ultramafic heterogeneities in the exhumed peridotites. Although the dunite network are usually small in volume of exposed mantle rocks, their mineral structures and chemical variations are widely observed in various tectonic environments and record complex mantle evolution. The Yushigou peridotite complex displays a 25 km2 mantle section belonging to a partly dismembered ophiolite in North Qilian suture zone, northwest China. It is mostly composed of depleted spinel harzburgite re‐equilibrated at temperatures of 900 °C, contain dunite veins and minor podiform chromite at its Moho level.Here we report the petrological observations and mineral chemical analyses of harzburgite‐dunite associations in the Yushigou peridotite complex. Based on the data we classified high Cr # (51–74) and low Cr# (19–34) dunite veins in this Yushigou complex. The podiform chromite are mainly high Cr# affinity and are production of melt‐harzburgite interaction. There are three main microstructures recorded the episodic melt‐rock interaction process: (1) complex spinel mineral inclusions both in harzburgite and two kind of dunite record widespread melt percolating. The observed primary inclusions (not related to later low temperature fluid activities) including hydrous and anhydrous silicates and minor sulphides, such as PGE and Fe‐Cu‐Ni related sulfidies, olivine, orthopyroxene, clinopyroxene, amphibole and phlogopite. (2) In all dunite samples, clinopyroxene dominated sieve‐textured rims around spinel are more abundant and more thicken than the harzburgite; implying stained/pulsated melt supply after the chromite crystallization. (3) Millimeter scale veined reaction patches in some dunite samples consist of clinopyroxene, amphibole, phlogopite, Fe‐Ni sulfidies and carbonates. Such metasomatic minerals assemblages may response for the later fluid infiltration after chromite mineralization. All these petrological observations and mineral chemical compositions (major and trace element contents) thus provide a wealth of information on the pathways of migrating melts in the mantle wedge, and provide constrain on nature and evolution of the uprising magma. Strong enrichment of incompatible trace elements in dunite clinopyroxene (LREE/HREE >10) possible reflect a slab origin of the percolating melt which is not observed elsewhere participating in chromite forming.

Geochemical and metamorphic record of the amphibolites from the Tuting–Tidding Suture Zone ophiolites, Eastern Himalaya, India: implications for the presence of a dismembered metamorphic sole

AbstractThe Tuting–Tidding Suture Zone (TTSZ), exposed along Dibang and Lohit river valleys in Arunachal Himalaya, NE India, is the easternmost continuation of the Indus–Tsangpo Suture Zone (ITSZ) and consists of ophiolites associated with metabasics and carbonates. Amphibolites, existing at the base of the ophiolite complex, were studied using whole-rock, mineral chemical analyses and pressure–temperature (P-T) pseudosection modelling to understand their metamorphic and petrogenetic history, and interpret the tectonic environment of their formation. They exhibit two-stage deformation, where D1 is depicted by polymineralic inclusion trails in former melt pools and the main foliation represents D2. Sub-alkaline tholeiitic character, high-field-strength element (HFSE) ratios and mid-oceanic ridge basalt (MORB) -like rare earth element (REE) patterns with negative Eu anomaly indicate that the protolith of these amphibolites originated in a spreading regime by extensive partial melting of a depleted mantle source at shallow depth. Petrography, mineral chemistry and P-T modelling indicate a three-stage metamorphic history for them. M1 is the prograde (c. 2.1 GPa, c. 450°C) defined by garnet centre compositions corresponding to the D1 event. The existence of former melts in the samples demarcates the M2 stage (1.4–1.8 GPa, c. 600°C). The rocks later underwent retrogression (M3: 0.8–1.0 GPa, 480–520°C), which corresponds to the D2 event. These observations suggest that the protolith of the TTSZ amphibolites originated in a mid-oceanic ridge setting, which accreted below a subduction zone where it underwent M1 metamorphism followed by M2 metamorphism, corresponding to partial melting of the rocks. Finally, the M3 event occurred during the obduction phase of the ophiolite complex, where the amphibolites were obducted as the metamorphic sole of the TTSZ ophiolites.

Mineralogical and geochemical studies of gabbroids in the East Central Desert, Egypt

Relevance of the work. The study area is limited to the Central-Eastern Desert of Egypt, represented by various rock units in addition to the gabbro rock containing iron oxides. Research Objective. This work is devoted to detailed studies of the geology of gabbroid rock minerals and iron oxides. Methodology. This work includes both field work (creation of a new geological map for various rock units in the study area) and laboratory work (preparation of thin sections for petrographic and mineralogical studies using a polarizing microscope and a scanning electron microscope SEM). Results and their application. Exploration and extraction of ore deposits and minerals in the eastern desert of Egypt is one of the most important goals for increasing the rate of mining in the country, which leads to economic recovery and meeting the needs of society. Previous work has focused on evaluating and studying economically mineral resources in the Central-Eastern Desert of Egypt. This place is considered one of the most promising mineral deposits in addition to the available iron oxide minerals. Geological studies show that the mineralization of iron oxide in the study area is mainly associated with gabbroid rocks, which carry iron oxide minerals in the form of groups and lenses in the lower part of the Earth’s layers, which is associated with magma rich in iron oxides. According to field studies, we found that iron minerals are represented by lenses up to 3 m thick, alternating with rock. Conclusions. The chemical analysis of minerals based on iron oxides was discovered using a scanning electron microscope (SEM) used to determine the chemical composition of these minerals, which are classified as minerals such as ilmenite and magnetite. Actually geochemical studies have shown that they have a high content of total iron oxides.

Lateral variations in the Unit 7\u20138 boundary zone of the Rum Eastern Layered Intrusion, NW Scotland: implications for the origin and timing of Cr-spinel seam formation

The Rum Layered Suite, NW Scotland, hosts Cr-spinel seams at the bases of peridotite–troctolite macro-rhythmic units in the eastern portion of the intrusion. Here, we present detailed field observations together with microstructural and mineral chemical analyses for the Unit 7–8 Cr-spinel seam and associated cumulates in the Eastern Layered Intrusion. Detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the Unit 7–8 boundary zone rocks. Although the Cr-spinel seam is laterally continuous over ~ 3 km, it is absent towards the centre and the margins of the intrusion. The compositional characteristics of Cr-spinel and plagioclase vary systematically along strike, exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the Rum intrusion; the Long Loch Fault. On the basis of our combined datasets, we propose that the upper part of the troctolite, the anorthosite layer underlying the Cr-spinel seam and the seam itself formed during a multi-stage magma replenishment event. The stages can be summarised as follows: (1) peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the Unit 7 troctolite. (2) The anorthosite layer then formed from the Unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma. (3) Subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the Unit 7–8 Cr-spinel seam, with (4) continued magma input eventually producing the overlying Unit 8 peridotite. In the central part of the Rum Layered Suite, the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer. However, the absence of anorthosite in close proximity to the Long Loch Fault can be explained by enhanced thermochemical erosion close to the feeder zone, and its absence close to the margins of the intrusion, at maximum distance from the Long Loch Fault, may be due to cooling of the magma and loss of erosion potential. In line with other recent studies on PGE-bearing chromitites in layered intrusions, we highlight the importance of multi-stage intrusive magma replenishment to the formation of spatially coupled anorthosite and Cr-spinel seams, as well as the lateral mineral chemical variations observed in the Unit 7–8 boundary zone cumulates.

Hydrothermal footprint related to regional-scale shear zone-controlled scheelite mineralization, Seridó W-skarn system, northeastern Brazil

Tungsten deposits and occurrences in the Seridó Mineral Province represent metasomatic assemblages that differ from the garnet-diopside skarn deposits described in the area. The six W(±Mo ± Au) selected for study show no spatial association with carbonate rocks or igneous intrusions, but are strongly controlled by regional-scale N20°E-trending shear zones formed during the Neoproterozoic-Ediacaran (ca. 615 Ma to 530 Ma) Brasiliano Cycle. Throughout the area, this orogenic event affected both basement and supracrustal rocks, forming scheelite-bearing rocks in both domains. The alteration process is characterized by three mineral associations: (1) diopside-amphibole-feldspars (DAF), (2) amphibole-feldspars (AF), and (3) biotite-rich (BR); and each deposit or occurrence may present one, two or all mineral assemblages. Some of the studied locations are also spatially associated with the mineralized garnet-diopside skarns (MGDS) of the province, where the three mineral associations generally occur tens to hundreds of meters below them. These associations represent a continuously evolving system that records changes in physicochemical properties over time. These changes are documented by mineral chemistry analyses for pyroxene, amphibole, feldspars, biotite, epidote group minerals, titanite and apatite. Earlier, higher temperature metasomatism enriched the system in Mg, Fe, Ca, Ba and REE, whereas later, lower temperature stages are characterized by an enrichment in Na, K, Si, W and Mo. The studied mines and occurrences provide important information on the distal, deeper hydrothermal alteration haloes of tungsten skarn deposits in the Seridó Mineral Province.

Stratabound skarn Pb-Zn mineralization in the Yunkai Domain (South China): The Fozichong case

The Fozichong Pb-Zn ore district, located in the Yunkai Domain, southern section of the Qin-Hang Metallogenic Belt (QHMB) in South China, consists of several large-size stratabound ore deposits. The ore bodies are mainly stratiform and stratoid, bounded within the Ordovician and Silurian strata that are intruded by the Yanshanian porphyries. These deposits are debated between syngenetic sedimentary exhalative (SEDEX)- and epigenetic skarn-type. Since the occurrence of ore bodies is accompanied by the green-color stratiform rock (GSR), the GSR origin may play an important role to resolve the controversy. This motivates us to conduct a set of petrographic investigations, whole-rock and mineral chemistry analyses of the GSR coupled with zircon U–Pb geochronology of the intrusions. Detailed petrographic observations reveal that the GSR is mainly composed of a mineral assemblage of garnet, pyroxene, epidote, amphibole and chlorite, in analogy with that of traditional skarns. In addition, the concentrations of ore-related Pb, Zn, Cu and Ba consistently decrease from rim to core of an individual GSR layer, supporting an origin of hydrothermal fluid replacement rather than comparatively homogeneous sedimentation. Therefore, the Fozichong GSR is referred to as the stratiform skarn in literature. The Mn-poor and Ca-rich andradite, diopside-hedenbergite and calcic amphibole in the skarn provide robust evidence for the magmatic hydrothermal origin. The skarn is believed to be formed by the infiltration of magmatic hydrothermal fluids since it is not confined in the intrusion contact zone. Where the hydrothermal fluids derived from is complicated due to the multi-stages magmatic intrusions present in the district. Detailed investigations indicate that the late Yanshanian porphyries (zircon U–Pb dated at ~ 100 Ma, including dacite porphyry and granite porphyry) have a genetic link with the Fozichong skarn mineralization, while the Indosinian Dachong pluton and the early Yanshanian Guangping pluton exhibit insignificant correlation with the mineralization. The redox state of the skarn system changes from oxidation (prograde and retrograde stages) to reduction (late retrograde and quartz-sulfide stages), as recorded by the Fe2+/Fe3+ ratios in skarn minerals (i.e., 0.05 for garnet, 0.01–0.23 for amphibole and 55.6–178.1 for chlorite, respectively). During the redox transition, the mineralization has been developed with the precipitation of chlorite at moderate temperatures (~255 to 278 °C). In combined with the above analyses, the Fozichong is representative of a stratabound skarn Pb-Zn mineralization system in the Yunkai Domain, South China.

Causes and Consequences of Wehrlitization Beneath a Trans‐Lithospheric Fault: Evidence From Mesozoic Basalt‐Borne Wehrlite Xenoliths From the Tan‐Lu Fault Belt, North China Craton

AbstractTrans‐lithospheric faults represent weak zones where some of the most intense interaction between the lithospheric mantle and melts occurs. We carried out detailed petrological observations and mineral chemical analyses of feldspar‐bearing wehrlites entrained in the Mesozoic basalts from Liaoyuan, northeastern North China Craton (NCC) to obtain new insights into the nature and consequences of wehrlitization. Wehrlites (n = 12) show textural evidence for replacement of orthopyroxene by clinopyroxene, combined with low Ti/Eu (692–4,425) and variable, though generally high, Ca/Al, (La/Yb)N, and Zr/Hf in clinopyroxenes. This is ascribed to interaction with a silica‐undersaturated, carbonated silicate melt. Feldspar with variable K2O (0.17–9.84 wt.%) and CaO (0.03–12.8 wt.%) and some clinopyroxene likely formed by decomposition of amphibole and mica during heating prior to entrainment. Integrated with data from peridotite xenoliths in Cenozoic basalts along the Tan‐Lu fault belt (TLFB), these observations indicate deep volatile‐rich melt metasomatism. This may have occurred during a known Late Cretaceous to Early Tertiary extensional stage and was precursory to basaltic magmatism in the north‐south direction along the TLFB. We suggest that wehrlitization beneath the TLFB—and elsewhere—monitors the passage and mobility of carbonated melts linked to lithospheric thinning up to decratonization. Upward transport of the wehrlitizing carbonated melts through the massive TLFB and ultimate degassing of CO2 at the surface during its extensional stage may have contributed to the mid‐Cretaceous greenhouse climate.

Deconstructing Earth's oldest ichnofossil record from the Pilbara Craton, West Australia: Implications for seeking life in the Archean subseafloor.

Microtextures of titanite (CaTiSiO5 ) in exceptionally preserved Archean pillow lavas have been proposed as the earliest examples of microbial ichnofossils. An origin from microbial tunneling of seafloor volcanic glass that is subsequently chloritized and the tunnels infilled by titanite has been argued to record the activities of subseafloor microbes. We investigate the evidence in pillow lavas of the 3.35Ga Euro Basalt from the Pilbara Craton, Western Australia, to evaluate the biogenicity of the microtextures. We employ a combination of light microscopy and chlorite mineral chemical analysis by EPMA (electron probe micro-analysis) to document the environment of formation and analyze their ultrastructure using FIB-TEM (focussed ion beam combined with transmission electron microscopy) to investigate their mode of growth. Petrographic study of the original and re-collected material identified an expanded range of titanite morphotypes along with early anatase growth forming chains and aggregates of coalesced crystallites in a sub-greenschist facies assemblage. High-sensitivity mapping of FIB lamellae cut across the microtextures confirm that they are discontinuous chains of coalesced crystallites that are highly variable in cross section and contain abundant chlorite inclusions, excluding an origin from the mineralization of previously hollow microtunnels. Comparison of chlorite mineral compositions to DSDP/IODP data reveals that the Euro Basalt chlorites are similar to recent seafloor chlorites. We advance an abiotic origin for the Euro Basalt microtextures formed by spontaneous nucleation and growth of titanite and/anatase during seafloor-hydrothermal metamorphism. Our findings reveal that the Euro Basalt microtextures are not comparable to microbial ichnofossils from the recent oceanic crust, and we question the evidence for life in these Archean lavas. The metamorphic reactions that give rise to the growth of the Euro Basalt microtextures could be commonplace in Archean pillow lavas and need to be excluded when seeking traces of life in the subseafloor on the early Earth.

Using Mineral Chemistry to Aid Exploration: A Case Study from the Resolution Porphyry Cu-Mo Deposit, Arizona

AbstractThe giant, high-grade Resolution porphyry Cu-Mo deposit in the Superior district of Arizona is hosted in Proterozoic and Paleozoic basement and in an overlying Cretaceous volcaniclastic breccia and sandstone package. Resolution has a central domain of potassic alteration that extends more than 1 km outboard of the ore zone, overlapping with a propylitic halo characterized by epidote, chlorite, and pyrite that is particularly well developed in the Laramide volcaniclastic rocks and Proterozoic dolerite sills. The potassic and propylitic assemblages were overprinted in the upper parts of the deposit by intense phyllic and advanced argillic alteration. The district was disrupted by Tertiary Basin and Range extension, and the fault block containing Resolution and its Cretaceous host succession was buried under thick mid-Miocene dacitic volcanic cover, obscuring the geologic, geophysical, and geochemical footprint of the deposit.To test the potential of propylitic mineral chemistry analyses to aid in the detection of concealed porphyry deposits, a blind test was conducted using a suite of epidote-chlorite ± pyrite-altered Laramide volcaniclastic rocks and Proterozoic dolerites collected from the propylitic halo, with samples taken from two domains located to the north and south and above the Resolution ore zone. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data of epidote provided indications of deposit fertility and proximity. Competition for chalcophile elements (As, Sb, Pb) between coexisting pyrite and epidote grains led to a subdued As-Sb fertility response in epidote, consistent with epidote collected between 0.7 and 1.5 km from the center of a large porphyry deposit. Temperature-sensitive trace elements in chlorite provided coherent spatial zonation patterns, implying a heat source centered at depth between the two sample clusters, and application of chlorite proximitor calculations based on LA-ICP-MS analyses provided a precisely defined drill target in this location in three dimensions. Drilling of this target would have resulted in the discovery of Resolution, confirming that epidote and chlorite mineral chemistry can potentially add value to porphyry exploration under cover.

Granulitic rocks at the Western Cordillera of Colombia: Evidence of metamorphism in the Colombian Caribbean Oceanic Plateau

The geological basament of the Western Cordillera of Colombia has been interpreted as an igneous province derived from the Colombian-Caribbean Oceanic Plateau (CCOP) with no high-grade metamorphic rocks. The Mistrató Quartz-diorite and Sabanalarga Batholith, however, as part of the CCOP, contain some rocks that evidence granulite facies metamorphism. Here we define the Mistrató Granulite as a different unit from the Mistrató Quartz-diorite and correlate with the Pantanillo Granulite, which has been previously defined as a different unit from the Sabanalarga Batholith. We use field relationships, petrography, mineral chemistry and whole-rock chemistry analyses to evidence the metamorphism. The Mistrató Granulite, which outcrops into the inner region of the Colombian Western Cordillera, hosts a Ca-richer recrystallized assemblage, consisting of plagioclase + clinopyroxene + ortopyroxene + quartz + ilmenite, and a Ca-poorer relict assemblage, which is composed of plagioclase + clinopyroxene + orthopyroxene + ilmenite + actinolite. The anhydrous nature and polygonal granoblastic texture of the recrystallized assemblage suggest metamorphism of a gabbroic protolith that reached granulitic conditions ( 900 ± 50 °C and < 1.1 GPa). Geochemical analyses reveal that these rocks may have formed in an island volcanic arc linked to the CCOP during the Late Cretaceous. We have found that the geochemistry of the Mistrató and Pantanillo granulites, together with regional trends, are comparable, allowing us to propose that both units could have derived from similar mafic oceanic rocks and formed under similar tectonic settings. This differs from previous interpretations that explain the Pantanillo Granulite as part of the continental basament of the Central Cordillera.

Lamprophyres and mineralization of the Koytash ore field (Southern Tien Shan)

Research subject. The Koytash ore field is located in the potentially productive Au, Ag, W, Mo, Ti, Fe, Cu, Pb, Zn and REE North Nuratau mineralisation zone of Tien Shan. The authors undertook a study of the composition of dikes breaking through the Paleozoic ore-bearing formations, as well as their petro- and ore-generating role in the formation of the Koytash-Ugat sulphide-rare-metal (W, Mo, Fe) specialised mineralisation. Materials and methods. A study of rock and mineral composition was performed at the Institute of Geology and Geophysics named after Kh.M. Abdullaev. The content of petrogenic and rare elements in rocks and sulphides was determined by ICP-MS using an ICPE-9000 mass-spec trometer in the Central Laboratory of the State Committee for Geology of the Republic of Uzbekistan. The chemical analysis of minerals was performed using a Jeol-8800Rh electronic microanalyser at the Institute of Geology and Geophysics named after Kh.M. Abdullaev. The micrographs of transparent sections were obtained using Nikon Optiphot 2 Pol and Polam R-311 microscopes. Results and conclusions. The conducted study showed that, in terms of their structure, the Koytash ore field dike formations can be regarded as lamprophyres. In terms of their chemical composition, these formations are mafic and intermediate rocks of the subalkaline series. It was found that the composition of lamprophyre dikes correlates with the size of the erosion section. Their melanocratic varieties are confined to the southern part of the intrusion (absolute elevations are 1000–1200 m), and leucocratic – to the northern (about 1900 m). This is assumed to be the result of crystallization differentiation of a single initial melt. The dikes of the Koytash ore field lamprophyres break through not only sulphide-rare-metal bodies of the Koytash-Ugat strip, but also skarn and carbonate rocks and, in turn, are broken through by quartz-polymetallic ore-bearing veins, which testifies to their inter-ore character.

Geochemistry of banded sulphidic cherts of Sandur greenstone belt, Dharwar Craton, India: Constraints on hydrothermal processes and gold mineralization

The banded iron formations which are hosted in the greenstone belts of Dharwar Craton are potential sources of iron and also major archives of gold mineralization at some places. The gold bearing banded sulphidic cherts from the Neoarchean Sandur greenstone belt of western Dharwar Craton have been reported in this paper in order to understand their geochemical characteristics, source of gold mineralization and depositional conditions. These rocks are predominantly characterized as mixed oxide-sulphide facies which are generally referred as sulphide facies iron formations, consisting of alternate mesoscale bands of silica and iron with sulphides such as pyrite, chalcopyrite, galena, arsenopyrite and sphalerite occurring as minor opaque phases. Mineral chemical analyses of pyrite having high Fe/(S + As), Co/Ni and Pb/As ratios suggests hydrothermal source. Native gold occurs as disseminations in these sulphides with its concentrations ranging from 0.05 to 1.48 ppm. Gold mineralization is sediment hosted variety, epigenetic, epithermal-hydrothermal in origin, where the sulphidic-auriferous fluids generated at Mid oceanic ridge (MOR)-hydrothermal system were transported and deposited in an oxygen-deficient environment. Low CaO, MgO, K2O and ƩREE contents suggests minor detrital input while La-Th-Sc relationship indicate minor volcaniclastic source from metabasic rocks. Low K/Rb and Th/U illustrate negligible effects of weathering in these samples. Geochemical proxies such as superchondritic Y/Ho ratios along with Post Archean Australian Shale (PAAS) and Mud from Queensland (MUQ) normalized REE + Y patterns with depleted LREE and positive Eu, La and Y anomalies suggest that these BIFs were generated by the chemical precipitation of iron and silica in sea water at MOR and continental margin setting which had suboxic to anoxic paleo-redox conditions.

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

The Rum Eastern Layered Intrusion (ELI; Scotland) is an open-system layered intrusion constructed of 16 macro-rhythmic units. Each of the macro-rhythmic units consists of a peridotite base and a troctolite (± gabbro) top, previously attributed to the fractional crystallisation of a single magma batch. This classic paradigm has been challenged, however, with evidence presented for the emplacement of peridotite sills in Units 9, 10, and 14, such as cross-cutting relationships, upward-oriented apophyses, and lateral discontinuities. To test whether the other major peridotites within the ELI represent sills, we have carried out new field, petrographic, and mineral chemical analyses of the peridotites in Units 7, 8 and 9. The peridotites display large- and small-scale cross-cutting relationships with the overlying troctolite, indicative of an intrusive relationship. The peridotites also show large-scale coalescence and lateral spatial discontinuities such that the ELI unit divisions become arbitrary. Harrisite layers and Cr-spinel seams found throughout Units 7, 8, and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma. Our observations allow for distinct subtypes of peridotite to be defined, separated by intrusive contacts, allowing for their relative chronology to be determined. Older, poikilitic peridotite, rich in clinopyroxene, is truncated by younger, well-layered peridotite, containing abundant harrisite layers. In addition to the new peridotite subtypes defined in this study, we find strong evidence for laterally oriented metasomatism within clinopyroxene-rich wehrlites at the top of the Unit 8 peridotite. The wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene-rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions. The observations presented in this study from the ELI cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre-existing feldspathic cumulate (gabbro or troctolite). The evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism, forming many of the unusual (often clinopyroxene-rich) lithologies that surround the sills. The broad-scale formation of the layered peridotites via incremental sill emplacement, suggested by the occurrence of upward-oriented apophyses, coalescence, and lateral discontinuity, could be applied to much larger ultramafic intrusions, which might have formed by similar mechanisms.

Strain partitioning in host rock controls light rare earth element release from allanite-(Ce) in subduction zones

AbstractCombined microstructural, mineral chemical, X-ray maps and X-ray single-crystal diffraction analyses are used to reveal the behaviour of individual grains of magmatic allanite relicts hosted in variably deformed metagranitoids at Lago della Vecchia (inner part of the Sesia-Lanzo Zone, Western Alps, Europe), which experienced high-pressure and low-temperature metamorphism during the Alpine subduction. X-ray single-crystal diffraction shows that none of the allanite crystals, irrespective of the strain state of the host rock, record any evidence of plastic deformation (i.e. intracrystalline deformation), as indicated by the shape of the Bragg diffraction spots, the atomic site positions, and their displacement around the centre of gravity. On the contrary, strong plastic deformation affected matrix minerals, such as quartz, white mica and feldspar of the hosting rocks, during the development of the Alpine eclogitic- and blueschist-facies metamorphism. Despite the strain-free atomic structures of allanite, different patterns of chemical zoning, as a function of strain accumulated in the rock matrix, are observed. As allanite occurs in magmatic and metamorphic rocks and it is stable at high-pressure and low-temperature conditions, we infer that allanite could behave as one of the main carriers of light rare earth elements into the mantle wedge during subduction of continental crust. In particular, the release of light rare earth elements from allanite, under high-pressure conditions in subduction zones, is facilitated by high strain accumulated in the host rock.

Rare earth elements and carbon-oxygen isotopes of calcite from the Tongjiachong Cu deposit, South China: Implications for fluid source and mineral precipitation

Calcite is a major gangue mineral in many types of hydrothermal deposits, and its chemical and carbon-oxygen isotopic signatures can shed light on the metallogenic evolution. The Tongjiachong Cu deposit, located in the northern margin of the Yangtze Block in South China, is hosted by dolostone of the Ediacaran Doushantuo Formation. Empirical geothermometric calculation indicates that the pre-ore chlorite was formed at 265–305 °C. Copper mineralization occurs mainly as veinlets in a calcite-chalcopyrite-bornite ± ankerite assemblage. Exsolution lamellae of chalcopyrite from bornite indicate that both minerals were formed coevally at 200–250 °C. Mineral chemical analysis on the syn-ore hydrothermal calcite from Tongjiachong suggests that all the calcite samples are enriched in rare earth elements and yttrium (REYs) (ΣREY = 53.7–119.9 ppm). They have similar Y/Ho (38.3–42.3) and varying La/Ho (21.8–86.7), and display consistent convex PAAS-normalized REY patterns. The distinct positive Eu anomalies (2.38–3.45) and lack of Yb anomalies (0.92–1.05) suggest initial fluid temperatures of 250 to 420 °C and calcites deposition temperatures of <250 °C. The lack of negative Ce anomalies reflects suboxic to anoxic conditions of the ore-forming fluids.Dolomite samples from the dolostone ore host show a marine carbonate C-O isotopic affinity (13CPDB = −4.0 to −2.7‰; δ18OSMOW = +19.8 to +20.2‰). Comparatively, the hydrothermal calcite contains lower 13CPDB (−7.4 to −6.1‰) and δ18OSMOW (+14.1 to +14.8‰), and display a positive 13CPDB vs. δ18OSMOW correlation. Simulated calculations of water-rock (W/R) interactions and ore geological constraints suggest that the initial fluids were likely originated from organic-rich formation water. We proposed that W/R reactions may have been the main drive for calcite precipitation at Tongjiachong, whilst decreasing temperature and W/R reactions may have led to the chalcopyrite precipitation.

Study of the chromite mineralization associated to ophiolites from Tinos Island, Attico-Cycladic Massif

This paper aims to study the chromitites, as well as their host rocks (meta-peridotites, meta-dunites and serpentinites) of the ophiolite complex of Mount Tsiknias, in Tinos Island. Recognition of their mineralogy and their textural features was carried out through detailed petrographic study. The mineral chemistry analysis contributed to the evaluation of the analyzed chromites, the chemical composition of which provides important information about the petrogenetic evolution of the chromitite ores. The chromites were in equilibrium with boninite melts derived from Supra-Subduction Zone, e.g., a depleted mantle wedge. In the binary classification diagram for spinels, the Tinos samples extend in the fields of Mg-chromite and chromite sensu strictu. In the TiO2 vs Al2O3 diagram, the chromites plot in the field of Supra-Subduction Zone (SSZ) peridotites and partly overlap the field of chromites in Back-Arc Basalts (BABB), however the same samples plot in the field of chromite of boninites. In the Al2O3 vs Cr2O3 diagram both groups of Tinos chromites plot in the field/extremity of “mantle chromites”.

Origin and sequence stratigraphic implications of high-alumina glauconite within the Lower Quartzite, Vindhyan Supergroup

Abstract An integrated field, petrographical and geochemical investigation reveals a shallow marine dys-oxic depositional setting for the glauconite at the top of the Mesoproterozoic Lower Quartzite Member of the Vindhyan Supergroup. A thorough facies and paleogeographic analysis indicates a storm-influenced, shallow subtidal depositional condition for the glauconitic sandstone, occupying the mid-level of a transgressive systems tract (TST) deposit, which is capped by the maximum flooding zone (MFZ). The X-ray diffractional parameters confirm the evolved character of the glauconite. The mineral chemical analysis indicates the glauconite is enriched in K2O, MgO and Al2O3 and depleted in Fe2O3(total), compared to the Phanerozoic variety. It formed by the pseudomorphic replacement of the K-feldspar, by the addition of Fe2O3 at consistently high content of K2O, unlike most Phanerozoic examples. The elevated level of H3SiO4, Cl, K, Na, Mg and Fe, possibly derived by the continental weathering and/or contemporary volcanism, facilitated the glauconitization process. The high content of Mg in the glauconite corresponds to the Mg-rich, Mesoproterozoic seawater. A dys-oxic depositional condition, associated with a major marine flooding surface, favoured the formation of glauconite. While the onset of anoxic regime caused the disappearance of glauconite and the deposition of pyritiferous and carbonaceous shale in the overlying condensed zone deposits.

Petrography and mineral chemistry of the Varena Iron Ore deposit, southeastern Lithuania: implications for the evolution of carbonate and silicate rocks and ore mineralization

The large Varena Iron Ore deposit of southeast Lithuania lies beneath 200–400 m thick sedimentary cover in the East European Craton. Several drilling projects have revealed that it contains ca. 70–200 million tons of iron ore. A metasomatic origin has been proposed by several investigators, with an igneous (mafic, ultramafic carbonatitic) origin also considered. Detailed micropetrography on a variety of ore-bearing and skarnitised samples through backscattered electron imaging, along with mineral chemical analysis and monazite dating by electron microprobe, provide support for a metasomatic origin. High-grade temperature (ca. 700–750° C) skarns containing forsterite, enstatite, spinel and diopside were formed preferentially in carbonate-rich rocks and were subsequently overprinted by amphibole-bearing (tremolite, actinolite, anthophyllite and hornblende) skarns. A low-temperature hydrous influx and changing redox conditions not only caused the serpentinization of the earlier skarns, but also deposited a considerable amount of iron ore in the form of magnetite. Iron was derived from various sources and was accompanied by sulphide, apatite, REE and other mineralizations. The ca. 1.78 Ga monazite grew during the regional metamorphism, while ca. 1.54 Ga is attributed to the last mineralization phases.

Ilmenites and their alteration products, sinkholes for uranium and radium in roll-front deposits after the example of South Tortkuduk (Kazakhstan)

The approximate determination of average Ra/U disequilibria in orebodies is one of the most common causes of errors in U reserve estimations. In roll-front deposits, the disequilibria are however frequently distributed following complex geometries, which must be fully understood to prevent major U reserve overestimates and costly unproductive extractive operations. The processes responsible for disruption of the radioactive equilibria and the U and Ra carriers in such complex natural systems remain poorly constrained. In this contribution, we propose an innovative approach, mixing orebody to sub-grain scale studies to unravel the distribution of U and Ra and the processes responsible for their concentration and uncoupling.Using mineral separations, gamma spectrometry and mineral-chemical analyses, we identified the Fe-Ti clusters (altered ilmenite + pyrite/marcasite) as the microsites for coffinite precipitation and Ra concentration. To understand the influence of such clusters on the distribution of U and Ra at the deposit scale, whole-rock Ra/U disequilibria were measured and mapped at a series of ten drill holes along a profile crosscutting the studied roll-front. The main Ra/U disequilibria are encountered around the mineralization in low U content zones. They are controlled by two main processes. (1) In the oxidized zones, the immobility of 230Th with respect to the U produces patches of Ra disequilibria (carried by the altered U minerals). (2) In the immediate vicinity of the roll-front, the dissolution of the mineralization produces an Ra flux trapped by the alteration products of ilmenites, as definitely confirmed by direct SIMS measurements. Such a process is responsible for the Ra disequilibria envelope located downstream of the richest ores, also known as Ra halo. The highest Ra/U ratios correspond to oxidized upstream samples, but most other high Ra/U ratios are from reduced downstream samples close to the mineralization. Such a low to medium U content envelope with high Ra/U ratios constitutes the main cause of U reserve overestimations.

The Potentials of Improving Mineral Source Additional Values in Lampung Province – A Preliminary Study

An initial research on non-metal mineral potential was conducted for stones inLampung Province. Surveys were conducted to some districts/towns in Lampung to takesample materials. Non-metal mineral chemical composition content analysis of stoneswas conducted with X-RF method. Lampung had mineral potentials of feldspar, zeolite,kaolin, and basalt of 400,500,000 m3; 18,945,000 m3; 9,750,000 m3; 419,071,833 m3respectively. These mineral source potentials were distributed in eight districts inLampung. Processing was required to improve additional values for these mineralsources, including crushing, grinding, classifying, and concentration process.Concentration improvements were required for feldspar and kaolin minerals. Wet highintensity magnetic separator process followed by froth flotation process becamesolutions to reduce high oxide iron content. Feldspar and kaolin could be used for suchindustries of ceramic, glass, paper, and refractory. Meanwhile, the zeolite mineral fromLampung could be used for agriculture, plantation, and fishery. Basalt stone wouldhave additional values when this was processed into cast basalt, fiber, and ceramiccomposite.Keywords: feldspar, kaolin, basalt, zeolite, Lampung