بررسی منشأ و شرایط شکل گیری آگات های خور و بیابانک (استان اصفهان)

Archean greenstones represent a large percentage of worlds total gold endowment and are actively mined on every continent barring Antarctica. Greenstone-hosted gold deposits often have complex deformation and alteration histories, and a general deposit model remains controversial. The aim of this thesis is to improve our understanding of gold mineralisation in Archean greenstone belts, based on a comprehensive case study of the world-class Geita Hill deposit in Tanzania. Geita Hill is one of the largest gold deposits within the Geita Greenstone Belt in north-western Tanzania and has been mined as an open pit since 2002. The deposit is hosted within a greenschist facies metamorphosed and complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately west dipping deformation zone of discontinuous shear fractures. Detailed structural studies have defined a deformation history for the deposit, providing an opportunity for an in-depth study of the hydrothermal alteration and fluids associated with gold mineralisation. The first component of this thesis builds a paragenetic framework for the Geita Hill deposit. The regional metamorphism is characterised by biotite + chlorite + actinolite + K-feldspar + magnetite ± pyrrhotite ± pyrite indicating upper greenschist facies metamorphism. The gold-related alteration overprints the regional metamorphism and is characterised by silicification and sulfidation fronts that end within one meter of the mineralised zone. Locally, the silicification and sulfidation of the wall rock occurs along a series of mineralised quartz veins which have a sub-vertical dip and tend E-W. Paleostress analysis of the mineralised shear fractures of the Geita Hill Shear Zone suggests vertical maximum compressive stress (σ₁) and northerly extensional stress (σ₃) consistent with the orientation of the mineralised quartz veins and indicating N-S extension. The composition of the mineralised quartz veins is characterised by quartz + biotite + K-feldspar + pyrite, which also overprints the metamorphic mineral assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but gold inclusions in biotite and along quartz grain boundaries are also present. The gold-bearing pyrite is associated with secondary biotite and K-feldspar. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite and inclusion free euhedral pyrite. It is common for the inclusion rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multi-phase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite + siderite + chlorite ± quartz ± pyrite ± barite. The thesis then builds on the paragenetic framework through silicate and sulfide geochemistry. Biotite was identified as a primary mineral both in the metamorphic assemblage and gold-related hydrothermal alteration assemblage. The study of silicates was conducted through detailed core logging, petrography, SEM mineral identification of alteration assemblages, SWIR measurements and microprobe analyses of biotite in order to identify the nature of the mineralising fluid and its spatial effect across the Geita Hill deposit. Results show that the mineralised assemblage is slightly more oxidized (pyrite + magnetite) compared to the metamorphic background (pyrrhotite + magnetite). The intense sulfidation within the ore zone resulted in the formation of Mg-rich biotite, which grades into more Fe-rich biotite away from the ore zone. This change in biotite composition can be detected using short wavelength infrared spectra, though a shift in the Fe-OH 2250 nm absorption feature to lower wavelengths. This shift is also correlated with an increase in gold grade within the mineralised zone. Halogen chemistry of the biotite implied the presence of multiple hydrothermal fluids during mineralisation, suggesting that within the ore zone a metamorphic fluid in equilibrium with the host rock was overprinted by and mixed with an infiltrating fluid that was enriched in fluorine. Study of the sulfides in the deposit was conducted through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of pyrite and magnetite grains. The gold distribution correlates well with Te, Ag, Bi and Pb consistent with its occurrence as micro- and nano-inclusions of gold bearing telluride minerals. As, Co and Ni distribution in pyrite grains hosted in quartz veins is patchy, whereas in pyrite grains from ironstone and diorite these elements show zoning characteristic of growth pattern. Elements including As, Ni, Co, Cu and Zn appear to be dominantly locally derived, and remobilised into the pyrite during sulfidation. The concentrations of these elements are highly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Sn, Bi and Pb appear to be dominantly externally derived, and closely correlate in all varieties of pyrite. The Se content is typical of pyrite from Archean gold deposits (~30ppm) and reflects to an average temperature of ~340°C for the mineralising fluid on the basis of temperature dependent incorporation into pyrite. Lastly, the gold-bearing hydrothermal fluids were studied directly through detailed microthermometry and raman microspectroscopy, and three principle fluids were identified: (1) A low salinity, carbonic-rich (XCO₂ > 0.8) fluid with minor N₂ (XN₂/(XCO₂ + XN₂) 20 wt.%; NaCl/(NaCl+CaCl₂) mass ratio > 0.45), aqueous brine that was interpreted to be magmatic in origin; and (3) A low salinity (NaCl < 5 wt.%) aqueous fluid that was interpreted to be meteoric in origin. Preserved fluid assemblages imply mineralisation occurred at pressures of less than 2 kbar, likely from 1.4 to 1.7 kbar, at temperatures of approximately 350 °C. C-O-H fluid modelling of the carbonicrich fluid has constrained ƒO₂ fluid to 1.5-1.8 log10 units above ƒO₂ FMQ corresponding to absolute values of 10- 30.5 bar. The gold was likely transported in the high salinity brine as Au-bisulfide complexes with tellurium, potentially introduced as a vapour. Deposition of Au was triggered via interaction of gold-bearing fluids with the relatively reduced Fe-rich host rocks and the low salinity CO₂-rich fluid.

The Mantoverde area of the III Región of northern Chile hosts numerous copper(–gold) deposits and prospects assigned to the iron oxide–copper–gold (IOCG) clan. These range from sulphide-poor magnetite–apatite–actinolite bodies (e.g. Carmen) to chalcopyrite-rich, hematite-cemented breccias and veins (e.g. Mantoverde, Cerro Negro and Palmira). The most important Cu-rich deposits are associated with the main or, more commonly, subsidiary structures of the plate-boundary-parallel Atacama Fault System, and are hosted by Middle to Upper Jurassic andesites and Lower Cretaceous dioritic to quartz dioritic plutons. The rocks in the wider Mantoverde area exhibit a complex evolution involving early sub-seafloor albitization (stage R1), subsequent, very low-grade diastathermal metamorphism (R2), and more local stage I K and Fe metasomatism and stage II hydrolysis, all preceding the emplacement of sulphide-bearing hematitic breccias and veins (i.e. ore-stage III). We develop a lithogeochemical exploration protocol specifically for Cu-rich IOCG-type deposits hosted by calc-alkaline volcano-plutonic terrains, based on the integration of whole-rock molar element ratios and oxygen isotope chemistry. Zirconium, the most conserved (least mobile) element in the volcanic and plutonic rocks of the area, was used as a common denominator for molar element ratios. The molar ratios Na/Zr, K/Zr and Al/Zr indicate that the host rocks proximal to the major deposits were affected by sodium depletion and potassium enrichment. The molar ratio (14Ca+19Na−14CO 2 )/(6Si+Al+2Fe+2Mg), recalculated to a percentage scale, constitutes a modified alteration index which can be used to quantify the degree of hydrothermal alteration in the host rocks. However, although this index differentiates barren from potentially prospective sectors (i.e. strongly altered host rocks), it does not identify the hydrolytically altered rocks that are closely associated with economic copper mineralization. The δ 18 O values of igneous rocks in the Mantoverde area vary widely from +4.2 to +14.1‰. Integrating the modified lithogeochemical alteration index with whole-rock δ 18 O values differentiates the least altered volcanic and plutonic rocks (i.e. alteration index &lt;30%), with δ 18 O values close to +7‰, from moderately altered volcanic rocks (alteration index of 30 to 65%) having higher δ 18 O values of +10 to +13.2‰ due to enrichment in 18 O during regional metasomatism hydrothermal alteration sub-seafloor related to albitization and metamorphism. A third group comprises strongly K-feldspathized, Fe-metasomatized and chloritized rocks localized in Cu-mineralized centres which have alteration indexes exceeding 65% and δ 18 O values of +4.2 to +14.1‰. The δ 18 O values of paragenetically related minerals indicate that barren, K- and Fe-metasomatized host rocks equilibrated with magmatically derived fluids at temperatures exceeding 420°C. Although no recognizable alteration haloes are associated with the ensuing lower-temperature Cu-(Au) ore stage (III), the genetically related hydrolytic alteration (II) which preceded it generated distinctive alteration indices and Na/Al and K/Al ratios, as well as δ 18 O values averaging +10‰, with chlorite O and H isotope compositions indicating equilibration with non-magmatic fluids. We propose, in combination, that these parameters can be used to vector onto Cu-rich IOCG-type mineralization in Andean, calc-alkaline terrains.

ناحیه مطالعاتی در شمال استان خراسان رضوی و 45 کیلومتری جنوب نیشابور قرار دارد. رگه های طلادار درون سنگهای گرانیت، گرانودیوریت، گرانودیوریت پورفیری، آندزیت، برش و توف نفوذ کرده اند. رگه ها از جنس کوارتز و کلسیت هستند. مناطق اصلی دگرسانی شامل منطقه سرسیتیک، سیلیسی، پروپلیتیک و منطقه کربنات است. کانی زایی به‌طور نزدیکی با دگرسانیهای سیلیسی و سرسیتیک به شکل رگه‌چه و با منطقه پروپلیتیک به شکل افشان همراه است. کانی شناسی کانیهای باطله کوارتز، کلسدونی، سرسیت، آدولاریا، کلسیت، دولومیت، ایلیت، دیکیت، آلبیت، کائولینیت و کلریت است. کانی زایی رگه ای عمدتا با رگه‌چه ها، برش، افشان و استوک ورک همراه هستند. رگه‌چه ها از پیریت، مارکازیت، آرسنوپیریت و کمتر کالکوپیریت، اسفالریت، گالن، مگنتیت و هماتیت تشکیل شده اند. بیشترین عیار طلا در رگه های سیلیسی دیده می شود. پیریت کانی سولفیدی اصلی در کانی سازی اولیه است. سه نوع پیریت بر اساس ترکیب شیمیایی شناسایی شده است: پیریت خالص، پیریت غنی از آرسنیک و پیریت تیتانیوم- وانادیوم دار. تجزیه های ریزکاوشگر الکترونی بیشترین غلظت آرسنیک، تیتانیوم و وانادیوم را به ترتیب 62/3، 91/3 و 53/0 درصد وزنی در پیریتهای رگه های طلادار نشان می دهند. طلا معمولا همراه با پیریت آرسنیک دار و پیریت تیتانیوم- وانادیوم دار است. براساس مطالعه بافت و ترکیب پیریتها، رگه‌چه‌های پیریت آرسنیک دار همراه با کانیهای سولفیدی آرسنوپیریت، کالکوپیریت، گالن، اسفالریت در دمای بالا و با کاهش دما و فشار، پیریتهای فرامبوئیدال با حاشیه حاوی آرسنیک و طلا شکل گرفته اند و در ادامه پیریتهای درشت تیتان دار به‌وجود آمده اند. پیریتهای خالص و تأخیری درشت بلور آخرین فاز کانی ساز در منطقه هستند.

The Şile Region contains discontinuous, cyclic, thin coal beds and industrial clay deposits that were accumulated in lacustrine basins which received extensive volcanoclastic sediments due to transport of highly weathered calc-alkaline volcanic rocks. The Sülüklü area has the largest kaolin deposit in this region. Cyclic kaolinization depended on the degree of leaching of Si and alkalis in cyclic swamp environments and, therefore, kaolinite contents vary in each discontinuous lens-shaped clay bed and underclay within the basin. The kaolins comprise disordered kaolinite, illite, smectite, gibbsite, quartz, pyrite, anatase, K-feldspar and goethite. Depth-related changes in the distribution of clay minerals, associated with coal beds, are indicative of organic acid-mineral reactions. Kaolinite crystallization initiated at the edges of sericitic mica sheets in the form of composite kaolinite stacks. The small size (&lt;1 µm), morphology and poor crystallinity of kaolinite crystals suggest that kaolinization post dated transportation. Primary or secondary origins of particles can be determined from the stacking sequences of kaolinite particles using high-resolution transmission electron microscopy images. Kaolinite stacks always contain a small amount of illite, but smectite is only present in the middle and upper levels. Gibbsite is a main constituent of refractory bauxitic clays locally found as discontinuous lenses and exploited from the lower level of the basin.Genesis of kaolin deposits took place in two stages: first, there was in situ weathering of the oldest andesitic agglomerates, tuffs and ashes at the base of the lacustrine basin coupled with discharge of shallow thermal waters which were initiated by local hydrothermal alteration; second, surface weathering enhanced transportation of altered rocks from the surrounding hills into the lacustrine basin. Kaolinization took place in cyclic swamp environments, as indicated by the presence of cyclic thin- to thick-bedded coals that provided necessary humic and fulvic acids for the post-depositional alteration of altered volcanic rocks to kaolin in dysaerobic, relatively low-pH conditions in saturated groundwater zones.

Geochemistry of hydrothermal illitizations in Eocene Kösedağ magmatic rocks, Zara-Suşehri area, NE Sivas, East-Central Anatolia: Origin and age of alteration

Introduction In the east and northeast of Sanandaj in the Qorveh-Bijar-Takab axis, there are series of basaltic composition volcanoes with Quaternary age. The study area is part of the Sanandaj-Sirjan zone and is located between 47°52' and 47°57' E longitudes and 35°26 and '35°30' N latitudes. Due to the location of the volcanic cone on Pliocene clastic sediments and Quaternary travertine, the age of these volcanoes is considered to be Quaternary. The cones mostly consist of low scoria, ash, volcanic bombs, lapilli deposits and basaltic lava (Moein Vaziri and Aminsobhani, 1985). Petrological and geochemical studies have been carried out to evaluate Quaternary magmatism in the area and to determine the nature of the lithological characteristics, such as the evaluation of source rocks and magma type, degree of partial melting and the tectonic setting of Ghezel Ghaleh rocks (Moein Vaziri, 1997). Simplified geological map of the study area is characterized by ER-Mapper software. Materials and methods In the course of field studies in the region, 40 samples were taken, 30 thin sections were prepared and polished. XRD analyses were performed on some whole rock samples. All major, minor and trace elements were assessed by ICP-MS at Lab Weft Laboratory in Australia. Results Based on the classification of structural zones, the area is located in the Sanandaj-Sirjan zone, hundred kilometers away from the main Zagros thrust along the NW-SE direction. After early Cimmerian orogeny, andesitic volcanic activity took place (Moein Vaziri and Aminsobhani, 1985). A major secondary mineral in these rocks is iddingsite, formed by hydration and oxidation of the olivine (Shelley, 1993). According to SiO2 against Na2O + K2O (TAS) diagram (Irvine and Baragar , 1971) and cationic R1 and R2 diagram (De La Roche et el., 1980), volcanic rocks of the area indicate alkaline series. Discussion To obtain more information on the tectonic setting of these rocks, the Zr/Y-Zr diagram by Pierce (Pearce and Norry, 1979) as well as Nb/Y versus Ti/Y diagram of Pierce (Pearce and Cann, 1973), show that alkali basalt rocks in the study area are fitted in the field of within plate basalts. To determine the genesis of rocks from melting curve of Aldanmaz and Colleagues (Aldanmaz et al., 2006) based on changes in REE (La on Sm/Yb), the samples show approximately 1 to 5% partial melting of garnet lherzolites. The spider diagrams indicate that the studied rocks are enriched in LREE and LILE, depleted in HFSE with no Eu anomaly, Cs, Sr, and Pb positive anomalies which are the characteristics of alkaline magmas and high concentrations of incompatible elements and alkaline elements in the lava, implying the melting of the lower part of the mantle source. Light rare earth elements, are incompatible with the primary crystallized phases such as olivine, clinopyroxene and plagioclase, consequently focused increasingly during phase crystallization and fractionation in the remaining fluid (Hirschman, 1998). Conclusions Based on microscopic and geochemical data, these rocks are alkali basalt, basanite and tephrite. The rocks contain olivine, pyroxene, feldspar, and minerals such as iddingsite, opaque and secondary minerals, calcite with porphyritic texture and microlitic and glassy matrix, vesicular and some glomeroporphyritic, vitrophyric and amygdaloidal textures. Most minerals have undulose extinction which indicates mantle deformation. Geochemical data for the rocks indicate high-K alkaline characteristic of the primary magma. The spider diagrams indicate that the studied rocks are enriched in LREE and LILE, depleted in HFSE with no negative Eu anomaly, positive anomalies of Cs, Sr, Pb which are characteristics of alkaline magmas. These rocks are produced by partial melting of garnet-lherzolite rich under lithospheric mantle. Based on tectonomagmatic diagrams, they are within plate basalts and by magmatic series graphs are alkali basalts. Microscopic evidence such as disequilibrium textures in the minerals (zoned state, solution and twinning) shows a magmatic contamination in mixing volcanic mass. References Aldanmaz, E., Koprubasi, N.O., Gurer, F., Kaymakci, N. and Gournaud, A., 2006. geochemical constraints on the Cenozoic, OIB-type alkaline volcanic rocks of NW Turkey: implications for mantle sources and melting processes. Lithos, 86 (1–2) pp. 50–76. De La Roche, H., Leterrier, J., Grand claude, P. and Marchel, M., 1980. A classification of volcanic and plutonic rocks using R1-R2 diagrams and major elements, it’s relationships with current nomenclature. Chemical Geology, 29(1-4): 183–210. Hirschman, M., 1998. Origin of the transgressive granophyres in the layered series of the Skaergaard intrusion, East Greenland. In: D.J. Geist and C.M. White (Editors.). Journal of Volcanology and Geothermal Research, 52(1-3): 185–207. Irvine, T.N. and Baragar, W.R.A., 1971. A guide to chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences, 5(8): 448– 523. Moein Vaziri, H., 1997. The history of magmatism in Iran. Tehran University Press, Tehran, 440 pp. (in Persian) Moein Vaziri, H. and Aminsobhani, A., 1985. Study of young volcanic region being involved in –Qorveh- Takab. Tehran University Press, Tehran, 350 pp. (in Persian) Pearce, J.A. and Cann, J.R., 1973. Tectonic setting of basaltic volcanic rocks determind using traceelements analysis. Earth and Planetary Science Letters, 19(2): 290– 300. Pearce, J.A. and Norry, M.J., 1979. Petrogenetic implications of Ti, Zr, Y and Nb variation in volcanic rocks. Contributions to Mineralogy and Petrology, 69(1): 33– 47. Shelley, D. (Translated by Mohamadzadeh, F.), 1993. Igneous and metamorphic rocks under the microscope, classification, textures, microstructures and mineral preferred-orientations. Chapman and Hall, Unwin, London, 445 pp.

At least two main stages of filling and subsidence have been identified in the Campo AlegreCorupá Basin (CACB), namely the Basin and the Caldera Stages. In the Basin Stage, the regional collisional tectonics triggered the far-field stress resulting in a local extension at ~605 ± 5 Ma through the reactivation of NNW-SSE inherited basement structures. The deposition of the sedimentary basin finishes with the Initial Volcanic Activity, corresponding to a bimodal mildly alkaline, predominantly mafic and effusive volcanism. The basaltic rocks are transitional to mildly alkaline, exhibiting OIB-like trace element enrichment patterns, with depletion in Nb and Ta, however, and crustal-like Sr-Nd isotopic signatures, suggesting that they were derived from low degrees (~5%) of partial melting of an enriched lithospheric mantle source. The silicic rocks are transitional to mildly alkaline trachydacites and minor rhyolites, with trace element compositions typical of A2-type granitoids, produced by fractional crystallization of the coeval basalts at the Moho. After the transition to a post-collisional setting, probably at ca. 595 Ma, regional extension led to the Caldera Stage of the basin, which had its volcanic peak at ca. 583-580 Ma, contemporaneous with the intrusive A-type magmatism of the nearby Graciosa Province. The Main Volcanic Activity, corresponding to the volcanic rocks from the Caldera Stage, is constituted by alkaline trachytes and rhyolites, occurring primarily as pyroclastic sequences with minor effusive lava flows, also exhibiting trace element compositions typical of A2-type granitoids. They are associated with minor transitional to mildly alkaline basalts exhibiting IAB-like trace element signatures. The basalts from the Caldera Stage result from the partial melting of ~15% of enriched lithospheric-mantle sources during the lithospheric root collapse of a cratonic terrane, and the silicic rocks are derived from their fractional crystallization at the Moho. However, an additional stage of differentiation in the upper crust is required to explain their silica-enriched compositions. The products from both volcanic activities were raised to the surface mainly through NNW-SSE and ENE-WSW oriented conduits, respectively reactivated and neo-formed during the collisional process. The crustal-scaled discontinuities associated with the development of the sedimentary basin have further controlled the subsidence of the caldera structure, which might be the main mechanism of preservation for these ancient volcano-sedimentary sequences. Lu-Hf isotopes from detrital zircon suggest an Andean arc-type tectonic setting during the Paleoproterozoic (~2,185 Ma) history of the Luis Alves Terrane (LAT) basement. This tectonic setting was responsible for the arc-like signatures of the intraplate lithospheric-derived rocks of both bimodal volcanic sequences. Crustal-like Sr-Nd-Hf isotopic characteristics result from a protracted isotope evolution of their mantle sources, and each tectono-magmatic stage results from a different extensional setting, which has implications for the metacratonization of the LAT. Finally, the CACB consists of a rare and excellent exposure of both caldera-forming pyroclastic rocks and their cogenetic underlaying plutons. Zircon trace element data for the volcanic rocks from all stages of the CACB indicate that the silicic volcanic rocks from the Caldera Stage represent highly evolved compositions extracted from a crystal-mush solidified as a syenitic intrusion (the Corupá pluton). During the evolution of the volcanic-plutonic system, zircon crystals were able to track compositional changes associated with the extraction of residual silicic liquids from crystal mushes and the formation of intermediate-silicic cumulates. Such a connection is revealed by the complementary behavior of Eu/Eu*, Zr/Hf, and Th/U ratios between volcanic and plutonic rocks, and interpreted to result from crystal-melt segregation processes.

Precise in situ zircon U‐Pb dating and Lu–Hf isotopic measurement using an LA‐ICP‐MS system, whole‐rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo‐Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle‐normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE (e.g. Th, U, Pb and La) and depletion of HFSE (e.g. Nb, Ta, Ti, Zr and Hi). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr (0.703457 to 0.708218) and ∊Nd(t) (–2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr–∊Nd(t) diagram. The involvement of crustal‐derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon ∊Hf(t) values, which range from –3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores (2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn‐volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U–Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo‐Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore‐forming elements. The magma and ore‐bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS‐type copper deposit.

The Middle Devonian carbonates of the Slave Point Formation, Hamburg field, northwestern Alberta, are composed mainly of stromatoporoid and Amphipora floatstones and rudstones, with interbedded mudstone and grainstone facies characteristic of deposition in open to slightly restricted marine platform environments. These carbonates have undergone a complex diagenetic history, from shallow to deep burial, as represented by fracturing, calcite cementation, silicification, and dolomitization. Petrographically, four different types of dolomite have been identified (from early to late): (1) fine-crystalline matrix dolomite; (2) pseudomorphic dolomite; (3) medium-crystalline pervasive dolomite; and (4) saddle dolomite. Fine-crystalline dolomite (5– 50 m) replaces the mud matrix and slightly penetrates the edges of allochems. It occurred in mud-supported facies and was precipitated by marine fluids. Oxygen isotope values range from 11.62 to 9.34 (Peedee belemnite), lower than postulated values for Devonian carbonates. The enriched 87Sr/86Sr isotope value from this phase (0.71002) suggests that later diagenetic fluids may have recrystallized this dolomite. Pseudomorphic dolomite (50–100 m) replaces crinoids and occurs as single, large dolomite crystals. Its oxygen and carbon isotopic values range from 10.58 to 9.65 and +4.24 to +4.49, respectively. Medium-crystalline pervasive dolomite (10–100 m) occurs along dissolution seams and obliterates all previous fabrics. It is proposed that this medium-crystalline dolomite formed during shallow to intermediate burial because of its association with dissolution seams and high iron content. The range of oxygen isotope values for this dolomite (11.74 to 9.5) suggests precipitation from a warm fluid, possibly in a burial environment, and/or later recrystallization by hydrothermal fluids. The relatively wide range of carbon isotope values (+1.19 to +4.49) and enriched strontium isotope ratio (0.710020) suggests recrystallization. Saddle dolomite (250–2000 m) partially to completely occludes void spaces (both fractures and vugs) and also occurs as a minor replacement mineral. The oxygen isotope values for saddle dolomite (13.95 to 11.97), as well as the nonradiogenic to enriched strontium isotope ratios for saddle dolomite (0.70494 to 0.710351), and the fluid-inclusion data (homogenization temperature, Th, range between 125 and 161C and estimated salinity, between 22.2 and 24.7 wt.% NaCl equivalent) indicate precipitation from hot, highly saline, hydrothermal fluids, which were probably expelled tectonically during the Late Devonian–Mississippian Antler thrust belt development.

The formation of agate is related to syngenetic or epigenetic magmatic hydrothermal fluids. Trace elements incorporated during the alteration of host rocks caused by hydrothermal magmatic fluids precipitate during their crystallization, reflecting the characteristics of the host rocks. While prior research has yielded extensive trace element data to differentiate agate types found in volcanic rocks, there remains a need for a more efficient method to identify the host volcanic rock of alluvial agate. In this study, a two-stage Random Forest approach was employed to classify the chemical compositions of agate originating from rhyolite, andesite, and basalt, with the aim of facilitating the determination of the host volcanic rock for unknown alluvial agate samples. A dataset comprising 203 agate compositional analyses, sourced from 16 distinct locations, was compiled and labeled for the purpose of training the Random Forest model. The classification results indicate that the developed models exhibit high accuracy (0.9524) and an F1 score of 0.9512, demonstrating their superior performance and efficiency. The feature importance analysis of these models reveals that U, Sb, and Sr contents are particularly crucial for discriminating between different types of volcanic rocks hosting agate. Furthermore, this study introduces a novel discriminant plot utilizing linear discriminant analysis (LDA) to evaluate the host volcanic rock of agate based on trace element data. Verifying the trace element data of agate samples related to basalt based on actual measurements shows that both the Random Forest (with accuracy of 0.813) and LDA plot underscore the effectiveness of using the trace elements found in alluvial agate for the identification of the host volcanic rock.

Silicic volcanic rocks at Hadjer el Khamis, near Lake Chad, are considered to be an extension of the Cameroon volcanic line (CVL) but their petrogenetic association is uncertain. The silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within‐plate granitoids. In situ U/Pb zircon dating yielded a mean 206Pb/238U age of 74.4 ± 1.3 Ma indicating the magmas erupted ∼10 million years before the next oldest CVL rocks (i.e., ∼66 Ma). The Sr isotopes (i.e., ISr = 0.7021–0.7037) show a relatively wide range but the Nd isotopes (i.e., 143Nd/144Ndi = 0.51268–0.51271) are uniform and indicate that the rocks were derived from a moderately depleted mantle source. Thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes. The silicic rocks are more isotopically similar to Late Cretaceous basalts identified within the Late Cretaceous basins (i.e., 143Nd/144Ndi = 0.51245–0.51285) of Chad than the uncontaminated CVL rocks (i.e., 143Nd/144Ndi = 0.51270–0.51300). The age and isotopic compositions suggest the silicic volcanic rocks of the Lake Chad region are related to Late Cretaceous extensional volcanism in the Termit basin. It is unlikely that the silicic volcanic rocks are petrogenetically related to the CVL but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the Central Atlantic Ocean and/or the Pan‐African Orogeny.

Massive sulfide deposits in the Vihanti-Pyhasalmi district, central Finland, are hosted by a highly deformed and metamorphosed bimodal volcanic sequence. Alteration zones that envelop these deposits are composed of cordierite + orthoamphibole ± garnet-bearing rocks (referred to as COR rocks) belonging to upper amphibolite to granulite facies. Outcrops of mafic and felsic COR rocks (MCOR and FCOR) in the vicinity of the Ruostesuo Zn-Cu deposit define mappable layers with distinctive metamorphic mineral assemblages: orthoamphibole + cordierite ± plagioclase ± quartz (M1COR); garnet + orthoamphibole + cordierite ± plagioclase ± quartz (M2COR); quartz + cordierite + biotite ± orthoamphibole ± cummingtonite ± garnet ± plagioclase (F1COR); and quartz + cordierite + garnet + sillimanite (F2COR). These rocks are interlayered with, or grade into, typical amphibolites (i.e., hornblende + plagioclase) and quartz + plagioclase-bearing rocks that are interpreted as least altered mafic and felsic metavolcanic rocks, respectively. Whole-rock geochemical data for least altered metavolcanic rocks and COR rocks at Ruostesuo define three mafic suites (primitive basalts, basaltic andesites, and ferrobasalts) and two felsic suites (R1 and R2 rhyolites). Using a combination of high field strength element ratios and rare earth element profiles, COR rocks can be assigned to all five volcanic suites. Calculated mass changes for the majority of exposed COR rocks at Ruostesuo are similar to those reported for chlorite + quartz assemblages found within the inner core of alteration pipes located below some sea-floor massive sulfide deposits. Relative to least altered precursors, mafic COR rocks have gained Fe, Mg ± S and lost variable amounts of Si, Ca, Na, K, Rb, Ba, Sr, and Eu, which resulted in an average net mass loss of about 10 percent. The bulk composition of most altered mafic COR rocks (near total loss of Ca, Na, K) reflects the proportion of chlorite to quartz within the premetamorphic alteration assemblages and the composition of the chlorite. Mass-change results for altered mafic rocks indicate that basalts at Ruostesuo were altered by an evolved, Fe-rich hydrothermal fluid at high water/rock ratios. Mass changes for COR rocks of the R2 felsic volcanic suite are similar to those of the mafic rocks (enriched in Fe ± Mg and depleted in Ca, Na, K, Rb, Ba, Sr, and Eu) except for significant additions of Si that resulted in net mass gains of 20 to 30 percent. Felsic COR rocks of the R1 volcanic suite show gains in K, Rb, and Ba that are attributed to the formation of premetamorphic sericite during an alteration stage dominated by hydrothermal seawater. In comparison to least altered metavolcanic rocks, mafic and felsic COR rocks are depleted in 18O, with {Delta}18O values 2 to 5 per mil lower than precursor rocks. Estimated temperatures of alteration were 270° to 350°C. These data indicate that the signature of a high-temperature (~300°C) sea-floor hydrothermal upflow zone is preserved within upper amphibolite to granulite facies rocks at Ruostesuo. The stratigraphic boundary between strongly chloritized mafic volcanic rocks and silicified felsic volcanic rocks marks a transition from net mass losses associated with hydrothermal fluid-dominated alteration to net mass gains associated with seawater-dominated alteration. Silicification at or near the boundary between mafic and felsic volcanic rocks may have been the result of conductive cooling or mixing with seawater from above. In the absence of textural or crosscutting relationships indicative of a younging direction, a stratigraphic model is proposed on the basis of the above geochemical constraints. Altered basalts at Ruostesuo represent the exposed base of a chloritic stockwork zone that passes upsection into altered felsic volcanic rocks that host massive Zn-Cu sulfides. This stratigraphic model has significance for the interpretation of other massive sulfide deposits in the Vihanti-Pyhasalmi district and exploration strategies that couple chemostratigraphy with mass-change results. More significantly, the preservation of premetamorphic oxygen isotope values for strongly metamorphosed and locally migmatitic COR rocks indicate that oxygen isotope mapping is a viable exploration tool in the district and in other mineralized upper amphibolite to granulite facies terranes worldwide.

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be re‐denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age: prehnite‐pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite‐pumpellyite to lower greenschist facies for the lower part of the Middle Ordovician volcanic rocks, and lower greenschist facies for the Lower Ordovician volcanic rocks. The P‐T conditions are estimated as T = 240 − 290°C and P = 1.5 − 4.5 kbar for the lower part of the Middle Ordovician rocks, and T = ∼ 300°C for the Lower Ordovician rocks. The variations of mineral assemblages occurring at different domains of the volcanic rocks were controlled by the variations of the effective bulk composition in those domains during metamorphism. The geochemical characteristics of Mg‐Al chromite in the Mayaxueshan volcanic rocks are consistent with an origin of island arc environment.

Three principal modes of formation are apparent for authigenic smectites in Recent marine sediments: alteration of volcanic rocks and glass, low-temperature combination of biogenic silica and Fe-oxyhydroxides, and direct precipitation from hydrothermal fluids. The latter two mechanisms are discussed with reference to new evidence from studies of sediments from the Bauer Deep of the equatorial eastern Pacific and the Atlantis II Deep in the Red Sea. In the Atlantis II Deep sediments, three sub-environments of smectite formation from hydrothermal fluids are recognized. In two of them nontronites are formed, whilst in the third an Fe-poor smectite, intermediate in composition between beidellite and montmorillonite, occurs.

The rare earth element geochemistry of acid-sulphate and acid-sulphate-chloride geothermal systems from Yellowstone National Park, Wyoming, USA