Compositional Variation of Hydrothermally Altered Volcanic Rocks in Hishikari Gold Epithermal System: A Useful Geochemical Indicator of Gold–Silver Epithermal Mineralization

The hydrothermally altered andesite hosting the Hishikari gold-silver vein deposits in southern Kyushu, Japan, is analyzed with respect to the spatial variation in chemical composition. The (CaO + Na₂O) content is found to be inversely correlated with the K₂O content as it progresses away from the site of mineralization. It was found that analytical data plotted on a (CaO + Na₂O) − K₂O diagram cannot be explained only by addition of K⁺ from the hydrothermal solution to the original rock and release of Ca²⁺ and Na⁺ from the original rock (K- alteration). Addition of Ca²⁺ and Na⁺ from the hydrothermal solution to the rock and release of K⁺ from the rock but release of K⁺, Ca²⁺, and Na⁺ to the hydrothermal solution (advanced argillic alteration) is important for causing the wide variations in K₂O, CaO, and Na₂O contents on the (CaO + Na₂O) − K₂O diagram. These variations can be explained by superimposed potassic, advanced argillic and calcium alterations. The altered rocks in the Honko-Sanjin area, Yamada area, and Masaki area analyzed by this study are characterized by their intermediate K₂O content and variable CaO content, high K₂O content and low CaO content, and low K₂O content and low CaO content, respectively. The K₂O, Na₂O and CaO contents and oxygen isotopic composition of altered andesite, in conjunction with the solubility of gold as a thio complex, suggest that both gold deposition and the observed compositional variation of altered andesite are the result of mixing between acidic groundwater and neutral gold-bearing hydrothermal solution. The present results indicate that the compositional variation of hydrothermally altered rocks may represent a useful geochemical indicator of epithermal gold–silver mineralization.     

Signature of potassium enrichment in granite of the Chitrial area, Nalgonda district, Andhra Pradesh: Inferences using its U, Th, K2O, Na2O, Rb, Ba and Sr contents

A medium tonnage unconformity proximal uranium deposit has been established at Chitrial by the Atomic Minerals Directorate in the Srisailam sub-basin. In this type of deposits, the association of uranium with potassic alteration (illitization) is well-documented. The present study is directed towards understanding such an association in the Chitrial area for which the uranium mineralized borehole core samples were collected and analyzed. It is observed that the average concentrations of K₂O, Na₂O, Rb, Ba and Sr in the granite of the Chitrial area are 5.35%, 1.78%, 252 ppm, 564 ppm and 52 ppm, respectively, and they show average critical elemental ratios of K/Rb, Ba/Rb and Rb/Sr as 191, 2.37 and 7.13, respectively. The granites show low K/Rb, low Rb/Ba and high Rb/Sr ratios compared to that of the average crust indicating its derivation from crustal source. The samples have higher values of uranium (av. 53 ppm), thorium (av. 66 ppm) and lead (av. 41 ppm). The U/Th ratio in the granite varies from 0.07 to 20.86 with an average of 1.68. They also exhibit high K₂O/Na₂O ratio typical of post-Archaean granite and very high values suggest the possibility of later potassium enrichment.     

Alteration Patterns Related to Hydrothermal Gold Mineralizaition in Meta-andesites at Dungash Area, Eastern Desert, Egypt

Abstract: The hydrothermal alteration patterns associating with the gold prospect hosted by metavolcanics in the Dungash area, Eastern Desert of Egypt, were investigated in order to assign their relationship to mineralization. The metavolcanics of andesitic composition are generated by regional metamorphism of greenschist facies superimposed by hydrothermal activity. Epidote and chlorite are metamorphic minerals, whereas sericite, carbonates, and chlorite are hydrothermal alteration minerals. The auriferous quartz vein is of NEE‐SWW trend and cuts mainly the andesitic metavolcanics, but sometimes extends to the neighbouring metapyroclastics and metasediments. Quartz‐sericite, sericite, carbonate‐sericite, and chlorite‐sericite constitute four distinctive alteration zones which extend outwards from the mineralized quartz vein. The quartz‐sericite and sericite zones are characterized by high contents of SiO₂, K₂O, Rb, and As, the carbonate‐sericite zone is by high contents of CaO, Au, Cu, Cr, Ni, and Y, and the chlorite‐sericite zone is by high contents of MgO, Na₂O, Zn, Ba, and Co. Gold and sulphide minerals are relatively more abundant in the carbonate‐sericite zone followed by the sericite one. The geochemistry of the alteration system was investigated using volume‐composition and mass balance calculations. The volume factors obtained for the different alteration zones, mentioned above (being 1.64, 1.19, 1.17, and 1.07, respectively), indicate that replacement had taken place with a volume gain. The mass balance calculations revealed addition of SiO₂, K₂O, As, Cu, Rb, Ba, Ni, and Y to the system as a whole and subtraction of Fe2O3 from the system. Initial high aK+ and aH+ for the invading fluids is suggested. As the fluids migrated into wallrocks, they became more concentrated in Mg, Ca, and Na with increasing activities of CO₂ and S. The calculated loss‐gain data are in agreement with the microscopic observations. Breakdown of ferromagnesian minerals and feldspars in the quartz‐sericite, sericite, and chlorite‐sericite zones accompanied by loss in Mg, Fe, Ca, and Na under acidic conditions and low CO2/H2O ratio may obstruct the formation of carbonates and sulphides, and the precipitation of gold in these zones. The role of metamorphic fluids in the area is expected to be restricted to the liberation of Au and some associated elements from their hosts.     

On the relationship between cumulus mineralogy and trace and alkali element chemistry in an Archean granite from the Barberton region,South Africa

A change in the liquidus mineralogy from plagioclase-quartz-biotite to plagioclase-quartz-K feldspar-biotite during the in situ fractional crystallization of a granitic magma has a marked effect on the abundance of and interrelationships between Ba, Rb, Sr, K₂O and Na₂O. During plagioclase fractionation, Ba and Rb enrich in successive solids and melt, while Sr is depleted. The K₂O content of the solid phase (around 1%) is very different from that of the melt (greater than about 3%) while Na₂O contents are similar (about 4–5%) so that variations in the amount of intercumulus melt result in wide variation in the Na₂O/K₂O ratio (from about 5 in cumulus-rich to about 1 in intercumulus-rich samples). The incoming of K feldspar as a cumulus phase causes Ba to be depleted along with Sr in successively formed solids, while Rb continues to be enriched. A pronounced compositional hiatus with respect to trace elements therefore results. The K₂O and Na₂O contents of melt and cumulate are now similar (around 5% K₂O and 4% Na₂O), so that little scatter in Na₂O/K₂O ratio (around 1) occurs as a result of variation in the amount of intercumulus melt. In general, trace element data from a natural example in the Barberton Mountain Land, South Africa, fit the models well, although the observed scatter of the data is somewhat greater than predicted by simple variation in cumulus-intercumulus proportions—possibly the result of the non-uniform distribution of biotite in the samples.     

Carbonatization of oceanic crust by the seafloor hydrothermal activity and its significance as a CO2 sink in the Early Archean

Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO₂ flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO₂ fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity.Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ¹³C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K₂O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K₂O, Rb, and Ba, and are depleted in Na₂O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO₂ enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO₂ in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO₂ ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite.The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 10¹³ mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10^-3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 10¹¹ cm²/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO₂ in the Early Archean.     

安徽沙坪沟斑岩钼矿床赋矿岩体钾质交代作用及其微量元素和Sr-Nd同位素响应

以沙坪沟钼矿主要的赋矿岩石——石英正长岩和花岗斑岩为对象,通过对比不同蚀变强度岩石的岩相学、岩石地球化学和同位素特征,研究该矿床的钾质交代作用-矿化特征,探讨不同热液蚀变的元素组合、蚀变过程中的元素迁移和Sr-Nd同位素的变化及其成因、不同蚀变的物理化学条件差异及其与矿化的关系,进而揭示蚀变-成矿热液流体的特征和起源。研究表明,石英正长岩和花岗斑岩的地球化学特征总体相似,显示其属同源岩浆演化产物,二者均受到钾质蚀变,但蚀变强度相差较大。钾质蚀变岩石的化学成分表现为高K_2O、Rb和低Na_2O、CaO、Sr、Ba,不同蚀变强度的岩石Rb/Sr和Sr同位素组成差别较大,花岗斑岩样品数据更显离散,甚至出现异常低的锶同位素初始值,表明热液蚀变强烈改造了Rb-Sr同位素体系,而Sm-Nd体系基本保持稳定。这一现象在东秦岭-大别钼矿带中典型的斑岩钼矿床也有出现,显示该成矿带具有相似的蚀变类型、热液起源和演化特征。而且钾长石化后期至黄铁云(绢)英岩化阶段也是最主要的钼成矿期,表明这期间流体系统pH值的降低致使Mo元素从流体中沉淀成矿。对比斑岩铜、铜-钼矿床和钼矿床的蚀变特征及其过程中元素和同位素的变化可以发现,这3种矿床均发育碱质交代作用,但蚀变强度、热液的Rb-Sr分异程度及其对原岩的改造程度存在较大差异,这暗示了各自特有的成岩、成矿物质和流体来源及大地构造背景。     

Loss of metals from pelites during regional metamorphism

In aluminous metapelites the ratio H₂O⁺/K₂O decreases with increasing metamorphic grade and degree of reaction. This ratio is a very practical indicator for the progress of the mineral reconstitution during progressive metamorphism. With decreasing values of the ratio H₂O⁺/ K₂O the Cu concentration and the following element ratios also decrease either continuously or in stepwise fashion: Tl/K₂O, Ba/K₂O, Pb/K₂O, Bi/K₂O, Hg/K₂O, Sr/Na₂O, Zn/(Fe²⁺+Mg), Cd/(Fe²⁺+Mg); Rb/K₂O remains approximately constant. In the aluminous metapelites of the Damara Orogen in Namibia the following losses occur between the biotite isograd and anatexis: 61% Cu, 20% Tl, 34% Ba, 59% Pb, 86% Bi, 46% Hg, 30% Sr, 25% Zn, 31% Cd. Thus the potential of regional metamorphism to form hydrothermal deposits in the low grade environment should not be neglected.     

Hydrothermal Alteration and Mineralization of Middle Jurassic Dexing Porphyry Cu-Mo Deposit, Southeast China

The Dexing deposit is located in a NE‐trending magmatic belt along the southeastern margin of the Yangtze Craton. It is the largest porphyry copper deposit in China, consisting of three porphyry copper orebodies of Zhushahong, Tongchang and Fujiawu from northwest to southeast. It contains 1168 Mt of ores with 0.5% Cu and 0.01% Mo. The Dexing deposit is hosted by Middle Jurassic granodiorite porphyries and pelitic schist of Proterozoic age. The Tongchang granodiorite porphyry has a medium K cal‐alkaline series, with medium K₂O content (1.94–2.07 wt%), and low K₂O/(Na₂O + K₂O) (0.33–0.84) ratios. They have high large‐ion lithophile elements, high light rare‐earth elements, and low high‐field‐strength elements. The hydrothermal alteration at Tongchang is divided into four alteration mineral assemblages and related vein systems. They are early K‐feldspar alteration and A vein; transitional (chlorite + illite) alteration and B vein; late phyllic (quartz + muscovite) alteration and D vein; and latest carbonate, sulfate and oxide alteration and hematite veins. Primary fluid inclusions in quartz from phyllic alteration assemblage include liquid‐rich (type 1), vapor‐rich (type 2) and halite‐bearing ones (type 3). These provide trapping pressures of 20–400 ´ 10⁵ Pa of fluids responsible for the formation of D veins. Igneous biotite from least altered granochiorite porphyry and hydrothermal muscovite in mineralized granodiorite porphyry possess δ¹⁸O and δD values of 4.6‰ and ?87‰ for biotite and 7.1–8.9‰, ?71 to ?73‰ for muscovite. Stable isotopic composition of the hydrothermal water suggests a magmatic origin. The carbon and oxygen isotope for hydrothermal calcite are ?4.8 to ?6.2‰ and 6.8–18.8‰, respectively. The δ³⁴S of pyrite in quartz vein ranges from ?0.1 to 3‰, whereas δ³⁴S for chalcopyrite in calcite veins ranges from 4 to 5‰. These are similar to the results of previous studies, and suggest a magmatic origin for sulfur. Results from alteration assemblages and vein system observation, as well as geochemical, fluid inclusion, stable isotope studies indicate that the involvement of hydrothermal fluids exsolved from a crystallizing melt are responsible for the formation of Tongchang porphyry Cu‐Mo orebodies in Dexing porphyry deposit.     

Mineralogy and geochemistry of the hydrothermally altered archaean granodioritic rocks of the ventersdorp dome,Western Transvaal,South Africa

Summary Archaean granodioritic rocks from the Ventersdorp Dome have been hydrothermally altered in two stages: epidotization and K-feldspatization. The alteration locally is both pervasive or occurs along veins. Mass balance calculations reveal CaO, Fe₂O₃, Sr and Y addition and loss of Na₂O,K₂O, MgO, Pb and Ba during epidotization. The combined epidotization and K-feldspatization involved a gain in SiO₂, Fe₂O₃, CaO, K₂O, Ba, Sr and Rb. A1₂O₃ and Nb were immobile during both alteration stages. The alteration is accompanied by a change in the An-content of the plagioclase (An₁₁ to An₁).

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Mineralogie und Geochemie der hydrothermal veränderten, archaischen Granodiorite des Ventersdorp Domes (westliches Transvaal, Südafrika)

Zusammenfassung Archaische Granodiorite des Ventersdorp Domes sind durch zwei Typen hydrothermaler Verdnderung, Epidotisierung und Kalifeldspatisierung überprägt worden. Die Veránderung tritt sowohl durchdringend als auch auf Adern auf. Massenvergleichsberechnungen ergaben, daß während der Epidotisierung CaO, Fe₂O₃, Sr und Y angereichert und Na₂O, K₂O, MgO, Pb und Ba abgeführt worden sind. Während der kombinierten Epidotisierung und Kalifeldspatisierung treten eine Zufuhr von SiO₂, Fe₂O₃, CaO, K₂O, Ba, Sr und Rb auf. A1₂O₃ und Nb verhielten sich während beider Verdnderungen immobil. Die hydrothermale Veränderung wurde von einer Verminderung des Ca-Gehaltes der Plagioklase begleitet (An₁₁ zu An₁).

     

Hydrothermal alteration and mass exchange in the hornblende latite porphyry,Rico, Colorado

The Rico paleothermal anomaly, southwestern Colorado, records the effects of a large hydrothermal system that was active at 4 Ma. This hydrothermal system produced the deep Silver Creek stockwork Mo deposit, which formed above the anomaly's heat source, and shallower base and precious-metal vein and replacement deposits. A 65 Ma hornblende latite porphyry is present as widespread sills throughout the area and provided a homogenous material that recorded the effects of the hydrothermal system up to 8 km from the center. Hydrothermal alteration in the latite can be divided into a proximal facies which consists of two assemblages, quartz-illite-calcite and chlorite-epidote, and a distal facies which consists of a distinct propylitic assemblage. Temperatures were gradational vertically and laterally in the anomaly, and decreased away from the centra heat source. A convective hydrothermal plume, 3 km wide and at least 2 km high, was present above the stock-work molybdenum deposit and consisted of upwelling, high-temperature fluids that produced the proximal alteration facies. Distal facies alteration was produced by shallower cooler fluids. The most important shallow base and precious-metal vein deposits in the Rico district are at or close to the boundary of the thermal plume. Latite within the plume had a large loss of Na₂O, large addition of CaO, and variable SiO₂ exchante. Distal propylitized latite samples lost small amounts of Na₂O and CaO and exchanged minor variable amounts of SiO₂. The edge of the plume is marked by steep Na₂O exchange gradients. Na₂O exchange throughout the paleothermal anomaly was controlled by the reaction of the albite components in primary plagioclase and alkali feldspars. Initial feldspar alteration in the distal facies was dominated by reaction of the plagioclase, and the initial molar ratio of reactants (alkali feldspar albite component to plagioclase albite component) was 0.35. This ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0.92 as the reaction progressed. Much of the alkali feldspar albite component in the proximal facies reacted while the, primary plagioclase was still unreacted, but the ratio for these assemblages increased to 1.51 when the plagioclase entered the reaction paragenesis. Plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of Na₂O. CaO is lost in the distal facies as hornblende reacts to chlorite, although some calcium may be fixed in calcite. CaO is added to the proximal facies as the quantity of chlorite replacing hornblende increases and epidote and calcite are produced.     

The geochemistry of wall rock alteration in turbidite-hosted gold vein deposits, central Victoria, Australia

Gold deposits hosted in Ordovician to Devonian turbidites in central Victoria, Australia, consist of steeply dipping quartz vein systems hosted mainly by reverse faults. Wall rock alteration of the host metasandstones, metasiltstones and shales (or slates) extends at least 20 m from the veins in the Bendigo-Ballarat zone (BBZ) and 10 m in the Melbourne zone (MZ) deposits. Alteration minerals include carbonates (ankerite, siderite and dolomite) chlorite, seriate, arsenopyrite, pyrite, chalcopyrite and sphalerite, with less common barite, albite and biotite in the BBZ and carbonates (siderite, ankerite, dolomite and ferromagnesite), sericite, chlorite, arsenopyrite, pyrite, and stibnite with less common chalcostibite in the MZ. SiO₂, Na₂O, MgO and Sr with P₂O₅ commonly decreasing during alteration while CO₂, S, As, Sb, Au, V, Al₂O₃, Ga, K₂O and Rb with Ni and Cr commonly increase. TiO₂, FeO, Fe₂O₃, MnO, Th, U, Nb, La, Ce, and Sc showed little change. Concentrations of Zn, Cu, Pb, and Ca are variable.The relatively large decrease of SiO₂ could account for most, if not all, quartz present in the ore veins. The Na₂O and MgO could have crystallized in the veins in the very minor albite and chlorite present. The addition of V in all and Ni and Cr in most deposits probably reflects a source enriched in these elements such as underlying greenstones. The source of both the volatile (S, As, Sb) and lithophile (K₂O and Rb) elements as well as Au is unknown, but they could have been derived from a magmatic source or from the metamorphism of Cambrian greenstones. CO₂, present as carbonate, was derived mainly by the reaction of graphite, originally present in the sediments, with the ore solutions. Al₂O₃, the only other major element after SiO₂, probably increased mainly due to the decrease of the latter.     

Rubidium,strontium content and strontium isotopic composition of strongly alkalic basaltic rocks from the Cape Verde islands

The Cape Verde islands are characterized by the presence of very strongly alkalic lavas. Cenozoic volcanics—covering the broadest compositional range present in the archipelago—and ranging from alkali-basalts to phonolites, associated with plutonic essexites and nepheline syenites, were analyzed for Sr isotopic compositions and concentrations in K, Rb and Sr. The close values of the Sr⁸⁷/Sr⁸⁶ ratios (ranging from 0.7029 to 0.7033) indicate a comagmatic origin for the different rock types; no correlation appears between the Sr isotopic composition and the K-content of the lavas, thus indicating that the lavas with high K₂O/K₂O + Na₂O ratio are generated from a primary magma by differentiation at shallow depths. The values of the Sr isotopic composition are distinctly lower than most values obtained for lavas of other oceanic islands. The origin of the magma type is discussed on the basis of these isotopic compositions and the K/Rb and Rb/Sr ratios: it is suggested that the primary magma has a nephelinitic composition and was formed by partial melting of a small fraction of undepleted mantle peridotite, containing phlogopite; the deeper part of the mantle where this nephelinitic magma generates would have a strontium isotopic ratio of about 0.703 and a Rb/Sr ratio lower than that of the upper part.     

Application of lithogeochemistry to exploration for deep VMS deposits in high grade metamorphic rocks, Snow Lake, Manitoba

Volcanic-associated massive sulphide deposits in the Snow Lake area of Manitoba are related to mineralogically and chemically distinct alteration zones. It is generally accepted that these zones represent crosscutting, subconformable or conformable synvolcanic alteration zones, which were coeval with and have been metamorphosed with the massive sulphides. Metamorphism ranges from upper greenschist facies to middle amphibolite facies. Surface lithogeochemical anomalies led to the discovery of small massive sulphide lenses at a vertical depth of 250 m in the Raindrop Lake area, southwest of Snow Lake, Manitoba. Variations in mineral assemblages of middle amphibolite facies alteration zones and analysis of variations in major and trace element chemistry were used to guide deep drilling at Raindrop Lake. The massive sulphide lenses are stratigraphically underlain by a low angle crosscutting “pipe” and a conformable footwall “apron” alteration.The alteration zones are composed of assemblages of garnet, staurolite and chlorite, and, less significantly, biotite, muscovite and kyanite. They are characterized by loss of Na and Ca, and addition of Fe, Mg, Cu and Zn. Mapping the alteration is aided by the application of the metamorphic AFM phase diagram for the appropriate metamorphic facies. Increasing intensity of alteration can be identified by the first appearance of new mineral phases, which are represented on the AFM diagram. These mineral trends coincide with loss of Na and Ca relative to Al, and increased Mg and Fe. Chemical alteration indices ACNK (molecular proportion Al₂O₃/(CaO + Na₂O + K₂O) and AI = 100 × [(MgO + K₂O)/(MgO + K₂O + CaO + Na₂O)] combined with Cu and Zn variation helped to quantify the intensity of alteration, despite being insensitive to Fe.The crosscutting pipe is dominantly Fe enriched, with a Cu-enriched core, Zn enriched margins and widespread Na and Ca depletion. Mineralogically it is identified by garnet, staurolite and chlorite and follows an iron and aluminum enrichment trend on the AFM diagram. The conformable alteration zone is characterized by local strong Mg enrichment, extensive Na and Ca depletion and variable Cu and Zn. Mineralogically it is characterized by the presence of chlorite and kyanite and follows a magnesium and aluminum enrichment trend on the AFM diagram.     

Lithogeochemistry in exploration for Proterozoic porphyry-type molybdenum and copper deposits, southern Finland

Over 450 samples were collected around four Proterozoic porphyry-type occurrences and analyzed for 30 elements. Relatively broad primary halos are associated with all four occurrences in spite of their moderate size and grade. The halos are characteristic of each particular occurrence depending on the geological and geochemical conditions. Copper forms extensive and highly contrasted anomalies, whereas Mo anomalies are of limited size. Gold, Zn, Pb, As, and Sb seem to be useful pathfinders for slightly eroded or blind occurrences. Pyritization and wall-rock alteration are weaker than in Phanerozoic porphyry deposits, consequently, S halos are small and often erratic. K₂O, Rb, and SiO₂ produce weak anomalies near the core of the occurrences. A density of a few tens of samples per km² is required for detailed follow-up exploration.The host granitoids have normal chemical compositions outside the mineralized areas with low and constant background contents of ore elements and are thus very suitable for lithogeochemical exploration. On a regional scale a density of one to five samples per km² is sufficient to identify mineralized intrusive phases and areas, because the porphyrytype occurrences were formed by extensive hydrothermal processes, which produced scattered anomalies over large areas.     

Geochemical characteristics of Tukureswari and Barbhita granitoid in Goalpara district,Assam

Late intrusive Tukureswari granitoids (TKG) and the Barbhita granitoids (BBG) of Goalpara district in western Assam constitute an important component of the continental crust of the Shillong Plateau. Thus, the geochemical study of these two granitoids involving their origin, classification and petrogenetic significance would be a contribution towards a better understanding of the evolution of continental crust of the Shillong Plateau.The major oxide and trace element geochemistry reveals several genetic issues on these two granitoids. The I-type affinity of the TKG is indicated from the geochemical features such as high TiO₂, P₂O₅ and K₂O contents, low normative corundum (< 1%), high Na₂O/K₂O ratios, and low concentrations of Ni, Co and Cr. Further, enriched LREE-LILE and HFSE depletion, as well as the normal calc-alkaline nature of arc affinity (e.g., enhanced LILE abundance and low HFSE/LILE ratios) of the TKG indicate subduction-related magmatism. TheTKG are also categorized as a deep-level pluton, being enriched in LREE and depleted in total REE and HFSE (Y, Nb, Ta, Zr, Hf). The high La/Nb ratio (1.9–8.6), negative Nb and Ti anomalies also suggest orogenic related magmatism.On the other hand, the geochemistry of the BBG reveal a high Niggli Si and Mg values, slightly high normative corundum values (2.16–3.41), high Th/Ta, Y/Nb, La/Nb, K₂O/Na₂O, and Rb/Sr ratios. It also shows ASI, K, Rb, and U contents, prominent depletion of Nb, Sr and Ti on the primitive mantle-normalized multi-element spider diagrams and a low concentrations of Cu, Cr, V and Na₂O (> 3.2%). All these geochemical characteristics provide strong evidences in support of a sedimentary parentage for Barbhita granitoids (BBG) and are dominantly of S-type.     

Ocean floor metamorphism in the Betts Cove ophiolite,Newfoundland

The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward and was reduced with depth. The mobilities of major and trace elements support the hypothesis of the interaction of seawater and basalt: Fe₂O₃, MgO, Na₂O and H₂O increase whereas CaO and Cu decrease in the rock after alteration; SiO₂, total iron, K₂O, Ba and Rb can either be depleted or enhanced in the altered material; TiO₂, P₂O₅, Zr, Y, Cr and Ni remain stable during the metamorphic episode. Finally, the occurrence of massive sulphides and incipient rodingitic gabbro is explicable in a circulatory seawater system.     

Fracture-related hydrothermal alteration of metagranitic rock and associated changes in mineralogy, geochemistry and degree of oxidation: a case study at Forsmark, central Sweden

Red-staining of rocks due to fluid–rock interaction during hydrothermal circulation in fractures is a common feature in crystalline sequences. In this study, red-stained metagranitic rock adjacent to fractures in Forsmark, central Sweden, has been studied with emphasis on the mineral reactions and associated element mobility occurring during the alteration. The main mineral reactions associated with the hydrothermal alteration are an almost complete saussuritization of plagioclase accompanied by total chloritization of biotite. Magnetite has been partly replaced by hematite whereas quartz and K-feldspar were relatively unaffected by the hydrothermal alteration. We show that redistribution of elements on the whole rock scale was very limited and is mainly manifested by enrichment of Na₂O and volatiles and depletion of CaO, FeO and SiO₂ in the red-stained rock. However, on the microscale, element redistribution was more extensive, with both intragranular and intergranular migration of e.g. Ca, K, Na, Al, Si, Fe, Ba, Cs, Rb, Sr, Ti and REEs. The altered rock shows a shift towards higher total oxidation factors, but the change is smaller than 1σ and the red-staining of the rock is due to hematite dissemination rather than a significant oxidation of the rock. An increase in the connected porosity is also observed in the altered rock.     

Occurrence of paragonite in the hydrothermalwall rock alteration zone of G. R. Halli gold deposit, Chitradurga Schist Belt, Western Dharwar Craton, Southern India

Paragonite (NaAl₂Si₃AlO₁₀(OH)₂), a dioctahedral sodium mica, occurs as a minor phase in the wall rock alteration assemblage of the Guddadarangavanahalli (G. R. Halli) gold deposit of Western Dharwar Craton, Karnataka. It occurs as colourless to pale yellowish grains and scaly aggregates in chlorite, ankerite and quartz rich matrix of the altered metabasalt. Its presence is noticed in the distal to near proximal wall rock alteration zone, mainly in the southern part of the G. R. Halli gold deposit. Presence of paragonite is identified by X-Ray Diffraction (XRD) and Electron Probe Micro-Analyzer (EPMA) studies. The Na₂O content of the paragonite varies between 5.96 wt% and 7.8 wt% whereas K₂O ranges from 0.44–2.68 wt%. Al₂O₃ is measured between 37.73 wt% and 39.72 wt% whereas SiO₂ varies from 45.06–47.19 wt%. The relative proportion of Na and K in paragonite varies between Na₉₆K₄ and Na₇₇K₂₃. The occurrence of paragonite in the wall rock alteration halo of the G. R. Halli gold deposit is proposed as a result of hydrolysis of metamorphosed plagioclase feldspars during the interaction of the hydrothermal fluids and the wall rock.     

Les caractères magmatiques du volcanisme plio-quaternaire des Andes dans le Sud du Pérou

Chemical data on late cenozoic lavas (83 new analyses) from southern of Peru indicate a zonal arrangement of lavas types according to their distance from the trench. The nearest belong to a calc-alkaline suite and the farthest are shoshonites rocks. The calc-alkaline rocks show progressive increase in K₂O content and in K₂O/Na₂O ratio in northeastern direction. The shoshonitic rocks are more potassic and titaniferous, and the K₂O/Na₂O ratio is approximatively constant. The potash content of the two series correlate with the depth of the inclined seismic zone beneath the volcanoes, but this correlation is less well established than in island arcs.     

The use of sorting curves in studying K2O alteration in interbedded greywacke and argillite

The chemical composition of unaltered greywacke-argillite sequences varies both abruptly and gradationally across short distances of a few centimeters to meters. These original variations are sufficient to mask changes in K₂O (and other oxides) resulting from hydrothermal alteration. Sorting curves for unaltered samples present K₂O contents in terms of two relatively immobile oxides SiO₂ and Al₂O₃. This allows an estimated of the original pre-alteration K₂O content of altered samples, from which gains and losses can then be calculated.