Geochemical and Environmental Evaluation of Um Shashoba Gold Mine Area,Southeastern Desert,Egypt

A suite of elements(Ag,Au,Ba,Bi,Cd,Co,Cr,Cu,Ga,Hf,Hg,Mn,Mo,Ni,Pb,Rb,Sb,Se, Sr,Te and Zn),total organic carbon(TOC)and pH were analyzed in stream sediment and tailing samples from Um Shashoba area,in order to evaluate Au placer and the sediments being impacted by old mining activities.Analytical results were examined using statistical,graphical and mapping methods.In spite of the results revealing that Au and most of the elements in sediments were in general significantly lower than those in tailing,Au wa...     

Identification of geochemical anomalies by using of number–size (N–S) fractal model in Bardaskan area, NE Iran

Geochemical anomaly separation and identification using the number–size (N–S) model at Bardaskan area, NE Iran is studied in this paper. Lithogeochemical data were used in this study which was conducted for the exploration for Au and Cu mineralization and enrichments in Bardaskan area. There are two major mineralization phases concluded epithermal gold and a disseminated systems. N–S log–log plots for Cu, Au, Sb, and As illustrated multifractal natures. Several anomalies at local scale were identified for Au (32 ppb), Cu (28 ppm), As (11 ppm), and Sb (0.8 ppm) and the obtained results suggest existence of local Au and Cu anomalies whose magnitudes generally are above 158 and 354 ppm, respectively. The most important mineralization events are responsible for presence of Au and Cu at grades above 1,778 and 8,912 ppm. The study reveals threshold values for Au and Cu are a consequence of the occurrence of anomalous accumulations of phyllic and silicification alteration zones and metamorphic rocks especially in tuffaceous sandstones and sericite schist types. The obtained results were correlated with fault distribution patterns, revealing a positive direct correlation between mineralization in anomalous areas and the faults present in the mineralized system.     

Mineralogy,composition and heavy metals’ concentration,distribution and source identification of surface sediments from the saline Maharlou Lake (Fars Province,Iran)

This study concerns the mineralogy, spatial distribution and sources of nine heavy metals in surface sediments of the Maharlou saline lake, close to the Shiraz metropolis in southern Iran. The sources for these sediments were studied by comparing the mineralogy and the distribution of heavy metals, using multivariate statistical analysis (correlation analysis and principal component analysis). The geochemical indices, including geo-accumulation index (Igeo), contamination factor (CF) and pollution load index (PLI), were used to assess the degree of heavy metal contamination in surface sediments. Sediment quality guidelines (SQGs) have also been applied to assess its toxicity. The XRD analysis shows that the main minerals of the surface sediments are aragonite, calcite, halite and quartz, with small amounts of montmorillonite, dolomite and sepiolite. The total heavy metal contents in surface sediments decrease in order of Sr?>?Ni?>?Cr?>?Zn?>?Cu?>?Co?>?Pb?>?As >?Cd and the average concentrations of Sr, Ni and As exceeded more than 10, 5 and 3 times, respectively, by comparing with the normalized upper continental crust (UCC) values. The results of pollution indices (Igeo, CF and PLI) revealed that strontium (Sr), nickel (Ni) and arsenic (As) were significantly enriched in those sediments. Based on the sediment quality guidelines (SQGs), Ni would infrequently cause toxicity. Multivariate statistical analysis indicated that the Ni, Co and Cr came mainly from natural geological background sources, while Cd, Cu, Pb, and Zn were derived from urban effluents (especially traffic emissions) and As originated from agriculture activities. Significant relationships of Sr with S, CaO and MgO in sediments suggest that Sr was derived from carbonate- and gypsum-bearing catchment source host rocks.     

“勉略宁”区金矿分带研究

勉略宁区（即略阳-勉县-宁强地区）金矿分布在北、中、南三带：北带钍铁-金-镍（钴）带;中带金-多金属矿带;南带金-铜与砂金-砂铂矿带.垂直分带：上部铁锰矿与铀矿带,分布在沉积岩和火山沉积岩中;中部铅锌-金矿带,下部金-铜矿带,分布在火山岩和火山碎屑岩中."勉略宁"区对金成矿有利,找矿前景乐观.     

Geochemical exploration for gold in the Nile Valley Block (A) area,Wadi Allaqi,South Egypt

The present work aims to estimate Au-anomalous distribution patterns, the optimum grain size fraction and pathfinder elements for gold placer. The obtained data of analyzed elements (Au, Ag, As, Ba, Cd, Co, Cr, Cu, Fe, Ga, Hg, Li, Mn. Mo, Ni, Pb, Rb, Sb, Se, Sr, Te, Ti, V, Y and Zn) in both the fine (?0.25 mm) and coarse (?1 mm+0.25 mm) grain size fractions of 32 dry stream sediment samples, which have been derived from metamorphosed Island Arc volcanic rocks of Late Proterozoic age, revealed that most of the analyzed elements in both grain size fractions are asymmetrically distributed and did not pass the tests of normality. The coarse grain size fraction appears to be the better size fraction for Au detection and can be considered as the optimum grain size fraction for future application in regional stream sediment surveys. The presence of Au anomalies in the upper part of the stream and beside the mine is either sourced by the main Au-mineralization or new potential extension of the mineralization in the study area. Silver, Cd, Se, Zn and Te can be considered as useful pathfinder elements for Au in the coarse grain size fraction, and they may be used for future geochemical exploration for Au in the area. Recommendation of the authors is pointing to perform lithogeochemical survey in the eastern and western parts of the mine.     

Distribution and enrichment of trace metals and arsenic at the upper layer of sediments from Lerma River in La Piedad,Mexico: case history

Lerma River is one of the largest rivers in Mexico. Over the past 20 years, unplanned population growth occurred along its course and the river has been used as the only outlet for industrial and domestic wastewater disposal. The aim of the present study was to determine trace metals such as Cr, Ni, Cu, Zn, Fe, Pb, and arsenic concentrations at the upper layer of sediments of the Lerma River meander in La Piedad, Michoacan, Mexico. Sediment samples were collected from eight different sites during the rainy and dry seasons. All samples were physically characterized, and concentration values of trace metals and As were determined. On the basis of protection criteria for freshwater sediments, concentrations of Fe, Zn, Cu, Ni, and Pb were found to exceed the lowest effect level; moreover, the concentrations were found to exceed the severe effect level at some sites, particularly for Cu. Statistical analyses showed significant differences between sampling seasons for Fe and As, and among sites for Ni, Cu, Zn, and Pb. In addition, the enrichment factor indicates the following order Zn > Cr > Cu > Ni > Pb > As, and the geoaccumulation index (I _geo) indicates contamination in the following order Zn > Cr > Cu > Ni > As > Pb. The Lerma River meander in La Piedad shows a reduction in pollution by trace metals and arsenic near the drain area and downstream of the meander. However, there are significantly higher concentrations of these elements in sediments of sites located in the middle part of the city.     

Indicative features of placer gold for the prediction of the formation types of gold deposits (east of the Siberian Platform)

Prediction and search for gold deposits in the east of the Siberian Platform are problematic because the study area is overlain by a thick cover of MZ-KZ deposits. Search for gold deposits by the largest geological institutions using conventional methods have not yielded positive results, because the main attention was focused on the discovery of ancient gold-bearing conglomerates of the Witwatersrand type and on the evaluation of the gold ore potential of basic magmatism. Typomorphism of placer gold bears huge information about the genesis of native gold, both its primary endogenous origin and its exogenous transformation, which makes it possible to identify the formation type of mineralization, increases the reliability of the prediction of gold deposits, and ensures their purposeful search in the platform areas. The revealed indicative features of placer gold made it possible to substantiate the formation of the gold ore sources of Precambrian low-sulfide gold-quartz, gold-iron-quartzite, porphyry gold-copper, and gold-PGE mineralization and Mesozoic gold-silver, gold-rare-metal, and gold-sulfide-quartz mineralization in the east of the Siberian Platform. We have established that high-fineness placer gold with microinclusions of pyrite, arsenopyrite, quartz, and carbonates with recrystallized structures and lines of plastic deformation is specific to the ore sources of low-sulfide gold-quartz mineralization. A high content of Cu (up to 4%) in flaky high-fineness gold is one of indicators of porphyry gold-copper mineralization. The angular shape of gold grains, the fine fraction and high fineness of gold, its completely recrystallized and regrown internal structure, and the permanent presence of Fe, Bi, and Cu microimpurities and hematite, ilmenite, and corundum microinclusions are typical of gold-iron-quartzite mineralization. Flaky and laminated high-fineness gold particles with steady Pt, Pd, and Ni impurities and Pt-mineral phases and Au-Pt intergrowths in them testify to the ore sources of gold-PGE mineralization. Laminated and cloddy gold fractions of > 0.25-2.0 mm, the medium and low fineness of gold, its single-crystal or, sometimes, porous internal structure, the wide range of microimpurities (Pb, Zn, As, Sb, Cu, Te, etc.), and microinclusions of native Ag, adularia, Sr-barite, and calcite are indicators of gold-silver mineralization. Laminated, dendritic, and cloddy-angular gold grains, wide variation in gold fineness (307-950‰), and the presence of microinclusions of native bismuth, maldonite, arsenopyrite, and silver tellurides are indicative of gold-rare-metal mineralization. Laminated and cloddy gold grains, their size varying from dust to > 0.25 mm, their mono- and coarse-grained internal structure, wide variation in gold fineness (600-900‰), and the presence of Hg microimpurities (up to 6% and more) and microinclusions of quartz, calcite, pyrite, arsenopyrite, tellurides, selenides, and REE phosphates point to gold-sulfide-quartz mineralization. The established placer gold indicators of the particular formation types of ore sources in the east of the Siberian Platform made it possible to predict Precambrian gold deposits with low-sulfide-gold-quartz mineralization similar to the Kirkland Lake and Porcupine mines and gold deposits with Mesozoic gold-silver mineralization similar to the Cripple Creek mine. The developed criteria for determining the types of mineralization by indicative features of placer gold give an insight into the ore genesis and can be successfully applied to prediction and search for gold deposits and to evaluation of their gold resources.     

Distribution and speciation of heavy metals in surface sediments from the Yangtze estuary and coastal areas

The concentrations and speciation of heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) in the sediments of the nearshore area, river channel and coastal zones of the Yangtze estuary, China, were systematically investigated in this study. The concentrations of all heavy metals except Ni in the sediments of the nearshore area were higher than those of the river channel and coastal zones. In the nearshore area, the concentrations of most heavy metals except Hg in the sediments of the southern branch were higher than those of the northern branch because of the import of pollutants from the urban and industrial activities around. When compared with the threshold effect level (TEL) and geochemical background levels, Cr, Ni and As accumulated and posed potential adverse biological effects. The speciation analysis suggested that Cd, Pb and Zn in the sediments of the three zones showed higher bioavailability than the other heavy metals, and thus posed ecological risk. Significant correlations were observed among Cr, Cu, Ni and Zn (r > 0.77) in the nearshore area, Ni, Cu, Zn and Pb (r > 0.85) in the river channel and Ni, Cu, Cr, Pb and Zn (r > 0.75) in the coastal zone. Principal component analysis (PCA) indicated that the discharge of unban and industrial sewage, shipping pollution and the properties of the sediments (contents of Fe, Mn, Al, TOC, clay and silt) dominated the distribution of heavy metal in the nearshore area, river channel and coastal zones of the Yangtze estuary.     

Regional geochemical baseline concentration of potentially toxic trace metals in the mineralized Lom Basin,East Cameroon: a tool for contamination assessment

The distribution of trace metals in active stream sediments from the mineralized Lom Basin has been evaluated. Fifty-five bottom sediments were collected and the mineralogical composition of six pulverized samples determined by XRD. The fine fraction (<?150 µm) was subjected to total digestion (HClO₄?+?HF?+?HCl) and analyzed for trace metals using a combination of ICP-MS and AAS analytical methods. Results show that the mineralogy of stream sediments is dominated by quartz (39–86%), phyllosilicates (0–45%) and feldspars (0–27%). Mean concentrations of the analyzed metals are low (e.g. As?=?99.40 µg/kg, Zn?=?573.24 µg/kg, V?=?963.14 µg/kg and Cr?=?763.93 µg/kg). Iron and Mn have significant average concentrations of 28.325 and 442 mg/kg, respectively. Background and threshold values of the trace metals were computed statistically to determine geochemical anomalies of geologic or anthropogenic origin, particularly mining activity. Factor analysis, applied on normalized data, identified three associations: Ni–Cr–V–Co–As–Se–pH, Cu–Zn–Hg–Pb–Cd–Sc and Fe–Mn. The first association is controlled by source geology and the neutral pH, the second by sulphide mineralization and the last by chemical weathering of ferromagnesian minerals. Spatial analysis reveals similar distribution trends for Co–Cr–V–Ni and Cu–Zn–Pb–Sc reflecting the lithology and sulphide mineralization in the basin. Relatively high levels of As were concordant with reported gold occurrences in the area while Fe and Mn distribution are consistent with their source from the Fe-bearing metamorphic rocks. These findings provide baseline geochemical values for common and parallel geological domains in the eastern region of Cameroon. Although this study shows that the stream sediments are not polluted, the evaluation of metal composition in environmental samples from abandoned and active mine sites for comparison and environmental health risk assessment is highly recommended.     

Study of late-Mesozoic magmatic rocks and their related copper-gold-polymetallic deposits in the Guichi ore-cluster district,Lower Yangtze River Metallogenic Belt,East China

ABSTRACT

The Guichi ore-cluster district in the Lower Yangtze River Metallogenic Belt hosts extensive Cu–Au–Mo polymetallic deposits including the Tongshan Cu–Mo, Paodaoling Au, Matou Cu–Mo, Anzishan Cu–Mo, Guilinzheng Mo and Zhaceqiao Au deposits, mostly associated with the late Mesozoic magmatic rocks, which has been drawn to attention of study and exploration. However, the metallogenic relationship between magmatic rocks and the Cu–Au-polymetallic deposits is not well constrained. In this study, we report new zircon U–Pb ages, Hf isotopic, and geochemical data for the ore-bearing intrusions of Guichi region. LA-ICP-MS U–Pb ages for the Anzishan quartz diorite porphyrite is 143.9 ± 1.0 Ma. Integrated with previous geochronological data, these late Mesozoic magmatic rocks can be subdivided into two stages of magmatic activities. The first stage (150–132 Ma) is characterized by high-K calc-alkaline intrusions closely associated with Cu–Au polymetallic ore deposits. Whereas, the second stage (130–125 Ma) produced granites and syenites and is mainly characterized by shoshonite series that are related to Mo–Cu mineralization. The first stage of magmatic rocks is considered to be formed by partial melting of subducted Palaeo-Pacific Plate, assimilated with Yangtze lower crust and remelting Meso-Neoproterozoic crust/sediments. The second stage of magmatism is originated from partial melting of Mesoproterozoic-Neoproterozoic crust, mixed with juvenile crustal materials. The depression cross to the uplift zone of the Jiangnan Ancient Continent forms a gradual transition relation, and the hydrothermal mineralization composite with two stages have certain characteristics along the regional fault (Gaotan Fault). Guichi region results from two episodes of magmatism probably related to tectonic transition from subduction of Palaeo-Pacific Plate to back-arc extensional setting between 150 and 125 Ma, which lead to the Mesozoic large-scale polymetallic mineralization events in southeast China.     

Geochemical anomaly definition using multifractal modeling,validated by geological field observations: Siah Jangal area,SE Iran

Recognition of geochemical anomalies is a pivotal assignment in exploration projects. This study aims to delineate different AuCu geochemical anomalies using number-size (N-S) and concentration-area (C-A) multifractal models in the Siah Jangal area, SE Iran. In this research, lithogeochemical datasets were applied for the exploration of Au and Cu. A comparison between geochemical anomaly maps based on the N-S and C-A fractal models shows the N-S fractal modeling is a powerful tool for separation of weak elemental geochemical anomalies in all of sampling zones. Based on a comparison between the results of these two methods and field studies, the geochemical anomaly zones, defined by the N-S fractal model, are more accurate than those recognized by the C-A fractal model. The obtained results of the N-S and C-A fractal models have been interpreted with the extensive set of information including structural interpretation, geological and alteration data. Au and Cu mineralization in the Siah Jangal area are hosted mainly by Oligocene-Miocene sub-volcanic rocks, especially strongly altered porphyric quartz diorite, hornblende diorite and diorite. Moreover, the positive dependence between various alteration zones and high concentrations of Au and Cu proves that strongly anomalous areas are correlated with these alteration zones. High grade Au (> 1000 ppb) and Cu (>150 ppm) are associated with the altered sub-volcanic rocks in the northern, eastern, and SW parts of the study area. Therewith, the strong anomaly populations are mostly occurred within the fault and fracture systems in the study area. This is a promising signal because quartz-sulfide veins and veinlets are associated with such structures.     

Geochemistry and genesis of sulphide ore deposits in Sharosh Village,Qandil Series,Kurdistan Region,NE Iraq

The low-grade base metal sulphide Cu–Zn–Pb and Fe mineralization of Qandil Series develop in shear zones that occur in formations of the north-western part of the Zagros Orogen. This sulphide mineralization occurs either as quartz vein type or disseminated type associated with metamorphic rocks (marbles and phyllites). This study aims to characterize these sulphide-rich ores by means of their mineralogical and geochemical features, including also the features of the corresponded host formations and those of marbles (calcitic and dolomitic) and phyllites. Petrographical data indicate the presence of Cu, Zn, Pb and Fe sulphides in hydrothermal quartz (±calcite) veins of different generations. Geochemical data of surface samples indicate enrichment of Cu and Fe in shear zones with low concentrations in Zn and Pb. The REE data indicate that the genesis of these sulphide ores took place in a hydrothermal system and was generally attributed to high temperature (> 250 °C).The mineralization seems to be fault-controlled, which is favoured by the significant tectonic deformation of the area.     

坦桑尼亚姆贝亚Sangambi地区土壤地球化学特征

坦桑尼亚的Sangambi地区位于姆贝亚的东北部、卢帕金矿田的东部。通过1∶10 000土壤地球化学测量,圈定了6个Au,Cu,Cr,Ni元素的组合异常,其中ZH-1和ZH-6组合异常与金、铜的矿化关系密切,而ZH-3和ZH-4组合异常与铬、镍(铜)矿化有一定关系。在片麻状花岗岩、细晶闪长岩和花岗闪长岩中发育大量NW向的韧性剪切带和断裂构造,其中的石英脉中经常见有黄铁矿、黄铜矿和金的矿化,刻槽取样最高品位w(Au)=11.5×10-6。土壤测量的结果证实,在卢帕金矿田的东部依然存在金成矿的潜力。     

Spatial geochemical variation of major and trace elements in the marine sediments of Mumbai Harbor Bay

The resulting concentration data sets of major (Na, K, Mg, Ca and Fe) and trace elements (Cu, Ni, Co and Mn) in bed and suspended sediments were used to evaluate the enrichment factor for anthropogenic influences and principal component analysis for identifying the origin of source contributions in the studied area. Normalization of metals to Fe indicated that high enrichment factors in the bed sediment were in the order of Co > Cu > Na > Ca > Ni except Mg, K and Mn while for suspended sediments, only Co has a high enrichment factor. High enrichment of Co and Cu reflected the contamination of sediments from anthropogenic sources. The high influence of Na and Ca in sediments may be caused for seawater salinization factor. A significant positive correlation among enrichment factors of various elements of interest suggests a common origin/identical behavior during transport in the sediment system.     

五台山—恒山绿岩带Au-Ag-Cu成矿作用地球化学特征

五台山—恒山绿岩带Ａｕ、Ａｇ、Ｃｕ矿床可分为二大类型：（１）再生型金银铜矿，产在包括岩浆岩在内的各类岩石断裂构造中，与岩浆期后热液有关；（２）变生型金银铜矿，产于各类变质岩中，具有层控特征（即绿岩型金矿）。在地球化学特征上，再生型矿床与变生型矿床相比，矿体及围岩中Ｍｏ、Ａｇ、Ｐｂ、Ｚｎ、Ｃｄ等成矿及伴生元素明显富集；Ｋ２Ｏ、Ｒｂ、Ｓｒ、Ｂａ、Ｔｈ、Ｕ也明显富集，是后期岩浆热液作用的结果；Ｈｇ、Ｆ的明显富集则与后期构造活动有关；Ｚｎ／Ｃｄ比值较低，说明受到后期岩浆侵入影响；Ｔｈ／Ｕ比值低，可能指示富钙的酸性岩环境。再生型Ａｕ矿化的元素组合为Ｃｄ、Ａｓ、Ｎｉ、Ａｇ、Ｓｂ、Ａｕ、Ｈｇ（Ｂｉ），再生型Ａｇ矿化的元素组合为Ａｓ、Ｓｂ、Ａｇ、Ｃｄ、Ｃｕ、Ｎｉ（Ｍｏ、Ｐｂ、Ｚｎ、Ｂｉ），变生型Ａｕ矿化的元素组合较简单，只为Ａｕ、Ｈｇ、Ａｓ或Ａｕ、Ｃｕ。上述地球化学特征不仅可以有效地区分矿化类型，而且可以作为地球化学找矿和评价的指标     

Distribution of noble metals in ore mineral assemblages of the Volkovsky gabbroic pluton, central Urals

Relationships between noble-metal and oxide-sulfide mineralization during the origin of the Volkovsky gabbroic pluton are discussed on the basis of geochemical data and thermodynamic calculations. The basaltic magma initially enriched in noble metals (NM) relative to their average contents in mafic rocks, except for Pt, is considered to be a source of Pd, Pt, Au, and Ag in the gabbroic rocks of the Volkovsky pluton. The ores were formed with a progressive gain of NM in the minerals during the fractionation of the basaltic magma. The active segregation of NM in the form of individual minerals (palladium tellurides and native gold) hosted in titanomagnetite and copper sulfide ore occurred during the final stage of gabbro crystallization, when the residual fluid-bearing melt acquired high concentrations of Cu, Fe, Ti, and V, along with volatile P and S. Copper sulfides—bornite and chalcopyrite—are the major minerals concentrating NM; they contain as much as 22.65–25.20 ppm Pd and 0.74–1.56 ppm Pt; 4.39–8.0 ppm Au, and 127.2–142.6 ppm Ag, respectively. The copper ore and associated NM mineralization were formed at a relatively low sulfur fugacity, which was a few orders of magnitude (attaining 5 log units) lower than that of the pyrite-pyrrhotite equilibrium. The low sulfur fugacity and the close chemical affinity of Pd and Pt to Te precluded the formation of pyrrhotite, pyrite, and PGE disulfides. The major ore minerals and NM mineralization were formed within a wide temperature range (800–570°C), under nearly equilibrium conditions. Foreign elements (Ni, Co, and Fe) affected the thermodynamic stability of Pd and Pt compounds owing to the difference in their affinity to Te and to elements of the sulfur group (S, Se, and As). The replacement of Pd with Ni and Co and, to a lesser extent, with Pt and the replacement of Te with S, As, and Se diminish the stability field of palladium telluride. Comparison of Pd tellurides from copper sulfide ores at the Volkovsky and Baronsky deposits showed the enrichment of the former in Au, Sb, and Bi, while the latter are enriched in Pt, Ni, and Ag. The enrichment of Pd tellurides at the Baronsky deposit in Ni is correlated with the analogous enrichment of the host gabbroic rocks.     

The origin of fluids and nature of fluid–rock interaction in mid-crustal auriferous mylonites of the Renco mine, southern Zimbabwe

Geothermometric constraints on auriferous shear zones of the Renco mine in the Northern Marginal Zone of the late-Archaean, granulite-facies Limpopo Belt in southern Zimbabwe indicate that deformation and associated mineralization occurred at temperatures of at least 600 °C up to more likely 700 °C. Mid- to upper-amphibolite facies conditions during mineralization correspond to the regional-scale retrogression of granulite facies wall rocks during the late-Archaean thrusting of high-grade metamorphic rocks of the Northern Marginal Zone onto low- to medium-grade granite-greenstone terrains of the Zimbabwe craton. Mineral assemblages indicate that the ore fluid was moderately oxidized with log f_O2 values between 10⁻¹⁷ and 10⁻¹⁸ bars with high H₂S activities of 0.25–0.75. Elements enriched in the shear zones include Au, S, Fe, Cu, Mo, Bi, Te, Ni, Co, and H₂O, Au and Cu being the most enriched. Geochemically, Au correlates with Cu but not with S, which, together with the fact that gold is only rarely intergrown or in direct contact with sulfides, possibly indicates a transport of gold as a chloride complex. The siting of gold along fractures or within implosion breccias suggests that gold was precipitated due to fluid immiscibility induced by catastrophic fluid pressure drops during seismic slip events. Fluid inclusions are predominantly CO₂ (±CH₄ ± N₂)-rich, but petrographic work indicates that fluid inclusions have undergone extensive post-entrapment modifications due to the pervasive recrystallization of mineral textures in the high-temperature shear zones. The mineralized shear zones are enriched in ¹⁸O compared to wall-rock enderbites, which is interpreted to represent an influx of externally derived fluids of probably metamorphic origin. Based on temporal and spatial relationships between mineralization, late-Archaean overthrusting of the Northern Marginal Zone onto the Zimbabwe craton, and coeval amphibolite-facies hydration of granulites, we suggest that the Renco mineralization formed in a mid-crustal environment from metamorphic fluids that were generated from dehydration of subcreted greenstone terrains of the Zimbabwe craton. Received: 27 October 1998 / Accepted: 13 August 1999     

Petrogenesis and metallogenic implications of Late Mesozoic intrusive rocks in the Tongling region,eastern China: a case study and perspective review

ABSTRACT

Large-scale Cu–Au mineralization is associated with Late Mesozoic intrusive rocks in the Tongling region of eastern China, which mainly comprise pyroxene monzodiorite, quartz monzodiorite, and granodiorite. To constrain the petrogenesis of the intrusive rocks and Cu–Au mineralization, detailed analyses of the geochronology, apatite in situ geochemistry, whole-rock geochemistry, and zircon Hf isotopic compositions were performed. Magmatic zircons from pyroxene monzodiorites, quartz monzodiorites, and granodiorites yield U–Pb ages of 136–149 Ma, 136–146 Ma, and 138–152 Ma, respectively, indicating that their formation ages are contemporaneous. Quartz monzodiorites and granodiorites (SiO₂ = 57.9–69.5 wt.%) are highly potassic calc-alkaline rocks with adakitic affinity and have low MgO and Y contents, low zircon ε_Hf(t) values (?11.7 to ?39.0), high apatite Cl contents (>0.2 wt.%), and log fO₂ values (?23.2 to ?8.23), indicating that they may have formed when metasomatized mantle-derived magmas mixed with slab-derived magmas before undergoing crustal assimilation and fractional crystallization. Pyroxene monzodiorites (SiO₂ = 48.4–53.0 wt.%) are shoshonitic and record high MgO, P₂O₅, and Y contents, high zircon ε_Hf(t) values (1.55 to ?7.87), high oxygen fugacity, low Nb and Ta contents, and low apatite Cl contents (mainly <0.2 wt.%), suggesting that they were primarily derived from a metasomatized lithospheric mantle-derived magma that experienced the assimilation of lower crustal materials. The results indicate that the intrusive rocks and associated large-scale Cu–Au mineralization of the Tongling region resulted from the partial melting of the subducted oceanic slab in an oxidizing environment.     

Geochemical indicators of gold-rich zones in the La Josefina epithermal deposit,Deseado Massif,Argentina

The gold deposit at La Josefina, in the Deseado Massif of Argentina, is a low-sulfidation epithermal deposit with some features of the intermediate sulfidation style; the Au occurs in quartz veins and hydrothermal breccias hosted by acid pyroclastic rocks produced by Jurassic bimodal volcanism. Exploration for this deposit type uses geochemical data for vectoring to mineralized rocks. Although a general suite of elements with anomalous concentrations around low-sulfidation deposits is known, that suite varies amongst individual deposits, which should be studied individually. The aim of this study was to determine, in the La Josefina deposit, geochemical indicators of Au-rich rocks at different scales and to assess the effects of weathering on those indicators. To reach these objectives, a mineralized zone (hosting Au-rich veins) and a non-mineralized zone (hosting barren veins) were compared in terms of vein mineralogy, alteration minerals and geochemistry. These zones were also compared with estimated element concentrations of the protolith. Element concentrations in the mineralized zone were then plotted against distance to the Au-rich veins, separating the dataset into two subsets: one from strongly oxidized rocks and the other from weakly oxidized rocks, considering the degree of oxidation as indicative of relative degree of weathering. Based on alteration and vein mineralogy, we interpret that the mineralized zone was hydrothermally active before, during and after Au-mineralization, while the non-mineralized zone was active only before Au-mineralization. Therefore, differences in element concentrations between the estimated protolith and the non-mineralized zone represent geochemical changes produced before mineralization, whereas differences between mineralized and non-mineralized zones represent changes produced during and after mineralization. We conclude that in rhyolitic to rhyodacitic pyroclastic rocks of the study area, affected by predominantly argillic and silicic hydrothermal alterations, high concentrations of Au, Cu, Hg, V, Co and P, and low concentrations of Ba are probably the best deposit-scale geochemical indicators to distinguish Au-mineralized zones from non-mineralized zones. High concentrations of As, Mn, Fe, Pb, Cr, Ni, MgO and Zn (as well as high values of chlorite-carbonate-pyrite index), and low concentrations of Be and CaO can also distinguish mineralized from non-mineralized zones. High concentrations of Au, Cu, Pb, As, Hg, and Co indicate Au-rich zones, independently of lithology and alteration, because they were introduced into the veins and host rocks together with Au. At detailed-scale, the concentrations of 15 elements vary with distance to Au-rich veins; therefore, those elements are considered potential indicators of proximity to Au-rich veins. Of those elements, Au, P, Cu, Pb, Hg, Ba and MgO are the strongest indicators of proximity to the Au-rich veins. In strongly oxidized rocks, some indicators of mineralized zones (Co, Mn, Pb, Zn and MgO) and some indicators of proximity to Au-rich veins (P, Au, Cu, Hg and Ba) should be used with caution or be avoided because weathering modifies their quality as indicators.     

PGE mineralization in Archean volcanic systems: geochemical evidence from thick, differentiated mafic-ultramafic flows, Abitibi greenstone belt, Ontario, and implications for exploration

PGE-rich disseminated zones with discrete platinum-group minerals (Pd, Pt and Rh mineral phases) have been discovered in three thick (80–130 m), differentiated (peridotite-gabbro) mafic-ultramafic flows of the Archean Abitibi greenstone belt, Ontario. Three mineralization zones (whole-rock ∑PGE + Au = up to 1000 ppb) occur along four stratigraphic cross sections through a 2 km strike-length of the Boston Creek Flow ferropicritic basalt. Their occurrence most strikingly correlates with lenticular-podiform concentrations of disseminated chalcopyrite (1 %) and clinopyroxene + interstitial magnetite-ilmenite intergrowths (15–20% oxide), high concentrations of related metals (3000 ppm Cu, 3000 ppm S, 1200 ppb Ag, and 1000 ppm V), strong PGE depletion in adjacent rocks and along strike, and lithological and textural complexity in the margins of the central gabbro-diorite layer. The mineralization zone (whole-rock Ir + Pt + Pd + Au = 110 ppb) within Theo's Flow tholeiitic basalt is somewhat similar in occurrence, style, and composition to those within the Boston Creek Flow. In contrast, the mineralization zone (whole-rock Ir + Pt + Pd + Au = 340 ppb) in Fred's Flow komatiitic basalt most strikingly correlates with vesicle-filling intergrowths of pyrrhotite + pentlandite ± chalcopyrite (2 modal %) and high whole-rock concentrations of Ni (2500 ppm), Cu (700 ppm), and S (1.1%) in the upper chilled margin of the flow.Although apparently uneconomic, these flow-hosted PGE mineralization zones are of interest in exploration, because they are more similar in stratigraphie setting, style, and composition to PGE-rich disseminated Fe-Cu sulfide mineralization zones within thick differentiated intrusions than to mineralization zones in other Archean volcanic rocks. The characteristics of the mineralization zones and their host rocks, especially high degrees of PGE enrichment, vertical and horizontal patterns of PGE depletion, and accumulation of clinopyroxene + magnetite-ilmenite intergrowths, indicate a critical genetic role for variations in the regime of melt flowage. The mineralization zones in the Boston Creek and Theo's Flows are interpreted to have formed by simultaneous in situ formation of PGE-rich Fe-Cu sulfide and Fe-Ti oxide from flowing silicate liquid in the margins of internal lava channels. The mineralization zone in Fred's Flow is interpreted to have formed by ponding and coalescence of PGE-enriched sulfurous vapor bubbles in the upper chilled margin during olivine accumulation on the base of a dynamic lava channel. The relative abundance of PGE mineralization zones and high degree of PGE enrichment in the Boston Creek Flow suggest that the most favorable exploration targets are rocks crystallized from late-stage, highly fractionated derivative liquids in large differentiated terropicritic units.