The world-class Wona-Kona gold deposit,Burkina Faso

The world-class > 4 Moz Wona-Kona gold deposit is hosted within the Paleoproterozoic Birimian Houndé greenstone belt which is the most important gold mineralized belt in the western part of Burkina Faso, with a cumulative reserve of ~ 11 Moz. The mineralization consists of a pervasive silicification with disseminated pyrite–arsenopyrite crosscut by quartz–carbonate veinlets (1 to 10 cm wide) forming a vertical, thick (up to 40 m) and laterally extensive (5 km) northeast trending orebody hosted within a large (200 m wide) shear zone of regional extent. Gold occurs in association with 3 generations of pyrite and 2 generations of arsenopyrite. Free gold, interpreted as the last mineralizing event, occurs as late fracture filling in the pervasive silicification zone.     

The Inata deposit,Belahouro District,northern Burkina Faso

The Inata gold deposit is hosted in the Bouroum greenstone belt of northern Burkina Faso and contains ca. 5 Moz of gold resource. The greenstone belt is divided into 4 distinct domains: The Pali West, Pali-Minfo and Fété Kolé domains comprised of variable proportions of mafic to intermediated volcanic, volcaniclastic and sedimentary rocks, and the Sona Basin comprised of feldspathic sandstones and turbidites. Potential Tarkwaian-like conglomerates are rarely observed on the eastern margin of the basin. The stratigraphy is crosscut by a series of intrusions between 2172 ± 15 Ma and 2122 ± 4 Ma. A complex deformation sequence is recorded in the rocks and has been interpreted in a five stage scheme: early syn-depositional basin margin faults reactivated through time and partitioning all subsequent regional deformation (De_B); N–S compression (D1_B > 2172 Ma); E-W compression (D2_B, < ca 2122 Ma); NW–SE compression (D3_B), and a late N–S compression (D4_B). D2_B-D4_B overprint all rocks, including those of the Sona Basin and Tarkwaian-like conglomerates. Peak metamorphism is mid- to upper-greenschist facies.Mineralisation at Inata is hosted in black shales and volcaniclastic rocks of the Pali-Minfo domain and comprises shear-zone hosted quartz-tourmaline-ankerite veins with associated sulphides dominated by pyrite and arsenopyrite. Three generations of pyrite (py1, py2, py3) and one generation of arsenopyrite (apy2) have been identified. Py1 is parallel to bedding and early D1_B foliation and not associated with gold. Py2 and apy2 are coeval, contain up to 1 ppm gold and are spatially associated with auriferous quartz veins. Py3 locally overprints previous assemblages and is also associated with Au. Fluid inclusions in quartz indicate H₂O to H₂O–CO₂–NaCl fluids in auriferous quartz veins.Microscopic to macroscopic observation of fabric-mineral-vein crosscutting relationships indicate that mineralisation is syn-D2_B, disrupted and remobilised during D3_B. All observations and data are consistent with Inata representing an orogenic style of gold mineralisation formed relatively late in the evolution of the host terrane.     

The geology of the Taparko gold deposit, Birimian greenstone belt, Burkina Faso, West Africa

The Taparko gold deposit, located in the eastern branch of the Proterozoic Birimian Bouroum-Yalogo greenstone belt (Burkina Faso) consists of a network of quartz veins developed in a N 170° trending shear zone (250 m wide, 4 km long) superimposed on the regional Birimian structural pattern. The quartz vein network is composed of: (a) a dominant array of quartz veins (type 1), parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation, layering, ribbon, tension gashes, etc.; (b) oblique and subparallel related veins (type 2) of gray to white weakly deformed quartz crosscutting the dominant quartz veins resulting in breccia structures; and (c) shallow dipping veins (type 3), cross-cutting veins types 1 and 2 and filled by undeformed white buck structure quartz. Cross-cutting relationships and different quartz types in different veins and within individual veins imply a concomitant filling of the veins during the progressive deformation. Initial sinistral transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 and 2 veins and were deposited in two stages, with native gold being related to second stage sulfides. Gold (and chalcopyrite) precipitated preferentially upon the surfaces of fractured pyrite grains in low-pressure sites (pressure shadow zones) around and/or within the sulfide grains (along subsequently annealed fractures). The formation of the South Taparko deposit can be divided into a succession of events: (a) during the first event, N 170°-directed sinistral transcurrent shearing resulted in a N 20° mylonitic foliation and fractured rock which allowed H₂O-, CO₂- and SiO₂-rich fluids to circulate and deposit quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane; and (c) during the third event, the sinistral transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile to brittle-ductile. During the last phase of deformation gold nucleated and deposited in low-pressure zones. Received: 9 July 1997 / Accepted: 23 March 1998     

The Nassara gold prospect,Gaoua District,southwestern Burkina Faso

The Nassara-Au prospect is located in the Birimian Boromo Greenstone Belt in southwestern Burkina Faso. It is part of a larger mineralized field that includes the Cu–Au porphyry system of Gaoua, to the north. At Nassara, mineralization occurs within the West Batié Shear Zone that follows the contact between volcanic rocks (basalt and andesite) and volcano-sediments (pyroclastics and black shales) at the southern termination of the Boromo Belt. Gold is associated with pyrite and other Fe-bearing minerals that occur disseminated within the sheared volcanic and volcano-sedimentary rocks. In particular, highest grades are distinguished in alteration halos of small quartz–albite–ankerite veins that form networks along the shear zone. Here, pyrites are marked by As-poor and As-rich growth zones, the latter containing gold inclusions. Gold mineralization formed during D2_NA. Subsequent shear fractures related to D3_NA related are devoid of gold. Nassara is a classical orogenic gold occurrence where gold is associated to disseminated pyrite along quartz veins.     

The Markoye Shear Zone in NE Burkina Faso

Birimian supracrustal sequences in NE Burkina Faso are dominated by meta-volcaniclastic greywacke, intercalated meta-conglomerate, siltstone and shale. The sequences where subjected to two phases of deformation and contact metamorphosed to hornblende–hornfels facies during emplacement of pyroxenite–gabbro–norite (Yacouba Mafic complex), granodiorite–tonalite (Tin Taradat granodiorite–tonalite) and dolerite dykes.Structural studies indicated that the NE-trending, first-order crustal-scale Markoye Shear Zone (MSZ; Markoye Fault of [Jeambrun, M., Delfour, J., Gravost, M., 1970. Carte géologique de L’Oudalan. Bureau De Recherches Geologiques et Miniéres, Burkina Faso.]) has undergone at least two phases of reactivation concomitant to two phases of regional deformation. The first phase of deformation, D1, resulted in the formation of NNW-NW trending folds and thrusts during dextral-reverse displacement on the MSZ. The deformation is termed the Tangaean Event and predates the Eburnean Orogeny. D2 phase involved a period of SE–NW crustal shortening and sinistral-reverse displacement on the MSZ, and is correlated to the Eburnean Orogeny 2.1 Ga. Deformation in D2 is characterised by NE-trending regional folds (F2) and a pervasive NE-trending foliation (S2-C to S2). Within the MSZ, deformation is characterised by NNE-trending zones of mylonite that are bordered in the hangingwall and footwall by pseudotachylite veins. Buck quartz-carbonate veins and quartz cataclasite veins crosscut the mylonite zones and are, in turn, crosscut by quartz–chlorite–(muscovite) shears that formed during reactivation of the MSZ late in D2. Several generations of veins are recognised at the Essakane main deposit (EMZ): Arsenopyrite–pyrite–gold mineralization in quartz veins formed in D1 during metasomatic alteration of the host rocks; Vein-stockwork gold mineralization is interpreted to have formed late in D2.     

The Bepkong gold deposit,Northwestern Ghana

The Bepkong gold deposit is located in the Wa–Lawra belt of the Paleoproterozoic Baoulé-Mossi domain of the West African Craton, in NW Ghana. It occurs in pelitic and volcano-sedimentary rocks, metamorphosed to greenschist facies, in genetic association with zones of shear interpreted to form during the regional D₃ deformational event, denominated D_B1 at the deposit scale. The ore zone forms a corridor-like body composed of multiple quartz ± carbonate veins surrounded by an alteration envelope, characterized by the presence of chlorite, calcite, sericite, quartz and disseminated pyrite, arsenopyrite plus subordinate pyrrhotite and chalcopyrite. The veins contain only small proportions of pyrite, whereas most of the sulphides, particularly arsenopyrite, occur in the altered host rock, next to the veins. Pyrite is also common outside of the ore zone. Gold is found in arsenopyrite, where it occurs as invisible gold and as visible – albeit micron-size – grains in its rims, and as free gold within fractures cross-cutting this sulphide. More rarely, free gold also occurs in the veins, in fractured quartz. In the ore zone, pyrite forms euhedral crystals surrounding arsenopyrite, but does not contain gold, suggesting that it formed at a late stage, from a gold-free hydrothermal fluid.     

The mineralogy and geochemistry of two copper-rich weathering profiles in Burkina Faso, West Africa

Two weathering profiles developed over disseminated Cu mineralization hosted by granodiorites (porphyry type) and felsic volcanics respectively, in a savannah tropical environment (Burkina Faso) have been studied in detail. A mineralogical and geochemical study was carried out in order to determine the characteristics of both profiles and the behaviour of Cu in such deeply weathered environments. Our investigation was focused on the upper part of the weathering profile, respectively 4.0 and 10.5 m below the surface.The mineralogical study reveals that in the first case (profile A) the predominant clay minerals are smectites and kaolinite while in the second (profile B) a more kaolinitic composition is indicative of more severe leaching. In fact, field observations seem to demonstrate that the latter situation is more clearly related to an ancient lateritic-type weathering while the first one results from more recent processes.In both cases the Cu contents through the profiles are high (several thousands of ppm) and in good agreement with the grades obtained in depth, in the mineralized rock. Nevertheless, some leaching can be observed in the upper soil horizons, but the contents still remain highly anomalous, in the 1000 ppm range.It is shown that Cu is distributed in the main secondary minerals constituting the weathering products, whether they are silicates (smectites, phyllites, kaolinites) or oxides (goethite, hematite, Mn oxides).The main stable Cu-bearing mineral seems to be the kaolinite: indeed, smectites turn into kaolinite in the upper part of the profiles while goethite seems to be depleted in Cu under the same conditions.As concerns geochemical exploration, two observations can be noted. Firstly, Cu is very stable in such supergene environments, and secondly, the best size fraction in which to detect the Cu secondary dispersion haloes in soil or stream sediment samples is the <63 μ fraction, in terms of anomaly intensity or contrast.     

The Kalana vein-hosted gold deposit,southern Mali

The Kalana gold deposit occurs within metamorphosed Paleoproterozoic volcano-sedimentary series of the Birimian Baoulé-Mossi domain of the West African Craton, in the south of Mali. These formations are intruded successively by small dioritic bodies, as well as andesitic and tonalitic dykes, which exhibit volcanic arc-setting signatures. Mineralization is hosted by two sets of quartz veins that intersect the regional schistosity. The first set of veins is the most important in terms of grades and size, and consists of thick veins (up to meter size) that range in trend from N–S to E–W with shallow plunges. The second set consists of much thinner (centimeter size) sub-vertical quartz veins oriented NE–SW. The two sets of veins are interpreted to have formed during the evolution of late, gently dipping thrust faulting. Two episodes of gold precipitation are recognized: a first episode, during early stages of vein growth, formed micron-size native gold inclusions in arsenopyrite; a later episode, during vein shearing and fracturing of the quartz lodes, precipitated native gold in free form in quartz, in fractured arsenopyrite, and associated with chalcopyrite, galena, sphalerite, Bi sulfides and native bismuth. All evidence suggests that the Kalana deposit represents orogenic gold mineralization formed during a relatively long-lived hydrothermal system, at a late stage of the tectonic history of the greenstone belt.     

Structural and gold mineralizing evolution of the world-class orogenic Mana district,Burkina Faso: Multiple mineralizing events over 150 million years

The Mana district, located in the northern part of the Birimian Houndé greenstone belt in western Burkina Faso, is a world-class Paleoproterozoic orogenic gold district (∼8 Moz) including five gold deposits (Fofina, Nyafé, Siou, Wona-Kona and Yaho). These deposits are located in specific lithostratigraphic domains, and gold is controlled by various structural features. Deposit- and regional-scale mapping, intrusion age and geochemistry, as well as airborne aeromagnetic and electrical resistivity geophysical data, were used to decipher the tectonic evolution of each gold deposit and the district. Five deformational and four gold mineralizing events were recognized.The first deformation event (D1_MD: E-W oriented shortening) affected the metamorphosed volcanic and sedimentary rocks of the Lower Birimian group. This early deformation episode was correlated with the formation of gently N-plunging folds (F_1MD) and N-S-striking thrusts faults coeval with emplacement of the pre- to synkinematic Wona-Kona and Siou plutons dated at ∼2172 Ma, under greenschist facies metamorphism. The quartz-carbonate veins (V_1MD) at Fofina and Siou formed during D1_MD at Eoeburnean time, manifesting the first gold event at approximately ∼2172 Ma.The following deformation event (D2_MD: E-W oriented extension) is associated with the deposition of the Upper Birimian group (Mana basin) overlying the Lower Birimian group. The geometry of the Mana basin is controlled by the Mana and Maoula shear zones. The Tarkwaian-type rock formation overlying the Upper Birimian group, controlled by the Wona-Kona and Siou shear zones, is constrained at the end of D2_MD or at the beginning of the D3_MD event with a maximum deposition age at ∼2113 Ma.The third deformation event (D3_MD: E-W to WNW-ESE transpression) affected the entire supracrustal rock. Such event is correlated with the formation of map-scale F_3MD folds and dextral shear zones during the Eburnean orogeny (∼2113–2090 Ma). A second gold mineralizing event occurred during D3_MD and is manifested by quartz-carbonate veins (V_3MD) and disseminated sulfides at the Yaho, Fofina and Nyafé and possibly Wona-Kona deposits.The fourth deformation event (D4_MD: NNW-SSE transpression) is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric NNE-striking folds (F_4MD) associated with vertical fold axes. Syn-D4_MD mineralization is characterized mainly by a strong silicification (Si_4MD) with disseminated pyrite and arsenopyrite along the Wona-Kona shear zone and by tiny quartz-carbonate veinlets (V_4MD). This event is considered the main gold-bearing event in the western margin of the Mana district.The fifth and last deformation event (D5_MD) is brittle in character and was responsible for the formation of E-W subvertical crenulation cleavages and reverse faults under overall N-S shortening. This late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the Wona-Kona and Siou deposits. This event could be as young as ∼2022 Ma, the age obtained from Ar-Ar datation of muscovite-schists at the Wona-Kona deposit.Our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit- and regional-scale mapping. It is interpreted that gold was introduced as early as ∼2172 Ma and possibly as late as ∼2022 Ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the Birimian belts.     

Gold mineralisation in the Essakane goldfield in Burkina Faso,West African craton

The West African craton is known for its structurally hosted Au deposits in Ghana, Burkina Faso, Côte d'Ivoire, Mali and Niger. The Essakane goldfield in northeast Burkina Faso has produced 1 606,000 oz of gold since 2010 from the Essakane Main Zone. The Essakane goldfield is made up of several exploration and artisanal sites that include; Essakane Main Zone, Gossey, Falagountou, Sokadie, Tin Zoubratan, Essakane North and South, Korizéna, Bom Kodjélé, Tin Taradat, Tassiri, Gaigou, and Takabangou. Gold mineralisation in sheeted and stockworks quartz–carbonate and tourmaline veins occurs with pyrite, arsenopyrite, and traces of pyrrhotite, galena and hematite. It is hosted in sheared, folded and contact metamorphosed volcanic, volcanoclastic and sedimentary Birimian Supergroup sequences. The maximum age of gold mineralisation in the Essakane goldfield is syn-deformational and formed during the Eburnean Orogeny (D2) at 2130–1980 Ma.     

北京市得田沟金矿床成因机制

得田沟金矿床产于太古宙四合堂群角闪岩相变质地层中。容矿围岩主要为角闪石岩、斜长角闪岩、片麻岩及变粒岩等。区域崎峰荷断裂带及边部次级剪切带控制着金矿床的形态、产状。矿石以含金多金属硫化物石英脉为主，其次为含金蚀变岩。矿床稳定同位素特征及金丰度的变化表明金与正长斑岩岩浆同源，来自地幔。成矿温度为７０－４９０℃，峰值区为１５０－２７０℃；成矿压力为１００－１５００ＭＰａ，在燕山中期（１４７×１０＾６ａ）     

甘肃鹿儿坝金矿床金赋存状态研究

甘肃鹿儿坝金矿处在西秦岭“岷礼金矿带”上,是我国卡林型金矿“西北金三角”组成部分。该矿床赋存于洮河复向斜北东翼之中三叠统浅变质浊积岩建造中,矿体受NWW向断裂构造控制明显,矿石以中-低温热液蚀变矿物组合为特征,是一个正在开发但难选的大型金矿床。为了解金的赋存状态,文章通过中-高倍矿相显微镜、金化学物相分析及电子探针(EMPA)测试等手段,研究了金在矿石中的赋存状态。结果表明,三类矿石中金的分布型式基本一致,载金矿物主要为毒砂、石英、黄铁矿和铁白云石;金以矿物包裹金为主,总占比达96.31%,其中硫化物包裹的金占61.45%~89.01%、硅酸盐中的金占有率为7.28%~25.49%,独立存在的金矿物较少,占1.03%~9.01%;独立金矿物主要以矿物粒间单体金形式存在,金成色987.05‰,为自然金,粒度以0.0009 mm×0.0061 mm~0.0078 mm×0.0114 mm为主。这为矿床研究提供了基础信息,并为改善矿山生产提供了一定的理论依据。     

The Loulo gold and tourmaline-bearing deposit

The stratabound polymorph Loulo gold deposits and occurrences were discovered in sedimentary rocks of the Early Proterozoic Birimian Formation, in the Kéniéba region of SW Mali. Early work showed reserves in the Loulo-0 and Loulo-3 deposits of 28 t of gold in ore averaging 4.43 g/t Au, to depths of 140 m; recent drilling has found mineralization to at least 300 m depth. Early tourmalinization of the sedimentary host rock is indicated by reworked tourmalinite pebbles in non-tourmalinized hanging-wall rocks. Mineralization, restricted to highly tourmalinized zones, occurs as: (a) pre-D1 (deformation) disseminated pyrite with micron-size inclusions of gold particles; (b) syn-/late-D2 stockworks with minor disseminated sulphides and native gold. Gold in the sulphides is small (10–40 gm), and occurs at pyrite-crystal edges, filling cracks and/or as inclusions. Post-D2 stockwork mineralization derived from the pre-D1 one that was emplaced with tourmaline, as is indicated by the small differences between pre-D1 and post-D2 sulphur isotopes, as well as by the type of quartz vein that indicates an almost closed system. In West Africa, the pre-D1 mineralization of Early Birimian age represents the first major introduction of gold into the Proterozoic. It attributes a major metallogenic role to the tourmalinized sandstone that indicates crustal accretion in West Africa during the Early Proterozoic. Editorial handling: D.D. Klemm     

广西龙头山金矿床矿石显微结构构造和金矿物成分特征

广西龙头山金矿床是热液型金矿床,运用扫描电子显微镜对其矿石的微裂隙特征、结构构造及成分进行分析。其结果表明,矿石的微裂隙是相通的;矿液在运移过程中,在相对封闭、酸碱度较适宜的条件下沉淀,形成了较自形的电气石、黄铁矿和金矿物;矿液经过了多期上升过程,对早期形成的矿物不断地进行叠加和改造,使不同期次形成的金矿物成分略有不同;金矿物有次生加大现象。进而认为,该金矿床应有三期以上的成矿期。     

The Mokrsko-West gold deposit,Bohemian Massif,Czech Republic: Mineralogy,deposit setting and classification

The Mokrsko-West deposit is unique among European Variscan gold deposits from the points of view of both the structure (an approx. 200 m thick complex of sheeted, several mm-thick, densely spaced quartz veins) and the economic viability (gold reserves of about 100 t). The deposit is hosted mainly by tonalite of the calc-alkaline Sázava tonalite suite (ca. 354 Ma) of the Central Bohemian Plutonic Complex. Mineralization is characterized by quartz-dominated gangue, no visible hydrothermal alteration, low sulfide content, high fineness native gold accompanied by maldonite, aurostibite, native bismuth and numerous Bi–Te–(S) phases. Five mineralogical stages are described in great detail. Arsenopyrite and chlorite thermometers, mineral phase stabilities and published isotope and fluid inclusion data are used to reconstruct the temperature and compositional evolution of the system. The role of liquid bismuth in the sequestration of gold is also discussed.The deposit shares the features of both orogenic gold (ORG) and intrusion-related gold (IRG) deposits. The IRG model is advocated by close spatial association between the ore zone and the tonalite host-intrusion, by the absence/scarcity of hydrothermal alteration, by the Au–Bi–Te–As elemental association and by marked thermal gradients from the early to late mineralization stages. The ORG model is advocated by an approx. 15–10 Ma gap between the intrusion of the tonalite-host and the ore formation, by isotope and geochemical evidence for a key role of metamorphic fluids in the mobilization and transfer of many elements/species (inclusive S and Au). The apparently ambiguous classification of the deposit can most probably be attributed to deposit formation at a depth of ≥ 9 km and to setting of the deposit at/inside a large-scale plutonic complex with multiple and prolonged tectonic and intrusive activity.     

新疆库布苏金矿元素地球化学特征

通过库布苏金矿不同岩脉、地层、石英脉内微量元素及稀土元素含量与组成的研究。,得出库布苏金矿岩脉形成与区域岩浆活动有关,岩脉与区域中基性岩脉,泥盆纪火山－沉积地层在成矿物质上具有同源性或继承性。石英脉与岩脉、地层在元素地球化学特征上差别较大。矿化与石英脉是不同阶段的产物,其为后期热液作用的产物。东准噶尔地区的金矿找寻应以围岩蚀变作为主要找矿标志。     

Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso

This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L⁻¹, they have a large range from <0.5 to 1630 μg L⁻¹. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L⁻¹. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O₂ concentrations <2 mg L⁻¹. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.     

The Gagarka gold deposit in the central Urals, Russia

The Gagarka gold deposit was formed in two stages. The gold-telluride ore of the main early stage was formed ～260 Ma ago synchronously with Permian collision, which was accompanied by retrograde metamorphism with mobilization of Au and Te from geochemically similar massive sulfide lodes in the rift zone. The Au-bearing argillic metasomatic rocks of the late stage presumably Mesozoic in age are distinguished by specific geochemistry and locally superposed on the ore related to the early stage. The upper part of the metasomatic column consists of quartz-kaolinite rock, which is confused in many cases with products of Mesozoic-Cenozoic weathering and because of this is not perceived as a guide for hidden Au-bearing argillic alteration, whose resource potential remains underestimated in the Urals.     

The Physico—Chemical Conditions for Mobilization of Gold in Mobin Gold Deposit,Southwest Hunan,China

Experiments on water-rock interaction were carried out on wall-rock samples fromtheMobin gold deposit,Southwest Hunan,China ,with the aim of determining the optimum physical and chemical conditions for the mobilization of gold in solution.Results indicate that gold is most easily mobilized from the wall rock-tuffaceous slate of the Mobin Deposit.Mobi-lization is optimized if fluids are neutral to slightly alkaline and contain both chlorine and sul-phur ions at the concentration and composition of about[0.25M(NH4)2 1M NaCl].The amount of gold leached from the tuffaceous slate increases with temperature although the effect decreases above about 200℃.     

新疆阿希金矿:古生代的低硫型浅成低温热液金矿床

新疆阿希金矿床为一形成于古生代的低硫型浅成低温热液金矿床,矿床产于伊犁—中天山板块北部中天山北缘活动大陆边缘的吐拉苏火山岩断陷盆地中。其赋矿围岩为大哈拉军山组安山质火山岩和火岩碎屑岩,矿体呈脉状产于古火山口外围的环形断裂带中,主要金属矿物有自然金、银金矿、黄铁矿、白铁矿、毒砂、赤铁矿、褐铁矿以及微量的浓红银矿、硒银矿、硫锑铜银矿、角银矿等,非金属矿物有石英、玉髓、菱铁矿、方解石、绢云母、冰长石等,围岩蚀变作用主要有硅化、绢云母化、碳酸盐化和青盘岩化。矿床以富集Au、Ag、As、Sb、Bi、Hg、Se、Te、Mo元素组合为特征,Ag/Au比值小,为0.46～11.1。氢、氧、碳、硫及稀有气体同位素组成特征显示其成矿流体主要为循环大气降水;成矿流体盐度主要为0.7%～3.1%NaCl_(eqv),平均为2.2%NaCl_(eqv);成矿温度为120～240℃,平均190℃;最大成矿深度约700 m。沸腾作用是引起成矿流体中矿质发生沉淀富集的主要成矿机制,成矿作用过程中流体处于近中性pH值的还原环境,成矿时代介于晚泥盆世晚期((363.2±5.7)Ma)到早石炭世维宪期。其一系列特征显示该矿是一个典型的、形成于古生代的低硫型浅成低温热液型金矿床。矿床得以保存与矿床形成后很快被阿恰勒河组沉积盖层覆盖有关,从上新世开始由于印度板块对欧亚板块的碰撞挤压作用,天山造山带被快速抬升遭受风化剥蚀作用使矿床重新露出地表而被发现。阿希金矿的发现对于在中、新生代以前的造山带中寻找浅成低温热液型金矿床具有重要的借鉴和指导意义。