Lithogeochemical and mineralogical indicators of Andean precious-metal and polymetallic vein mineralization

Proximal and distal lithogeochemical alteration patterns associated with precious-metal and polymetallic vein mineralization in the Andes of Bolivia and Peru are delineated both by sulphophile elements such as Ag, As, Pb, S, Sb, Sn and Zn, and also by hydrothermal alteration indicators such as CaO/MgO, K₂O/Na₂O, Rb/K and Rb/Sr. In some cases, single-element, ratio and normalized data additive plots permit the detection of a vein at a distance of more than 100 m even when the visible alteration extends for less than 10 m. Asymmetric hanging-wall-footwall halos in both volcanic and sedimentary rocks provide a useful supra-ore/sub-ore indicator for inclined veins. The K₂O/Na₂O ratio appears to be the most reliable lithogeochemical guide to ore-related hydrothermal alteration.     

藏东日扎山一带马拉松多组流纹岩年龄、地球化学特征及其地质意义

为确定藏东日扎山一带马拉松多组流纹岩的形成时代及成岩构造环境,探讨古特提斯洋闭合时间,应用LA-ICP-MS方法对其进行锆石U-Pb精确定年,并开展岩石学和地球化学研究。结果表明,该区流纹岩岩浆锆石206Pb/238U年龄加权平均值为244±1.2Ma,较精确地限定了马拉松多组流纹岩的形成时代,为中三叠世早期。岩相学及地球化学研究结果显示,马拉松多组流纹岩具高硅(Si O_2=72.72%～76.88%),富碱((K_2O+Na_2O)=6.64%～7.41%,K_2ONa_2O),过铝质(Al_2O_3=11.76%～13.03%,A/CNK=1.17～1.31)特征;岩石富集大离子亲石元素K、Rb,高场强元素Th、U,而亏损大离子亲石元素Sr和Ba,高场强元素Nb、Ta、P、Zr、Hf、Ti等;稀土元素配分模式表现出轻稀土元素富集(LREE/HREE=1.93～2.89),轻稀土元素分馏程度稍高于重稀土元素的右倾V字形分布模式,具有明显的负Eu异常(δEu=0.36～0.41);其成因可能与幔源岩浆的底侵和加厚地壳的减薄有关,是幔源基性岩浆底侵导致地壳物质脱水发生部分熔融的产物,即主要是陆壳和硅铝质源岩部分熔融形成的,岩浆在上升过程中经历了结晶分异作用。其化学性质具有S型向A型流纹岩演化的趋势,以及同碰撞弧火山岩与碰撞后A型流纹岩的双重特性。综合研究认为,研究区在早中三叠世处于弧-陆碰撞后活动大陆边缘短暂的后造山伸展构造环境,古特提斯洋(金沙江洋)在此之前已经闭合。     

Volcanic lithogeochemistry and alteration at the Delbridge massive sulfide deposit, Noranda, Quebec

The Delbridge orebody occurs within a thick sequence (> 1 km) of porphyritic to aphyric massive rhyolite and rhyolite breccia of the Archean Blake River Group. The orebody produced ≈ 370,000 tonnes grading 0.61% Cu, 9.6% Zn, 110 g/t Ag and 2.1 g/t Au (1969–1971). The footwall consists of massive quartz porphyritic rhyolite mantled by proximal rhyolite breccias. An irregular chloritic alteration pipe with mineralization is subvertical to the ore lens. The orebody occurs at a thick cherty horizon within rhyolite breccia, and is overlain by a succession of mafic debris flows, porphyritic to aphyric massive rhyolite flows, and finally andesite. The main alteration assemblage in the rhyolite units is quartz-albite-sericite-chlorite-carbonate. Immobile element plots and rare-earth element data indicate that the footwall rhyolite flows and proximal breccias are tholeiitic to transitional (Zr/Y = 3.5–5.5; La_N/Yb_N = 1.7–2.6), whereas hangingwall rhyolite flows are mildly calc-alkaline (Zr/Y = 6.5–7.5; La_N/Yb_N = 2.8–3.8). These two rhyolite types also have separate alteration lines in Ti-Zr space and in various immobile element plots. The identification of chemically different rhyolites above and below the orebody provides markers that can be identified and traced even where strongly altered. An intrusive rhyolite mass in the footwall is chemically identical to the hangingwall aphyric rhyolite flows, and is interpreted as the feeder to these flows. Calculated mass changes in the footwall rhyolite commonly are large, and result from major silica change (±30%), significant loss of Na₂O + CaO, and important additions of K₂O and FeO + MgO. The margins of the pipe show net mass gain, whereas the interior of the pipe shows net mass loss. Hangingwall rhyolite shows mass changes that generally are much smaller than in the footwall. Felsic rocks in the silica-sericite alteration zone up to ≈ 200 m from the orebody have high δ¹⁸O values of 10–12‰, reflecting low-temperature alteration. The orebody occurs near the contact between a mainly tholeiitic rhyolite footwall and an overlying sequence of mildly calc-alkaline rhyolite then andesite.     

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.     

Re–Os and U–Pb Geochronology of the Dawan Mo–Zn–Fe Deposit in Northern Taihang Mountains,China

The Dawan Mo–Zn–Fe deposit located in the Northern Taihang Mountains in the middle of the North China Craton (NCC) contains large Mo‐dominant deposits. The mineralization of the Dawan Mo–Zn–Fe deposit is associated with the Mesozoic Wanganzhen granitoid complex and is mainly hosted within Archean metamorphic rocks and Proterozoic–Paleozoic dolomites. Rhyolite porphyry and quartz monzonite both occur in the ore field and potassic alteration, strong silicic–phyllic alteration, and propylitic alteration occur from the center of the rhyolite porphyry outward. The Mo mineralization is spacially related to silicic and potassic alteration. The Fe orebody is mainly found in serpentinized skarn in the external contact zone between the quartz monzonite and dolomite. Six samples of molybdenite were collected for Re–Os dating. Results show that the Re–Os model ages range from 136.2 Ma to 138.1 Ma with an isochron age of 138 ± 2 Ma (MSWD = 1.2). U–Pb zircon ages determined by laser ablation inductively coupled plasma mass spectrometry yield crystallization ages of 141.2 ± 0.7 (MSWD = 0.38) and 130.7 ± 0.6 Ma (MSWD = 0.73) for the rhyolite porphyry and quartz monzonite, respectively. The ore‐bearing rhyolite porphyry shows higher K₂O/Na₂O ratios, ranging from 58.0 to 68.7 (wt%), than those of quartz monzonite. All of the rock samples are classified in the shoshonitic series and characterized by enrichment in large ion lithophile elements; depletion in Mg, Fe, Ta, Ni, P, and Y; enrichment in light rare earth elements with high (La/Yb)_n ratios. Geochronology results indicate that skarn‐type Fe mineralization associated with quartz monzonite (130.7 ± 0.6 Ma) formed eight million years later than Mo and Zn mineralization (138 ± 2 Ma) in the Dawan deposit. From Re concentrations in molybdenite and previously presented Pb and S isotope data, we conclude that the ore‐forming material of the deposit was derived from a crust‐mantle mixed source. The porphyry‐skarn type Cu–Mo–Zn mineralization around the Wanganzhen complex is related to the primary magmatic activity, and the skarn‐type Fe mineralization is formed at the late period magmatism. The Dawan Mo–Zn–Fe porphyry‐skarn ores are related to the magmatism that was associated with lithospheric thinning in the NCC.     

Geology, petrography and geochemistry of rhyolite-hosted uranium mineralization at Mohar, Shivpuri district, Madhya Pradesh

In the westernmost part of the Bundelkhand Granitoid Complex (BGC), a mesa structure represents a unique outlier, surrounded by brecciated granite and filled with Vindhyan sedimentary rocks locally known as the Dhala Formation near Mohar village of Shivpuri district, Madhya Pradesh. Uranium mineralisation located in the area is mostly associated with rhyolite of peralkaline to peraluminous in nature, that has a high average uranium concentration (30 ppm). The mineralization is in or adjacent to caldera and is hydrothermal vein-type. Radioactivity is mainly due to coffinite with limited radioactivity due to U-Ti complex, uranium adsorbed in clay and labile uranium along fracture. Coffinite occurs in association with pyrite and chalcopyrite or chlorite with presence of fluorite. Features such as chloritisation, clay formation and sulfide mineralisation manifest hydrothermal alteration. Chemical analysis indicates the aluminous nature of the rock and their high K₂O/Na₂O (3.81–12.84) ratios are suggestive of predominance of potash feldspar over sodic. The alteration index varies from 49.88–92.40, which, reflects high intensity of hydrothermal alteration. Chlorite-carbonatepyrite index (CCPI), a measure of the intensity of replacement of sodic feldspars and glass by sericite, chlorite, carbonate, and pyrite associated with hydrothermal alteration proximal to the ore bodies varies from 3.84–49.66. On the basis of core study, geochemistry and mineralogy, it is envisaged that epigenetic hydrothermal solutions were responsible for concentration of uranium as coffinite, radioactive carbonaceous matter and adsorbed uranium phases in rhyolite with sulfide confined to weak planes.     

云南铜厂沟斑岩型铜钼矿矿化蚀变过程中成矿元素迁移特征

铜厂沟斑岩型铜钼矿是格咱岛弧燕山晚期Mo多金属成矿作用的典型代表,矿床中与Cu-Mo矿化相关的蚀变作用广泛发育。文章在对该矿床岩相学研究的基础上,选择TiO₂作为蚀变过程中惰性组分、运用Gresens方程对元素迁移进行定量分析。主量元素迁移特征显示在钾硅酸盐化蚀变阶段和石英—绢云母化蚀变阶段中Al₂O₃、K₂O、Na₂O、P₂O₅、SiO₂持续迁入,青磐岩化阶段Fe₂O₃、MgO、CaO迁入富集;微量元素在钾硅酸盐化阶段显示为Mo、Ba、W、Ra、Nb、Ta、LREE元素明显迁入,且Ba、W与Mo的富集有明显的相关性,石英—绢云母化蚀变阶段Pb、Zn富集成矿,青磐岩化阶段Cu富集成矿,铜厂沟斑岩型铜钼矿矿化蚀变与成矿元素迁移特征研究对该区深部地球化学勘查指标制定、成矿信息及矿体边界确定等具有重要的参考意义。     

东天山玉峰金矿热液蚀变作用与元素迁移规律

玉峰金矿位于中亚造山带东天山东缘,是近年来新发现的含银高品位金矿床。该矿床目前已探明6个金矿体,均赋存在石英斑岩中。矿区热液蚀变作用发育,与成矿关系最密切的为黄铁绢英岩化和硅化,显示明显的蚀变分带:以石英硫化物脉为中心,黄铁绢英岩化带在其两侧大致对称分布。本文选取矿体上盘和下盘的石英斑岩、黄铁绢英岩进行了全岩的主、微量元素及成矿元素测试,并对其中的长石和绢云母进行电子探针成分分析。测试结果表明,黄铁绢英岩中的Au含量较蚀变前呈指数级增长,Ag、Cu、As等成矿元素大量增加,CaO、Na_2O、P_2O_5、P_2O_5、Sr、Pb、Th、U、Sb等元素显著迁出,而SiO_2、Al_2O_3、TiO_2含量和稀土元素含量变化较小,表现稳定。热液蚀变过程中,石英斑岩中71%～76%的正长石发生绢云母化,导致K_2O大量迁出;而钠长石几乎全部蚀变为绢云母,造成Na_2O大量迁出。热液流体的贡献使得蚀变岩中MgO、Fe_2O_3~T含量成倍增加,并主要富集在绢云母和/或黄铁矿晶格中。综合分析认为,绢英岩化蚀变带,Au、Cu、As和Bi等元素的综合化探异常,低电阻率、高激化率的地球物理特征可以作为玉峰矿区深部和外围找矿的标志。研究区内的石炭纪石英斑岩带,尤其是在构造叠加部位,热液活动使其更有利于矿化富集,是找矿勘探的有利部位。     

羌北-昌都地块那益雄组火山岩锆石U-Pb年代学及地球化学特征

本文对青藏高原羌北-昌都地块阿布日阿加措地区的晚二叠世那益雄组火山岩进行了年代学和地球化学研究。该火山岩主要由玄武岩、安山玄武岩、安山岩、流纹岩、凝灰岩组成,具有碱性玄武岩到酸性熔岩的特征。锆石U-Pb年代学研究表明该火山岩的形成时代为251. 1±4. 8～249. 6±1. 3 Ma之间。地球化学分析结果表明,该地区的流纹岩具有高的Si O2(74. 85%～77. 55%)和Na2O+K2O(5. 40%～6. 61%)含量,较低的MgO、K2O和Ca O含量,Al2O3含量低且稳定,里特曼指数平均为1. 15,小于3. 3。安山岩Si O2含量55. 13%～56. 28%,Na2O+K2O含量4. 13%～6. 15%,里特曼指数平均为2. 20,小于3. 3,属于钙碱性安山岩。碱性玄武岩Si O2含量51. 49%,Na2O+K2O含量6. 34%,里特曼指数为4. 73,属于碱性系列。稀土元素配分曲线为富集LREE的右倾型。另外,富集大离子亲石元素(LILE) Th、U,亏损高场强元素(HFSE) Nb、Ta等特征,均说明羌北-昌都地块阿布日阿加措地区的火山岩形成于陆缘岛弧环境。     

安徽庐枞盆地酸性蚀变岩帽地质地球化学特征研究

酸性蚀变岩帽是浅成低温热液系统演化的产物,形成于酸性高氧化性流体的化学条件下;在高硫化型浅成低温热液金矿床中广泛发育,是该类矿床的显著识别特征。通过对酸性蚀变岩帽的野外地质特征、矿物共生组合和地球化学特征研究,能较好阐明浅成低温成矿热液系统的特征、性质、发生和发展演化及成矿作用过程。庐枞矿集区是长江中下游成矿带重要的矿集区之一,盆地内广泛发育以明矾石为特征蚀变矿物的酸性蚀变岩帽,面积超过30km~2,指示盆地内高硫化浅成低温热液系统的存在。目前为止,前期工作主要针对明矾石矿床地质特征和明矾石资源储量进行,该酸性蚀变岩帽的地质地球化学特征研究尚未开展。本次工作通过对酸性蚀变岩帽系统的野外采样、全岩地球化学分析和短波红外光谱测试分析技术(PNIRS测试)分析,确定其主要赋存在砖桥组火山岩中,组成矿物为石英、明矾石、高岭石、地开石,此外有少量绢云母、伊利石、珍珠陶土、叶蜡石、褐铁矿,极少数的叶腊石和黄钾铁矾等,在钻孔深部存在浸染状和半自形粒状黄铁矿。由于受到地表风化剥蚀和不同热热中心的影响,水平方向从矾山明矾石矿床向外围发育石英+明矾石带、石英+高岭石/地开石+明矾石带、石英+高岭石/地开石带、硅化带以及最外围的泥质带即高岭石±绢云母±伊利石带。根据酸性蚀变岩帽的矿物组合和主量元素特征,可将其分为三类:硅质蚀变岩、明矾石蚀变岩和粘土蚀变岩。硅质蚀变岩中SiO_2含量发生明显的富集作用,其余主量元素(K_2O、Na_2O、Al_2O_3、Fe_2O_3、P_2O5)含量显著降低;明矾石蚀变岩和粘土蚀变岩具有相似的地球化学特征,SiO_2、Al_2O_3、Fe_2O_3、P_2O_5元素含量范围变大,K_2O和Na_2O含量降低,且Na_2O降低更加明显;而钛为不活泼元素,在岩石发生蚀变过程中TiO_2含量变化很小。矾山地区的酸性蚀变岩帽的产状、蚀变类型、地球化学特征受构造和地层的双重控制。     

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.     

Paleozoic Amphibolites,Kreuzeck Mountains,Austria: Geochemical variations in the vicinity of mineralization

Strata-bound antimony, mercury, tungsten and massive sulfide mineralization occurs within a metamorphosed Paleozoic volcanosedimentary sequence in the Kreuzeck Mountains, Austria. Amphibolites are a significant constituent of that sequence; on the basis of stable trace element data, they can be classified as metamorphic equivalents of recent olivine tholeiites. In the ore environment of strata-bound mineralization, intensive alteration associated with submarine hydrothermal activity has affected the wall rocks. This includes depletion in SiO₂, a change in the oxidation stage of iron and pronounced enrichment in Na, Sr, Ba and CO₂. Microprobe analyses reveal lower MgO content in chlorites and higher Na₂O content in feldspars in rocks from the ore environment. Hydrothermal halos are thus documented by changes in mineral and whole-rock compositions and represent guides to exploration.     

SECONDARY DISPERSION HALOS OF A COPPER-COBALT DEPOSIT IN THE SOUTHERN URALS

Landscape-geochemical analysis of the expressions of blind ore bodies, in the semi-arid grassland environments of the Southern Urals, shows a depression of the migration capacities of copper, nickel, and cobalt and a relative weakness of the corresponding secondary dispersion halos in the geochemical profiles. Such halos are derived from the ore bodies' mineralized host rocks, however, rather than from the ore bodies themselves, in ortho-eluvial and trans-ortho-eluvial landscapes, and, as such, have a certain prospecting usefulness, under the circumstances, particularly where they lie in immature soils or in certain horizons of thin chernozems typical of the region. On the other hand, geochemical prospecting in neo-eluvial landscapes of the area is definitely not recommended. Mechanical dispersion halos and trains are the principal forms of the metals' migration in the region but they too represent mainly the anomalous disseminated mineralization in the host rocks, right next to the ore bodies themselves.--V. P. Sokoloff.     

A comparative study of stream water and stream sediment as geochemical exploration media in the Rio Tanama porphyry copper district,Puerto Rico

To test the relative effectiveness of stream water and sediment as geochemical exploration media in the Rio Tanama porphyry copper district of Puerto Rico, we collected and subsequently analyzed samples of water and sediment from 29 sites in the rivers and tributaries of the district. Copper, Mo, Pb, Zn, SO₄^2?, and pH were determined in the waters; Cu, Mo, Pb, and Zn were determined in the sediments. In addition, copper in five partial extractions from the sediments was determined. Geochemical contrast (anomaly-to-background quotient) was the principal criterion by which the effectiveness of the two media and the five extractions were judged.Among the distribution patterns of metals in stream water, that of copper most clearly delineates the known porphyry copper deposits and yields the longest discernable dispersion train. The distribution patterns of Mo, Pb, and Zn in water show little relationship to the known mineralization. The distribution of SO₄^2? in water delineates the copper deposits and also the more extensive pyrite alteration in the district; its recognizable downstream dispersion train is substantially longer than those of the metals, either in water or sediment. Low pH values in small tributaries delineate areas of known sulfide mineralization.The distribution patterns of copper in sediments clearly delineate the known deposits, and the dispersion trains are longer than those of copper in water.The partial determinations of copper related to secondary iron and manganese oxides yield the strongest geochemical contrasts and longest recognizable dispersion trains. Significantly high concentrations of molybdenum in sediments were found at only three sites, all within one-half km downstream of the known copper deposits. The distribution patterns of lead and zinc in sediments are clearly related to the known primary lead-zinc haloes around the copper deposits. The recognizable downstream dispersion trains of lead and zinc are shorter than those of copper.     

新疆东天山白山钼矿深部岩体地球化学特征及成因意义

白山钼矿深部钻探表明矿体下部存在着矿化花岗斑岩体,所获岩芯显示其岩性主要为似斑状钾长花岗岩、黑云母斜长花岗岩和花岗斑岩.该岩体具有高Al、Na和Sr,低Mg、Y和Yb,以及高Sr/Y和La/Yb比值等特点,类似于埃达克质岩的地球化学特征,暗示来源于较深的含石榴子石的源区;岩石形成时代和地球化学构造判别图解表明,岩体形成于造山后的构造环境.因此,白山钼矿岩体可能为加厚下地壳熔融的产物.白山钼矿床的成矿作用可能与埃达克质岩浆活动有关,其成矿过程得益于高Mo丰度的地壳源区、成岩过程中提供流体和岩浆高氧逸度环境等.     

早子沟金矿成矿过程中的元素迁移规律及金富集机制探讨

早子沟金矿位于秦岭造山带西段夏河—合作断裂带上,成矿作用与断裂构造关系密切,蚀变带受断裂构造控制,本文通过对矿区蚀变带内常量元素的迁移规律研究,通过计算得到矿区明显带出的组分为SiO2、Al2O3、K2O、MnO,而Fe2O3、Na2O则明显带入,探讨了元素迁移的性状及其与蚀变类型的关系,并对矿床金富集机制做了简要分析。     

胶东新立构造蚀变岩型金矿床元素地球化学行为

新立金矿位于华北克拉通东部胶东金矿省内,是一个典型的蚀变岩型(焦家型)金矿床,矿床受区域三山岛-仓上断裂带控制,产出于蚀变的玲珑花岗岩中。在主断裂带下盘蚀变矿化发育,可以划分为五个阶段:钾长石化阶段、绢英岩化阶段、黄铁绢英岩矿化阶段、多金属硫化物矿化阶段和碳酸盐化阶段。不同的蚀变矿化过程导致各阶段样品具有不同的矿物组合和地球化学特征。钾长石化花岗岩以高钾钠含量为特征,因为样品中含有新生成的二次钾长石及残留的钠长石;绢英岩的钙钠含量明显低于钾长石化花岗岩,是绢英岩化阶段斜长石蚀变分解所致;黄铁绢英岩FeT2O3含量普遍较高且SiO 2含量波动明显,与该阶段样品中大量黄铁矿的产出及部分样品中包含石英团块或石英脉相一致;多金属硫化物矿化样品的元素含量与黄铁绢英岩基本类似;碳酸盐化蚀变样品以高CaO含量为特征,与该阶段大量沉淀析出的方解石相符。在原始地幔标准化微量元素配分图上,不同阶段矿化蚀变样品,大离子亲石元素Rb、Ba、K、Sr差异性明显,而高场强元素Zr、Hf、Nb、Ta、P差异性较小,表明热液蚀变过程中大离子亲石元素活动性较强,高场强元素相对稳定。在稀土元素球粒陨石标准化图中,不同阶段样品均呈明显的右倾性,蚀变早期(钾长石化阶段)样品铕异常不明显,蚀变矿化主期(绢英岩化阶段至多金属硫化物矿化阶段)样品普遍具有负铕异常,而蚀变晚期(碳酸盐化阶段)样品普遍具有正铕异常。元素相关性分析表明,金与银、砷、铋和硫具明显正相关,与铜、铅具有一定的相关性。成矿元素因子分析也显示出金与砷、硫、铁、钴在矿化过程中具有类似的地球化学行为。实测剖面上不同位置样品元素地球化学特征的差异性主要受构造及蚀变矿化类型、强度控制,成矿元素在主断裂附近达到峰值,在远离主断裂方向上,其含量整体呈下降趋势,但在次级矿化断裂发育地段,含量又明显回升;另外在断裂附近由于微裂隙及节理相对发育,有利于热液的渗透,蚀变较强,对成矿元素的空间分布亦具有控制作用。     

内蒙古贺根山缝合带后造山作用——满克头鄂博组火山岩锆石U-Pb年龄和地球化学制约

以出露于贺根山缝合带梅劳特乌拉蛇绿岩中的白音瑞满克头鄂博组火山岩为研究对象,通过野外地质调查、岩石学、地球化学和LA-ICP-MS锆石U-Pb年代学研究,探讨火山岩成因、构造环境与贺根山缝合带后造山作用。岩石地球化学研究表明,白音瑞地区满克头鄂博组火山岩主要为流纹岩,岩石具有较高SiO_2、K_2O和Na_2O+K_2O含量,以及较高Ga/Al值,相对贫CaO、MgO、Sr、Ba、Eu、Ti和P的特征。稀土元素含量较低,配分曲线为微弱右倾的海鸥式分布。岩石学和地球化学特征表明,该区满克头鄂博组流纹岩为A型流纹岩,形成于后造山伸展构造环境,为后造山A型花岗岩浆作用的产物。LA-ICP-MS锆石U-Pb同位素定年结果显示,该流纹岩的形成年龄为158.0±0.7Ma,表明满克头鄂博组流纹岩喷发于晚侏罗世,反映了贺根山缝合带晚侏罗世后造山A型花岗岩浆作用事件。结合二连-贺根山缝合带石炭纪蛇绿岩、石炭纪—二叠纪岛弧岩浆岩和中三叠世—早白垩世后造山A型岩浆岩的时空分布与演化关系,认为二连-贺根山缝合带在中三叠世—早白垩世经历了后造山伸展作用演化过程。     

安徽马头斑岩型钼铜矿床蚀变带常量元素迁移规律及其定量计算

文章对安徽马头斑岩型钼铜矿床花岗闪长斑岩体和粉砂岩在矿化蚀变过程中常量元素的迁移规律进行了探讨。马头矿床元素质量迁移计算结果表明,在钼铜矿化蚀变过程中常量元素均发生不同程度的带入带出,迁移规律明显。在花岗闪长斑岩中,从钾化带、绢英岩化带到青磐岩化带,均为带入的常量元素有K₂O、CaO、SiO₂、H₂O⁺,而Na₂O明显带出,说明矿化与去钠化、钾长石化等蚀变密切相关。在粉砂岩中,蚀变岩明显带出的有Na₂O、CaO、MgO、Fe₂O₃、H₂O⁺等,而且黄铁辉钼矿化粉砂岩在常量元素的迁移程度远比青磐岩化粉砂岩大。马头矿床成矿系统质量变化定量计算结果表明,花岗闪长斑岩和粉砂岩系统的质量变化整体上表现为净带入与净带出,且净带出量大于净带入量,因此马头矿床成矿系统总体上是净带出的,即马头矿床钼铜矿化产出在元素总体负异常体系中。该项研究成果为揭示矿化蚀变过程中元素的作用机制提供了线索,并且可能成为深部成矿潜力定量评价的有效途径之一。     

Early Paleozoic magmatism and metallogeny related to Proto-Tethys subduction: Insights from volcanic rocks in the northeastern Altyn Mountains,NW China

Remnants of the Proto-Tethys are mainly preserved in the region between south of the North China-Tarim Block and north of Qiangtang-Sibumasu/Baoshan Blocks. Magmatic-metallogenic events related to the Proto-Tethyan subductions were rarely reported, and the subduction history and polarity of the Proto-Tethyan are still under debate. Here, we presented new data of zircon UPb ages, whole-rock Sr–Nd–Pb isotopes, major and trace elements and zircon Hf isotopes for the volcanic rocks in the northeastern Altyn Mountains. Information of over 14 volcanic-hosted deposits/prospects in the region has been compiled. These volcanic ore hosts consist mainly of basaltic andesite, andesite, dacite and rhyolite rocks. The andesite and rhyolite rocks are newly zircon UPb dated to be Late Cambrian-Early Ordovician (andesite: 490.5 ± 5.2 Ma; rhyolite: 492.6 ± 2.9 Ma and 491.6 ± 5.6 Ma), representing the timing of volcanism and VMS (Volcanogenic Massive Sulfide) mineralization. All the volcanic rocks belong to the high-K calc-alkaline and shoshonite series: the andesite rocks from the Kaladawan area in north of the region display arc geochemical affinities and contain (⁸⁷Sr/⁸⁶Sr)_i (0.7082–0.7083) and ε_Nd(t) (−9.7 to −7.6), indicating that they were likely formed by partial melting of the mantle wedge with subducted sediment inputs. The rhyolite rocks from the Kaladaban area in south of the region are characterized by high SiO₂ (64.46–78.55 wt%), low alkali (Na₂O + K₂O, 3.46–7.17 wt%), and contain (⁸⁷Sr/⁸⁶Sr)_i (0.7063–0.7095), ε_Nd(t) (−6.6 to −1.5), and zircon ε_Hf(t) (−5.5 to 5.4), indicating that they were likely derived from partial melting of the lower crust with depleted mantle inputs. Rock assemblage and geochemistry suggest that volcanic rocks in the northeastern Altyn Mountains may have formed in a continental arc setting. Their spatial distributions with respect to the ophiolites in the region suggest that the subduction was likely south-dipping. This subduction-related arc magmatism may have formed the many important VMS and porphyry–skarn deposits in the region.