Middle Proterozoic Vein-Hosted Gold Deposits in the Pontes e Lacerda Region,Southwestern Amazonian Craton,Brazil

Structural, geochemical, and isotope studies were carried out on the gold deposits of the Pontes e Lacerda region (Mato Grosso state, Brazil), where rocks of the Aguapei and Rondoniano mobile belts (southwestern Amazonian craton) occur. The orebodies are hosted in metavolcanic, gneiss-granite, quartzite, tonalite, and granite units. Tectonics involve oblique overthrusting (from northeast to southwest), which led to the formation of recumbent folds and thrusts (pathways for the mineralizing fluids), upright folds, and faults with dominant strike-slip component. These unconformities represent potential sites for mineralization. During geological mapping, it was observed that the orebodies consist of quartz, pyrite, and gold, and that the hydrothermal alteration zone contains quartz, sericite, pyrite (altered to limonite), and magnetite (altered to hematite). Chalcopyrite, galena, and sphalerite occur only in the Onca deposit. Chemical analysis of sulfides indicates high contents of Bi, Se, and Te in sulfides and gold, suggesting plutonic involvement in the origin of hydrothermal solutions.

K-Ar dating of hydrothermal sericites from gold veins yielded ages in the range from 960 to 840 Ma, which may indicate the age of original crystallization of sericite. Pb-Pb dating in galenas yielded model ages in the range from 1000 to 800 Ma for the Onca deposit, which is in agreement with K-Ar ages. Pb-isotopic ratios indicate high U/Pb and low Th/Pb for the upper-crustal Pb source before incorporation in galena crystals. The Pontes e Lacerda gold deposits yielded ages correlated to the Aguapei event and probably were formed during a Proterozoic contractional tectonic period in the southwestern part of the Amazon craton, which may characterize an important metallogenic epoch in the Pontes e Lacerda region.     

K-Ar Ages of Tin-Polymetallic Mineralization in the Oruro Mining District, Central Bolivian Tin Belt

Abstract. K-Ar age determinations were carried out on vein- and rock-forming minerals from five vein-type tin-polymetallic ore deposits of the Oruro mining district in the central part of the Bolivian tin belt. The sericite from vein selvedges and an altered host rock provides good estimates of the ages of hypogene mineralization, and supergene alunite and jarosite provide ages for erosional and weathering episodes. It is concluded that hypogene mineralization in the Oruro mining district took place during the early to middle Miocene: 15.8±0.8 Ma at San José, 20.1±l.l Ma at Morococala, 20.5±1.0 Ma at Avicaya, and 19.6±1.0 Ma at Llallagua. Fine grained supergene alunite (δ³⁴S = -10.1 960) and jarosite yield K-Ar ages of 6.7±0.7 Ma at Avicaya and 3.9±0.7 Ma at Bolivar, respectively, suggesting that erosion and chemical weathering were active at those times.     

K-Ar Ages of Granitic Magmatism and Related Pegmatite Formation at the Umanotani-Shiroyama Mine, Shimane Prefecture, SW Japan and Their Bearings on Cooling History

Abstract. The Umanotani-Shiroyama pegmatite deposits, the largest producer of K-feldspar and quartz in Japan, are of typical granitic pegmatite. Ilmenite-series biotite granite and granite porphyry, hosting the ore deposits, and biotites separated from these rocks yielded K-Ar ages ranging from 89.0 to 81.4 Ma and 95.2 to 93.7 Ma, respectively. Muscovite and K-feldspar separated from the ore zone yielded K-Ar ages with the range of 96.2 to 93.1 Ma and 87.3 to 80.7 Ma, respectively. Muscovites from quartz-muscovite veins in the ore zone and in the granite porphyry yielded K-Ar ages of 90.4 and 76.3 Ma, respectively. K-feldspar is much younger in age than coexisting muscovite. It is noted that the K-Ar ages of biotite separates and the whole-rock ages are identical to those of muscovite and K-feldspar in the ore zone, respectively. These time relations, as well as field occurrence, indicate that the formation of the pegmatite deposits at the Umanotani-Shiroyama mine is closely related in space and time to a series of granitic magmatism of ilmenite-series nature. Using closure temperatures of the K-Ar system for biotite and K-feldspar (microcline), cooling rate of the pegmatite deposits is estimated to be about 82C/m.y. at the beginning, but slowed down to about 15C/m.y. in the later period.     

江西大吉山钨矿成矿年代学研究

大吉山钨矿床是一个大型的石英脉型钨矿床。矿床在空间上与燕山早期复式花岗岩相伴。利用云母单矿物的K Ar同位素年龄测试方法,测定了矿床有关的花岗岩及矿脉的同位素年龄。结果表明,黑云母花岗岩中黑云母K Ar年龄为(160 3±3 03)Ma和(164 2±3 5)Ma,二云母花岗岩中白云母K Ar年龄为(160 6±2 8)Ma,含钨石英脉中白云母K Ar年龄为(152 6±2 35)Ma和(158 1±2 8)Ma。钨成矿与花岗岩的成岩基本不存在时差,推测含钨石英脉的形成可能与岩浆分异出的富含矿化剂和成矿元素的流体有关。     

南岭地区油山岩体和坪田岩体形成年龄及其地质意义

南岭地区的油山岩体和坪田岩体位于诸广山复式岩体东侧,以前的研究认为它们都属于燕山期花岗岩。对油山岩体和坪田岩体进行的LA-ICP-MS 锆石U-Pb 年代学研究结果表明,油山岩体的成岩年龄为（213.4±3.0） Ma,坪田岩体（西北部）的成岩年龄为（238.8±2.2） Ma,因此油山岩体应属于印支期岩体,而坪田岩体可能是一个印支-燕山早期复式岩体,而非前人所划定的燕山期岩体。此外,两个岩体内含有太古代和541 Ma~1 642 Ma 的继承锆石,暗示物源区曾存在太古代和元古代地壳,经历过多期次的岩浆作用。部分油山岩体锆石在CL图像上表现出具有暗色边部的特征,这些暗色边可能与后期热液作用影响有关,其时代为燕山期,表明油山岩体在燕山期受到了流体作用的影响,坪田岩体锆石也存在较窄的暗色边,也是受后期热液作用影响所造成。     

冈底斯带门巴区晚白垩世花岗岩年代学、地球化学及构造背景

门巴区晚白垩世花岗岩主要分布于冈底斯中段的扎雪、金达和桑巴附近,由黑云母钾长花岗岩、钾长花岗岩和二云母钾长花岗岩及花岗闪长岩等组成。其中花岗闪长岩SHRIMP 锆石U--Pb 年龄为68. 8 ± 1. 6 Ma; 钾长花岗岩黑云母K--Ar 年龄为78. 2 ± 1. 2 Ma 和81. 5 ± 1. 4 Ma; 黑云母钾长花岗岩和二云母钾长花岗岩的黑云母K--Ar 分别为90. 8 ± 1. 81 Ma 和91. 2 ± 1. 8 Ma。岩石学和地球化学分析结果表明,S 型和低Sr 低Yb 特征的黑云母钾长花岗岩和二云母钾长花岗岩形成于拉萨地块与羌塘地块碰撞造山过程中的地壳加厚背景; S 型和总体低Sr、高Yb 特征的钾长花岗岩形成于造山带晚期阶段的伸展背景; I 型花岗闪长岩应是新特提斯洋俯冲作用的结果,形成于岛弧构造环境。     

Petrographic, geochemical and geochronological investigation on granitic pebbles from Permotriassic metasediments of the Tisia terrain (eastern Papuk, Croatia)

Granitic pebbles occurring in the Permotriassic metasedimentary sequence of eastern Papuk, Slavonian Mountains, Croatia, were recognized to represent a coherent group of felsic, muscovite-albite metagranites. Fabrics, modal compositions and geochemical data imply that the rocks are derivatives of S-type granites formed through a combination of igneous and subsequent metasomatic processes. A Variscan formation age is demonstrated by K-Ar dating on coarse muscovite (range of 329?C317?Ma) as well as by electron microprobe based Th-U-Pb monazite dating (338?±?15?Ma). Additionally to the Variscan metasomatic processes of albitization and greisenisation, which led to an almost complete replacement of K-feldspar and biotite by albite and coarse muscovite, pebbles were affected by a younger phase of alteration resulting in the formation of a fine-grained sericitic matrix. The fine sericite yields K-Ar ages of 91?C83?Ma. A substantial reheating of the rocks during the Cretaceous is also indicated by the growth of new monazite dated at 106?±?10?Ma. Yttrium-contents of the Cretaceous monazite from the granite pebbles (0.3?C0.9?wt% Y₂O₃) are compatible with metamorphic temperatures of ~350?C400°C. These data confirm recent concepts according to which large parts of the Slavonian Mountains received a pervasive Cretaceous low-T regional metamorphic overprint. Furthermore, the pebbles provide useful information on the nature of the eroded Variscan crust of the Tisia Terrain, which has obviously contained considerable amounts of evolved high-level S-type granites modified through albitization and greisenization.     

Timing of the cataclastic deformation along the Akaishi Tectonic Line,central Japan

This paper describes K-Ar ages of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. Petrological and mineralogical features of these rocks are also examined. Based on the results, we test the hypothesis that the K-Ar ages of these rocks represent the age of hydrothermal alteration associated with the fault movement. Intensity of deformation and alteration increase passing from host rock into cataclasite and finally into fault gouge. This increase corresponds to an increase of the value of crystallinity index (Kübler index) of the micaceous minerals contained in these rocks. Furthermore, the degree of rejuvention of K-Ar ages of the micaceous minerals increases in the same sense. A correlation of K-Ar ages (plotted on the y axis) versus Kübler index (plotted on the x axis) yields a concave curve asymptotically parallel with the x axis at approximately 15 Ma. This curve is interpreted to represent the mode of decrease of the relative amounts of inherited argon in the K-Ar system of the micaceous minerals, corresponding to an increase in the intensity of deformation and alteration. Inclination of the curve becomes zero if all the inherited argon is lost from the K-Ar system of the micaceous minerals. Thus, it is concluded that the hydrothermal alteration occurred at approximately 15 Ma. The strike-slip basin along the ATL formed the middle Miocene. The K-Ar dating of ATL gouges indicates that the ATL was active simultaneously with formation of the strike-slip basin along its trace during the middle Miocene.     

The age of the Kagenfels granite (northern Vosges) and its bearing on the intrusion scheme of late Variscan granitoids

In the Saxothuringian part of the Vosges (France), a first series of Variscan plutonic rocks (diorites to granites) has been intruded by several younger granites. Rocks of both the older generations have been cross-cut by the late orogenic Kagenfels granite. The averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 Ma, respectively, whereas Rb-Sr and K-Ar dates around 290 Ma have been reported for the Kagenfels granite. Because of the unlikely large age hiatus, a redetermination of the intrusion age of the Kagenfels granite formation appeared to be irrevocable. The newly obtained mineral ages on the Kagenfels granite (K-Ar and ⁴⁰Ar/³⁹Ar biotite ages as well as single zircon radiogenic ²⁰⁷Pb/²⁰⁶Pb data: 331 ± 5 Ma) are about 40 Ma older than the previous results. They are interpreted as giving the time of emplacement of the Kagenfels granite during the latest Visan. The mineral ages of the earlier plutonic rocks in this part of the Variscan Orogeny in all probability are not significantly different from their ages of intrusion. Therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the Lower-Upper Carboniferous boundary.     

航空高光谱CASI-SASI数据蚀变矿物信息提取与应用 ——以甘肃省敦煌市小金窝子地区为例

航空高光谱遥感数据具有“空谱合一”的优势,深入挖掘并应用高光谱数据中蕴含的蚀变矿物地质信息具有重要意义。本文应用航空高光谱CASI-SASI数据,建立了基于“整体谱形-特征参量协同”的蚀变矿物信息提取流程,并在甘肃省敦煌市小金窝子地区开展了蚀变矿物信息提取、地质涵义分析和野外查证。基于高光谱CASI-SASI数据成功提取了褐铁矿、绿泥石、绿帘石、短波云母、中短波云母、中长波云母、长波云母共7种蚀变矿物。高光谱CASI-SASI数据提取的大面积区域性分布的蚀变矿物多为区域热液作用产物,小范围定向分布的蚀变矿物与断裂、岩脉等成矿热液活动关系密切,局部产出且具定向性的中短波云母与断裂构造及酸性脉体关系密切,基于野外地质调查和ASD光谱实测的蚀变矿物野外查证工作证实蚀变矿物提取结果可信。褐铁矿+中短波云母的蚀变矿物组合具有金矿化指示意义,为基础地质调查和矿产资源勘查提供了重要的遥感信息。     

江苏省东海县片麻状碱性花岗岩的成因及年代学研究

江苏省东海地区位于苏鲁造山带的西南缘,区内广泛出露片麻状碱性花岗岩.本文对东海地区磨山、虎山和房山片麻状碱性花岗岩的产状、岩石结构构造、碱性铁镁矿物、元素地球化学和锆石成因及SHRIMP定年等综合研究结果表明：该类岩石具有高硅、富碱、富铁贫镁和含碱性铁镁矿物等特征,说明其为碱性花岗岩;岩体还保留有沉积岩特有的层理（走向北东、倾向南东等一致的产状）,长石等造岩矿物显示在固相条件下结晶的特征,以及继承锫石大部分是不同时代的岩浆或变质锆石碎屑等,均证实其原岩是沉积岩.SHRIMP定年对变质锆石的边部获得两组₂₀₆Pb-₂₃₈U加权平均年龄,分别为224.7±9.4 Ma和209.4±2.5～212±3.5Ma,反映了东海片麻状碱性花岗岩成岩时间属于印支期,相当于晚三叠世,变质锆石的核部₂₀₆Pb-₂₃₈U年龄变化于866～248 Ma,反映原岩时代不会早于晚古生代.     

北淮阳新开岭地区花岗岩锆石U-Pb年龄和氧同位素组成

对大别造山带北麓的北淮阳新开岭地区岩浆岩进行了锆石阴极发光显微结构观察和SHRIMP法锆石微区UPb定年.在锆石阴极发光图像中, 一个花岗岩样品中的大部分锆石颗粒具有明显的初始岩浆振荡环带, 为典型的岩浆锆石, 少有蚀变的颗粒和/或区域; 而另一个花岗岩样品中的锆石虽然同样具有振荡环带, 但是大部分颗粒中心的初始岩浆环带被扰动, 指示这些锆石为岩浆锆石, 受到了较强的后期热液蚀变的改造.对锆石具有初始岩浆环带和溶蚀结构的区域分别进行SHRIMP法UPb微区定年结果表明, 这些岩浆岩的形成年龄为(820±4) Ma, 热液蚀变作用发生的时间为(780±4) Ma.新开岭地区新元古代花岗质岩石的形成和后期超固相热液蚀变作用分别对应于超大陆裂解之前的约830~795Ma岩浆活动和裂解过程中约780~745Ma的岩浆作用.单矿物激光氟化氧同位素分析结果表明, 这些岩浆岩具有非常低的δ18O值, 其中锆石为1.90‰~5.78‰, 石英为-2.88‰~-7.67‰, 斜长石为-4.01‰~-11.40‰.锆石和其他矿物之间表现出强烈的氧同位素不平衡, 而其他矿物之间则达到了氧同位素的再平衡.结合不同δ18O值锆石的内部结构特征, 认为该地区的热液蚀变作用为超固相条件下的高温热液蚀变.这一过程不但改变了石英等矿物的氧同位素组成, 同时也不同程度地改变了锆石的氧同位素组成, 所以这些样品中低δ18O值锆石可能是超固相条件下热液蚀变的结果.石英中具有异常低的δ18O值表明蚀变流体来源应为寒冷气候大气降水.所以, 新开岭地区亏损18O蚀变岩石的形成与裂谷岩浆作用和雪球地球事件相耦合的高温大气降水热液蚀变有关.      

Kübler Index and K-Ar Ages of Illite in the Yinshan Polymetallic Deposit, Jiangxi Province, South China: Analyses and Implications

Abstract. The Yinshan polymetallic deposit is a hydrothermal vein-type deposit closely related to Late Jurassic felsic-inter-mediate volcanic-subvolcanic activity in Jiangxi Province, South China. Illite is a major alteration mineral observed in the deposit. Our study shows that the Kübler index of the illite has a close relation to ore-forming fluids of different stages of hydrothermal alteration and mineralization. The early Pb-Zn-Ag mineralization dated at 130–136 Ma is characterized by relatively low water/rock ratios and diffusive fluid movement within phyllite, whereas the later Cu-Au-S mineralization at 122 -125 Ma was accompanied by higher water/rock ratios and localized fluid flow through fractures and channels. Illite formed in the early Pb-Zn-Ag mineralization stage contains swelling layers while the illite formed in the later Cu-Au-S mineralization stage has no swelling layers but was associated with intensive chloritization. The last stage of mineralization (at 104 Ma) was minor and did not produce significant amounts of illite.     

三江盆地绥滨断陷基底花岗岩的锆石U-Pb年代学及其构造意义

根据三江盆地绥滨断陷滨参1井基底花岗岩锆石LA-ICP-MS U-Pb定年结果,探讨了盆地基底的性质。滨参1井基底花岗岩中的锆石呈自形-半自形晶,并具有典型的岩浆生长环带;锆石的Th/U值为0.22~0.82,表明其岩浆成因。花岗岩中锆石22个测点的²⁰⁶Pb/²³⁸U年龄值为427~502 Ma,其加权平均年龄分为2组,分别为（430±8） Ma和（496±6） Ma。（430±8） Ma的年龄代表了三江盆地绥滨断陷基底花岗岩的形成时代为早志留世晚期;而（496±6） Ma可能是花岗岩浆形成或侵位过程中的捕获锆石年龄,并且它与佳木斯地块麻山群和泛非期片麻状花岗岩的形成时间一致。上述结果表明：三江盆地绥滨断陷基底中存在加里东期花岗岩,三江盆地西部基底与佳木斯地块的属性相同。     

Dating Magmatic Hornblende and Biotite and Hydrothermal Sericite by Laser Probe Technique：Constraints on Genesis of Wangershan Gold Deposit, Eastern Shandong Province, China

INTRODUCTIONEasternShandongProvincerepresentsthelargestgoldconcentrationregioninChina ,withannualproduc tion >5 5 - 6 0tgoldandpresentreserveofmorethan90 0tgold ,accountingforaquarterofthetotalgoldproductioninthecountry .Amajorityofthegoldde positsishostedintheJurassic Cretaceousgranitoidin trusions .TimingofgoldmineralizationineasternShan donghasbeenamajorfocusofmanypreviousstudies .K Ar,Rb SrandconventionalU Pbisotopicdatinginthelasttwodecadeshaveyieldedawiderangeofminer alizationag…     

冈底斯带东段鲁朗—墨脱地区中新世花岗岩 的地球化学、年代学及成因

冈底斯东段的鲁朗-色季拉和墨脱-崩崩拉一带花岗岩的岩石类型主要为二长花岗岩、黑云母花岗岩、花岗闪长岩、石英闪长岩等.墨脱花岗岩的K-Ar年龄为19～22Ma;鲁朗花岗岩的40Ar-39Ar年龄为14～18Ma.岩石地球化学研究结果表明,本区花岗岩主要属于高钾钙碱性系列和钙碱性系列,同时具有某些埃达克岩的特征,表现为高SiO2(65.60%～76.40%)、Al2O3(12.32%～17.23%)、Sr/Y(2.41～86.46)、(La/Yb)n(6.65～56.14)比值,低Y(4.23×10-6～39.40×10-6)等特点.呈典型的LREE和LILE富集型分配模式,Eu为正到弱负异常.本区中新世花岗岩主要来源于中下陆壳的硅铝质成分和镁铁质成分的重熔,不同于具埃达克岩成分的冈底斯中新世含矿花岗斑岩.以中新世花岗岩侵位为标志,东喜马拉雅构造结地区的初始伸展可能在22Ma左右,早于冈底斯中段(20Ma左右).     

Authigenic sericite record of a fossil geothermal system: the Offenburg trough,central Black Forest,Germany

A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K–Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of ¹⁸O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150–210 °C. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.     

诸广山南体桃金洞花岗岩成因和铀成矿潜力探讨

文中对位于湘赣粤三省交界处的诸广山南体桃金洞花岗岩进行了锆石U-Pb年代学和岩石地球化学的研究,并将其与诸广山南体东部其他印支期非产铀和产铀花岗岩进行了对比。LA-ICP-MS锆石U-Pb年龄为204±2.1 Ma,为印支晚期岩浆活动的产物。岩石地球化学组成呈过铝质,硅和碱含量偏高(Si O_2=69.7%~75.0%,K_2O+Na_2O=7.74%~9.08%),富铁、贫镁,属于碱钙性/钙碱性-过铝质-铁质花岗岩。稀土元素总量较高(∑REE=226~272×10~(-6)),LREE富集(LREE/HREE=6.27~11.4,(La/Yb)_N=4.01~15.0),Eu亏损较明显(δEu=0.15~0.42),富集Rb、Th和U,亏损Ba、Sr、Ti和Eu,属于典型的低Ba、Sr花岗岩;(~(87)Sr/~(86)Sr)i值较高(0.71922~0.72040),εNd(t)值较低(-10.0~-10.2),两阶段Nd模式年龄为1.80~1.82 Ga。上述特征表明,桃金洞花岗岩属于典型的壳源型花岗岩,是在地壳伸展-减薄构造背景下,由古元古代地壳岩石演变而成的变质杂砂岩组分岩石经中低程度部分熔融形成。对比研究显示,诸广山南体印支期产铀花岗岩蚀变作用强,FeO~T/(Fe O~T+MgO)比值变化较大,CaO含量低,主要为碱钙性花岗岩,Ba、Sr、Ti和Eu亏损更强烈,ε_(Nd)(t)值更低和Nd模式年龄更古老。非产铀花岗岩源岩以砂质岩为主,U含量相对较低。桃金洞花岗岩未经后期明显热液蚀变作用,不具有产铀花岗岩蚀变强烈的特点,地球化学特征相似于诸广山南体印支期非产铀花岗岩,铀成矿潜力可能不大。     

Rb−Sr,K−Ar and fission track ages for granites from Penang Island,West Malaysia: an interpretation model for Rb−Sr whole-rock and for actual and experimental mica data

Penang Island represents the northwestern extension of the western magmatic belt of Peninsular Malaysia. Thirty-one samples of highly evolved biotite-and biotite-muscovite granites were used in an integrated study to unravel the complex magmatic, tectonic and cooling histories of these rocks. Highly distorted Rb–Sr whole-rock age patterns are evident. These are attributed to the partial post-magmatic Sr homogenization within the granite batholith which led to the rotation of isochrons towards younger ages and higher ^(87/86)Sr intercepts. The recognition of this mechanism allowed the establishment of a new Rb–Sr interpretation model. The intrusion ages of the granites can be extrapolated based on the evolutionary trend of the initial ^(87/86)Sr. Including the data of Bignell and Snelling, three episodes of granite emplacement at 307±8 Ma, 251±7 Ma and 211±2 Ma are suggested for Penang and the NW Main Range. The late-Triassic intrusive induced a hydrothermal conductive convection system which affected all the granites. It is considered to be responsible for the Rb–Sr whole-rock age distortion, the Rb–Sr and K–Ar biotite age resetting and the textural and mineralogical changes in the granites. The duration of the hydrothermal convections, deduced from the Rb–Sr whole rock ages, is about 6 Ma and 20 Ma in the northern and southern parts of Penang respectively. Fast regional cooling to 350±50°C within a time span of 1–3 Ma is recognized for the late-Triassic Feringgi intrusive from the mica ages, followed by a generally slow cooling rate of about 1°C/Ma. Fission track ages, in addition, indicate blockwise uplift along the N-S and NW-SE tending faults, thus resulting in the exposure of deeper crustal levels in southern and eastern Penang. A change in the tensional regime since Oligocene/Miocene, accompanied by a southwest tilting of the island, is indicated by the fission track apatite ages. Variable sometimes younger K–Ar, respectively Rb–Sr biotite ages mainly depend on the degree of hydrothermal overprint at different crustal levels. An increase of the reaction surface by grain size reduction influences Rb–Sr and K–Ar mica ages in similar ways, as has been demonstrated by experimental data.     

Petrology and alteration geochemistry of the epithermal Balya Pb-Zn-Ag deposit,NW Turkey

The 1.5 km-large hydrothermal system of Balya is characterized by three alteration styles which from the outer halo towards the center are: (i) propylitic alteration with the hydrothermal mineral assemblage of calcite-daphnite-albite-epidote-quartz-pyrite; (ii) argillic/phyllic alteration with the hydrothermal mineral assemblage of sericite/muscovite-kaolinite-rutile-quartz ± pyrite; (iii) advanced argillic alteration with the hydrothermal mineral assemblage of alunite-jarosite-kaolinite-quartz-sericite ± pyrite. Hornblende andesite is the protolith of the hydrothermal alteration system. Enrichment in Si, Sb and Rb, and depletion in Na, Ca, Mg, Fe, Mn, P, Ba, Sr, and Zn distinguishes the argillic/phyllic and advanced alteration types from propylitic alteration and the unaltered hornblende andesite protolith. REE distribution patterns indicate an essentially immobile behaviour of REEs during the alteration cycle. K-Ar age data for unaltered and hydrothermally altered rocks define a synchronous age of 25.3 ± 1.2 Ma for both igneous and hydrothermal activity.