Geochemical evidence for the hydrothermal origin of sulphur,base metals and gold in Proterozoic metamorphosed black shales,Kainuu and Outokumpu areas,Finland

Metamorphosed black shale formations 120–150 m thick with median concentrations of 7% non-carbonate carbon and 6–8% sulphur are abundantly met in drill cores in the Kainuu and Outokumpu areas. Carbon isotope ³C values are comparable to the average isotopic composition of non-carbonate carbon in sedimentary rocks. The rare earth element patterns show cerium depletion, indicating a marine origin. Base metal concentrations in black shales are high in the Talvivaara mineralization (ore estimate: 300 Mt with 0.26% Ni, 0.14% Cu and 0.53% Zn) and in the vicinity of ophiolite complexes. In the Kainuu schist belt, the median value for gold is 35 ppb (max 170 ppb) in the western part and 16 ppb (max 180 ppb) in the east. Highest platinum and palladium concentrations encountered are 60–70 ppb. It is shown that in the Kainuu black shales, concentrations of sulphur, base metals and precious metals have been increased by hydrothermal processes, as indicated by positive europium anomalies, elevated mercury concentrations (max 7.5 ppm), high sulphur isotope ³⁴S values and the enrichment of sulphur and base metals relative to carbon. The processes operative during the deposition of these shales find an analogy in recent processes in ocean ridge spreading axes.     

Pillow structures in serpentinite,Tagadur, Nuggehalli schist belt,Karnataka

Pillow structures developed in serpentinite is described from the Nuggehalli schist belt, Karnataka. Chemistry of the rock reveals that the serpentinite is a direct representation of mantle material.     

Cobalt as an exploration tool in the Outokumpu zone, Finland

The Outokumpu region in eastern Finland is an integral part of the Precambrian formations of the Karelidic orogeny. The copper-cobalt ore deposits discovered in the region are associated with a lithologic complex that consists of serpentinites, skarns, carbonate rocks and quartzites. The outer zone of this rock association adjacent to the surrounding mica schists is commonly occupied by black schists. The association constitutes the coherent stratigraphic sequence known as the Outokumpu zone, which is the environment in which the ore deposits of the Outokumpu type occur. The total length of the ribbon-like zone is about 240 km. Three Cu---Co ore deposits: Outokumpu, Vuonos and Luikonlahti are currently being exploited and four sub-economic ore showings have been found in the zone. All the deposits and ore showings are of the polymetallic sulphide type with copper, zinc, cobalt and nickel. There are also small amounts of silver, gold, tin and selenium in the orebodies. These stratabound ore deposits are submarine volcanic exhalative in origin. The immediate host rock of the ore is commonly quartzite, interpreted as a chemical silica precipitate.The Outokumpu copper-cobalt ore deposit was discovered in 1910. Owing to its economic potential, the Outokumpu zone is one of the most thoroughly studied parts of the Finnish Precambrian. More than 1000 holes have been drilled from the surface into the zone and provide a large source of material for lithogeochemical studies.The association does not lend itself easily to geophysical investigations because of its complex geology, characterized by black schists and other rocks giving a strong geophysical response. For this reason, and because of the extensive drilling, lithogeochemistry has become an important exploration tool in the region.The discovery of the blind Vuonos ore deposit in 1965 was the result of a comprehensive lithogeochemical study carried out in the early 1960's. Several old prospects were sampled in the Outokumpu zone and the sulphide phase was analyzed for Cu, Co, Ni and Zn.Clustering of the analytical data gives five groups of rock types: (1) quartzite-skarn-dolomite; (3) black schists; (4) mica schists; (5) copper-cobalt ore (Huhma and Huhma, 1970). The locations of these groups in Ni---Co and Cu---Co diagrams and in an Co---Cu---Ni triangular diagram are shown in Figs. 1, 2 and 3. The nickel content of serpentinites varies between 1500 and 2200 ppm and that of cobalt between 70 and 110 ppm. Thus the Ni/Co ratio averages 20:1. In the quartzite-skarn-dolomite-group the nickel content ranges from 900 to 3000 ppm and the cobalt content from 50 to 120 ppm. The Ni/Co 150 to 500 ppm and the cobalt content from 20 to 60 ppm. The Ni/Co ratio is about 10:1. Mica gneisses are poor in sulphides. Their nickel content averages from 40 to 90 ppm and the cobalt content from 15 to 30 ppm.The copper-cobalt ore occupies a discrete area in the Ni---Co diagram. Its nickel content varies between 1000 and 2000 ppm and the cobalt content between 1000 and 3000 ppm. The copper-cobalt diagram shows that the cobalt content of the serpentinites and the quartzite-skarn-dolomite group is fairly constant varying between 60 and 140 ppm. The copper content ranges from zero to 100 ppm in the former and from 10 to 100 ppm in the latter. In black schists the copper content varies from 100 to 300 ppm, the cobalt content being some tens of ppm. The mica gneisses are somewhat poorer in their Cu and Co contents. In this case too, the copper-cobalt ore has a Cu---Co content distinctly apart from those of the other groups.In the Cu---Co---Ni triangular diagram the serpentinite and the quartzite groups plot near the Ni apex of the triangle, the relative Ni content being 94–96% and that of Cu less than 1%. The black schists and mica gneisses have their own area near the centre of the Cu---Ni join with the Co content not exceeding 10%. There are several exceptions where points in the diagrams described above plot outside the normal field. Most of these anomalous points are located between the normal area of the rock type and that of the Cu---Co ore. The Ni---Co diagram in particular demonstrates that the deviations are due to the increase in the Co content.In summary, the anomalies of the Outokumpu type have: (1) a Ni/Co ratio lower than 15:1; and (2) a Cu percentage of the sum Cu+Co+Ni = 100 higher than 5. These anomaly units are applied to rank the analytical data of the Outokumpu zone. It is evident from the diagrams that the increase in cobalt content outside its normal field is the prime indication of the proximity of the ore. Being rather constant, the nickel content is used as a reference. Thus a decrease in the Ni/Co ratio indicates the presence of the ore. In some cases the decrease in the Ni/Co ratio may be due to a local decrease in the Ni content. The anomaly can be checked by the Cu/Cu+Ni+Co ratio. Except when testing the anomalies with Co, Cu and Ni as described above, the Co content alone can be used as an indicator of the proximity of a Cu---Co orebody. This was tested in one section of the Vuonos orebody (Fig. 4). The pyrite phase of the quartzites was selectively leached and its Co content analyzed. It was noted that the Co content of pyrite increased somewhat when the orebody was approached along strike.Before this method can be used more widely, the stratigraphic position and the structure of the potential ore horizon must be known fairly accurately. The proximity of an orebody can also be evaluated by means of the Co content of the black schists. As shown above, the Co content of the black schists is usually considerably under 100 ppm; it is usually over 100 ppm only adjacent to an orebody. Consequently, the extensive data on black schists in the Outokumpu zone can be sorted into potential and less potential ones.     

Structure of the Outokumpu ore district and ophiolite-hosted Cu–Co–Zn–Ni–Ag–Au sulfide deposits revealed from 3D modeling and 2D high-resolution seismic reflection data

The Outokumpu district within the North Karelia Schist Belt in eastern Finland hosts a number of Cu–Co–Zn–Ni–Ag–Au sulfide deposits that are associated with Palaeoproterozoic ophiolitic metaserpentinites derived from depleted mantle peridotites that were subsequently tectonically interleaved with allochthonous metaturbidites. The metaperidotites have been extensively metasomatized to quartz–carbonate–calc–silicate rocks of the Outokumpu assemblage. The Outokumpu area has been affected by a multiple-phase tectonic history comprising various phases of folding and shearing followed by several faulting events. Future exploration has to expand the search into deeper areas and requires knowledge of the subsurface geology. In order to unravel the complex structure 3D geologic models of different scales have been built using a variety of information including geological aeromagnetic and gravity maps, digital terrain models, and mine cross sections as well as data like drill core logs combined with observations from underground mine galleries, structural measurements, aeromagnetic data, and seismic surveys. For crustal structures, data from seismic surveys lines have been reprocessed for our purpose. Both deposit-scale and regional-scale models allow the reconstruction of a sequence of structural events. The mined ore has formed during remobilization of a proto-ore and is closely related to shear zones (thrusts) that truncate the Outokumpu assemblage. Later faults dismembered the ore explaining the variable depth of the different ore bodies along the Outokumpu ore zone. On larger scale at least four km-scale thrust sheets, separated by major listric shear zones can be identified in the ore belt, which are internally further imbricated by subordinate shear zones. These thrusts separate a number of lens-shape metaperidotite bodies that are probably surrounded by Outokumpu assemblage rocks. Thrust stacking was followed by at least three stages of faulting that divided the ore belt into fault-bounded blocks with heterogeneous displacements: (i) faulting along NW-dipping faults with unresolved kinematics, (ii) reverse faulting along c. 50°–60° SE-dipping faults, and (iii) SW–NE to SSW–NNE striking faults which may have formed at an earlier stage and have been reactivated.The specific Outokumpu alteration assemblage around metaperidotite bodies combined with shear zones acting as path ways for fluids are the main vectors to mineralization. Seismic reflection data do not provide a simple tool to directly detect the sites of Outokumpu assemblage bodies at depth but they identify strong reflector zones which are characteristic for though not exclusive to the assemblage, shear zones can be recognized as curved dislocations in the seismic lines. Our study shows that 3D modeling, when used in combination with surface geology and other geophysical data and good knowledge about the structural evolution clearly improves the interpretation of reflectors and enables the identification of strong reflector packages as Outokumpu assemblage that, due to absent geological control, have first been mapped as “unknown reflector”. It thus enhances the chances for locating potentially economic horizons at depth and to delineate target areas for detailed exploration.     

元谋尹地金矿化区控矿条件及找矿前景

从地球化学特征分析中提取出矿化区控矿因素,矿化存在于海资哨岩组千枚岩、石英岩、石英片岩中,受地层岩性、韧性剪切破碎带的控制。圈出Ⅰ级靶区1处、Ⅱ级靶区2处。     

南苏鲁造山带的超高压变质岩及岩石化学研究

在南苏鲁造山带核部，古老的表壳岩和花岗质侵人岩经历了三叠纪的超高压变质作用，在超高压变质岩石抬升过程中经历了强烈的角闪岩相退变质作用改造。据岩相学和岩石化学研究，可以区分出六大类典型超高压变质岩：榴辉岩、石榴石橄榄岩、石英硬玉岩、石榴石多硅白云母片岩、硬玉石英岩和石榴石绿辉石文石岩。这些岩石的角闪岩相退变质产物分别是斜长角闪岩、蛇纹岩、长英质片麻岩、长石石英云母片岩、石英岩和大理岩。地球化学研究揭示，榴辉岩的原岩很可能是形成在大陆内部构造环境的拉斑玄武岩，而石榴石橄榄岩可能是起源于亏损的残余地幔。石英硬玉岩原岩包括正变质的花岗岩和奥长花岗岩、副变质的酸性火山碎屑岩和长石石英砂岩。大面积分布的古老花岗岩很可能是形成在大陆或大陆边缘环境。长石石英云母片岩、石英岩和大理岩的原岩为沉积岩，与副变质的长英质片麻岩和基性火山岩—起构成了古老的表壳岩组合。双峰式的酸性和基性火山岩组合的存在也证明部分表壳岩是形成在大陆环境。因此，可以推测南苏鲁造山带核部的超高压变质岩原岩为形成在大陆板内环境的沉积岩—酸性和基性火山岩—花岗岩和奥长花岗岩建造。     

内蒙古北山小红山岩组变形特征及地质意义

为了研究红石山—百合山—蓬勃山古生代构造带(蛇绿岩带)物质组成和变形特征,利用数字填图方法,在内蒙古北山地区大红山一带,开展1:5万区域地质调查。发现大红山南出露一套变质基底,主要岩石组合为一套海相细碎屑岩夹中基性火山岩及磁铁石英岩组合,以含硅铁建造及变质基性火山岩为特征。该岩石组合无底无顶,变形变质强烈,分不出层序,故称之为小红山岩组。各类中低级变质岩说明变质作用达到了高绿片岩—低角闪岩相。该组北部被上石炭统白山组角度不整合覆盖,南部被早泥盆世、晚石炭世侵入岩侵入。区内出露面积约140 km²,厚约2 890 m。明显有4次变形: 前3次变形为韧性变形,形成了透入性面理S₁、大型褶皱、韧性剪切带等构造形迹; 后一次变形以脆性变形为特征,形成了较大规模的断层带。小红山岩组多期次变形、变质说明了红石山—百合山—蓬勃山古生代构造带(蛇绿岩带)确实存在,并经历了多期次变形和变质。     

Petrology,geochemistry and metamorphic evolution of Lancang Group in the Changning-Menglian complex belt and its implications on the tectonic evolution of the Paleo-TethysEI北大核心CSCD

Lancang Group within the Changning-Menglian complex belt in the Sanjiang area, Yunnan Province involves many kinds of meta-sediments, including staurolite-kyanite-bearing garnet-mica schist, garnet-mica schist, chloritoid-white mica schist and chlorite-glaucophane-albite schist. Detailed petrographic observation, mineral chemistry analysis and phase equilibrium modelling have shown that these meta-sediments preserve distinctly metamorphic evolutions. The staurolite-kyanite-bearing garnet-mica schist records the decompression and cooling histories related to retrograded metamorphic processes from middle-temperature eclogite facies to amphibolite facies with a peak mineral assemblage of garnet + kyanite + phengite + jadeite formed at the P-T condition of about 19 similar to 30kbar and 600 similar to 750 degrees C. For the garnet-mica schist, the peak metamorphic mineral association constrained by X-Prp and X-Grs in garnet, and Si content in phengite includes garnet + phengite + omphacite + lawsonite + paragonite and the related P-T condition is around 17 similar to 19. 5kbar and 430 similar to 475 degrees C . The chloritoid-white mica schist is characterized by the mineral assemblage of chloritoid + phengite + paragonite + chlorite whereas the peak mineral assemblage includes phengite + paragonite + carpholite. The peak P-T condition defined by Si content in phengite is limited in the range of 17 similar to 19kbar and 300 similar to 330 degrees C. Both garnet-mica schist and chloritoid-white mica schist consistently record heating and decompression processes from lawsonite-blueschist facies to epidote-blueschist facies. Metamorphic reactions and mineralogy of chlorite-glaucophane-albite schist roughly give the P-T condition of 9 similar to 11kbar and 430 similar to 520 degrees C. Studies on the geochemistry of Lancang Group reveal that these meta-sediments show the geochemistry affinity to the continental arc, active continental margin and upper crust sediments. The protoliths are mainly mud rock and sandstone with low maturity and a little of mafic-intermediate volcanic rock. The sediment sources are predominantly intermediate-acid magmatic rocks with old sedimentary contamination to different degree. Tectonic discrimination diagrams show that meta-sediments in the Lancang Group are mainly derived from the continental island arc or active continental margin tectonic setting. Combined with the metamorphism and geochemistry characteristics of these rocks in the Changning-Menglian complex belt, it is therefore inferred that the meta-sediments of Lancang Group display various metamorphic evolutions. Lancang Group are considered to have experienced multi-phase/stage and complex tectonic evolution histories.     

新疆萨尔托海糜棱岩化石英菱镁岩元素地球化学特征及其对金成矿作用的贡献

新疆萨尔托海石英菱镁岩产在达拉布特蛇绿混杂岩带中,是蛇纹岩在剪切带深部发生热液交代作用的产物。蛇纹岩先转变为滑石片岩,再进一步转变为石英菱镁岩。石英菱镁岩局部发生剪切变形,形成糜棱岩化石英菱镁岩。与蛇纹岩相比,石英菱镁岩的MgO和SiO_2含量降低,Al_2O_3和CaO含量升高;而糜棱岩化石英菱镁岩的MgO含量较石英菱镁岩降低,Al_2O_3和SiO_2含量较石英菱镁岩升高。微量元素地球化学对比研究表明石英菱镁岩继承了蛇纹岩的微量元素特征;而糜棱岩化石英菱镁岩的微量元素含量较蛇纹岩发生了显著变化,稀土元素、高场强元素和自然金的含量明显升高,指示这些元素在剪切变形过程中发生迁移富集。剪切变形伴随强烈的流体/岩石反应,并导致剪切带流体组成和物理化学性质发生改变,从而影响矿物结晶或分解,并控制微量元素的带入迁出。蛇纹岩转变为石英菱镁岩的过程释放Au,可为糜棱岩化石英菱镁岩中的金矿化提供成矿元素。     

西昆仑甜水海地块大红柳滩铁矿床成因浅析及找矿意义

位于西昆仑甜水海地块东段的大红柳滩赤铁矿是近几年发现的大型铁矿床,产于震旦纪甜水海岩群滨浅海相浅变质碎屑岩-碳酸盐岩中,赋矿岩性主要为含铁白云质大理岩、白云母石英片岩和硬绿泥石白云母石英片岩。通过矿体形态特征、矿物组合和矿石结构构造分析,认为该时期存在缺氧富铁洋盆或者深水盆地,矿床的形成经历了铁质沉积和变质改造两个阶段,属于新元古代沉积变质成因条带状硅铁建造矿床(BIF),找矿潜力巨大。该矿床是继塔什库尔干县一带发现了多个与火山岩建造密切相关的大型规模Algoma型BIF之后的重大找矿突破,也是西昆仑地区首次发现该类型矿床。深入开展该矿床的地质特征及勘探工作,能够指导西昆仑铁矿的下一步找矿方向,推进新疆地区条带状含铁建造(BIF)研究和西昆仑构造格局演化的认识。     

内蒙古北山地区百合山蛇绿混杂岩带的厘定及其洋盆俯冲极性——基于1∶5万清河沟幅地质图的新认识

造山带内蛇绿混杂岩带结构与组成的精细研究可为古板块构造格局重建和古洋盆演化提供最直接证据。北山造山带内存在多条蛇绿混杂岩带,记录了古亚洲洋古生代以来的俯冲和闭合过程,然而其大地构造演化长期存在争议。红石山—百合山蛇绿混杂岩带位于北山造山带北部,主要由蛇绿(混杂)岩和增生杂岩组成,具典型的"块体裹夹于基质"的混杂岩结构特征,发育紧闭褶皱、无根褶皱、透入性面理和双重逆冲构造。蛇绿混杂岩带中岩块主要由超镁铁质-镁铁质岩(变质橄榄岩、辉石橄榄岩、异剥辉石岩、蛇纹岩)、辉长岩、玄武岩、斜长花岗岩、硅质岩等洋壳残块以及奥陶纪火山岩、灰岩等外来岩块组成,基质则主要为蛇纹岩、砂板岩及少量的绿帘绿泥片岩;在蛇绿混杂岩带北侧发育有台地相灰岩与深水浊积岩组成的沉积混杂块体,具滑塌堆积特征。蛇绿混杂岩带内发育三期构造变形,前两期为中深构造层次下形成的透入性变形,第三期为浅表层次的脆性变形,未形成区域性面理。空间上,由增生杂岩和蛇绿(混杂)岩组成的百合山蛇绿混杂岩带共同仰冲于绿条山组浊积岩之上,具有与红石山地区蛇绿混杂岩带相似的岩石组成、构造变形和时空结构特征。百合山蛇绿混杂岩带南侧发育同期的明水岩浆弧,由晚石炭世石英闪长岩-花岗闪长岩-二长花岗岩以及白山组岛弧火山岩组成,其与百合山蛇绿混杂岩带共同构成了北山造山带北部石炭—二叠纪的沟-弧体系,指示了红石山—百合山洋盆向南俯冲的极性。     

河南桐柏围山城金、银成矿带矿床地质特征及找矿标志

围山城金银成矿带矿床位于桐柏―大别山(北坡)金银成矿带北亚带。矿体主要赋存于上元古界歪头山组炭质绢云石英片岩内,严格受构造条件控制,含矿层受热液作用影响产生不同蚀变,尤其硅化与矿化的关系最为密切。矿区成矿地质条件优越,资源丰富。笔者在对矿带内地层、岩石、构造、围岩蚀变等较为详细分析的基础上提出了找矿标志。     

Metamorphism of the Basement of the Qilian Fold Belt in the Minhe-Ledu Area, Qinghai Province, NW China

The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me     

吉林中部呼兰群泥质岩递进变质带特征及其地质意义

下古生界呼兰群变质岩系位於吉林褶皱带南端,其中泥质岩变质片岩段具有明显的递进变质作用,并可划分为:铁铝石榴石带、十字石带、蓝晶石带、硅线石带四个矿物带,呈中压中温巴洛式变质带特征。本文对该变质带矿物组合、原岩建造、变质作用演化、成因机理及其地质意义进行了探讨。     

Magnetic Mapping of Banded Iron Formation of Sandur Schist Belt,Dharwar Craton,India

High ground magnetic anomalies are observed at western basin of the Sandur schist belt (SSB) near Muraripura village, Karnataka. Sources for these anomalies are inferred due to presence of exposed banded iron formations hosted by metabasalts, arenaous and argillaceous rocks associated with the Donimali Formation of the schist belt. Two varieties of BIF bands are observed in the study area viz. Band 1 and Band 2. The bands 1 and 2 are displaced by lateral strike-slip fault. Band 1 is dominantly composed of banded iron formation, rich in iron ore minerals and silica chert. Band 2 is largely composed of ferruginous quartzite and quartz arenites. Qualitative analysis of the magnetic data indicates that the study area can be divided into three lithologically important BIF/BQ formations. Spectral and quantitative analysis of the magnetic data indicates, the average depth of band 1 is 70 m and band 2 is 130 m. The magnetic data results are well correlated with chemical analysis of borehole and surface rock samples data.     

A metasomatic setting,the Russian River Arch,and gravitational emplacement in the history of eclogites at the classic eclogite locality of Jenner,California, USA

ABSTRACT

Blocks of metamorphic rock designated as ‘high-grade’ blocks, commonly less than 100 m in diameter, consisting of garnet-glaucophane- and hornblende-schists and gneisses and rare eclogite, are widely distributed within mélanges of the Franciscan (accretionary) Complex of California. Eclogite-glaucophane schist blocks present at Jenner, California, have been studied for petrographic, geochemical, structural, and age characteristics, but their relationship to associated Franciscan rocks is poorly understood. The studied blocks are not in situ, but rather occur in landslide deposits and beach sands. The landslide deposits overlie the low to middle slope exposures of sandstone-rich broken formations of the Franciscan Complex that are not known to contain high-grade blocks. Geochemical studies suggest a serpentinite host for the blocks. Upslope, a serpentinite-matrix mélange contains numerous high-grade blocks, including rare retrograded eclogite, and is the likely block source. The Jenner terrain as a whole was uplifted relative to rocks to the north and south near Annapolis and Freestone, respectively, by uplift along the post-Pliocene Russian River (anticlinal) Arch, as indicated by the regional distribution of arching, wave-cut, post-Franciscan surfaces with overlying Miocene/Pliocene marine sedimentary rocks. Local uplift increased landsliding and colluvial downslope movement of the blocks. In addition, local, wave-influenced transportation of smaller blocks, together with the downslope mass movements, brought the high-grade blocks to their present positions. The high-grade blocks are thus displaced from upslope exposures of the original serpentinite-matrix mélange host, in which the blocks likely experienced the metasomatism that converted eclogite to glaucophane schist. In general, the relationship of blocks to the original serpentinite host is a critical element of subduction zone architecture related to subduction zone processes and history, and should be analysed, in any studies that seek to explain the architecture and history of any accretionary complex with similar high-grade blocks.     

内蒙古苏布格塔蓝闪石片岩基本特征及成因分析

苏布格塔蓝闪石片岩赋存于奥陶纪哈拉哈河组地层中,岩石组合类型多样,据蓝闪石光学特征及电子探针分析结果,认为本区的蓝闪石属铁蓝闪石。蓝闪石片岩的标型矿物组合以蓝闪石+绿帘石+阳起石+多硅白云母+榍石为特点,由泥质碎屑岩变质重结晶形成,原岩形成环境为大陆边缘海盆地。其变质成因,属于与板块运动有关的区域低温中高压动力变质作用形成。     

Blueschist ophiolites in the melange zone,northern New Caledonia

A regional melange zone, 150 km long and 30 km wide, forms the southern boundary and structural capping to a high-pressure blueschist belt in northern New Caledonia. The disrupted country rocks in the melange zone are Mesozoic metagrey-wackes and Eocene chert-limestone sequences which have been penetrated from below by tectonically-injected ophiolite slivers containing metamorphosed serpentinite, gabbro, dolerite, basalt, tuff, chert and shale. An ocean crust origin for these rocks is indicated by chemical, mineralogical and radiometric data from coastal outcrops at Anse Ponandou on the northeast coast. The age (41 m.y.), metamorphic environment (350 ° C at 7 kb), and mineral association (acmitic jadeite-riebeckite-pyropic spessartine-pistacitic epidote-lawsonite-high Si phengite) are significantly different from those of the adjacent regional high-pressure schist belt, indicating a separate structural site for blueschist metamorphism of buried ophiolitic ocean crust during early Tertiary orogenesis.     

Evolution of a kyanite-bearing belt within a HT-LP orogen: the case of NW Variscan Iberia

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al-poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite-bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.     

Geological controls on refractory ore in an orogenic gold deposit, Macraes mine, New Zealand

The Macraes mine is hosted in an orogenic (mesothermal) gold deposit in metasedimentary rocks of the Otago Schist belt. Much gold occurs within altered schist with minimal silica-addition, and this study focuses on altered schist ore types. The unmineralized host schists are chemically and mineralogically uniform in composition, but include two end-member rock types: feldspathic schist and micaceous schist. Both rock types have undergone hydrothermal alteration along a shallow-dipping foliation-parallel shear zone, but their different rheological properties have affected the style of mineralisation. Micaceous schist has been extensively recrystallized and hydrothermally altered during ductile deformation, to form ores characterized by abundant, disseminated millimetre-scale pyrite cubes (typically 1–2 wt% S) and minor silicification. The earliest pyrite contained Ni and/or As in solid solution and no gold was imaged in these pyrites or later arsenopyrite grains. The ore type is refractory and gold recovery by cyanide leaching is less than 50%, with lowest recovery in rocks that have been less affected by later brittle deformation. In contrast, hydrothermally altered feldspathic schist is characterized by mineralised black microshears and veinlets formed during shear-zone related brittle deformation. Microsheared ore has relatively low sulphur content (<0.7 wt%) and muscovite has been illitised during hydrothermal alteration. Pyrite and arsenopyrite in microshears are fractured and deformed, and contain 1–10 m blebs of gold. Later pyrite veinlets also contain micron- to submicron-scale inclusions of sphalerite, chalcopyrite, galena, and gold (10 microns). Gold in microsheared ore is more readily recoverable than in the refractory ore, although encapsulation of the fine gold grains inhibits cyanidation. Both microsheared ore and disseminated pyritic ore pass laterally into mineralised black shears, which contain hydrothermal graphite and late-stage cataclastic sulphides. This black, sheared ore releases gold readily, but the gold is then adsorbed on to gangue minerals (preg-robbed) and net cyanidation recovery can be less than 50%. Hence, low gold recovery during cyanidation results from (1) poor liberation of gold encapsulated in microcrystalline quartz and unfractured sulphide grains, and (2) preg-robbing of liberated gold during cyanidation. Introduction of pressure-oxidation of ore prior to cynidation has mitigated these issues.