Fluid inclusion studies of the Abawso gold prospect,near the Ashanti Belt,Ghana

The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en échelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H₂O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H₂O and H₂O-CO₂ inclusions over CO₂ inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO₂ inclusions from a H₂O-CO₂ parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO₂±N₂ inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.Editorial handling: E. Frimmel     

Quartz and feldspar zoning in the eastern Erzgebirge volcano-plutonic complex (Germany, Czech Republic): evidence of multiple magma mixing

Zoned quartz and feldspar phenocrysts of the Upper Carboniferous eastern Erzgebirge volcano-plutonic complex were studied by cathodoluminescence and minor and trace element profiling. The results verify the suitability of quartz and feldspar phenocrysts as recorders of differentiation trends, magma mixing and recharge events, and suggest that much heterogeneity in plutonic systems may be overlooked on a whole-rock scale. Multiple resorption surfaces and zones, element concentration steps in zoned quartz (Ti) and feldspar phenocrysts (anorthite content, Ba, Sr), and plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the eastern Erzgebirge volcano-plutonic complex. Generally, feldspar appears to be sensitive to the physicochemical changes of the melt, whereas quartz phenocrysts are more stable and can survive a longer period of evolution and final effusion of silicic magmas. The regional distribution of mixing-compatible textures suggests that magma mingling and mixing was a major process in the evolution of these late-Variscan granites and associated volcanic rocks.

Quartz phenocrysts from 14 magmatic phases of the eastern Erzgebirge volcano-plutonic complex provide information on the relative timing of different mixing processes, storage and recharge, allowing a model for the distribution of magma reservoirs in space and time. At least two levels of magma storage are envisioned: deep reservoirs between 24 and 17 km (the crystallisation level of quartz phenocrysts) and subvolcanic reservoirs between 13 and 6 km. Deflation of the shallow reservoirs during the extrusion of the Teplice rhyolites triggered the formation of the Altenberg-Teplice caldera above the eastern Erzgebirge volcano-plutonic complex. The deep magma reservoir of the Teplice rhyolite also has a genetic relationship to the younger mineralised A-type granites, as indicated by quartz phenocryst populations. The pre-caldera biotite granites and the rhyodacitic Schönfeld volcanic rocks represent temporally and spatially separate magma sources. However, the deep magma reservoir of both is assumed to have been at a depth of 24–17 km. The drastic chemical contrast between the pre-caldera Schönfeld (Westfalian B–C) and the syn-caldera Teplice (Westfalian C–D) volcanic rocks is related to the change from late-orogenic geotectonic environment to post-orogenic faulting, and is considered an important chronostratigraphic marker.     

First record of 1.2 Ga quartz dioritic magmatism in the Archaean Yilgarn Craton,Western Australia,and its significance

Farming of southern bluefin tuna in South Australia currently contributes to more than 30% of the value of the aquaculture production in Australia. This study investigated the natural sedimentary setting of the area designated for this important industry in coastal waters off Port Lincoln, and explored the links between the natural distribution of sediments and potential environmental effects and risks to the industry. Sediments were mostly composed of poorly sorted silts and fine sands, predominantly skeletal remains of carbonate-secreting organisms. The contribution of plankton to the organic matter remaining in the sediments was calculated to be in excess of 80% using concentration-dependent stable-isotope mixing models. An erosional area was identified south of Rabbit Island where sediments contained up to 50% siliciclastic material, grainsize distributions were better sorted and coarser, and organic carbon and total nitrogen contents were very low. In contrast, deeper waters north of Cape Donington were identified as a depocentre for fine sediments, which contained organic matter levels twice those elsewhere in the region despite the extremely high carbonate contents (>80%). The heavier stable isotopic signature of nitrogen suggested that this organic matter comprised a greater fraction of weathered components, probably advected to the area by suspended and bedload transport. This local variability of sediment characteristics in the farming zone suggests that the benthic assimilative capacity of farmed sites will depend on their location. Wastes from pens located south of Rabbit Island in particular are likely to be quickly winnowed out by wave and tidal action. These pens are also less likely to be affected by resuspension of fine sediments that might be associated with unusually severe storms.     

铜绿山铜铁矿床地质特征及岩浆岩与成矿关系探讨

铜绿山铜铁矿床位于鄂城-大幕山新华夏复合隆起带中部、阳新复式岩体西北端。矿体赋存于下三叠统大冶群第四至第七岩性段。矿床由12个矿体组成。矿区内岩浆活动频繁。通过对区内岩浆演化、岩石化学、岩性特征以及微量元素特征等方面系统研究,认为矿床成矿物源为铜绿山石英正长闪长玢岩岩体,含碱值偏高的中酸岩类是控制成矿元素富集及各类矿床组合的内在因素,岩浆的成矿专属性由岩浆本身特性决定。     

青海夏日哈木矿区中泥盆世闪长玢岩锆石U-Pb年代学、地球化学及其地质意义

夏日哈木矿区的闪长玢岩主要出露于矿区北部。闪长玢岩LA-MC-ICP-MS锆石U-Pb年龄为（381.7±1.9） Ma,MSWD=0.20,属中泥盆世;结合闪长玢岩侵入成矿杂岩体的野外产状,研究表明夏日哈木镍矿的形成早于381.7±1.9 Ma。岩石学及化学成分显示其属于钙碱性、偏铝质系列闪长玢岩。岩体富Na（K2O/Na2O值为0.45~0.60）,稀土元素总量较低（ΣREE为128.28×10-6 ~ 141.65×10-6）,轻重稀土分馏明显,（La/Yb）N值为6.86 ~ 7.96;根据其较高的La/Nb值（3.32~3.76,平均3.57）、Th/Nb值（0.95～1.02,平均为0.99）以及Th/La值（0.266～0.291,平均0.278）,结合源区判别图解,研究表明该岩石为下地壳部分熔融的产物。综合区域地质演化背景,表明夏日哈木矿区闪长玢岩形成于造山后伸展向板内过渡的环境。     

湖北狮子立山锶(铅锌)矿区石英闪长玢岩锆石SHRIMP U-Pb定年及其意义

湖北狮子立山锶(铅锌)矿床是我国重要的超大型锶矿床之一,产出于鄂东矿集区北部铁山岩体东缘的狮子立山岩体(主要为石英闪长玢岩)与三叠纪蒸发岩地层(嘉陵江组)的接触带或附近。本文对矿区内的石英闪长玢岩与成矿作用之间的关系和该岩体的年代学开展了详细的研究工作。结果表明,狮子立山岩体与锶矿成矿作用可能存在密切的联系,矿床中的部分Sr可能来自于石英闪长玢岩的强烈蚀变。锆石SHRIMP U-Pb的定年工作得到2个石英闪长玢岩样品的形成时代分别为138.6±0.8Ma和138.9±1.2Ma,二者在误差范围内基本一致,这些数据与前人获得的铁山岩体的成岩时代基本一致,暗示本次测得的石英闪长玢岩的样品年龄可靠性较高,且很可能与铁山岩体是同一地质背景下的产物,即形成于区域晚侏罗-早白垩世岩石圈由挤压到伸展的转换环境。狮子立山岩体的侵位地层及矿床中硫同位素组成的特征与程潮和金山店矽卡岩型铁矿具有明显的相似性,暗示在狮子立山矿床附近具有寻找矽卡岩型铁矿床的潜力。     

江西德兴斑岩铜矿石英闪长玢岩成因及成矿学意义

为探究石英闪长玢岩成因及幔源基性岩浆对斑岩铜矿的贡献,本文选取德兴矿床石英闪长玢岩开展了锆石U-Pb定年、Hf同位素和全岩地球化学研究。获得石英闪长玢岩LA-ICP-MS锆石U-Pb年龄为169 Ma,与成矿花岗闪长斑岩侵位时间一致,岩体为中侏罗世岩浆活动的产物。石英闪长玢岩具有低的SiO₂(58.41%~63.12%)和K₂O(1.68%~2.94%)含量及A/CNK值(0.85~1.04),富集大离子亲石元素和轻稀土元素,亏损高场强元素Nb、Ta、Ti和重稀土元素,属于钙碱性到高钾钙碱性系列岩石。具有相对亏损的锆石Hf同位素组成,εHf(t)=2.20~7.93(最大值7.93),指示其源区为岩石圈地幔。锆石稀土元素配分模式图显示出明显的正Ce异常,岩浆氧逸度(lg f_O2)为-20.05~-6.66,达到磁铁矿-赤铁矿氧逸度等级,指示石英闪长玢岩结晶自高氧逸度岩浆。全岩地球化学特征显示,德兴石英闪长玢岩与成矿花岗闪长斑岩及其暗色包体符合岩浆混合的演化趋势,说明成矿花岗闪长斑岩可能是中侏罗世幔源基性岩浆和地壳酸性岩浆大规模混合作用的产物,并且石英闪长玢岩代表了岩浆混合过程中的幔源基性端员。结合前人研究成果,认为在中侏罗世伸展构造背景下,软流圈物质上涌导致新元古代受交代的岩石圈地幔部分熔融形成幔源基性岩浆,基性岩浆的底侵作用诱发下地壳物质熔融并与之发生一定程度的岩浆混合作用,形成了花岗闪长斑岩的母岩浆。高氧逸度幔源岩浆的加入可抑制斑岩体系硫化物的过早饱和,同时为德兴矿床注入了成矿所需的部分挥发分和金属元素。     

四川瀑布沟地区断裂破碎带的物质成分与断裂活动年代

本文主要研究断层泥中新生的粘土矿物特征,并研究断层泥中石英碎砾表面溶蚀的显微形貌结构特征。粘土矿物以X-射线衍射分析为主,石英碎砾则利用扫描电子显微镜对其显微形貌结构进行分析,并根据石英表面溶蚀的形貌特征来探讨断裂活动的相对年代     

Fluid Inclusion Feature of Gold-Bearing Quartz Veins from the Jinniushan Gold Deposit, Shandong, China

The auriferous veins at Jinniushan occurs within the Jinniushan faulted zone in the Kunyushan Granite. Optical observation reveals that gold ore body formed during the main stage of hydrothermal activity. Detailed geothermometric studies of fluid inclusions from the veins show that the forming temperature ranges between 130℃ and 370℃ and the salinity is from 4.01 to 15.21 wvt percent NaCl. The ore-forming fluid is featured by low to moderate salinity, and low to moderate temperature. According to investigations of the values of vapor/liquid and temperatures of the ore-forming fluids, we propose that the boiling fluid inclusions exist in the main mineralization stages. Fluid boiling is suggested as a mechanism for the precipitation of gold from the hydrothermal fluid in the Jinniushan gold deposit.