非传统金矿化探的理论与方法技术研究

传统的金矿化探方法已不能满足找矿的要求，特别是寻找隐伏矿的要求。为此我们根据过去十几年的研究提出了如下新的理论认识及相应的方法：（１）金在表生条件下不仅有其惰性的一面，而且更且具有很强的活动性；（２）金不仅呈不均匀的颗粒存在，而且还大量做为超微细的分散金存在；（３）区域性大规模的金异常和隐伏矿上方的叠加含量异常是由易呈活动的超微细分散金形成的；（４）为研究这类存在形式的金应大幅度降低金分析的检出限     

Gold in the weathering crust at the Suzdal' deposit (Kazakhstan)

A comparative analysis of morphology and geochemistry was made for gold from the primary ores and weathering crust of the Suzdal' gold deposit, Eastern Kazakhstan. The deposit is localized in Carboniferous carbonaceous-terrigenous strata and is of gold-sulfide type. Study of gold from primary ores showed that it occurs mainly in two species: free and so-called invisible. Free gold is crystallomorphic segregations and irregular-shaped grains up to tens of microns in size; it occurs in intergrowths with sulfides, quartz, carbonate, and mica-chlorite aggregate. Most of gold particles have a fineness of 930–980‰, with some grains showing wide variations in composition. Invisible gold (probably chemically combined) is present in fine-acicular arsenopyrite and, less frequently, pyrite.Being transported to the weathering crust, all this gold served as a source for “neogenic” gold of diverse morphologic forms. We recognized crystalline (isometric, prismatic, acicular, and tabular) particles and drusoid gold aggregates in the form of exotic intergrowths of crystallomorphic and sinter-shelly grains. The grains tend to coarsen from bottom to top of the weathering crust. Several generations of gold of different granulometric classes are observed. We have revealed seed and layer growth and dissolution structures in crystals of early generations overgrown with fine grains. All these gold varieties are associated with hypergene minerals. Most of this gold is of high fineness (on the average, 995‰). The hypergene gold particles are chemically homogeneous high-grade, without rims.The results of studies suggest that the high-grade hypergene gold formed in the weathering crust as a result of the dissolution of invisible gold of sulfides and its local redistribution and deposition in oxidizing media. This is also evidenced from the tendency of gold to coarsen from bottom to top of the weathering crust. A distinctive feature of secondary gold is well-expressed crystals and their great diversity.     

紫木凼金矿床载金矿物及金的赋存状态

紫木凼金矿床主要载金矿物为含砷黄铁矿和毒砂,氧化矿石中,金呈显微粒状自然金－显微金嵌布于褐铁矿,石英水云母,方解石的颗粒间或孔隙中,原生矿石中,金主要呈次显微粒自然金－次显微金包裹体,其次是类质同像形式赋存在含砷黄铁矿和毒砂矿物中。     

Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle-ductile crystal-plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.     

安徽铜陵狮子山矿田夕卡岩型金矿床的矿物组合、化学成分及成因意义

包村和朝山金矿床位于安徽铜陵狮子山矿田，属于夕卡型岩金矿床，侵入岩体分别为包村石英闪长岩体和白芒山辉石闪长岩体。通过矿相鉴定和电子探针分析，对矿床中的自然金进行了详细的观察和研究。结果表明，在两个矿床中，金矿物均以独立的自然金颗粒形式产出，且常具有“银边结构”，但在自然金的嵌布方式、载体矿物、形态、大小、金银含量变化和成色等方面表现出显著差别。根据自然金的不同特征，对金矿床的成因进行分类，包村金矿床属于Cu-Au共生，而朝山矿床为Cu-Pb-Zn-Au共生型金矿床。不同矿床不同的金产状主要是由于金的运移、络合物种类、温度、pH和金银络合物活度比值等因素赞成的。此外，包村矿床和朝山矿床在地表均遭受氧化作用形成表生矿床，其表生金成色均明显高于原生金，且成分均一。     

Glacially-crushed quartz grains in late Quaternary deposits in the Virunga Mountains,Rwanda -indicators of wind transport from the north?

Quartz sand grains in late Quaternary deposits of the Virunga Mountains, northwestern Rwanda, were found to contain glacial-crushing microtextures. Glacially-crushed subangular grains generally lack sharp edges, and carry mainly low frequency, linear crushing features. Because there are no known tills in the Virunga Volcanoes (and because quartz is rare in volcanic terrain), it is likely the grains were delivered by aeolian transport from the Ruwenzori Massif, -150 km to the north. All the grains studied are consistent with crushing under low shear stress, and over short transport distances, in alpine glaciers. An increase in the percentage of glacially-crushed quartz from Holocene to Pleistocene beds likely reflects on the increased strength of the paleowind system (trade winds) during the last glacial maximum. Alternatively, it could reflect on the increased source area for glacially-crushed grains resulting from an increase in the extent of the Afroalpine belt during the last glaciation.     

The nature and provenance of Golestan loess deposits in northeast Iran

Mineralogical, textural, geochemical, and weathering characteristics of loess deposits in Golestan province of Iran suggest that they are mostly derived from felsic igneous rocks and are related to Quaternary palaeoclimate. Whole‐rock analyses indicate heavy minerals such as zircon, tourmaline and phyllosillicate minerals (e.g. muscovite, chlorite, illite) exert a significant control on the chemical composition. The loess samples display uniform chemical composition, indicative of similar alteration history. Chemical index of alteration suggests a weak to moderate degree of weathering in a felsic source area. Scanning electron micrographs of quartz grains reveal abundant silt‐sized quartz particles, a result of glacial grinding during the Late Pleistocene in Central Asia. Subsequently, these silt particles were transported from Central Asia to their depositional site by wind and paraglacial processes. Local topography of northeast Iran (Alborz Mountains) acted as a major barrier, entrapping the airborne particles on the plains of Golestan province. Copyright © 2012 John Wiley & Sons, Ltd.     

胶东昆嵛山二长花岗岩副矿物成因矿物学研究

根据胶东昆嵛山二长花岗岩主要的副矿物组合（磁铁矿、榍石、绿帘石、褐帘石、磷灰石、锆石及自然金等）及相应的含量，结合有关矿物的形态、成分，特别是３２６粒锆石群形态学的研究，对岩石成因与金矿化的关系等，提出了矿物学标志。确定昆嵛山岩体为岩浆成因，岩浆物质来源为胶东群部分重熔，物质组成以壳源为主，为Ⅰ型磁铁矿系列花岗岩类；确定岩体结晶温度跨度大，因而富含水分，岩体富含Ｓ及其他挥发分，富含碱质，早期矿物结晶处于较氧化碱性环境，以上条件对形成金矿有利。对胶东三大主要含金岩体，即昆嵛山、郭家岭及玲珑岩体副矿物特点进行了对比。     

福建尤溪肖板金矿床金的赋存状态及金矿物特征

肖板金矿床属受构造控制的中低温岩浆热液矿床,矿化类型为构造蚀变岩型。金多呈独立金矿物形式出现,少许呈分散状;金矿物以自然金为主,平均成色９３０,有少量银金矿和碲金矿。金矿物以包体金、裂隙金、连生金和粒间金等形式嵌布于黄铁矿、黄铜矿、石英、方铅矿及方解石等主要载金矿物中,且石英、方解石中较金属硫化物中占优势。金矿物形态各异,粒度以中细粒为主。     

石榴石U-Pb定年对山西义兴寨金矿床角砾岩筒时间的限制和金矿成因的指示

山西义兴寨金矿床是五台山-恒山金矿集区内最为典型的金矿床,也是山西省最大的金矿床.矿区产出有4个角砾岩筒,与金矿的形成具有紧密的联系.然而,有关该矿床角砾岩筒的形成时间和机制的研究十分欠缺,角砾岩筒与金矿化的关系仍不清楚.通过对角砾岩筒内的矽卡岩角砾中的石榴石和切割角砾岩的石英斑岩中的锆石开展了LA-ICPMS原位U-Pb同位素分析,结果表明角砾岩和石英斑岩的形成时间分别为140±2 Ma和141±1 Ma,从而精确限制了角砾岩筒的形成时间.以上研究表明,义兴寨矿区4个角砾岩筒的形成时间与石英斑岩侵位时代和金矿形成时间完全一致,表明角砾岩筒是岩浆分异的气液流体导致围岩发生隐爆而形成,而金矿化也可能与岩浆释放的热液流体直接相关.      

云南大理点苍山末次冰期冰碛物石英砂扫描电镜形态特征分析

云南大理点苍山是大理冰期的命名地,保存有确切的末次冰期冰川作用遗迹.对点苍山冰碛物石英砂进行了扫描电镜形态特征分析,获得了其石英砂形态特征及其频率,并在冰川成因的典型特征的频率上取得了新的认识.扫描电镜分析表明,点苍山冰碛物石英砂形态特征主要为：石英砂形状主要为次棱角状、尖棱角状和多棱角状,边缘多可见次棱脊和棱脊磨损,表面起伏度高;机械特征主要有贝壳状断口、平行解理面、裂隙、粘附碎片、擦痕、机械V形坑;化学特征主要有蚀坑和蚀缝、蜂窝状溶蚀表面、无定形硅沉淀和硅质薄膜.在石英砂表面与冰川作用密切相关的擦痕的频率为8%～32%;粘附碎片的频率为16%～40%;裂隙的频率为12%～32%.通过对点苍山冰碛物的石英砂扫描电镜分析,得出该第四纪海洋性冰斗冰川和悬冰川的石英砂形态特征及其频率.该冰碛物石英砂原始形态特征明显,机械特征频率不高,是近源堆积的冰碛物石英砂的典型形态特征.     

Mineralogy of the Boroo Gold Deposit in the North Khentei Gold Belt,Central Northern Mongolia

Mineral assemblages and chemical compositions of ore minerals from the Boroo gold deposit in the North Khentei gold belt of Mongolia were studied to characterize the gold mineralization, and to clarify crystallization processes of the ore minerals. The gold deposit consists of low‐grade disseminated and stockwork ores in granite, metasedimentary rocks and diorite dikes. Moderate to high‐grade auriferous quartz vein ores are present in the above lithological units. The ore grades of the former range from about 1 to 3 g/t, and those of the latter from 5 to 10 g/t, or more than 10 g/t Au. The main sulfide minerals in the ores are pyrite and arsenopyrite, both of which are divisible into two different stages (pyrite‐I and pyrite‐II; arsenopyrite‐I and arsenopyrite‐II). Sphalerite, galena, chalcopyrite, and tetrahedrite are minor associated minerals, with trace amounts of bournonite, boulangerite, geerite, alloclasite, native gold, and electrum. The ore minerals in the both types of ores are variable in distribution, abundance and grain size. Four modes of gold occurrence are recognized: (i) “invisible” gold in pyrite and arsenopyrite in the disseminated and stockwork ores, and in auriferous quartz vein ores; (ii) microscopic native gold, 3 to 100 µm in diameter, that occurs as fine grains or as an interstitial phase in sulfides in the disseminated and stockwork ores, and in auriferous quartz vein ores; (iii) visible native gold, up to 1 cm in diameter, in the auriferous quartz vein ores; and (iv) electrum in the auriferous quartz vein ores. The gold mineralization of the disseminated and stockwork ores consists of four stages characterized by the mineral assemblages of: (i) pyrite‐I + arsenopyrite‐I; (ii) pyrite‐II + arsenopyrite‐II; (iii) sphalerite + galena + chalcopyrite + tetrahedrite + bournonite + boulangerite + alloclasite + native gold; and (iv) native gold. In the auriferous quartz vein ores, five mineralization stages are defined by the following mineral assemblages: (i) pyrite‐I; (ii) pyrite‐II + arsenopyrite; (iii) sphalerite + galena + chalcopyrite; (iv) Ag‐rich tetrahedrite‐tennantite + bournonite + geerite + native gold; and (v) electrum. The As–Au relations in pyrite‐II and arsenopyrite suggest that gold detected as invisible gold is mostly attributed to Au⁺¹ in those minerals. By applying the arsenopyrite geothermometer to arsenopyrite‐II in the disseminated and stockwork ores, crystallization temperature and logfs₂ are estimated to be 365 to 300 °C and –7.5 to –10.1, respectively.     

Correlations between mineralogical and chemical compositions of fine stream sediments: application to geochemical exploration in tropical rain forests, Bolívar State, Venezuela

In this work we have studied stream sediments that differ in hydrodynamic energy of depositional environments and in the lithologies of source areas. The first type consists of sediment sampled in stream channels in a relatively high energy environment (samples CS); the other type is from sediments deposited laterally, along margins of channels, where stream energy decreases (samples LS). Eighteen CS samples and 36 LS samples, sieved to 230 mesh, taken from streams and rivers that drain different lithologies, were analyzed for mineralogical and chemical compositions.Samples CS and LS have different chemical compositions, although they are composed of the same minerals. CS samples have low concentrations of absorbing phases (e.g., clay and Fe-Mn oxides), low total and extractable Cu, Co, Ni and Cr, and high zircon and quartz. Conversely, LS samples have greater abundances of absorbing phases, transition elements, and lower zircon and quartz. The sample types can be discriminated by compositional differences in Zr, Si and Al.Differences in proportions of minerals of the fine sediment samples (Fe-Mn oxides, clays, quartz and zircon) result from differences in source areas and hydrodynamic characteristics of depositional sites. It is difficult, if not impossible, to collect all samples from sites with exactly the same hydrodynamic conditions or from the same source area; it is therefore necessary to classify samples according to mineralogical compositions, and to know proportions of absorber phases, clays and oxides, to the inert phase, quartz, before interpretation.     

胶东苏鲁地体范家埠金矿成矿作用与矿床成因浅析:兼与胶北地体金矿对比

范家埠金矿是胶东半岛苏鲁地体内规模最大的石英脉型金矿。含金石英脉中锆石阴极发光和LA-ICP-MS微量元素及U-Pb同位素分析结果表明,含金石英脉中的锆石均为捕获围岩(新元古代威海片麻状花岗岩)的岩浆锆石,其中15个颗粒在U-Pb图解上位于谐和线上,其206Pb/238U加权平均年龄为(726±14)Ma(MSWD=4.6);另外7颗锆石位于不一致线上,其上交点年龄为(758±25)Ma,下交点年龄为(123±11)Ma(MSWD=0.45)。这些锆石的下交点年龄与金矿脉旁侧斜闪煌斑岩脉的角闪石40Ar/39Ar年龄分别为(118.8±1.6)Ma和(117.5±1.5)Ma,在误差范围内一致,表明范家埠金矿的成矿时代为120Ma左右。由此认为,苏鲁地体和胶北地体的金矿成矿作用均发生于早白垩世晚期120Ma前后,与目前多数人认为的华北克拉通岩石圈减薄的峰期时间一致,表明苏鲁地体的金矿成矿受岩石圈减薄的构造背景控制。范家埠金矿床成矿流体具中高温、中高盐度15.5%～23.2%(NaCleq)、低δ18OH2O(-3.84‰～-4.05‰)和低δDH2O(-82.5‰～-80.8‰)值等特点,载金矿物黄铁矿富32S(δ34S=-5.5‰～-9.1‰),与含油气盆地热卤水的组成相近,暗示范家埠金矿的成矿流体可能来源于中生代胶莱盆地。这种盆地流体沿断裂——五莲—米山深大断裂及其次级构造迁移,同时萃取基底岩石中的金形成含矿热液并在还原的环境中成矿。相反,胶北地体大量金矿床成矿流体具有低盐度、高δ18OH2O和δDH2O,δ34S(‰)为正值等特点。胶北和苏鲁地体早白垩世金成矿强度和成矿作用的差异可能反映了两者在地壳基底、流体来源和成矿环境等方面的不同。     

Au–Ag–Te Mineralization of the Low‐Sulfidation Epithermal Aginskoe Deposit,Central Kamchatka,Russia

Mineralogic studies of major ore minerals and fluid inclusion analysis in gangue quartz were carried out for the for the two largest veins, the Aginskoe and Surprise, in the Late Miocene Aginskoe Au–Ag–Te deposit in central Kamchatka, Russia. The veins consist of quartz–adularia–calcite gangue, which are hosted by Late Miocene andesitic and basaltic rocks of the Alnei Formation. The major ore minerals in these veins are native gold, altaite, petzite, hessite, calaverite, sphalerite, and chalcopyrite. Minor and trace minerals are pyrite, galena, and acanthine. Primary gold occurs as free grains, inclusions in sulfides, and constituent in tellurides. Secondary gold is present in form of native mustard gold that usually occur in Fe‐hydroxides and accumulates on the decomposed primary Au‐bearing tellurides such as calaverite, krennerite, and sylvanite. K–Ar dating on vein adularia yielded age of mineralization 7.1–6.9 Ma. Mineralization of the deposit is divided into barren massive quartz (stage I), Au–Ag–Te mineralization occurring in quartz‐adularia‐clays banded ore (Stage II), intensive brecciation (Stage III), post‐ore coarse amethyst (Stage IV), carbonate (Stage V), and supergene stages (Stage VI). In the supergene stage various secondary minerals, including rare bilibinskite, bogdanovite, bessmertnovite metallic alloys, secondary gold, and various oxides, formed under intensely oxidized conditions. Despite heavy oxidation of the ores in the deposit, Te and S fugacities are estimated as Stage II tellurides precipitated at the log f Te₂ values ?9 and at log fS₂ ?13 based on the chemical compositions of hypogene tellurides and sphalerite. Homogenization temperature of fluid inclusions in quartz broadly ranges from 200 to 300°C. Ore texture, fluid inclusions, gangue, and vein mineral assemblages indicate that the Aginskoe deposit is a low‐sulfidation (quartz–adularia–sericite) vein system.     

山东省平度市大庄子金矿区黄铁矿、方铅矿与金的共生关系研究

通过对山东省平度市大庄子金矿区黄铁矿、方铅矿三个蚀变阶段特征研究,发现金矿化主要发生在第二阶段,强烈的石英-黄铁矿-碳酸盐化与金矿化的关系最为密切。金多数以包体的形式赋存于黄铁矿等金属硫化物中,并且在黄铁矿、方铅矿、闪锌矿等矿物共同产出时更有利于金的富集。研究认为石英和黄铁矿组合以及黄铁矿、黄铜矿、方铅矿、闪锌矿的组合可作为金的富集指示性矿物组合,伴生矿物之中方铅矿含金性最好。方铅矿、黄铜矿、闪锌矿和磁黄铁矿呈细脉状、网脉状、斑杂状产于含金石英脉和团块状黄铁矿的裂隙中,研究发现细脉状较团块状黄铁矿含金性要好。     

内蒙卓资-凉城地区古元古代变质过铝/强过铝花岗岩的形成温度:区域高温/超高温变质作用证据

内蒙古卓资-凉城地区孔兹岩系出露大量古元古代过铝/强过铝花岗岩。野外地质和岩石地球化学特征均显示它们存在两种类型。一类来自孔兹岩部分熔融,与孔兹岩呈渐变过渡关系并以含有粗大的石榴石、石英和堇青石斑晶为特征,出露广泛(A类);另一类花岗岩为花岗侵入岩体,矿物粒度相对细小,无矿物斑晶,其起源尚待进一步查证(B类)。电子探针(EPMA)分析石英单颗粒Ti含量并运用钛温度计进行计算,结果显示两类花岗岩石英Ti温度存在较明显差异:A类中石英钛温度多集中在680℃～800℃和>800℃间,而B类花岗岩多集中在650℃～750℃间,前者较后者普遍存在高的Ti温度值;两类花岗岩的锆饱和温度也存在差异,两者分别集中在820℃～890℃和700℃～780℃间。结果表明,来自孔兹岩部分熔融的A类花岗质岩石形成温度应高于900℃,B类花岗岩侵入体的石英和锆饱和温度均反映出正常花岗质侵入体结晶温度范围。由于A类花岗岩广泛分布在卓资-凉城地区,因此我们认为本区发生的超高温变质作用属于区域高温/超高温变质作用的结果。     

Gold Mineralization of the Gatsuurt Deposit in the North Khentei Gold Belt,Central Northern Mongolia

Mineral assemblages, chemical compositions of ore minerals, wall rock alteration and fluid inclusions of the Gatsuurt gold deposit in the North Khentei gold belt of Mongolia were investigated to characterize the gold mineralization, and to clarify the genetic processes of the ore minerals. The gold mineralization of the deposit occurs in separate Central and Main zones, and is characterized by three ore types: (i) low‐grade disseminated and stockwork ores; (ii) moderate‐grade quartz vein ores; and (iii) high‐grade silicified ores, with average Au contents of approximately 1, 3 and 5 g t^?1 Au, respectively. The Au‐rich quartz vein and silicified ore mineralization is surrounded by, or is included within, the disseminated and stockwork Au‐mineralization region. The main ore minerals are pyrite (pyrite‐I and pyrite‐II) and arsenopyrite (arsenopyrite‐I and arsenopyrite‐II). Moderate amounts of galena, tetrahedrite‐tennantite, sphalerite and chalcopyrite, and minor jamesonite, bournonite, boulangerite, geocronite, scheelite, geerite, native gold and zircon are associated. Abundances and grain sizes of the ore minerals are variable in ores with different host rocks. Small grains of native gold occur as fillings or at grain boundaries of pyrite, arsenopyrite, sphalerite, galena and tetrahedrite in the disseminated and stockwork ores and silicified ores, whereas visible native gold of variable size occurs in the quartz vein ores. The ore mineralization is associated with sericitic and siliceous alteration. The disseminated and stockwork mineralization is composed of four distinct stages characterized by crystallization of (i) pyrite‐I + arsenopyrite‐I, (ii) pyrite‐II + arsenopyrite‐II, (iii) galena + tetrahedrite + sphalerite + chalcopyrite + jamesonite + bournonite + scheelite, and iv) boulangerite + native gold, respectively. In the quartz vein ores, four crystallization stages are also recognized: (i) pyrite‐I, (ii) pyrite‐II + arsenopyrite + galena + Ag‐rich tetrahedrite‐tennantite + sphalerite + chalcopyrite + bournonite, (iii) geocronite + geerite + native gold, and (iv) native gold. Two mineralization stages in the silicified ores are characterized by (i) pyrite + arsenopyrite + tetrahedrite + chalcopyrite, and (ii) galena + sphalerite + native gold. Quartz in the disseminated and stockwork ores of the Main zone contains CO₂‐rich, halite‐bearing aqueous fluid inclusions with homogenization temperatures ranging from 194 to 327°C, whereas quartz in the disseminated and stockwork ores of the Central zone contains CO₂‐rich and aqueous fluid inclusions with homogenization temperatures ranging from 254 to 355°C. The textures of the ores, the mineral assemblages present, the mineralization sequences and the fluid inclusion data are consistent with orogenic classification for the Gatsuurt deposit.     

In situ U–Pb dating of zircon formed from retrograde garnet breakdown during decompression in Rogaland, SW Norway

Regionally metamorphosed metapelites from Rogaland, SW Norway, contain zircon formed during the decompression reaction garnet + sillimanite + quartz → cordierite. The zircon, which occurs as inclusions in cordierite coronas around garnet, is texturally, chemically and isotopically distinct from older zircon in other textural settings in the matrix. A SHRIMP U–Pb age of 955 ± 8 Ma based on analyses in thin section on the decompression zircon from the cordierite coronas, therefore dates a point on the retrograde path, estimated from garnet–cordierite equilibria to be 5.6 kbar, 710 °C. This population was under‐represented in conventional SHRIMP analyses of individual zircon in a mono‐mineralic grain mount and, in the absence of a textural context, its significance unknown. The dominant age identified from SHRIMP analyses of the grain mount, in combination with analyses from matrix zircon in thin section, was 1035 ± 9 Ma. Based on the lack of consistent textural relationships with any specific minerals in thin section, as well as rare earth element chemistry, the 1035‐Ma population is interpreted to represent zircon growth during incipient migmatization of the rocks at 6–8 kbar and c. 700 °C. This is consistent with previous estimates for the age of regional M₁ metamorphism during the Sveconorwegian Orogeny. The most important outcome of this study is the successful analysis of zircon grains in a specific, well‐constrained reaction texture. Not only does this allow a precise point on the regional P–T path to be dated, but it also emphasizes the possibility of zircon formation during the retrograde component of a typical metamorphic cycle.     

Gold nugget morphology and geochemical environments of nugget formation,southern New Zealand

Gold nuggets (centimetre scale) have formed in a supergene alteration zone on hydrothermal gold deposits, and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals, and along fractures in quartz and host rocks. The supergene alteration was driven by groundwater-driven water-rock interaction near to a regional unconformity beneath fluvial sediments, and involved clay alteration and oxidation that extended up to 50 m below the unconformity. Oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals. The nuggets have some crystalline form, and internally they consist of anhedral grains, elongated gold plates, and intimate intergrowths of gold and iron oxyhydroxide. Nugget surfaces have further micron scale overgrowths of microparticulate gold, gold plates, and gold crystals. Nuggets were eroded and recycled into nearby proximal Miocene quartz pebble conglomerates, where they concentrated in placers near the basal unconformity. Later recycling transferred gold into Pleistocene fluvial channels. Gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles, with little change in mass. All groundwater maintained high pH throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation. Hence, gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets, albeit with lower dissolved sulphur concentrations. Despite aridification of the climate in the late Cenozoic, with resulting localised high dissolved chloride concentrations, chloride complexation did not contribute to gold mobility.