9: Processes of tectonism, magmatism and mineralization: Lessons from Europe

Metallogenic provinces in Europe range in age from the Archaean to the Neogene. Deposit types include porphyry copper and epithermal Cu–Au, volcanic-hosted massive sulphide (VMS), orogenic gold, Fe-oxide–Cu–Au, anorthosite Fe–Ti-oxide and sediment-hosted base-metal deposits. Most of them formed during short-lived magmatic events in a wide range of tectonic settings; many can be related to specific tectonic processes such as subduction, hinge retreat, accretion of island arcs, continental collision, lithosphere delamination or slab tear. In contrast, most sediment-hosted deposits in Europe evolved in extensional, continental settings over significant periods of time. In Europe, as elsewhere, ore formation is an integral part of the geodynamic evolution of the Earth's crust and mantle. Many tectonic settings create conditions conducive to the generation of water-rich magma, but the generation of ore deposits appears to be restricted to locations and short periods of change in temperature and stress, imposed by transitory plate motions. Crustal influence is evident in the strong structural controls on the location and morphology of many ore deposits in Europe. Crustal-scale fault–fracture systems, many involving strike-slip elements, have provided the fabric for major plumbing systems. Rapid uplift, as in metamorphic core complexes, and hydraulic fracturing can generate or focus magmatic–hydrothermal fluid flow that may be active for time spans significantly less than a million years. Once a hydrologically stable flow is established, ore formation is strongly dependent on the steep temperature and pressure gradients experienced by the fluid, particularly within the upper crust. In Europe, significant fracture porosity deep in the crystalline basement (1%) is not only important for magmatic–hydrothermal systems, but allows brines to circulate down through sedimentary basins and then episodically upward, expelled seismically to produce sediment-hosted base-metal deposits and Kupferschiefer copper deposits. Emerging research, stimulated by GEODE, can improve the predicting power of numerical simulations of ore-forming processes and help discover the presence of orebodies beneath barren overburden.     

新型充气钻井液在新疆严重漏失地层的应用

利用特殊植物纤维KWS作为泥浆处理剂的新型充气钻井液，不仅具有常规充气钻井液低密度、排粉能力强、节约用水等优点，还可以不受钠、钙离子的影响，提高钻井泡沫稳定性和钻井液抗盐能力。通过采用此种新型充气钻井液，利用其低密度特性、特殊泥饼、泡沫的群体封堵作用和泡沫的疏水屏蔽作用，成功解决了裂隙地层的漏失问题，恢复了钻探生产。在我国西部干旱缺水漏失地区具有良好的应用前景。     

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.     

塔里木盆地西部阿克莫木气藏地层压力特征

阿克莫木气田目前已有多口井完钻,各井在钻揭白垩系砂岩储层前,对地层压力纵向上的变化规律认识不清,在什么层位及深度下7″套管意见仍不统一。本文根据目前研究现状和生产面临问题,对白垩系各组地层分布规律、压力特征进行了详细研究,认为白垩系克孜勒苏群、库克拜组分布稳定,压力窗口相近,白垩系东巴组与上覆古近系阿尔塔什组压力窗口相近。建议今后该区钻探7″套管应下至库克拜组顶部-东巴组底部,减少地层漏失和油气勘探风险。     

The genus Perissocytheridea Stephenson, 1938 (Crustacea: Ostracoda) and evidence of brackish water facies along the Oligo-Miocene,Pirabas Formation,eastern Amazonia,Brazil

Perissocytheridea Stephenson is characteristic of brackish water facies. In 57 samples from the Upper Oligocene to Lower Miocene, from five localities, Pirabas Formation, Pará State, Brazil eleven species have been identified. Among these species, four are new reports: Perissocytheridea punctoreticulata n. sp., Perissocytheridea largulateralis n. sp., Perissocytheridea colini n. sp. and Perissocytheridea pirabensis n. sp.; five species in open nomenclature: Perissocytheridea sp. 1, P. sp. 2, P. sp. 3, P. sp. 4, and P. sp. 5 and two species left in “aff.” abbreviation: Perissocytheridea aff. Perissocytheridea pumila and Perissocytheridea aff. Perissocytheridea brachyforma subsp. excavata. The distributional pattern of the Perissocytheridea combined with the occurrence of foraminifera Elphidium and Ammonia in the studied sections supports the presence of the brackish water facies to the respective layers. Their quantitative variation through the studied sections indicate more than one phase of salinity reduction (about >5 and <30 ppm) and/or the decrease of the sea level, forming lagoon along the Pirabas Formation. This genus has a wide paleobiogeographical occurrence and stratigraphic distribution ranging from the Cretaceous to Recent, and already been recorded in the northern of South America, especially in the Neogene of Solimões Basin, but this is the first report of a neotropical genus to the Oligo-Miocene deposits of Pirabas Formation, northern Brazilian Coast, Pará State.     

Selenium attenuation via reductive precipitation in unsaturated waste rock as a control on groundwater impacts in the Idaho phosphate patch

Phosphate mining in southeastern Idaho has historically resulted in the release of dissolved metals and inorganics to groundwater and surface water, primarily due to leachate from waste rock in backfilled pits and overburden storage piles. Selenium (Se) is of particular concern due to its high concentration in leachate and its limited attenuation downgradient of source zones under oxic conditions. Assessments of potential groundwater/surface water impacts from waste rock typically involve laboratory characterization using saturated and unsaturated flow columns packed with waste rock. In this study, we compare the results of saturated and unsaturated column tests with groundwater quality data from the Mountain Fuel, Champ, South and Central Rasmussen Ridge Area (SCRRA), Smoky Canyon, Ballard, Henry, and Enoch Valley Mines, to understand the release and attenuation of Se in different geochemical environments. Column studies and field results demonstrate that the ratio of aqueous Se to aqueous sulfate (Se:SO₄ ratio) is a useful metric for understanding Se release and attenuation, where the extent of sulfate reduction is much less than Se reduction. Comparison of dissolved Se and sulfate results suggests that the net leachability of Se from unsaturated waste rock is variable. Overall, Se concentrations in groundwater directly beneath waste rock dumps is not as high as would be predicted from unsaturated columns. Lower Se:SO₄ ratios are observed immediately beneath waste rock dumps and backfilled pits relative to areas receiving shallow waste rock runoff. It is hypothesized that Se released in the oxic upper portions of the waste rock is subsequently attenuated via reductive precipitation at depth in unsaturated, low-oxygen portions of the waste rock. This highlights an important mechanism by which Se may be naturally attenuated within waste rock piles prior to discharge to groundwater and surface water. These results have important implications for mining practices in the region. A better understanding of Se dynamics can help drive waste rock management during active mining and capping/water management options during post-mining reclamation.     

Tectonostratigraphic history of the Neogene Maimará basin,Northwest Argentina

This paper presents the tectonostratigraphic evolution of the Maimará Basin and explores the relationship between the clastic sediments and pyroclastic deposits in the basin and the evolution of the adjacent orogeny and magmatic arc. The sedimentary facies in this part of the basin include, in ascending order, an ephemeral fluvial system, a deep braided fluvial system and a medial to distal ephemeral fluvial system. We interpret that Maimará Formation accumulated in a basin that has developed two stages of accumulation. Stage 1 extended from 7 to 6.4 Ma and included accelerated tectonic uplift in the source areas, and it corresponds to the ephemeral fluvial system deposits. Stage 2, which extended from 6.4 to 4.8 Ma, corresponds to a tectonically quiescent period and included the development of the deep braided fluvial system deposits. The contact between the Maimará and Tilcara formations is always characterized by a regional unconformity and, in the study area, also shows pronounced erosion.Rare earth element and other chemical characteristics of the tuff intervals in the Maimará Formation fall into two distinct groups suggesting the tuffs were erupted from two distinct late Miocene source regions. The first and most abundant group has characteristics that best match tuffs erupted from the Guacha, Pacana and Pastos Grandes calderas, which are located 200 and 230 km west of the study area at 22º-23º30′S latitude. The members the second group are chemically most similar to the Merihuaca Ignimbrite from the Cerro Galán caldera 290 km south-southwest of the studied section. The distinctive geochemical characteristics are excellent tools to reconstruct the stratigraphic evolution of the Neogene Maimará basin from 6.4 to 4.8 Ma.     

胶东玲珑金矿田175号脉群深部地质特征与成矿预测

玲珑金矿属于以石英脉型为主的金矿床,其矿脉受玲珑断裂和破头青断裂控制。区内脉群发育,175号脉群是玲珑金矿的主要大型金矿脉群之一。文章通过总结175号脉群成矿地质背景、矿床地质特征,分析了控矿因素、矿体赋存规律、矿石类型等;阐述了175号脉群深部成矿规律,并根据见矿工程利用地质块段法预测175号脉群11—101勘探线范围-800 m标高以下(332+333)金矿石量192.44×10~4 t,金金属量8.5t。     

江苏省南京市江宁区杨库村铜矿地质特征及找矿前景

通过对江苏省南京市江宁区杨库村铜矿的地质背景、矿床地质特征、围岩蚀变及矿床成因、成矿控矿条件的分析研究,并与相邻已知矿区进行对比,结合近几年的工作成果,阐述了杨库村铜矿的找矿前景。