Alkali-rich igneous rocks and related Au and Cu large and superlarge deposits in China

The alkali-rich igneous rocks in China occur as fifteen linear distributed belts of each extending several hundreds to several thousands kilometers in length. These include most types of alkali-rich igneous rock categories discovered worldwide. The related Au, Cu large and superlarge deposits or metallogenic focus-areas include Dongping, Guilaizhuang, Yulong, etc. Direct and indirect genetic links have been found between alkali-rich igneous rocks and Au, Cu mineralization. The petrogenesis and metallogenesis of the alkali-rich igneous rocks are mainly controlled by (1) mantle enrichment, (2) strong interaction between mantle and crust, (3) lower contens of sulfur, high and high contents of volatiles, (4) the significant turn of regional tectonic framework from compress to extension and (5) the strong change of regional lithosphere structure.     

Large and superlarge silver deposits associated with volcanic rocks in China

This paper deals with the geology and geochemistry of the Gacun and Laochang large-sized marine volcanic rock-type Ag deposits in the Sanjiang (Tri-River) area of southwestern China and of the continental volcanic-subvolcanic rock-type Ag deposits in the Tianshan area of Xinjiang, and in the East area, China. It is considered that the marine volcanic rock-type Ag deposits occur mainly in the second-ordered volcano-sedimentary basins developed in island-arc and rift tectonic environments. The Ag deposits show an obvious zonation, with vein-network mineralization in the lower parts and hot water sedimentary rock-hosted stratified mineralization in the upper parts. From the Earth's surface downwards the ore-forming elements follows the order of As(Au))→Ag, Pb, Zn→Cu. The whole rock Rb-Sr isotopic isochron age of layered orebodies in the Gacun deposit is 204±14 Ma, indicating that the main stage of mineralization is Late Triassic in age. The continental volcanic-subvolcanic (porphyry) rock-type Ag deposits were formed later than the country rocks. The ores exhibit disseminated, veinlet disseminated, network and lumped structures. In addition, this study also deals with the geochemical characteristics of the continental volcanic-subvolcanic rock-type Ag deposits and the relations between Ag deposits or silver itself and fluorite, halogen-family elements and manganese.     

Large and superlarge silver deposits associated with volcanic rocks in China

This paper deals with the geology and geochemistry of the Gacun and Laochang large-sized marine volcanic rock-type Ag deposits in the Sanjiang (Tri-River) area of southwestern China and of the continental volcanic-subvolcanic rock-type Ag deposits in the Tianshan area of Xinjiang, and in the East area, China. It is considered that the marine volcanic rock-type Ag deposits occur mainly in the second-ordered volcano-sedimentary basins developed in island-arc and rift tectonic environments. The Ag deposits show an obvious zonation, with vein-network mineralization in the lower parts and hot water sedimentary rock-hosted stratified mineralization in the upper parts. From the Earth’s surface downwards the ore-forming elements follows the order of As(Au)) →Ag, Pb, Zn→Cu. The whole rock Rb-Sr isotopic isochron age of layered orebodies in the Gacun deposit is 204±14 Ma, indicating that the main stage of mineralization is Late Triassic in age. The continental volcanic-subvolcanic (porphyry) rock-type Ag deposits were formed later than the country rocks. The ores exhibit disseminated, veinlet disseminated, network and lumped structures. In addition, this study also deals with the geochemical characteristics of the continental volcanic-subvolcanic rock-type Ag deposits and the relations between Ag deposits or silver itself and fluorite, halogen-family elements and manganese.     

Bio-reef-chert suite and superlarge ore deposits

The bio-reef-chert suite is an important ore-bearing rock assemblage and one of the metallogenic rock suites of superlarge ore deposits. It is formed as a fixed and ordered suite in space and time, and composed of different rocks formed by different geological processes. It is the product of basin evolution at special stage in a special geological setting. It is also the comprehensive product of normal sedimentary process, biological process in basin, hydrothermal sedimentary process under basin base and magmatic process in the deep lithosphere.     

Geo-tectonic conditions of the formation of Proterozoic large and superlarge ore deposits along northwestern margin of North China Plate

Four large and superlarge copper-multimetal ore deposits hosted in the Mid-Proterozoic Zhartaishan Group along northwestern margin of North China Plate are stratabound hydrothermal sedimentary deposits, and these deposits formed in mid-late stage of the Mid-Proterozoic Langshan-Zhartaishan rifting system. Four necessary conditions are proposed for the formation of these large and superlarge ore deposits: (i) favorable geological background and tectonic environment; (ii) abundant sources of metallogenic material; (iii) favorable sedimentary lithofacies and paleogeographic conditions; (iv) sudden change events during metallogenic processes.     

Anoxic environment and formation of superlarge ore deposits

The formation of many metallic and non-metallic ore deposits is a result of coupling of mineralization, related to anoxic environment and regional, or global background. Inter act~on, concordance and continual development of the mineralization and environmental background control the large to superlarge scale of ore deposits. Three kinds of ore-forming models related to anoxic environment and three important background patterns are suggested. The turning period of geological history is favorable to the formation of large to superlarge ore deposits.     

Charge generation and propagation in igneous rocks

Various electrical phenomena have been reported prior to or concurrent with earthquakes such as resistivity changes, ground potentials, electromagnetic (EM), and luminous signals. Doubts have been raised as to whether some of these phenomena are real and indeed precursory. One of the reasons for uncertainty is that, despite decades of intense work, there is still no physically coherent model. Using low- to medium-velocity impacts to measure electrical signals with microsecond time resolution, it has now been observed that when dry gabbro and diorite cores are impacted at relatively low velocities, 100 m/s, highly mobile charge carriers are generated in a small volume near the impact point. They spread through the rocks, causing electric potentials exceeding +400 mV, EM, and light emission. As the charge cloud spreads, the rock becomes momentarily conductive. When a dry granite block is impacted at higher velocity, 1.5 km/s, the propagation of the P and S waves is registered through the transient piezoelectric response of quartz. After the sound waves have passed, the surface of the granite block becomes positively charged, suggesting the same charge carriers as observed during the low-velocity impact experiments, expanding from within the bulk. During the next 2–3 ms the surface potential oscillates, indicating pulses of electrons injected from ground and contact electrodes. The observations are consistent with positive holes, e.g. defect electrons in the O²⁻ sublattice, traveling via the O 2p-dominated valence band of the silicate minerals. Before activation, the positive holes lay dormant in the form of electrically inactive positive hole pairs (PHP), chemically equivalent to peroxy links, O₃X/^OO\XO₃, with X=Si⁴⁺, Al³⁺, etc. PHPs are introduced into the minerals by way of hydroxyl, O₃X–OH, which all nominally anhydrous minerals incorporate when crystallizing in H₂O-laden environments. The fact that positive holes can be activated by low-energy impacts, and their attendant sound waves, suggests that they can also be activated by microfracturing. Depending on where in the stressed rock volume the charge carriers are activated, they will form rapidly moving or fluctuating charge clouds that may account for earthquake-related electrical signals and EM emission. Wherever such charge clouds intersect the surface, high fields are expected, causing electric discharges and earthquake lights.     

Metallogenic focus-area and superlarge mineral deposits in the bordering zones between China,Russia and Mongolia

Two deep-sea cores in the northwest Pacific have been analysed for sedimentology, mineralogy and environmental magnetism. The results show that after eliminating the interference from volcanism, several proxies such as quartz content, mass susceptibility and anhysteretic magnetic remanence can be used to indicate the eolian deposit from East Asia, and provide information on paleo-atmospheric circulation. A comparison of eolian record in Cure RC10-175 with its oxygen isotopic curve has revealed that the peaks of eolian accumulation occurred at the climatic “optimum” of the Holocene and the last interglaciation, showing the fairly complex nonlinear relationship between continental/pelagic eolian records and the global glacial cycles as well as within the climatic and environmental system. Project supported by the National Natural Science Foundation of China (Grant No. 9291104)     

Metallogenic focus-area and superlarge mineral deposits in the bordering zones between China,Russia and Mongolia

A correlative study of geology and metallogenic process in the bordering zones between China, Russia and Mongolia shows that the region of the western slope of the Da Hingan Mts. -Eastern Transbaikalia-Eastern Mongolia is a metallogenic focus-area associated with Late Mesozoic intraplate tectono-magmatic activation. During the period of 160–120 Ma a great number of multiplex deposits with similar genesis including large and superlarge mineral deposits were formed.

     

Normative composition and classification of lunar igneous rocks and glasses,I. Lunar igneous rocks

678 major element analyses and all available trace element determinations of lunar rocks with igneous textures were collected from the literature. Rittmann norms were calculated by an ALGOL program. The norm values, grouped according to increasing clinopyroxene contents, were plotted into quartz-plagioclase-orthopyroxene and olivine-plagioclase-orthopyroxene triangles, respectively. The plots indicate that all lunar rocks form a compositional continuum that starts from rocks very high in plagioclase and continues, with increasing clinopyroxene, to plagioclase-poorer and orthopyroxene-richer rocks containing partly quartz, partly olivine.According to apparent clusters in the plots, and taking into account lunar rock types defined by previous authors, the continuum of normative compositions was subdivided into five major rock groups (I to V). The averages of these groups can be characterized by clinopyroxene contents and plagioclase/orthopyroxene ratios (I: 3% cpx, plag/opx = 30; II: 4% cpx, plag/opx = 7; III: 8% cpx, plag/opx = 2; IV: 26% cpx, plag/opx = 0.8; V: 34% cpx, plag/opx = 1.8). According to the contents in K-feldspar, from groups III and V K-rich subgroups were separated. Average contents of major elements and trace elements were calculated for main groups and subgroups.For the normative groups of lunar igneous rocks, names are proposed which conform to the nomenclature of terrestrial rocks.     

Characteristics of melt inclusions in skarn minerals from Fe,Cu(Au) and Au(Cu) ore deposits in the region from Daye to Jiujiang

The skarns and skarn deposits are widely distributed at home and abroad. The skarn deposits include many kinds of ores and higher ore grade. Some of them are broad in scale. Scientists of ore deposits from different countries have paid and are paying grea…     

Characteristic magnetic properties of titanomagnetites in continental igneous rocks

Curie temperatures, hysteresis, alternating field properties and anhysteretic and ordinary susceptibilities have been used to characterize the titanomagnetites in a large collection of continental granites, diorites, syenites, anorthosites, gabbros, diabases and basalts. Low-Curie-point titanomagnetites or titanomaghemites were found only in basalts. In all shallow and deep-seated intrusive rocks, the predominant magnetic phase was nearly-titanium-free titanomagnetite with a Curie point of 520–580°C. Most felsic plutonic rocks owed their magnetic properties to coarse, discrete titanomagnetites with truly multidomain properties. Many mafic plutonic rocks (anorthosites, gabbros, norites) displayed bimodal magnetic properties, strong-field properties being due to the discrete titanomagnetites and weak-field properties being due to fine magnetite inclusions in deuterically altered silicates. The Lowrie-Fuller test and the anhysteretic induction curve were the most diagnostic tests of this bimodal behaviour. Grain-size variation within a single diabase dike or sill had a strong expression in all magnetic properties, except H_R/H_c and the Lowrie-Fuller test. On the other hand, the Lowrie-Fuller test was a sensitive indicator of changes in “effective” grain size in basalts due to the subdivision of grains by ilmenite lamellae.     

Metallogenesis of superlarge gold deposits in Jiaodong region and deep processes of subcontinental lithosphere beneath North China Craton in Mesozoic

The study of ore-forming chronology indicates that the superlarge gold deposits in the Jiaodong region were formed in 120±10 Ma. Sr-Nd-Pb isotopic compositions from typical gold deposits suggest that ore-forming materials were derived from the multisources, mantle component was partly involved in mineralization, the deep dynamic processes are the major geological background of large-scale metallogenesis in the Jiaodong region in Mesozoic. The deep processes mainly include the effect of post deep-subduction of continental crust of the central orogen belt and the distant effect of subduction of the paleo-Pacific plate underneath the Eurasian continent. However, lithosphere thinning, crust-mantle interaction, crustal extension and formation of large-type ore-controlling structures would be the comprehensive consequences of the above-mentioned geodynamic processes in the region.     

Metallogenesis of superlarge gold deposits in Jiaodong region and deep processes of subcontinental lithosphere beneath North China Craton in Mesozoic

The study of ore-forming chronology indicates that the superlarge gold deposits in the Jiaodong region were formed in 120±10 Ma. Sr-Nd-Pb isotopic compositions from typical gold deposits suggest that ore-forming materials were derived from the multisources, mantle component was partly involved in mineralization, the deep dynamic processes are the major geological background of large-scale metallogenesis in the Jiaodong region in Mesozoic. The deep pro- cesses mainly include the effect of post deep-subduction of continental crust of the central orogen belt and the distant effect of subduction of the paleo-Pacific plate underneath the Eurasian continent. However, lithosphere thinning, crust-mantle interaction, crustal extension and formation of large-type ore-controlling structures would be the comprehensive consequences of the above- mentioned geodynamic processes in the region.     

Magnetic effects associated with chemical changes in igneous rocks

Experimental evidence and theory indicate that chemical changes occur in many igneous rocks at sufficiently low temperatures to significantly affect the remanent magnetization. Some chemical changes lead to self-reversals of magnetization that are not reproducible in laboratory experiments. Such self-reversals appear to be very rare in subaerially-erupted basalts, but they probably are much more common in some other rock types, such as granites and diorites. The stability of the natural remanent magnetization in igneous rocks can be decreased, left unaltered, or increased by chemical changes. In addition, chemical changes will usually affect the intensity of magnetization in a rock; the intensity can increase, decrease, or (rarely) be left unaltered by a chemical change. Such changes are important to consider in the development of improved techniques for obtaining reliable estimates of the intensity of the Earth's magnetic field in the past and in correctly interpreting marine magnetic anomalies. Finally, experiments and theory are presented which suggest that many of the chemical changes in igneous rocks will only occasionally produce significant changes in the direction of the magnetization.     

Geophysical targeting of Fe-oxide Cu–(Au) deposits west of Kiruna,Sweden

Ore deposits are frequently associated with various types of rock alteration that may affect the physical properties of rocks, and result in a characteristic signature of the deposits. The present work focuses on the use of geophysical data in targeting areas hosting Fe-oxide Cu–Au (IOCG) deposits west of the Kiruna mining district, northern Sweden. It is noted that IOCG deposits in different areas worldwide (e.g., Australia, Canada) are preferentially located in proximity to faults associated with deformation zones of regional relevance, and that the deposits are hosted in areas affected by potassic to sodic alteration. This study shows that at a semi-regional scale IOCG deposits occur within areas of high positive gravity anomalies, high values of estimated potassium content and high K/Th ratios, which is an expression of potassic alteration. High magnetic anomalies are associated with magnetite occurrences, while low linear magnetic anomalies mark fault/shear zones, in proximity of which the majority of the IOCG deposits occur.     

Alkali-rich intrusive rocks in the Qinghai-Xizang Plateau and its vicinities as exemplified by Kuganzi and Taihe plutons

The alkali-rich intrusive rocks in the Qinghai-Xizang Plateau can be divided into two series in terms of dark-colored rock-forming minerals, petrochemical composition and trace elements: ∣) sodic alkaline series rocks consisting of alkaline rocks and alkali granite which contain dark-colored minerals, such as aegirine augite, aegirine and riebeckite, and being petrochemically enriched in alkali and sodium but low in Ca, Sr and Ba; ║) potassic alkaline series rocks consisting of alkaline rocks and alkali granite which contain non-alkaline dark-colored minerals (diopside, edenite), and being petrochemically enriched in alkali, K and Ca with high abundances of Sr and Ba.     

Electrical conductivity of igneous rocks: Composition and temperature relations

The conductivity of four igneous rocks with, 49, 65, 77, and 84% SiO₂ was measured as a function of temperature in the interval from 20° to 1280°C; measurements were made in a vacuum of 10^?3 torr. No simple relationships were found between conductivity and SiO₂ content or versus major element groupings such as Na₂O=K₂O=CaO and TiO₂=Cr₂O₃=Al₂O₃=Fe₂O₃=FeO. An analytical expression was obtained between conductivity and the albite-quartz ratio, valid for temperatures between 300° and 1200°C. It was necessary to compute the CIPW norm in order to obtain the albite and quartz percentages. The onset of melting apparently occurred between 600° and 700°C. Petrography performed on two samples after cooling showed 70 and 85% partial melting. Three conduction regions were identified: 1) below 300°C, 2) between 300°C and 600°C, and 3) above 600°C. Different activation energies obtained for the heating and cooling intervals confirm that the sample undergoes textural changes in the heating-cooling cycle. Activation energy increments of 0.1 and 0.2 eV per decade of albite-quartz ratio were obtained.