Intermediate-acid Rocks Research Articles

Petrogenesis and Chronology of Intermediate-Acidic Intrusive Rocks in Tongling Area, Anhui Province, China

The intermediate-acidic intrusive rocks in Tongling area of Anhui province can be divided into two magmatic series, namely the shoshonitic series and the high-potassium(K) calc-alkaline series. The shoshonitic series is characterized by an association of pyroxene monzodiorites + monzodiorites + quartz monzonites, and high Rittmann indices. This series has common association with gold deposits. The high-potassium(K) calc-alkaline series is characterized by an association of diorite + quartz monzodiorites + granodiorites. It has lower Rittmann indices and REEs than the shoshonitic series and is associated with copper deposits. Results obtained from the studies show that the former is derived from the differentiation of the alkaline basic magma in mantle and the later is mixture between the differentiated mantle magma and crustal magma. The chronology of intermediate-acidic intrusive rocks is about 140Ma.

A Genesis Analysis of the Regional Gravity and Magnetic Anomalies in the Northern Part of Eastern Xinjiang, Northwest China

The authors discuss the genesis of the regional gravity and magnetic anomalies in the northern part of Eastern Xinjiang, a correlation analysis was carried out between the regional gravity anomaly and the regional magnetic anomaly. Through data processing and integrated interpretation of the gravity and magnetic data in the study area, the Bouguer gravity anomaly and the magnetic anomaly by reduction to the pole were separated. Then, the regional gravity and magnetic anomalies across multiple scales were extracted; in addition, the correlation coefficients between the regional gravity anomaly and the regional magnetic anomaly at different scales were calculated. Finally, the features of the gravity and magnetic fields, their geological significance, and the origins of these regional gravity and magnetic anomalies were analyzed. The results showed that the regional negative gravity anomaly in the Turpan-Hami basin and the Santanghu basin was mainly caused by Cenozoic and Mesozoic strata; the regional positive magnetic anomaly was mainly caused by the Precambrian metamorphic basement. The study revealed that the regional positive gravity and magnetic anomalies resulted from the crust and the mantle substances in the eastern part of the Junggar basin. While the regional positive gravity anomaly was mainly caused by Pre-Mesozoic strata, and the regional negative magnetic anomaly by the sedimentary formation and intermediate acid rocks in the Bogeda-Harlike folded zone and the Jueluotage anticlinorium. The regional negative gravity and magnetic anomalies in the active zone of the northern margin of Tarim were mainly caused by the sedimentary formation and intermediate acid rocks.

Evaluation of Gold Geochemical Anomalies in the Liaodong Paleorift

89 Au geochemical anomalies are delineated by using 1/200000 regional geochemical exploration data. By researching regional geochemical characteristics and the relationship with the geological background, the author points out that: the main factors causing high background of Au geochemical anomalies are Gaixian and Dashiqiao formation of Liaohe group, intrusions of Mesozoic intermediate-acid intrusive rocks. The elements combination types of typical anomalies are determined by using factorial analysis,cluster analysis and other mathematical methods with the combination of elements association in typical anomalies:the composite anomaly of Baiyun gold deposits is Au-As-Sb, Maoling gold deposit is Au-As-Bi-Mo, Wulong gold deposits is Au-As-Bi-W, Xiaotongjiapuzi gold deposit is Au-As-Bi-Mo-Sb. By using multivariate statistical analysis method,62 ore-caused anomaly are preferred in 89 Au geochemical anomalies delineated. On this basis, the 62 anomalies are divided into 4 kinds of anomaly types reference to elements combination types of typical anomalies,the classification results of ore-caused anomalies are: 4 geochemical anomalies of Baiyun type,36 geochemical anomalies of Maoling type,11 geochemical anomalies of Wulong type, 11 geochemical anomalies of Xiaotongjapuzi type. According to the results, the prospecting direction is provided for the future of gold exploration.

LA‐ICP‐MS Zircon U‐Pb Dating of Intermediate‐Acidic Intrusive Rocks and Molybdenite Re‐Os Dating from the Bangpu Mo (Cu) Deposit, Tibet and its Geological Implication

AbstractThe multi‐stage intrusions of intermediate‐acid magma occur in the Bangpu mining district, the petrogenic ages of which have been identified. The times and sequences of their emplacement have been collated and stipulated in detail in this paper by using the laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) zircon U‐Pb dating method. The ages of biotite monzogranite that were formed before mineralization in the southwest of this mining district are 70±1 Ma (mean square of weighted deviates (MSWD) = 9.5, n= 8) and 60.60±0.31 Ma (MSWD=3.8, n= 16), which belong to the late Cretaceous–early Paleocene in age. That means, they are products of an early tectonic‐magmatic event of the collision between the Indian and Asian continentals. The ages of ore‐bearing monzogranite porphyry and ore‐bearing diorite porphyrite are 16.23±0.19 Ma (MSWD= 2.0, n= 26) and 15.16±0.09 Ma (MSWD=3.9, n= 5) separately, which belong to the middle Miocene in age; namely, they are products of the Gangdese post‐collision extensional stage when crust‐mantle materials melted and mixed as well as magmatic intrusion simultaneously occurred. Some zircons with ages of 203.6±2.2 Ma (MSWD=1.18, n= 7) were captured in the ore‐bearing diorite porphyrite, which shows that there had been tectono‐magmatic events in the late Triassic–early Jurassic. Molybdenum (copper) ore‐bodies produced in the monzogranite porphyry and copper (molybdenum) ore‐bodies produced in the diorite porphyrite are the main ore types in this ore deposit. The model ages of Re‐Os isotopic dating for the 11 molybdenite are 13.97–15.84 Ma, while isochron ages are 14.09±0.49 Ma (MSWD=26). The isochron ages of seven molybdenite from molybdenum (copper) ore with monzogranite porphyry type are 14.11±0.31 Ma (MSWD=5.2). There is great error in the isochron ages of four molybdenite from copper (molybdenum) ore with diorite porphyrite type, and their weighted average model ages of 14.6±1.2 Ma (MSWD=41), which generally represent the mineralization age. The results about the Re‐Os isotopic dating of molybdenite in the ore of different types have limited exactly that, the minerlazation age of this ore deposits is about 14.09 Ma, which belongs to the middle Miocene mineralization. The Bangpu deposit has a uniform metallogenic dynamics background with the porphyry type and skarn‐type deposits such as Jiama, Qulong and others.

Cambrian rift-related magmatism in the Ossa-Morena Zone (Iberian Massif): Geochemical and geophysical evidence of Gondwana break‐up

Volcanic rocks of Cambrian age from Zafra (Ossa-Morena Zone, Iberian Massif) are the result of rift processes that affected Cadomian arc units accreted to the NW edge of Gondwana during the Neoproterozoic–Early Cambrian transition. Tephrite to rhyolite volcanics define an alkaline transitional association (Coombs type). Basic–ultrabasic rocks exhibit typical alkaline REE-patterns, strongly enriched in LREE with respect to HREE. Two parental magmas are identified, one with a mantle signature, lack of Nb negative anomaly and εNd500Ma +3.8 to +4.2; another with crustal contribution, minor Nb negative anomaly and εNd500Ma +0.8 to +1.8. Intermediate-acid rocks show variable REE fractionation and share geochemical characteristics of both basic–ultrabasic groups with restricted εNd500Ma +2.2 to 3.1 and general absence of Nb negative anomaly. Basic–ultrabasic melts resulted from different amounts of partial melting of a homogeneous source and segregation at the garnet–spinel transition zone. We argue that the “Hales transition” recently recognized in reflection seismic experiments of SW Iberia might image such a source region. Mantle-derived magmas ponded at the base of the crust and weakly interacted with crustal rocks/melts, whilst intermediate-acid rocks were generated by plagioclase±clinopyroxene±amphibole fractionation. Melt ascent took place through fractures, with limited crustal interaction. Based upon the new geochemical results and complementary cartographic and geophysical data, a model is presented for the Cambrian break-up of North Gondwana due to magma ascent from the mantle.

Metallogenic elements of the Halasu-Hadansun copper belt in the Junggar region, northern China

The Halasu-Hadansun copper belt is located in the Central Asian-Mongolian metallogenic domain, and represents a highly endowed ore-forming environment. At present, a variety of mineralisation types have been identified in the belt. Devonian porphyry-related mineralisation occurs in the northern part of the belt. In Xileketehalasu and Yulekenhalasu deposits, mineralisation occurs close to the porphyry, and with distinct alteration zones. The major economic elements are Cu, Mo and Au. Mineralisation in altered rocks of Permian age occurs in the central belt; and this mineralisation is related to the host rocks which are strongly silicified and include Permian intermediate-acidic rocks. Mineralisation in basic rocks, also of Permian age, is present in the southern part of the belt. The economic elements there are mainly Cu and Fe and with associated Ti. Quartz vein mineralisation of Triassic age is well developed and widely distributed, and is enriched in Cu in the northern part of the belt and forms Au deposits in the central belt.

Re-Os Geochronology of Molybdenite from Yuanzhuding Porphyry Cu-Mo Deposit in South China

The Yuanzhuding porphyry Cu-Mo deposit, discovered in 2008, is located within the southern segment of the Chenzhou-Huaiji fault belt, South China. The deposit is hosted within the Upper Cambrian neritic facies sandstone and shale formation that is the strata of the axis and wings of within the Yuanzhuding anticline. Both of the porphyry and exo-contact zone contain Cu-Mo mineralization. Economic orebodies occur mainly in the exo-contact zone around the porphyry. We dated 11 molybdenite samples obtained from two borehole cores of the deposit, using the Re-Os method. Analyses of eight molybdenite samples from borehole ZK12-112 yield a Re-Os isochron age of 157.3 ± 4.3 Ma (2σ), and analyses of 11 samples from boreholes ZK12-112 and ZK16-104 yield a Re-Os isochron age of 155.6 ± 3.4 Ma (2σ). These isochron ages are within the error of the Re-Os model ages, demonstrating that the age results are reliable and that the Yuanzhuding porphyry Cu-Mo deposit formed during the late Middle Jurassic. The formation age of the Yuanzhuding Cu-Mo deposit is similar to the age of intermediate-acid rocks and W-Sn polymetallic deposits along the Chenzhou-Huaiji fault belt. This concordance suggests the same geodynamic process to the igneous activity and related mineralization, raising the potentiality of the porphyry Cu-Mo mineralization in the belt.

Lead in K-feldspar and its relations to PB metallogenesis

In intermediate-acid magmatic rocks, alkaline magmatic rocks, gneisses and migmatitic rocks K-feldspar is a rock-forming rock in which the contents of Pb are highest, just 2 –10 times those of the whole rock,3–16 times those of mica minerals and 6–32 times those of quartz. The lowest contents of K-feldspar are recognized in Early Proterozoic and Achaean rocks, with Pb in the K-feldspar accounting for less than 10% of that in the whole rock; in post-Middle Proterozoic alkaline magmatic rocks, K-feldspar-rich granites and metamorphic rocks the contents of K-feldspar tend to increase, with the proportion of Pb over that in the whole rock being obviously increased. In the alkaline rocks in which K-feldspar accounts for 50% –70% of the total in the whole rock, the contents of Pb in K-feldspar account approximately for 70% –95% of the total lead in the whole rock. Being accessible to hydrothermal alteration in the late periods, K-feldspar was conversed to sericite, calcite, quartz, etc. In the process of such conversion the lead would be leached out and then find its way into fluid phase. This kind of transformation can provide sufficient ore-forming material for later Pb metallogenesis.

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Many of the Yanshannian intermediate-acid intrusive rocks related to Cu-Au mineralization in the Eastern Yangtze Block are characterized by high Al2O3, Sr contents, while low in Y, Yb contents, thus with high Sr/Y, and La/Yb ratios, and variational isotope signatures in particular, e.g. eNd(t) = −11.92–1.96, (143Nd/144Nd)i = 0.5120–0.5125, TDM = 0.70–1.71 Ga, (87Sr/86Sr)i = 0.7043 –0.7076. The geochemical characteristics of these rocks suggest that: (1) these rocks are geochemically similar to adakite, which might have been stemmed from the partial melting of thickened basaltic lower crust due to basalt underplating; and (2) the high pressure (1.2–4.0 GPa) and high temperature (850–1150°C) surroundings of the lower crust favor both the fluid and adakite-like magma to generation. Not only can the adakite-like magma carry abundant fluid and Cu-Au ore-froming materials, but also can it bring them to the shallow part with ease and contributes to the Cu-Au mineralization.

Geological implications for the deep-derived mafic enclaves from Cenozoic shoshonitic rocks in Jianchuan-Dali area, west Yunnan

There exist intermediate to intermediate-acid shoshonitic rocks dated at 26. 3–36.7 Ma in the Early Tertiary extensional basins along the NE flank of the Dali-Jianchuan section of the Honghe-Ailaoshan left-lateral strike-slip ductile shear zone, where some high- and medium-high grade metamorphosed mafic enclaves have been found. According to theP-T conditions and mineral assemblage characteristics, the enclaves are grouped into three types: type-I, garnet diopsidite, formed at lower crust or in the transitional section between the mantle and the crust with the metamorphic conditionP= (10.47–11.51) × 108 Pa andT =771-932°C; type-II, garnet- and diopside-bearing amphibolite, formed at the depth of middle to lower crust with the metamorphic condition:P = (6–10 ) × 108 Pa andT = 780–830°C; type-III, metamorphosed gabbro, formed at relatively higher levels than the above 2 types with lower metamorphic condition. Type-I and type-II enclaves had experienced a decompressional retrograde metamorphism caused by tectonic elevation associated with the metasomatism of SiO2, Na2O-rich fluid phase with the new equilibrated temperature atT = 761–778°C. Geochemical evidence indicates that both deep-derived mafic enclaves and host rocks have similar EMII properties, which are related to the involvement of subducted materials. The sustaining subduction, compression and strike-slip ductile shearing between the India plate and the Yangtze craton are the main constraints on the subcontinental mantle properties, interaction between crust and mantle, and the formation and evolution of alkali-rich magma.

Geology, geochemistry and genesis of the Mazhuangshan gold deposit in Hami, East Tianshan, Xinjiang, China

The Mazhuangshan area is located in the east of the Aqikekuduke Island Arc, where there are distributed intermediate-acid magmatic rocks emplaced during the Middle-Late Carboniferous. There are more than 20 orebodies in the area with an average gold grade 6.4 × 10−6 at present. The dominant metallic minerals are natural gold, auriferous silver, natural silver, pyrite and galena. Pyrite is the key gold carrier, high in Fe and low in S. Wall-rock alterations mainly include pyritization, silicification, and sericitization. Carbonation alteration was extensive at the late stage, often resulting in a high-grade orebody. Three mineralization stages may be distinguished.

Characteristics of magmatic rocks and tectonic setting of Mazhuangshan area, East Tianshan, China

The Mazhuangshan area lies in northeast to Aqikekuduke, where are distributed intermediate-acid rocks, which were formed in the Middle-Late Carboniferous epoch. The magmatic rocks are peraluminous and belong to calcareous series. All values of ACNK(1.03–2.12), ANK (1.67–2.93), SI(4.1–33), δ(0.71–4.82), La/ΣREE(0.09–0.26) ΣLREE/ΣHREE(1.76–11.01), (La/Sm)N(l.60–6.35), (Sm/Nd)N(0.33–0.58), (La/Yb)N(4.7–12.1), LaN/LuX(5.9–11.8), (Ce/Yb)N(3.14–6.64), δEu(0.38–1.06), RbN/SrN(42–120) and Sr* (0.02–0.2) demonstrate they vary with D_ I(45–90). S-B diagram also shows that the lithophile elements are abundant in acidic magmatic rocks. Moreover, it is concluded that the primitive magma originated from a subduction zone at the continental edge, and underwent fractional crystallization and contamination, then generated the magmatic rocks. The multi-cation index (R1: 3055–3993, R2: 286–438) shows that the primitive magma originated from the mantle. All these conclusions are beneficial for exploring gold deposits in the area.

Characteristic elemental compositions of the Yangtze River and Yellow River surface sediments and their geological background

Significant differences are noticed in major and trace element compositions between the Yangtze River and the Yellow River surface sediments. The former sediments are rich in some major elements such as K, Fe, Mg, Al, and most of the trace elements which show wide variations in element concentrations, whereas the Yellow River sediments only have higher Ca, Na, Sr, Ba, Th, Ga, Zr, Hf contents and show slight variations in element contents. In the Yangtze River Basin are widely distributed intermediate-acid igneous rocks and complicated source rocks together with strong chemical weathering which determine the elemental compositions of the Yangtze River sediments, while the elemental compositions of the Yellow River sediments are decided by the chemical composition of loess from the Loess Plateau and intense physical weathering. Cu, Zn, Sc, Ti, Fe, V, Ni, Cr, Co, Li and Be can be used to distinguish the Yangtze River sediments from the Yellow River sediments and be treated as tracers for both the sediments to study the processes of their mixing and diffusion in the coastal zones of China.

Discovery of granulite from the Fuping area in southern Qinling Mountains and its geological significance

Two-pyroxene-bearing granulites were discovered for the first time in the Hercynian-lndosinian metamorphic belt of the southern Qinling Mountains. The granulites occur In the lower part of the Fuping complex and are dominated by intermediate-acidic rocks intercalated with a small amount of bands and lenses of basic granulites. The main metamorphic minerals include orthopyroxene, clinopyroxene, biotite, plagioclase and quartz, and orthopyroxenes are often retrogressively transformed Into amphiboles. The metamorphic conditions have been estimated to beT = 720–780°C andP = 0. 6 GPa. The granulite-bearing Fuping complex probably belongs to early Proterozoic in age. Whether it occurs as the crystalline basement of the southern Qinling Hercynian-lndosinian orogenic belt or as a thrusted slice in the collisional process needs further study.

Geology and geochemistry of reworking gold deposits in intrusive rocks of China—I. Features of the intrusive rocks

Most gold deposits in intrusive rocks were formed as a result of reworking processes. The intrusive rocks containing gold deposits and consisting of ultramafic-mafic, intermediateacid and alkaline rocks of the Archean, Proterozoic, Caledonian, Hercynian and Yenshanian periods occur in cratons, activated zones of cratons and fold belts. Among them, ultramaficmafic rocks, diorite, alkaline rocks, and anorthosite are products of remelting in the mantle or mantle-crust or mantle with crustal contamination. However, auriferous intermediate-acid rocks are products of metasomatic-remelting in auriferous volcanic rocks or auriferous volcanosedimentary rocks in the deep crust.

The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales

Summary The major and trace element geochemistry of the early to middle Proterozoic meta-igneous rocks of the Willyama Complex in the Broken Hill Block indicates that the basic rocks originated by partial melting of a depleted mantle source, followed by fractional crystallization along a trend of strong iron enrichment coupled with some crustal assimilation. At least some of the intermediate-acid rocks were derived from fractionation of these basic parents, but major crustal involvement may be necessary to explain the large quantities of felsic volcanics in the lower part of the Complex. The volcanic rocks in the Broken Hill mine area show an unusually high degree of primary iron enrichment (up to 25 wt % total iron as Fe 2 O 3 ) and a lesser degree of bimodality than elsewhere. By analogy with Recent ferrobasalt provinces, this extreme iron enrichment may indicate an origin in a propagating rift where small, high-level magma chambers separated from the magma source. If so, the consequent high geothermal gradient and high fracture permeability may have provided the necessary conditions for genesis of the Broken Hill ore deposit.

Xenoliths from southern Africa: a perspective on the lower crust

Summary Granulite-facies xenoliths from southern African kimberlites yield temperatures from 780–900°C and pressures from 10–15 kb. They have Nd model ages of 1.4–3.7 Ga and retain Rb-Sr mica and Sm-Nd garnet ages from 0.14–1 Ga, significantly older than kimberlite emplacement ages. It is argued that low abundances of U, K and Rb in lower crustal rocks can be either primary features resulting from plagioclase-dominated cumulate processes, or secondary effects caused by the granulite-facies breakdown of mica in wet intermediate-acid rocks. Combined trace element and isotope arguments suggest that contamination with lower crustal material does not significantly affect the geochemistry of flood basalts from the Karoo, the Parana and the Deccan.

Experimental studies of leaching and albitization in intermediate-basic rocks

Experimental studies combined with geological evidence indicate that gaseous thermal fluids enriched in NaCl (or other chlorides) leach and metasomatize intermediate-basic and intermediate-acidic rocks. In this process, part or all of the rock- and ore-forming materials are mobilized and enriched to form skarns and Fe deposits. When gaseous thermal fluids enriched in NaCl (or other chlorides) act on intermediate-basic rocks, the mobilities of the main components in the rocks are, in decreasing order K (or Na), Fe, Ca, Si, Mg and Al. The mobility of Al is extremely small and the mobility of Mg is not large either. The leaching capacities of high-temperature fluids for rock- and ore-forming elements, depend mainly on the kinds and numbers of negative and positive ions, pH and Eh, temperature and pressure, etc. Acidic solution and high temperature favor the leaching of Fe whereas alkaline solution and high pressure favor the leaching of SiO 2. At 550°C and 750–1000 bar, weak alkaline solution enriched in NaCl favors the formation of albite. Acidic solution favors the formation of minerals such as amphiboles and chlorites while wollastonite can form in wider ranges of pH. The mobility of Fe and albitization represent the change in pH-values of the solutions at different mineralized stages.

REE geochemistry of sea-floor sediments of the continental shelf of the East China Sea

REE geochemistry has been studied of sea-floor sediments of the continental shelf of the East China Sea. The average content of RE2O3 is estimated at 175 ppm based on 68 sediment samples from the continental shelf. The absolute concentrations of Y and La-Lu in the sediments are shown in Table 3. The REE distribution patterns in the sediments demonstrate a distinct depletion in Eu and a negative slope. It is considered that the fragments of Mesozoie intermediate-acid igneous rocks widely distributed in southeastern China are the main source of REE in sea-floor sediments of the East China Sea continental shelf.

Considerações sobre a origem dos grãos "olhos-de-quartzo" nas rochas porfiroidais do distrito de Western Mines, Ilha de Vancouver, Canadá

Quartz-eye bearing porphyroidal occur in straight association with the vent ore, in the vent zone, of the base metal massive sulphide deposits of the Western Mines District. Microscopic observations on these indicated two stages of development, which are part of a continuous process of rock alternation. In the earlier stage (first generation quartz) there was a total or quasi-total replacement of chloritecalcite spherulites or a filling of vesicles by quartz. It was accompanied by development of strong vertical foliation which superimposed to the primary bedding. In the final stage (second generation quartz) the quartz overprinted the vertical schistosity, and deformation was minimum. Quartz-eye typical of the earlier stage, termed first generation quartz, is surrounded by a bowed micaceous matrix; this is not the case when it is examined in a section perpendicular to the foliation. Composite grains and recrystallized single round grains may be in association with deformed or broken grains of original quartz and with latter or second generation grains. A preserved nucleus of calcite is occasionally observed. Hypidiomorphic and idiomorphic coarse grains of quartz are typical of the final fase of the vent alteration process, they are termed second generation quartz. Overgrowth of second generation quartz without optical character of continuity upon first generation quartz is commonly observed. In addition most of the round rims of quartz represent the outer zone of the crystallized second generation quartz. It becomes distinguishable from the rest of the quartz grain because of the abundance of microinclusions. Since both stages of quartz generation affected either mafic-intermediate or intermediate-acidic rocks, it is suggested that the term quartz-eye bearing porphyroidal rocks might be better regarded as a textural terminology than a compositional one.