New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites. A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.

/pdf/a-type-granites-geochemical-characteristics-discrimination-489z52bx3o.pdf

A-type granites: geochemical characteristics, discrimination and petrogenesis

. Granitoids and related rocks of eastern Australia can be classified according to their metallogenic potential using a scheme based on compositional character, degree of compositional evolution, degree of fractionation, and oxidation state. The scheme is based on empirical and theoretical considerations and satisfactorily describes the known distribution of granite-related mineralisation.

The granitoids range from unevolved, mantle compatible compositions to highly evolved and fractionated. They exhibit age- and region-specific variations in silica content, compositional evolution and oxidation state. The most unevolved intrusive igneous rocks comprise those of the Ordovician of the Lachlan Orogen, and the Devonian of the New England Orogen. Strongly fractionated and evolved I-type granites occur in western Tasmania, the southern New England Orogen, and far north Queensland. Other fractionated suites tend to occur relatively rarely in the Lachlan Orogen and elsewhere. Oxidation states vary markedly. The most consistently oxidised rocks occur in the Ordovician of the central Lachlan Orogen, and the northernmost New England Orogen. The Carboniferous I-types of the northeastern Lachlan Orogen are consistently more oxidised than other Lachlan Orogen I-types.

Gold-rich, Cu-poor systems associated with felsic I-types in eastern Australia are associated with W-Mo mineralised suites with gold occurring within a predictable metallogenic zonation. Gold mineralised I-types comprise weakly to moderately oxidised, high-K granitoid suites that, at least in the east Australian context, have low K/Rb ratios and show strong fractionation trends. Gold is readily removed from granitic magmas through the early precipitation of sulfides, or to a lesser extent by magnetite. Crystallisation of Fe-poor, silica-rich granitic magmas in a relatively narrow oxidation window between the FMQ and NNO buffers may provide conditions where retention of Au in magmas in felsic granitic magmas is optimised.

/pdf/redox-and-compositional-parameters-for-interpreting-the-g3nrxqkzz2.pdf

Redox and Compositional Parameters for Interpreting the Granitoid Metallogeny of Eastern Australia: Implications for Gold-rich Ore Systems

The 37 � 1 Ma breccia-hosted porphyry Mo(^Nb) deposit at Cave Peak is genetically related to a mafic, alkaline intrusion nearby (the Marble Canyon Stock), thus providing an excellent opportunity to study the influence of mafic magmatism on porphyry Mo formation. Laser ablation inductively coupled plasma mass spectrometry analysis of crystallized melt inclusions allowed reconstruction of pristine concentrations of Mo and other fluid-mobile elements in melts ranging from phonotephrite (� 50 wt % SiO2 )t o rhyolite (� 75 wt % SiO2) in composition. Incompatible trace element abundances in these melt inclusions define smooth, linear trends reaching up to 100 times average upper continental crust, implying that the felsic magmas evolved from a mafic parent by fractional crystallization.The mafic parent has a geochemical signature typical of primitive melts generated in within-plate tectonic settings. Molybdenum increases from 4 ppm in the mafic endmember to 12 ppm in felsic melts immediately before Mo mineralization. After breccia formation, Mo concentrations in the residual melt decreased gradually to 5 ppm, suggesting that Mo was removed from the residual melts by exsolving fluids. This interpretation is supported by fluid^melt partition coefficients (DMo, fluid/melt ¼ 17^20) measured on assemblages of coexisting fluid and melt inclusions. A high sulfur fugacity in the mafic input magma is indicated by early pyrrhotite saturation. To account for the 0·4^1·5 � 10 6 t Mo present in the deposit a volume of � 30^100 km 3 alkali feldspar granite magma is needed, or up to 10 times more of the mafic melt. Significantly less magma is required to provide all the sulfur present in the deposit.The results of this study provide direct evidence for derivation of metals (and sulfur) in porphyry Mo deposits from associated mafic, alkaline magmas through fractional crystallization.

Source and evolution of molybdenum in the porphyry Mo(-Nb) deposit at Cave Peak, Texas

In this contribution we discuss a series of porphyry and porphyry-skarn mineral systems, which constitute important metallogenic belts along major lithospheric breaks and/or collision zones that form the margins of the North China craton and the Yangtze craton. We describe a selection of porphyry and porphyry-skarn deposits in the Great Xing’an Range in northeast China, the Yanshan-Liaoning, Xilamulum belts along the northern margin of the North China craton, the Qinling orogen, and the Middle-Lower Yangtze River Valley along the bonudaries between the North China and the Yangtze cratons. The occurrence of platform carbonate successions may have played some role in the evolution of ore fluids that resulted in the formation of porphyry-skarn deposits. The porphyry deposits of eastern China, although sharing some common features with classic porphyry systems that are related to convergent margins, do exhibit unusual features, particularly in terms of ore fluid compositions, the common presence of fluorite, hydrothermal alteration assemblages, which tend to be less hydrous than the porphyry at convergent margins. On the other hand, the porphyry mineral systems of eastern China have some similarities with the Climax-type porphyry. A major tectonothermal event (Yanshanian), which took place in the Jurassic-Cretaceous, affected eastern China and most of the eastern margin of Asia. This Yanshanian event was responsible for the magmatism and associated mineral systems in the region. The granitoid intrusions and accompanying porphyry and porphyry-skarn deposits were developed along orogenic belts and major structural discontinuities mostly along the tectonic boundaries between the North China and Yangtze cratons and between the northern margin of the North China craton and the Mongolia-Siberia plate. The porphyry and porphyry-skarn deposits herein described include: Chalukou in the Great Xing’an Range orogen, Jiguanshan in the Xilamulun metallogenic belt, Xiaojiayingzi, Lanjiagou, Chehugou, Yuanbaoshan in the Yanshan-Liaoning belt, Nanninhu, Yuchiling, Jinduicheng, Lemeingou in the East Qinling orogen, Tongchakoou, Shaxi from the Middle-Lower-Yangtze River Valley. The role of mantle-lithosphere dynamics in eastern China, which led to the formation of these porphyry and porphyry-skarn mineral systems, is discussed and reviewed.

Intracontinental Porphyry and Porphyry-Skarn Mineral Systems in Eastern China: Scrutiny of a Special Case “Made-in-China”

This chapter focuses on S in porphyry-type ore deposits, layered-mafic-intrusion-hosted ore bodies, and magmatic sulfide deposits. Porphyry-type ore deposits, e.g., Bingham Canyon, Utah, U.S.A. and Grasberg, Irian Jaya, are important hosts of Cu, Mo, Au, and Ag. Ore deposits hosted in layered mafic intrusions, e.g., the Bushveld and Stillwater complexes, contain significant quantities of Ni, Cu, Cr, Au and the platinum group elements (PGE: Pt, Pd, Rh, Re, Ir, Ru). Magmatic sulfide deposits, differentiated from layered mafic intrusions in that the former evince more clearly a role for immiscible sulfide accumulation without the possible presence of an aqueous fluid(s), e.g., Noril’sk and Voisey’s Bay, contain significant quantities of Cu, Ni, and the PGE. Allowing for some variability within any given ore deposit type, each of these types is unique in terms of the range of pressure and temperature of ore deposit formation, tectonic setting, and the compositional type(s) of parental causative magma. However, most porphyry-type ore deposits, layered-mafic-intrusion-hosted ore bodies, and magmatic sulfide deposits share the following features: 1) they are related chemically and physically to silicate magma; 2) they are the byproduct of differentiation of magma; 3) metals are hosted dominantly in sulfide minerals and the deposits can be thought of primarily as S anomalies (e.g., the Butte and Bingham Canyon porphyry ore deposits, located in Montana and Utah, U.S.A., respectively, contain 60 and 100 times, respectively, the ~20 Mt of S emitted during the 1991 eruption of Mt. Pinatubo); and 4) the metal(s) and S in each deposit type, albeit not necessarily the total quantity of S, are together derived from the same magmatic source. It is the connectivity of these commonalities that serves as the basis for this review chapter.

The ubiquitous presence of S in magmatic systems is manifested in the commonly observed mass of S …

The Role of Magmatic Sulfur in the Formation of Ore Deposits

Stockwork molybdenite deposits in the western conterminous United States may be divided into two groups based on the chemistry of the source pluton: granodiorite systems and granite systems. These two groups show significant differences in age, tectonic setting at time of formation, and orebody geometry.Granite molybdenite systems develop in, and adjacent to, epizonal plutons that meet Tuttle and Bowen's (1958) definition of granite. Cenozoic source plutons for these systems were emplaced in the eastern Rockies and Great Basin coincident with, or just prior to, regional extensional tectonism. Mineralization and alteration patterns in granite molybdenite systems generally conform to those described at the Climax and Henderson deposits.Major element data displayed on Ab-Or-Q and A-K-F plots 1 can characterize potassic, quartz-sericite, argillic, and silicic alteration assemblages and lead to evaluation of the probable depth to mineralization in prospects. Observed alteration reflects hydrogen and alkali metasomatism in fluorine-rich systems. Minor elements concentrated in granite molybdenite systems include Be, Cs, F, Li, Mo, Nb, Rb, Sn, Ta, Th, U, and W.Geophysical and geological evidence indicates that the cylindrical source plutons associated with known granite molybdenite systems are cupolas on the tops of batholiths, suggesting a genetic link between batholiths and ore. A granite molybdenite system appears to be the result of the following: lithophile element concentration by convection-aided thermogravitational diffusion in the upper part of a batholithic magma chamber; transfer of the resulting high silica, volatile-rich magma from the roof zone of the batholith to a high-level cylindrical cupola; establishment of thermogravitational diffusion in the cupola causing further lithophile element concentration; formation of potassic alteration and an ore shell as a result of volatile saturation, resurgent boiling, hydrofracturing, and boiling; and the evolution of peripheral alteration assemblages as meteoric waters enter the magmatic fluid plume above the cupola. Stacked orebodies represent discrete episodes of resurgent boiling, hydrofracturing, and mineralization.Computer-assisted evaluation of major element analyses of igneous rocks, using chemical criteria developed from known granite molybdenite systems, may locate areas with a high exploration potential. These techniques suggest that several late Precambrian bimodal basaltrhyolite terrains in the eastern United States and Canada have exploration potential for molybdenum.

Granite molybdenite systems

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Granite molybdenite systems