Diorite Intrusions Research Articles

Evolution of an Early Permian extensional detachment fault from synintrusive, mylonitic flow to brittle faulting (Grassi Detachment Fault, Orobic Anticline, southern Alps, Italy)

Abstract Lower Permian volcanic and sedimentary rocks of the Collio Formation in the Orobic Anticline do not rest with a depositional contact on the Variscan basement but are separated from it by the subhorizontal Grassi Detachment Fault, consisting of a cataclasite layer underlain by mylonite. Field relations indicate that both the cataclasite and the mylonite are Early Permian in age. The mylonite formed in a continuous process before, during, and after the intrusion of the Val Biandino Quartz Diorite in the footwall of the detachment fault. Microstructure and quartz texture of the mylonite indicate top-to-the-southeast displacement. Quartz textures of mylonite close to the intrusive bodies are characterized by c-axis single maxima near the Y-direction of the finite strain, indicating prism <a>glide as the dominant gliding system and hence high temperatures (above c. 500 °C) during mylonitization. This is explained by heat advection through the rising quartz diorite melt. During detachment faulting, the footwall of the Grassi Detachment Fault was bowed up to form a metamorphic core complex. The Ponteranica Conglomerate was deposited as a proximal, syntectonic fan-delta on the southeast side of the metamorphic core complex late in its evolution. The unconformity of the Verrucano Lombardo over the Collio Formation and the basement results from erosion of the topography created by detachment faulting, core complex updoming, and block tilting. These results indicate dramatic SE–NW stretching (in present-day coordinates) of the South-Alpine crust during the Early Permian. The return from the thickened, orogenic crust at the end of the Hercynian orogeny to the normal crustal thickness ( c. 30 km) of Late Permian and Early Triassic times was accommodated to a large extent by crustal extension, at least in this part of the southern Alps.

A review of the Cu–Ni sulphide deposits in the Chinese Tianshan and Altay orogens (Xinjiang Autonomous Region, NW China): Principal characteristics and ore-forming processes

Several Cu–Ni sulphide deposits and occurrences have recently been discovered along parallel deep faults in the Chinese Tianshan and Altay orogenic belts, Xinjiang Province, NW China. The Kalatongke and several other Cu–Ni mineralized intrusions are located along the Irtysh fault, which separates the Altay orogenic belt from the Junggar basin. The Huangshan, Huangshandong, Xiangshan, Tudun, Erhongwa, Tula’ergen, and Hongling deposits occur along the Kanggurtag suture, which separates the Jueluotage orogenic belt from the Turpan-Hami (Tuha) basin. The Baishiquan, Tianyu, and Tianxiang deposits are located along the Arqikekuduke fault, which separates the Jueluotage orogenic belt from the Central Tianshan Precambrian terrane. The Poyi, Poshi, and Luodong deposits are located along the Baidiwa fault, which separates the Central Tianshan Precambrian terrane from the Beishan Paleozoic rift. Re–Os dating of Cu–Ni sulphide ores reveals that these Cu–Ni ore belts formed in a narrow age range of 298–282 Ma. This age range is about the same as those of associated intrusions and dykes dated by the SHRIMP zircon U–Pb method. Tectonic and geochronological constraints suggest that the amalgamation of continental blocks mainly occurred during the Late Carboniferous in the Central Asian orogenic belt. Large-scale hydrothermal and magmatic metallogenesis in the region occurred during post-collisional stages of Latest Carboniferous to Early Permian age. The Cu–Ni sulphide deposits are part of this metallogenic event. The Cu–Ni orthomagmatic sulphide deposits in northern Xinjiang are represented by: (1) net-textured type deposits by segregation of a Cu–Ni metal sulphide melt and (2) magma conduit type. Some mafic–ultramafic suites exhibit lithological zoning caused by strong differentiation. Stratiform orebodies are hosted by ultramafic rocks at the base of the magma chamber. Good examples are the Kalatongke, Huangshandong, and Poshi deposits. Others, such as the Xiangshan, Baishiquan, Tianyu, and Tula’ergen deposits, are hosted by magma conduits, consisting of peridotite, troctolite, and pyroxenite. These ultramafic rocks either occur within faults or are surrounded by gabbroic and dioritic intrusions. These two types of orthomagmatic Cu–Ni sulphide deposits are also distributed along the same ore belts. For instance, the differentiated sill-related Huangshandong deposit co-exists with the magma conduit type Tula’ergen deposit in the Jueluotage orogenic belt. Orthomagmatic Cu–Ni sulphide deposits in northern Xinjiang formed during post-collisional extension and are possibly related to a Late Carboniferous–Early Permian mantle plume event. The mafic–ultramafic suites and associated Cu–Ni deposits are commonly accompanied by dyke swarms and are characterized by elongated outcrops occurring along parallel E–W-trending regional faults. These mafic–ultramafic suites and accompanying dyke swarms are generally fractionated, implying that they were feeders of presently eroded flood basalts.

Intrusive rocks and tectono-metamorphic evolution of the Mako Paleoproterozoic belt (Eastern Senegal, West Africa)

The Kedougou Kenieba Inlier (KKI) (Paleoproterozoic of Eastern Senegal) is a portion of the West African Craton (WAC) containing a granite-greenstone terrain that experienced three distinct periods of magmatic activity, peaking at 2200, 2160–2130 and 2100–2070 Ma. In the Inlier, Paleoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Our structural data and deformation sequence indicate that the Mako Greenstone Belt and the adjacent granitoid complexes have undergone a multi-phase evolutionary history that is spread over a prolonged period. The available geochronological data and field studies allowed classification of the granitoids of the KKI into four generations GI, GII, GIII and GIV. The current data suggest that the oldest rocks in the KKI, the Badon granites (2198 ± 2 Ma) and the tonalitic gneisses from Tonkouto (2200–2198 Ma) (GI), could be correlated with an early Birimian magmatic event. The gneisses, crystallized at depth, record the earliest deformation and in contrast to other tonalites, do not appear to have intruded volcanic rocks. The second manifestation of magmatism was intrusion of mafic diorite – the Gabbro Sandikounda Layered Igneous Complex type (GII) and development of the Laminia Kaourou Plutonic Complex (LKPC) (2160–2130 Ma). These bodies pre-date or are sometimes synchronous with a major deformational episode, and may, therefore, have formed very early in convergent Birimian orogenesis. The third major peak of magmatic activity occurred after the above major episode with the development of the oval shaped Diombalou and Bouroumbourou plutons (GIII). The orientation of these plutons parallel to the regional strike of the schistosity indicates structural control on granite emplacement. Eburnean magmatism was terminated in the Mako Belt following compressional Eburnean deformation, with the emplacement of the Tinkoto, Mamakono plutons (GIV) in the east of the complex and continued in the Dialé–Daléma supergroup with the syntectonic emplacement of the Saraya batholith. Garnitiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2080 Ma. Field observations suggest the early plutons of the complex granitic (Kakadian) batholith intruded during convergent deformation whereas later igneous activity accompanied regional orogen-parallel extension, followed by exhumation. In the Mako Belt, thickening of the crust was proposed to have caused heating and the ‘apparent diapiric rise’ of the Diombalou and Bouroumbourou plutons.

Interaction of adakitic melt-peridotite: Implications for the high-Mg# signature of Mesozoic adakitic rocks in the eastern North China Craton

Abundant dunite and harzbugite xenoliths are preserved in Early Cretaceous high-Mg# [63–67, where Mg# = molar 100 × Mg/(Mg + Fe tot)] diorite intrusions from western Shandong in the North China Craton (NCC). Dunite and some harzburgite xenoliths typically preserve areas of orthopyroxenite (sometimes accompanied by phlogopite) either as veins or as zones surrounding chromite grains. Harzburgite is chiefly composed of olivine, orthopyroxene, minor clinopyroxene and chromian-spinel. High Mg#'s (averaging 91.4) and depletions in Al 2O 3 and CaO (averaging 0.52 wt.% and 0.29 wt.%, respectively) in harzburgite and dunite xenoliths suggest that they are residues formed by large degrees of polybaric melting. However, olivines and orthopyroxenes from dunite xenoliths spatially associated with orthopyroxenite display lower Mg#'s (i.e., 82–87 and 83–89, respectively), suggesting that an adakitic melt–peridotite reaction has taken place. This is consistent with the production of veined orthopyroxene or orthopyroxene + phlogopite in dunite and some harzburgite xenoliths in response to the introduction of adakitic melt into the previously depleted lithospheric mantle (i.e., harzburgite and dunite xenoliths). The presence of orthopyroxene in veins or as a zones surrounding chromite in peridotite xenoliths is thought to be representative of adakitic melt metasomatism. The dunite and harzbugite xenoliths are relatively rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), poor in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and lack Eu anomalies on chondrite normalized trace element diagrams. The initial 87Sr/ 86Sr ratios and εNd( t) values for the xenoliths range from 0.7058 to 0.7212 and + 0.18 to − 19.59, respectively. Taken together, these features, combined with the strong depletion in HFSE and the existence of Archean inherited zircons in the host rocks, suggest that the adakitic melt was derived from the partial melting of early Mesozoic delaminated lower continental crust. The interaction of the adakitic melt with peridotite is responsible for the high-Mg# character of the early Cretaceous diorites in western Shandong.

Post-collisional melting of crustal sources: constraints from geochronology, petrology and Sr, Nd isotope geochemistry of the Variscan Sichevita and Poniasca granitoid plutons (South Carpathians, Romania)

The Sichevita and Poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the Getic Nappe in the South Carpathians. The present work provides SHRIMP age data for the zircon population from a Poniasca biotite diorite and geochemical analyses (major and trace elements, Sr–Nd isotopes) of representative rock types from the two intrusions grading from biotite diorite to biotite K-feldspar porphyritic monzogranite. U–Pb zircon data yielded 311 ± 2 Ma for the intrusion of the biotite diorite. Granites are mostly high-K leucogranites, and biotite diorites are magnesian, and calcic to calc-alkaline. Sr, and Nd isotope and trace element data (REE, Th, Ta, Cr, Ba and Rb) permit distinguishing five different groups of rocks corresponding to several magma batches: the Poniasca biotite diorite (P1) shows a clear crustal character while the Poniasca granite (P2) is more juvenile. Conversely, Sichevita biotite diorite (S1), and a granite (S2*) are more juvenile than the other Sichevita granites (S2). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within P1 and S1 groups. Dehydration melting of a number of protoliths may be the source of these magma batches. The Variscan basement, a subduction accretion wedge, could correspond to such a heterogeneous source. The intrusion of the Sichevita–Poniasca plutons took place in the final stages of the Variscan orogeny, as is the case for a series of European granites around 310 Ma ago, especially in Bulgaria and in Iberia, no Alleghenian granitoids (late Carboniferous—early Permian times) being known in the Getic nappe. The geodynamical environment of Sichevita–Poniasca was typically post-collisional of the Variscan orogenic phase.

Early Cretaceous U–Pb zircon ages for the Copiapó plutonic complex and implications for the IOCG mineralization at Candelaria, Atacama Region, Chile

Four of the major plutons in the vicinity of the Candelaria mine (470 Mt at 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag) and a dike–sill system exposed in the Candelaria open pit have been dated with the U–Pb zircon method. The new geochronological data indicate that dacite magmatism around 123 Ma preceded the crystallization of hornblende diorite (Khd) at 118 ± 1 Ma, quartz–monzonite porphyry (Kqm) at 116.3 ± 0.4 Ma, monzodiorite (Kmd) at 115.5 ± 0.4 Ma, and tonalite (Kt) at 110.7 ± 0.4 Ma. The new ages of the plutons are consistent with field relationships regarding the relative timing of emplacement. Plutonism temporally overlaps with the iron oxide Cu–Au mineralization (Re–Os molybdenite ages at ∼115 Ma) and silicate alteration (ages mainly from 114 to 116 and 110 to 112 Ma) in the Candelaria–Punta del Cobre district. The dated dacite porphyry and hornblende diorite intrusions preceded the ore formation. A genetic link of the metallic mineralization with the quartz–monzonite porphyry and/or the monzodiorite is likely. Both of these metaluminous, shoshonitic (high-K) intrusions could have provided energy and contributed fluids, metals, and sulfur to the hydrothermal system that caused the iron oxide Cu–Au mineralization. The age of the tonalite at 110.7 Ma falls in the same range as the late alteration at 110 to 112 Ma. Tonalite emplacement may have sustained existing or driven newly developed hydrothermal cells that caused this late alteration or modified 40Ar/39Ar and K/Ar systematic in some areas.

The Lindeques Drift and Heidelberg Intrusions and the Roodekraal Complex,Vredefort, South Africa: comagmatic plutonic and volcanic products of a 2055 Ma ferrobasaltic magma

The Lindeques Drift and Heidelberg Intrusions and the volcanic Roodekraal Complex belong to a high-Ti mafic igneous suite (HITIS) that was emplaced in the Kaapvaal Craton as km-sized bodies, to the north and north-east of the Vredefort impact structure, prior to impact. New zircon SHRIMP ages for the Lindeques Drift Intrusion (2054.8 ± 5.7 Ma) and Roodekraal Complex (2053 ± 9.2 Ma) are indistinguishable from that of the Bushveld magmatic event (~2.06 to 2.05 Ga). The dominant rock types in the Lindeques Drift and Heidelberg Intrusions are even-grained and porphyritic spessartites which essentially are clinopyroxene-magnetite (-ilmenite) ± olivine cumulates with variable fractions (estimated 15 to 30%) of interstitially crystallized magma. The concentration of magmatic amphibole (edenite-magnesiohastingsite), responsible for the porphyritic texture of the spessartite, is directly proportional to the fraction of trapped magma, which also crystallized interstitial plagioclase, sphene, sulphide, apatite, microperthite and biotite. The Lindeques Drift spessartite is cut by fine-grained dykes and sills(?) of diorite and syenodiorite. Low-silica diorite in the Lindeques Drift Intrusion was produced by the reaction of the parental magma with the dolomitic country rock of the Chuniespoort Group through desilicificaton and de-alumination. The Roodekraal Complex is composed of multiple flows of predominantly mugearite lava with sub-flow intrusions of diorite. The diorite of Lindeques Drift and the lava and diorite of Roodekraal are chemically comparable in terms of both major and trace elements, suggesting a co-magmatic derivation. The magma parental to the three bodies is proposed to be a low-Al ferrobasalt with alkaline affinities (MgO = 4.8%, TiO 2 =1.8%, P 2 O 5 = 0.52%, Zr/(P 2 O 5 · 10 4 ) = 0.02, Nb/Y=1.47, K 2 O/Yb =1.3, Ta/Yb = 1.1). The latter was probably derived by deep crustal fractionation of amphibole and plagioclase from an alkali basalt(?) precursor in an embryonic continental rift setting. The probability that a gabbro dyke, stratigraphically below and perpendicular to the Lindeques Drift Intrusion, is a feeder to the latter is regarded as small.

Geochemistry and evolution of ore-forming fluids of the Yueshan Cu–Au skarn- and vein-type deposits, Anhui Province, South China

The Yueshan mineral belt is geotectonically located at the centre of the Changjiang deep fracture zone or depression of the lower Yangtze platform. Two main types of ore deposits occur in the Yueshan orefield: Cu–Au–(Fe) skarn deposits and Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits. Almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the Yueshan dioritic intrusion. In the vicinity of the Zongpu and Wuhen intrusions, there are many Cu–Pb–Zn–Au–(S) vein-type and a few Cu–Fe–(Au) skarn-type occurrences. Fluid inclusion studies show that the ore-forming fluids are characterised by a Cl −(S)–Na +–K + chemical association. Hydrothermal activity associated with the above two deposit types was related to the Yueshan intrusion. The fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water. In comparison, the hydrothermal activity related to the Zongpu and Wuhen intrusions was characterised by low salinity fluids. Chlorine and sulphur species played an important role in the transport of ore-forming components. Hydrogen- and oxygen-isotope data also suggest that the ore-forming fluids in the Yueshan mineral belt consisted of magmatic water, mixed in various proportions with meteoric water. The enrichment of ore-forming components in the magmatic waters resulted from fluid–melt partitioning. The ore fluids of magmatic origin formed large Cu–Au deposits, whereas ore fluids of mixed magmatic-meteoric origin formed small- to medium-sized deposits. The sulphur isotopic composition of the skarn- and vein-type deposits varies from − 11.3‰ to + 19.2‰ and from + 4.2‰ to + 10.0‰, respectively. These variations do not appear to have been resulted from changes of physicochemical conditions, rather due to compositional variation of sulphur at the source(s) and by water–rock interaction. Complex water–rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing. Lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore-forming fluids. Hydrodynamic calculations show that magmatic ore-forming fluids were channelled upwards into faults, fractures and porous media with velocities of 1.4 m/s, 9.8 × 10 − 1 to 9.8 × 10 − 7 m/s and 3.6 × 10 − 7 to 4.6 × 10 − 7 m/s, respectively. A decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the Triassic sedimentary rocks led to the deposition of the ore-forming components. The major species responsible for Cu transport are deduced to have been CuCl, CuCl 2 −, CuCl 3 2− and CuClOH, whereas Au was transported as Au 2(HS) 2S 2−, Au(HS) 2 −, AuHS and AuH 3SiO 4 complexes. Cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of Cu deposition. Gold deposition was related to decrease of pH, total sulphur concentration and fO 2, which resulted from fluid boiling and mixing. Geological and geochemical characteristics of the two deposit types in the Yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism. Geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 Ma and represent a continuous process during the Yanshanian time. The cooling of the intrusions and the mineralisation event might have lasted about 6 Ma. The cooling rate of the magmatic intrusions was 80 to 120 °C my − 1 , which permitted sufficient heat supply by magma to the ore-forming system.

Geochemical patterns of arsenic-enriched ground water in fractured, crystalline bedrock, Northport, Maine, USA

High mean As concentrations of up to 26.6 μmol/L (1990 μg/L) occur in ground water collected from a fractured-bedrock system composed of sulfidic schist with granitic to dioritic intrusions. Sulfides in the bedrock are the primary source of the As in the ground water, but the presence of arsenopyrite in rock core retrieved from a borehole with As concentrations in the ground water barely above the detection limit of 2.0 μmol/L, shows that there are complicating factors. Chemical analyses of water from 35 bedrock wells throughout a small watershed reveal spatial clustering of wells with high As concentrations. Stiff diagrams and box plots distinguish three distinct types; calcium-bicarbonate-dominated water with low As concentrations (CaHCO 3 type), sodium-bicarbonate-dominated water with moderately high As concentrations (NaHCO 3 type), and calcium-bicarbonate-dominated water with very high As concentrations (High-As type). It is proposed that differences in recharge area and ground-water evolution, and possible bedrock composition difference are responsible for the chemical distinctions within the watershed. Lack of correlation of As concentrations with pH indicates that desorption of As is an insignificant control on As concentration. Correlations of As concentrations with Fe and redox parameters indicates that reductive dissolution of Fe(III) oxyhydroxides may play a role in the occurrence of high As concentrations in the NaHCO 3 and High-As type water. The oxidation of sulfide minerals occurs within the ground-water system and is ultimately responsible for the existence of As in the ground water, but there is no correlation between As and SO 4 concentrations, probably due to precipitation of Fe(III) oxyhydroxides and adsorption of As under oxidizing conditions.

The El Galeno and Michiquillay porphyry Cu–Au–Mo deposits: geological descriptions and comparison of Miocene porphyry systems in the Cajamarca district, northern Peru

El Galeno and Michiquillay are early to middle Miocene Cu–Au–Mo porphyry-related deposits located in the auriferous Cajamarca district of northern Peru. The El Galeno deposit (486 Mt at 0.57% Cu, 0.14 g/t Au and 150 ppm Mo) is associated with multiple dioritic intrusions hosted within Lower Cretaceous quartzites and shales. Emplacement of the porphyry stocks (17.5–16.5 Ma) in a hanging wall anticline was structurally controlled by oblique faults superimposed on early WNW-trending fold-thrust structures. Early K-feldspar–biotite–magnetite (potassic) alteration was associated with pyrite and chalcopyrite mineralisation. A quartz–magnetite assemblage that occurs at depth has completely replaced potassically altered rocks. Late- and post-mineralisation stocks are spatially and temporally related to weak quartz–muscovite (phyllic) alteration. High Au grades are associated with early intrusive phases located near the centre of the deposit. Highest Cu grades (~0.9% Cu) are mostly associated with a supergene enrichment blanket, whilst high Mo grades are restricted to contacts with the metasedimentary rocks. The Michiquillay Cu–Au–Mo deposit (631 Mt at 0.69% Cu, 0.15 g/t Au, 100–200 ppm Mo) is associated with a Miocene (20.0–19.8 Ma) dioritic complex that was emplaced within the hanging wall of a back thrust fault. The intrusive complex is hosted in quartzites and limestones. The NE-trending deposit is crosscut by NNW-trending prospect-scale faults that influenced both alteration and metal distribution. In the SW and NE of the deposit, potassic alteration zones contain moderate hypogene grades (0.14 g/t Au and 0.8% Cu) and are characterised by chalcopyrite and pyrite mineralisation. The core of the deposit is defined by a lower grade (0.08 g/t Au and 0.57% Cu) phyllic alteration that overprinted early potassic alteration. Michiquillay contains a supergene enrichment blanket of 45–80 m thickness with an average Cu grade of 1.15%, which is overlain by a deep leached cap (up to 150 m). Cu–Au–Mo (El Galeno-Michiquillay) and Au-rich (Minas Conga) deposits in the Cajamarca region are of similar age (early–middle Miocene) and intrusive rock type (dioritic) associations. Despite these geochronological and geochemical similarities, findings from this study suggest variation in metal grade between the hybrid-type and Au-rich deposits result from a combination of physio-chemical factors. These include variations in temperature and oxygen fugacity conditions during hypogene mineralisation resulting in varied sulphide assemblages, host rock type, precipitation of ubiquitous hydrothermal magnetite, and late hydrothermal fluid flow resulting in a well-developed phyllic alteration zone.

Geophysical and petrophysical study of an iron oxide copper gold deposit in northern Sweden

A geophysical–petrophysical study has been performed in an area WSW of the city of Kiruna, northern Sweden. The sub-regional tectonic setting is dominated by two important shear zones, which define the boundary of a granitic body. Many Cu–Fe-occurrences are located in proximity of faults related to these major deformation zones. Particular attention has been given to the Tjårrojåkka iron oxide copper gold (IOCG) deposit. Here the bedrock is characterised by intermediate to mafic meta-volcanics, metamorphosed intermediate to mafic dykes, and gabbroic–dioritic intrusions of Svecofennian ages (∼1.96–1.75 Ga). The major Cu- and Fe-occurrences are hosted by the meta-andesites. The aim of the study is to put the deposits into a tectonic framework and test existing hypotheses for their occurrences. Glacial deposits cover almost the entire area, leading to a scarcity of outcrops and inferring that geophysical data are fundamental for geological understanding. In addition to this, petrophysical analysis is vital for the interpretation of geophysical data (gravity, airborne magnetics and radiometrics, very low frequency) and for the definition of geophysical signatures of the deposits. The anisotropy of magnetic susceptibility (AMS) was also studied for the tectonic analysis. More than 150 oriented samples were collected in a number of outcrops along a profile intersecting the major structures in the Tjårrojåkka area. From the airborne magnetic data, two major linear features are interpreted as deformation zones. The strike of these deformation zones is approximately NW–SE and E–W, respectively. The same trends have been defined from other geophysical data such as airborne VLF and ground gravity data. A third important structural trend striking SW–NE has been defined by K/Th data and ground magnetic data. Very good agreement has been found between geophysical lineaments and AMS directions. Magnetic foliations determined by AMS measurements confirm the existence of three major trends in the study area: SW–NE, E–W and NW–SE. The major Fe-orebody shows approximately a SW–NE strike direction as defined from ground magnetic data. This is parallel to the strike of magnetic foliation determined in outcrops ∼1 km NW of the deposit. The epigenetic nature of the Cu and Fe occurrences in Tjårrojåkka and their spatial relationship with deformation zones suggest a connection between the formation of the deposits and a tectonic event. A later tectonic episode resulted in E–W trending deformation in the central area, affecting the orebodies themselves. Other, probable, compressive deformations have been indicated from petrophysical and geophysical analyses. Thermomagnetic measurements indicate that Fe-oxides (Ti-magnetite) are common in the area, while Fe-sulphides are almost absent. Multi-domain magnetite has been identified as the most common Fe-oxide in different rock types, while an unstable magnetic mineral has been detected in metamorphosed volcanics. A good spatial correlation has been observed between Cu-deposits and high K/Th values from radiometric data, values that are expressions of potassic alteration.

Intra-arc transpression in the lower crust and its relationship to magmatism in a Mesozoic magmatic arc

Structural observations and U–Pb geochronology from Fiordland, New Zealand support a model of partitioned transpression within the lower crust of an early Mesozoic magmatic arc called the Median Batholith. We use this lower crustal section to test whether transpression was an efficient mechanism for transporting magma through the deep lithosphere. A continentward migration of magmatic activity occurred within the margin of Gondwana after ∼140 Ma followed by a period of concentrated magmatism in a vertical, 12–15 km wide lower crustal shear zone after ∼119 Ma. The shear zone, named the Indecision Creek Shear Zone, contains variably oriented dioritic intrusions and displays systematic variations in the three-dimensional orientation of ductile structures. From the margins to the center of the shear zone the pitch of stretching lineations on foliation surfaces changes from 10–35° to 55–82° with increasing finite strain. This increase in pitch is accompanied by a steepening and counter-clockwise rotation of foliation planes. These and other structural patterns indicate that arc-parallel sinistral oblique-slip and strike-slip displacements occurred at the shear zone margins and that deformation in its center was dominated by horizontal arc-normal shortening and near vertical extrusion. This style of partitioned transpression reflects the effects of rheological contrasts created by a heterogeneous pattern of magmatism within the arc. Field relationships and U–Pb dates on zircon suggest that the shear zone formed along the boundary between outboard (older) and inboard (younger) parts of the batholith and facilitated the transfer of small volumes of magma vertically through the lower crust until at least ∼111 Ma, when convergence and arc magmatism waned.

Petrogenesis of syenites at a rifted continental margin: origin, contamination and interaction of alkaline mafic and felsic magmas in the Astrophyllite Bay Complex, East Greenland

The Astrophyllite Bay Complex in East Greenland (part of the Palaeogene North Atlantic Igneous Province) consists of an alkaline diorite plug, with detached trachyandesitic pillows, surrounded by co-magmatic syenite that was emplaced into Archaean basement. The diorite intrusion has yielded a 47.11 ± 0.68 Ma Rb-Sr isochron age. Saw-cut profiles through pillow-syenite-gneiss sections have been taken to resolve close spatial elemental and isotopic (Sr-Nd-Hf-Pb-O) variations. The diorite and syenite formed from alkaline basaltic, mantle-derived, melts with complex histories of prolonged assimilation and fractional crystallisation. Each evolved to different extents in separate magma chambers during the establishment of new plumbing systems in the Kangerlussuaq area. The diorite is dominated by lower crustal, granulite facies contamination, whereas the syenite shows evidence for greater degrees of upper crustal amphibolite facies contamination, indicating stalling and fractionation of magmas at different levels within the crust. The syenite and diorite magmas were subsequently emplaced as separate pulses into the basement gneisses at Astrophyllite Bay giving rise to superimposed local contamination trends between pillow/syenite and syenite/gneiss, respectively.

Age of the Pueblo Viejo Gold-Silver Deposit and Its Significance to Modelsfor High-Sulfidation Epithermal Mineralization

Isotopic analyses were undertaken to clarify age relationships between mineralization in the Pueblo Viejo high-sulfidation epithermal deposit and its host Los Ranchos Formation. This study is important to models for high-sulfidation epithermal deposits because the Los Ranchos Formation is a largely submarine, island-arc tholeiite sequence, very unlike the subaerial, calc-alkaline volcanic sequences that host most high-sulfidation epithermal deposits. Previous U-Pb analyses of zircons from the Los Ranchos Formation show that it formed between about 118 and 111 Ma and was intruded by the Cotui stock at about 112 Ma. New U-Pb isotope analyses of zircons from quartz porphyries at two different levels in the ore-hosting, upper Los Ranchos Formation indicate an age of 111.4 ± 0.5 Ma (MSWD = 1.92), essentially the same as that of the Cotui stock, confirming that the stock was probably the source of the quartz porphyry magmas and mineralizing fluids. Rb-Sr analyses of sphalerite from gold-bearing veins reflect mixing between magmatic and seawater Sr reservoirs but yield no age constraints. Ar-Ar analyses of illite from advanced argillic alteration in the Moore orebody yield a plateau age of 58 Ma interpreted to represent complete resetting or neocrystallization, probably related to the thermal effect of small diorite intrusions immediately south and west of the district. These measurements, combined with geologic relationships, confirm that high-sulfidation mineralization at Pueblo Viejo was coeval with deposition of its host Los Ranchos Formation, part of the island-arc tholeiite sequence that forms the base of the Greater Antilles arc. This greatly expands the types of volcanic terranes that might be prospective for high-sulfidation mineralization beyond subaerial calc-alkaline sequences to more mafic, dominantly submarine, island-arc tholeiite sequences. At present, relatively few high-sulfidation deposits are known in island-arc tholeiite sequences. This could reflect the greater difficulty of developing large advanced argillic alteration zones in these dominantly mafic rocks or simply a failure to recognize island-arc tholeiite sequences in volcanic arcs that contain mineralization. Exploration might also have been discouraged by the fact that most island-arc tholeiite sequences are dominantly submarine and contain exhalative mineralization. That both types of mineralization might form in these arcs is suggested by relationships in the Tonga-Kermadec ridge and adjacent areas, where dacitic calderas in island-arc tholeiite sequences host exhalative and high-sulfidation mineralization. The presence of an unusually large deposit such as Pueblo Viejo in an island-arc volcanic sequence is attributable to the iron-rich nature of the wall rocks, which promoted early deposition of gold by sulfidation, and to the presence of carbonaceous sedimentary rocks that isolated the hydrothermal system and promoted ore deposition. The age and isotopic data show further that these carbonaceous sedimentary rocks were deposited during ocean anoxic event (OAE) 1b, encouraging speculation that other island-arc tholeiite volcanic sequences containing ocean anoxic event sediments might also be favorable for high-sulfidation epithermal mineralization.

Geometric aspects of synkinematic granite intrusion into a ductile shear zone — an example from the Yunmengshan core complex, northern China

Abstract The Cretaceous Yungmengshan core complex in northern China contains a large syntectonic granodiorite batholith that intrudes a slightly older diorite intrusion. A major gently dipping ductile décollement shear zone is developed along the contact of the diorite and granodiorite. The shear zone is invaded by a large volume of granitic and pegmatite veins associated with the main granodiorite batholith during activity of the shear zone under high-grade metamorphic conditions. Progressively older veins are more strongly deformed into tight cylindrical fold structures rotated into parallelism with the lineation and foliation in the shear zone. Parallelism of veins to the foliation is partly due to this rotation, but also to foliation-parallel injection of younger syntectonic pegmatite veins. Several small-scale structures have been recognized that allow distinction of solid-state deformation of veins. Granite veins do not extend much above the ductile shear zone that seems to act as a lid and an effective depository to intruding granite veins from the underlying batholith. There was considerable volume increase in the footwall and lower part of the shear zone by vein intrusion.

Late-magmatic calcite-pyrrhotite-chalcopyrite mineralisation associated with magmatically-evolved diorite, Marble Hall, South Africa

Calcite-pyrrhotite-chalcopyrite veins are pervasively developed in a diorite intrusion (~2.06 Ga), associated calc-silicate hybrid rock, and carbonate country rock in the direct vicinity of Marble Hall, Mpumalanga Province, South Africa. The diorite intrusion is probably sill-like in shape and related to a series of sills that formed a ~500m thick package within the impure siliceous carbonate rocks of the Malmani Subgroup, Transvaal Supergroup. The intrusion comprises a hybrid rock at the base, followed upward by cumulate-layered diorite and a capping of diorite pegmatite. Fluid build-up due to crystal differentiation and fractionation of mainly amphibole and plagioclase gave rise to the development of H2S-bearing hydrous fluids that permeated and reacted with the dolomitic country rocks, thereby releasing CO2 into the fluids. Scavenging of aluminium, titanium, iron, calcium, sodium, potassium and zirconium (rare) from the early-formed minerals took place as the fluids channelled into contraction cracks and fissures. Early-formed magmatic and metamorphic (in diorite-related hybrid rocks) amphibole, andesine, magnetite and ilmenite were transformed into secondary assemblages containing actinolite-ferroactinolite, albite, chlorite, calcite, apatite, sphene, zircon (rare), pyrrhotite, chalcopyrite and pyrite. With increasing degree of alteration, magmatic-textured mesocratic diorite went to bleached diorite, allotriomorphic-textured diorite and finally to granoblastic albite-rich diorite. Final crystallisation of the fluids gave rise to aggregates of calcite, pyrrhotite, rare chalcopyrite and sphene. The existence at Marble Hall of coincident gravity and magnetic anomalies, nickeliferous ultrabasic rocks, the presence of sulphur (manifested by the calcite-pyrrhotite veins), and the apparent overprint of a high temperature metamorphic-metasomatic event , collectively enhance the area’s prospectivity for base metal deposits.

The Falémé Iron District, Senegal

The Faleme iron district in the Kedougou-Kenieba inlier of the Paleoproterozoic Birimian Supergroup of West Africa consists of nine major and 19 minor orebodies, distributed in a belt 65 km long and 15 km wide. Two major exoskarn orebodies have total reserves of 320 million metric tons (Mt) magnetite ore with 42 percent Fe, and seven major supergene-enriched orebodies, which overlie endoskarn, have reserves of 310 Mt with 59 percent Fe. Previous workers advocated an (exhalative)-sedimentary origin for the primary ore. In this paper, however, we present strong evidence for contact-metasomatic skarn mineralization associated with microdiorite intrusions, with an emphasis on the Karakaene-Ndi and Goto deposits. The ore deposits of the Faleme district are the only major Precambrian magnetite skarn deposits worldwide that have not been subjected to postore metamorphism. The deposits of the Faleme district have many similarities to Phanerozoic magnetite skarn deposits that are associated with dioritic intrusions and also lack postore metamorphism. The endoskarn of the Karakaene-Ndi deposit is hosted by microdiorite with pervasive albitization. Garnet (And 56 –100 Grs 0 –43 Alm 0 –2 Sps 0 –1 Pyr 0 –1 ) and clinopyroxene (Di 58 –100 Hed 2 –42 ) are early phases. Individual grains of garnet and clinopyroxene commonly are inhomogeneous, with Fe being concentrated at the margin of the grains rather than in the core. Fe-rich zones in clinopyroxene are also enriched in Na. Biotite and magnetite precipitated relatively late, followed by pyrite (2 vol %) ± chalcopyrite ± pyrrhotite ± iss. Aqueous-carbonic fluid inclusions in quartz from one sample have CO 2 concentrations of 5 to 14 mol percent and indicate pressures of 300 to 900 bars and temperatures of 281° to 373°C at total homogenization. The magnesian exoskarn of the Goto deposit is hosted by dolomitic and calcitic marble, which locally contains graphite. Magnetite is associated with prograde clinopyroxene (Di 89 –100 Hed 0 –11 ) and phlogopite-rich biotite (Mg/(Mg + Fe)~0.89) as well as retrograde serpentine. Garnet is scarce. Sulfur concentrations average 1 wt percent. Pyrrhotite and subordinate pyrite ± chalcopyrite ± arsenopyrite ± pentlandite ± cobaltite postdate most of the magnetite. The exoskarn formed under more reducing conditions and at lower temperatures than the endoskarn. Magnetite in the exoskarn has trace element concentrations similar to those of magnetite in the endoskarn but is distinguished from igneous magnetite in granodiorite and magnetite in andesite (possibly of igneous origin) by low concentrations of Cr 2 O 3 and V 2 O 5 . Microdiorite is the most likely source of iron although no igneous magnetite in microdiorite could be identified. Microdiorite with only moderate hydrothermal alteration has lower iron concentrations (1.6–4.5% Fe 2 O 3(total) ) than unaltered granodiorite and weakly altered andesite in the area (avg ~7% Fe 2 O 3(total) ). Iron was probably leached from the moderately altered microdiorite and transferred into zones of intense alteration in microdiorite and host-rock marble. The low degree of hydrothermal alteration of granodiorite and small areal extent of andesite argue against a granodioritic or andesitic source of iron. The supergene ore is derived from endoskarn with about 30 percent Fe and consists of hematite and hydrous iron oxides. The largest orebodies have a thickness of 100 m and are located in the central part of hills that rise 250 m above the surrounding peneplain; the bottom of the enrichment zone usually is centered deep below the top of the hills. The absence of supergene enrichment in exoskarn orebodies probably is due to unfavorable structural conditions. Supergene enrichment of endoskarn was probably not exclusively by residual enrichment, in which Ca, Mg, Na, and Si were removed from the orebody, but also involved addition of Fe from descending solutions to the site of enrichment. Exploration for new supergene iron ore should be focused outside the exoskarn belt between Goto and Safa, as the deposits in this area are unlikely to have experienced major supergene enrichment. The Faleme district has some similarities to the geologic setting of iron oxide-copper-gold districts in which the deposits are hosted by dioritic rocks and may be representative of an iron oxide-rich end member of the iron oxide-copper-gold class. However, only about 3 t of gold have been mined from alluvial deposits in the area and the district is relatively poor in copper.

Platinum-group element mineralization in an As-rich magmatic sulphide system, Talnotry, southwest Scotland

AbstractArsenic-rich magmatic sulphide mineralization is hosted by a diorite intrusion at Talnotry, southwest Scotland. A relatively abundant and diverse platinum-group mineral assemblage is present and is dominated by sperrylite, irarsite and electrum with subordinate merenskyite, michenerite and froodite. Early euhedral gersdorffite is enriched with respect to Rh, Ir and Pt and in some cases contains exsolved blebs of irarsite or euhedral grains of sperrylite. Sperrylite is also enclosed within silicates and sulphides indicating that it crystallized directly from an As-rich sulphide liquid. Pyrrhotite-chalcopyrite mineral assemblages are consistent with the fractional crystallization of monosulphide solid solution and are overlain by PGE-, Ni- and As-rich mineral assemblages indicative of crystallization from a NiAs liquid. Late-stage, cross-cutting, electrum-bearing chalcopyrite veins are consistent with the crystallization of Cu- and Au-rich intermediate solid solution. The chemistry, mineralogy and lithological relationships of the diorite suggest that it may be an appinite and as such is potentially analogous to the Au-rich lamprophyre dykes present within southwest Scotland.

Late Neoproterozoic to Early Palaeozoic palaeogeography of Avalonia: some palaeomagnetic constraints from Nuneaton, central England

Palaeomagnetic studies have been carried out on Neoproterozoic, Cambrian and Ordovician rocks in the Nuneaton inlier, England (52.5° N, 1.5° W). Three magnetic components were recognized, which provide a consistent structural and magnetic history of the inlier. Neoproterozoic volcaniclastic and intrusive rocks acquired a characteristic remanent magnetization (ChRM) dated at 603Ma. Late Ordovician rocks are represented by lamprophyre and diorite intrusions and their ChRMs were probably imprinted during their emplacement, at about 442 Ma. The Lower Cambrian sedimentary sequence of the Hartshill Sandstone Formation, which unconformably overlies the Neoproterozoic rocks and hosts the Ordovician intrusions, does not preserve a primary magnetization but shows the imprints of the Late Ordovician (442 Ma) remagnetization, as well as a probable end-Carboniferous remagnetization. Palaeolatitudes calculated for the late Neoproterozoic rocks and Ordovician intrusions are in good agreement with other palaeolatitudes calculated for Avalonia during those times. Both the late Neoproterozoic and Late Ordovician rocks additionally show ChRMs with declination anomalies indicating a large tectonic rotation of the Nuneaton area, possibly during one of the Caledonian phases of deformation affecting southern Britain.

THE MELILITE (Gh50) SKARNS OF ORAVITA, BANAT, ROMANIA: TRANSITION TO GEHLENITE (Gh85) AND TO VESUVIANITE

Almost monomineralic Mg-rich gehlenite (similar toGh(50)Ak(46)Na-mel(4)) skarns occur in a very restricted area along the contact of a diorite intrusion at Oravita, Banat, in Romania, elsewhere characterized by more typical vesuvianite-garnet skarns. In the vein-like body of apparently unaltered gehlenite, the textural relations of the associated minerals (interstitial granditic garnet and, locally, monticellite, rare cases of exsolution of magnetite in the core zone of melilite grains) suggest that the original composition of the gehlenite may have been different, richer in Si, Mg, Fe (and perhaps Na), in accordance with the fact that skarns are the only terrestrial type of occurrence of gehlenite-dominant melilite. The same minerals, monticellite and a granditic garnet, appear in the retrograde evolution of the Mg-rich gehlenite toward compositions richer in Al, the successive stages of which are clearly displayed, along with the final transformation to vesuvianite. These changes include (1) the local development of small rounded patches with Al-rich compositions (Gh(60) to Gh(85)), accompanied by monticellite, spurrite (or tilleyite, afwillite, kilchoanite) and, at a later stage, a granditic garnet, and (2) the complete transformation of gehlenite to vesuvianite, associated with minor clintonite (+ monticellite and probably ellestadite), usually along a sharp front typically rimmed by a 0.5-mm-wide zone of Al-rich gehlenite (Gh(85)). The gehlenite rim, as well as the gehlenite in the local modifications, show a remarkable correlation between the akermanite and Na-melilite contents, probably of crystal-chemical origin. The local modifications (1) are interpreted as nearly closed-system (except for Na and Fe) retrograde reactions at moderate temperature (500-600degreesC), controlled by the localized presence of small amounts of fluid at low pressure, mainly involving the transformation of the akermanite component to monticellite. The silica released by this transformation resulted in the formation of spurrite (among others) and of the Na-melilite component at first, and garnet later. The subsequent transformation (2) of geldenite to vesuvianite and clintonite, which involved silica metasomatism, may result from the more pervasive infiltration of the same fluid, at higher pressure, probably related to the development of the garnet-vesuvianite skarns elsewhere along the intrusive contacts.