Diorite Stock Research Articles

The gabbro-diorite magmatism from the Narm area, western Kuh-e-Sarhangi (Central Iran): Evolution from Eocene magmatic flare up to Miocene asthenosphere upwelling

Geodynamically, the Kashmar-Kerman Tectonic Zone (KKTZ) is one of the most perplexing tectonomagmatic belts of the Central Iran Microcontinent (CIM), comprising two important districts, Bafq and Kuh-e-Sarhangi. The Late Neoproterozoic-Early Cambrian granitoids, metamorphic rocks, and mildly metamorphosed volcansedimentary sequences are the oldest geologic outcrops in the Narm area, which is located in the western part of Kuh-e-Sarhangi. Alkaline gabbro-diorites with relatively high contents of K2O (1.99–3.03 wt%) and Na2O (2.7–5.99 wt%) are among the youngest intrusive rocks in the area, representing a within-plate provenience. These rocks were emplaced into Paleozoic sedimentary units as mafic-intermediate stocks, sills and dykes. Geochemically, these rocks could have resulted directly from partial melting (e.g., FeOT/MgO>1, Nb/La>0.5) with no considerable indication of assimilation with crustal materials (e.g., Ti/Zr > 30, Ti/Y > 200). Assimilation and fractional crystallization cannot account for magma evolution of gabbro-diorite rocks in the Narm area, using rare earth element ratios and geochemical models. There are also some geochemical signatures of an asthenospheric origin for the Narm gabbro-diorite rocks, such as the low ratios of La/Nb (1.5) and La/Ta (22). U–Pb zircon ages show that the Narm gabbro-diorites formed during two major episodes of magmatism in Central Iran: 40.3 ± 0.1 Ma in the Late Eocene (Bartonian) for gabbroic units and 8.04 ± 0.05 and 7.86 ± 0.05 in the late Miocene (Tartonian) for diorite stocks and diorite sills, respectively. Despite a time difference of more than thirty million years, geochemical similarities between the Eocene gabbro rocks and the Miocene diorite from the Narm area are striking. It is proposed that the best scenarios for the west of Kuh-e-Sarhangi mafic-intermediate magmatic pulses along with the deep faults of the Central Iran, are an Eocene magmatic flare up and a Miocene asthenosphere upwelling. Temporally and spatially, these rocks are comparable to the Cenozoic alkaline intrusive rocks of the Bafq region.

Spatial-temporal heterogeneity of magma emplacement process and its constraints on localization of associated orebody: A case study in the Shizishan orefield of the Tongling Ore Cluster, East China

We present a study on the Dongguashan quartz diorite stock which is the largest Early Cretaceous intrusion associated with skarn-porphyry polymetallic ore deposit in the Tongling Ore Cluster, Middle-Lower Yangtze Metallogenic Belt in East China. The Dongguashan quartz diorite show massive texture without obvious foliation, and intrusive contact was locally observed inside of the stock. The anisotropy of magnetic susceptibility (AMS) results of the stock show two distinct groups. The Group I (G-I) is dominated by NE-SW-striking magnetic foliation with variably oriented magnetic lineation. The Group II (G-II) that intruded into G-I is characterized by steep NW-SE-striking magnetic foliation and lineation, which are parallel to the vein-like orebody developed in the stock. The 3D geometrical modelling of the stock displays a triangular shape in plan-view with an eastward bulge and irregular stock boundary in the eastern side and the contact surface is steeper in the west than that in the east, denoting that the stock was constructed with an eastward magma accretion trend. Furthermore, the Dongguashan quartz diorite has a wide range of composition and geochronological data, suggesting a multiple magma pulses emplacement model. Accordingly, we propose that the Dongguashan stock was constructed by at least two stages magma pulses. The earlier stage magma pulses intruded along the NE-SW-striking pre-emplacement structures in the country rock and partly intruded into the lithological and mechanical discontinuous interfaces which yielded the stratabound skarn orebody. The eastward magma accretion produced a highly deformed and longer heated country rock on the eastern side of the stock, favoring the orebody development along the eastern stock-country rock contact interface. The late-stage magma pulse and the parallelism among the trend of magnetic foliation of G-II, extensional structure and vein-like orebody suggest that the vein-type orebodies may controlled by both magma emplacement and regional tectonics.

Anatomy of Shaku–dake high–Mg diorite, southwest Japan: Lithofacies variations and growth process of high–Mg diorite stock

Anatomy of Shaku–dake high–Mg diorite, southwest Japan: Lithofacies variations and growth process of high–Mg diorite stock

Evidence of iron oxide-copper–gold mineralization in the Torud-Chahshirin Magmatic Belt, northern Iran: Insight from the Robaie area

The Torud-Chahshirin Magmatic Belt in northern Iran is defined by Tertiary porphyry, IOCG, Skarn, Manto, MVT, and Au, Ag and base metal-rich epithermal systems, and it is a part of the eastern Alborz Orogenic Belt of Iran. Mineralization in the Torud-Chahshirin Magmatic Belt occurs as veins, veinlet, and disseminated mineralization hosted by the Eocene-Oligocene volcanic, subvolcanic, and volcanoclastic rocks. Mineralization prospects of the Torud-Chahshirin Mgmatic Belt are spatially and temporally accompanied with magmatic rocks of early to middle Eocene (∼55–37 Ma.) calc-alkaline and early to late Oligocene (∼34–24 Ma.) alkaline rocks that have continually evolved through assimilation and fractional crystallization (AFC) processes at shallow crustal levels. The Robaie area is located in the Torud-Chahshirin Magmatic Belt. The dominant stratigraphic unit is composed of Eocene volcanic rocks (trachyandesitic to andesitic in composition), which are intruded by subvolcanic rocks. The subvolcanic rocks have features typical of shoshonitic affinity and are metaluminous to weakly peraluminous. These rocks are characterized by enrichment in large-ion-lithophile elements (LILEs) and light rare-earth-elements (LREEs), depletion of heavy rare-earth-elements (REEs, LaN/YbN ≈ 9.65–24.34), and high-field-strength-elements (HFSEs). Values of εNd(i) commonly show a small variation between +0.2 and +0.7, whilst 86Sr/87Sr (i) goes from 0.7056 to 0.7063. The whole set of evidence agrees with derivation of intrusive rocks of Robaie from primitive magma resulting from partial melting in a mantle wedge above a subduction zone. A sample from a dyke of biotite-hornblende diorite porphyry, with a negative εNd(i) value (−1.7), probably corresponds to a magma that underwent assimilation of crustal material. U-Pb data of zircons from dioritic stock and dyke show ages of 49.99 Ma and 50.43 Ma, respectively. Mineralization in Robaie is related to the biotite-hornblende diorite intrusions and is associated with hydrothermal alterations including argillic, argillic-silicification, argillic-sericitic, and propylitic alteration. The ore-bearing veins, veinlets, and disseminated mineralization are controlled by structures. There are two types of veins and veinlets mineralization in the Robaie area including: 1) quartz-chalcopyrite-pyrite-specularite ± gold veins, and 2) quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets. The highest amount of copper (>1%) and gold (>5 g/t) is associated with quartz-chalcopyrite-pyrite-specularite ± gold veins. Microthermometric study of fluid inclusions shows that homogenization temperatures are 181–422 °C and salinities of ore-forming fluids ranged from 11.1 to 17 wt% NaCl equivalent. The temperature and salinity of ore-forming fluids were lower in the formation of the quartz-chalcopyrite-pyrite-specularite ± gold veins than in the quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets, and fluid mixing was the most important mechanism for mineralization in the area. The mineralogy, alteration, geochemistry, fluid inclusion studies, and petrogenesis of subvolcanic rocks of the Robaie area indicate that the Robaie deposit could be classified as a member of the Iron Oxide-Copper-Gold (IOCG) deposits.

Geology, mineralization, fluid inclusion, and stable isotope characteristics of the Zaghar and Sarabadan Cu-(Au) skarn deposits, Tafresh, Central Iran

The Zaghar and Sarabadan copper-(gold) deposits are located in the northern part of the Urumieh-Dokhtar magmatic belt. Mineralization occurred in contact of the post-Eocene diorite-tonalite stocks with the upper Triassic-Jurassic shale and sandstone series in the Zaghar, and lower Eocene volcanic and pyroclastic rocks in the Sarabadan. Based on field observation and mineralogical studies, three stages of mineralization can be distinguished: (1) prograde skarn; (2) retrograde skarn; and (3) supergene stages. The prograde skarn is characterized by association of andradite, diopside, cordierite, and minor actinolite. The retrograde skarn stage is divided into two sub-stages: (1) retrograde skarn I; and retrograde skarn II. The retrograde skarn I, is marked by deposition of the main alteration minerals (e.g., tremolite, actinolite, chlorite, epidote), magnetite, ilmenite, pyrrhotite, pyrite, barite, and small amount of other sulfide minerals. The retrograde skarn II or quartz-sulfide-carbonate stage, is characterized by quartz and calcite vein-veinlets associated with Cu-Au mineralization. The supergene stage is marked by Fe oxy-hydroxides, covellite, malachite, azurite, and minor clay minerals and calcite. Ore-forming fluids associated with the quartz, calcite, and sphalerite from the retrograde II stage of mineralization is represented by LVS fluid inclusions with the temperature of 218–512 °C and salinity of 6.73–59.09 wt% NaCl, and LV type fluids with the temperature of 130–215 °C and salinity of 1.70–30.00 wt% NaCl equivalent. Stable isotope analysis of quartz yielded values of −85.30 to −60.70 δD‰, and +7.82–9.60‰ δ18Ofluid, indicating that the ore-forming fluids were mainly magmatic fluids with some contributions from meteoric waters. The δ34S values of pyrite vary from +4.90–8.10‰, demonstrate that the sulfur in deposit was probably originated from the magma. The geological and geochemical features introduce the Zaghar and Sarabadan as Cu-(Au) skarn deposits are related to the magmatic-hydrothermal activity of diorite stocks within continental magmatic arc.

U–Pb systematics of the western Dharwar Craton - Glimpses of a billion year history of crustal evolution and relations to ancient supercratons

The Dharwar Craton developed progressively over a billion years, through two main stages of crustal growth separated by a few-hundred million year long period of relative quiescence. The first stage between 3.4 and 3.0 Ga developed a proto-craton, which was considerably amplified during the second main stage between 2.7 and 2.4 Ga, through extensive magmatism, tectonism, and crustal consolidation. This paper reports U–Pb dating results obtained in four specific areas of the craton, with the data encompassing key moments in this long development. Rocks formed during the proto-craton stage include a 3089 Ma augen gneiss and a 2973 Ma evolved granite, the latter of which marks the final cratonization event of the proto-craton. The beginning of the second main stage is recorded in this study by 2650 Ma tonalite and trondhjemite, a 2623 Ma granite dyke cutting augen gneiss, and 2614, 2602 and 2588 Ma volcanic rocks. Titanite responded differently to the long evolution, as a function of location and type of overprint. It preserved an original 2973 Ma magmatic age in the west, but was reset and/or crystallized during secondary events in central domains of the craton, yielding ages between 2590 and 2360 Ma. A diorite stock intruded at 2207 Ma in the consolidated crust. It is correlated with the Anantapur-Kunigal mafic dyke swarm, one of a series of such events in the Dharwar Craton between 2.35 and 1.79 Ma. In terms of its overall evolution the Dharwar Craton has an affinity with the Slave clan, which includes the Wyoming and Zimbabwe cratons. It also matches many features in the evolution of the São Francisco Craton, a probable other member of Sclavia. This is in contrast to the Amazonian Craton, which has more affinity with the Superior clan.

Zircon U-Pb dating and stable isotopic compositions for constraining the genesis of the Chagangnuoer magnetite deposit in western Tianshan, NW China

The Chagangnuoer magnetite deposit in western Tianshan (NW China) is hosted by submarine volcanic rocks and marble lens, with a resource of >220 Mt Fe at 38 wt%. Two major orebodies FeI and FeII are current in production. Pervasive garnet skarn, magnetite, actinolite–epidote, and marble zones are developed at Chagangnuoer. Paragenetic sequences are divided into garnet–pyroxene (I), magnetite–pyrite ± chalcopyrite (II), actinolite–epidote–K-feldspar (III), garnet ± tremolite ± scapolite (IV), magnetite ± hematite (V), actinolite–epidote (VI), and sulfide–calcite–quartz (VII) stages. A diorite beneath the FeII and a granodiorite with skarn alteration have zircon U-Pb ages of 329 ± 2 and 317 ± 4 Ma, respectively. These intrusions have high potassic to shoshonitic, metaluminous affinities, and significant Nb, Ta, Ti and Sr anomalies. Stage II magnetite has δ18OV-SMOW compositions of 1.9–3.5‰, higher than Stage V magnetite (0.5–2.1‰), indicating a hydrothermal fluid source of magmatic origin. The δ18O values of garnet, actinolite, epidote and quartz range from 4.1 to 12.2‰. Stage VII calcite has δ13CV-PDB of −2.0 to −0.8‰ and δ18O of 8.9 to 9.9‰, lower than for the marble (average δ13C + 2.3‰, δ18O 12.8‰). δ34SV-CDT compositions for pyrite and chalcopyrite from Stages II and VII vary between 0.8 and 13.1‰. The isotopic compositions imply that minor non-magmatic sources (e.g., marine carbonate, seawater, and meteoric water) have been circulated into hydrothermal fluids producing the skarn alterations and associated iron mineralization. The early-stage skarn alteration and main Fe mineralization relate to the emplacement of diorite stock at depth, whereas the late-stage skarn-magnetite assemblages are intimately associated with the granodiorite. The Chagangnuoer deposit is concluded to be magnetite skarns formed by magmatic-hydrothermal fluids that emanated from the dioritic and granodioritic magmas intruding above a sub-arc mantle wedge and that metasomatized intensely with the Carboniferous andesitic-rhyolitic volcanic rocks and carbonate.

Unveiling growth histories of multi-generational garnet in a single skarn deposit via newly-developed LA-ICP-MS U[sbnd]Pb dating of grandite

Although garnet UPb dating method has been reported recently, yet the accurate concordia 206Pb/238U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS UPb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS UPb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept 206Pb/238U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia 206Pb/238U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations ([U]avg > 80 ppm) contents. The precision UPb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon UPb ages of the quartz dioritic stock and 40Ar39Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite UPb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia UPb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns.

Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a suitable area for considering the effects of tectonic structures on development of the volcanic facies in the magmatic provinces. The main structure in the area is a dextral fault (so called Rahjerd Fault) that has produced the local extensional basins and also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline suite that had been changed and evolved in the magma chambers of local extensional basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite and magnesiohornblende) reveal that the AFC process is a dominant process in the magma chamber. Also, according to geothermobarometric calculations, the investigated volcanic rocks of the area could be grouped into two types: one type with higher P-T (>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600- 800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA) and the dioritic stock.

Whole-Rock Geochemistry of Host Rocks and K/Ar Age of Hydrothermal Mineral of the Co-O Epithermal Gold Deposit, Mindanao, Philippines

Whole-rock chemical composition of host rocks and recently acquired K/Ar age of hydrothermal mineral of the Co-O epithermal gold deposit in Mindanao Island of the Philippines are herein reported. Located along a Pliocene-Quaternary calc-alkaline magmatic zone at eastern Mindanao region, the Co-O gold deposit is of intermediate sulfidation epithermal Au (+Ag ± Cu ± Pb ± Zn) quartz vein type. Geological units in the area are probable Eocene to Oligocene basaltic-andesitic to andesitic volcanic flows and volcaniclastic rocks, Oligocene andesitic to dioritic stocks and dikes, a diatreme-maar complex, and an overlying sedimentary sequence. The mineralized quartz ± calcite veins are mainly hosted in the intrusive rocks and surrounding volcanic rocks that are hydrothermally altered generally to K-feldspar, chlorite and other clay minerals. Discrimination diagram using immobile elements such as Zr/TiO2 vs. Nb/Y indicates that these rocks belong to sub-alkaline andesite and basaltic-andesite to basalt and alkali basalt in composition. The volcanic rocks plot in the island arc tholeiite and calc-alkaline fields of generally basaltic andesite and andesite composition, with a few in basalt and dacite. In the “alteration box plot”, the samples mostly plot within the least altered intermediate volcanic host rocks in the hydrothermal alteration field. Plots of each rock unit show a general chlorite-carbonate (-pyrite) alteration trend, with plots of volcanic rocks more dispersed and a few fallen outside the least altered box. K/Ar dating of hydrothermal minerals from the andesite porphyry and polymictic diatreme breccia samples yielded ages of 28.6 ± 0.9 Ma (Late Oligocene) and 31.7 ± 1.9 Ma (Early Oligocene), respectively. Age dating of these hydrothermally formed minerals gives the age of the hydrothermal activity associated with the mineralization. This suggests that the hydrothermal activity associated with the Co-O epithermal vein system transpired immediately after or during the Oligocene magmatism in a tectonic setting that produced island arc tholeiitic to calc-alkaline magmas, prior to drifting to its present location and accretion with the central and western parts of the Mindanao Island; in contrast to more prominent Miocene and Pliocene to Pleistocene mineralization ages along the Philippine archipelago.

The Paleoproterozoic Copper-Gold Deposits of the Gaoua District, Burkina Faso: Superposition of Orogenic Gold on a Porphyry Copper Occurrence?

The Gaoua mining district, underlain by Paleoproterozoic volcanic and intrusive rocks of the Boromo-Goren greenstone belt in southern Burkina Faso, is a rare case where economic copper and gold resources occur together in the Birimian terrane of West Africa. The main country rock lithologies consist of flows and volcaniclastic sediments of basaltic to andesitic compositions. These are intruded by subvolcanic diorite and quartz diorite stocks and dikes, granophyre dikes and, less commonly, gabbroic dikes. The stocks are porphyritic in texture, locally brecciated, and characterized by development of a carbonate, anhydrite, and quartz stockwork system. Structural observations, detailed petrography, and sulfide geochemistry reveal that copper and gold in the stockworks and surrounding altered rocks cannot be associated with the same mineralizing event. Copper mineralization, mainly in the form of chalcopyrite, is typically observed in hydrothermal breccia and is associated with early arsenic-poor and gold-free pyrite. The geochemistry of the plutonic rocks and associated basalts and andesites implies formation in an arc setting. Textural evidence indicates that gold mineralization is closely tied to a second generation of sulfides, with the later pyrite hosting visible gold and minor chalcopyrite that clearly postdates the copper ore. Analyses of these pyrites by LA-ICP-MS reveal compositions rich in Ag, As, Au, Bi, Cu, and Pb. Pyrite with this signature was only observed in local shear and fracture zones that crosscut the host rocks and copper-rich breccia. Therefore, our data indicate the strongly deformed and metamorphosed porphyry copper mineralization in the Gaoua district is overprinted by orogenic gold mineralization. Formation of the early porphyry copper deposit took place at 2165 ± 24 Ma based upon Re-Os dating of the early pyrite; the age of the younger gold event, which is similar in style to that described throughout the West African craton, is not known but may be related to one of the lode gold episodes that are widespread through the West African craton at ~2150 and ~2100 to 2040 Ma. The Gaoua copper deposit, one of very few Precambrian porphyry copper deposits known in West Africa, formed in a volcanic-arc setting that predated the Eburnean orogeny in the Boromo-Goren greenstone belt.

Distal Pb-Zn-Ag veins associated with the world-class Donggou porphyry Mo deposit, southern North China craton

The southern North China craton hosts numerous world-class porphyry Mo and Pb-Zn-Ag vein deposits. Whether or not the Pb-Zn-Ag veins are genetically associated with the porphyry Mo system remains contentious. Here we focus on the genetic relationships between the Sanyuangou Pb-Zn-Ag vein deposit and the world-class Donggou porphyry Mo deposit, and discuss the potential implications from the spatial and temporal relationships between porphyry and vein systems in the southern North China craton.At Sanyuangou, vein-hosted sulfide mineralization mainly comprises pyrite, sphalerite, and galena, with minor chalcopyrite, pyrrhotite, bornite, tetrahedrite, covellite, polybasite and argentite. The mineralization is hosted by a quartz diorite stock, which has a zircon U-Pb age of 1756±9Ma. However, sericite from alteration selvages of Pb-Zn-Ag sulfide mineralization yields a well-defined 40Ar/39Ar plateau age of 115.9±0.9Ma. Although nominally younger, the sericite 40Ar/39Ar age is similar to the age of the nearby Donggou porphyry Mo deposit (zircon U-Pb age of 117.8±0.9; molybdenite Re-Os ages of 117.5±0.8Ma and 116.4±0.6Ma). Pyrite from Donggou has elevated contents of Mo and Bi, whereas pyrite from Sanyuangou is enriched in Cu, Zn, Pb, Ag, Au, and As. This trace element pattern is consistent with metal zonation typically observed in porphyry related metallogenic systems. Pyrite grains from Sanyuangou have lead isotopes overlapping those from Donggou (17.273–17.495 vs. 17.328–17.517 for 206Pb/204Pb, 15.431–15.566 vs. 15.408–15.551 for 207Pb/204Pb, and 37.991–38.337 vs. 38.080–38.436 for 208Pb/204Pb). Collectively, the geological, geochronological, and geochemical data support a magmatic-hydrothermal origin for the Sanyuangou Pb-Zn-Ag deposit and confirm that the Pb-Zn-Ag veins and the Donggou Mo deposit form a porphyry-related magmatic-hydrothermal system.Given the widespread Pb-Zn-Ag veins and Mo mineralized porphyries in many districts of the southern North China craton, the model derived from this study has broad implications for further exploration of Mo and Pb-Zn-Ag resources in the area.

Mineralization time and tectonic setting of the Zhengguang Au deposit in the Duobaoshan ore field, Heilongjiang Province, NE China

The Zhengguang deposit, a representative large gold deposit in the Duobaoshan ore field in NE China, is located in the northeast of the Central Asian Orogenic Belt (CAOB). Ore body emplacement was structurally controlled and occurs mainly at the contact zone between the strata of Duobaoshan Formation and an Ordovician diorite stock. The diorite rocks have a close genetic relationship with Au mineralization. Re–Os isotope dating of Au-bearing pyrite yields an isochron age of 506 ± 44 Ma (MSWD = 15). Based on present and previous dating results, it can be concluded that the Zhengguang deposit formed at ~480 Ma. The mineralization time of the Zhengguang deposit is nearly identical to those of the Duobaoshan and Tongshan deposits, indicating they are all derived from the same metallogenic system. The Duobaoshan-style porphyry Cu–Mo mineralization may exist at deeper levels at Zhengguang. The geochemical characteristics of the Zhengguang dioritic rocks presented in this paper are similar to those of bajaitic high-Mg andesite, and the magmas originated from a mantle wedge metasomatized by melts from a subducting oceanic slab at an active continental margin setting. The Ordovician magmatic–metallogenic events in the Duobaoshan ore field were caused by the westward subduction of an oceanic slab located between the Xing’an and Songliao blocks. It is worth pointing out that the Zhengguang deposit is the oldest known Phanerozoic Au deposit in NE China. Further studies of this deposit will improve understanding of the regularity of ore formation and aid mineralization forecast across the Duobaoshan region.

Role of evaporitic sulfates in iron skarn mineralization: a fluid inclusion and sulfur isotope study from the Xishimen deposit, Handan-Xingtai district, North China Craton

The Xishimen iron skarn deposit in the Handan-Xingtai district, North China Craton, contains 256 Mt @ 43 % Fe (up to 65 %). The mineralization is dominated by massive magnetite ore along the contact zone between the early Cretaceous Xishimen diorite stock and middle Ordovician dolomite and dolomitic limestones with numerous intercalations of evaporitic beds. Minor lenticular magnetite-dominated bodies also occur in the carbonate rocks proximal to the diorite stock. Hydrothermal alteration is characterized by extensive albitization within the diorite stock and extreme development of magnesian skarn along the contact zone consisting of diopside, forsterite, serpentine, tremolite, phlogopite, and talc. Magmatic quartz and amphibole from the diorite and hydrothermal diopside from the skarns contain abundant primary or pseudosecondary fluid inclusions, most of which have multiple daughter minerals dominated by halite, sylvite, and opaque phases. Scanning electron microscopy (SEM) and laser Raman spectrometry confirm that pyrrhotite is the predominant opaque phase in most fluid inclusions, in both the magmatic and skarn minerals. These fluid inclusions have total homogenization temperatures of 416–620 °C and calculated salinities of 42.4–74.5 wt% NaCl equiv. The fluid inclusion data thus document a high-temperature, high-salinity, ferrous iron-rich, reducing fluid exsolved from a cooling magma likely represented by the Xishimen diorite stock. Pyrite from the iron ore has δ34S values ranging from 14.0 to 18.6 ‰, which are significantly higher than typical magmatic values (δ34S = 0 ± 5 ‰). The sulfur isotope data thus indicate an external source for the sulfur, most likely from the evaporitic beds in the Ordovician carbonate sequences that have δ34S values of 24 to 29 ‰. We suggest that sulfates from the evaporitic beds have played a critically important role by oxidizing ferrous iron in the magmatic-hydrothermal fluid, leading to precipitation of massive magnetite ore. A synthesis of available data suggests that oxidation of Fe2+-rich, magmatic-hydrothermal fluids by external sulfates could have been a common process in many of the world’s iron skarn deposits and other magnetite-dominated ores, such as iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) systems.

A hydrothermal origin for the large Xinqiao Cu-S-Fe deposit, Eastern China: Evidence from sulfide geochemistry and sulfur isotopes

The Xinqiao Cu-S-Fe deposit in the Tongling ore district, Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB; Eastern China), is located along the northern margin of the Yangtze Craton. The stratiform- and skarn-type Xinqiao mineralization comprises five stages, namely the early skarn (Stage I, garnet and diopside), late skarn (Stage II, epidote-dominated), iron oxides (Stage III, hematite and magnetite), colloform pyrite (Stage IV) and quartz-sulfides (Stage V). There are three pyrite types at Xinqiao, i.e., colloform (Py1; Stage IV), fine-grained (Py2, from Py1 recrystallization; Stage V) and coarse-grained (Py3; Stage V) pyrites.Scanning Electron Microscope (SEM) imagery for Py1 reveals that they are cubic microcrystalline pyrite aggregates, and the EDS and XRD data indicate that some Py1 contain minor siderite impurities. Electron Microprobe Analysis (EMPA) and LA-ICP-MS geochemical data demonstrate that the three pyrite types have relatively high Fe/S ratios and distinctly high Mn, Cu and As concentrations. Compared to Py2 and Py3, Py1 has higher Pb, Bi and Ag, but lower Co, Ni, Se, Cd, Te and Au. Ratios of Fe/S (0.837 to 0.906), Se/Te (2.39 to 14.50) and Co/Ni (0.67 to 4.67) of the Xinqiao pyrites resemble typical hydrothermal pyrites. δ34SCDT of Py1 (−0.6‰ to 2.7‰, average 0.58‰), Py2 (1.8‰ to 2.5‰, average 2.1‰) and Py3 (1.9‰ to 4.4‰, average 3.5‰) are close to those of the Xinqiao skarn-type orebodies (1.3‰ to 4.1‰), but distinct from those of the Upper Carboniferous Huanglong Formation limestone (−9.5‰ to −15.4‰), suggesting that the three pyrite types (especially Py1) were genetically linked to the Yanshanian (Jurassic-Cretaceous) magmatic-hydrothermal events, with Py1 probably reflecting rapid crystallization during fluid mixing. We interpret that the Xinqiao stratiform mineralization may have been associated with the Jitou quartz diorite stock, as may be the case also for the skarn-type mineralization hosted in the contact between the Yanshanian Jitou stock and the Lower Permian Qixia Formation limestone. Overall, the Xinqiao Cu-S-Fe mineralization may have been generated by the Jurassic-Cretaceous tectono-thermal event in Eastern China.

Epithermal and arc-related layered mafic platinum-group element mineralisation in the mafic–ultramafic rocks of eastern Papua

ABSTRACTThis paper describes contrasting platinum-group element (PGE) mineralisation in the mafic–ultramafic rocks of eastern Papua, discovered nearly 100 years after the first record of detrital ‘osmiridium’ in alluvial gold production from the region. The two mineralisation types include: (1) the Doriri Pd–Pt–Ni lode, a new type of epithermal shear zone-hosted PGE mineralisation from the Ada'u Valley; and (2) stratiform Pt–Pd ± Au mineralisation hosted in an arc-related layered mafic intrusion at the Goroa prospect in the Waria Valley. Doriri is the first description of the transport and deposition of PGE in epithermal conditions, confirming the case long made by geochemists for the solubility, transport and deposition of significant concentrations of Pd and Pt in hydrothermal fluids up to 300°C. Doriri's temperature of formation (100–220°C) is based on chlorite thermometry, clearly placing the deposit in the epithermal field. Other epithermal-hydrothermal features of the deposit include formation at a shallow depth, episodic mineral deposition and the development of rhythmically layered chlorite and magnetite, hydrothermal brecciation, cross-cutting vein textures, the presence of low-temperature marcasite in the lode's pyrite–pentlandite–pyrrhotite sulfide mineralisation and a Pd:Pt ratio >7. The stratiform Goroa Pt–Pd ± Au mineralisation contains concentrations up to 311 ppb Pt + Pd over a 3 m interval in a plagioclase-rich gabbro. Grey–black micrograbbo commonly forms alternating thin layers within plagioclase-rich gabbro and many of these mafic layers contain visible sulfides. Sulfide content of the plagioclase-rich gabbro is ≪1 vol%. A Pt:Pd ratio of ∼1 is typical of the cumulate layered mineralisation. The association of Goroa mineralisation with (1) plagioclase-rich gabbroic rocks, rather than ultramafic rocks, (2) trace sulfides, rather than chromite, and (3) the occurrence with Pt–Pd detrital alloys, suggests a genetic relationship with arc-related layered mafic intrusions. The Goroa diorite–gabbro–ultramafic sequence may be genetically linked to late Paleocene–middle Eocene tonalite, quartz diorite and porphyry arc-related stocks present in the lower Waria River district.

Evidence for hybridisation in the Tynong Province granitoids, Lachlan Fold Belt, eastern Australia

ABSTRACTThe role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ϵHf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ϵHf in two of the three analysed samples (8.8 and 10.1 ϵHf units) exceeds that expected from a single homogeneous population (∼4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ϵNd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.

The Yatela gold deposit in Mali, West Africa: The final product of a long-lived history of hydrothermal alteration and weathering

The Yatela gold deposit is located in the Kédougou-Kénieba inlier (KKI), a window of ca. 2200-2050 Ma rocks that are exposed in eastern Senegal and western Mali. The geology of the KKI differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Yatela deposit occurs within 8 km of the regional-scale Senegal–Mali Shear Zone. Country rocks in the Yatela region have been subjected to polycyclic deformation and regional greenschist-facies metamorphism. A syn-kinematic diorite stock has intruded the metasedimentary sequences in the open pit and is associated with a hornblende-hornfels contact aureole. Field relationships and micro-textural data indicate that the primary gold mineralisation is shear-hosted. The similar relative timing and structural setting between the Yatela primary gold mineralisation and other world-class deposits in the region (e.g., Loulo, Lawrence et al., 2013a; Massawa, Treloar et al., 2014; Sadiola Hill, Masurel et al., in press) suggest that regional orogenic gold mineralisation occurred during a period of transcurrent tectonics, after the cessation of regional compressional deformation. The primary gold mineralisation at Yatela, however, is low-grade and sub-economic. It is hosted by marbles and, to a lesser extent, diorite. The primary ore is pyrite-rich, with abundant chalcopyrite, minor arsenopyrite and accessory Zn–Pb–Sb–Fe–Ag–Co–Ni-bearing mineral species. Post-Birimian surficial dissolution of hydrothermally altered and mineralised host marbles resulted in the creation of troughs, which were draped and infilled with a ferruginous dissolution residue enriched in gold. This auriferous residuum formed the economic resource mined at Yatela until decommissioning in 2013. The Yatela gold deposit is unique with respect to mineralisation types encountered in West Africa because an auriferous residuum of economic interest (>1 Moz) derives from an underlying sub-economic Birimian occurrence. This study provides a new appreciation for the economic interest of Yatela-style orebodies and their prospectivity in other gold provinces of the West African Craton where carbonates are present, such as in the Siguiri basin in Guinea and the Ity region in western Côte d’Ivoire.

Time–space focused intrusion of genetically unrelated arc magmas in the early Paleozoic Ross–Delamerian Orogen (Morozumi Range, Antarctica)

The growth of continental crust in accretionary orogenic belts takes place through repeated cycles of subduction–accretion of rock units from continental and oceanic magmatic arcs, supra-subduction zone backarcs and forearcs loaded with continent-derived materials. An ancient example relevant to magmatic arc accretion models is represented by the remnants of the Cambrian–Ordovician Ross Orogen in the Morozumi Range, Victoria Land (Antarctica). There, late Neoproterozoic phyllites host an intrusive complex which preserves a remarkably uncommon record of genetically unrelated magma pulses emplaced under a variable stress regime in a short time span: (1) a dominant K-feldspar–phyric granite, (2) fine-grained dioritic stocks and dykes, (3) a peraluminous granite; and (4) a tonalitic–granodioritic dyke swarm. Laserprobe U–Pb zircon dates cluster at late Cambrian times for all these units, yet they carry differential cargoes of relict cores. Unique geochemical–isotopic signatures for both the less evolved magmas (diorite and dyke tonalite) and the most acidic ones (granite and peraluminous granite) indicate that each one of them originated from distinct sources at depth. Additionally, field relationships and chemical evolutionary trends testify for a variety of shallow level open-system processes, such as magma mingling/mixing between diorite and main granite magmas, as well as progressive incorporation of the host schists by the dyke tonalite magma. In summary, crustal growth in the Morozumi intrusive complex was contributed by fresh mantle magma issuing from the metasomatised mantle wedge, while the production of other melts did recycle different crustal portions/layers: the main granite derived from Grenville-age granulitic lower crust; the peraluminous granite from late Proterozoic upper crust, and the tonalite magmas derived from subduction erosion-enriched subarc mantle and evolved by ingestion of local metasedimentary rocks. Overall, the Morozumi intrusive complex yields evidence for emplacement in the same site at the same time of magmas issuing from different sources that are usually found at a different depth in the arc lithospheric section. A likely scenario to activate this specific mechanism of melt production is a subduction zone affected by subduction erosion.

Cretaceous granitoids and their zircon U–Pb ages across the south‐central part of the Abukuma Highland, Japan

AbstractZircons separated from Cretaceous granitoids are dated from a south‐central transect of the Abukuma metamorphic and granitic terrane. The zircon ages do not follow ‘older’ and ‘younger’ granitoid ages that are used conventionally. In the western part of the study area (Zones I, II and III) where the Takanuki and Gosaisho metamorphic rocks are exposed, the Iritono quartz dioritic stock intruding the greenschist facies rocks in Zone III exhibits the oldest age of 121 Ma in the studied region. Quartz diorite located northward shows 112 Ma, but the other four granitoids intruding into the Takanuki and Gosaisho metamorphic rocks are younger and 103–99 Ma. Two‐mica and biotite granites belong to the youngest age group of 99 Ma. The granitic activities of both the Abukuma and Ryoke belts were initiated by intrusion of quartz dioritic magmas and were ended by two‐mica granite activity. The ages of the eastern two batholiths vary from 110 to 106 Ma (four samples), and show no age common to the Kitakami granitoids farther to the north. Throughout the Japanese Islands arc, Cretaceous granitic activities became younger toward the marginal sea side from the Kitakami Mountains, to the Abukuma Highland, and the Ryoke Belt, then to the Sanin belt of the Inner Zone of Southwest Japan.