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The North China craton was stabilized in the late Paleoproterozoic but experienced significant removal of ancient lithospheric keel in the late Mesozoic that resulted in the formation of numerous world-class gold deposits with combined reserves of more than 7000 t of gold. However, it remains uncertain whether the North China craton contains older gold deposits formed during generation and final stabilization of the craton. Here, we show that the Shanggong gold deposit (105 t Au at 5.31 g/t) on the southern margin of the North China craton formed in the late Paleoproterozoic during the collision between the Eastern and Western blocks that led to formation of the Trans–North China orogen and final stabilization of the craton. The Shanggong deposit is hosted in amphibolite-facies rocks of the Neoarchean to early Paleoproterozoic Taihua Group and overlying volcanic rocks of the late Paleoproterozoic Xiong’er Group. Gold mineralization is structurally controlled by NE-striking faults and occurs in four segments: the Liuxiugou, Hugou, Shanggong, and Qiliping segments. The ores consist mainly of quartz-ankeritesulfide stockworks and sulfide disseminations in hydrothermally altered wall rocks. Gold is mostly contained in arsenian pyrite that is variably associated with minor sphalerite, galena, and chalcopyrite. Ore-related alteration assemblages comprise mainly quartz, ankerite, K-feldspar, sericite, and tourmaline. Both the stockworks and mineralized alteration assemblages contain hydrothermal accessory minerals, including monazite, apatite, and rutile. Paragenetic relations and textural data show that these accessory phases precipitated synchronously with gold-bearing sulfides. Laser ablation–inductively coupled plasma–mass spectrometry spot analyses of monazite and apatite from the Shanggong segment yielded reproducible U-Pb dates of 1747 ± 20 Ma (2σ, mean square of weighted deviates [MSWD] 0.46) and 1788 ± 200 Ma (2σ, MSWD = 11.3), respectively. These dates are indistinguishable within errors from an apatite U-Pb date of 1743 ± 79 Ma (2σ, MSWD = 1.6) at the Liuxiugou segment and a rutile U-Pb date of 1804 ± 52 Ma (2σ, MSWD = 0.77) at the Hugou segment. These new dates suggest that the Shanggong deposit formed at ca. 1.80–1.74 Ga, coeval with or immediately after formation of the Trans–North China orogen and final stabilization of the North China craton. Sulfides from the Shanggong gold deposit have δ34S values ranging from −18.5‰ to −6.9‰, whereas the coexisting ankerite has δ13CPDB of −6.81‰ to −1.61‰ and δ18OSMOW of 15.70‰–17.62‰. The stable isotope data are distinctively different from values of the Early Cretaceous gold deposits in the southern North China craton, indicating contrasting hydrothermal systems responsible for these two categories of gold deposits. The results presented here, combined with independent geologic evidence, allow Shanggong to be the first confirmed Paleoproterozoic orogenic gold deposit in the North China craton. Recognition of Paleoproterozoic orogenic gold mineralization provides significant new insights into the gold metallogeny of the well-endowed North China craton and has implications for future gold exploration along the three Paleoproterozoic orogenic belts in this craton.

Gold deposits in Precambrian cratons were mostly generated during the formation and stabilization of the cratons, but the North China craton is unusual in that its gold deposits were mainly formed ∼1.7 b.y. after its stabilization. A magmatic-hydrothermal origin or mantle-derived fluid source has been proposed for the giant gold deposits of the Jiaodong District in the eastern North China craton, but direct evidence is sparse, and the mineralization processes remain controversial. Here, we present the results of a comprehensive geological, geochronological, and geochemical study of the gold mineralized Xiejia diorite beneath the Linglong ore field at Jiaodong to link the gold mineralization to underlying magmatism. Magmatic zircon and titanite grains from the Xiejia diorite have laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages of 121.3 ± 0.9 Ma to 120.8 ± 1.1 Ma and 121.7 ± 3.9 Ma, respectively, which are indistinguishable from the time of gold deposition throughout the Jiaodong District as constrained by previous studies. The diorite has a shoshonitic composition and is characterized by strong enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) along with significant depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs). Samples of the diorite have high initial 87Sr/86Sr ratios, but low εNd(t) and ɛHf(t) values and low Pb isotope ratios. These geochemical characteristics are akin to those of contemporaneous mafic dikes in most gold mines at Jiaodong, indicating that the Xiejia diorite was most likely derived from an enriched lithospheric mantle source. The upper part of the diorite intrusion is pervasively altered and mineralized, containing an average of 0.32 g/t Au, but locally up to 7.59 g/t. Hydrothermal titanite from the mineralized diorite has a LA-ICP-MS U-Pb age of 122.3 ± 4.3 Ma, which is consistent with the gold-bearing pyrite Re-Os isochron age of 122.5 ± 6.7 Ma. Ore-related sericite aggregates from the Dongfeng gold deposit above the Xiejia diorite have a 40Ar/39Ar plateau age of 122.6 ± 1.3 Ma. Pyrite from the mineralized diorite yielded δ34SCDT values of 2.1‰−9.7‰, which are comparable with those of pyrite (δ34SCDT = 5.8‰−8.1‰, where CDT indicates the Canyon Diablo troilite standard) from gold ores of Dongfeng. Pyrite grains from both groups also have similar Pb isotope compositions. The S and Pb isotope data are consistent with values of mafic dikes that are spatially and temporally associated with gold veins in the Linglong ore field. The results presented here thus indicate a possible genetic link between gold mineralization in the Xiejia diorite and underlying magma presumably represented by the Xiejia diorite. The auriferous fluids exsolved from that magma and migrated upward along the Potouqing fault to form the Dongfeng gold deposit above the Xiejia diorite. The mineralized diorite thus links shallow gold mineralization to deep-seated mantle-derived magmatism generated during the extensive destruction of the North China craton induced by the rollback of the subducted paleo-Pacific plate.

The Troodos Massif has played a unique role in the development of the ophiolite concept. The importance of Troodos derives from its coherent nature, its excellent exposures and easy access, and the fact that it was one ofthe first such bodies to be studied in detail. Early studies demonstrated the genetic relationship between ophiolitic units and elucidated the nature and significance of sheeted dikes, leading to the recognition that ophiolites are fragments of ocean lithosphere formed in spreading environments. Information derived from Troodos played a key role in formulating a new definition of ophiolites in 1972, one that has remained essentially unchanged for 30 years. The suprasubduction zone nature of ophiolitic lavas was first recognized in Troodos, providing a new understanding of where ophiolites originate in the ocean basins and raising questions as to the validity of applying the ophiolite model to the greater part of the oceanic lithosphere. Detailed mapping and drilling of Troodos has shown that the ophiolite was constructed from multiple subrift magma chambers, and has elucidated the relationships between the plutonic complex and the sheeted dikes. The base metal orebodies of Troodos were shown to be hydrothermal deposits before black smokers were discovered on the seafloor, and they now provide opportunities for detailed, three-dimensional studies of fossil hydrothermal systems. Finally, study of Troodos provided convincing evidence for uplift of oceanic lithosphere by partial underthrusting of continental lithosphere, a model now widely favored for many cases of ophiolite emplacement. Although our understanding of ophiolites has been greatly enhanced by studies of Troodos, many questions remain and we expect that the massif will continue to play a significant role in ophiolite research.

Ocean Drilling Program ODP Hole 735B, drilled on Legs 118 and 176, 1508 m of oceanic layer 3 on a transverse ridge adjacent to the Atlantis II Fracture Zone, Southwest Indian Ridge. The cored sequence consists predominantly or olivine gabbro and troctolite and lesser amounts of gabbro, and gabbronorite rich in oxides. The section contains live major blocks of relatively primitive olivine gabbro and troctolite, composed of many smaller igneous bodies. Each Of these composite blocks shows a small upward decrease in Mg# [defined as 100 x Mg/(Mg + Fe 2+)] and contains more fractionated Fe- and Ti-rich gabbros near the top.Small, crosscutting bodies of olivine gabbro and troctolite with diffuse boundaries may represent conduits through crystal mushes for melts migrating upward and feeding individual intrusions. Oxide gabbros and gabbronorites are commonly associated with shear zones of intense deformation, which crosscut the section at all levels, However, oxide-rich rocks decrease in abundance downward and are nearly absent in the lower 500 m of the section. The gabbros and gabbronorites appear to have formed from late-stage, Fe- and Ti-rich, intercumulus melts that were expelled out of fractionating olivine gabbros into the shear zones. The fabrics of the recovered gabbros are consistent with synkinematic cooling and extension of the crustal section in a mid-ocean ridge environment. However, thick intervals of the core have only a weak magmatic foliation. The magmatic foliation is commonly overprinted by a weak, parallel, deformational fabric probably reflecting the transition from a largely magmatic to a largely crystalline state. Deformation in this crustal section decreases markedly downward. Metamorphism and alteration also decrease downward, and much of the core has less than 5% background alteration. Major zones of crystal-plastic (ductile by dislocated creep) deformation in the upper part of the core probably formed under conditions equivalent to granulite-facies conditions when there was little or no melt present. Late-magmatic and hydrothermal fluids produced a variety of plagioclase, amphibole, and diopside veins. Late-stage, low-temperature veins of calcite, smectite, zeolite, prehnite are present in a few intervals. The fact that the cored is unlike ophiolite as defined by the Penrose Conference Participants suggests that no ophiolite representing an ultra-slow-spreading-ridge environment like the Southwest Indian Ridge may be preserved.

Any comprehensive model for the origin of the Troodos complex and other Mideast ophiolite complexes must explain the geochemical evidence for subduction-zone involvement, the thin inferred oceanic crust, the extensional environment indicated by sheeted dikes, the existence of fault zones perpendicular to the inferred spreading axes, and the discontinuous nature of ophiolite exposures around the Arabian block. The Andaman Sea region of the Indian Ocean may provide an actualistic model for the origin of these ophiolites. There, spreading takes place in short segments above a subduction zone in the region of active spreading. The Andaman Sea model substantially accounts for the known geologic and geophysical data from the circum-Arabian ophiolite belt, and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt. The model also predicts that Andaman Sea spreading-center magmas may be similar in composition to those found in the Mideast ophiolites.

Fresh volcanic glass is preserved throughout the extrusive section of the Troodos ophiolite, which indicates that the lavas have not been pervasively metamorphosed. Glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals. The basal 400–500 m of the sequence consists of an andesite-dacite-rhyolite assemblage containing abundant hyaloclastites. The remainder of the section comprises a basalt–basaltic andesite assemblage with high MgO and low TiO 2 and total iron. The lower sequence is interpreted as an evolved arc-tholeiite suite; the upper has some similarities to boninitic lavas. The close association in time and space of these two suites is similar to that observed in the Mariana and Bonin arcs and suggests that all of the Troodos lavas were erupted in a subduction-zone environment, most probably in an incipient arc or fore-arc.

The Salton Sea geothermal field lies in the Salton Trough, the landward extension of the Gulf of California, an area of active crustal spreading. Surface volcanic rocks of the field consist of five small rhyolite domes extruded onto Quaternary sediments of the Colorado River delta. Two domes are linked by subaqueous pyroclastic deposits; the others are single extrusions with or without marginal lava flows. The domes are low-calcium, alkali rhyolite with 1 to 2 percent crystals. Similar silicic rocks found in wells have been extensively altered by geothermal brines. Basaltic rocks occur as xenoliths in the domes and as subsurface dikes, sills, or flows. The xenoliths consist of low-potassium tholeiitic basalt similar to that of the East Pacific Rise. Subsurface basaltic rocks are mineralogically similar to the xenoliths but have undergone extensive hydrothermal alteration. Numerous partly melted granitic xenoliths in the domes show various degrees of either cotectic melting along quartz-feldspar grain boundaries or disequilibrium incongruent melting of hydrous ferromagnesian minerals. These rocks are sodic granite containing notably higher SiO/sub 2/, CaO, and Na/sub 2/O and lower total iron than the enclosing rhyolite. Compositions and textures suggest that the granite xenoliths are fragments of basement rather than the crystallized equivalentsmore » of the rhyolite magma. The bimodal basalt-rhyolite assemblage of the Salton Sea geothermal field is believed to have formed by partial fusion in two stages of mantle peridotite, forming successive rhyolitic and basaltic melts. After formation, the rhyolitic magma was partly contaminated by continental crust material.« less

Aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics. Five zones are recognized in the Troodos ophiolite and in situ oceanic crust: (1) a sea-floor weathering zone (SWZ), (2) a low-temperature zone (LTZ), (3) a transition zone (TZ), (4) an upper dike zone (UDZ), and (5) a mineralized zone (MZ). The MZ may be overlain by the SWZ, LTZ, or TZ, but in the known examples it is always underlain by the UDZ. The development of the SWZ is dependent upon the duration of cold-seawater circulation, which is dependent upon seafloor topography and the rate and nature of sedimentation. The boundary between the low- and high-temperature zones is largely controlled by permeability contrasts. Lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust. This, along with the evidence for disequilibrium assemblages, implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects.

The Silver Peak volcanic center is part of a late Tertiary volcanic province in western Nevada and eastern California characterized by slightly alkaline, potassic lavas. The rocks of the Pliocene Silver Peak center have an alkali-lime index of 55.5, a K2O/K2O + Na2O ratio of 0.5 at 60 percent SiO2, and range in composition from trachybasalt to rhyolite, with latite the most voluminous. Chemical and mineralogical variations in the intermediate and silicic lavas are compatible with differentiation by crystal fractionation. The differentiation trend is similar to calc-alkaline suites with progressive enrichment in silica and alkalis but no relative enrichment in iron. Relatively high water pressures during crystallization are indicated by the ubiquitious presence of biotite or hornblende. The parent magma of the intermediate and silicic part of the series is believed to have a composition of trachyandesite with approximately 56 percent silica. The potassic nature of trachyandesite in this province is tentatively related to depths of a Tertiary subduction zone beneath western North America. Associated trachybasalts of the Silver Peak center are nepheline normative and characterized by high strontium contents, low Rb/K and Rb/Sr ratios, and relatively high Sr87/Sr86 ratios (.7061). These are believed to represent a separate parent magma derived from material previously depleted in rubidium.