Metallogenic Provinces of Northeast Pacific: ABSTRACT

Large-scale porphyry-type mineralization in the Central Asian metallogenic domain: A review

2090–2070 Ma A-type granitoids in Zanhuang Complex: Further evidence on a Paleoproterozoic rift-related tectonic regime in the Trans-North China Orogen

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

The first Metallogenic Chart of Mexico has been prepared in compliance with the Metallogenic Chart of the World Project and in keeping with the North America Subcommittee commitments. It portrays the location of mines and/or mineral districts, and through special symbols and colors from an ample and explicit legend, shows the type of ore deposits, their age, and depositional environment, etc. Thus, on the basis of mineral-deposit environment, the author tentatively proposes to divide the Mexican territory into six metallogenic provinces. (These later may be subdivided into subprovinces and smaller units.) 1. The Baja California province, on the north, with an approximate surface area of 92,000 sq km, is made up of one or more very large granodioritic batholiths, vast areas of metamorphic rocks of undetermined age, and restricted volcanic-neck areas. The southern part shows extensive andesitic and ignimbrite flows as well as silicic and mafic intrusive rocks. 2. The Sierra Madre Occidental province is a 266,000-sq km area of volcanic rock intruded by granites and granodiorites and subordinate mafic rocks. Mineral deposits are present as veins in the intrusive End_Page 1456------------------------------ and extrusive rocks. It is assumed that the intrusives were the origin of the mineral deposits in the volcanic rocks. Some mineralization occurs in Cretaceous limestone by contact metasomatism. 3. The Sierra Madre Oriental province, a 379,000-sq km area, occupies the great Mexican geosynclinal folded belt of Laramide age. Intrusive rocks and some volcanoes establish the metallogenic processes, which are mostly by contact metasomatism and vein filling. Metallogenesis appears to be of late Tertiary age. 4. The Sierra Madre del Sur area extends for 114,000 sq km from the State of Michoacan ESE toward the State of Oaxaca. Geologically it is very similar to the Sierra Madre Occidental province, but seems to constitute a different block of generally lower topography and with more sedimentary Cretaceous limestone. Limestone remnants overlie extensive intrusive rocks. Some are mineralized. Large areas of metamorphic rocks seem to carry mineralization of Precambrian or pre-Paleozoic age. Mineralization in the volcanic and sedimentary rocks seems to be of late Tertiary age. 5. The Mesa Central province extends over an area of 105,000 sq km from the northern edge of the volcanic axis on the south to northern Zacatecas and to part of the Durango on the Central Plateau. The geology is made up of very vast flows of andesite in the southern part, and predominantly rhyolite in the northern part. The thick sequence of volcanic rocks shows low-temperature and pressure mineralization. This is specially noticeable where intrusives, as in Guanajuato and affect the extrusive rocks. Mercury and fluorite deposits are abundant. 6. Eje Neo-Volcanico, a 190,000-sq km volcanic zone or volcanic chain crosses the continent from Bahia Banderas, in the vicinity of Puerto Vallarta, on the Pacific coast, to the Sierra of San Andres Tuxtla on the Gulf of Mexico. Some authors have postulated the thesis that a large transverse fault crosses the continent, as a continental expression of the Clarion fault. The author's recent paper on ERTS-1 image interpretation does not show evidence of this effect. The famous Taxco, Pachuca, Angangueo, El Oro and Tlalpujagua silver deposits are along this volcanic belt. Much more research on their origin is needed, and the Metallogenic Chart of Mexico will enhance this research. End_of_Article - Last_Page 1457------------

Geochemical characteristics of sedimentation and metamorphism of the Dongchuan copper deposit, Yunnan Province, China

The Cimmerian orogenic belt of North Dobrogea (East Romania) is located in the Carpathian foreland between the Moesian and Scythian Platforms. In the absence of reliable geochemical data on the different magmatic rock-types, various geodynamic models have been suggested for the Triassic-Jurassic evolution of this belt. Geochemical studies on mafic dykes emplaced in different Hercynian basement rocks, as well as on the Early-Middle Triassic Niculitel Formation basalts have been performed in order to provide new constraints for the geotectonic setting of this belt. A Permo-Triassic phase of crustal thinning of the Hercynian basement is suggested by the still poorly known alkaline magmatism. The Triassic magmatic history involves intrusion of tholeiitic dykes in the Hercynian basement of the Macin Zone, and extrusion of pillow basalts (Niculitel Formation) that most likely occurred above the carbonate compensation depth in a rifted basin with a thinned crust, as suggested by facies characteristics of carbonate rocks interbedded with basalts. This basin could have corresponded either to an aborted rift, or to a passive margin related to back-arc opening. Our data indicate that Macin and Niculitel basalts are derived from a MORB-type asthenospheric mantle source variably influenced by a plume-type component. The less enriched character of the Macin dykes reflects a lesser influence of plume source on magma composition with respect to the Niculitel basalts. Modern chemical analogues are found in the South West Indian and American-Antarctic Ridges, where composition of basalts range from pure plume-type ocean island basalts (OIBs) to pure MORBs, depending on the influence of the Bouvet mantle plume on MORB source. Regardless of the geochemical differences, which can reasonably be related to local variations of the plume component influence on the MORB source, a common geodynamic setting can be postulated for the origin of the two basaltic series. In the hypothesis of the aborted rift, an evolution of the mantle sources, starting with a predominating plume activity followed by an uprise of the primitive asthenospheric mantle, can be postulated for the North Dobrogea Triassic basalts. By contrast, in the back-arc basin model, the plume activity may have played a major role in weakening the lithosphere and preparing the back-arc spreading. The close geochemical similarities between basalts from North Dobrogea and basalts from various ophiolitic complexes and oceanic ridges do not necessarily imply that North Dobrogea Triassic basalts represent an ophiolitic sequence or, in other words, the hypothesized uprise of asthenospheric primitive mantle does not imply an oceanic spreading steady-state. Our data indicate that the Macin and Niculitel basalts originated in an extensional tectonic setting in which the transition from alkaline to E-MORB magmatism was a consequence of the mantle plume evolution through time.

The late Archaean Shimoga schist belt in the Western Dharwar Craton, with its huge dimensions and varied lithological associations of different age groups, is an ideal terrane to study Archean crustal evolution. The rock types in this belt are divided into Bababudhan Group and Chitradurga Group. The Bababudhan Group is dominated by mafic volcanic rocks followed by shallow marine sedimentary rocks while the Chitradurga Group is dominated by greywackes, pillowed basalts, and deep marine sedimentary rocks with occasional felsic volcanics. The Nb/Th and Nb/La ratios of the studied metabasalts of the Bababudhan Group indicate crustal contamination. They were extruded onto the vast Peninsular Gneisses through the rifting of the basement gneiss. The Nb/Yb ratios of high-magnesium basalts and tholeiitic basalts of Chitradurga Group suggest the enrichment of their source magma. Based on the flat primitive mantle-normalized multi-element plot with negative Nb anomalies and Th/Ta-La/Yb ratios, the high-magnesium basalts and tholeiitic basalts are considered to have erupted in an oceanic plateau setting with minor crustal contamination. The high-magnesium basalts and tholeiitic basalts formed two different pulses of same magma type, in which the first pulse of magma gave rise to high-magnesium basalts which were derived from deep mantle sources and underwent minor crustal contamination en route to the surface, while the second pulse of magma gave rise to tholeiitic basalts formed at similar depths to that of high-magnesium basalts and escaped crustal contamination. The associated lithological units found with the studied metavolcanic rock types of Bababudan and Chitradurga Groups of Dharwar Supergroup of rocks in Shimoga schist belt of Western Dharwar Craton confirm the mixed-mode basin development with a transition from shallow marine to deep marine settings.

Numerous polymetallic volcanogenic massive sulfide (VMS), vein, and replacement deposits are distributed along the Changning–Menglian suture zone in Sanjiang Tethyan metallogenic province, SW China. Laochang is the largest Pb–Zn–Ag vein and replacement deposit in this area, with a proven reserve of 0.51 Mt Pb, 0.34 Mt Zn, and 1,737 t Ag. Its age and relationship to magmatic events and VMS deposits in the region, however, have long been debated. In this paper, we present pyrite Re–Os and titanite U–Pb ages aiming to provide significant insights into the timing and genesis of the Pb–Zn–Ag mineralization. Pyrite grains in textural equilibrium with galena, sphalerite, and chalcopyrite from stratabound Pb–Zn–Ag and Cu-bearing Pb–Zn–Ag orebodies have a Re–Os isochron age of 45.7 ± 3.1 Ma (2σ, mean square weighted deviation (MSWD) = 0.45), whereas titanite grains intergrown with sulfide minerals yield a weighted mean 206Pb/238U age of 43.4 ± 1.2 Ma (2σ, n = 8). A Mo-mineralized granitic porphyry intersected by recent drilling below the Laochang Pb–Zn–Ag ores yields a zircon U–Pb age of 44.4 ± 0.4 Ma (2σ, n = 12). Within analytical uncertainties, the ages of the Pb–Zn–Ag deposit and the concealed Mo-mineralized porphyry are indistinguishable, indicating that they are products of a single magmatic hydrothermal system. The results show that Laochang Pb–Zn–Ag deposit is significantly younger than the host mafic volcanic rock (zircon U–Pb age of 320.8 ± 2.7 Ma; 2σ, n = 12) and Silurian VMS deposits along the Changning–Menglian suture zone, arguing against its origin as a Carboniferous VMS deposit as many researchers claimed. The initial 187Os/188Os ratio (0.540 ± 0.012) obtained from the pyrite Re–Os isochron suggests that metals were likely derived from the granitic porphyry that formed from a hybrid magma due to mixing of crustal- and mantle-derived melts, rather than from the mafic volcanic host rocks as previously thought. Our results favor that the Laochang Pb–Zn–Ag deposit is the shallow product of a porphyry Mo system. Thus, there is potential for discovery of porphyry Mo or Cu–Mo deposits below Laochang and similar Pb–Zn–Ag deposits in the Changning–Menglian suture zone.

Petrogenesis of Cenozoic high–Sr/Y shoshonites and associated mafic microgranular enclaves in an intracontinental setting: Implications for porphyry Cu-Au mineralization in western Yunnan, China

Le complexe volcano-plutonique calco-alcali de la rivière daléma (Est Sénégal): discussion de sa signification géodynamique dans le cadre de l'orogénie eburnéenne (protérozoïque inférieur)

The Silurian?Middle Devonian history of the Lachlan Orogen is characterised by the formation of rift basins and the emplacement of large amounts of granite. Many rift basins contain felsic or mixed felsic and mafic volcanic rocks, indicative of crustal as well as mantle melting being involved in lithospheric extension. However, there are several large rift basins that are filled by siliciclastic sedimentary rocks in which volcanics occupy < 1 % of the basin fill and may be buried. For the latter basins the question is: was rifting amagmatic, or are products of melting present at depth below the surface, either in deep basin sediments or in basement below the basin. In this paper, we attempt to address this question for sedimentary basins in the Cobar?Louth region of western New South Wales. For the Late Silurian?Early Devonian Cobar Basin, we use 1989 explosion?generated seismic reflection data that have been reprocessed using a new semblance filtering technique to improve the data quality. For the Nelyambo Trough, part of the Devonian Darling Basin in western NSW, we used Vibroseis deep seismic reflection data recently acquired in cooperation with Geoscience Australia and the Predictive Mineral Discovery Cooperative Research Centre. Gravity profiles were acquired along the Cobar lines. For the Nelyambo Basin, gravity data were extracted from a statewide dataset to match the seismic lines. The combined seismic and gravity data sets suggest that bright reflectors in the seismic sections represent mafic volcanics. These reflectors lie within inferred rift fill near the base of the Nelyambo Trough, but also occur in basement under the southwestern margin of the Cobar Basin.

Quartz diorites represent the earliest (ca. 540 Ma) and most primitive plutonic rocks in the Pan African Damara belt and they pre-date the main phase of high-T regional metamorphism. Two suites of synorogenic quartz diorites are unusual among Damaran intrusive rocks in their elemental and isotopic features. Comparison of the diorite compositions with melts from amphibolite-dehydration melting experiments points to a garnet-bearing meta-tholeiite, probably enriched in K2O, as a likely source rock. Partial melting processes generated mafic (ca. 50 wt% SiO2) quartz diorites in the deep crust at temperatures of between 1,000 and 1,100 °C, based on comparison with experimental results and similar temperature estimates based on P2O5 solubility in mafic rocks. Subsequently, the quartz diorites evolved by multistage, polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses. Although their chemical characteristics (high LILE, low HFSE) resemble those of other quartz diorites with calc-alkaline affinities, they differ in their enriched Sr (initial 87Sr/86Sr: 0.70943–0.71285), Nd (initial e Nd: –9.1 to –15.2 ) and O (δ18O: 6.8–8.1‰) isotope compositions. Neodymium model ages (TDM) that range from 1.7 to 2.2 Ga and large variation in 207Pb/204Pb relative to 206Pb/204Pb indicates involvement of ancient crustal material. Lead (206Pb/204Pb: 17.08–17.23, 207Pb/204Pb: 15.53–15.62, 208Pb/204Pb: 37.71–38.16) isotope compositions are strongly retarded, indicating that the source underwent a pre-Pan-African U/Pb fractionation and U depletion. It is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust. This contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during Pan-African times with characteristics modified by subduction of oceanic crust and sedimentary rocks.

Plate tectonics, the hypothesis of multiple crustal plates floating on a viscous layer called the asthenosphere, provides rationale for viewing the earth's outer shell as a system of shifting continents and growing ocean basins. The idea of diverging plates (or sea-floor spreading) implies that plates converge elsewhere at compatible rates. Estimated convergence rates range up to 4 in. per year, or 140 mi in the 2 m.y. since the beginning of Pleistocene time. Convergence between oceanic crust and continental crust may result in thermal generation of oil and gas in sediments as young as Pleistocene age because of rapid deep burial associated with subduction. Mountainous source areas for sediment and steep continental slopes favor rapid burial of organic material with turbidites. Rapid subduction of oceanic crust under continental margins may carry sediments to depths which provide requisite thermal environments for generation of oil and gas from organic matter disseminated in the sediment. Continued subduction of oceanic crust under continental slopes may cause reverse faulting such that oil and gas accumulations are uplifted toward the ocean bottom. Core samples obtained adjacent to the Aleutian Trench in the western Gulf of Alaska apparently show effects of subduction and reverse-fault uplift on a section of Pleistocene sediment. Although this Pleistocene sediment is only a few hundred feet below the ocean bottom, organic matter carbonization suggests previous burial of at least 8,500 ft and late pregeneration stage of organic carbonization. In contrast, noncommercial oil production from uplifted deep-water sediment of early Tertiary age at Katalla, Alaska, suggests formerly significant accumulations have been dissipated by faulting, uplift, and erosion. Late Tertiary rocks beneath outer continental shelves and/or upper continental slopes at convergent margins may be in the End_Page 1459------------------------------ optimum stage of current petroleum expulsion but still buried deeply enough for entrapment of giant oil accumulations. Regions for analogous exploration application of this hypothesis, in addition to the western Gulf of Alaska, include continental or island margins adjacent to other deep oceanic trenches such as the Japan, Mindanao, Java, Solomon Sea, Peru-Chile, and Central American trenches, and the southern end of the Puerto Rico Trench northeast of Trinidad. End_of_Article - Last_Page 1460------------

Mineral deposits of the southeastern Pacific region include: (1) hypogene, volcanogenic, and sedimentary deposits of the Andean region, (2) scattered copper deposits in the Antarctic Peninsula; and (3) metal-enriched sea-floor sediments and volcanic rocks. Andean metalliferous deposits are related spatially and genetically to calc-alkaline plutons and volcanic rocks emplaced during the Andean orogeny of Jurassic to Quaternary age. These deposits are components of a single metallogenic province, superimposed on two or more pre-Andean Paleozoic and Precambrian metallogenic provinces. Scattered copper deposits of the Antarctic Peninsula are similar in age and origin to deposits in the Andes and are considered to be in the Andean province. The sea-floor deposits include: (1) etal-enriched rocks and sediments that have formed at an accreting plate margin, such as the East Pacific Rise or near sea-floor volcanism; (2) sea-water precipitates; and (3) concentrations of heavy minerals in clastic sediments along the continental margin. End_Page 1435------------------------------ The Andean metallogenic province can be divided into dominant metal (or metals) subprovinces, each parallel with the Andes and the continental margin. The central Andes of Peru, northern Chile, and Bolivia contain the greatest concentration of exploitable deposits and greatest variety of ore types, and have five linear, partly overlapping, subprovinces. These subprovinces, from west to east, are characterized by deposits of: (1) iron; (2) copper, with or without gold; (3) polymetallic base metals (Zn, Pb, Cu), generally containing silver; (4) tin; and (5) gold. Iron deposits are near the coast from central Chile to southern Peru. The copper and polymetallic provinces are continuous throughout most of the central Andean region and extend south into Chile and north into Colombia. A disc ntinuous gold province, overlapping the copper and iron provinces, can be traced from central Chile to northwestern Colombia; another belt of gold deposits is in the eastern Andes from Bolivia to Ecuador. Tin deposits are restricted essentially to the eastern Andes of Bolivia. Magmas of the calc-alkaline rocks of the Andes are believed to have formed by melting of mantle, oceanic sediments, and oceanic crust at depths of 100 to 200 km along the Benioff zone. These igneous rocks generally decrease in age from west to east, though nonuniformly. Rocks of Jurassic and Cretaceous age are most abundant near the coast, whereas those of Tertiary and Quaternary age are dominant in the Andes; but locally, rocks and ore deposits of different ages are juxtaposed. These relations suggest variations in rates of subduction, in inclination of the subduction zone, and in position relative to the continental margin. Metals associated with the calc-alkaline rocks were supplied by the source rocks in the Benioff zone; some may have been enriched in metals, and from metal-rich ones in the overlying mantle and continental crust. The rising magmas probably assimilated or caused mobilization of previously formed ore deposits. End_of_Article - Last_Page 1436------------

SummaryThe ophiolite belts in both Egypt and Saudi Arabia between Latitude 22° N and 24° N, having approximately the same direction of extension, NNW or N—S.The ophiolite belts are folded, the fold axes trending NW to E W in Egypt, while NE—SW is the main trend in Saudi Arabia. The shear zones of E—W strike are mineralized (Cu, Zn, Pb minerals) in the Egyptian side, while N—S or NE—SW shears are mineralized in Saudi Arabia (Cu minerals in Umm ad Damar and J. Sumran). The tension faults play a role for gold mineralization in both Egypt and Saudi Arabia. The directions and magnitudes of folds and faults in Egypt and Saudi Arabia are different, probably that is due to the difference in directions and magnitude of the forces and stresses on both sides of the Red Sea. The sulfides in the ophiolites have to be taken to represent typical “Cyprus type” volcanic exhalative mineralization during sea floor spreading. Later on some of these sulfides were remobilized by epigenetic hydrothermal solutions to be localized along pre-existing shears.In Egypt, the ring complexes zone is located west to the main ophiolitic suite; that let the present writer to be tempted to say, the ring complexes are probably existing in a zone east of the western ophiolitei mass of Southern Hijaz Quadrangle, but now is covered by Tertiary basaltic and andesitic lavas.