MORB-like Source Research Articles

Pb isotope bearing on the metallogenesis of sulfide ore deposits in central and southern Peru

Twenty-five Pb isotope determinations are presented on galena, chalcopyrite, pyrite, chalcocite, and cerussite from 15 mineral deposits in the copper, iron, and polymetallic provinces of central and southern Peru. The results are evaluated in comparison with the Pb isotope signatures of potential source materials which include plutonic rocks of the Coastal batholith, Precambrian basement rocks of the Arequipa massif, as well as midocean ridge basalts (MORB) of the Nazca plate, and Pacific Ocean pelagic-metalliferous sediments subducted at the Peru-Chile trench.Several types of mineral deposits are recognized in the copper province which roughly coincides areally with exposures of the Early Jurassic to Paleogene Coastal batholith. Amphibolitic Cu-Fe skarns with the magnetite-pyrite-chalcopyrite mineral association define an array with a Pb isochron age of 3.2 Ga in the 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb covariation diagram. Mineral deposits of the copper veins in granitoids association at the Cata-Canete and Cobre-Acari districts plot along the same array. The isochron age of 3.2 Ga is too old to have chronological significance. The array of points is therefore interpreted as the product of mixing between two end members. Enriched subcontinental mantle (produced by mixing in the subduction zone of pelagic and/or metalliferous sediments and material from a MORB-like source) and supracrustal sedimentary materials are the most likely end members.The Pb isotope compositions of disseminated porphyry Cu-Mo deposits are virtually identical to those of the batholithic rocks with which they are genetically linked. Thus the El Molino prospect has high Pb ratios, typical of plutons near the Lima-Arequipa segment boundary in the Coastal batholith (Mukasa, 1986a). The Pb content of the plutons is chiefly from mixtures of a depleted, MORB-like component and subducted pelagic-metalliferous sediments. The Cerro Verde deposit, like its host rocks, has low 206 Pb/ 204 Pb values most easily explained by mixing of materials derived from the Precambrian basement with those from the subduction-generated enriched subcontinental mantle keel.Deposits of the auriferous vein association and the unclassified Unturos-Chacaya galena and pyrite deposit show a wide Pb isotope scatter in the Pacific Ocean sediment field, as well as both above and below it. Similarly, galena and chalcopyrite from the polymetallic province show major Pb isotope heterogeneities.Extensive metal scavenging from local supracrustal materials occurred demonstrably. Accordingly, metal sulfides along Andean-type plate margins do not have a unique source.

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

The western part of the Mesozoic Peruvian Trough, the Huarmey Basin, has a fill of pillow lavas, sheet lavas, hyaloclastites, tuffs and minor cherts, siliceous and calcareous oozes. Maximum subsidence occurred in the Albian when 9000 meters of basin fill accumulated. This was later intruded by gabbros and dikes and then the Coastal Batholith. The basin is an extensional marginal basin continuous with other basins of similar age to the south. Facies analyses indicate that the basin was relatively deep, with no continental influx, and not dissimilar to spreading and off-axis systems on the ocean floor. Structures at the surface and at depth indicate the crust has split and the basin was floored by mantle material. The basin shows a marked polarity, with tholeiitic basaltic rocks at the center and high-K acid rocks at the eastern margin, with intermediate types between. These changes in petrology and chemistry relate to lateral changes in source composition. Secular variations are also present and indicate a calcalkali source giving way to a more MORB-like source with a variable continental component. The basin is part of the major rifting event in the Cretaceous which affected the whole western margin of South America and was an important and necessary precursor to major batholith intrusion.

The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model

New REE data, and new Nd, O, Sr, and Pb isotopic data are presented and integrated with previous data for this low-K intra-oceanic arc suite. Geochemically, the arc tholeiites and basaltic andesites range from extremely HFS element depleted (northern Tonga) to near N-MORB-like HFS element abundances in L'Esperance (southern Kermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristic selective enrichment generally recognized in arc magmas, and thus indicate decoupling of the HFS and LIL elements. Modelling suggests a compositionally variable source (mantle wedge) along the arc, ranging from restite after remelting an N-type MORB source (northern end), to progressively less depleted, MORB-like sources southwards. Thus, the low HFS/LIL element ratios are interpreted in terms of HFS depletion followed by LIL element enrichment associated with subduction; broad correlations occur between Zr/Ba and Sr/Nd ratios (fractionation corrected) and 87Sr/86Sr and Nd/Nd ratios.Derivation of the arc magmas from depleted peridotote requires superimposed fractional crystallization, which has been modelled thermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentally produced partial melts from depleted lherzolite (Jaques & Green, 1980). It is shown that the arc tholeiites and basaltic andesites (and also high Mg-andesites) are potentially developed at low pressures ≤ 5 kb), from parental magmas also generated at relatively low pressure (≤ 10 kb). These data further suggest that a southward increasing depth of magma segregation (correlating with Benioff Zone geometry) could account for differences in chemistry between the Tonga and Kermadec arc segments.The mechanism of LILE enrichment is still highly problematic, but it is suggested that the model of Tatsumi et al. (1986) may account for much of the geochemical data; this involves relatively shallow release, via fluids, of LIL elements into overlying peridotite beneath the fore-arc region. Induced convection in the mantle wedge moves the metasomatized mantle into the zones of magma generation.The development of the inferred, variably depleted mantle wedge source is here related to active back-arc spreading which is slightly older and more rapid behind the northern region of the arc. The lherzolite restite from this spreading process is interpreted to undergo further partial melting in the metasomatized mantle wedge overlying the subduction zone, involving induced convection.The back-arc island of Niua fo'ou is geochemically quite distinct from the arc magmas, being similar to N-MORB in its trace elements, but to OIB in its isotope ratios.