Marcos Zentilli

Geochemistry and origin of volcanic rocks of the Andes (26°–28°S)

Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ° and 28 ° South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest from the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.

Geology of the Darreh-Zerreshk and Ali-Abad Porphyry Copper Deposits, Central Iran

This paper describes the petrology, mineralogy, alteration, structural characteristics, and geological evolution of the Darreh-Zerreshk and Ali-Abad copper deposits within the central Iranian volcano-plutonic belt, Yazd Province, central Iran. Intrusions in this area, a result of subduction magmatism, range in composition from quartzmonzodiorite to granite, yet copper-molybdenum porphyry—type mineralization is restricted to quartzmonzodioritic to granodioritic plutons. The mineralizing intrusions cut Cretaceous and Eocene strata, and have been dated as Oligocene—Miocene. Strong negative anomalies for Nb, Ta, P, and Ti and enrichment in the LREE, LILE, Ba, Rb, Sr, and Pb for the mineralizing igneous rocks suggest they are of I type and have magmatic arc affinity. REE patterns, major-, and trace-element data suggest that these rocks were derived from partial melting of enriched upper mantle, modified by variable crustal contamination. Hornblende fractionation was an important control on the evolution of copper-bearing plutonic rocks. Structural observations suggest that these deposits were emplaced in tensional pull-apart domains formed between two secondary dextral strike-slip faults of the regional Dehshir-Baft shear system. Rapid exhumation of the porphyry copper systems allowed for the development of only minor supergene enrichment.

Evolution of an Active Ductile to Brittle Shear System Controlling Mineralization at the Chuquicamata Porphyry Copper Deposit, Northern Chile

The giant Eocene-Oligocene Chuquicamata porphyry copper deposit in northern Chile illustrates a remarkable degree of structural control from the time of porphyry emplacement through mineralization-alteration and supergene enrichment. The porphyry system developed within a transtensional faultbend related to the > 2000-km-long, trench-parallel shear system known as the Precordilleran Fault System, which has been active in the Precordillera since at least the Eocene. Detailed structural sampling and mapping at scales from 1:10 to 1:1000 within representative mineralized structural domains defined within the Chuquicamata open pit reveals that the bulk of mineralization is contained in fractures with defined preferred orientations that impart an anisotropy to ore grades at certain scales. Cross-cutting relationships in one domain, Estanques Blancos, establish a relative chronology of alteration-mineralization pulses related to an older (34 Ma), deeper potassic and a younger, superimposed (31 Ma), shallower qu...

Field characteristics of laterally emplaced dikes: Anatomy of an exhumed Miocene dike swarm in Reydarfjördur, eastern Iceland

Abstract Many dikes of the Tertiary Breiddalur dike swarm in Reydarfjordur have a great lateral extent but are vertically (upward and/or downward) discontinuous. Field relationships indicate that they are not the feeders of the lava pile they intrude. Their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading Krafla volcanic center of the neovolcanic zone. Average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other Tertiary dike swarms in eastern Iceland. It is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures.

Fission track thermochronology of Neogene plutons in the Principal Andean Cordillera of central Chile (33-35°S): Implications for tectonic evolution and porphyry Cu-Mo mineralization

Apatite fission track data for Miocene plutons of the western slope of the Principal Andean Cordillera in central Chile (33-35°S) define a distinct episode of enhanced crustal cooling through the temperature range of the apatite partial annealing zone (~125-60°C) from about 6 to 3 Ma. This cooling episode is compatible with accelerated exhumation of the plutons at the time of Pliocene compressive tectonism, and mass wasting on the western slope of the Principal Andean Cordillera in central Chile. The timing coincides with the southward migration of the subducting Juan Fernandez Ridge and the development of progressive subduction flattening northward of 33°S. It also corresponds to the time of active magmatic-hydrothermal processes and rapid unroofing of the world class Rio Blanco-Los Bronces and El Teniente porphyry Cu-Mo deposits. Zircon fission track ages coincide with previous 40 Ar/ 39 Ar dates of the intrusions, and with some of the apatite fission track ages, being coherent with igneous-linked, rapid cooling following magmatic intrusion. The thermochronologic data are consistent with a maximum of about 8 km for Neogene exhumation of the plutons.

Oxygen isotopic compositions of Central Andean plutonic and volcanic rocks, latitudes 26°–29° south

Abstract Oxygen isotope data are reported for 27 igneous rocks of Mesozoic to Quaternary age from the Central Andes. 26–29°S. The plutonic rocks, and most of the volcanics, have δ 18 O values between 6.2 and 8.3‰. The whole-rock δ 18 O values show a weak correlation with initial 87 Sr/ 86 Sr data. This O-Sr array differs from documented trends for calc-alkaline plutonic suites from California, Scotland and northern Italy, but overlaps with data for volcanic and plutonic rocks from Ecuador, northern Chile and southern Peru. The oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks (e.g., rocks that experienced 18 O enrichment during surficial weathering). The available O, Sr and Pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust. From the Late Mesozoic on, the 87 Sr/ 86 Sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of Late Precambrian to early Mesozoic age.

Apatite Fission Track Thermochronology of Paleozoic Sandstones and the Hill-Intrusion, Northern Linksrheinisches Schiefergebirge, Germany

A fission track investigation was carried out on apatite from Paleozoic sandstones along a NW-SE transect and the Hill-intrusion in the northern Linksrheinisches Schiefergebirge. The study included age determinations, confined track length and chemical analysis as well as computer modeling of the thermal history. Apatite fission track ages vary between 130±11 Ma and 239±13 Ma. Mean confined track lengths range from 12.35±0.36 tm to 13.20±0.73 µm. Fluorapatite grains with no chlorine content are the main components. Single grain ages of the Paleozoic sandstones and the tonalite decrease with increasing fluorine and decreasing OH and chlorine content. Single grain ages from wallrock sandstones of the galena-sphalerite-calcite-quartz veins show no relation with the chemical composition. Computer modeling of the thermal history using an inverse model allowed to distinguish an Upper Paleozoic to Lower Mesozoic component (up to 220 Ma), a Mesozoic component, and a post-Campanian component. An average cooling rate below 1.5°C/Ma was estimated for the Upper Paleozoic thermal history. The subsidence of the Graben of Malmedy led to the accumulation of 3,000m of sediments during Permian and Lower Triassic. Apatite grains of the Devonian sandstones of the Venn-Weser-Inde nappe were completely annealed after deposition and reached the PAZ at 260 Ma. With a normal geothermal gradient this temperature would imply 3,000m to 4,000m of now eroded deformed Devonian and Carboniferous units.

Sulphur in Pennsylvania coals of Atlantic Canada: geologic and isotopic evidence for a bedrock evaporite source

Abstract Higher sulphur contents in coals are commonly explained by the proximity of the original peat to marine waters during deposition, as the sulphate ions in seawater provide an abundant source of sulphur. However, Pennsylvanian coals of the Maritimes Basin, which commonly contain 5–8% sulphur, are associated with a predominantly freshwater biota and alluvial sedimentary features, and evidence for marine beds is scare. Several workers have suggested that the sulphur was derived from dissolution of thick sulphate-evaporites in the underlying Mississippian Windsor Group, rather than from Pennsylvanian seawater. We set out to test this hypothesis using a biogeochemical model derived from studies of sulphur isotopes in modern peat swamps. During the bacterial decomposition of plant material, the H 2 S generated from groundwater sulphate is fixed as metal sulphides which are enriched in the lighter 32 S isotope by 15% or more. Sulphides formed chemically during burial will be much less fractionated. Isotope ratios of sulphur-bearing products can thus be used to trace the original source of the sulphur by comparison with a source of known isotopic composition. Eight evaporite samples from the upper Windsor Group in the Sydney Basin have a mean δ 34 S value of 15%. Sixteen pyrite samples from five coal seams in the overlying Morien Group range from 14.8% to −5% δ 34 s. The pyrite values follow the pattern predicted for derivation from Windsor sulphates. Disseminated pyrite generally contains more highly fractionated sulphur than nodular pyrite, suggesting that much of the former was generated by bacterial action whereas the latter was produced during later burial. Isotopic ratios for soluble sulphate in the coal correspond closely to those for the associated pyrite. Because Mississippian and Pennsylvanian seawaters showed a similar range of δ 34 S values, the test does not preclude a Pennsylvanian marine source of sulphate. Both geological and isotopic data, however, are consistent with a Windsor evaporite source.

Hydrocarbon involvement in the genesis of ore deposits; an example in Cretaceous stratabound (manto-type) copper deposits of central Chile

The analysis of the empirical association of organic carbon in its various forms (e.g., oil, gas, coal) with a variety of ore-deposit types can be a rewarding exercise in the formulation of genetic models and exploration. In many ore provinces, organic matter, residing in its source rocks, migrating in the form of hydrocarbons, or in transient, permanent, leaking, or degrading reservoirs, is likely to have interacted with ground waters, with hydrothermal solutions of diverse origin, or with magmas. After a brief review of these associations, we describe the ubiquitous occurrence of remnant petroleum in two representative “manto-type” stratabound copper deposits in Chile and suggest its possible genetic role and exploration significance. Overmature petroleum (solid bitumen, pyrobitumen) is found in association with copper ores in the southern orebodies of the El Soldado copper deposit in central Chile. Textures indicate that petroleum occupied the host-rock porosity before copper mineralization. Isotopic d...

Age of mineralization by basinal fluids at the El Soldado manto-type copper deposit, Chile: 40Ar/39Ar geochronology of K-feldspar

Abstract Manto-type copper deposits of the southern Central Andes are strata-bound, epigenetic mineral concentrations hosted by bimodal volcanic and volcaniclastic rock suites. They are economically important, but since they are not obviously related to intrusions, their genesis has been controversial, to some extent, because it has proven difficult to date the ore-forming event and distinguish it from the age of low-grade regional metamorphism. El Soldado, the largest manto-type copper deposit in central Chile, is hosted by marine volcanic and subvolcanic intrusive rocks of the Lower Cretaceous extensional arc. To discriminate between regional metamorphism and mineralization, we dated 11 samples of K-feldspar by the stepwise-heating 40 Ar/ 39 Ar method. Hydrothermally precipitated K-feldspar associated with copper sulfides precipitated at temperatures of >300 °C and range in age from 100.5±1.5 to 106.1±1.1 Ma (mean 103±1.6 Ma; n =8). Syn-mineralization K-feldspar gave an age of 102.5±2.0, which is interpreted to be the age of mineralization. K-feldspar unrelated to ore yields older ages between 109.4±1.1 and 112±2 Ma (mean=110.3±1.4 Ma; n =3) is variably reset by the younger ca. 103 Ma mineralization episode. The above ages are interpreted to represent the age of alkali redistribution and crystallization due to burial metamorphism of the host sequence. Fission track dating of apatite in the host rocks yields an age of ca. 90 Ma, indicating fast cooling of the system to less than ca. 100 °C after mineralization. Time–temperature modeling of fission track length data are compatible with initial fast exhumation, followed by slow exhumation and cooling to surface temperatures. High levels of atmospheric Ar were detected in all K-feldspar samples and are tentatively attributed to connate meteoric fluids incorporated into the basinal metamorphic mineralizing fluids.