Porphyry Copper Ore Deposits Research Articles

Identification of processes in Cu-ore heap leaching using Cu isotopes and leachate chemistry at Tschudi mine, northern Namibia

Copper isotopic fractionation (in δ65Cu) and leachate characterization were studied in the context of heap leaching at the Tschudi copper mine in northern Namibia. The leached solution is of Mg-SO4 type with high Al and Fe concentrations. The source of Mg and Al in the leachate can be from the alteration of micas such as Mg-bearing muscovite confirmed by X-ray diffraction and scanning electron microscopy; however, the source of Mg cannot be determined with certainty. The principal secondary minerals identified in the leached ore are gypsum and jarosite. The value of pH in leachate is ∼1.21 and the concentration of dissolved Cu, occurring mostly as CuSO40 and Cu2+, is about 2 g/L. In comparison with unleached ore (avg. δ65Cu −1.47 ‰), leached ore exhibits lighter isotopic values (avg. δ65Cu −6.01 ‰) with apparent isotopic fractionation Δ65Culeached ore-unleached ore of about −4.54 ‰. In contrast, there is isotopic enrichment of leachate in heavier 65Cu isotope (leachate δ65Cu 0.34 ‰) with apparent isotopic fractionation Δ65Culeachate-unleached ore value of +1.81 ‰. These results are in good agreement with Cu isotopic fractionation and depletion in heavier 65Cu isotope reported for dissolution experiments in laboratory and groundwater linked to the porphyry copper ore deposits around the world. The leaching of heaps can be considered an analogy of upper part of gossans, but here the supergene enrichment zone is missing due to extremely low pH and oxidizing conditions.

PETROCHEMICAL AND STRUCTURAL CHARACTERISTICS OF PORPHYRY COPPER MINERALIZATION IN THE ASTANEH ORE DEPOSIT, MIDDLE PART OF THE URUMIEH-DOKHTAR MAGMATIC ARC (IRAN)

Within the Urumieh-Dokhtar Magmatic Arc in the central part of Iran, the formation of which is associated with the Neotethys closure, there are many porphyry copper deposits and ore occurrences. One of them is the Astaneh porphyry copper ore deposit, located in the central part of the Saveh-Ardestan ore region southeast of Ardestan city. The purpose of this study is to investigate the petrochemical characteristics of rocks and to determine the relationship between the distribution of porphyry copper mineralization and tectonic position of faults within the study area. To achieve the goal, there were used the structural and geological data obtained in the fieldwork, as well as the results of mineralogical and geochemical analyses. The obtained results show that rocks of different composition of the Astaneh ore deposit (andesite, andesite-basalt, basalt, trachybasalt) were formed in the suprasubduction zone, and probably in the environment prior to the collision of the of continental plates. Paragenetic relationships and mineralogical analysis show that the evolution of mineralization of the Astaneh ore deposit can be divided into three stages: pre-ore, hypogene and supergene mineralization. Geochemical research based on the study of the content of the major chemical elements in the rocks of the region shows that igneous rocks belong to calc-alkaline basalts and geodynamically can be attributed to the products of magmatism of the ensial island arc. The results concluded that the main stages of the formation of a porphyry copper ore deposit in the study area attain maximum spatio-temporal similarity with the tectonomagmatic phases of the development of the Neotethys Ocean. In addition, the Southern Ardestan fault, running through the study area and intersecting the basement structures, forms wide permeable zones favorable for the formation of porphyry copper deposits therein.

Hypogene enrichment in Miduk porphyry copper ore deposit, Iran

This study was planned with the aim of identifying the nature and circumstances of the high-graded central core and increasing trend of copper content through depth of 1000 m in Miduk PCD. Mechanisms of high-grading, refer to hypogene enrichment (HE), in PCDs poorly understood. Two main hypotheses for hypogene enrichment formation assumed addition of extra copper to the system, alternatively hypogene leaching and enrichment. In order to obtain alteration-mineralization-geochemical pattern both horizontally and vertically, all macroscopic data extracted from relogging of 6800 m’ drill core along an east–west profile, compiled with microscopic observations from studying of 550 thin-polished sections and copper grades of 3400 samples analyzed by XRF and ICP-OES. Our findings proved hypogene enrichment events at deposit. HE evidences in macroscopic and microscopic scales identified almost as various replacement textures between Fe-Cu sulphides and also vein-reopening by later Cu-mineralization and new generation of disseminated or vein type mineralization. In addition, appearance of dark halo, as consuming intermediate chalcocite phase, around pyrite and chalcopyrite which gradually evolves as bornite, also extruding extra iron as fibrous hematite at the outer edge of bornite product replaced chalcopyrite, partially replacement of bornite and chalcopyrite to hypogene chalcocite and covellite-digenite in deep potassic are other HE evidences in the case study. Here, we draw on microscopic observations and SEM-BSE-EDS results, secondary hypogene genesis for some of bornite and chalcopyrite as a hypogene enrichment evidence. Observations from relogging show that potassic alteration has a relatively good preservation in the center of the deposit from depth to surface, but affected by intense overprinting of subsequent alterations towards margins. Evident function of ore-leaching at margins, also elevated copper grades in central parts of the deposit strongly suggest leaching-fixation mechanism. Where buffer potential of the rock is preserved copper fixation and where it totally eliminated almost complete leaching of copper happened. Consequently, we introduce leaching-fixation as index processes in hypogene enrichment at the case study. We suggest that identifying the nature of hypogene enrichment processes and its characterizations not only improve understanding about PCD’s hydrothermal evolution, but also achieve exploration indicators, furthermore, industrial benefits in the production line.

Analysis of the Geomechanical Conditions at the Kalmakir Porphyry Copper Ore Deposit (Uzbekistan)

The parameters of the open-pit mining of the Almalyk deposit of porphyry copper ores of the Kalmakir open-cast are analyzed. Three-dimensional model of the quarry is constructed by means of modern geoinformation modeling method. Basic factors affecting the deformation processes of the rock mass are identified. The zones of potential deformations of the quarry-sides with the depth increase are determined, recommendations for effective deformation processes monitoring to provide safe mining operation process are developed.

Contrasting hydrological processes of meteoric water incursion during magmatic–hydrothermal ore deposition: An oxygen isotope study by ion microprobe

Meteoric water convection has long been recognized as an efficient means to cool magmatic intrusions in the Earth's upper crust. This interplay between magmatic and hydrothermal activity thus exerts a primary control on the structure and evolution of volcanic, geothermal and ore-forming systems. Incursion of meteoric water into magmatic–hydrothermal systems has been linked to tin ore deposition in granitic plutons. In contrast, evidence from porphyry copper ore deposits suggests that crystallizing subvolcanic magma bodies are only affected by meteoric water incursion in peripheral zones and during late post-ore stages. We apply high-resolution secondary ion mass spectrometry (SIMS) to analyze oxygen isotope ratios of individual growth zones in vein quartz crystals, imaged by cathodo-luminescence microscopy (SEM-CL). Existing microthermometric information from fluid inclusions enables calculation of the oxygen isotope composition of the fluid from which the quartz precipitated, constraining the relative timing of meteoric water input into these two different settings. Our results confirm that incursion of meteoric water directly contributes to cooling of shallow granitic plutons and plays a key role in concurrent tin mineralization. By contrast, data from two porphyry copper deposits suggest that downward circulating meteoric water is counteracted by up-flowing hot magmatic fluids. Our data show that porphyry copper ore deposition occurs close to a magmatic–meteoric water interface, rather than in a purely magmatic fluid plume, confirming recent hydrological modeling. On a larger scale, the expulsion of magmatic fluids against the meteoric water interface can shield plutons from rapid convective cooling, which may aid the build-up of large magma chambers required for porphyry copper ore formation.

Controls on porphyry Cu mineralization around Hanza Mountain, south-east of Iran: An analysis of structural evolution from remote sensing, geophysical, geochemical and geological data

Hanza Mountain in Urmia–Dokhtar Magmatic Arc, southeast of Iran, consists of monocline of Eocene volcanic rocks into which the Oligocene granitoid rocks have been intruded. This area has excellent potential for economic porphyry copper deposits with Bondar Hanza, Daralu, and Sarmesk deposits among them. Hanza Mountain is located between NW–SE horsetail thrust faults derived from the Gowk and Sabzevaran strike-slip faults. The analysis of the kinematics of these strike-slip faults shows that they were not the cause of the formation of the pull-apart basin; thus they have not directly played any effective role in localizing the final emplacement of porphyries responsible for the formation of these copper deposits, but the Cu mineralization occurred mainly within a set of normal and thrust faults in the region. The alteration types and faults in Bondar Hanza were distinguished using detailed local geology, including distribution of known mineralization, supported by remote sensing (ASTER), airborne geophysics, and topography; the relationship between mineralization and faults was examined using Rose diagrams and Fry Analysis. This investigation of Bondar Hanza deposit has revealed that the trend of faults and dykes, as well as the distribution of copper analyses within drill cores, is aligned with the main trend of mineralization. The NW–SE trending faults in the Urmia–Dokhtar Magmatic Arc are effective in localizing the emplacement of porphyry copper ore deposits and those that trend between N125°–N145° are key to further exploration.

Geochronology and Geochemistry of the Tinggong Porphyry Copper Ore Deposit, Tibet

AbstractWe have determined the ages of the ore‐bearing Tinggong porphyries and the Eocene granites using the LA‐ICPMS zircon U‐Pb method. Zircons from one adamellite porphyry and two diorite porphyries yield ages of 15.54±0.28 Ma, 15.02±0.25 Ma and 14.74±0.22 Ma, respectively. The ages of two granites are 50.48±0.71 Ma and 50.16±0.48 Ma. Light Rare Earth Elements (LREE) are enriched in the ore‐bearing adamellite porphyries, which are high‐K calc‐alkaline and metaluminous, while Heavy Rare Earth Elements (HREE) and Y are strongly depleted, indicating an adakitic affinity. The Large Ion Lithophile Elements (LILE) of the adamellite porphyries are highly enriched, whereas some High Field Strength Elements (HFSE) are depleted. The diorite porphyry in this study is chemically similar to the adamellite porphyries, except that the Mg# of the diorite porphyry is a little higher, demonstrating more mantle contamination. Four samples from different rocks are selected for in situ zircon Hf isotopic analyses. The samples show positive εHf(t) values and young Hf model ages, indicating their derivation from juvenile crust However, the adamellite porphyry and diorite porphyry formed in the Miocene exhibit more heterogeneous Hf isotopic ratios, with lower εHf (t) values than the granites formed in the Eocene, suggesting the involvement of old Indian continent crust in their petrogenesis. The geochronology and geochemistry of the adamellite porphyries and the diorite porphyries indicate that they formed from the same source region in a post‐collisional environment, but contaminated by crust and mantle materials in different ratios. The metallic minerals formed mainly during the older adamellite porphyry stage, but they were recycled and reactivated by the diorite porphyry intrusion.

Multiple-point geostatistical simulation of dykes: application at Sungun porphyry copper system, Iran

Variogram-based methods are not capable of capturing high (>2) order statistics since the variogram measures the relationship between two points at a time only. Multiple-point geostatistics (MPS) has brought new insights into many geological modeling problems. The application of MPS methods has been well documented in realizing complex geological patterns. These methods have often been used in reservoir characterization since their advent in recent decades. The frequent non-linear behaviors of geologic continuity are not limited to reservoirs, but mineral deposits bear complicated formations in many cases. Relying on the power of MPS methods and considering the complexity of geological scenarios in mineral deposits, we have applied MPS in the modeling of mineral deposits. A training image (TI) is produced using geological data from upper horizons of a porphyry copper ore deposit which have been mined out during the previous mining operations. In this study, the SNESIM algorithm has been used. A number of realizations are produced using this multiple-point geostatistical method. Extensive validation steps are performed considering the TI as the reference model. These validations first show that the TI is representative for the domain under study and also illustrates some degrees of similarity between the TI and the realizations. Despite simplifications made to the problem, the application of MPS in mineral deposit modeling still faces many challenges.

Porphyry copper deposits: strike–slip faulting and throttling cupolas

Porphyry copper deposits sometimes form during the solidification of stocks of relatively oxidized magma of intermediate composition. Most workers have assumed ore-forming systems have special chemical attributes, but none has been found that is useful to guide exploration efforts. Stocks can form where strike–slip movements generate pull-apart pathways into which intrusions can rise from batholithic magma chambers. Upwelling of buoyant, bubble-bearing magma along the sides of a stock brings magmatic fluid to shallow depths where large bubbles can separate and pool under the cupola separating solidified igneous rock from mobile magma. Where rapid seismogenic movement on the bounding strike–slip fault ruptures the solidified, but hot and ductile carapace, downward propagating extension fractures can drain an accumulation of magmatic fluid. Decompression and cooling of fluid that jets upward into extension fractures causes mineral precipitation. Where strike–slip movements cause pull-aparts to dilate with sufficient recurrences – from decades to perhaps a century or so, throttling of the fluid accumulation acts as a safety valve that prevents explosive detonation of the system. Concurrently, the upward infiltration of magmatic fluid from the cupola is strongly focused into the pull-apart and generates the characteristic concentric alteration zones that guide exploration drilling. We conclude that porphyry copper ore deposits form where strike–slip movements are concurrent with the early stages of deep-seated bubbling (≳6 km) along the walls of a rapidly cooling stock of magma. Supergiant deposits form where the bubbling front extends into the top of a parent batholith.

Geological Characteristics and Ore‐forming Time of the Dexing Porphyry Copper Ore Mine in Jiangxi Province

Abstract:The Dexing porphyry copper ore mine is located in the Qin‐Hang metallogenic belt between the Yangtze block and the Cathaysia block. It is a giant porphyry copper mine in China, including 3 ore districts: Tongchang, Fujiawu and Zhushahong. Our analyses of Re in molybdenite indicate that the ore‐forming material of the copper ore deposits in Dexing should be mainly mantle‐derived. Our study fills in a gap in the study of formation time of the Dexing copper mine, and further proves that the copper ore deposits in the three ore districts should be formed simultaneously, about 170 Ma, belonging to the early Yanshan period, and that the formation time of the copper ore deposits should be consistent with the formation time of granodiorite porphyry in which the copper ore deposits are hosted. Promising areas for seeking porphyry copper ore deposits is predicated to be the west or southwest of Dexing.

Reconstructing physico-chemical parameters of hydrothermal mineralization of copper at the Malanjkhand deposit, India, from mineral chemistry of biotite, chlorite and epidote

Mineral chemistry of biotite, chlorite and epidote associated with the granitoid ore body of the Malanjkhand copper-molybdenum deposit have been studied to constrain temperature, oxidation and sulfidation states as well as fugacity ratios of HF, HCl and H2O of the hydrothermal mineralizing fluid. Calculated mineralizing temperatures from biotite, chlorite and epidote are mutually consistent and agree well with earlier estimates derived from fluid inclusion studies. Major element chemistry of biotite indicates low oxidation state (FMQ-NNO) and re-equilibrated nature (∼300°C). However, exchange of Cl-F-OH between biotite and the fluid continued to lower temperatures. Subtle differences between the Malanjkhand deposit and known porphyry copper deposits have been evidenced by the fugacity ratios of halogens and water. Chlorite registers equilibrium with fluid down to 200°C and, together with biotite composition, indicates that copper was transported as chloride complex in the fluid. Sulfide deposition occurred during a prompt rise in fugacity of H2S at 300°C, possibly resulting from a coupled oxidation of Fe2+ and reduction of SO42-. Interaction of ore fluid with epidote in the wall-rock favored the deposition of chalcopyrite, enhanced the Ca2+ activity in the fluid and inhibited a fall in pH and a subsequent acid alteration of K-feldspar in the wall-rock. These physicochemical characteristics of the mineralized fluid suggest that the Malanjkhand deposit represents an ancient granitoid-associated geothermal system rather than a classic porphyry copper ore deposit.

Re–Os isotope systematics: Sources and age of epithermal and porphyry copper ore deposits

Re–Os isotope systematics: Sources and age of epithermal and porphyry copper ore deposits

Gold in porphyry copper deposits: its abundance and fate

Porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits. Whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust. Cu/Au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5,000,000 with a median near 40,000, which separates gold-rich and gold-poor deposits. Gold is found in porphyry copper deposits in solid solution in Cu–Fe and Cu sulfides and as small grains of native gold, usually along boundaries of bornite. SIMS (ion probe) analyses of ore minerals from the gold-rich Batu Hijau, Kingking and Skouries porphyry copper deposits show that bornite contains about 1 ppm Au, whereas chalcopyrite contains about an order of magnitude less. Chalcocite and covellite contain 10–20 ppm Au, but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits. The amount of gold presently in solid solution in Cu–Fe sulfides is not adequate to account for all the gold in porphyry copper deposits, and the remainder is present as micron-scale grains of native gold. Experiments in the Cu–Fe–S–Au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600–700°C, and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200–300°C. In contrast, Cu/Au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary (potassic) ore and alteration are thought to form. This indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in Cu–Fe sulfides when the deposit forms at high temperature, and that gold is not commonly added later from other sources, although it can be redistributed during cooling or later events. The experimental data also suggest that high-temperature (600–700°C) vapors can extract considerably more gold from porphyry copper systems than can low-temperature (300°C) alteration. Comparison to Cu/Au ratios of volcanic emissions suggests further that high-temperature processes remove copper (relative to gold) from porphyry copper systems, whereas low-temperature processes remove gold preferentially and that this can account for deposits with extremely low and high Cu/Au ratios, respectively. Deposits with Cu/Au ratios between about 20,000 and 100,000, however, probably reflect different degrees of removal of gold or copper by immiscible sulfides.

Using mineralogy to optimize gold recovery by flotation

Gold is an important by-product of many porphyry copper ore deposits. Precise mineralogical characterization of the unfloated gold in what is typically very low grade (i.e., <0.1 g/t Au) flotation tails provides a clear picture of the carriers and causes for gold losses in these large tonnage operations, thereby identifying the means to reduce such losses. Analyzing quantitative gold deportments in flotation tails enables the carriers of unfloated gold to be ranked, the most appropriate carrier identified, and test work designed to address specific causes of gold losses. Typically, 10–20% of the gold in the tails can be recovered without additional grinding or by regrinding a rougher or cleaner scavenger concentrate.

Illumination of vein quartz textures in a porphyry copper ore deposit using scanned cathodoluminescence: Grasberg Igneous Complex, Irian Jaya, Indonesia

Vein quartz from the Grasberg Igneous Complex in Irian Jaya, Indonesia, displays a wide range of features when observed using scanned cathodoluminescence imaging on the electron microscope. Examination of seventeen samples revealed concentric growth zoning, quartz-filled microfractures, dark luminescence near sulfides, changes in crystal size and orientation from vein edges to vein centers, turbid growth zoning, and the truncation of concentric growth zoning. Individual features observed along two large photomosaic traverses from vein margins to vein centers coupled with trace element analyses from the electron microprobe allow determination of a detailed history of fracture infilling. The two traverses indicate a simple, two-stage history of crystal growth into an open fracture followed by mechanical closing of the fracture with resulting microfractures. Electron microprobe trace-element analyses show that the Fe content of quartz reaches values of up to 4900 ppm in dark-luminescent regions near sulfide crystals and decreases smoothly to ~200 ppm along traverses away from the sulfides. The solution to the diffusion equation coupled with existing experimental data about Fe diffusion in SiO2 indicate the duration of the diffusion was perhaps on the order of thousands of years.

Bubbling Magma Chambers, Cupolas, and Porphyry Copper Deposits

Porphyry copper deposits are the major source of copper and significant sources of molybdenum, gold, and other metals. They are associated with the near-surface intrusion of small stocks of intermediate composition. They can form when H2O-unsaturated magma is emplaced into wall rock that is cool enough that steep lateral thermal gradients create a narrow solidification front. At depths less than ∼4 km, cooling and crystallization cause fluid saturation to occur within sidewall magma that is mobile because it contains less than ∼25% suspended crystals. After a sufficient volume of bubbles forms, mobile sidewall magma buoyantly rises instead of sinking. The bubbles expand as they decompress, and at depths of ∼2 km they become large enough to rise on their own. separate from the upwelled magma, and charge the cupola at the top of the stock with magmatic fluid. The partially degassed magma sinks into the interior of the stock. Upwelling of saturated sidewall magma entrains deeper-seated, nearly saturated magma, which decompresses and saturates as it rises. As the system cools, the depth of H2O saturation and sidewall upwelling increases. Bubbles of copper-rich fluid are generated where the saturation front extends to depths of ∼6 km or more. Overall, the system is cooling, but the upward advection of heat maintains the cupola region at roughly constant position for the life of convective upwelling along the sidewalls. Porphyry copper ore deposits can form where draining of the fluid pocket beneath a cupola is steady and a large volume of magma is cycled through the system. Magma in the stock that escapes to intrude commonly has a porphyritic texture because crystal growth is enhanced, and nucleation is suppressed when the magma is H2O saturated. Porphyry copper deposits of common size can form during the solidification of large stocks. Super-giant porphyry copper deposits can form where the saturation front propagates from a stock into an underlying batholithic chamber with a magma volume on the order of 1000 km3 and a top at depths of 10 to 15 km.

Orogens and slabs vs. their direction of subduction

Subduction zones appear primarily controlled by the polarity of their direction, i.e., W-directed or E- to NNE-directed, probably due to the westward drift of the lithosphere relative to the asthenosphere. The decollement planes behave differently in the two end-members. In the W-directed subduction zone, the decollement of the plate to the east is warped and subducted, whereas in the E- to NNE-directed, it is ramping upward at the surface. There are W-directed subduction zones that work also in absence of active convergence like the Carpathians or the Apennines. W-directed subduction zones have shorter life (30–40 Ma) than E- or NE-directed subduction zones (even longer than 100 Ma). The different decollements in the two end-members of subduction should control different PTt paths and, therefore, generate variable metamorphic assemblages in the associated accretionary wedges and orogens. These asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast `eastward' migrating structural wave along W-directed subduction zones, whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones. The magmatic pair calc-alkaline and alkaline–tholeiitic volcanic products of the island arc and the back-arc basin characterise the W-directed subduction zones. Magmatic rocks associated with E- or NE-directed subduction zones have higher abundances of incompatible elements, and mainly consist of calc-alkaline–shoshonitic suites, with large volumes of batholithic intrusions and porphyry copper ore deposits. The subduction zones surrounding the Adriatic plate in the central Mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab. The western margin of the Adriatic plate contemporaneously overridden and underthrust Europe toward the `west' to generate, respectively, the Alps and the Apennines, while the eastern margin subducted under the Dinarides–Hellenides. These belts confirm the characters of the end-members of subduction zones as a function of their geographic polarity similarly to the Pacific subduction zones.

The Precordilleran fault system of Chuquicamata, Northern Chile: evidence for reversals along arc-parallel strike-slip faults

The Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of Northern Chile, was the site of the Andean magmatic arc from the late Cretaceous to the Eocene-Oligocene boundary. Magmatism came to an end during the Incaic tectonic phase (38 Ma), which caused arc-normal shortening and the development of longitudinal dextral strike-slip faults (Precordilleran Fault System). This magmatic arc tectonism is also related to the formation of the Chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the Precordillera. Structural investigations in and around the Chuqicamata open-pit mine have shown that wrench tectonics determined the kinematics of the area. The NS-striking West Fissure, which separates a 35-Ma-old non-mineralised pluton to the west from a central late Paleozoic basement ridge containing the mineralization, became a sinistral fault along with other parallel faults in the area. The central part is separated from similar Paleozoic rocks to the east by the Messabi-Este fault and a narrow faulted and sheared syncline of Mesozoic-Cenozoic sediments. This fault bears structures indicating dextral movements, which probably are of an age that is similar to the mylonites (34.8 Ma) in the western pluton. The dextral movements preceded the sinistral shear. Thus, the fault system of Chuqicamata displays a reversal of arc-parallel shear movements. According to the orientation of quartz veins in the mineralized body, it is presumed that the sense of displacement of these strike-slip motions reversed, when mineralization started at about 32 Ma. During this time the stress field must have changed fundamentally. The Incaic Phase dextral transpression is supposed to have been induced by the oblique vector of plate motion. The following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling. As, however, the vector of plate motion remained unchanged during that time, oblique subduction cannot be used as an argument for arc-parallel sinistral shear movements.

Structural evidence of orogen-parallel strike slip displacements in the Precordillera of northern Chile

The Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of northern Chile, is made up of several elongate basement ridges following the trend of the Andes. These ridges, which morphologically rise above the Mesozoic-Tertiary cover rocks, are developed as anticlinal cores or as pressure ridges bounded by reverse faults and, thus, show considerable orogennormal shortening. Several nearly vertical faults cut through the Precordillera, parallel or at a very low angle to the mountain ranges. From the following structures it can be inferred that orogen-parallel transcurrent movements took place along the faults: (a) asymmetric en echelon fault arrays, (b) stratigraphic and structural discontinuities across major faults, (c) fabrics in fault rocks, and (d) vertical folds. A dextral displacement of the order of tens of km is probable. The orogen-parallel strikeslip movements as well as the orogen-normal shortening are considered as phenomena of magmatic arc tectonics due to the focussing of central Andean igneous activity on the Precordillera from the Late Cretaceous until the paroxysm of deformation (45-30 Ma). Deformation along the Precordilleran Fault System is related with the development of the large porphyry copper ore deposits of that area. The structural evolution of the Precordillera can be explained by oblique subduction resulting in dextral transpression.

Nd and Sr isotope study of hydrothermally altered granite at San Manuel, Arizona; implications for element migration paths during the formation of porphyry copper ore deposits

A study of the Nd and Sr isotope composition of the Late Cretaceous monzogranite stock at San Manuel, Arizona, and its mineralized alteration halo was undertaken in order to assess the transport of Nd and Sr that accompanied the hydrothermal alteration. Sampling was along a 1-km transect perpendicular to the alteration zones extending from the potassically altered core to the propylitically altered fringe of the system. Large isotopic differences between the stock and the enclosing 1.4-b.y.-old granitic wall rock, coupled with the original homogeneity of the latter, provide optimal conditions for detecting evidence of chemical transport. The mineralizing porphyry stock has a lower 87 Sr/ 86 Sr ratios (0.708) and a higher epsilon Nd (-6) than the Precambrian granite wall rock (0.782, -11). The 87 Sr/ 86 Sr ratios of the altered wall rock decrease regularly from 0.782 to 0.715 from the propylitic alteration zone to the stock contact, indicating that material flow was primarily from the stock into the wall rock, consistent with an orthomagmatic model for porphyry copper ore genesis. The Sr in the ore zone is 75 percent stock derived, and evidence of stock-derived Sr is found up to 700 m into the wall rock. The Sr and Rb concentration variations are mainly controlled by the alteration mineralogy and do not correlate with the isotopic variations. The epsilon Nd values and the Sm and Nd concentrations of the altered wall rock show subtle changes that are mostly confined to the potassically altered zone. Only one wall-rock sample, located 0.5 m from the stock contact, is strongly disturbed. The results suggest that Sr isotopes may provide a useful tool for studying or locating fossil hydrothermal systems, whereas the relatively immobile Nd isotopes are reliable indicators of the magmatic provenance of altered intrusions.