Porphyry Copper Systems Research Articles

3DU–TA: A new multifractal model involving surface topography for separation of geochemical anomalies

A novel anomaly recognition technique was propounded to help reconcile mineralization-related geochemical patterns with topographic non-stationaries within the study area. The topography variations were characterized by elevation and slope factors of grid cells in a digital elevation model (DEM). In the first step of the technique architecture, the spatial U-statistics formulation was expanded for a 3D framework to integrate elevation-related information into geochemical measurements. Slope angles were then modeled across the cell-based geochemical map using DEM information to modulate planimetric areas into topographic attributes. Three-dimensional spatial U-values and topographic attributes were integrated in a multifractal model called 3DU–TA, providing power-law relations that aid in decomposition of geochemical patterns. The mathematical formulation established for the 3DU–TA model was implemented by programming in MATLAB platform and it was applied to geochemical data pertaining to a deposit-scale porphyry copper system. Core drilling data were used as benchmark to appraise the performance of the 3DU–TA model in the recognition and separation of metal enrichment levels from background and it was compared with the well-established concentration–area (C–A) fractal model. We defined mineralization rate Mr and normalized productivity Np as criteria for judging the integrity of anomalous geochemical patterns derived by the 3DU–TA model in reflecting subsurface mineralization. We also adapted a success-rate plot for deposit-scale experiments and the relevant areas under the curve (AUC) as gauges for quantitative evaluation of model relevance in reflecting subsurface metal enrichment. The derived values of Mr and Np imply that the geochemical populations decomposed with the 3DU–TA model, compared with those decomposed with the C–A model, yielded anomaly patterns that were more consistent with subsurface metallogenic realities. The success-rate curve for topography-enhanced results had an AUC = 0.940, which was higher than that for the C–A model (AUC = 0.867), indicating that the former revealed anomalies with substantially stronger spatial correlations with subsurface metal contents. Intuitive geochemical and quantitative metallogenic assessments suggest that the proposed multifractal modeling technique is capable of providing more practical insights into mineralization realities in geosystems with complex topographic variations.

Numerical Modeling of Structurally Controlled Ore Formation in Magmatic‐Hydrothermal Systems

AbstractFaults and fractures can be permeable pathways for focused fluid flow in structurally controlled ore‐forming hydrothermal systems. However, quantifying their role in fluid flow on the scale of several kilometers with numerical models typically requires high‐resolution meshes. This study introduces a modified numerical representation of m‐scale fault zones using lower‐dimensional elements (here, one‐dimensional [1D] elements in a 2D domain) to resolve structurally controlled fluid flow with coarser mesh resolutions and apply the method to magmatic‐hydrothermal ore‐forming systems. We modeled horizontal and vertical structure‐controlled magmatic‐hydrothermal deposits to understand the role of permeability and structure connectivity on ore deposition. The simulation results of vertically extended porphyry copper systems show that ore deposition can occur along permeable vertical structures where ascending, overpressured magmatic fluids are cooled by downflowing ambient fluids. Structure permeability and fault location control the distribution of ore grades. In highly permeable structures, the mineralization can span up to 3 km vertically, resulting in heat‐pipe mechanisms that promote the ascent of a magmatic vapor phase to an overlying structurally controlled epithermal system. Simulations for the formation of subhorizontal vein‐type deposits suggest that the major control on fluid flow and metal deposition along horizontal structures is the absence of vertical structures above the injection location but their presence at greater distances. Using a dynamic permeability model mimicking crack‐seal mechanisms within the structures leads to a pulsating behavior of fracture‐controlled hydrothermal systems and prevents the inflow of ambient fluids under overpressured conditions.

Correction: Gardaneh Salavat porphyry copper system: Is it about metal endowment or fertility of the porphyritic intrusion?

Correction: Gardaneh Salavat porphyry copper system: Is it about metal endowment or fertility of the porphyritic intrusion?

Controls on the metallogenesis of the Lhasa–Mozugongka district, Gangdese Belt, Tibetan Plateau: Constraints on melt distribution and viscosity from the 3-D electrical structure of the lithosphere

Some of the largest and most significant Miocene porphyry copper systems in China are within the Gangdese metallogenic belt on the southern Tibetan Plateau. It has been recognized that the crustal architecture and rheology, derived from regional tectonic events, has direct implications for the evolution and transport of fluids and magmas, and thus for the metallogenesis and prospectivity. Using data from a magnetetolluric array, which intersects the Lhasa–Mozugongka district of the Gangdese metal belt, a 3-D electrical resistivity model was generated, with the goal of investigating the tectonic and rheological controls on the magmatic mineral system. The lithospheric temperature distribution was estimated by applying the Arrhenius equation to conductivity profiles generated from 1-D Monte-Carlo models of long-period magnetotelluric data. The conductivity of partial melts in the lower and middle crust (30–60 km depth) was estimated for local conditions by applying the experimentally-derived equation of X. Guo et al. (2018). Subsequently, we estimated the melt fraction required to explain the observed bulk resistivity in each part of the study area. Variations in the effective viscosity of the lower and middle crust were constrained by the electrical resistivity model by applying the empirical relation of Liu and Hasterock (2016). Beneath the Miocene Cu–Mo deposits in the Lhasa terrane, conductive features in the lower and middle crust are attributed to partial melt fractions of more than 5% and viscosity reductions of 1–2 orders of magnitude. These conductive features may represent the signatures of (ore-controlling) melt/fluid migration channels and deeper melt/fluid source regions in the form of extensive crustal reservoirs of partial melt. Based on the interpretations of the geophysical model, and other available geological and geochemical evidence, a model of the metallogenic dynamics of the Miocene porphyry Cu–Mo deposits is proposed. Overall, the study highlights the applicability of electromagnetic geophysical methods to reliably link resistivity structures to melt/fluid transport channels and sources within a mineral system and supports the hypothesis that crustal rheology exerts a major control on the distribution of ore deposits.

Formational types of porphyry copper deposits and their ore-magmatic systems

Two individual ore-formational types have been identified within the porphyry copper family: the gold-copper-porphyry and molybdenum-copper-porphyry ones. The fundamental difference between them lies in their geotectonic position that determines the origin and composition of fertile magmatic source, as well as other features the composition of ore-bearing porphyry intrusions, the mineralogical and geochemical features of ores, and the pattern of metasomatic and ore zonation. In this study, we investigated porphyry copper ore-magmatic systems with complex metallogeny, or complex porphyry copper systems (CPCS), where porphyry copper deposits constitute the core elements spatially associated with mineralization of other ore-formational types. Based on a set of conjugated and combined types of ore mineralization we established the differences between diorite-related gold-copper-porphyry CPCS, formed in basaltic volcano-plutonic belts, and granodiorite-monzonite-related molybdenum-copper-porphyry CPCS, formed in andesite belts. These differences confirm the validity of the distinction of the two ore-formational types of deposits of the porphyry-copper family.

Detection of alteration zones using the Dirichlet process Stick-Breaking model-based clustering algorithm to hyperion data: the case study of Kuh-Panj porphyry copper deposits, Southern Iran

Detection of hydrothermal alteration zones (HAZs) associated with porphyry copper systems using remote sensing imagery is a crucial stage for discovering high potential zone of ore mineralization. Statistical model-based clustering methods have great potential for automatic and accurate detection of hydrothermal alteration minerals using hyperspectral remote sensing imagery. In this research, the Dirichlet Process based on Stick-Breaking (DPSB) model-based clustering algorithm was implemented to hyperion remote sensing imagery to discriminate HAZs associated with the Kuh-Panj porphyry copper deposit, south, Iran. The DPSB clustering algorithm was implemented and subsequently compared with the k-means algorithm, CLARA clustering, hierarchical clustering, Gaussian finite mixture model (GFMM), Gaussian model for high-dimensional (GMHD) and spectral clustering as well as spectral angle mapping (SAM). Results derived from the DPSB model-based clustering algorithm show 88.6% accuracy in distinguishing propylitic, argillic, advanced argillic, propylitic-argillic and phyllic alteration zones. The DPSB algorithm can be broadly implemented to hyperspectral remote sensing imagery for detecting alteration zones associated with porphyry systems.

ГЕОХІМІЧНІ ОСОБЛИВОСТІ РУДНО-ПОРФІРОВОЇ МІНЕРАЛІЗАЦІЇ У МАГМАТИЧНІЙ СИСТЕМІ ГОШГАРЧАЙ (МУРОВДАРСЬКИЙ РАЙОН, АЗЕРБАЙДЖАНСЬКА ЧАСТИНА МАЛОГО КАВКАЗУ)

The geochemical features of intrusive rocks and the surrounding rocks associated with porphyry copper mineralization in the Goshgarchay magmatic system were studied. Major factors of concentration of copper and copper-porphyritic mineralization in rocks of gabbro-diorite-granodiorite formation has been revealed. It has been established that the structure of this ore region was formed as a result of successive alteration of some deformation stages accompanied by development of fractures in intrusive massive, by dike intrusions, locks displacements along faults and the fractures composed of various mineral associations. Main ore components were defined and relations hips among elements were considered. Geochemical associations allowing the localization of the development area of mineralization have been analyzed in various formation stages of porphyry copper system according to the results of cluster analysis. In this study, Cu, Mo, Pb, Zn, Ag, Ni, Co, Mn, Ti, Cr, V, and Sr both in the surrounding rocks and in ore-bearing intrusive rocks were considered as stable elements. This group of elements can be regarded as indicator elements in porphyry copper. The results of the claster analysis show that the elements which are less characteristical for hydrothermal solutions (Mn, Ti, Cr, V, Sr) keep their freedom in all wallrocks and the existence of their grouping is probably associated with their transportation process from silicate rocks with surrounding basic-average content. The elements such as Cu, Mo, Ag, Pb and Zn which form free group are connected with the generation from the magmatic source in the course of crystallization differentiation process and exposure of rocks to the hydrothermal activity which is considered the source of these elements.The identification of reasons of the association of V with Cu and Zn and Pb with Fe group elements in the intrusive facies rocks is characterized with its uncertainty. The analysis of the schedule made according to the results of Cu and basic ore components (Mo, Ag, As, Bi, Gd, Co, Cr, Se, Ge, Li, Nb and etc.) in the ores of Goshgarchay deposit show their dependence on one another equally. The amount of Cu in the ores have Ag, As, Bi, Cd, Ce, Ge, Sr, Mo, Sb, Th and Ti positive correlation relationship, but it has negative correlation relationship with Co, Cr, Cs, La, Li, Pb, Rb, Ni, Nb and Zn. Direct dependence is observed for ore mass between concentrations of Cu, Mo, Pb, Zn, Ag, Se, Te and other elements in the Goshgarchay porphyry copper deposit. Three various mineral associations differing from one another participate in stock work type copper-porphyry mineralization: 1) primary sulfide minerals; 2) oxidized ores; 3) primary sulfide minerals significantly enriched with products of hydrothermal ore carrying solutions.

Chemical and short‐wave infrared characteristics of white mica associated with the Gajah Tidur porphyry copper system at the deep Grasberg Cu—Au—(Mo) deposit, Indonesia

AbstractThe Grasberg Cu—Au—(Mo) deposit comprises the shallower Main Grasberg porphyry Cu—Au and the deeper Gajah Tidur (GT) porphyry Cu—Mo—(Au) systems. The GT porphyry preserves various types of white mica whose geochemical variations provide insights into the white mica‐dominated alteration of porphyry systems. The white mica assemblages within the GT porphyry system comprise: (1) muscovite‐anhydrite‐chlorite (MAC), (2) muscovite‐chlorite‐anhydrite (MCA), and (3) muscovite‐quartz ± pyrophyllite (MQP). These assemblages display zonation from central and deep parts of the system to its shallower and peripheral parts. The MAC alteration white micas are characterized by high Na, Fe, Ti, and V concentrations, and with short‐wave infrared Al—OH absorption wavelengths of 2203–2208 nm. The MCA white micas have higher Mg content than the other two GT white mica assemblages but similar Al—OH absorption wavelengths to the MAC white micas. The MQP alteration white micas have low Na, Fe, Mg, and Ti, but relatively high Si, Al, and F, and Al—OH absorption wavelengths are largely shorter than 2202 nm. We interpret that the high Fe and Ti content of the MAC white micas is due to inheritance of these elements from mafic minerals they replaced. The higher Fe content of these white micas explain their longer wavelength Al—OH absorption positions relative to the MQP white micas. In contrast, lower Fe content and shorter Al—OH wavelengths of the MQP white micas are caused by their higher Si and Al content, which reduces iron occupancy in the white mica crystal structure. White micas in this assemblage formed at lower temperature and probable lower pH condition that may have led to a replacement of Fe by Al. The short‐wave infrared Al—OH position of white mica together with the associated hydrothermal assemblage can be used as a proximitor for porphyry Cu hydrothermal centres. White mica associated with chlorite, anhydrite, and chalcopyrite, which commonly occur overprinting or adjacent to the potassic alteration center, are characterized by Al—OH absorption positions at 2200–2215 nm. By contrast, white mica associated with quartz‐pyrite are characterized by Al—OH wavelengths shorter than 2202 nm. In the distal part of porphyry Cu system, white micas may be associated with chlorite and have Al—OH absorption positions longer than 2204 nm.

Hybrid Fuzzy-Analytic Hierarchy Process (AHP) Model for Porphyry Copper Prospecting in Simorgh Area, Eastern Lut Block of Iran

The eastern Lut block of Iran has a high potential for porphyry copper mineralization due to the subduction tectonic regime. It is located in an inaccessible region and has harsh arid conditions for traditional mineral exploration campaigns. The objective of this study is to use Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) remote sensing data for porphyry copper exploration in Simorgh Area, eastern Lut block of Iran. Hydrothermal alteration zones such as argillic, phyllic and propylitic zones associated with porphyry copper systems in the study were identified using false color composition (FCC), band ratio (BR), principal component analysis (PCA) and minimal noise fraction (MNF). The thematic alteration layers extracted from FCC, BR, PCA and MNF were integrated using hybrid Fuzzy-AHP model to generate a porphyry copper potential map for the study area. Four high potential zones were identified in the central, western, eastern and northeastern of the study area. Fieldwork was used to validate the approach used in this study. This investigation exhibits that the use of hybrid Fuzzy-AHP model for the identification of hydrothermal alteration zones associated with porphyry copper systems that is typically applicable to ASTER data and can be used for porphyry copper potential mapping in many analogous metallogenic provinces.

Crustal structure control on porphyry copper systems in accretionary orogens: insights from Nd isotopic mapping in the Central Asian Orogenic Belt

Crustal structure control on porphyry copper systems in accretionary orogens: insights from Nd isotopic mapping in the Central Asian Orogenic Belt

Stream Sediment Datasets and Geophysical Anomalies: A Recipe for Porphyry Copper Systems Identification—The Eastern Papuan Peninsula Experience

Airborne magnetic and radiometric datasets have, over the past few years, become powerful tools in the identification of porphyry systems which may host economic porphyry copper–gold–molybdenum ore bodies. Magnetisation contrasts with the unaltered host rocks, coupled with the elevated radiometric signature, compared to the host rock, makes identification of large-scale porphyry copper systems possible. Integrating these two different datasets with stream sediment data and other geochemical exploration methods results in a higher degree of confidence. Stream sediment data were analysed to see the distribution of copper and gold elements throughout the study area, located within the Eastern Papuan Peninsula of Papua New Guinea. Airborne geophysics data over the same area were also processed for magnetic and radiometric responses. The processing of the magnetic data revealed several magnetic anomalies related to concealed intrusive rock units, with associated radiometric signatures. The distribution of gold and copper anomalism was correlated with the geology and geophysical signatures. Results indicate varying degrees of correlation, with some areas showing a strong correlation between gold/copper occurrence and geophysical signatures, compared to other areas. Some factors that we believe impact the level of correlation may include tectonic history, volcanic cover, and weathering patterns. We recommend caution when applying multi-data exploration for porphyry copper systems.

The geophysical signature of Oyut deposits, Oyu Tolgoi, Mongolia

The well-known Oyu Tolgoi Cu-Au group deposits can be divided into three main deposits: Hugo Dummett deposit (Hugo North and Hugo South), Oyut deposits (South Oyu, Southwest Oyu and Central Oyu), and Heruga deposit in the south. These deposits sit along 26 km long, north-northeast trending belt termed as the Oyu Tolgoi trend. This paper reviews investigations on geophysical signatures of the South Oyu, Southwest Oyu and Central Oyu deposits and compares geophysical models of the mineral deposits with their lithology, alteration, mineralization, and structures. A variety of datasets including induced polarization, ground magnetic, gravity survey are used in the study and generated inversion products of ground magnetic and gravity data with integrated interpretation. Typical responses from the Oyut deposits are: up to 0.1 mGal positive gravity anomaly above background, 100–200 nT low or high magnetic anomaly compared to background depending on the geological situations, and from 12 mV/V to 30 mV/V chargeability anomalies and low resistivity signatures from 100 ohm.m to 400 ohm.m. The interpreted geological-geophysical models of porphyry Cu-Au deposits presents in this study have emphasis on integrated interpretation of geophysical techniques, and inversions of gravity and magnetic data in gold rich porphyry copper system.

Predicting the emplacement of Cordilleran porphyry copper systems using a spatio-temporal machine learning model

Porphyry copper (Cu) systems occur along magmatic belts derived in subduction zones. Our current understanding of their formation is restricted to observations from the overriding plate, resulting in a knowledge gap in terms of processes occurring in convergence zones through time. An association between key tectonic processes and the timing and location of porphyry Cu systems requires linking geological observations to plate tectonic subduction models. We connect the evolution of subduction zones and downgoing slab properties with the history of porphyry ore deposition across the Americas by using a spatio-temporal machine learning approach. We use these spatio-temporal properties to apply a wide range of prominent machine learning methods and show the results in terms of accuracy of predictions on the test dataset. We then provide spatial visualisations in deep-time showing highly prospective areas for porphyry Cu mineralisation along areas not related with known mineral occurrences along the subduction margins of North and South America. We show that a high orthogonal plate convergence speed is the most important feature related to the formation of porphyry Cu systems. This parameter is linked with the thickening of continental arcs which has been shown to enhance Cu transport within plutons. Convergence speed also controls the volume of plate material subducted into the upper mantle, including carbonate phases in the upper volcanic portion of the ocean crust and water-rich pelagic deep sea sediments, which have been demonstrated to enhance sulphur and metal transport, respectively, boosting metasomatic enrichment of the mantle wedge in volatiles, sulphur, and fluid-mobile large ion lithophile elements (LILE), and promoting the formation of porphyry Cu systems in the overriding plate. We incorporate plate tectonic subduction models with machine learning models for the generation of copper mineralization prediction maps. We demonstrate the potential of this application in global mineral exploration along Cordilleran subduction zones and provide open source code and data with our results.

Mapping alteration minerals in the Pulang porphyry copper ore district, SW China, using ASTER and WorldView-3 data: Implications for exploration targeting

• The PCA + S-A method with WorldView-3 data has good potential for alteration minerals extraction. • New potential porphyry mineral alteration zonings were found. • The results ars helpful for understanding the Pulang mineralization system. Identifying alteration minerals of porphyry Cu deposits by using multispectral remote sensing data is a significant approach for ore prospecting. However, obtaining refined alteration information related to a porphyry Cu system is still challenging, especially in an area with complex geological background. In this study, the effectiveness of spectroscopic techniques for mapping alteration minerals associated with the Pulang porphyry copper deposit, SW China, which is located in high altitude and with tough terrain, were discussed by using ASTER and WorldView-3 data. The method of Principal Component Analysis (PCA) + Spectrum-Area (S-A) was developed to enhance the remote sensing information extraction of the alteration zones with quartz, sericite, chlorite and epidote. The results suggested that the extraction accuracy of porphyry alteration minerals based on WorldView-3 data with the method of PCA + S-A is significantly improved comparing with that of ASTER data. Using the method, we identified a new alteration zone caused by sericite anomalies in the Pulang deposit, which was afterward confirmed by field investigation. Hence, the PCA + S-A method based on WorldView-3 data is an effective approach for mapping alteration zones and ore prospecting in the porphyry copper system.

Microthermometry and noble gas isotope analysis of magmatic fluid inclusions in the Kerman porphyry Cu deposits, Iran: constraints on the source of ore-forming fluids

This paper reports microthermometric and noble gas isotope data for fluid inclusion assemblages (FIAs) with evidence of phase separation, i.e. coexisting vapor-rich and halite-saturated inclusions, hosted in the early-formed quartz stockwork veins and post-magmatic quartz eye crystals in two economic porphyry Cu deposits (PCDs; Sar Cheshmeh and Miduk) and two sub-economic prospects (Sar Kuh and Abdar) from the Kerman porphyry copper belt (KPCB), Iran. The multiphase halite-saturated inclusions (i.e., Type I) in all studied PCDs and prospects had the highest homogenization temperature (Th = 525–594 °C) and salinities (63–73 wt% NaClequiv), whereas vapor-rich inclusions (Type II) had lower Th (362–460 °C). Fluid inclusion data show that like economic PCDs, the sub-economic prospects were formed in a fertile hydrothermal system and benefited from a mineralizing fluid, which evolved from a primary hot (mostly > 400 °C), metal-rich and oxidized fluid (as evidenced by the presence of opaque- and hematite-bearing fluid inclusions) of unknown salinity, which underwent a phase separation process to form both brine and vapor phases in the early stage of mineralization. The helium abundance and its isotopic composition document a mantle-derived magmatic source for the primary ore fluid in the formation of the studied PCDs and prospects (3He/4He ratios ranging from 0.46 to 2.8 Ra, corresponding to a mantle He contribution in ore fluids between ~ 7 and 45%). However, subsequent hydrothermal processes, i.e., vapor–brine phase separation, fluid-rock interaction with crustal rocks, and mixing with meteoric pore water containing dissolved atmospheric (e.g., Ne and Xe) and some crustal noble gases (e.g., Ar), changed the initial noble gas composition of the magmatic ore fluid to predominantly atmospheric- and crustal-like compositions. A significant proportion of mantle-derived He (up to 45%) in high-temperature (513–594 °C) and high-salinity (61.5–73 wt% NaClequiv) FIAs may indicate the existence of buried, economic, porphyry Cu mineralization in the Abdar prospect; therefore, it is suggested to be a possible target for further exploration. Comparing the He and Ar noble gas isotope composition in porphyry copper systems of different size and economic importance in this study showed that the ore-forming fluids of the outsized PCD (i.e., Sar Cheshmeh) have higher contributions of crustal-derived fluids characterized by predominantly radiogenic noble gas signatures (4He and 40Ar) than the smaller PCDs. This could have been achieved by a prolonged hydrothermal circulation in a large volume of crustal rocks containing radiogenic noble gases under a long-lived heat regime resulting from a deeply emplaced and slowly cooled composite intrusive body.

Duration of Hydrothermal Alteration and Mineralization of the Don Manuel Porphyry Copper System, Central Chile

The Don Manuel porphyry copper system, located in the Miocene–Pliocene metallogenic belt of central Chile, contains spatially zoned alteration styles common to other porphyry copper deposits including extensive potassic alteration, propylitic alteration, localized sericite-chlorite alteration and argillic alteration but lacks pervasive hydrolytic alteration typical of some deposits. It is one of the youngest porphyry copper deposits in the Andes. Timing of mineralization and the hydrothermal system at Don Manuel are consistent with emplacement of the associated intrusions (ca. 4 and 3.6 Ma). Two molybdenite samples yielded consistent ages of 3.412 ± 0.037 and 3.425 ± 0.037 Ma. 40Ar/39Ar ages on hydrothermal biotites (3.57 ± 0.02, 3.51 ± 0.02, 3.41 ± 0.01, and 3.37 ± 0.01 Ma) are associated with potassic alteration. These ages are younger than the youngest intrusion by ~300 k.y. recording the cooling of the system below 350 °C. Such a time gap can be explained by fluxing of hot magmatic fluids from deeper magmatic sources.

Recognition of a Middle–Late Jurassic arc-related porphyry copper belt along the southeast China coast: Geological characteristics and metallogenic implications

AbstractRecent exploration has led to definition of a Middle–Late Jurassic copper belt with an extent of ∼2000 km along the southeast China coast. The 171–153 Ma magmatic-hydrothermal copper systems consist of porphyry, skarn, and vein-style deposits. These systems developed along several northeast-trending transpressive fault zones formed at the margins of Jurassic volcanic basins, although the world-class 171 Ma Dexing porphyry copper system was controlled by a major reactivated Neoproterozoic suture zone in the South China block. The southeast China coastal porphyry belt is parallel to the northeast-trending, temporally overlapping, 165–150 Ma tin-tungsten province, which developed in the Nanling region in a back-arc transtensional setting several hundred kilometers inboard. A new geodynamic-metallogenic model linking the two parallel belts is proposed, which is similar to that characterizing the Cenozoic metallogenic evolution of the Central Andes.

PCA analysis of soil geochemical data and environmental assessment of the pre-mining Shuteen area, South Mongolia

There are widespread mineralized systems, occurrences, deposits that occurred in the South Gobi, Mongolia, where many of them already undergone mining activity. However, not all of them have a pre-mining environmental assessment. The Shuteen area is one of these highly prospective areas for mining; due to the porphyry copper system observed, prospecting and exploration projects have been carried out since 1980. Numerous prospecting and exploration works have been done during 1997–2007 and suspended last decade because of economic and political reasons. The area is located in a Gobi-desert environment and wind plays a main role in the transport and mobility of the elements. This research was conducted to assess the environmental condition of the area, based on soil, dry river sediment, and drinking water geochemistry. We are expecting the area has a naturally high concentration of some heavy metals in soil, concerning the geology and mineralization. The results show that the heavy metal content in the soil does not exceed the permissible limit of the Mongolian National standard on soil quality. Whereas, the water samples contain F- above the permissible limit guided in the Mongolian National Standard on drinking water quality. The unusual chemical composition of the water is related to the soil composition characteristic of the Gobi region. They are characterized by low alkali, high salinity and mineral/water ratios are high. Environmental assessment surveys determine the characterization of an areal geochemical base prior the development of later exploration or mining project, might have affect it and establish the initial environmental status.

Structural Controls on Alteration Stages at the Chuquicamata Copper-Molybdenum Deposit, Northern Chile

AbstractAll existing bench and tunnel vein and fault structural data with identified mineral infill, acquired in Chuquicamata, were georeferenced, digitized, and, according to their mineralogy, assigned to one or more of the major alteration events developed between 35 and 31 Ma. Veins and faults were separated into two main stages: (1) the late magmatic and potassic stage that comprises the background potassic and the propylitic alteration and (2) the hydrothermal stage composed by early (intense potassic), main (principal and late sericite; hydrothermal stages H1 through H4), and late (advanced argillic alteration) hydrothermal events. The spatial distribution of the propylitic to late-hydrothermal events that plotted within the major fault framework indicate these had either permeable or impermeable (±barrier) behavior through time. The area of the deposit was divided into 600 square grids measuring 100 × 100 m, and a stress orientation analysis was carried out for every propylitic to late-hydrothermal alteration event. The analysis indicates that the local principal horizontal stress (σH) trajectories are nonlinear and noncoaxial through the successive alteration events, differing from the previous and following stages, and in the majority of cases do not coincide with the approximate east-northeast orientation of the inferred tectonic far-field stress orientation. The differences between the stress trajectories, away from the far-field stress orientation throughout the evolution of the system, are considered to be principally related to the dynamic variations experienced by the stress components, such as thermal-magmatic stresses linked to temperature fluctuations due to cooling or heating by progressive igneous/hydrothermal activity and/or elastic, overburden-related stresses associated with reaccommodations developed during uplift and erosion. The estimated stresses resulting after erosional unroofing and decreasing temperature indicate that the maximum horizontal stress varied as the system evolved from the commonly accepted depth of emplacement of ~6 km. During the late magmatic, background potassic, and intense potassic stages, the calculated differential stress was contractional, decreasing to an isotropic state at the contraction-extension stress reversal that hosted the main hydrothermal H1 through H3 events, to finally become extensional at the shallow late-hydrothermal event. The most significant mineralization occurred at the time of stress reversal, coincidental with the sericite and quartz-sericite events (H1-H4), associated with hydrothermal fluid accumulation, overpressuring, and multiple-orientated hydraulic fracture development.The Chuquicamata study suggests that the local stress control involved in the emplacement of porphyry copper systems is fundamentally related to variable and progressive heat energy release, associated with igneous and hydrothermal activity, and to the elastic stresses derived from uplift and unloading, rather than to a constant far-field tectonic stress. The continuous local stress fluctuations led to bulk stress readjustments and cyclical stress-fluid interactions for local fault reactivation, damage zone modification, brecciation, permeability creation/destruction, and fluid focusing, as well as the discharge of hydrothermal fluids throughout the evolution of the system.

Exhumation history of the La Caridad and Suaqui Verde porphyry copper deposits in the eastern Basin and Range province of Sonora: Insights from thermobarometry and apatite thermochronology

The Arizona-Sonora porphyry copper cluster is one of the most productive provinces of this type on Earth. There, upper-crustal deposits emplaced between Late Cretaceous and Early Eocene, subsequently exhumed and developed supergene enrichment profiles. Despite the Basin and Range regional extension, these shallow intrusions and associated mineralization were preserved in eastern Sonora in the exhumed footwalls of normal faults. In order to better constrain their exhumation history, we performed a thermobarometrical (Al-in-hornblende) and thermochronological (apatite U–Pb and fission-track) study on the La Caridad and Suaqui Verde porphyry copper deposits. On the one hand, thermobarometry allows us to determine that the Suaqui Verde porphyry copper emplaced at 4.8 ± 0.9 km-depth, which implies that Cu–Mo mineralization may occur deeper than the ~2–4 km depth classically proposed for porphyry copper. On the other hand, using thermobarometry and inverse modeling of thermochronologic data obtained for individual and drill core samples, we reconstruct the thermal history of these porphyry copper. After a very fast post-emplacement cooling, numerical models suggest that cooling of rocks was slow and mainly monotonous during exhumation. Indeed, our data do not record an acceleration of the exhumation in eastern Sonora during the Basin and Range regional extension (between ~25 and 12 Ma). However, inverse modeling of fission-track data obtained along a drill core in La Caridad allows us to reconstruct the evolution of the geothermal gradient through time, which is in agreement with the regional geodynamics. Overall, it cannot be excluded that Oligocene volcanic rocks protected the ore deposits from erosion, even if they remained relatively thin (<500 m-thick). We propose that eastern Sonora was characterized by Basin and Range extension distributed on numerous normal faults with small offsets, which allowed to preserve the upper crustal part of the Cretaceous-Paleogene magmatic arc and its numerous associated porphyry copper systems.