Widespread anhydrite saturation in Laramide-age arc magmas of the southwestern USA

Reports of magmatic anhydrite are relatively rare, with only ~30 occurrences documented worldwide so far. However, magmatic anhydrite saturation is difficult to recognize because anhydrite decomposes rapidly in near-surface environments. In most cases, only anhydrite inclusions shielded within other phenocryst phases were able to survive. Alternatively, since anhydrite phenocrysts preserved in fresh volcanic rocks are characteristically intergrown with apatite phenocrysts, the former presence of anhydrite phenocrysts can be recognized based on the occurrence of lath-shaped cavities that show a strong spatial association with apatite phenocrysts. These cavities can be either empty or filled with low-temperature, secondary minerals such as zeolites, carbonates, or microcrystalline silica. A systematic search for the occurrence of such cavities, combined with optical and Raman-spectroscopic identification of anhydrite inclusions preserved within apatite, hornblende and quartz phenocrysts, demonstrates that most of the Laramide-age magmas associated with the Santa Rita and Hanover-Fierro porphyry–skarn Cu (Zn, Mo, Au, Pb) deposits were saturated in magmatic anhydrite. The anhydrite typically coexisted with monosulfide solid solution (MSS), suggesting oxygen fugacities of ~2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer. The magmas range from andesitic to rhyodacitic in composition, and from shortly pre-mineralization (~61 Ma) to shortly post-mineralization (~57 Ma) in age. In three samples with particularly well-recognizable former anhydrite phenocrysts, their modal abundance could be quantified based on high-resolution scans of polished hand specimens. The observed modal anhydrite abundances of 0.63 to 1.8 vol % translate into minimum magma sulfur contents of 0.20 to 0.56 wt % S. The highest sulfur content of 0.56 wt % S is difficult to reconcile with available anhydrite solubility models, but it could be reproduced in an anhydrite solubility experiment performed at 950°C and 1.15 GPa on a natural latite containing 13.1 wt % dissolved H2O. The sample with the second-highest sulfur content of 0.26 wt % S requires ~10 wt % H2O in the silicate melt, and, consequently, a minimum pressure of ~0.5 GPa. Taken together, the results suggest that the magmas of the Central Mining District were extremely hydrous and thus originated from great depth. Indeed, their major element compositions and reconstructed H2O and S contents agree well with experimentally observed and numerically predicted compositions of residual silicate melts after 50 to 70 wt % crystallization of ordinary arc basalts at high pressure and high oxygen fugacities.

The Cretaceous-Eocene Mexican Magmatic Arc: Conceptual framework from geochemical and geochronological data of plutonic rocks

The U.S. Geological Survey conducted an assessment of resources associated with porphyry copper deposits in the western Central Asia countries of Kyrgyzstan, Uzbekistan, Kazakhstan, and Tajikistan and the southern Urals of Kazakhstan and Russia as part of a global mineral resource assessment. The purpose of the study was to (1) delineate permissive areas (tracts) for undiscovered porphyry copper deposits; (2) compile a database of known porphyry copper deposits and significant prospects; (3) where data permit, estimate numbers of undiscovered deposits within those permissive tracts; and (4) provide probabilistic estimates the amounts of copper (Cu), molybdenum (Mo), gold (Au), and silver (Ag) that could be contained in those undiscovered deposits. Western Central Asia, a region diverse in its geologic complexity, is situated north of the Tarim and North China tectonic blocks and sandwiched between the East European and Siberian cratons. The Ural Mountains form the western margin of the region; the southern margin is formed by the high-standing ranges that make up the Tian Shan mountain range in the border regions of Kazakhstan, Kyrgyzstan, and western China, where the effects of collisional tectonics are well displayed. The tectonic collage that makes up the core of western Central Asia is perhaps the geologically least understood part of the region. There is broad agreement that the early Paleozoic is made up of tectonically juxtaposed blocks that vary from Precambrian-cored microcontinents to magmatic arc and related complexes to subduction-related accretionary complexes. The rudiments of an incipient, contiguous single Kazakhstan block were formed by the end of the Silurian. In the middle to late Paleozoic, the block was unconformably superposed by two large, nested magmatic-arc belts, one Devonian, the other Carboniferous. Both magmatic-arc complexes were folded into a horseshoe-shaped, southeast-opening orocline in response to the final collisions of the various surrounding cratonic blocks with the Kazakhstan block. Additional deformation in the upper Cenozoic derived from the collision of India and China significantly redistributed fragments of the various mosaicked blocks, particularly in the central and southern parts of the western Central Asian region. Porphyry copper deposits are associated with many of the magmatic-arc fragments and belts throughout the geologically complex region, and economically important deposits are found in arc sequences of all Paleozoic Periods. The economically most productive arcs are Carboniferous. The assessment includes a discussion of the tectonic and geologic setting of porphyry copper deposits in western Central Asia (chapter 1), an application of remote sensing data for hydrothermal alteration mapping as a tool for porphyry copper assessment in the region (chapter 2), and a probabilistic assessment of undiscovered porphyry copper resources in four areas that represent Ordovician and Late Paleozoic (Carboniferous-Permian) magmatic arcs (chapter 3). The principal litho-tectonic terrane concept used to delineate permissive tracts was that of a magmatic arc that formed in the subduction boundary zone above a subducting plate. Eight permissive tracts are delineated on the basis of mapped and inferred subsurface distributions of igneous rocks assigned to magmatic arcs of specified age ranges that define areas where the occurrence of porphyry copper deposits within 1 kilometer of the Earth’s surface is possible. These tracts range in area from about 8,000 to 200,000 square kilometers and host 18 known porphyry copper deposits that contain about 54 million metric tons of copper. Available data included geologic maps, the distribution of significant porphyry copper occurrences and potentially related deposit types, the distribution of hydrothermal alteration patterns that are consistent with porphyry copper mineralization, and information on possible subsurface extensions of permissive rocks. On the basis of analyses of these data, the assessment team estimated a mean of 25 undiscovered porphyry copper deposits for the study area. Estimates of numbers of undiscovered deposits were combined with grade and tonnage models in a Monte Carlo simulation to yield a mean estimate of 95 million metric tons of copper in undiscovered porphyry copper deposits; this represents about twice the amount of identified porphyry copper resources (54 million metric tons). Detailed descriptions of each permissive tract, including the rationales for delineation and assessment, are given in appendixes, along with a geographic information system (GIS) that includes permissive tract boundaries, point locations of known porphyry copper deposits and significant occurrences, and hydrothermal alteration data based on analysis of remote sensing data.

The O 18 /O 16 , D/H and C 13 /C 12 values, where applicable, were measured on quartz (40 samples), K-feldspar (12), biotite (16), sericite (23), and calcite (7) from potassic, sericitic and argillic alteration assemblages and from fresh igneous rocks and veins in 9 North American porphyry copper and molybdenum deposits, and in 4 other hydrothermal mineral deposits. The porphyry copper deposit at Santa Rita, New Mexico was sampled in detail.Both sericites and biotites appear to preserve their O 18 /O 16 and D/H values, as these isotope ratios are not affected by later low-temperature exchange with local ground waters. A systematic correlation is, however, observed between the D/H ratios of sericites (and clays) from most Tertiary porphyry deposits and those of meteoric ground waters; this requires the presence of a significant meteoric water component in the hydrothermal fluids involved in sericitization (and argillization). In contrast to the sericites, the very restricted range of D/H ratios of biotites from Bingham, Ely and Santa Rita (--74 + or - 10ppm) indicates the probable dominance of magmatic waters in the fluids associated with biotite alteration. The oxygen isotope data for the quartz-K-feldspar-biotite-chalcopyrite assemblages also support a high temperature of formation in the presence of waters with delta O 18 -values in the magmatic water range. However, at the Butte deposit meteoric waters were apparently associated with the early biotitization.Many problems are associated with the application of isotopic geothermometers to the alteration assemblages because of the general presence of: (1) retrograde exchange, especially of the K-feldspars, (2) isotopically heterogeneous minerals, such as quartz of different origins, that are intimately intergrown and thus difficult to separate physically, and (3) the common lack of independent criteria for recognizing isotopic equilibrium. In the quartz-sericite assemblage the delta O 18 of the igneous quartz is preserved and is different from that of quartz precipitated during the sericitization processes.The Santa Rita isotopic data are discussed in terms of two simplified end-member models to evaluate the importance of temperature variations relative to variations in the O 18 /O 16 ratio of the hydrothermal fluids; both types of variations are probable. Calculated temperature ranges for the combined models are 580 degrees to 390 degrees C for potassic alteration (note that these orthoclases are very K-rich, Or (sub 90-96) ), and 390 degrees to 285 degrees C for sericite alteration, with delta O 18 -water ranging from 7.3 to 3.9 per mil. Similar temperatures for sericite formation were obtained independently by direct isotopic analysis of some quartz-sericite assemblages, utilizing the quartz-muscovite O 18 /O 16 geothermometer. C 13 /C 12 values of hydrothermal vein carbonates in the porphyry copper deposits (delta = -- 2.6 to --5.9) tend to be a little heavier than primary igneous calcites from carbonatites (which typically have delta = -- 5 to --8ppm). The similarities between the isotopic variations in the calculated hydrothermal waters associated with sericitization and argillization and those of saline formation waters of North America suggest that heated Na-Ca-Cl brines originally present in the neighboring sedimentary and volcanic sections may be an important component of the hydrothermal fluids. The quantities of meteoric ground water that have interacted with the hot igneous stocks of the porphyry ore deposits, though very significant, are much smaller than the amounts involved in the vein systems at Butte and in many epizonal Tertiary igneous intrusions emplaced into permeable volcanic country rocks.

The importance of H2O in arc magmas for the formation of porphyry Cu deposits

The porphyry-copper systems are represented by ore-bearing magmatic complexes which to a greater or lesser extent are the host rocks of mineralization, and the processes involved in the differential formation of intrusive and subvolcanic bodies are similar to those leading to the release and concentration of useful components during some of the subsequent postmagmatic processes. Producers of porphyry-copper ore are mostly the intermediate and slightly acid rocks, the majority showing some tendency towards increased potassic alkalinity. The magmatic systems involved usually manifest several pulses of different compositions and/or fabric grouped into larger and more complex intrusive centres, sometimes called "batholiths". Copper, however, concentrates in separate, small and relatively late magmatic differentiates of increased acidity. Porphy.ry-copper mineralizations are characteristic of magmatic systems of homodromous evolution during subduction. Porphyry-copper deposits associated with the bimodal basalt-rhyolite magmatism of the epicontinental riftogenesis or with MORB are not known. In the Eurasian copper belt, the porphyry-copper systems are typically associated with massive and disseminated copper (polymetallic) - pyrite (±Au) mineralizations in propylites and secondary quartzites and with copper-gold (±Ag) base-metal veins and skarns. All three types are closely associated in space forming, along with the productive volcano-intrusive centres, an integral polygenic and polyfacial ore-magmatic system. The segment discussed in this study includes 65 deposits located exclusively within the Eurasian Plate active margin and associated with the formation of mature magmatic arcs during the period from the Upper Cretaceous till the Neogene inclusive. The most typical feature of the geodynamic setting is that a number of small continental blocks (micro-continents) have been also involved in the process of ocean (Neotethys) closing accompanying the collision of the major lithospheric plates (the Afro-Arabian and Eurasian ones). These blocks probably broke off the Afro-Arabian Plate and migrated to the north where they stuck to the Eurasian Plate southern margin forming an accretionary collage. The formation of porphyry-copper systems in the magmatic arcs is exclusively a post-accretion process at advanced subduction and collision. An especially characteristic feature is the localization of the largest and best defined deposits inside the accreted exotic blocks. Deposits formed in the back-arc thrust zone of the Eurasian Plate active margin are comparatively less widespread. In Romania, the porphyry-copper systems formed during the Upper Cretaceous and the Neogene are located in a block collage traced back, rather guestionably, to the Gondwana. The porpltyry-copper systems in East Serbia occur in a similar setting. As to the Tertiary deposits in North Macedonia, Krusha Mountain and Halkidiki Peninsula, they definitely belong to the Serbo-Macedonian accreted terrane. On the territory of Bulgaria, the majority of the Upper Cretaceous porphyry-copper deposits are lo;atd in the Central Srednogorie, Sakar Strandja and East-Thracia terranes. Separate deposits are situated (possibly as parts of allochthons) in the Balkaoid's thrust-nappe – back-arc belt. In Turkey (Strandja Mt.), a group of complex copper-molybdenum-tungsten porphyry deposits are located not far away from the Bulgarian ones, again in the East Thracia terrane. Anotller, less conspicuous group of Upper Cretaceous (and m1y be even younger) age is located in the active southern margin of the Eurasian Plate. Further east, all porphyry-copper deposits of the USSR, Iran and Pakistan occur in accreted exotic terranes including those of Lesser Caucasus, Northwest Iran, Central Iran and Hilmend. Local petrochemical correlation is based on the K2O+Na2O/SiO2 diagram on which fields of porphyrycopper deposits from the Circum-Pacific metallogenic belt and from the Caribbean have been plotted prior to analysis. Compared to these plots, the porphyry-copper systems in the Eurasian belt show analogous acidity of magmatism. Alkalinity is variable but generally the Eurasian (Alpine, Mediterranean) belt as compared to the Pacific one is characterized by a pronounced potassic alkilinity (high·potassic calc-alkaline series). This is consistent with the overall Mediterranean petrochemistry. Outside it, e.g. further east in Pakistan, the ore-bearing magmatic complexes are of normal calc-alkaline character. A general petrochemical correlation shows that compared to the Pacific belt the porphyry-copper systems in the Romania-Pakistan segment of the Eurasian copper belt are formed in a mixed geodynamic setting involving both continental and island arcs. It is suggested that the present-day mixing of geodynamic settings is a secondary phenomenon caused by Late Alpine collision processes which have deformed the primary tectonic features and have pushed to the north many of the magmatic arcs involved.

The following personalised narrative aims to document the highlights of my involvement in some of the ground breaking developments in Economic Geology and their direct application to mineral exploration and discovery over the past half century. The story begins with my introduction to geology at secondary school and university, followed by doctoral research based on fieldwork in the Andes of South America. Then, as an employee of the Chilean Geological Survey, I got my introduction to porphyry copper deposits before returning to the UK to take up a post-doctoral research fellowship. This formative period concluded with my starting out as an independent geological consultant to the global exploration and mining industry. These early years happened to coincide with the plate tectonics revolution and its radical implications for metallogeny. I realised that porphyry copper and related deposits are integral parts of volcano-plutonic arcs generated during subduction of oceanic lithosphere, and volcanogenic massive sulphide deposits in ophiolite complexes must have formed at oceanic spreading centres. At approximately the same time, application of K-Ar dating to copper deposits led to definition of metallogenic belts and corresponding epochs in the Andes, and then established the timing of their economically important supergene oxidation and enrichment. Subsequently, using more modern and precise U-Pb zircon and Re-Os molybdenite methods, collaborative attempts were made to determine porphyry copper deposit lifespans and ages of various copper belts, deposits and prospects around the world, including the Zambian Copperbelt. The focus on porphyry copper deposits led first to an appreciation of the linkage between them and subaerial volcanism and the importance of potassic alteration as a major host of hypogene copper mineralisation, and then to geological characterisation of the increasing number of gold-rich examples. Appreciation of the importance of hydrothermal breccias in porphyry copper deposits, including recognition of mineralised diatremes, resulted in a classification scheme for breccias that may be extended to related deposit types. Extensive fieldwork showed that zones of advanced argillic alteration, termed lithocaps, constitute the shallow parts of porphyry copper systems. The role of tectonic uplift in both porphyry copper formation and subsequent supergene modification was also charted. The end result of this body of work was a porphyry copper model that can be used as a basic exploration guide. In response to a marked increase in the world gold price in the late 1970s, more effort was devoted to gold concentrations in magmatic arc terranes, commencing with epithermal gold deposits in the shallow lithocaps of porphyry copper systems. This led to an input to classification schemes for epithermal precious metal deposits and, eventually, to assignment of the three main epithermal types to specific tectono-magmatic settings. After years of speculation, porphyry gold deposits were recognised for the first time in northern Chile, followed by definition of a new gold deposit class in association with relatively reduced granitic intrusions. A magmatic-hydrothermal origin for Carlin-type gold deposits was steadfastly supported over many years, notwithstanding its unpopularity until relatively recently, culminating in a proposal for modern analogues. For decades, metallogenic provinces and corresponding epochs have been widely appreciated, but debate concerning their origin(s) persists. The nature of accompanying magmatism could well provide an adequate explanation for at least some provinces (e.g., tin, molybdenum and possibly silver), but precursor metal enrichment in the lowermost crust and/or subcontinental lithospheric mantle may well be required in the case of gold and copper provinces. The story concludes with brief commentary on mineral exploration, which has been my lifelong (pre)occupation. Requirements for success in mineral exploration are discussed, based primarily on familiarity with the circum-Pacific region, followed by analysis of the types of companies and individuals involved and the burgeoning challenges to the exploration process, which, if not remedied, will have dire consequences for future metal production and global plans for the energy transition. My involvement as a lone practitioner in both mineral exploration and metallogenic research probably says something about my independent character traits, but nonetheless has been largely unstructured and certainly unplanned. I have no hesitation in recommending a similar career path for any recent graduate who relishes adventure and is prepared to endure significant work-life imbalance.

The middle Eocene (51.8 ± 0.6 Ma) hypogene protore underlying the supergene orebody of the Cerro Colorado porphyry Cu (-Mo) deposit, I Region, Chile, exhibits features not commonly documented in such hydrothermal systems. Early-stage alteration of Upper Cretaceous plagioclase-phyric andesite generated a sub-horizontal blanket of pervasive, extremely fine grained but texture-preserving biotite (≤35 modal %)-albite (≤40%)-magnetite (≥3%) alteration, 8 km 2 in area but lacking sulfide minerals. At least seventy percent of the chalcopyrite > pyrite stockwork mineralization was emplaced during the subsequent Main-stage alteration, which comprises, with decreasing depth, quartz-albite, sericite-chlorite-clay (smectite), quartz-sericite-clay, and andalusite-diaspore-pyrophyllite assemblages. The deposit is apparently unique among documented central Andean porphyry systems in the association of the highest grade copper mineralization with intermediate argillic alteration. The subsequent Transitional-stage phyllic (i.e., quartz-sericite-pyrite ± tourmaline) alteration was associated with the emplacement of molybdenite-rich breccia bodies. The occurrence of undumortieri-tized tourmaline veinlets cutting andalusite-diaspore assemblages confirms that much of the advanced argillic alteration took place during the Main stage. Early-stage alteration was the product of nonboiling, cool (trapping temperature ≤ca. 380°C), low-salinity (≤8 wt % NaCl equiv) fluids which added substantial K, Na, Mg, Fe 2+ , Cl, F, and water to the host andesite. The initial Main-stage fluids, boiling at a paleodepth of ca. 2.5 to 3.0 km, were up to 160°C hotter (≤544°C) and highly saline (≤52 wt % NaCl equiv). As these fluids rose, they cooled to ≤320°C, were diluted (to ≤37 wt % NaCl equiv), deposited sulfides, and leached K, Na, Ca, Mg, Fe 2+ ,and Cl from the host rocks, yielding diverse, broadly contemporaneous, intermediate and advanced argillic alteration facies. Pressure estimates require that low-density (≤0.3 g/cm 3 ), and thus more acidic, fluids were primarily responsible for the formation of the quartz-sericite-clay and shallow advanced argillic alteration. Subsequent phyllic alteration was similarly caused by boiling fluids which were hot (≤486°C) and saline (≤47 wt %) at depth but cooler (≤334°C), dilute (≤8 wt %), and vapor dominated at shallower levels. Even Terminal-stage pyrite veins formed at temperatures as high as 450°C, albeit from low-salinity (≤8 wt %) fluids. Following a major prograde thermal transition from the Early to the Main stage, each sulfide-depositing alteration episode at Cerro Colorado was generated by a pulse of high-temperature fluid which cooled and diluted as it rose. Such changes in fluid characteristics, temperature, and alteration relationships are well documented in numerous geothermal fields, where potassic alteration generally develops at ca. 270° to 350°C and are likely to occur at an early stage in any hydrothermal system in which magmatic fluids, exsolving at relatively high pressures, ascend into the near-surface environment. The initial alteration at Cerro Colorado plausibly developed under conditions similar to those in nonexplosive geothermal systems, in which hydrothermal fluids cool and disperse laterally at shallow depths. Extensive zones of biotite-rich alteration, generally barren and with a hornfelsic appearance, occur in many andesite-hosted porphyry copper deposits, but few data are available elsewhere for mineralogical, pressure, temperature, or metasomatic exchange relationships in such systems. Cerro Colorado may, however, be representative of a subclass of porphyry copper deposits exhibiting unusually close analogies with geothermal systems.

Generally, during the preliminary exploration stages of porphyry copper deposits (PCDs), the mineralization and exploration targets will be evaluated using mineral potential modeling that includes recognizable mineralization determination criteria, data preparation, generating factor maps, and combining of factor maps into the appropriate inference networks. The most important PCDs are related to subduction activity (Guilbert, 1986; Sawkins, 1990). Urmia– Dokhtar Magmatic Arc (UDMA) Porphyry copper deposit belt, which passes through Iran, is one of the most important examples of this kind of geodynamic setting. Several PCDs include Sarcheshmeh, which is the world’s eighth in terms of contained copper and gold (Cook et al, 2005), are located in the NW-SE trending (UDMA). This study area is located in southern part of UMDA in Southeast of Iran in 1:100000 Sarduiye geology sheet. There are some occurrences and PCDs in this region among which are Sarmashk, Daralou, and Bondar Hanza. In fact, Sarduiyeh area is a monocline from volcanic rocks, such as rhyolite and andesite, of Eocene age with intrusions of younger granodiorite and granite rocks. The aim of this paper is the predication of copper porphyry mineralization in the study area by using of the Analytical Hierarchy Process (AHP) method in GIS. To this end, geological data, Aster satellite images, airborne geophysics, and stream sediment geochemical data have been used.

Although porphyry copper-gold mineralization in the region has been known for approximately 40 years, the Chagai belt has only recently revealed its true potential with the discovery of a world-class deposit at H14-H15 at Reko Diq. The ~300-km-long, east-west–trending Chagai belt comprises several superimposed magmatic arcs and corresponding porphyry copper- (gold, molybdenum) mineralization generated in consecutive events at 43 to 37 Ma (middle-late Eocene), 24 to 22 Ma (early Miocene), 18 to 16 Ma (early Miocene), 13 to 10 Ma (middle Miocene), and 6 to ~4 Ma (late Miocene-early Pliocene) The tectonomagmatic evolution of the region was marked by major, continental-scale events associated with (1) the northward drift of India and its interaction and final collision with the southern margin of Eurasia, and (2) the collision of Arabia with central Iran resulting in final closure of the Neo-Tethyan Ocean. Initial, precollisional stages of arc evolution between the Middle Jurassic and Paleocene formed kilometer-thick submarine volcanic sequences with important flyschlike packages and intercalations of biohermal limestone. The subalkaline basalt and basaltic andesite of this stage possess a tholeiitic lineage of oceanic island-arc affiliation. Small-scale, submarine volcanogenic copper mineralization of manto- and Kuroko-type formed during the late, mature stages of this arc just prior to collision. Early contact of the leading edge of India and the island arcs in the late Paleocene (~55 Ma) resulted in a reorganization of the arc in the Chagai belt, with the consequent extrusion of subaerial andesite followed by emplacement of magnetite-series, calc-alkaline composition batholiths during the late Eocene. This plutonism accompanied the first manifestations of porphyry-type alteration and mineralization at ~43 Ma, in a rapidly emerging arc of Andean type. The belt is defined by 48 porphyry systems with copper-(gold, molybdenum) mineralization associated with calc-alkaline, biotite- and amphibole-bearing porphyry stocks of predominantly quartz diorite to granodiorite composition. Hydrothermal alteration includes potassic, propylitic, sericite-clay-chlorite, sericitic, and advanced argillic assemblages, the last in transitional epithermal environments. Calcic-potassic assemblages are developed locally at the expense of more mafic, commonly dioritic, rocks. Widespread porphyry copper and copper-gold mineralization during the Miocene, at 24 to 22 and 18 to 16 Ma, formed in a subaerial subvolcanic environment during moderate tectonic uplift, most notably during the early Miocene at Saindak and Tanjeel. Faster rates of regional uplift and exhumation, in conjunction with a period of conspicuous volcanic quiescence, characterized large-scale porphyry copper-gold mineralization at H14-H15 at Reko Diq during the middle Miocene between 13 and 10 Ma. Regional, tectonically triggered uplift and consequent exhumation may have been the result of collision of Arabia with central Iran. In general, mature, cumulative supergene enrichment and oxidation zones seem to be absent in the porphyry copper deposits of the Chagai belt, primarily due to the high neutralization potentials and low pyrite contents of the ore-related potassic alteration. Nevertheless, a supergene chalcocite blanket formed at Tanjeel during the Pliocene, at ~4 Ma, in sericitic alteration that accompanied the hypogene mineralization. The tectonomagmatic characteristics of the Chagai porphyry copper belt, including the multimillion-year history of subduction along an Andean-type margin, together with conspicuous short-lived contraction events and concomitant volcanic quiescence during arc construction and evolution are similar to those in contractional belts at convergent margins containing large-scale porphyry copper deposits elsewhere. Superposition of the Chagai magmatic arcs and corresponding porphyry copper events occurred during the last 55 m.y., a situation that implies that the focus of magma generation above the subducting slab remained essentially stationary. Possible mechanisms responsible for this include seaward migration of the trench axis during much of the Tertiary, especially since the Oligocene, and consequent flattening of the subducted slab.

Water-rich high-alumina basalts are widely implicated in models of subduction zone magma genesis and porphyry copper mineralisation, yet their phase relationships at high pressures have been very little studied since the pioneering work of Yoder and Tilley (1963). To fill this gap an experimental study has been carried out on water-saturated phase relationships of a high-alumina basalt (HAB) from St. Kitts, Lesser Antilles volcanic arc (Eastern Caribbean), in the pressure range 1.5 to 20 kbar. Experimentally produced glasses, mineral compositions and mineral assemblages match whole rock data of Lesser Antilles high-alumina basalts and high-alumina basaltic andesites, phenocryst assemblages and mineral chemistry. I show that the liquidus silicate mineral phase changes from olivine and plagioclase at low pressures, through clinopyroxene and amphibole at intermediate pressures, to garnet above 14 kbar. Experimentally produced mineral assemblages correspond well to plutonic xenolith found in Lesser Antilles and change from dunnite, and olivine and hornblende gabbro dominated lithology at pressure ≤ 10 kbar to hornblende pyroxenite, hornblendite and eclogite at pressures ≥ 10kbar.Melt compositions describe liquid lines of descent that resemble those of many magmatic arc sequences; only alumina shows any strong correlation with pressure. Water saturation (as determined by the difference method) is lower than predicted by most solubility models. The evolved melts generated at low crustal conditions from these experimental series do not resemble silicic melts associated with porphyry copper deposits.I also address the long-debated origin of high-An plagioclase (An > 90), which occurs in many volcanic and plutonic rocks associated with arc magmatism. Additional experiments on HAB at water-undersaturated and fluid-saturated experiments with 2M CaCl2 solution and CaCO3 have been carried out at 7 kbar to evaluate fluid composition and aH2O on An content of plagioclase. These experiments, combined with existing experiments on broadly basaltic compositions, demonstrate that high-An plagioclase (An ≥ 90) crystallises readily from fluid-saturated high-alumina basaltic melts at pressures of 1 to 10 kbars and temperatures 850 – 1100 ºC. However, the limit of An content produced by experiments is An96. Addition of CaCl2 to the fluid and CaCO3 to the system does not have a significant effect on An content. To produce plagioclase with An content more than 96 mol% a secondary process such as re-melting/re-crystallisation must be involved (Melekhova et al 2022).Melekhova E., et al. (2022) Journal of Petrology 63.5 (2022): egac033.

K–Ar dating demonstrates that all but eight of 41 dated porphyry copper and related ore deposits of Mexico were emplaced during the Laramide episode of maximum plate convergence. One older deposit is related to the Jurassic volcanic arc of western North America, one is pre-Laramide Cretaceous, four are Oligocene in age, and two late Cenozoic deposits are within the modern trans-Mexican–Chiapenecan volcanic arc. Thirty-three of the deposits lie within a long narrow belt that continues into Arizona and New Mexico, and widens from 100 km to over 300 km in the region of maximum extension in the southern Basin and Range Province. Eighty-five percent of the deposits were emplaced during the eastward transgression of the Cordilleran volcanic arc in middle Cretaceous through Eocene time.The occurrence of the porphyry copper deposits of Mexico appears to be independent of the terrane intruded and the copper content of the wall rocks where the wall rocks predate the volcanic arc, which is syngenetic with the porphyry stock. However, strontium is significantly more radiogenic where the host porphyry has intruded terrane having a Precambrian crystalline basement. Most frequently, the porphyry pluton can be observed to have intruded penecontemporaneous volcanic rocks or the batholith itself. The porphyries appear to be apophyses of the batholiths. The relationships suggest that the ore components are contained within the calc-alkaline batholiths and concentrated in the subvolcanic porphyries and wall rocks during transport of hydrothermal fluids to the volcanic orifice.The shape of the Cordilleran copper belt is controlled by magma composition, existence of a protective capping of dominantly volcanic rock, uplift, time, and erosion. As the continental volcanic arc that produced the porphyry copper deposits progressed eastward, the associated magma became more alkalic and copper poor. Thus, enrichment to ore grade became increasingly improbable to the east. Uplift and ample time for erosion prior to the return of the continental volcanic arc reduced the probability of ore preservation to the west. Optimum conditions for preservation were present within the belt where burial of calc-alkalic porphyry plutons under a thick volcanic cover occurred before removal of the ore zone by erosion. The broader width of the porphyry belt to the north is probably the result of both more extensive basin-and-range extension and basin-and-range taphrogeny that exposed some of the porphyries to relatively recent denudation and consequently made them available for economic exploitation.

Porphyry copper deposits in Arizona are genetically associated with Late Cretaceous and early Tertiary igneous complexes that consist of older intermediate volcanic rocks and younger intermediate to felsic intrusions. The igneous complexes and their associated porphyry copper deposits were emplaced into an Early Proterozoic basement characterized by different rocks, geologic histories, and isotopic compositions. Lead isotope compositions of the Proterozoic basement rocks define, from northwest to southeast, the Mojave, central Arizona, and southeastern Arizona provinces. Porphyry copper deposits are present in each Pb isotope province. Lead isotope compositions of Late Cretaceous and early Tertiary plutons, together with those of sulfide minerals in porphyry copper deposits and of Proterozoic country rocks, place important constraints on genesis of the magmatic suites and the porphyry copper deposits themselves. The range of age-corrected Pb isotope compositions of plutons in 12 Late Cretaceous and early Tertiary igneous complexes is 206 Pb/ 204 Pb = 17.34 to 22.66, 207 Pb/ 204 Pb = 15.43 to 15.96, and 208 Pb/ 204 Pb = 37.19 to 40.33. These Pb isotope compositions and calculated model Th/U are similar to those of the Proterozoic rocks in which the plutons were emplaced, thereby indicating that Pb in the younger rocks and ore deposits was inherited from the basement rocks and their sources. No Pb isotope differences distinguish Late Cretaceous and early Tertiary igneous complexes that contain large economic porphyry copper deposits from less rich or smaller deposits that have not been considered economic for mining. Lead isotope compositions of Late Cretaceous and early Tertiary plutons and sulfide minerals from 30 metallic mineral districts, furthermore, require that the southeastern Arizona Pb province be divided into two subprovinces. The northern subprovince has generally lower 206 Pb/ 204 Pb and higher model Th/U, and the southern subprovince has higher 206 Pb/ 204 Pb and lower model Th/U. These Pb isotope differences are inferred to result from differences in their respective post-1.7 Ga magmatic histories. Throughout Arizona, Pb isotope compositions of Late Cretaceous and early Tertiary plutons and associated sulfide minerals are distinct from those of Jurassic plutons and also middle Tertiary igneous rocks and sulfide minerals. These differences most likely reflect changes in tectonic setting and magmatic sources. Within Late Cretaceous and early Tertiary igneous complexes that host economic porphyry copper deposits, there is commonly a decrease in Pb isotope composition from older to younger plutons. This decrease in Pb isotope values with time suggests an increasing involvement of crust with lower U/Pb than average crust in the source(s) of Late Cretaceous and early Tertiary magmas. Lead isotope compositions of the youngest porphyries in the igneous complexes are similar to those in most sulfide minerals within the associated porphyry copper deposit. This Pb isotope similarity argues for a genetic link between them. However, not all Pb in the sulfide minerals in porphyry copper deposits is magmatically derived. Some sulfide minerals, particularly those that are late stage, or distal to the main orebody, or in Proterozoic or Paleozoic rocks, have elevated Pb isotope compositions displaced toward the gross average Pb isotope composition of the local country rocks. The more radiogenic isotopic compositions argue for a contribution of Pb from those rocks at the site of ore deposition. Combining the Pb isotope data with available geochemical, isotopic, and petrologic data suggests derivation of the young porphyry copper-related plutons, most of their Pb, and other metals from a hybridized lower continental crustal source. Because of the likely involvement of subduction-related mantle-derived basaltic magma in the hybridized lower crustal source, an indiscernible mantle contribution is probable in the porphyry magmas. Clearly, in addition, Pb was contributed from the local country rocks. This is most evident in sulfide minerals in veins that are late stage, hosted in Proterozoic gneiss, and/or peripheral to the porphyry copper deposit.

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How to make porphyry copper deposits