Christoph A. Heinrich

Bio: Christoph A. Heinrich is an academic researcher from ETH Zurich. The author has contributed to research in topics: Fluid inclusions & Porphyry copper deposit. The author has an hindex of 69, co-authored 199 publications receiving 15885 citations. Previous affiliations of Christoph A. Heinrich include École Polytechnique Fédérale de Lausanne & University of Zurich.

100th Anniversary Special Paper: Vapor Transport of Metals and the Formation of Magmatic-Hydrothermal Ore Deposits

Abstract: In most published hydrothermal ore deposit models, the main agent of metal transport is an aqueous liquid. However, there is increasing evidence from volcanic vapors, geothermal systems (continental and submarine), vapor-rich fluid inclusions, and experimental studies that the vapor phase may be an important and even dominant ore fluid in some hydrothermal systems. This paper reviews the evidence for the transport of metals by vapor (which we define as an aqueous fluid of any composition with a density lower than its critical density), clarifies some of the thermodynamic controls that may make such transport possible, and suggests a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapor or a vapor-derived fluid. Analyses of vapor (generally >90% water) released from volcanic fumaroles at temperatures from 500° to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (from ppm to wt %) are observed in vapor inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperatures (400°–650°C). Moreover, concentrations of some metals, notably Cu and Au, are commonly higher in vapor inclusions than they are in inclusions of coexisting hypersaline liquid (brine). Experiments designed to determine the concentration of Cu, Sn, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) partly explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems, and furthermore that it increases sharply with increasing water fugacity and correlates positively with the fugacity of HCl. Thermodynamic analysis shows that metal solubility is greatly enhanced by reaction of the metal with HCl and by hydration, which results in the formation of species such as MeCl m . n H2O. Nonetheless, the concentrations measured by these experiments are considerably lower than those measured in experiments involving aqueous liquids or determined for vapor fluid inclusions. A possible explanation for this and for the apparent preference of metals such as Cu and Au for the vapor over the coexisting brine in some natural settings is suggested by limited experimental studies of metal partitioning between vapor and brine. These studies show that, whereas Cu, Fe, and Zn all partition strongly into the liquid in chloride-bearing sulfur-free systems, Cu partitions preferentially into the vapor in the presence of significant concentrations of sulfur. We therefore infer that high concentrations of Cu and Au in vapor inclusions reflect the strong preference of sulfur for the vapor phase and the formation of sulfur-bearing metallic gas species. Phase stability relationships in the system NaCl-H2O indicate how vapor transport of metals may occur in nature, by showing a range of possible vapor evolution paths for the conditions of porphyry-epithermal systems. At the world-class Bingham Canyon porphyry Cu-Au deposit, evidence from fluid inclusions supports a model in which a single-phase fluid of intermediate to vapor-like density ascends from a magma chamber. On cooling and decompression, this fluid condenses a small fraction of brine by intersecting the two-phase surface on the vapor side of the critical curve, without significantly changing the composition of the expanding vapor. Vapor and brine reach Cu-Fe sulfide saturation as both phases cool below 425°C. Vapor, which is the dominant fluid in terms of the total mass of H2O, Cu, and probably even Cl, is interpreted to be the main agent of metal transport. The evolution of fluids leading to high-grade epithermal gold mineralization is initiated by an H2S-, SO2-, Au-, and variably Cu- and As-rich vapor, which separates from an FeCl2-rich brine in a subjacent porphyry environment. In the early stages of the hydrothermal system, vapor expands rapidly and on reaching the epithermal environment, condenses, producing hypogene advanced argillic alteration and residual vuggy quartz and, in some cases, coeval high-sulfidation precious metal mineralization (e.g., Pascua). More commonly, the introduction of precious metals occurs somewhat later, after the site of magmatic fluid exsolution has receded to greater depth. Because of the relatively high pressure, the vapor separating from brine at this stage cools along a pressure-temperature path above the critical curve of the system, causing it to contract to a liquid capable of transporting several parts per million Au to temperatures as low as 150°C.

Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions

Abstract: The major and trace element compositions of individual fluid inclusions from a range of magmatic-hydrothermal ore deposits were analyzed by laser-ablation inductively coupled plasma-mass spectrometry, to explore the behavior of ore-forming components during fluid phase separation (“boiling”) in high-temperature saline fluid systems. Data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior. Na, K, Fe, Mn, Zn, Rb, Cs, Ag, Sn, Pb, and Tl preferentially partition into the brine (probably as Cl complexes), whereas Cu, As, Au (probably as HS complexes), and B selectively partition into the vapor. Fluid phase separation is probably a major, previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic-hydrothermal systems, from deep plutons through porphyry-style and greisen deposits to epithermal mineralization and volcanic fumaroles.

SILLS: A MATLAB-based program for the reduction of laser ablation ICP-MS data of homogeneous materials and inclusions

Abstract: Mineralogical Association of Canada Short Course 40, Vancouver, B.C., p. 328–333. INTRODUCTION This paper describes a software package named SILLS (Signal Integration for Laboratory Laser Systems) designed for data reduction and concentration calculation of transient Laser Ablation ICP–MS signals, written in MATLAB (The MathWorks, Inc.). The software is primarily designed for user friendly and flexible data reduction to obtain quantitative analyses of fluid, melt or mineral inclusions enclosed in mineral or glass host, but can also be used for homogeneous samples. The transient nature of these signals demands a dedicated data reduction procedure, particularly if signals from inclusions and their chemically distinct host minerals need to be separated quantitatively from each other (Heinrich et al. 2003, Longerich et al. 1996). The program includes a convenient and versatile option to display raw transient signals from the ICP–MS. Inclusion analysis, in particular, requires the visualization of each inclusion signal and the possibility to select integration intervals for gas blank, host mineral and inclusions precisely and flexibly. In contrast to other data reduction approaches, SILLS yields quantitative analyses of host and inclusion for a range of possible data reduction approaches without error-prone copy– paste steps between spreadsheets. The software was first developed and used by M. Allan at Leeds University (Allan et al. 2005) and later intensively redesigned, extended and tested in a collaborative effort at the Institute of Isotope Geochemistry and Mineral Resources at ETH Zürich. The SILLS data import is tested to work with files from Agilent and Perkin Elmer ICP–MS directly and with Thermo Element 2 data after a MATLAB-based convert script, editable for other instrument output files. The procedures and equations for concentration calculation are taken from the literature (Halter et al. 2002, Heinrich et al. 2003, Longerich et al. 1996) and are summarized in documents included with the software. All calculations are implemented according the most current knowledge, based on literature and in-house experience, but may still contain errors, thus requiring critical inspection of the results in every case. In particular, the calculation of uncertainties (Luo et al. 2007) on individual analyses and the calculation of limits of detection are undergoing continuous improvement, because no general applicable method has been published to date. For further information about obtaining the software package and future updates, please visit http://www.igmr.ethz.ch/research/fluids/software.

Porphyry Copper Systems

Abstract: Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

The role of magmas in the formation of hydrothermal ore deposits

Abstract: Magmatic fluids, both vapour and hypersaline liquid, are a primary source of many components in hydrothermal ore deposits formed in volcanic arcs. These components, including metals and their ligands, become concentrated in magmas in various ways from various sources, including subducted oceanic crust. Leaching of rocks also contributes components to the hydrothermal fluid—a process enhanced where acid magmatic vapours are absorbed by deeply circulating meteoric waters. Advances in understanding the hydrothermal systems that formed these ore deposits have come from the study of their active equivalents, represented at the surface by hot springs and volcanic fumaroles.