Mineralization (geology)

Abstract: oC to ~300 o C, from the surface to as deep as 1 to 2 km. Here we highlight the general characteristics of the two principal styles of epithermal mineralization in which gold is the dominant economic metal. We base our generalizations on observations of many deposits and prospects in the circum-Pacific region. Distinguishing between the two styles is crucial for effective exploration. Although they show similar alteration mineralogies, the distribution of the alteration zones is different, and the economic mineralization is associated with different parts of the system. The alteration zoning can be used as a pointer towards the most prospective part of the system, but only when the style has been correctly recognized. In addition, the two styles of mineralization have differences in their geochemical associations. Figure 1. Generalized sketches showing the relation of fluid types to alteration zoning in the two styles of epithermal deposits. (a) In low-sulfidation systems, the fluid at 1-2 km depth is near-neutral pH and reduced, and in equilibrium with the host rocks at greater depths. The boiling fluid rises along permeable zones, depositing ore and gangue minerals, and may discharge from near-neutral pH hot springs. The separated vapor with CO2 and H2S condenses in the vadose zone to form a steam-heated water, acidic from oxidation of H 2S. (b) In high-sulfidation systems, magmatic volatiles ascend to the epithermal environment where they are absorbed by meteoric water, and the HCl and SO2 form a highly acidic solution that leaches the rock outward from the fluid conduit. Ore metals may be introduced into this leached rock by later magmatic fluids (see Hedenquist et al., 1994).

Abstract: Many diverse ore systems are classified together as iron oxide copper-gold (lOCG) deposits based on an empirical definition arising primarily from geochemical features that do not specify tectonic setting, geologic environment, or sources of ore-forming fluid, metals, or other ore components. Such deposits have (1) Cu, with or without Au, as economic metals; (2) hydrothermal ore styles and strong structural controls; (3) abundant magnetite and/or hematite; (4) Fe oxides with Fe/Ti greater those in most igneous rocks and bulk crust; and (5) no clear spatial associations with igneous intrusions as, for example, displayed by porphyry and skarn ore deposits. IOCG deposits commonly have a space-time association with Kiruna-type apatite-bearing oxide Fe ores and many examples of the latter contain sulfide minerals, Cu, and Au. Most IOCG deposits display a broad space-time association with batholithic granitoids, occur in crustal settings with very extensive and commonly pervasive alkali metasomatism, and many are enriched in a distinctive, geochemically diverse suite of minor elements including various combinations of F, P, Co, Ni, As, Mo, Ag, Ba, LREE, and U. Iron oxide Cu-Au systems are numerous and widely distributed in space and time; they occur on all continents and range in age from the present at least back into the Late Archean. In economic terms, the most important ICOG deposits are those the Carajas district, Brazil (Archean, Amazon craton); in the Gawler craton and Cloncurry districts, Australia (late Paleoproterozoic to Mesoproterozoic debated intracratonic or distal subduction-related settings); and in the Juarassic-Cretaceous extended continental margin arc of the coastal batholitic belt in Chile and Peru. IOCG deposits and associated features define distinct metallogenic belts in which other types of Cu and Au deposits are rare or absent. The largest deposits include Salobo, Cristallino, Sossego, and Alemao (Carajas), Olympic Dam (Gawler craton), Ernest Henry (Cloncurry district), and Candelaria-Punta del Cobre and Manto Verde (Chile), and have resources greater than 100 million metric tons (Mt), ranging up to more than 1,000 Mt with metal grades that exceeds those in most porphyry-style Cu ± Au deposits. A comparison of larger and well-described IOCG deposits illustrates the geologic diversity of the class as a whole. They occur in a wide range of different host rocks, among which plutonic granitoids, andesitic (meta)volcanic rocks, and (meta)siliclastic-metabasic rock associations are particularly prominent. Host rocks may be broadly similar in age to the ore (e.g., Olympic Dam, Candelaria-Punta del Cobre, Raul-Condestable) but in other cases significantly predate mineralization such that ore formation relates to a quite separate geologic event (e.g. Salobo, Ernest Henry). Mineralization is interpreted to have occurred over a wide depth range, from around 10km (e.g., several deposits in the Cloncurry district) to close to the surface (e.g., Olympic Dam); where systems have been tilted and exposed in cross section (such a s at Raul-Condestable in Peru), they can display strongly zoned mineral parageneses. Structural and/or stratigraphic controls are pronounced, with deposits characterically localized on fault bends and intersections, shear zones, rock contacts, or breccia bodies, or as lithology-controlled replacements. Host rocks in the vicinity of orebodies display intense hydrothermal alteration. In the immediate vicinity of the ore, the variable pressure-temperature conditions of alteration and mineralization are reflected in a spectrum of deposits ranging form those in which the dominant Fe oxide is magnetite and alteration is characterized by minerals such as biotite, K-feldspar, and amphibole though to hematite-dominated systems in which the main silicate alteration phases are sericite and chlorite. Where present, Na and Na-Ca alteration tends to be developed deeper or more distal from ore, is more extensive, and commonly predates K-Fe alteration and mineralization. Carbonates are commonly abundant, particularly in assoication with, or postdating, Cu-bearing sulfides that tend to be paragenetically late and postdate high-temperature silicate alteration in the deeper seated deposits. Independent variation in fO2- fS2-(T) conditions during mineralization produced deposits ranging from pyrite-poor examples, with complex Cu mineral associations, including chalcopyrite, bornite, and chalcocite (e.g., Salobo, Olympic Dam), to others in which pyrite and chalcopyrite are the main sulfides (e.g., Ernest Henry, Candelaria). Fluid inclusion evidence suggests that geochemically complex brines, commonly with a carbonic component, were involved in IOCG genesis. However, the ultimate sources of water, CO2, metals, sulfur, and salinity have yet to be well constrained, and it is possible that these components may have different origins from deposit to deposit. Brines and metals may be sourced directly from underlying magmas, indirectly by interaction of magmatic fluids with country rocks or other fluids, or independently through modification of basinal or metamorphic fluids. Ore deposition may primarily involve interaction of voluminous fluid with wall rocks and cooling. However, several studies have emphasized the role of mixing sulfur-poor, metal-rich brines with sulfur-bearing fluids at the site of ore deposition, although characterization of the causative fluids has proven problematic. Uncertainty also exists about the original tectonic settings of several major IOCG districts, and considerably more research is needed before it will be clear whether these deposits are linked by a single family of related genetic mechanisms or whether they can form in a range of fundamentally different geologic environments from fluids of different sources.

Abstract: Preface. Introduction: Mineral Resources:. Introduction And Aims. What Makes A Viable Mineral Deposit?. Some Useful Definitions And Compilations. Natural Resources, Sustainability And Environmental Responsibility. Summary And Further Reading. Part I: Igneous Processes:. 1. Igneous Ore-Forming Processes:. Introduction. Magmas And Metallogeny. Why Are Some Magmas More Fertile Than Others?...The 'Inheritance Factor'. Partial Melting And Crystal Fractionation As Ore Forming Processes. Liquid Immiscibility As An Ore-Forming Process. A More Detailed Consideration Of Mineralization In Mafic Magmas. A Model For Mineralization In Layered Mafic Intrusions. Summary And Further Reading. 2. Magmatic-Hydrothermal Ore-Forming Processes:. Introduction. Some Physical And Chemical Properties Of Water. Formation Of A Magmatic Aqueous Phase. The Composition And Characteristics Of Magmatic-Hydrothermal Solutions. A Note On Pegmatites And Their Significance To Granite-Related Ore-Forming Processes. Fluid-Melt Trace Element Partitioning. Water Content And Depth Of Emplacement Of Granites: Relationships To Ore-Forming Processes. Models For The Formation Of Porphyry-Type Cu, Mo And W Deposits. Fluid Flow In And Around Granite Plutons. Skarn Deposits. Near-Surface Magmatic-Hydrothermal Processes: The 'Epithermal Family Of Au-Ag-(Cu) Deposits. The Role Of Hydrothermal Fluids In Mineralized Mafic Rocks. Summary And Further Reading. Part II: Hydrothermal Processes:. 3. Hydrothermal Ore-Forming Processes:. Introduction. Other Fluids In The Earth's Crust And Their Origins. The Movement Of Hydrothermal Fluids In The Earth's Crust. Further Factors Affecting Metal Solubility. Precipation Mechanisms For Metals In Solution. More On Fluid/Rock Interaction: An Introduction To Hydrothermal Alteration. Metal Zoning And Paragenetic Sequence. Modern Analogues Of Ore-Forming Processes: The VMS-SEDEX Continuum. Mineral Deposits Associated With Aquo-Carbonic Metamorphic Fluids. Ore Deposits Associated With Connate Fluids. Ore Deposits Associated With Near Surface Meteoric Fluids (Ground Water). Summary And Further Reading. Part III: Sedimentary/Surficial Processes:. 4. Surficial and Supergene Ore-Forming Processes:. Introduction. Principles. Lateritic Deposits. Clay Deposits. Calcrete-Hosted Deposits. Supergene Enrichment Of Cu And Other Metals In Near-Surface Deposits. Summary And Further Reading. 5. Sedimentary Ore-Forming Processes:. Introduction. Clastic Sedimentation And Heavy Mineral Concentration: Placer Deposits. Chemical Sedimentation: Banded Iron Formation, Phosphorites, And Evaporites. Fossil Fuels: Oil/Gas Formation And Coalification. Summary And Further Reading. Part IV: Global Tectonics And Metallogeny:. 6. Ore Deposits in a Global Tectonic Context:. Introduction. Patterns In The Distribution Of Mineral Deposits. Continental Growth Rates. Crustal Evolution And Metallogenesis. Metallogeny Through Time. Plate Tectonics And Ore Deposits: A Summary. Summary And Further Reading. References. Index