Abstract The Paracatu deposit in Brazil is a shallowly dipping, bulk-tonnage, low-grade, vein-style orogenic Au orebody hosted in very strongly deformed Neoproterozoic carbonaceous phyllite of the southern Brasília fold belt. At regional to district scales, the gold orebody lies along the eastern, hanging-wall edge of a major thrust of the ~630 Ma Brasiliano orogeny. This thrust cuts through a facies transition between clastic-dominated rocks of the Canastra Group and carbonate-dominant rocks of the Vazante Group, deposited at ~1000 Ma in a rift to passive-margin environment on the flank of the São Francisco craton. At the same scales, the footwall of this major thrust system hosts numerous structurally controlled zinc deposits including Vazante and Morro Agudo. At Paracatu, ore genesis occurred primarily by the formation of early tectonic quartz sulfide-carbonate veins, prior to substantial ductile deformation (boudinage), local physico-chemical reworking of these veins, and redistribution of some gold. Structural, geochemical, and isotopic data indicate a strong influence of the local rocks (cm to 100-m scales) on many ore ingredients, and the quartz and carbonate in ore veins were most likely derived locally (cm to m scales). However, the coassociation of gold and arsenic with the boudinaged veins and a major thrust, and the absence of metal enrichments normally associated with syngenetic metalliferous black shales, supports a model of far-field derivation of gold within this metasedimentary package (km to 10-km scales). Transport of metal-bearing fluids toward a favorable structural and chemical site during thrusting and orogenesis was possibly focused, during precipitation to ore grades, by the position of transverse structures in the basement, which also influenced deposition of the adjacent zinc deposits. Successful mining of the low-grade resource was initially favored by the subhorizontal orebody geometry and weathering characteristics, and subsequently by high production rates from the 100-m-thick mineralized zone.

Abstract Normal faults commonly represent one of the principal controls on the origin and formation of sedimentary rock-hosted mineral deposits. Their presence within rift basins has a profound effect on fluid flow, with their impact ranging from acting as barriers, causing pressure compartmentalization of basinal pore fluids, to forming conduits for up-fault fluid flow. Despite their established importance in controlling the migration and trapping of mineralizing fluids, we have yet to adequately reconcile this duality of flow behavior and its impact on mineral flow systems within basinal sequences from a semiquantitative to quantitative perspective. Combining insights and models derived from earthquake, hydrocarbon, and mineral studies, the principal processes and models for fault-related fluid flow within sedimentary basins are reviewed and a unified conceptual model defined for their role in mineral systems. We illustrate associated concepts with case studies from Irish-type Zn-Pb deposits, sedimentary rock-hosted Cu deposits, and active sedimentary basins. We show that faults can actively affect fluid flow by a variety of associated processes, including seismic pumping and pulsing, or can provide pathways for the upward flow of overpressured fluids or the downward sinking of heavy brines. Associated models support the generation of crustal-scale convective flow systems that underpin the formation of major mineral provinces and provide a basis for differences in the flow behavior of faults, depending on a variety of factors such as fault zone complexities, host-rock properties, deformation conditions, and pressure drives. Flow heterogeneity along faults provides a basis for the thoroughly 3D flow systems that localize fluid flow and lead to the formation of mineral deposits.

Abstract The Neoproterozoic successions of the São Francisco Craton are primarily represented by the Bambuí and Una groups, deposited in cratonic epicontinental basins, and by the Vazante and Vaza Barris/Miaba groups, which accumulated on passive margins on the edges of the craton. The epicontinental basins comprise three megasequences: glaciogenic, carbonate platform (marine) and dominantly continental siliciclastics. Possible correlative sequences are observed in the passive margin deposits. At least two major transgressive–regressive sea-level cycles occurred during the evolution of the carbonate megasequence, which lies above glaciomarine diamictites of probable early Cryogenian (i.e. Sturtian) age. C, O, Sr and S isotope trends from analyses of well-preserved samples, together with lithostratigraphic observations, provide reasonable correlations for most of the Neoproterozoic successions of the São Francisco Craton. The 87 Sr/ 86 Sr record of these successions, ranging from 0.70769 to 0.70780, supports the proposed correlation with the Bambuí, Una and Vaza/Barris successions, and with the basal units of the Vazante Group. In addition, C-isotope positive excursions ranging from +8.7 to +14‰ and negative excursions from –5.7 to –7‰ VPDB in the Bambuí, Una and Vaza-Barris successions provide key markers for correlations. The precise ages of the sedimentation in these successions remains a matter of debate, but organic shales of two units of the Vazante Group have been dated by Re–Os techniques in two different laboratories, both yielding Mesoproterozoic ages. The Neoproterozoic and Mesoproterozoic successions preserve significant glaciogenic deposits.

Abstract The Quadrilátero Ferrífero district, located on the southern portion of the San Francisco craton in Minas Gerais, Brazil, comprises Archean greenstone terranes of the Nova Lima Supergroup and the Paleoproterozoic cratonic cover sequences of the Minas Supergroup that consist of quartzites, metaconglomerates, phyllites, dolomites, and banded iron formations. The Minas Supergroup was affected by two orogenic events—the Paleoproterozoic Transamazonian-Mineiro (2.1–2.0 Ga) orogeny and the Neoproterozoic to Early Paleozoic Brasiliano-Araçuaí (0.65–0.50 Ga) orogeny, resulting in complex deformation and metamorphic grades that increase from greenschist facies in the West to amphibolite facies in the East. Metamorphosed iron formations, referred to as itabirites, are found in three compositionally distinct lithofacies, namely quartz itabirite, dolomitic itabirite, and amphibolitic itabirite; these lithofacies are host to a large number of economically important high-grade iron ore deposits that give rise to the name Quadrilátero Ferrífero, or "Iron Quadrangle." High-grade iron ores replace itabirites in tectonically favorable, low-strain sites. faults acted as conduits while large fold hinges were sinks for mineralizing fluids. Hard and fine-grained hematite and/or magnetite orebodies are in the western low-strain domain of the Quadrilátero Ferrífero. Subsequent deformation led to recrystallization and development of distinctly schistose high-grade hematite ores characteristic of the eastern high-strain domain. A combination of hypogene and geologically recent supergene processes is thus invoked to explain the formation of the high-grade iron ores of the Quadrilátero Ferrífero. Three stages of hypogene ore formation are distinguished. The first two of these stages took place early during the Transamazonian orogeny (2.1–2.0 Ga) and are well preserved in the western low-strain domain. During the first stage metamorphic fluids leached SiO 2 and carbonates and mobilized iron, which resulted in the formation of massive magnetite bodies, Fe oxide veins, and Fe-rich itabirite bodies; during the second stage, low-temperature, low-salinity fluids caused oxidation of magnetite and Fe-rich dolomite to hematite. The resulting ore is porous to massive and has a granoblastic fabric. The third and final hypogene stage of ore formation is related to thrusts of uncertain age (Transamazonian or Brasiliano orogeny), which dominate the tectonic structure of the eastern high-strain domain of the Quadrilátero Ferrífero. Crystallization of tabular hematite and large platy specularite crystals that overprint the preexisting granular fabric in the presence of high-salinity hydrothermal fluids are characteristic of this stage. During the Neogene, supergene residual enrichment processes gave rise to the formation of soft to friable hematite orebodies. The larger soft orebodies that surround some smaller hard high-grade orebodies are typically associated with dolomitic itabirite. Together, both ore types comprise the giant high-grade iron deposits typical for the Quadrilátero Ferrífero, resulting from the superposition of both hypogene and supergene processes. Pure supergene deposits are considerably smaller and do not extend to deeper levels below the erosion surface.

Abstract Iron enrichment in banded iron formation (BIF)-hosted high-grade iron deposits is the final result of sequential removal or replacement of gangue minerals from the host by hydrothermal and supergene processes. Apart from the presence of the host BIF, structure is the most important control on the location of these deposits. Also, the distinct structural setup of the mineralizing environment results in iron ore of distinct textural features and consequently variable physical properties. In the Hamersley province of Western Australia pre-Upper Wyloo Group extensional faults are most often associated with high-grade hematite deposits in the Paleoproterozoic Brockman Iron Formation. The most important faults provide a fluid pathway between underlying dolomites of the Wittenoom Formation, through a sequence of shales and cherts, and into the overlying BIF. Iron ore in the Kaapvaal province of South Africa is hosted within BIFs of similar age to the Pilbara craton. The BIFs in the Kaapvaal province rest directly on dolomite, and Paleoproterozoic karst structures form the main spatial control on the high-grade iron ore. In contrast, low-angle thrust faults are the principal structural control on large deposits in the Marra Mamba BIF in the Hamersley province. These structures provided a more effective fluid pathway between the BIF and the overlying dolomites. A very similar structural scenario controls the very large Paleoproterozoic iron deposits in the Quadrilátero Ferrífero province in Brazil, although individual deposits are often highly complex due to postmineralization deformation during the Brasiliano orogeny. Structural reconstruction suggests that early structures, particularly thrust faults and tight folds that link a potential fluid source such as the dolomites of the Gandarela Formation with the underlying BIFs, form the most important control on ore formation in this province. Iron deposits hosted by Archean BIFs are less well understood. In the Carajás province of Brazil, fluids derived from granitoid intrusions are interpreted to have caused the initial hypogene alteration of the BIF which later focused the supergene ore fluids that led to high-grade hematite formation. Major structures that linked these granitoids with the BIF were crucial in the formation of the protores. In all these districts, mineralizing structures are those that provided the most effective link between a source of hydrothermal, silica-undersaturated fluids and iron formation, or allowed the influx of surface-derived meteoric waters to control the sites of ore formation in the BIF. Another important effect of structures is that they locally caused a differential pressure gradient during deformation and concentrated fluids into low-strain or dilational sites of iron ore formation. Most high-grade iron deposits formed close to (paleo)-unconformity surfaces and are, therefore, prone to rapid erosion. The structural setting can play a major role in preservation of these deposits. Ore deposits near normal faults in extensional grabens and karst structures are particularly favorable to ore preservation because the faults usually caused downthrow of the mineralized zones and burial by younger sediments. Compressional structures such as thrusts were far less favorable, because they usually caused uplift and erosion of the orebodies within them. Orebodies controlled by these structures require postmineralization preservation events, such as a major postore orogeny, or formed relatively recently, and therefore erosion did not progress far enough to erode them.

Abstract Sediment-hosted Pb-Zn deposits contain the world’s greatest lead and zinc resources and dominate worldproduction of these metals. They are a diverse group of ore deposits hosted by a wide variety of carbonate andsiliciclastic rocks that have no obvious genetic association with igneous activity. A range of ore-forming processes in a variety of geologic and tectonic environments created these deposits over at least two billion years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic to low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host rocks that includes stratiform, strata-bound, and discordant ores. These ores are divided into two broad subtypes: Mississippi Valley-type (MVT) and sedimentary exhalative (SEDEX). Despite the “exhalative” component inherent in the term “SEDEX,” in this manuscript, direct evidence of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX. The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores. The distinction between some SEDEX and MVT deposits can be quite subjective because some SEDEX ores replaced carbonate, whereas some MVT deposits formed in an early diagenetic environment and display laminated ore textures. Geologic and resource information are presented for 248 deposits that provide a framework to describe and compare these deposits. Nine of the 10 largest sediment-hosted Pb-Zn deposits are SEDEX. Of the deposits that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits and 15 MVT (excluding Irish-type) deposits. Despite the skewed distribution of the deposit size, the two deposits types have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn), with a fairly consistent ratio of about 10/1, regardless of the size of the deposit or district. Zinc grades are approximately the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits. The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX deposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT deposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits hosted in carbonate rocks are virtually indistinguishable. The distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon. The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in the Phanerozoic. MVT deposits dominantly formed in platform carbonate sequences typically located within extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intracontinental or failed rifts, and rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their host rocks; however, a few are close (<~5 m.y.) to the age of their host rocks. In the absence of direct dates for SEDEX deposits, their age of formation is generally constrained by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation. Fluid inclusion, isotopic studies, and deposit modeling suggest that MVT and SEDEX deposits formed from basin brines with similar temperatures of mainly 90° to 200°C and 10 to 30 wt percent NaCl equiv. Lead isotope compositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal sources. Lead isotope compositions do not provide criteria that distinguish MVT from SEDEX subtypes. However, sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVT sulfur isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive isotopic compositions from 0 to 20 per mil. Isotopic values for MVT ores extend over a wider range and include more data with negative isotopic values. Given that there are relatively small differences between the metal character of MVT and SEDEX deposits and the fluids that deposited them, perhaps the most significant difference between these deposits is their de-positional environment, which is determined by their respective tectonic settings. The contrasting tectonic setting also dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology, deposit morphology, and ore textures. Brief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a subset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct ore type (Irish type). There are no significant differences in grade tonnage values between MVT deposits and the subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical MVT ores. The dominance of bacteriogenic sulfur in the Irish ores commonly ascribed as uniquely Irish type is in fact no different from several MVT deposits or districts. A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain significantly higher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits are almost three times more enriched in Ag and one and a half times more enriched in Pb compared to other SEDEX deposits. Metamorphism is a characteristic feature but not a prerequisite for inclusion in the Broken Hill-type category, and known Broken Hill-type examples appear to occur in Paleo- to Mesoprotero-zoic terranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to support their inclusion as a separate category of SEDEX deposits.

We have evaluated the temporal distribution of Mississippi Valley-type (MVT) Zn-Pb deposits with special attention to the nature and number of deposits of Precambrian age. Our evaluation is based on the widely used model for MVT mineralization involving metal-bearing brines that lack reduced S and that deposit sulfides only where they encounter a reservoir of sulfide or where sulfate in the metal-bearing brine is reduced to sulfide. For MVT systems of this type, basins with abundant sulfate would be most favorable for development of MVT mineralization because these would allow transport of metals in sulfate-rich brines and deposition of metals in areas where the sulfate was reduced. Because abundant sulfate requires abundant atmospheric oxygen, the distribution of MVT deposits through time might reflect compositional changes in Earth's atmosphere, especially the suggested Great Oxidation Event (GOE). A compilation of new data for the Bushy Park-Pering district in the Transvaal Supergroup of South Africa, the world's oldest known MVT province, and published information on other Precambrian MVT deposits in the Ediacara, Berg Aukas/Abenab, Gayna River, Warrabarty, Nanisivik, Kamarga (Century), McArthur River (Coxco), Ramah, and Esker districts shows that they are generally similar in geologic setting and mineralogy to those in Phanerozoic rocks. Fluid inclusions in some Neo-proterozoic deposits, including Berg Aukas/Abenab, Gayna River, Warrabarty, and Nanisivik, record higher temperatures and salinities than found in most Phanerozoic deposits, possibly reflecting igneous activity or a more proximal basinal setting during Precambrian time. Fluid inclusion leachate data for several Precambrian MVT deposits suggest that their parent brines formed by evaporation of seawater, and S isotope compositions indicate that the S was derived largely from coeval seawater sulfate. Comparisons of data from all deposits show no evidence for a gradual increase in temperature or salinity backward through time, such as might be caused by higher heat flow during early stages of Earth history, although the magnitude of this effect might be lost in the uncertainty of most fluid inclusion measurements. These observations confirm that MVT deposits reflect the chemistry of their source basins, which are as old as 2.6 Ga. No MVT deposits or suitable host rocks of an older age are known. Precambrian MVT deposits do differ from their Phanerozoic analogues in the magnitude of mineralization. Precambrian deposits and districts formed at an estimated rate of 5.5 per billion years versus a significantly larger rate of ∼60 per billion years for Phanerozoic deposits, and the Phanerozoic deposits are considerably larger. Furthermore, the transition from low-magnitude, Precambrian-type to high-magnitude, Phanerozoic-type MVT mineralization took place at the beginning of Cambrian time rather than at the 2.3 Ga GOE. This appearance of widespread MVT mineralization is closer to the time at which sulfate concentrations in the world ocean are estimated to have reached present-day levels. Although these conclusions are subject to considerable uncertainty because of the limited number of Precambrian deposits, the lack of an increase in the frequency of MVT mineralization at the GOE suggests that widespread MVT mineralization requires higher levels of sulfate than could have been provided by this event, or that the appearance of sulfate in the ocean was considerably delayed. Finally, the presence of MVT deposits in basins that formed considerably before the GOE suggests that local sulfate concentrations were available at even early points in Earth's history.