Widespread Brecciation Research Articles

Geology and Structure of the Río Blanco Cu-Mo Porphyry Deposit, Central Chile

Abstract The Río Blanco-Los Bronces deposit is the largest Cu-Mo porphyry deposit in the world in terms of contained Cu metal. It is the product of protracted superposed magmatic and hydrothermal activity associated with multiple intrusive and brecciation events, with simultaneous regional uplift, erosion and unroofing, and decompression. Magmatism resulted in three major mineralization-alteration stages. The premineralization stage occurred during the emplacement of the San Francisco batholith, resulting in late magmatic and early hydrothermal events. The synmineralization stage corresponds to the main hydrothermal events associated with the Río Blanco-Los Bronces porphyry and breccia complexes, which were related to three intrusion phases, widespread brecciation, and an epithermal-style advanced argillic alteration. Late-stage magmatism, followed by hydrothermal activity, was associated with the emplacement of subvolcanic rhyolite complexes and late-stage porphyry intrusions. The synmineralization intrusions are associated with high-grade breccia bodies that have well-defined alteration-zonation patterns. Compilation and analysis of the historical Río Blanco structural data sets from the different mines, tunnels, and pits have allowed the assignment of all mapped structures to four hierarchical orders based on their continuity, crosscutting relationships, and infill compositions. The larger structures (orders 0 and 1) have along-strike continuity, correlate between drifts and/or mine levels, whereas smaller structures (orders 2 and 3) were grouped according to their dimensions and distributions within the larger-order structure-defined panels. All orders 0 and 1 structures were modeled in three dimensions, while orders 2 and 3 were in two dimensions. The structures mapped at Río Blanco have an intimate relationship with the pre- to the late-stage geologic evolution of magmatism and mineralization. The regional-and to a lesser extent district-scale structural evolution was related to premineralization basin-opening and subsequent tectonic inversion, whereas at the camp scale, syn- to late mineralization intrusions and related hydrothermal features were superimposed on this inherited structural architecture.

The Sukari Gold Deposit, Egypt: Geochemical and Geochronological Constraints on the Ore Genesis and Implications for Regional Exploration

AbstractThe Sukari gold deposit (>15 Moz Au) in the Eastern Desert of Egypt is hosted by a deformed granitoid stock (Sukari tonalite-trondhjemite intrusion) and mainly occurs as a network of crosscutting sulfide-bearing quartz (± carbonate) veins and intensely sulfidized-silicified-sericitized wall rock. Emplacement of the Sukari intrusion into a tectonized Neoproterozoic accretionary complex was controlled by a system of NE- to NNE-trending oblique faults that are related to a deep-seated positive flower structure. A robust genetic model has been hampered by the poorly understood relationships between gold mineralization and host rocks. In this study, zircon U-Pb ages of three samples from the Sukari intrusion define a crystallization age of ~695 ± 2 Ma. In contrast, hydrothermal sericite from the ore zone yields an 40Ar/39Ar age of ~625 ± 3 Ma, which coincides with the onset of major sinistral transpression in the region.Features including sigmoidal morphology of gold quartz veins and abundant subhorizontal tension gashes alongside widespread brecciation and recrystallization suggest that quartz veining occurred during renewed shortening and exhumation through the brittle-ductile transition. Petrographic and micro-X-ray fluorescence (µXRF) studies indicate that disseminated gold and sulfides, commonly associated with sericite and carbonate alteration, are mostly confined to stylolitic bands in the quartz veins. Oscillatory and sector zoning patterns, irregular As-rich bands, and truncations between early- and late-genetic pyrites reflect variations in temperature and mechanism of ore deposition, demonstrated by variable As/S and Co/Ni ratios in the different pyrite generations. Laser ablation-inductively coupled plasma-mass spectrometry analysis pinpoints the covariance of gold and arsenic contents in pyrite, but free milling gold inclusions in microfractures consistently have a mercury-bearing electrum composition, depicting different ore formation stages.Ore fluids with δ34SH2s values of –1.9 to –3.0‰, modeled from gold-associated pyrite and arsenopyrite assemblages with nearly identical δ34S values, suggest a likely single source of sulfur. Alternatively, multisourced sulfur could have extensively mixed and equilibrated by fluid reaction with carbonaceous wall rock. Gold deposition was triggered by abrupt changes in fluid pH and fO2. As an implication for future exploration, sites of maximized strain gradients adjacent to granitoid bodies along extensive transpression zones in the district could be highly prospective targets, particularly where imprinted by sericite-carbonate ± graphite alteration.

Geology, geochemistry and geochronology of the Songwe Hill carbonatite, Malawi

Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 × 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.

The conjunction of factors that lead to formation of giant gold provinces and deposits in non-arc settings

It is quite evident that it is not anomalous metal transport, nor unique depositional conditions, nor any single factor at the deposit scale, that dictates whether a mineral deposit becomes a giant or not. A hierarchical approach thus is required to progressively examine controlling parameters at successively decreasing scales in the total mineral system to understand the location of giant gold deposits in non-arc environments. For giant orogenic, intrusion-related gold systems (IRGS) and Carlin-type gold deposits and iron oxide-copper-gold (IOCG) deposits, there are common factors among all of these at the lithospheric to crustal scale. All are sited in giant gold provinces controlled by complex fundamental fault or shear zones that follow craton margins or, in the case of most Phanerozoic orogenic giants, define the primary suture zones between tectonic terranes. Giant provinces of IRGS, IOCG, and Carlin-type deposits require melting of metasomatized lithosphere beneath craton margins with ascent of hybrid lamprophyric to granitic magmas and associated heat flux to generate the giant province. The IRGS and IOCG deposits require direct exsolution of volatile-rich magmatic-hydrothermal fluids, whereas the association of such melts with Carlin-type ores is more indirect and enigmatic. Giant orogenic gold provinces show no direct relationship to such magmatism, forming from metamorphic fluids, but show an indirect relationship to lamprophyres that reflect the mantle connectivity of controlling first-order structures.In contrast to their province scale similarities, the different giant gold deposit styles show contrasting critical controls at the district to deposit scale. For orogenic gold deposits, the giants appear to have formed by conjunction of a greater number of parameters to those that control smaller deposits, with resultant geometrical and lithostratigraphic complexity as a guide to their location. There are few giant IRGS due to their inferior fluid-flux systems relative to orogenic gold deposits, and those few giants are essentially preservational exceptions. Many Carlin-type deposits are giants due to the exceptional conjunction of both structural and lithological parameters that caused reactive and permeable rocks, enriched in syngenetic gold, to be located below an impermeable cap along antiformal “trends”. Hydrocarbons probably played an important role in concentrating metal. The supergiant Post-Betze deposit has additional ore zones in strain heterogeneities surrounding the pre-gold Goldstrike stock. All unequivocal IOCG deposits are giant or near-giant deposits in terms of gold-equivalent resources, partly due to economic factors for this relatively poorly understood, low Cu-Au grade deposit type. The supergiant Olympic Dam deposit, the most shallowly formed deposit among the larger IOCGs, probably owes its origin to eruption of volatile-rich hybrid magma at surface, with formation of a large maar and intense and widespread brecciation, alteration and Cu-Au-U deposition in a huge rock volume.

The “Brecciated Limestones” of Maiella, Italy: Rheological implications of hydrocarbon-charged fluid migration in the Messinian Mediterranean Basin

The upper Messinian “Brecciated Limestones” of the Maiella area (Central Italy) reveal clues to the rheological behavior of the sedimentary column during fluid migration processes, as well as the timing of a major phase of methane release. Seep limestones have been studied worldwide in ancient and present-day settings. However, investigations of the complex rheologic behavior shown by the wide array of brecciated facies have received less attention. In this work special emphasis was placed on fabric and facies analyses, comparisons of the various scales of observation, and integrated stable isotope analyses (δ18O and δ13C), all within the context of the precisely-dated stratigraphy of the late Messinian sediments of the Mediterranean Basin.Above the basin scale Messinian Erosional Surface (MES), widespread brecciated limestone buildups and minor concretions constitute microbial carbonate bodies laterally embedded or passing to a marly-pelitic succession. In situ fluid-driven brecciation, together with the occurrence of fluid migration paths, are interpreted to represent vestiges of a mud volcano feeder system. However, chaotic textures testify to complex rheological behavior, where brittle and plastic responses in the same micro-environments co-occurred during fluid migration through a partially lithified sedimentary column.Different patterns of tar-bearing facies and degrees of brecciation, coupled with a geochemical dataset yielding wide ranges of δ18O (from +4 to −10‰ PDB-1) and δ13C values (from +4 to −40‰ PDB-1), point to two distinct stages of ascending hydrocarbon-charged fluids: in an earlier phase, fluids were mostly typified by low-molecular-weight hydrocarbons and induced widespread brecciation in the sedimentary column; in a later phase, fluid migration was accompanied by crude oil migration. During the early phase, fluid migration through the sedimentary column developed both: (a) with large vigorous fluxes, creating neoformed feeder channels and causing high degrees of brecciation; and (b) via slow seepage, favouring authigenic carbonate precipitation in the host sediment and causing negligible brecciation.In our proposed scenario, the Maiella area witnessed a hydrocarbon-rich fluid migration event (mainly methane) during the late Messinian, accompanied by authigenic carbonate precipitation and widespread brecciation. We suggest that this event was triggered by the sudden depressurization induced by the major drawdown of the Mediterranean Sea in late Messinian times.

Constraints on hydrothermal fluid pathways within Mary Kathleen Group stratigraphy of the Cloncurry iron-oxide–copper–gold District, Australia

Widespread brecciation and Na-(Ca) alteration in the metacarbonate-dominated Mary Kathleen Group characterise some of the district-scale host-rocks to iron-oxide–copper–gold (IOCG) mineralisation in the Proterozoic Cloncurry District of north-eastern Australia. Structural and mineralogical observations combined with stable isotope data indicate that within the Mary Kathleen Group, late- to post-metamorphism brecciation was preferentially developed in calcite-poor lithologies, while marbles and to a lesser degree calc-silicate rocks were not prone to fracturing and acted as impermeable barriers to fluid flow. These impermeable layers helped maintain high fluid pressures promoting brecciation in adjacent lithologies. Only in areas of localised high shear strain did carbonate-rich lithologies allow for appreciable fluid flow. Such carbonate-rich rocks contain isotopically and mineralogically distinct skarn-like assemblages, the recognition of which may be important in identifying high fluid flux corridors in the vicinity of major IOCG deposits.

Mineralogical and Isotopic Zonation in the Sur-Sur Tourmaline Breccia, Rio Blanco-Los Bronces Cu-Mo Deposit, Chile: Implications for Ore Genesis

The Sur-Sur tourmaline breccia is located in the southeast part of the Rio Blanco-Los Bronces porphyry copper- molybdenum deposit, central Chile. The breccia hosts approximately one-quarter of the total resource of 57 Mt of fine copper at Rio Blanco. The breccia is hosted within, and contains altered clasts of, granodiorite from the 12 to 8 Ma San Francisco batholith, which intruded a sequence of Miocene volcanic and volcaniclastic rocks. A series of weakly mineralized to barren felsic porphyries cut the breccia and indicate a minimum age of approximately 6 Ma for mineralization at Sur-Sur. The Sur-Sur breccia dike is at least 3 km long, 0.2 km wide, and has a vertical extent of at least 1 km. The breccia has been cemented by early biotite and anhydrite at depth and by tourmaline and specularite at higher altitudes. These early-formed cements have been overgrown and in some cases replaced by chalcopyrite, magnetite, pyrite, and quartz. Mineralogical zonation in the breccia includes a transition from biotite cement and related biotite alteration upward to tourmaline cement and quartz-sericite-tourmaline alteration at approximately 3,000-m elevation. Iron-oxide minerals are also zoned, with a transition upward from a magnetite-dominated zone below 3,330 m to a specularite-dominated zone above 3,600 m. Pyrite is the dominant sulfide at altitudes above 4,000 m. Secondary liquid-rich, vapor-rich, and hypersaline fluid inclusions are preserved in quartz and tourmaline cement. Measured homogenization temperatures are mostly between 300 degrees and 450 degrees C, and salinities range from 0 to 69 wt percent NaCl equiv. Sulfur isotope compositions of sulfide cement range from -4.1 to +2.7 per mil. The lowest delta 34S(sulfide) values are in samples from between 3,700- and 4,000-m elevation, where they correspond to the highest copper grades in the tourmaline breccia. This high-grade zone also contains abundant specularite (locally replaced by magnetite). Modeling of sulfate-sulfide equilibrium indicate that approximately 150 degrees C of cooling over a vertical interval of 100 m would be required to account for the zonation of sulfide isotope compositions at Sur-Sur, making conductive cooling an unlikely ore-forming mechanism. Measured 206Pb/204Pb values of lead in anhydrite cement in the Sur-Sur tourmaline breccia and the Rio Blanco magmatic breccia range from 17.558 to 18.479. 207Pb/204Pb values range from 15.534 to 15.623, and 208Pb/204Pb values range from 37.341 to 38.412. The lead in anhydrite is considerably less radiogenic than that indicated by values obtained previously for lead in sulfide ores and igneous host rocks at Rio Blanco-Los Bronces. The source of lead in anhydrite must have been from rocks external to the main magmatichydrothermal system, probably the Precordilleran basement. A magmatic-hydrothermal explosion from a deep-seated crystallizing intrusion triggered breccia formation at Sur-Sur. Hydrostatic pressures catastrophically exceeded lithostatic load plus the tensile strength of the confining granodiorite, leading to widespread brecciation and subsequent invasion by large volumes of magmatic gas and hypersaline brine. The low-density gas phase (carrying H2O, SO2, HCl, and B2O3) separated physically from the dense copper-bearing brine and flushed through the breccia column first, where it condensed into ground waters of uncertain derivation. Anhydrite, specularite, and tourmaline were deposited from this lowsalinity, acidic, oxidized hybrid solution. Subsequent upwelling of magmatic-hydrothermal brine resulted in sulfide deposition. High-grade copper deposition is interpreted to have occurred in response to mixing of the oxidized, acidic water with the copper-bearing magmatic-hydrothermal brine.

Carbon and oxygen isotope study of hydrothermal carbonates in the zinc-lead deposits of the San Vicente district, central Peru: a quantitative modeling on mixing processes and CO 2 degassing

Carbon and oxygen isotope studies of the host and gangue carbonates of Mississippi Valley-type zinc-lead deposits in the San Vicente District hosted in the Upper Triassic to Lower Jurassic dolostones of the Pucará basin (central Peru) were used to constrain maiels of the ore formation. A mixing model between an incoming hot saline slightly acidic radiogenic (Pb, Sr) fluid and the native formation water explains the overall isotopic variation (δ 13C = −11.5 to + 2.5‰ relative to PDB and δ 18O = + 18.0 to + 24.3‰ relative to SMOW) of the carbonate generations. The dolomites formed during the main ore stage show a narrower range (δ 13C = −0.1 to + 1.7‰ and δ 18O = + 18.7 to +23.4‰) which is explained by exchange between the mineralizing fluids and the host carbonates combined with changes in temperature and pressure. This model of fluid-rock interaction explains the pervasive alteration of the host dolomite I and precipitation of sphalerite I. The open-space filling hydrothermal white sparry dolomite and the coexisting sphalerite II formed by prolonged fluid-host dolomite interaction and limited CO 2 degassing. Late void-filling dolomite III (or calcite) and the associated sphalerite III formed as the consequence of CO 2 degassing and concomitant pH increase of a slightly acidic ore fluid. Widespread brecciation is associated to CO 2 outgassing. Consequently, pressure variability plays a major role in the ore precipitation during the late hydrothermal events in San Vicente. The presence of native sulfur associated with extremely carbon-light calcites replacing evaporitic sulfates (e.g., δ 13C = −11.5‰), altered native organic matter and heavier hydrothermal bitumen (from −27.0 to −23.0‰ δ 13C) points to thermochemical reduction of sulfate and/or thiosulfate. The δ 13 C- and ° 18O-values of the altered host dolostone and hydrothermal carbonates, and the carbon isotope composition of the associated organic matter show a strong regional homogeneity. These results coupled with the strong mineralogical and petrographic similarities of the different MVT occurrences perhaps reflects the fact that the mineralizing processes were similar in the whole San Vicente belt, suggesting the existence of a common regional mineralizing hydrothermal system with interconnected plumbing.

The geology of the Sulitjelma ore field, Northern Norway; some new interpretations

The 20 or more Sulitjelma orebodies are recognized as stratiform, strata-bound pyritic Cu-(Zn) sulfide ores, the products of volcanic-associated hydrothermal sedimentary exhalative formation. The ores are interpreted as having been formed at a single stratigraphic interval on the Ordovician sea floor. Each has a well-developed subjacent zone of alteration, in which the core of each alteration zone appears to be abnormally enriched in potassium relative to the surrounding strata. The core is enveloped by a more typical chloritic alteration zone characterized by an increasing Fe/(Fe + Mg) ratio away from the deposit. Distinct ore facies are recognized as products of primary hydrothermal zonation and of regional amphibolite--grade metamorphism and accompanying penetrative tectonic deformation. The metals in the ores are interpreted to result from leaching of the enclosing basalts. These show widespread physical and chemical effects of hydrothermal leaching. The widespread brecciation associated with the orebodies can partly be explained by tectonism; it may also be evidence of a cyclic catastrophic stage during the evolution of the sea-floor hydrothermal system. The Sulitjelma Gabbro Complex shows textural features in accordance with rapid cooling by seawater and it is believed that the cooling gabbroic magma represents the heat source which drove the hydrothermal convection cell.

Painirova and Mertainen iron ores; two deposits of the Kiruna iron ore type in northern Sweden

Apatite iron ores of the Kiruna type have been described from Painirova and Mertainen in northern Sweden. It is considered important that the ores are associated with volcanic rocks of a particular composition, i.e., iron-rich trachytes, which do not form part of a magmatic differentiation series. It is suggested that such a trachyte has been formed from the assimilation of iron-rich material during its formation. The ore, mostly in the form of magnetite, occurs in the host trachytes as disseminations, coarse aggregates, globular and spherical concentrations, and as a fracture infilling component. It is thought possible that the iron ores are the result of immiscibility aided by a high volatile content. The associated widespread brecciation in Painirova and Mertainen may have been caused by the mechanical effect of the iron-rich fluids in a differential stress field during consolidation of the trachyte.

The Creighton pluton, Ontario: an unusual example of a forcefully emplaced intrusion

The Creighton granitic pluton, which is located along the southern margin of the Sudbury Basin within the Southern Province, was affected by a local deformation event prior to the widespread brecciation which accompanied the formation of the Sudbury Basin ('Sudbury brecciation event'). Detailed mapping has shown that foliation trends within the eastern portion of the Creighton pluton form a complete elliptical loop parallel to the contact of the pluton. Within the foliation loop is a large deformed inclusion. The deformation within the inclusion is very in homogeneous, and two sets of deformation structures, both of which predate the Sudbury brecciation event, can be recognized. The structures within the inclusion are totally absent from lithologically similar rocks only a few kilometres away, outside the pluton. The structure of the pluton and the inclusion indicate that the pluton was forcefully emplaced and that the inclusion was deformed by constriction during the emplacement of the pluton. The granitic rocks of the pluton must have been rigid enough to transmit stresses which were capable of deforming the rocks of the inclusion, and therefore it appears likely that the Creighton pluton was emplaced as a diapir.

Ozark Pre-Cambrian-Paleozoic Relations

Throughout most of the past century, the topographically high felsitic rocks of the St. Francois Mountains in southeastern Missouri have been interpreted as exhumed pre-Cambrian mountain peaks which had been buried beneath Cambrian and Ordovician sediments. Prevalence in part of the region of sub-horizontal (contouring) felsite-Paleozoic contacts which define a regional, mildly undulating surface, truncation of Cambrian and Lower Ordovician formations by these felsites, and abundant evidence that such contacts are tectonic rather than depositional indicate that these topographically high felsites lie above the Paleozoic strata, and are in regional thrust relationship. Among previously enigmatic, incongruous, or misinterpreted phenomena attributable to this tectonism are: (1) numerous klippen and other probable klippen composed almost entirely of pre-Cambrian igneous and metamorphic rocks; (2) northeasterly directed, shallow-rooted, compressional structure, including minor thrusting, in the autochthonous Paleozoic rocks; and (3) widespread brecciation, prolific silicification, lead-zinc, barite, and other mineralization, together with some alkaline-mafic intrusions. These associated phenomena involve surface-exposed strata from the Cambrian to the Permian, and occur in several areas including at least western Kentucky, southern Illinois, southern Missouri, northern Arkansas, northeastern Oklahoma, and eastern Kansas. A granite-felsite complex of autochthonous pre-Cambrian is exposed in the northeastern (structurally higher) part of the St. Francois Mountains. The roots of this great thrust (or thrusts) are most probably in the Ouachita geosynclinal belt of southern Arkansas and Oklahoma; the age of the thrusting appears to be latest Permian and(or) later--perhaps Appalachian.