Chloritic Breccia Research Articles

A shear-hosted Au-Cu-Bi metallogenic event at ~1660 Ma in the Tennant Creek goldfield (northern Australia) defined by in-situ monazite U-Pb-Th dating

High-grade Au, Cu and Bi ores in the Tennant Creek goldfield have been mined from hydrothermal magnetite- and/or hematite-rich ironstone bodies. Less well known is a style of Au-Cu-Bi mineralisation hosted by quartz vein systems within shear zones outside ironstones. We report ion probe (SHRIMP) U-Pb-Th ages of 1659 ± 13 Ma and 1659 ± 15 Ma for hydrothermal monazite associated with this mineralisation style at the Orlando East Au-Cu-Bi deposit and Navigator 6 Au prospect, respectively. The dated monazite at Orlando East is intergrown with Cu and Bi sulfides and Au in quartz-chlorite veins within a shear zone. At Navigator 6 the monazite occurs both in late-stage chalcopyrite-bearing quartz-hematite veins and within earlier-formed sheared chloritic breccia that also contains anomalous Au.Although the full significance of the ~1660 Ma monazite ages has yet to be determined, the results indicate the introduction of Au, Cu and Bi in quartz vein systems at ~1660 Ma, or remobilisation of earlier, ~1850 Ma, main-stage Au-Cu-Bi from mineralised ironstones and into quartz veins 10 s–100 s of metres from the initial mineralisation. In either case, hydrothermal fluids capable of transporting significant concentrations of ore metals were present in the Tennant Creek goldfield at ~1660 Ma, synchronous with deformation. A key implication is that rock sequences up to ~200 million years younger than the ironstone-hosted Au-Cu-Bi deposits may now be considered prospective for a second stage of Au ± Cu ± Bi mineralisation. The results also validate non-ironstone shear-hosted deposits in the Tennant Creek region as an alternative exploration target to the well known high-grade but low-tonnage Tennant Creek ironstone-hosted deposits.

The Louzidian Normal Fault near Chifeng, Inner Mongolia: Master Fault of a Quasi-Metamorphic Core Complex

A special metamorphic core complex underlain by a low-angle strike-slip ductile shear zone is present near Chifeng in eastern Inner Mongolia, northern China. The geology of the study area is similar to that of several Cordilleran metamorphic core complexes, but contrasts in significant ways as well. A major ESE-dipping normal fault, the Louzidian Range frontal fault, formed during Late Cretaceous extension. This fault separates a crystalline footwall locally containing mylonitic basement gneisses and granitic rocks (0 to >3 km thick) from a non-metamorphic hanging wall that is distended by normal faults. However, the shear sense of the underlying mylonitic shear zone, a low-angle strike-slip zone, is not compatible with the Louzidian fault. It may be related to a pre-Cretaceous regional sinistral strike-slip event rather than the Late Cretaceous regional crustal extension common throughout eastern China. Pre-existing mylonitic fabric anisotropy appears to have controlled the development of the Louzidian normal fault. Chloritic breccias locally developed along the fault indicate that it cut deeply into the crust of northern China.

Neogene exhumation of the Variscan Szeghalom Dome, Pannonian Basin, E. Hungary

AbstractThe Pannonian Basin consists of several deep sub‐basins separated by uplifted basement highs. One of the largest and deepest sub‐basins is the Békés Basin in SE Hungary which is surrounded to the north by several crystalline domes. Of these, the Szeghalom Dome is the best known petrologically, and has been used for studying the Neogene exhumation history of the area.On the basis of metabasite bulk chemistry, the Szeghalom Dome can be subdivided into four distinct zones, the borders of which correspond closely to geophysical anomalies. The four zones exhibit different pre‐Variscan and Variscan pressure–temperature–time (P–T–t) histories. In zone I, high‐pressure relics (600°C, > 12 kbar) occur. Peak‐metamorphic conditions of the main Barrovian overprint were c. 700°C and 6 kbar, and amphibole K/Ar ages scatter around 260 Ma. Zone II shows similar characteristics, with the formation of anatectic granite at peak conditions. K/Ar ages are as low as 220 Ma. Zones III and IV show similar evolution to each other with high‐grade Barrovian relics (700°C), peak metamorphism at 570°C and amphibole ages of 300 Ma.Throughout the dome, fingerprints of a three‐fold Neogene tectonic evolution can be observed. An early detachment fault formed quartz–feldspar–mylonite in zone IV, while chloritic breccia developed at the border of zones II and III. Following the thermal subsidence phase of the basin evolution, the area was dominated geodynamically by the reactivation of older strike‐slip fault systems. These movements were responsible for the horizontal transport of the crystalline blocks at both the southern and northern flanks of the dome. Copyright © 2000 John Wiley & Sons, Ltd.

Structural development of the Mid-Tertiary Doi Suthep Metamorphic Complex and Western Chiang Mai Basin, Northern Thailand

The northern Western Ranges of Thailand contain mylonitic gneisses of the Doi Suthep Metamorphic Complex (DSMC). Near Chiang Mai, mylonitic foliations outline a southeast-trending dome, and stretching lineations trend N80°W. Low-grade metasedimentary rocks and Tertiary fanglomerates flank the dome on the east, and widespread chloritic breccia marks the intervening Doi Suthep detachment fault. We interpret that the metamorphic complex developed between Triassic and early Miocene, with detachment faulting, mylonitization, and uplift during Oligocene to Miocene. Within the DSMC, dikes of mutually cross-cutting, hence coeval, granitic pegmatite and aplite cut mylonitic orthogneiss. Although the contacts of the dikes-are sharp, the dikes contain a mylonitic fabric that parallels the fabric in the surrounding mylonitic orthogneiss; thus, we infer that intrusion occurred during mylonitization. East of Doi Suthep, Tertiary fanglomerates of the Mae Rim Formation overlie the Doi Suthep detachment fault. Variable strikes, with dips of 10°–50°, and several outcrop-scale fold hinges suggest that the Mae Rim Formation is deformed into a set of non-cylindrical folds. Numerous, variably-striking, high-angle normal faults suggest either multiple deformations or deformation in a three-dimensional strain field. The discovery of an intraformational angular unconformity confirms that the Mae Rim Formation accumulated during folding.

Footwall refrigeration along a detachment fault: implications for the thermal evolution of core complexes

Oxygen isotope compositions of epidote and quartz from chloritic breccias that underlie the detachment fault in the metamorphic core complex of the Whipple Mountains yielded quartz-epidote fractionations that range from 4.1 to 6.4 per mil and increase systematically toward the fault. These fractionations give mean temperatures that decrease from approximately 432 degrees C at 50 meters below the fault to approximately 350 degrees C at 12 meters below the fault. This extreme thermal gradient of 82 degrees C over 38 meters (2160 degrees C per kilometer) is best explained by advective heat extraction by means of circulating surface-derived fluids. Models of lithospheric extension consider only conductive cooling resulting from tectonic denudation and thus require revision to include fluid-induced fault-zone refrigeration.

Late Devonian-Carboniferous detachment faulting and extensional tectonics in western Cape Breton Island, Nova Scotia, Canada

Shallow-dipping detachment faults have played a key role in the evolution of the Late Devonian to Carboniferous Maritimes Basin of western Cape Breton Island. The faults are responsible for large-scale post-orogenic crustal thinning and denudation of mid-crustal levels. Tournaisian and Westphalian clastic sedimentation can be related to detachment faulting along the Margaree Shear Zone (MSZ) and the Ainslie Detachment (AD), respectively. The MSZ consists of thick shallow-dipping retrogressive mylonite overprinted by cataclastic horizons, brittle detachment faults, and chloritic breccia. The shear zone crosscuts and transports low-grade Late Devonian bimodal volcanic rocks and redbeds of the Fisset Brook Formation in its hanging wall towards the west, and juxtaposes them against exhumed medium- to high-grade metamorphic rocks and basement in its footwall. Locally the mylonite is 200–700 m thick and outcrops along strike from north to south for approximately 60 km. Tournaisian coarse clastic units and debris flow deposits of the Horton Group unconformably overlie ductile mylonite of the MSZ due to incision of brittle faults into the MSZ. Regional transgression, and marine flooding of the extensional complex and exhumed units are marked by the thick Visean carbonate and evaporite deposits of the Windsor Group. The carbonates reflect a relatively quiescent tectonic regime, and widespread subsidence was likely due to the cessation of extension-related thermally induced buoyancy in a tectonically thinned crust. The Ainslie Detachment nucleated as a regional flat-lying fault along the carbonate-evaporite contact at the top of the Macumber Formation, forming calc-mylonite near the base of the Windsor Group. Significant stratigraphic omissions in the Windsor Group above the detachment provide clear evidence of an extensional regime during faulting. Clastic deposits of the Westphalian Cumberland Group are bounded by the detachment, and sedimentation is related to listric normal faults which cut down-section towards the west-southwest. The documentation of detachment faults has widespread implications for the interpretation of the post-orogenic evolution of the northern Appalachians. An extensional regime can now be recognized. Deep reflection seismic data are interpreted in light of the new findings and can be related to Late Devonian—Carboniferous basin formation.

Seismic reflectivity of the Whipple Mountain shear zone in southern California

The Whipple Mountains shear zone of southeastern California comprises part of the Whipple Mountains metamorphic core complex of the North American Cordillera. The 3.9‐km‐thick shear zone displays excellent exposures of ductilely deformed mid‐Tertiary mylonitic gneisses, as well as kinematically related, but younger cataclasites which underlie a major low‐angle normal fault (the Whipple detachment fault). Prominent crustal reflections beginning at 3–4 s were recorded on CALCRUST seismic profiles southwest of the Whipple Mountains. In order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others, we collected oriented rock specimens from all major structural units in the Whipple Mountains and determined P wave velocities for these samples parallel to three principal fabric directions under laboratory‐induced confining pressures up to 500 MPa. Using a geologic section through the mylonitic gneisses from a previous investigation, we then constructed a detailed acoustic impedance section for the Whipple Mountain shear zone based upon laboratory results. This section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the CALCRUST records. Significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses. The finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the “seismic fabric” of strong, laterally discontinuous but subparallel reflections as seen in the CALCRUST profiles southwest of the Whipple Mountains. Chloritic breccias beneath the Whipple detachment fault (WDF) have much lower seismic impedance than that of rocks occurring both structurally above and below. However, the significant thinning of the chloritic breccia zone to the southwest of the Whipple Mountains may have caused the poor image of the WDF on the CALCRUST profiles. The absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone.

K-metasomatism and detachment-related mineralization, Harcuvar Mountains, Arizona

The Bullard detachment fault, a regional low-angle normal fault exposed in the Harcuvar Mountains of west-central Arizona, separates lower-plate mylonitic rocks and chloritic breccia from upper-plate volcanic and sedimentary rocks. Areally extensive K-metasomatism has converted upper-plate mafic flows and felsic ash-flow tuffs into rocks with 8 to 12 wt. % K 2 O, 2 O, and a simple K-feldspar-hematite-quartz mineralogy. The secondary K-feldspar is very pure (Or 95 to 99.5), monoclinic, and structurally similar to orthoclase. Differences in δ 18 O values between secondary K-feldspar replacing sanidine phenocrysts (9‰ to 11‰) and K-feldspar replacing groundmass (11‰ to 14‰) in the tuffs imply differential O-exchange with migrating fluids. Whole-rock δ 18 O values for tuffs (10‰ to 14‰) and mafic flows (6‰ to 9‰) do not, therefore, represent primary igneous values. The rocks apparently became K-metasomatized and 18 O-enriched while interacting with low- to moderate-temperature, neutral to alkaline, oxidizing brines that accumulated in an extensional basin above the detachment fault. Cu-Au-Ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault. Fluid-inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 °C along the detachment fault, 240 to 290 °C in mafic flows above the detachment fault, and 100 to 130 °C in barite-calcite-Mn-oxide veins farther from the detachment fault. The predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt.% NaCI More dilute brines with 6 to 12 equivalent wt. % NaCI formed the barite-calcite-Mn-oxide veins. Inferred δ 18 O values of the mineralizing fluids range from +3‰ for high-temperature quartz-sulfide veins to -5‰ for lower-temperature barite-calcite-Mn-oxide veins. The high salinities, oxygen isotope compositions, and geologic setting indicate that the mineralizing fluids were basinal brines. The mineralizing fluids apparently evolved from early deep-level, reducing basin brines to a later stage marked by the influx of higher-level, oxidizing basin brines. Relatively minor amounts of less evolved, lower- 18 O meteoric water entered the system during the very late stages of mineralization. Paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously K-metasomatized rocks. Mineralization and K-metasomatism may be indirectly linked, however, because both occurred during detachment faulting and both involved basinal brines. Specifically, K-metasomatism of upper-plate units liberated elements, such as Cu, Pb, Zn, Mn, Sr, and Na, that were incorporated into the mineralizing basin brines. Multiple fluid regimes during detachment faulting are indicated, because basin-brine-dominated mineralization overprinted lower-plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system.

Detachment zones of Cordilleran metamorphic core complexes: thermal, fluid and metasomatic regimes

Metamorphic core complexes in Arizona are characterized by extensional tectonics, in which listrically faulted and unmetamorphosed volcanic rocks of an upper plate are tectonically superimposed on high-grade rocks of a lower plate along low angle detachment faults. The detachment faults are marked by microbreccias and chloritic breccias which developed by hydraulic fracturing and metasomatism that overprinted subjacent amphibolite facies mylonites. Oxygen and hydrogen isotope data indicate that the mylonites formed at ∼500 °C under conditions of low water/rock ratio. Chloritic breccias developed at 300–350 °C during incursion of hydrothermal fluids along the detachment fault. Hydrothermal alteration involved massive additions of Fe, Mn, and Mg, with concomitant depletions of K and Na, reflecting the hydrolysis of K-feldspar and plagioclase to chlorite.

The transition from amphibolite-facies mylonite to chloritic breccia and role of the mylonite in formation of Eocene epizonal plutons, Bitterroot dome, Montana

The Bitterroot mylonite is a ductile-deformed amphibolitefacies mylonite (A-mylonite), abruptly capped by ductile- to brittle-deformed greenschist-facies mylonite (G-mylonite). The movement picture of the A-mylonite from its lineation and S-C-surfaces is strongly focused; the average orientation for the G-mylonite is similar but much more diffuse.

Fluid motion associated with Tertiary mylonitization and detachment faulting: 18O/16O evidence from the Picacho metamorphic core complex, Arizona

Major crustal detachment faults of Tertiary age in the Picacho metamorphic core complex of southern Arizona demark three tectonic plates in a structural section characterized by the transition from undeformed granitic basement through a mylonitic carapace to brecciated and hydrothermally altered counterparts. A lower detachment fault defines the top of the lower plate and is coplanar with the overprinting of mylonites to chloritic breccias. Variably altered and fractured granite of an overlying middle plate is bounded by an upper detachment fault on which allochthonous Miocene volcanics have been superposed; the volcanics display intense oxidative potassic alteration. This overall transition is accompanied by a 10‰ increase in δ18O and a 400 °C decrease of temperature. Undeformed Oracle Granite of the lower plate (δ18O ≈7.8) retains near-magmatic mineral fractionations, but these are disturbed in mylonitic equivalents, where ambient temperatures were ∼520 °C, δ18OH2O ≈4‰, and conditions of low water/rock ratios prevailed. Chloritic breccias record a whole-rock shift to +8.5‰, accompanied by diminished temperatures of 300 °C, and the infiltration of fluids where δ18OH2O = 3‰ ± 1‰ under conditions of high water/rock ratios. A pronounced isotopic discontinuity occurs at the upper detachment fault where intensely oxidized and K-metasomatized volcanics of the upper plate are shifted to 18‰ at temperatures that diminished to ∼ 150 °C. Two distinct fluid reservoirs were involved in alteration of the lower two plates and the upper plate, respectively, the tectonic section recording an upward transition from high to low temperature, low to elevated water/rock ratios, ductile creep to brittle fracturing, and an interface of deep “exchanged” fluids with a shallow oxidized surface aqueous reservoir.

Thermal and fluid regimes in the Bitterroot lobe–Sapphire block detachment zone, Montana: Evidence from 18O/16O and geologic relations

In western Montana, a 15-km-thick sequence of allochthonous Belt Series metasedimentary rocks is thought to have moved eastward off the Northern, or Bitterroot, lobe of the emergent Idaho batholith. The zone of detachment south of Missoula, defined by mylonitic rocks ∼500 m thick, is largely in the batholith but traverses the granite-metasediment boundary. Later chloritic breccias, characterized by intense transgranular fracturing, overlie and locally transect the mylonites. In granite and pegmatite mylonites, quartz (δ18O = 10.4 to 11.1) and coarse muscovite (8.4‰) yield isotopic temperatures of 550 ± 50 °C, interpreted as the temperature of ductile deformation, and would have involved fluids of 9.5 ± 0.5‰, which is close to the magmatic or high-temperature metamorphic range. K-feldspar (7.4 to 1.3‰), fine muscovite (8.3 to 6.8‰), and biotite (2.0 ± 0.2‰) have been shifted by variable magnitudes to low δ18O, which reflects post-mylonite, meteoric-water incursion resetting, less retentive minerals, an effect also observed in undeformed precursors to mylonites. Mylonitic metasedimentary rocks 25 km from the Bitterroot Lobe have not been isotopically disturbed by exchange with meteoric water but retain isotopic concordancy for coexisting quartz, k-feldspar, and muscovite, corresponding to estimated temperatures of 450 to 500 °C the ambient conditions of ductile deformation. In chloritic breccias, quartz and feldspar have undergone shifts of about −10‰ relative to their values in mylonitic granites and exhibit disequilibrium fractions (Δ = 5.3 to 9.6‰), due to preferential exchange of feldspar down to lower temperatures. Albite and chlorite yield temperatures of 370 °C (Δ = 4.5‰) to 250 °C (Δ = 6.3‰), and calculated δ18O of fluids in equilibrium with albite is −7 (370 °C) to −11.8 (250 °C). This temperature range is corroborated by fluid-inclusion data. High deduced temperatures and fluid isotopic compositions in the mylonites are commensurate with a pluton roof-zone environment at near magmatic conditions, providing enhanced ductility in mylonites. The chloritic breccia is regarded as a structural domain that accommodated late movement of the overlying rocks subsequent to removal of the main cover and provided conduits for incursion of low-temperature meteoric waters. The structural sequence reflects the change from high-temperature ductile deformation of mylonites, under conditions of crust-equilibrated fluids at low water/rock ratio, to a regime of brittle fracturing at diminished temperatures and lower confining stress. In the latter structural environment, hydrological communication to the surface promoted elevated water/rock ratios in the domains of fracturing.

Cordilleran metamorphic core complexes in Arizona: A contribution from geomorphology

Cordilleran metamorphic core complexes consist of granitic cores surrounded by, in an outward direction, mylonite, chloritic breccia, and microbreccia. These rocks are separated by a low-angle detachment fault from typically unmetamorphosed but normally faulted rocks. Rocks above the detachment fault have been transported, relative to the core rocks, in one consistent direction, giving a distinct lineation to rocks below the detachment fault. Dome-shaped metamorphic core-complex mountains show considerable structural control of landforms. Mylonitic rocks develop triangular dome facets around the margins and low relief areas in the central parts of the complexes. Chloritic breccia is much less resistant to erosion and has been almost completely removed from many complexes. Drainage patterns in metamorphic core complexes are mainly radial, but an older set of drainage lines parallel to mylonitic lineation, especially channels that drain down the crests of antiformal ridges, suggests that drainage developed on a ramp on the detachment fault and the underlying breccia prior to lineation-parallel arching.

Origin of breccias in the Mount Painter area, South Australia

Abstract Field and petrographic studies on granitic, hematitic and chloritic breccias in the central portion of the Mount Painter Inlier, South Australia, indicate that: (i) breccias and brecciated basement extend to depths exceeding 400 m and have gradational contacts; (ii) clasts are mainly autochthonous and contain fine‐scale hematite, chlorite or quartz veinlets and fractures; (iii) K‐metasomatism preceded hematitisation and chloritisation; (iv) hematitic breccia intrudes a pegmatite dyke correlated with the Ordovician Arkaroola Pegmatite; and (v) U, F and REE‐containing minerals are present in the Proterozoic basement rocks, and concentrated in the breccias. With a single exception, δ34S values for pyrite from the breccias and brecciated granites fall in the narrow range —2.9% to +3.5%, implying formation from magmatic emanations or reducing fluids that leached sulphide minerals of magmatic derivation. δ34S values for three barite samples are all close to +16%o, and firm conclusions cannot be drawn f...