Interflow Sediments Research Articles

Cheakamus basalt lavas, British Columbia: a Pleistocene record of rapid, continuous eruption within a mountainous drainage system

The Cheakamus basalts are a voluminous (1.65 km3) set of Late Pleistocene, valley-filling lavas erupted from a vent situated near present-day Conflict Lake, in the alpine Callaghan Valley near Whistler, British Columbia. Geochemical and petrographic properties suggest these olivine–plagioclase porphyritic basalt lavas derive from a single batch of magma affected by minor sorting of phenocrysts and xenocrystic plagioclase. Thirty-four sites sampled for paleomagnetic directions record a mean pole direction of 345.2°/73.0° (α95 = 1.3°) and show no statistical variation nor drift with stratigraphic position. These data suggest that the Cheakamus basalt lavas were emplaced in a single paleomagnetic moment—a period of time significantly less than 2000 years. 40Ar/39Ar geochronometry on three lava samples returns a weighted mean age estimate of 15.95 ± 7.9 ka (2σ) and field evidence, including well-glaciated lava flow surfaces overlain by till, indicate the eruption coincided with the early stages of the Fraser glaciation (∼20–18 ka). The lavas preserve features indicative of a landscape hosting diverse and dynamic paleoenvironments. Subaerial eruption of basalt lava filled an ice-free Callaghan Creek drainage system before inundating and damming of the paleo-Cheakamus River creating an upstream rising body of water. Periodic overtopping of the lava dam resulted in syn-eruptive intermittent flooding and overtopping of lavas expressed by discontinuous lenses of interflow sediment. Rare instances of enigmatic cooling columns may also indicate localized ice contact with glaciers that partially filled the Cheakamus Valley. The displacement of the modern Cheakamus River and the long-term damming and formation of Callaghan and Conflict lakes remain direct indicators of the control and impact these basaltic eruptions have had on the geomorphology of the present-day Callaghan and Cheakamus valleys.

The ~1.1 Ga St. Ignace Island complex, Northern Ontario, Canada: Evidence for magma mixing and crustal melting in the generation of Midcontinent Rift-related bimodal magmas and implications for regional metallogeny

Abstract The St. Ignace Island complex in Northern Ontario is a package of dominantly felsic rocks emplaced within the upper portions of the Osler Volcanic rocks of the ~1.1 Ga Midcontinent Rift System. The Osler volcanic rocks are predominantly tholeiitic basalts intercalated with rare interflow sediments and rhyolites. The St. Ignace Island complex is a ~26 km2 stock with a felsic core of quartz-feldspar-phyric rhyolites and dacites and an outer ring of anorthosite and gabbro. Textures at a variety of scales within the rocks of the complex show clear evidence of the mingling and mixing of partially crystallized mafic and felsic liquids. Two multigrain (zircon/baddeleyite) fractions from a sample of the gabbro define a Discordia line with an upper intercept date of 1107±8.9 Ma. The core of the complex consists of dacites and rhyolites with similar REE abundances with negative Nb anomalies whereas the surrounding mafic rocks are gabbros to monzogabbros that are less LREE-enriched than the felsic rocks but with similar HREE. Felsic units have a narrow range of 87Sr/86Sri (0.7032-0.7045) and 143Nd/144Ndi (0.51051-0.51057) whereas the mafic end members have similar 87Sr/86Sri (0.7040-0.7061) but more radiogenic 143Nd/144Ndi (0.51067-0.51085). Very well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene and some plagioclase phenocrysts from the complex. These represent some of the oldest unrecrystallized silicate melt inclusions described to date. Melt inclusions within quartz from the felsic volcanics are broadly rhyolitic in composition whereas MI from plagioclase in the mafic volcanics range from basalt to basaltic andesite; these felsic and mafic melt compositions are interpreted to represent the end-member liquids in the system and bulk rock analyses affirm mixtures of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in the mafic and felsic MI than in continental crust. Bulk rock metal concentrations are also significantly lower than in the MI, suggesting that the melt inclusions may preserve pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The whole rock and MI geochemistry of the St. Ignace complex are broadly similar to the Central Osler Group and, given the broad similar ages, suggests they may have been derived from a similar mantle source, but distinct from the source of rhyolites in the Black Bay Peninsula. The negative Nb anomalies and negative εNd values for the St. Ignace complex are consistent with mixing with older continental crust during ascent and emplacement. The rocks of the St. Ignace Island complex likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate the rhyolite end members. The felsic melts ascended to shallower levels in the crust where they mixed with mafic magmas derived directly from the deeper chamber. Generally, melt inclusions in the complex have very high Cu and Ag contents, similar to those observed in arc-related and extremely oxidized early rift-related rocks and may account for the world-class volcano-sediment-hosted Cu-(Ag) deposits within the rift and the presence of small porphyry-style deposits.

A simple recipe for red bole formation in continental flood basalt provinces: weathering of flow-top and flow-bottom breccias

Continental flood basalt (CFB) provinces such as the Deccan Traps show numerous inter-flow horizons called boles, most commonly red or brown. These have been variably interpreted as altered glassy bases of basalt flows, paleosols developed on exposed flow tops, altered volcanic ash beds, or local inter-flow sediments. We believe that each of these mechanisms may apply, singly or in combination, to specific red boles, and propose an additional mechanism, namely the weathering of flow-top and flow-bottom breccias. The breccias are inherently highly prone to weathering, and evidence for the mechanism is a close observed correspondence between breccia zones and red boles. The mechanism explains the absence of red boles in the compound pāhoehoe flows which characteristically lack breccias, and their abundance in rubbly pāhoehoe and ‘a’a lava flows. The latter observation has been misinterpreted as indicating significant eruptive hiatuses and a low eruption frequency. Importantly, even all breccia-derived boles do not have the same significance: those derived from flow-top breccias are paleosols, formed by subaerial weathering during eruptive breaks. Those derived from flow-bottom breccias, thus not in contact with the atmosphere, were formed due to groundwater ingress along flow contacts and fractures. These boles have developed slowly over long time periods. Significant development of all bole types must also have occurred in post-volcanism time and is arguably ongoing even today in the exposed and unexposed parts of CFB sequences. Recognizing multiple and complex origins of superficially similar-looking red boles is of critical importance in CFB volcanology and flow stratigraphy, and a warning sign to straightforward application of mineral chemical, mineral magnetic, geochemical, and similar data on boles to interpreting paleoweathering, paleoclimates, and paleoenvironments.

Magnesium isotopic composition of altered oceanic crust and the global Mg cycle

To investigate the behavior of Mg isotopes during low-temperature alteration of oceanic crust and to further understand its role in the global Mg cycle, we measured the Mg isotopic compositions (25Mg/24Mg and 26Mg/24Mg) of a set of samples of altered oceanic crust (AOC) recovered from the Ocean Drilling Program Hole 801C, the reference site for old crust (∼170 Ma) subducting in the Pacific. The measured δ26Mg values range from −1.70‰ to 0.21‰, deviating from that of pristine oceanic basalts (−0.25 ± 0.07‰). Composite samples of volcanoclastic breccia that have experienced relatively intense alteration have larger variation in δ26Mg values (−1.01‰ to 0.15‰) than composite samples of massive basaltic flows (−0.53‰ to −0.04‰), indicating significant Mg isotope fractionation during low-temperature alteration of the oceanic crust. Moreover, the upper off-axis basement has on average lower δ26Mg values (−1.70‰ to −0.04‰) than the lower on-axis basement (−0.16‰ to 0.21‰). These findings, combined with the co-variations between MgO content and FeO∗/CaO ratio and between δ26Mg and FeO∗/CaO ratio, suggest that formation of Mg-bearing minerals (i.e., saponite and calcite) during low-temperature alteration of the oceanic crust accounts for the highly variable δ26Mg of AOC. Early formation of saponite under anoxic condition preferentially takes up heavy Mg isotopes and accounts for Mg enrichment and relatively high δ26Mg in the on-axis basement. Subsequent precipitation of carbonates results in the dilution of Mg and relatively low δ26Mg in the off-axis basement. In addition, accumulation of carbonate-rich interflow sediments in the upper basement may contribute further to the low δ26Mg. A weighted average δ26Mg value of 0.00 ± 0.09‰ is estimated for the AOC at Site 801, implying that low-temperature alteration of oceanic crust drives the ocean to a lighter Mg isotopic composition, and thus requires additional carbonate precipitation to maintain a steady-state Mg isotopic composition of seawater. A mass balance calculation suggests that the Mg output flux due to low-temperature alteration of the oceanic crust equals ∼12% of the annual Mg riverine input, indicating that AOC is a significant sink of Mg in seawater. Our study further highlights that recycling of AOC with highly variable δ26Mg along with overlying marine sediments into the mantle through subduction may generate Mg isotopic heterogeneity in the mantle at small scales.

Paleomagnetism of the Teel basalts from the Zavkhan terrane: Implications for Paleozoic paleogeography in Mongolia and the growth of continental crust

A narrow extensional basin on the Zavkhan terrane of Mongolia exposes a >1.8-km-thick succession of basalt flows within the Teel Formation, along with rhyolites and interflow sediments. We present new U-Pb zircon ages of 446.03 ± 0.21 Ma (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) on a rhyolite in the Teel Formation and 286 ± 5 Ma (laser ablation–inductively coupled plasma–mass spectrometry) on a nearby granitic intrusion (Tonkhil Complex). New paleomagnetic data yield a magnetite remanence that is likely primary, acquired during cooling of flows. The mean direction is statistically improved after tilt corrections; however, the tilt test significance is limited given the low variation in tilt between flows. We interpret a second remanence, held by hematite, as an overprint that was likely acquired later in the Paleozoic Era. The tilt-corrected magnetite direction implies a paleolatitude of ∼20°, while the hematite overprint is equatorial in both geographic and tilt-corrected coordinates. The ca. 446 Ma Teel remanence is consistent with an Ordovician paleogeographic position near Siberia; however, the hematite direction requires subsequent drift to the equator, indicating that these Mongolian terranes were not continuously connected to Siberia, which moved away from the tropics during the Paleozoic Era. This result is consistent with biogeographic constraints and a previously proposed model wherein Amuria traveled with North China during the Permian Period and collided with Siberia during the Jurassic to Triassic closure of the Mongol-Okhotsk Ocean. In this model, continental growth occurred through the collision and oroclinal buckling of a ribbon continent rather than long-lived accretion on the margin of a major craton.

Hanging-Wall Vectoring for Buried Volcanogenic Massive Sulfide Deposits, Paleoproterozoic Flin Flon Mining Camp, Manitoba, Canada

The Paleoproterozoic Flin Flon mining district is one of the world’s most prolific volcanogenic massive sulfide (VMS) camps and includes a single stratigraphic interval that hosts the 85.5 million tonne (Mt) Flin Flon, 777, and Callinan Zn-Cu-(Au) deposits. Rapid seafloor burial of the VMS hydrothermal system by a thick succession of pillowed basalt resulted in the hanging-wall strata being affected to varying degrees by the still upward migrating fluids. This hanging-wall alteration hydrothermal fingerprint allows delineation of the regionally metamorphosed paleohydrothermal system, and its characterization has the potential to lead to discovery of buried, stacked, or structurally displaced mineralization. Evidence for the presence of continued seafloor hydrothermal activity above the Flin Flon-Callinan VMS horizon is observed in the pillowed flows, interlayered hyaloclastite-rich flow tops, and also within finely bedded interflow volcaniclastic sediment. A 30% to 60% metalliferous exhalative component was detected through geochemical and mineral analysis in interpillow volcaniclastic rocks, chert, and epidosite in the hanging-wall sequence. The regional distribution of Fe- and Mg-rich chlorite, epidote-clinozoisite, biotite-annite, actinolite-hornblende-ferrotschermakite, and stilpnomelane and albite-oligoclase modifies metamorphic isograds and defines discrete vertical fluid pathways controlled by synvolcanic growth faults and associated sill-dike swarms. Silica-enriched hanging-wall alteration zones are proximal to Fe-Ti basalt sills and occur as discrete hanging-wall zones parallel to the plunge of the 62 Mt Flin Flon deposit. Anomalous concentrations of Hg, Sb, Ag, Pb, Te, As, Au, and Bi form within these hanging-wall halo alteration zones, indicating migration of the more volatile metals present in the underlying VMS deposits. Synvolcanic depressions, dike swarms, and hydrothermal-metamorphic fluid corridors are detectable through trace element anomalies, trace mineral chemistry, and 18O isotope geochemistry. Oxygen isotope analysis of the Flin Flon-777-Callinan VMS hanging-wall strata defines a number of high δ O18 anomalies extending 1,200 m above that indicate that <300°C subseafloor hydrothermal activity continued after burial of the massive sulfide deposits. Coupled with the geochemical and mineral chemical anomalies, this is indicative of the presence of continued, relatively low temperature hydrothermal fluid “leakage” from a robust seafloor hydrothermal event that generated the VMS deposits. A combination of techniques, including mineral chemistry, isotope, and trace element data, is demonstrated to be successful in identifying and delineating zones of hanging-wall hydrothermal alteration in greenschist- to amphibolite-grade metamorphic rocks of the Flin Flon mining camp. Use of these, coupled with mapping to define periods of quiescence as marked by horizons of sedimentary rocks in the hanging-wall basalts of the Hidden Formation, has the potential to lead to discovery of deeply buried deposits on the Flin Flon horizon or deposits at higher stratigraphic levels. Our findings indicate that the basaltic hanging wall on the Flin Flon-777-Callinan hydrothermal system was an efficient cap on the system, with vestiges of continued hydrothermal fluid flow detected in the interpillow and interflow components. These volumetrically minor components are critical sampling media and are pertinent to global exploration for detection of VMS mineralization buried beneath thick mafic volcanic sequences.

Paleoweathering and depositional conditions in the inter-flow sediment units (bole beds) of Deccan Volcanic Province, India: A mineral magnetic approach

A total of 54 samples from thirteen different profiles of interflow sediment units occurring in the Deccan Volcanic Province (DVP, India) are studied using an integrated mineral magnetic and field documentation approach. The occurrence and distribution of ferri- and antiferromagnetic oxides associated with these sediments infer depositional conditions dominated by various detrital modes and recycling of baked sediments under limited authigenic environments. The baking of underlying sediments is conditioned by aqueous or dry nature of the substrate vis-à-vis the viscosity of the overlying lava flow that can be identified by a combination of brown/red boles and compact/pahoehoe type flows. The presence of antiferromagnetic superparamagnetic (SP) fraction in the ‘red bole capped brown boles’ indicates the depth of baking to ~20cm; while the SP fraction also marks infilling, recycling and pigmentation in other occurrences of the red boles studied here.Overall, the mixed magnetic mineralogy and sediment characters depict frequent recycling and re-deposition by surface run-off, infiltration and ponding as the most common processes of bole bed formation in DVP governed by shallow streams or laterally migrating channels. Infiltration of the baked or unbaked sediments into fragmentary topped basalts is frequently shown by the red and brown boles, while infilling into the interstices and cavities is common with green boles. The red boles are also characteristic of detrital mixing of oxidized/partially oxidized ferrimagnets, many times coated with pigmentary hematites. The present results envisage a great scope for the routine mineral magnetic studies (integrated with field observations) for rapid characterization of the extensively occurring bole bed horizons in the DVP and their study as analogs of Martian soils and sediments.

Paleoproterozoic, turbidite-hosted, gold deposits of the Ashanti gold belt (Ghana, West Africa): Comparative analysis of turbidite-hosted gold deposits and an updated genetic model

The Paleoproterozoic gold deposits of the Ashanti gold belt, Ghana, occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics (alternately block-faulted Tarkwaian sediments). They are suggested in this article to belong to that class of gold deposits recognized as ‘turbidite-hosted’. Hence, evidence from previous investigations of turbidite-hosted and turbidite-associated gold deposits occurring in circumstances roughly similar to the Ashanti deposits are used, where relevant, to produce an updated genetic model for the latter. Given the relatively significant amounts of graphite associated with the major Ashanti deposits, the role of non-carbonate carbon is assessed in the light of conclusions reached by work on other turbidite-hosted and turbidite-associated gold belts. Conclusions as to the origins of mineralizing fluids and temporary repositories of gold (auriferous interflow sediments) reached by work on these other belts are also analysed. A concept of the tectonic development of the Ashanti belt-Kumasi basin area, introduced in this article, is deemed to be critical to the origin of the Ashanti gold deposits. It encompasses the role of non-carbonate carbon, turbidites, and the primary sources and secondary repositories of Au and As in the genesis of the major Ashanti gold deposits. It examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments, the occurrences of felsic metasandstones in transition-zone sediments, and TTG granitic material and fragments of chemical sediments in turbidites: all of which are incompatible with one or the other of existing concepts of tectonic development in the area.

Transport and Precipitation of Gold in Phanerozoic Metamorphic Terranes from Chemical Modeling of Fluid-Rock Interaction

This study reports results of mass transfer calculations using chemical modeling software (HCh) to determine chemical parameters that may have had a significant effect on gold deposition in turbidite-hosted Phanerozoic metamorphic terranes. The chemical system modeled was Al-As-Au-C-Ca-Cl-Cu-Fe-H-K-Mg-N-Na-O-S-Sb-Si to simulate fluid-rock interaction, gas partitioning, and mineral precipitation in veins. Each modeling run takes into consideration both (1) the composition of the vein fluid and the minerals predicted to precipitate in the vein and (2) the composition of the fluids during fluid-rock interaction and the predicted alteration assemblage of the host rocks. Results of the modeling are in good agreement with observed mineral assemblages in variably endowed Paleozoic orogenic gold provinces (central Victoria and northeast Tasmania, Australia; Buller terrane, New Zealand; Meguma terrane, Canada; Sierra de Rinconada, Argentina), and illustrate that gold is precipitated efficiently and over a wide temperature range (400°–200°C) from low-salinity, mixed aqueous-carbonic fluids. The modeling also allowed us to vary the critical components of the fluid (e.g., confining pressure that affects phase separation, ∑CO2, ∑S, f O2, and pH) to investigate how each of these parameters influences the amount of gold predicted to precipitate in the vein and the associated mineral assemblage. With the exception of the low ∑S fluid, the modeling scenarios predict the precipitation of gold mainly in the vein due to desulfidation processes. Carbon dioxide and other gases in the fluid play an important role: they limit the f O2 to a range where elevated gold concentrations can be maintained and transported in the fluid and also have an important effect on fluid immiscibility. We also investigated what effect a range of possible source rock compositions (i.e., granite, turbidites, greenstones, auriferous exhalative interflow sediments) have on gold solubility in fluids equilibrated with these rocks and hence, how rock compositions may influence ore transport and deposition. The results indicate that, at high temperatures, fluids in equilibrium with auriferous exhalative interflow sediments or greenstones will contain the highest gold concentrations. This outcome suggests that tectonic settings that enable preconcentration of gold in sulfide-rich siliciclastic or exhalative interflow sediments might provide more favorable conditions for the formation of well-endowed gold provinces.

Volcanology and evolution of the Werribee Plains intraplate, basaltic lava flow-field, Newer Volcanics Province, southeast Australia

Volcanism in the Werribee Plains at the eastern end of the Cenozoic Newer Volcanics Province began ca 4.6 Ma, the formation of eruptive fissures partly controlled by the orientation of pre-existing basement structures. The exposed eruption centres are small-volume scoria cones and lava shields, and both produced significant lava flows. Calculations of the emplacement times of the lava flows suggest the lava shields were active for about 6 months on average. Scoria cone eruptions probably lasted days to weeks. Despite relatively long repose periods between eruptions (about one eruption per 97 000 years), few interflow sediments are preserved. Minimal physical erosion of the lavas probably reflects the shallow gradient of the Werribee Plains and the onset of arid climatic conditions in the late Cenozoic. Inflated pahoehoe lavas are the dominant volcanic facies in the Werribee Plains. Other lava facies identified include spongy (S-type) pahoehoe, ponded lavas, transitional lavas, and a thick, distal aa lava facies. The characteristics of the volcanic facies and their distribution in this volcanic province resemble those in the type example of basaltic plains provinces, the Snake River Plain, USA. Basaltic plains provinces are produced by a unique style of volcanism which has been largely unexplored compared with basaltic volcanism in other tectonic settings (such as flood basalt and ocean island volcanism). This study expands our understanding of the processes involved in the evolution of basaltic plains provinces. The volcanic facies association typical of basaltic plains provinces is better characterised here, thus aiding the future identification of these unique lava flow-fields elsewhere, in both modern and ancient sequences.

Natural environmental changes indicated by Late Glacial and Holocene sediments from Lake Constance, Germany

Abstract The sediments of Lake Constance are a detailed archive for manifold environmental changes of the lake and its catchment area for the period after the last deglaciation. They are dated by means of sedimentological characteristics, secular variations of the declination of the earth's magnetic field, AMS-14C and counting of sediment laminations. In total, the sediments represent a time span of approximately 17,500 cal. yr, i.e. 14,600 14C yr. After meltwater-dominated sedimentation during retreat of the Rhine glacier, wind-transported material settled down between 17,500 and 15,600 cal. yr B.P. (approximately 13,100 14C yr B.P.) and formed beige-brown rhythmites. Overlying dark clays were deposited until the beginning of the Holocene. The lack of wind-transported sediments indicates increasing vegetation around the lake. A subdivision of the dark clay indicated by a change in magnetic volume susceptibility is the result of the Younger Dryas event, which increases the sedimentation rate. Early Holocene sediments contain the highest autochthonous calcite concentrations. Three maximum concentrations occurred around 11,100, 9300 and 7800 cal. yr B.P. (9600, 8300, 6900 14C yr B.P.). Several horizons with dark gray interflow sediments indicate periods with numerous flood events related to the Alpine River Rhine around 4100, 3300 and 2600 cal. yr B.P. (3700, 3100, 2500 14C yr B.P.).

Turbidite-hosted gold deposits of Central Victoria, Australia: their regional setting, mineralising styles, and some genetic constraints

Victoria is a gold province of international stature with primary gold mineralisation richly developed in the pre-Carboniferous rocks of the Lachlan Fold Belt. Total recorded production is in the order of 2,460,000 kg gold of which some 60% was sourced from extensive alluvial deposits, both of deep and shallow lead origin. Eleven mining centres each produced over 30 tonnes of gold with the major producer being Bendigo (684,300 kg). The Palaeozoic sequence in central Victoria is characterised by Cambrian metabasites or greenstones with associated volcanogenic and pelagic sequences followed by a Cambro–Ordovician to Early Devonian sequence of silici-clastic sediments. This succession is characterised by north–south trending open to tight folds and west-dipping thrust faults. Extensive S- and I-type granites, were emplaced from Late Silurian to Late Devonian, with the latter group, in part, associated with acid volcanic complexes. Mineralising styles include mesothermal deposits, either turbidite-hosted or dyke-affiliated, and these account for the bulk of primary gold mineralisation in the State. Less significant epithermal prospects and deposits are associated with either Cambrian calc-alkaline volcanics or sediments, typically of Siluro–Devonian age. Other styles of minor importance include Cambrian volcanogenic exhalative Cu–Zn mineralisation with gold as a minor credit (0.15–0.6 g/t Au), granite-hosted gold, and a potentially important group comprising disseminated gold in sediments. Examples of this latter group may exhibit spatially-related coticule development and by analogy with Nova Scotian disseminated intrametallic deposits, may have implications for a hitherto unrecognised style of gold mineralisation in Victoria. A gradation in lode-Au mineralising styles occurs across central Victoria. To the west in the Stawell metallogenic province, mineralisation was emplaced structurally and stratigraphically lowest in the crust. Mesothermal mineralisation is shear-hosted in Cambrian meta-sediments and greenstones exhibiting repeated deformation, multiple foliations, and extensive shearing. Host sediments were dominated by greenschist grade regional metamorphic conditions. Within the Ballarat metallogenic province, mesothermal mineralisation is hosted by dilational-site's style, in Ordovician sediments which were subjected to upper prehnite–pumpellyite facies conditions to the west and zeolite facies conditions to the east adjacent to the Melbourne metallogenic province. Ore fluids were of the low salinity C–O–H type with 1–10 wt% NaCl equivalent. CO 2 and CH 4 contents of fluid inclusions tend to be low with variable CO 2/CH 4 ratios. Depth of deposition (based on evidence from Wattle Gully mine) was in the order of 7 km and isotopic data suggest that ore fluids were derived either from an evolved igneous or from a devolatilised metamorphic source. Towards the eastern margin of the Ballarat metallogenic province and within the adjoining Siluro–Devonian Melbourne metallogenic province, epithermal style Au mineralisation is commonly emplaced in structurally and stratigraphically higher rocks. Trapping temperatures are lower than in deposits to the west and ore fluids are of low salinity C–O–H–N type with N 2>15 vol%. Gangue pyrite has elevated Sb contents and ore fluids were dominated by evolved meteoric water. δ 13 C values from vein carbonates suggest their derivation from oxidised organic carbon from within the Siluro–Devonian marine sediments. Alteration styles in gold deposits across central Victoria include sericitisation, carbonatisation, sulphidation, chloritisation, and (de-) silicification. Owing to the primary mineral assemblages and the physico-chemical characteristics of the ore-bearing fluids, visible evidence for wall rock alteration is commonly limited to 5–10 m wide zones of discolouration containing disseminated arsenopyrite and pyrite porphyroblasts and carbonate spotting. However, current petrographical and geochemical studies have demonstrated that the effects of pervasive wall rock alteration are more pronounced and extensive than previously recognised. Structural deformation within the Stawell and Ballarat metallogenic provinces, both from field evidence and 40 Ar / 39 Ar dating of cleavage phyllosilicates, indicates an Early Silurian event which migrated progressively eastwards with time. This Silurian event probably dates accretion of the proto Lachlan Fold Belt with the Kanmantoo Group of foreland Australia. Preliminary dating of hydrothermal sericites and felsic dykes associated with mineralisation, suggests episodic mineralising events 20–70 million years after accretion. These events demonstrate a spatial and temporal relationship with low-pressure regional metamorphism and/or granite plutonism; the latter being concentrated in two main pulses (400 Ma and 365 Ma) in central Victoria. The source of gold is equivocal with both igneous and devolatilised metamorphic sources being suggested. One potential reservoir source is the basement Cambrian greenstones of both MORB and boninitic affinity with associated sulphidic interflow sediments. Initial geochemical data indicate mean values of 65 ppb Au in these sulphidic interflow sediments which could have reacted with CO 2-rich fluids developed during Siluro–Devonian regional metamorphism and magmatism. Subsequent deposition of gold from such fluids could have occurred in structurally higher level dilational traps.

Metatholeiites and interflow sediments from the Cambrian Heathcote greenstone belt, Australia; sources for gold mineralization in Victoria?

Metatholeiites and interflow sediments of Cambrian age are exposed in the Lake Cooper quarry located in the northern segment of the Heathcote greenstone belt, central Victoria, Australia. The exposed sequence is 281 m thick and comprises nine massive to pillowed lava flows and seven interflow sequences. The metabasalts from the Lake Cooper quarry have strong petrological and geochemical similarities to normal midocean ridge basalts (N-MORB) and to metabasalts from greenstone suites elsewhere in Victoria, suggesting that they have not been exposed to extensive hydrothermal alteration. The interflow sediments in the quarry are subdivided into units comprising finely bedded sulfides, layers of sulfidic carbonaceous argillites, chert, and sedimentary carbonate rocks. Unusual mineral assemblages in these sediments, as well as their trace element concentrations and rare earth element signatures, suggest an exhalative volcanogenic origin. Whereas gold values in the tholeiites from the Lake Cooper quarry generally remain below 2 to 5 ppb, gold contents in the interflow sediments, which are also characterized by elevated As, Cu, and Zn values, range from <5 to 243 ppb and average 67 ppb. On the basis of sedimentological features and sulfide petrology, accumulation of the ore metals in these sediments is inferred to have occurred during deposition of the exhalative precipitates, possibly as a result of extraction and transfer of ore metals from the thick boninite sequence known to occur below the tholeiites in Victoria. Mass balance calculations show that remobilization of gold from the interflow sediments by metamorphic fluids and subsequent redeposition within dilatation zones and structural traps at higher crustal levels during the early to middle Paleozoic could account for a significant component of the central Victorian gold province. Thus, these exhalative metasediments might have played a vital role in accumulating, preconcentrating, and providing the gold required to form the world-class turbidite-hosted lode deposits in the Lachlan fold belt of Victoria.

The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift

The southernmost exposed rocks of the North American Midcontinent rift system (1100 Ma) consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the St. Croix River in Minnesota and Wisconsin. The flows are mostly high-Fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene. Abundant secondary chlorite, epidote, and actinolite indicate the group was metamorphosed to greenschist facies (~350 °C). Low sodium (M4 site) and tetrahedral aluminum (AlIV) contents of actinolite indicate low-pressure metamorphism (0.25 GPa) and imply a geothermal gradient of 45 – 50 °C/km. Low magnesium (Mg# = 0.37–0.58) and Ni contents (36–185 ppm) indicate the basalts have undergone significant fractionation and are not primary mantle melts. Incompatible element abundances are inversely correlated with Mg#, and most samples plot within either high or low trace element groups (e.g., Ti, P, Zr). The basalts are enriched in the light rare earth elements and Th, and are variably depleted in Ta and Nb relative to La and Th. Initial 143Nd/144Nd compositions of the group range from 0.51099 to 0.51122 (initial εNd = −4.5 to +0.1). Most flows have isotopic compositions within a relatively limited range (initial εNd = −2.5 to −1.6), but exhibit variable trace element abundances. Flows with the highest and lowest initial 143Nd/144Nd ratios have isotopic compositions that are inversely correlated with trace element abundances and ratios (e.g., La/Yb, Th/La, Th/Ta). The combined geochemical data suggest that the Chengwatana basalts originated from plume-derived melts and underwent variable fractional crystallization and crustal contamination. These melts may have interacted with lithospheric mantle enriched during Penokean subduction.

REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia

The 2.7 Ga-old volcanic sequence at Kambalda comprises a lower formation of massive and pillowed tholeiitic basalts (Lunnon basalt); a middle formation (Kambalda komatiite) of channel- and sheet-flow facies komatiitic peridotites with thin, intercalated sedimetns (Silver Lake member) and thin, massive, aphyric and differentiated spinifex-textured komatiites (Tripod Hill member); upper formations of ocellar; pillowed and massive komatiitic basalts (Devon Consuls basalt) and thin, massive and pillowed komatiitic basalts and thick layered sills/flows (Paringa basalt). The komatiites represent a regressive lava sequence, reflecting decreasing flow rates and increasing magma viscosities with time. These mantle-derived lithologies are intruded and overlain by crustally-derived lithologies: felsic and intermediate dikes and plutons (e.g. Kambalda granodiorite) and felsic and intermediate lavas, breccias, tuffs and epiclastic sediments (Black Flag group). Whole-rock major-element, trace-element and Nd-isotopic compositions of the mafic and ultramafic lavas vary systematically with stratigraphic location. REE have been mobile in certain strongly carbonated komatiites, but alteration cannot explain the systematic variations. Lunnon basalts are characterized by low MgO (5–8%), moderately low [La/Sm] n (0.7–0.9) and moderately high ϵ Nd values (+2.1 to +3.7); they are uncontaminated, slightly fractionated, low-degree partial melts of a mixture of depleted and undepleted mantle, and were probably derived from the head of a mantle plume. Spinifex-textured lavas in the channel-flow facies of the Silver Lake peridotite are characterized by very high MgO (16–31%), low [La/Sm] n (0.4–0.7) and variably high ϵ Nd values (+1.8 to +5.4); they are variably fractionated, only slightly contaminated, high-degree partial melts of highly depleted mantle, and were probably derived from the tail of a mantle plume. The parental komatiite is inferred to have contained ⩾30% MgO, [La/Sm] n ⩽ 0.5 and ϵ Nd ⩾ +5. Pyroxene-spinifex-textured and porphyritic-pyroxene lavas in sheet-flow facies are characterized by moderated MgO (21 to 12%), moderately low [La/Sm] n (0.6–1.0) and a moderately high ϵ Nd value (+2.3); they were contaminated by 2–5% of granitic crust and/or interflow sediment ± basalt and fractionated during eruption and emplacement. Aphyric and olivine-spinifex-textured lavas in the Tripod Hill komatiite have variably high MgO (15–32%, generally <26% MgO), moderately low [La/Sm] n (0.6–0.8) and moderately high ϵ Nd values (+2.7 to +4.4); they were contaminated by 2–5% of granitic crust and fractionated during ascent. Devon Consuls and Paringa basalts are characterized by low to moderate MgO (4–16% and 12–16%, respectively), high to very high [La/Sm] n (1.3–1.4 and 2.5–2.8) and moderate to very low ϵ Nd values (+3.3 to +1.2 and −1.2 to −2.4); they were contaminated by 5–7% and 20–30% of granitic crust, respectively, and fractionated during ascent. Thus, the degree contamination-accelerated fractional crystallization increased with time (Lunnon basalt→Silver Lake peridotite→Tripod Hill komatiite→Devon Consuls basalt→Paringa basalt), as magma conduits became heated, as erupted rates declined, and the magmas cooled and became more viscous. This produced the observed regression in lava facies upwards through the sequence. Age dates and thermal constraints indicate that this was only indirectly related to crustal underplating by the source magma (plume), but crustal heating eventually culminated in wholesale melting of the crust, intrusion of felsic plutons and eruption of felsic-intermediate volcanics. The degree of contamination also varied with location and time within individual flow units. The channel-flow facies of the Silver Lake peridotite, which thermally-eroded the most footwall rocks, contains some of the least-contaminated lavas, whereas the adjacent sheet-flow facies, which overlies uneroded sediment, containes some very contaminated komatiites and rare sediment xenomelts. The contaminants in the sheet-flow facies must be derived from an earlier stage of turbulent flow and thermal erosion in a turbulently-flowing sheet flow upstream from Kambalda, which evolved with increasing distance and time into a turbulently-flowing channel-flow facies flanked by a laminarly-flowing sheet-flow facies at Kambalda. Both facies crystallized and accumulated significant amounts of olivine during emplacement, forming thick lower cumulate zones, but eventually ponded and fractionated, producingk highly differentiated spinifex-textured zones. The sheet-flow facies crystallized relatively early, preserving the contaminants, whereas the channel-flow facies was replenished by more primitive magmas and crystallized later, after eruption rates declined and the conduits and lava formed chilled margins.

REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia

The 2.7 Ga-old volcanic sequence at Kambalda comprises a lower formation of massive and pillowed tholeiitic basalts (Lunnon basalt); a middle formation (Kambalda komatiite) of channel- and sheet-flow facies komatiitic peridotites with thin, intercalated sedimetns (Silver Lake member) and thin, massive, aphyric and differentiated spinifex-textured komatiites (Tripod Hill member); upper formations of ocellar; pillowed and massive komatiitic basalts (Devon Consuls basalt) and thin, massive and pillowed komatiitic basalts and thick layered sills/flows (Paringa basalt). The komatiites represent a regressive lava sequence, reflecting decreasing flow rates and increasing magma viscosities with time. These mantle-derived lithologies are intruded and overlain by crustally-derived lithologies: felsic and intermediate dikes and plutons (e.g. Kambalda granodiorite) and felsic and intermediate lavas, breccias, tuffs and epiclastic sediments (Black Flag group). Whole-rock major-element, trace-element and Nd-isotopic compositions of the mafic and ultramafic lavas vary systematically with stratigraphic location. REE have been mobile in certain strongly carbonated komatiites, but alteration cannot explain the systematic variations. Lunnon basalts are characterized by low MgO (5–8%), moderately low [La/Sm]n (0.7–0.9) and moderately high ϵNd values (+2.1 to +3.7); they are uncontaminated, slightly fractionated, low-degree partial melts of a mixture of depleted and undepleted mantle, and were probably derived from the head of a mantle plume. Spinifex-textured lavas in the channel-flow facies of the Silver Lake peridotite are characterized by very high MgO (16–31%), low [La/Sm]n (0.4–0.7) and variably high ϵNd values (+1.8 to +5.4); they are variably fractionated, only slightly contaminated, high-degree partial melts of highly depleted mantle, and were probably derived from the tail of a mantle plume. The parental komatiite is inferred to have contained ⩾30% MgO, [La/Sm]n ⩽ 0.5 and ϵNd ⩾ +5. Pyroxene-spinifex-textured and porphyritic-pyroxene lavas in sheet-flow facies are characterized by moderated MgO (21 to 12%), moderately low [La/Sm]n (0.6–1.0) and a moderately high ϵNd value (+2.3); they were contaminated by 2–5% of granitic crust and/or interflow sediment ± basalt and fractionated during eruption and emplacement. Aphyric and olivine-spinifex-textured lavas in the Tripod Hill komatiite have variably high MgO (15–32%, generally <26% MgO), moderately low [La/Sm]n (0.6–0.8) and moderately high ϵNd values (+2.7 to +4.4); they were contaminated by 2–5% of granitic crust and fractionated during ascent. Devon Consuls and Paringa basalts are characterized by low to moderate MgO (4–16% and 12–16%, respectively), high to very high [La/Sm]n (1.3–1.4 and 2.5–2.8) and moderate to very low ϵNd values (+3.3 to +1.2 and −1.2 to −2.4); they were contaminated by 5–7% and 20–30% of granitic crust, respectively, and fractionated during ascent. Thus, the degree contamination-accelerated fractional crystallization increased with time (Lunnon basalt→Silver Lake peridotite→Tripod Hill komatiite→Devon Consuls basalt→Paringa basalt), as magma conduits became heated, as erupted rates declined, and the magmas cooled and became more viscous. This produced the observed regression in lava facies upwards through the sequence. Age dates and thermal constraints indicate that this was only indirectly related to crustal underplating by the source magma (plume), but crustal heating eventually culminated in wholesale melting of the crust, intrusion of felsic plutons and eruption of felsic-intermediate volcanics. The degree of contamination also varied with location and time within individual flow units. The channel-flow facies of the Silver Lake peridotite, which thermally-eroded the most footwall rocks, contains some of the least-contaminated lavas, whereas the adjacent sheet-flow facies, which overlies uneroded sediment, containes some very contaminated komatiites and rare sediment xenomelts. The contaminants in the sheet-flow facies must be derived from an earlier stage of turbulent flow and thermal erosion in a turbulently-flowing sheet flow upstream from Kambalda, which evolved with increasing distance and time into a turbulently-flowing channel-flow facies flanked by a laminarly-flowing sheet-flow facies at Kambalda. Both facies crystallized and accumulated significant amounts of olivine during emplacement, forming thick lower cumulate zones, but eventually ponded and fractionated, producingk highly differentiated spinifex-textured zones. The sheet-flow facies crystallized relatively early, preserving the contaminants, whereas the channel-flow facies was replenished by more primitive magmas and crystallized later, after eruption rates declined and the conduits and lava formed chilled margins.

Crustal structure of the central graben of the Midcontinent Rift beneath Lake Superior

Travel times and amplitudes of refracted and reflected arrivals observed during GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) from the Midcontinent Rift in Lake Superior have been modeled using two-dimensional ray-tracing techniques. Forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau-sum analysis of the travel-time data at each station combined with information from a coincident common midpoint (CMP) reflection profile. When converted to time, the resulting model agrees quite well with the CMP reflection profile. Within the central graben of the Midcontinent Rift, approximately 2 km of sedimentary rock (2.8–4.6 km/s) overlie an 8-km-thick sequence of volcanics and interflow sediments (5.0–6.5 km/s) within the central rift graben observed on the reflection data. Beneath this sequence is a 6–8-km-thick sequence of 6.6–7.0 km/s material interpreted to represent metamorphosed volcanics. The velocity of the material at the base of the rift graben is not well constrained (approx. 7.0–7.2 km/s), but probably comprises an additional 10–12 km of metavolcanic rocks and intrusions that extend to the base of the graben as imaged on the coincident seismic reflection profile. The central graben is ranked by reverse faults, the Isle Royal and Keweenawan faults, that probably represent normal faults associated with graben formation that were reactivated during the late Precambrian. A 50–100 ms time advance anomaly associated with the Isle Royal fault is observed at every station. This anomaly has been modeled as shallow, high-velocity blocks located directly beneath the fault. The blocks correlate well with the walls of a steep-sided bathymetric trough and are believed to represent highly indurated upper Keweenawan sediments resulting from hydrothermal alteration. A marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at offsets exceeding 100 km, and has been modeled as pre-rift continental crustal rocks with a velocity of about 6.5 km/s adjacent to the 7.0 km/s material in the graben. Calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high-density rocks occupying the central half-graben imaged on the CMP profile. Wide-angle reflections from about 15–30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift-related detachment surfaces and/or pre-rift structures within the pre-rift crust. Additional wide-angle reflections define the top and bottom of high-velocity lower-crustal layer. The high-velocity lower-crustal layer is thickest beneath the rift graben and is interpreted to represent magmatic material underplated to the crust during rifting, as discussed by Tréhu et al. (1991). Total crustal thickness beneath the lake varies from about 55 km beneath the graben to about 35 km beneath the southern shore and 40 km beneath the northern shore.

Sediment and shear-hosted gold mineralization of the Tartoq Group supracrustals, southwest Greenland

Abstract Gold mineralization occurs in the Archaean Tartoq Group supracrustals, which are dominated by greenschist-amphibolite facies metamorphosed mafic rocks. Although deformed, recognizable features of the stratigraphy (pillowed lavas, intercalated sills and interflow sediments), suggest shallow to deep sea-floor deposition. Large, intensely carbonatized structural zones cut through the greenstones, subparallel to regional layering, and postdating major folding. They are interpreted as major shear zones and are characterized by internal ductile deformation due to high fluid pressures with brittle deformation of peripheral zones. Erratic high-grade gold mineralization is associated with quartz-pyrite lenses and veins that occur subparallel to layering within the shear zones. Moderate, but more consistent values are found in more extensive pyrite-arsenopyrite-quartz layers, also subparallel to layering. These lenses and layers occur adjacent to, or within the major carbonatized shear zones. Microscopically, gold is found as small (10 μm) grains in both pyrite and arsenopyrite. Low, but consistently anomalous grades of gold are also found in strongly contorted banded, muscovite-rich, quart-zpyrite rocks. These are thought tobe thin, chemically deposited interflow sediments within the greenstone pile, analogous to oxide-facies “banded iron formation” which is found nearby. Metamorphism has obscured critical textures, but the gold was either chemically deposited along with exhalative silica and sulphides, or may have been introduced later, during deformation and alteration of primary BIF. The Tartoq Group rocks are similar in most respects to those of other Archaean greenstone belts, being distinguished only by their excellent outcrop exposure. Current models for the origin of Archaean gold deposits are adequate to explain the Tartoq gold mineralization. A multistage model of gold concentration is proposed. Gold is initially leached from mafic volcanics (and possibly underlying gneisses) and redeposited at anomalous levels in chemical sediments. These may be further enriched by sulphidation reactions. High-grade gold mineralization is typically epigenetic in style, where very large volumes of fluid are channeled through shear zones undergoing brittleductile deformation.

Metallogeny of the midcontinent rift system of North America

Abstract The 1.1 Ga Midcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ∼ 1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments; (2) sediment-hosted copper sulfide and native copper; (3) copper sulfide veins and lodes hosted by rift-related volcanic and sedimentary rocks; and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Magmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include CuNiPGE and TiFe (V) in the Duluth Complex, U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks; the interaction between magma and country rocks was important in generation of the magmatic CuNi sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. More than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift, providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30–40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift, or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift.

Crustal Contamination Identified in Keweenawan Osler Group Tholeiites, Ontario: A Trace Element Perspective

In the Lake Superior area, voluminous Middle Proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the Midcontinent Rift and overlie Archean and older Proterozoic basement. The 3000 m-thick section of gently dipping (~10°SSE) basalts on the Black Bay Peninsula is divided into three suites based on the relationship between stratigraphy, petrography, and geochemical variations (e.g., Mg-number, $$A1_{2}O_{3}$$, La/Sm, and Gd/Yb). The Lower suite consists of augite-olivine porphyritic lavas with high Mg-number, $$SiO_{2}, TiO_{2}$$ and Gd/Yb, but low La/Sm. The overlying Central suite flows are either aphyric or plagioclase porphyritic and have lower Mg-number. $$SiO_{2}$$, La/Sm and Gd/Yb decline upward through the Central suite. The Upper suite lavas are either aphyric or plagioclase porphyritic, and element ratios plot on slightly different trends compared to the Central suite. The Lower suite has a narrow compositional range; the average of the four most primitive lavas is used...