Late Eocene Age Research Articles

Age and depositional environment of the Rock River coal basin, Yukon Territory, Canada

Poorly exposed Late Eocene strata in the Rock River basin, 115 km northeast of Watson Lake, accumulated in an intermontane valley with a geometry and history controlled by subsidence associated with the Rock River Fault. The sequence, as seen in one outcrop and five borehole sections, is dominated by drab mudrocks with minor sandstones and some thick lenses of coal. The mudrocks accumulated in floodplain marsh and pond settings associated with a low-gradient, possibly anastomosed, fluvial system. River banks were stable owing to the abundance of plant roots in the channel walls. Although channel sandstone and conglomerate were not identified in the core, the abundance of coarsening- and fining-upwards sets of sandstone of splay origin indicates pronounced levee development. Woody coals accumulated in areas well away from the main channel, in a series of elongate forested swamps, which were periodically inundated by flood water.The overall palynological assemblage is typical of the Eocene and Early Oligocene. A Late Eocene age is inferred from the presence of Gothanipollis in combination with the absence of index species for the Early–Middle Eocene and the latest? Eocene and Oligocene. The low miospore diversity indicates a temperate climate. The dominance of the palynological assemblage by Taxodiaceae–Cupressaceae pollen indicates wet–humid conditions.

The significance of incision and fluvial sedimentation in the Basal White River Group (Eocene-Oligocene), Badlands of South Dakota, U.S.A.

A newly defined lithostratigraphic unit, the Chamberlain Pass Formation (CPF), records the initial episode of incision, fluvial sedimentation, and pedogenesis in SW South Dakota following the retreat of the Cretaceous Interior Seaway. The CPF is Middle(?) to Late Eocene in age, and consists of fluvial sandstone and mudstone. Pedogenic modification of the unit has created a distinctive pedostratigraphic unit, the Interior Paleosol Series. The CPF thickens from west to east, achieving a maximum channel-belt thickness of ⩾ 11 m. Paleoflow data indicate that deposition of the CPF was restricted to a fault-controlled basin southeast of the Black Hills uplift. Sandstones in the CPF were derived from a recycled sedimentary rock source area to the west. In contrast, sandstones in the overlying Chadron Formation (Late Eocene) had a variety of sources including the Precambrian core rocks of the Black Hills uplift. Deposition of the CPF brackets four significant Paleogene changes in baselevel that occurred in this region. These events were: (1) Late Cretaceous to Middle(?) Eocene relative baselevel fall, weathering and erosion of the Cretaceous Pierre Shale to form the Yellow Mounds Paleosol, and fluvial incision; (2) Middle(?) to Late Eocene relative baselevel rise and deposition of the CPF; (3) Late Eocene relative baselevel fall, weathering and erosion of the CPF to form the Interior Paleosol, and fluvial incision; and (4) Late Eocene to Oligocene relative baselevel rise and deposition of the Chadron Formation. The first event was probably eustatic, the second was controlled primarily by local subsidence in a fault-bounded basin, the third records the tectonic uplift and unroofing of the Black Hills, and the fourth event was probably primarily controlled by eustasy, but other factors may have been important.

Paleoclimatological Analysis of Late Eocene Core, Manning Formation, Brazos County, Texas: ABSTRACT

ABSTRACT A core of the basal part of the Manning Formation was drilled to provide a baseline for paleoclimate analysis of the expanded section of siliciclastic rocks of late Eocene age in the outcrop belt. The interdeltaic Jackson Stage deposits of this area include more than 20 cyclic units containing both lignite and shallow-marine sedimentary rocks. Depositional environments can be determined with precision; and the repetitive nature of cycles allows comparisons of the same environment throughout, effectively removing depositional environment as a variable in interpretation of climate signal. Underlying Yegua strata contain similar cycles, providing more than 35 equivalent environmental transects within a 6-m.y. time interval of Jackson and Yegua section, when additional cores are taken. The core is from a cycle deposited during maximum flooding of the Jackson Stage. Deposits range from shoreface (carbonaceous) to midshelf, beyond the range of storm sand deposition. Rocks are leached of carbonate but contain foraminifer test linings, agglutinated foraminifers, fish debris, and rich assemblages of terrestrial and marine palynomorphs. All samples examined contain marine dinoflagellates, which are most abundant in transgressive and maximum flood zones, along with agglutinated foraminifers and fish debris. This same interval contains two separate pulses of reworked palynomorphs. The transgressive interval contains Glaphyrocysta intricata, normally present in Yegua rocks. Pollen indicates fluctuating subtropical to tropical paleoclimates, in three short cycles of cooler temperatures, indicated by abundance peaks of alder pollen (Alnus) in transgressive, maximum flood, and highstand deposits. End_of_Record - Last_Page 776-------

The geology, petrography and palynology of Tertiary coals from the Eureka Sound Group at Strathcona Fiord and Bache Peninsula, Ellesmere Island, Arctic Canada

Coal seams of substantial thicknesses and lateral continuity are developed in the Expedition and Iceberg Bay Formations at Strathcona Fiord and in the Expedition Formation at Bache Peninsula. The coals are lignite/subbituminous in rank and have low to moderate sulphur and variable ash contents. At Strathcona Fiord resources have been estimated to be in the order of 1 billion tonnes, whereas at Bache Peninsula estimated resources are 100 million tonnes. The coals at Strathcona Fiord are characterized by the dominance of wood-derived macerals of the humotelinite group, whereas at Bache Peninsula the coals have high amounts of detrital macerals (humodetrinite) and also have significant amounts of inertinite macerals. Pollen and spore assemblages in coals from the Iceberg Bay Formation at Strathcona Fiord are dominated by Taxodiaceace pollen with less and variable amounts of Pinaceae pollen, pteridophyte spores and angiosperm pollen. The pollen and spore assemblages indicate a Late Paleocene-Early Eocene age of the Iceberg Bay Formation coals. The Expedition Formation coals at Bache Peninsula have in general a somewhat less diverse pollen and spore assemblage and show a larger input of fern and Sphagnum spores. The pollen and spore assemblages in Expedition Formation coals at Bache Peninsula are indicative of (Early?) Paleocene age. The depositional environment for Iceberg Bay Formation coals at Strathcona Fiord is that of a forested swamp, which formed on a broad coastal plain. At Bache Peninsula peats formed in broad valleys, in which a fluviatile-lacustrine environment of deposition prevailed. The mires were likely less forested than at Strathcona Fiord as indicated by the abundance of ferns and sphagnum spores and the very low content of wood-derived macerals of the humotelinite group. The pollen assemblages in both Expidition and Iceberg Bay Formation coals indicate a temperate climate and moderate precipitation during peat accumulation.

Zircon U‐Pb geochronology of the Zambales and Angat Ophiolites, Luzon, Philippines: Evidence for an Eocene arc‐back arc pair

Two basement terranes, the Zambales ophiolite in the west, and the Angat ophiolite in the east, are exposed on the island of Luzon, separated by a circa 10 km thick and circa 100 km wide sedimentary basin. The structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution proposed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zircon fractions from tonalite in the Acoje block of the Zambales ophiolite give concordant U‐Pb ages at 44.2 (±0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U‐Pb ages of 45.1 (±0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Aksitero Formation. No age difference is discerned between the arc‐like Acoje block and MORB‐like Coto block of the Zambales ophiolite. Four zircon fractions from two sample sites in the Angat ophiolite give concordant ages of 48.1 (±0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike‐pillow lava‐sediment sequence south‐southeast of the main ophiolite. The small age difference between the Zambales and Angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the Southern Sierra Madre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the Eocene Angat ophiolite implies that there are two ophiolitic basements exposed in the Southern Sierra Madre. The relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cretaceous ophiolitic basement. This suggests that the Zambales‐Angat ophiolite represents a preserved Eocene back‐arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In this model, the Zambales‐Angat ophiolites are therefore not allochthonous terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.

Iridium Anomalies of Late Eocene Age at Massignano (Italy), and ODP Site 689B (Maud Rise, Antarctic)

Iridium anomalies of Late Eocene age are here reported in sequences from northern Italy and Weddell Sea, Antarctic. We have analyzed 270 samples of marly limestones from the Eocene/Oligocene boundary stratotype section at Massignano, Italy, ranging from 35.6 to 34.5 Ma (NP 18 Zone to NP 19/20 Zone). We discovered a distinct Ir peak of 156 ± 19 parts per trillion (ppt) in the lower calcareous nannofossil NP 19/20 Zone, corresponding to the mid-lower part of foraminiferal P16 Zone, and the top of magnetic Chron 16n2. The age of this Ir anomaly derived from several dated volcanic ashes found in the same section is 35.7 ± 0.4 Ma

Timing and kinematics of deformation in the Malaguide Complex, internal zone of the Betic Cordillera, southeast Spain

The Betic Cordillera of southern Spain forms the westernmost of the Mediterranean Alpine orogenic belts and is divided, traditionally, into Internal and External Zones. In the External Zone fold and thrust belt, unmetamorphosed cover rocks of the Iberian margin were deformed from the early Miocene onward. In contrast, Internal Zone rocks are dominantly metamorphic and are generally assumed to have been deformed and metamorphosed in an Alpine event resulting from Late Cretaceous/Palaeogene convergence between Africa and Europe. In the Sierra Espuña, at the northern edge of the Internal Zone in the Eastern Betics, the uniquely preserved Permo‐Triassic to Miocene stratigraphy of the unmetamorphosed Malaguide Complex, offers a rare opportunity to date the Internal Zone deformation. Kinematic data, thrust geometries and structural facing of folds suggest that the imbricate thrust stack of Malaguide rocks formed as a result of NNW‐directed motion, within present geographical coordinates. A fossilized front to this thrust stack, preserved beneath a thick sequence of Oligocene conglomerates, indicates that the timing of the thrust deformation is late Eocene in age. Deformation propagated into the foreland basin in the late Oligocene with renewed thrusting. Subsequently, the whole stack has been folded into a N‐NW vergent regional recumbent fold, the Espuña fold, during the early Miocene.

Eocene succession on the East Shetland Platform, North Sea

A sand-prone Middle Eocene sequence is recognized in three cored boreholes on the East Shetland Platform. Three lithostratigraphic divisions are recognized: Lower Sand Unit, Main Sand Unit and Upper Claystone Unit. The Lower Sand Unit is of fluvio-deltaic origin, occurring on the southern part of the East Shetland Platform. The Main Sand Unit represents the subsequent transgression of the delta. The unit comprises a set of highstand shelf systems tracts separated by glauconitic bands and reflected by palynological assemblage breaks. The presence of reworked palynofloras in sediments of late Middle Eocene age implies derivation of sediment from the north-west part of the East Shetland Platform. Although it is tempting to ascribe sea level changes to the sediment pattern, a tentative correlation with the Hampshire Basin only highlights the difficulties involved.

Paleobiology of the oligopithecines, the earliest known anthropoid primates

Anthropoid primates of the subfamily Oligopithecinae are late Eocene in age, and have a known distribution of Northeast Africa and the Arabian Peninsula. Body sizes of the three known oligopithecine species are estimated from allometric molar size regressions to be 700–1000 g forOligopithecus savagei, 600–900 g forCatopithecus browni, and 500 g for the least well-known and smallest species,Proteopithecus sylviae. Occlusal features of the molar teeth, considered in conjunction with body size, suggest that all three species were frugivorous and insectivorous. The orbital size ofCatopithecus indicates a diurnal activity cycle. A relatively broad interobital region in this species may indicate prosimian-like or callitrichid-like olfactory adaptations. Structural features of the crushed skull suggest thatCatopithecus had a smaller cranial capacity than those of extant anthropoids with a similar body size. Fossil plants and birds from localities yielding oligopithecines suggest a wet, warm, tropical, forested, swampy environment. These paleobiological inferences about the extinct oligopithecines are discussed in relation to questions about primate adaptations near the prosimian-anthropoid transition.

Eocene microplankton from La Meseta Formation, northern Seymour Island

Twenty two samples collected from the Tertiary La Meseta Formation of Cape Wiman, Seymour Island, Antarctic Peninsula yielded abundant and moderately diverse assemblages of marine palynoflora, dominated by dinoflagellate cysts, together with acritarchs and chlorophyta. The assemblages can be divided into three association: Association 1, characterized by low diversity dinoflagellate assemblage of late Early Eocene age which are dominated by Enigmadinium cylindrifloriferum; Association 2 characterized by more diverse dinoflagellate cyst assemblages, which show a marked decrease in the dominance of E. cylindrifloriferum, and an increase in relative abundance of Areosphaeridium cf. diktyoplokus; and Association 3 which is characterized by a decrease in dinoflagellate cyst diversity up section. Changes in dinoflagellate cyst dominance and diversity throughout the section suggests a gradation from a stressed, shallow marine palaeoenvironment to a more open near-shore, shallow marine system becoming progressively more nearshore up section. The assemblages are no older than late Early Eocene in age, and possibly as young as Mid–Late Eocene.

Thrusting and wedge growth, Southern Alps of Lombardia (Italy)

A south-vergent fold-thrust belt of Miocene-Recent age accompanies the south slope of the Lombardian Alps and is partly buried beneath Plio-Pleistocene Po Valley basin fill. The belt is probably detached along a trans-crustal thrust, named Main South Alpine Thrust (MSAT), with an estimated dip slip of 70–100 km. Transport on this thrust piggybacks the Adamello pluton of Late Eocene age, pre-Adamello folds, and Oligocene-Miocene Insubric strike-slip structures, by ramping up through 12–15 km of Austro-Alpine (Adria) crust and through 8–10 km of Triassic to Eocene sediments. Folds in the Front Ranges are ascribed to MSAT ramping, not to pre-Adamello compression. The MSAT soles upward in a blind thrust beneath 3–4 km of Oligocene-Pliocene foredeep fill. Initial regional failure along the MSAT implies substantial and pre-existing topographic relief near the Insubric line. An average of 25% wedge thickening during MSAT transport is consistent with the requirement of Coulomb critical taper. Progression of the south-Alpine detachment from the MSAT to the base of the foreland sediments has added a thickness of 6–12 km in footwall imbrications to the base and the toe of the thrust wedge. This addition in wedge volume is consistent with wedge dynamics only if a mid-Miocene or younger spike of excess Alpine topography is admitted.

Late Eocene age of Fayum mammal faunas

Re-evaluation of the stratigraphic boundaries of the non-marine Jebel Qatrani Formation (Fayum, Egypt) indicates that the entire formation should be dated to Late Eocene, Priabonian age. The Bartonian shallow-marine microfaunas in the immediately underlying formations are now assigned to the Middle Eocene, suggesting that the subsequent transition to coastal-plain deposition is best placed in the lower part of the Upper Eocene. The erosional surface at the top of the Jebel Qatrani correlates to the global hiatus in inner-shelf sequences which was initiated by eustatic decline at the end of the Eocene. Paleontology supports a late Eocene age for the Jebel Qatrani fauna, in that the fossil mammals show no signs of the radical modernization of African faunas that took place in the early to middle Oligocene.

Interpreting compositional variations of calciturbidites in relation to platform-stratigraphy: an example from the Paleogene of SE Spain

In the Sierra de Aitana (Betic Cordilleras, Southern Spain) a sequence of massive platform-limestones outcrops. Study of these limestones indicates a Late Eocene age and has resulted in distinguishing four lithofacies-associations. Several features suggest tilting, uplift and partial exposure of this carbonate platform at the end of the Eocene. In a more basinward section this time-interval is represented by calciturbidites, the composition of which has been analysed microscopically. The Upper Eocene turbidites are mainly composed of loose, skeletal grains derived from the platform-interior. Around the Eocene-Oligocene boundary the composition changes abruptly and the Lower Oligocene turbidites mainly consist of platform-derived limestone clasts and skeletal debris from the platform-margin. This compositional change is accompanied by a large decrease in volume of turbidite-input. It is concluded that both composition and amount of calciturbidites reflect the history of the adjacent carbonate platform. This case-study therefore provides new evidence for the mechanism of “highstand shedding” in carbonate depositional systems.

New Late Eocene mollusks from localized limestone deposits formed by subduction-related methane seeps, southwestern Washington

The trochid archaeogastropodMargarites(Pupillaria)columbianan. sp., the mytilid bivalveModiolus(Modiolus)willapaensisn. sp., and the vesicomyid bivalveCalyptogena chinookensisn. sp. are described from the earliest known fossil communities associated with subduction-related methane seeps. The communities are in very localized limestones of late middle to late Eocene age along the southwestern margin of Washington. These limestones contain large numbers of chemosynthetic bivalves and worm tubes, as well as other macrobenthos, that colonized around cool-temperature methane seeps along the landward slopes of an ancient subduction-zone complex.Calyptogena chinookensisn. sp.is the earliest known species of this genus, which was previously known from Miocene to Recent.

An example of an accretionary forearc basin from northern Central Anatolia and its implications for the history of subduction of Neo-Tethys in Turkey

Deformed sedimentary sequences of Late Cretaceous-middle Eocene age located between the Sakarya continent and the Anatolian Complex have been interpreted as the fill of a forearc The history and structure of this basin are critical for understanding the evolution of the Neo-Tethyan subduction in the Middle East. I test and elaborate upon this interpretation on the basis of three basic outcrop areas of Upper Cretaceous-lower Tertiary sedimentary sequences confined to a northeast-southwest-trending belt in north-west Ankara. These are the Alci, Kinik and areas. They expose two different sedimentary sequences called Alci and Orhaniye. In the Alci area, the sequence starts at the base with the continental red clastic rocks on the erosional surface of an underlying ophiolitic melange and continues upward into the shallow-marine reefal buildup of the late Maastrichtian age. The upper half of the Alci sequence is separated from its lower half by a local disconformity and dominated by the continental debris-flow deposits composed mostly of volcanic material. By contrast, both the Kinik and areas are characterized by a thick, continuous and coarsening-upward sequence overlying conformably the underlying ophiolitic melange. The lower half of the sequence is dominated by a deep-marine flysch, its middle part by continental clastic material, and the topmost part by the shallow-marine reefal buildup. Lower contact relationships, lithologic composition, facies distribution, internal structures, and the regional tectonic settings of both sequences strongly suggest that the above-mentioned three outcrops preserve the remnants of a peripheral accretionary forearc basin forming the north-northwestern margin of the Haymana-Polath accretionary forearc basin during middle Campanian-middle Eocene time. This peripheral basin is here named Orhaniye accretionary forearc basin.

The Naga Hills and Andaman ophiolite belt, their setting, nature and collisional emplacement history

The Indo-Burmese Range and the Andaman-Nicobar Island Arc, form a continuous arcuate trend along which several ophiolite occurrences have been reported. In Naga Hills (NHO) and Andaman (ANO), these ophiolites are represented by dismembered mafic and ultramafic rocks with closely associated oceanic pelagic sediments. They occur as folded thrust slices occupying the highest tectonic levels and are brought to lie over distal shelf sediments of Eocene to Oligocene age. Ophiolites are unconformably overlain by ophiolite-derived clastics of Middle to Late Eocene age. The ophiolites preserved along this belt are remnants of a continuous, narrow, one or several intra-continental ocean basin(s) of broadly comparable age, created during the Late Mesozoic rifting of the Greater India Gondwana continent. Rifting and creation of oceanic crust date between Cretaceous and Early Eocene. In the initial stages, the ocean floor had been deeper than Carbonate Compensation Depth (CCD). Subsequently it had become uneven, when oceanic crust was being added through several seamounts or seamount chains and on top of which calcareous pelagic sediments were deposited. Both tholeiitic and alkaline volcanic rocks are present in these ophiolites. In NHO, the two groups of lavas have generated from different sources in different tectonic settings. The alkalic and some tholeiitic lavas in NHO are similar to off-axis seamount basalts. Tholeiitic lavas from ANO and some NHO resemble MORB or backarc basin basalts and on the basis of certain chemical characters these are suggested to have generated in marginal basin setting. Significant volume of acid differentiates are associated in ANO which also support the marginal basin character of the basalts. The suite of rocks in ANO indicates fractionation in a shallow level magma chamber. Closure of the small ocean basin(s) and emplacement of ophiolites took place in two stages. In the initial stage, the seamount chain brought to the subduction zone collided with the Burmese block prior to Middle Eocene. Part of the ophiolites represent clipped seamounts which got accreted to the leading edge of the eastern continental block. With continued closure, this eastern block with accreted ophiolite slices was brought in juxtaposition with distal shelf sediments of the western block marking the terminal continent-continent collision. The thrust front of ophiolitic rocks apparently advanced further westward in Andaman to the south compared to the northern sector, and thus an imbricated zone and melange involving the Eocene floor sediments (Lipa Fm) has been created, whereas in the Naga Hills the floor sediments (Disang Fm) remained virtually passive. The time of terminal continental collision is represented as the regional Late Oligocene unconformity. The entire thrust stack got deformed and folded into upright geometry after being blocked. The present subduction of oceanic crust beneath the Andaman island arc appears to be a westward jump of subduction zone due to sustained post-collisional NE drive of the Indian plate.

Joint analyses of calcite twins and fault slips as a key for deciphering polyphase tectonics: Burgundy as a case study

Determination of regional paleostresses based on calcite twin analysis has been successfully applied for the first time in the Burgundy platform, France. Paleostress tensors obtained with this method are internally consistent along the profile studied. Moreover, the same directions of paleostresses are independently inferred from fault striation analysis. The results clearly show that the late Mesozoic-Cenozoic evolution of the area has involved polyphase deformation, including 1. (1) NNE-SSW extension, probably late Mesozoic in age, 2. (2) N-S compression related to a major orogenic event in the Pyrenees and Provence, late Eocene in age, 3. (3) E-W extension related to the Oligocene rifting event (the Rhine-Saône rift system), 4. (4) WNW-ESE compression related to the Late Miocene westward thrusting of Jura. Calcite twin and striated fault analyses are two complementary tools for paleostress determination: their combined application will allow accurate mapping of paleostress trajectories in intraplate tectonics setting. Moreover, calcite twins can allow stress and paleostress determination when macroscopic features are not observable, which is often the case in very weakly deformed areas and in drill holes.

Single-Crystal 40 Ar/ 39 Ar Dating of the Eocene-Oligocene Transition in North America

Explanations for the causes of climatic changes and associated faunal and floral extinctions at the close of the Eocene Epoch have long been controversial because of, in part, uncertainties in correlation and dating of global events. New single-crystal laser fusion (SCLF) (40)Ar/(39)Ar dates on tephra from key magnetostratigraphic and fossilbearing sections necessitate significant revision in North American late Paleogene chronology. The Chadronian-Orellan North American Land Mammal "Age" boundary, as a result, is shifted from 32.4 to 34.0 Ma (million years ago), the Orellan-Whitneyan boundary is shifted from 30.8 to 32.0 Ma, and the Whitneyan-Arikareean boundary is now approximately 29.0 Ma. The new dates shift the correlation of Chron C12R from the Chadronian to within the Orellan-Whitneyan interval, the Chadronian becomes late Eocene in age, and the North American Oligocene is restricted to the Orellan, Whitneyan, and early Arikareean. The Eocene-Oligocene boundary, and its associated climate change and extinction events, as a result, correlates with the Chadronian-Orellan boundary, not the Duchesnean-Chadronian boundary.

First record of the family Ringiculidae (Gastropoda) from the Middle Tertiary of Antarctica

A new species of the late Mesozoic–Cenozoic family Ringiculidae (Ringicula(Ringicula)cockburnensisn. sp.) is described from basal glauconitic beds of late Eocene age of Cockburn Island, Antarctica, and is the first reported occurrence of the family Ringiculidae from the continent of Antarctica.Ringicula (R.) cockburnensisn. sp. most closely resemblesR. castigatafrom the middle Oligocene Duntroonian Stage of New Zealand and provides further support for the strong provinciality (Weddellian Province) that existed along the southern margin of the Pacific during the Late Cretaceous and early Tertiary.

The Kelian disseminated gold deposit, East Kalimantan, Indonesia

Kelian is one of a number of recently discovered Tertiary volcanic-hosted gold deposits which occur within a 400-km-long, northeast-trending belt in the interior of Kalimantan, the Indonesian part of Borneo. It consists of two main orebodies, namely West and East Prampus, and four smaller mineralized ore zones. These have a combined resource potential of +75 Mt at 1.8 g/t Au, making Kelian the largest known gold deposit in Indonesia. The deposit was discovered in 1976 during follow up work of alluvial gold occurrences within the Kelian River, using traditional exploration techniques of stream sediment, pan concentrate, rock float and outcrop sampling. Subsequent detailed investigations included soil sampling, trenching, deep augering, ground magnetics, induced polarization and 60,000 m of diamond drilling. The geology of the deposit consists of a pile of silicic pyroclastics grading upwards into a series of sediments of Late Eocene age. The sequence was folded and faulted along northerly and northeasterly trends and intruded by a number of subvolcanic andesitic to trachyandesitic bodies in the Early Miocene. Some time after the emplacement of the andesites a hydrothermal system was established. It brought about extensive alteration, mineralization and hydrothermal brecciation. The preserved part of this system is 1 km 2 in area and has a vertical extent of at least 600 m. Limited radiometric age data suggest an age of around 20 Ma for the alteration. Four partly overlapping stages of alteration/mineralization are recognized: 1. Stage I: chlorite+carbonate+sericite+pyrite±epidote alteration in the cores of the larger andesite bodies. 2. Stage IIA: localized fracturing and sericite±quartz deposition. Stage IIB: a major period of polyphasal fracturing and hydrothermal brecciation with associated boiling of hot (290–330°C), dilute (0.5–4.2 eq. wt.% NaCl) and CO 2-rich fluids, and development of replacement/vein alteration of adularia+sericite+quartz+pyrite accompanied by subordinate As-Ag mineralization and possibly some gold mineralization. Progressive loss of CO 2 upon boiling caused a progressive increase in adularia and a corresponding decrease in sericite deposition. Deeper in the system a quartz-sericite-carbonate assemblage was formed without adularia. 3. Stage III: renewed magmatic activity, focused on East Prampus, released volatiles to the hydrothermal system, producing a hot (300–330°C), relatively saline (> 10 wt.% NaCl), CO 2-rich brine, and extensive fracturing and hydrothermal brecciation. This was accompanied by boiling, deposition of various carbonates±quartz±sericite and gold-base metal mineralization; preliminary stable isotope data on the carbonates support the presence of a magmatic component. Mixing of the upwelling hot, saline fluids with descending cooler, relatively low pH, steam-heated fluids has resulted in a complex zonation in carbonate mineralogy and a wide range of fluid salinities and homogenization temperatures. Away from the centre of brecciation and boiling, gold and base metals were deposited in a more passive environment together with carbonate and/or quartz. 4. Stage IV: late-stage widespread influx of relatively cool (<200°C), low pH (3–4), CO 2-rich, steam-heated fluids from the top and margins of the system forming a kaolinite+Fe/Mn carbonate+pyrite ±hematite±cinnabar assemblage. The magmatic-hydrothermal event was followed by uplift and erosion of more than 900 m during the Midddle Late Miocene and basaltic volcanism in the Plio-Pleistocene. A second cycle of erosion, which began in the Pleistocene, has removed most of the young volcanic cover exposing the deposit as known today.