Palaeomagnetism applied to magnetic anomaly interpretation: a new twist to the search for mineralisation in northern Chile

The Chilean Iron Belt is an important location for Fe and Cu(Fe)-Au deposits and includes the recently developed Candelaria deposit, which is located some 20 km south of the city of Copiapó in northern Chile. This mine is now a major Cu producer in Chile but its discovery in the late 1980s was relatively fortuitous. The exploration programme included a ground magnetic survey from which lessons can be learnt in the search for further such deposits. Palaeomagnetic studies throughout the northern part of the Chilean Iron Belt indicate major crustal rotation of the region, probably, related to oblique convergence and transpression at the Andean Margin. The application of palaeomagnetic techniques to a magnetite-apatite deposit, Mina Fresia, indicates that magnetite-rich ores in this area are capable of maintaining a significant remanence component which will contribute to their magnetic anomaly. As with the volcanics, intrusives and sediments of the region this, remanence is clearly rotated clockwise. Using 2.5D and 3D magnetic modelling, it is demonstrated that the magnetic anomaly associated with the Candelaria deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity. Recognition of clockwise-rotated, remanence-dominated anomalies should provide a new key to the search for deposits in this part of Chile and elsewhere. An example of an unexplored anomaly showing this key feature of a clockwise-rotated remanence component dominating over the induced component is presented. Modelling of this anomaly indicates a pair of sources which are either ten times as big or ten times as magnetic as the Candelaria deposit. It is suggested that the low cost of palaeomagnetic study of an exploration target should be a prerequisite to magnetic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern. Received: 9 April 1999 / Accepted: 5 October 1999     

Periglacial climate at the 2.5 Ma onset of Northern Hemisphere glaciation inferred from the Whippoorwill Formation, northern Missouri, USA

The Whippoorwill Formation is a gleyed diamicton that is present locally within bedrock depressions beneath the oldest glacial till in northern Missouri, USA. Stratigraphy, paleomagnetism, and cosmogenic-nuclide burial ages show that it was deposited between the Matuyama-Gauss magnetostratigraphic boundary at 2.58 Ma and the first advance of the Laurentide ice sheet into Missouri at 2.47 ± 0.19 Ma. High cosmogenic-nuclide concentrations also show that the constituents of the Whippoorwill Formation experienced long exposure at a stable landscape surface with erosion rates of 1-2 m/Ma. However, cosmogenic-nuclide concentrations are invariant with depth below the Whippoorwill Formation surface, indicating active mixing of the soil profile shortly before burial by till. The Whippoorwill Formation retains numerous features indicative of cryoturbation. Therefore, we interpret it as a buried Gelisol, a soil formed under periglacial conditions in the presence of permafrost. At the onset of Northern Hemisphere glaciation, climate cooling established permafrost conditions and accelerated erosion by inducing landscape instability. Thus, weathered regolith materials were mobilized and redeposited by gelifluction shortly before the ice sheet overrode the landscape.     

Last glacial climate instability documented by coarse-grained sediments within the loess sequence, at Fanjiaping, Lanzhou, China

Optically Stimulated Luminescence dating, grain-size analysis and magnetic susceptibility measurements were conducted on the Fanjiaping loess section, from the western Chinese Loess Plateau. The results confirm that last glacial high-frequency climatic shifts were documented in mid-latitude continental archives. The grain-size record indicated that coarse-grained sediments with horizontal bedding and channel-fill structures were only deposited in several short intervals, equivalent to the beginning of marine oxygen isotope stage (MIS) 4 and the early to middle MIS 3. This probably implies brief rainfall intensification of the Asian summer monsoon, and its disappearance since the late MIS 3 to MIS 2 may have been a response to significant glacial cooling in the Northern Hemisphere. Previous investigations revealed high sea-surface temperatures at high latitudes at the start of MIS 4, and the early to middle MIS 3 intensification of summer insolation in the Northern Hemisphere, implying evident climate amelioration. Climate improvement favors boreal forest recovery, enhancing both winter and summer air temperatures. The resultant smaller equator-polar temperature gradient probably helped the moisture-laden summer monsoon to penetrate northward. This study thus provides new significant information about the response of terrestrial loessic palaeoenvironments to millennial-timescale climatic fluctuations during the last glacial period.     

Porosity evolution of the Cambrian Bonneterre Dolomite, south-eastern Missouri, USA

The Cambrian Bonneterre Dolomite of south-eastern Missouri, USA, hosts the large Mississippi Valley type (MVT) lead-zinc-copper ore deposits of the region. The Bonneterre Dolomite consists of dolomitized algal bioherms, oolitic grainstone and associated lithologies that were deposited on a carbonate platform surrounding the Precambrian age St Francois Mountains. Porosity was determined by point counting thin sections from cores and mines in the Bonneterre Dolomite and by gas porosimetry. Volumes of epigenetic cements were estimated by point counting cement filling micro- and mesoporosity using cathodoluminescence. Cement volumes were added to present porosity to estimate porosities during various stages of mineralization. Prior to the onset of mineralization, micro- and mesoporosity in the Bonneterre Dolomite averaged approximately 19%. Precipitation of early dolomite cement (roughly concurrent with the main period of sulphide deposition) reduced average porosity to approximately 7% and closed off much of the intercrystalline pore space. Later cementation by dolomite (prior to late stage sulphides) reduced porosity to approximately 5%, and late cementation by quartz further reduced porosity to the present average value of <4%. Periods of carbonate dissolution during MVT mineralization enhanced large scale megaporosity associated with fractures and breccias but did not significantly increase smaller scale porosity. Dolomite cementation associated with MVT mineralization, porosity and permeability were facies controlled. Today, as a result of mineralization, large scale fractures and breccias control porosity and permeability. This study indicates that dolomite porosity may undergo significant change during basinal brine migrations associated with MVT mineralization.     

The quaternary glacial history of the Lahul Himalaya,northern India

This paper presents the first glacial chronology for the Lahul Himalaya, Northern India. The oldest glaciation, the Chandra Glacial Stage, is represented by glacially eroded benches at altitudes greater than 4300 m above sea-level. This glaciation was probably of a broad valley type. The second glaciation, the Batal Glacial Stage, is represented by highly weathered and dissected lateral moraines, which are present along the Chandra valley and some of its tributaries. This was an extensive valley glaciation. The third major glaciation, the Kulti Glacial Stage, is represented by well-preserved moraines in the main tributary valleys of the Chandra valley. This represents a less extensive valley glaciation. Two minor glacial advances, the Sonapani I and II, are represented by small sharp-crested moraines, which are within a few hundred metres or few kilometres of the present-day glaciers. The change in style and extent of glaciation is attributed to an increase in aridity throughout the Quaternary, due either to global climatic change or uplift of the Pir Panjal mountains to the south of Lahul, which restricted the northward penetration of the south Asian summer monsoon. © 1996 John Wiley & Sons, Ltd.     

Numerical simulation of the paleohydrology of glacial Lake Oshkosh, eastern Wisconsin, USA

Proglacial lakes, formed during retreat of the Laurentide ice sheet, evolved quickly as outlets became ice-free and the earth deformed through glacial isostatic adjustment. With high-resolution digital elevation models (DEMs) and GIS methods, it is possible to reconstruct the evolution of surface hydrology. When a DEM deforms through time as predicted by our model of viscoelastic earth relaxation, the entire surface hydrologic system with its lakes, outlets, shorelines and rivers also evolves without requiring assumptions of outlet position. The method is applied to proglacial Lake Oshkosh in Wisconsin (13,600 to 12,900 cal yr BP). Comparison of predicted to observed shoreline tilt indicates the ice sheet was about 400 m thick over the Great Lakes region. During ice sheet recession, each of the five outlets are predicted to uplift more than 100 m and then subside approximately 30 m. At its maximum extent, Lake Oshkosh covered 6600 km² with a volume of 111 km³. Using the Hydrologic Engineering Center-River Analysis System model, flow velocities during glacial outburst floods up to 9 m/s and peak discharge of 140,000 m³/s are predicted, which could drain 33.5 km³ of lake water in 10 days and transport boulders up to 3 m in diameter.     

Metamorphism of a Late Jurassic volcano-plutonic arc, northern California, USA

Abstract The metamorphic history of the Middle to Upper Jurassic volcanic and hypabyssal rocks exposed in the Klamath Mountains and Sierra Nevada of California is related, in part, to the rifting of a volcano-plutonic arc. The Callovian to Kimmeridgian rocks exposed in the region consist of, from north-west to south-east, a back-arc ophiolite, a rifted volcanic arc and a volcanic arc complex. All of these units have been metamorphosed and contain various combinations of the phases chlorite, amphibole, epidote, prehnite and pumpellyite. Projection of coexisting phases onto the composition plane MgO/(MgO + FeO) and Al₂O₃+ Fe₂O₃ - 0.75 CaO - Na₂O through quartz, water, albite and epidote results in consistent mineralogical compatibilities within each region, but crossing tie-lines between regions. This suggests that the volcanic and hypabyssal rocks from each region have equilibrated under different intensive conditions. The back-arc ophiolite in the north has suffered subseafloor high-T/P hydrothermal metamorphism with geothermal gradients on the order of 100° C km^?1. The rifted volcanic arc has undergone synchronous burial, hydrothermal and contact metamorphism. Metamorphic field gradients in the region pass through the prehnite-pumpellyite and greenschist facies suggesting geothermal gradients on the order of 30° C km^?1. The southernmost volcanic arc complexes contain metavolcanics of the pumpellyiteactinolite and greenschist facies suggesting moderate- to high-P/T metamorphism and geothermal gradients on the order of 20° C km^?1. The apparent increase in rifting and calculated geothermal gradients from south-east to north-west suggest that the observed very low- and low-grade metamorphism may be a response to enhanced thermal gradients during extension of the volcanic arc. This correlation between the extent of rifting and metamorphism is consistent with a model of diastathermal metamorphism of a propagating rift along the western margin of North America during the Late Jurassic. The plate tectonic setting may be analogous to the present-day Andaman Sea region.     

A glacial chronology for the Fish Creek drainage of Boulder Mountain, Utah, USA

Boulder Mountain, located in South Central Utah, is one of several mountain ranges on the Colorado Plateau that was glaciated during the late Pleistocene. Using ³He exposure-age dating (corrected for non-cosmogenic ³He with shielded samples), we determined ³He exposure-ages for boulders from the most well-preserved moraines in the Fish Creek drainage of Boulder Mountain. ³He exposure-ages indicate a last glacial maximum (LGM) advance ∼23,100 ± 1300 to 20,000 ± 1400 yr ago and a later and smaller advance ∼16,800 ± 500 to 15,200 ± 500 yr ago. This chronology is very similar to other cosmogenic glacial chronologies from the Western U.S. and suggests that the timing of glacial advance and retreat on the Colorado Plateau was generally in phase with the rest of the Western U.S. during the late Pleistocene.     

Geomorphic,sedimentary and hydraulic reconstruction of a glacial lake outburst flood in northern Alberta,Canada

Glacial lake outburst floods occurred frequently during the last deglaciation of the Laurentide Ice Sheet. Within the Interior Plains, these floods carved large spillway systems; however, due to a lack of abundant sediment, deposits within prairie spillways are rarely preserved. Here, we present geomorphic and sedimentary evidence and hydraulic modelling of the eastern Beaver River Spillway, formed by the catastrophic drainage of the ice‐dammed glacial Lake Algar, in north central Alberta. During this flood, coarse‐grained sediment eroded from local till formed large pendant bars. Within the first ~50 km of the spillway (Reach 1), pendant bars contain downstream orientated foresets overlain by horizontally bedded coarser gravels. The remaining pendant bars (Reach 2), present downflow of a moraine barrier, differ, comprising massive, matrix‐supported, inversely graded gravels capped by a boulder layer. We use a HEC‐GeoRAS/HEC‐RAS system in conjunction with palaeostage indicators to estimate the steady‐state water surface elevation. Modelling results show that peak discharge within Reach 1 of the eastern Beaver River Spillway was approximately 14 000–21 000 m³ s^?1. For Reach 2, 30 km downstream, the peak discharge was estimated at 23 000–40 000 m³ s^?1 (n_bulked 18 000–26 000 m³ s^?1). The downstream discharge increase, consistent with the sedimentary change in pendant bar deposits, is attributed to sediment bulking of the flood flow. This provides the opportunity to observe a range of flow conditions, and associated sedimentology, from a single flood event. The reconstructed flow conditions, coupled with lake volume estimates from the ponding above the moraine barrier suggest a minimum flow duration of 3–5 days.     

Palaeomagnetism,rock magnetism and magnetic fabrics in Precambrian metamorphic terranes of the Huabei Shield,China

This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a₉₅=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a₉₅=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a₉₅=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.     

Sediment magnetic properties of glacial till deposited since the Little Ice Age maximum for selected glaciers at Svartisen and Okstindan, northern Norway

Samples of glacial till deposited since the Little Ice Age (LIA) maximum by two glaciers, North Bogbre at Svartisen and Corneliussen-breen at Okstindan, northern Norway, were obtained from transects running from the current glacier snout to the LIA (c. AD 1750) limit. The samples were analysed to determine their sediment magnetic properties, which display considerable variability. Significant trends in some magnetic parameters are evident with distance from the glacier margin and hence length of subaerial exposure. Magnetic susceptibility (χ) decreases away from the contemporary snout, perhaps due to the weathering of ferrimagnetic minerals into antiferromagnetic forms, although this trend is generally not statistically significant. Trends in the ratios of soft IRM/hard IRM which are statistically significant support this hypothesis, suggesting that antiferromagnetic minerals are increasing relative to ferrimagnetic minerals towards the LIA maximum. Backfield ratios (IRM -100 mT/SIRM) also display a significant and strong trend towards magnetically harder behaviour with proximity to the LIA maximum. Thus, by employing a chronosequence approach, it may be possible to use sediment magnetics data as a tool for reconstructing glacier retreat in areas where more traditional techniques, such as lichenometry, are not applicable.     

冀北尚义—隆化区域断裂形成与演化历史探讨

尚义—隆化区域断裂呈近EW向分布于冀北地区,活动于新太古代晚期(阜平晚期),全新世至今仍有活动,探讨其形成与演化历史,对进一步研究冀北地区地质构造、矿产分布及新构造运动对人类活动的影响具有重要意义。通过统计该断裂中的构造岩及指向构造,发现尚义—隆化区域断裂为复杂的强变形带,韧性变形与脆性变形相间,具有长期及多期活动性、倾向及性质多变等特点; 尚义—隆化区域断裂是冀北地区不同时段不同级别构造单元的重要分界线之一,对两侧地质历史的发展及演化具有重要的控制作用。根据构造叠置关系、糜棱岩类变质程度与区域变质作用的对应关系,以及剪切指向组构反映的活动方式与区域构造运动主应力的对应关系,重新厘定了构造叠加改造关系及变形期次,结合前人构造岩的变质矿物年龄,将该断裂划分为2个构造阶段、9个构造活动期。     

Discussion on the formation and evolutionary history of Shangyi-Longhua regional fault in northern Hebei Province

Shangyi-Longhua regional fault was EW distributed near northern Hebei Province, which was first active in the Late Neoarchean (Late Fuping) and still active in the Holocene and even today. Therefore, exploring its formation and evolutionary history is of great significance to study its geological structure, mineral distribution and the influence of Neotectonic movement on human beings in northern Hebei Province. The regional faults in Shangyi-Longhua are strong complex deformation zones through the census of the tectonic rocks and all kinds of directional structures in the fault zone. The ductile deformation and brittle deformation are alternating with characters of chronicity, multiphase activity, propensity and nature variation. Shangyi-Longhua regional fault is one of the important dividing lines of tectonic units of different levels in different periods in northern Hebei Province, which plays an important role in controlling the development and evolution of geological history on both sides. The authors redefined the structure superposition transformation relation and the development period, according to the superimposed relationship of the sturcture, the corresponding relationship between metamorphic degree of mylonites and the regional metamorphism, and the corresponding relationship between the movement model reflected by the shear-pointed fabric and the principal stress of the regional tectonic movement. Combined with the metamorphic mineral age of the predecessor tectonites, the authors divided the fault structure into two construction phases and nine tectonic active periods.     

Influence of a glacial buzzsaw on the height and morphology of the Cascade Range in central Washington State, USA

Analysis of climatic and topographic evidence from the Cascade Range of Washington State indicates that glacial erosion limits the height and controls the morphology of this range. Glacial erosion linked to long-term spatial gradients in the ELA created a tilted, planar zone of 373 cirques across the central part of the range; peaks and ridges now rise ≤600 m above this zone. Hypsometric analysis of the region shows that the proportion of land area above the cirques drops sharply, and mean slopes >30° indicate that the areas above the cirques may be at or near threshold steepness. The mean plus 1σ relief of individual cirque basins (570 m) corresponds to the ∼600-m envelope above which peaks rarely rise. The summit altitudes are set by a combination of higher rates of glacial and paraglacial erosion above the ELA and enhanced hillslope processes due to the creation of steep topography. On the high-precipitation western flank of the Cascades, the dominance of glacial and hillslope erosion at altitudes at and above the ELA may explain the lack of a correspondence between stream-power erosion models and measured exhumation rates from apatite (U-Th/He) thermochronometry.     

Reef margin collapse, gully formation and filling within the Permian Capitan Reef: Carlsbad Caverns, New Mexico, USA

An area of reef margin collapse, gully formation and gully fill sedimentation has been identified and mapped within Left Hand Tunnel, Carlsbad Caverns. It demonstrates that the Capitan Reef did not, at all times, form an unbroken border to the Delaware Basin. Geopetally arranged sediments within cavities from sponge–algal framestones of the reef show that the in situ reef today has a 10° basinwards structural dip. Similar dips in adjacent back-reef sediments, previously considered depositional, probably also have a structural origin. Reoriented geopetal structures have also allowed the identification of a 200-m-wide, 25-m-deep gully within the reef, which has been filled by large (some  >15 m), randomly orientated and, in places, overturned blocks and boulders, surrounded by finer reef rubble, breccias and grainstones. Block supply continued throughout gully filling, implying that spalling of reef blocks was a longer term process and was not a by-product of the formation of the gully. Gully initiation was probably the result of a reef front collapse, with a continued instability of the gully bordering reef facies demonstrated by their incipient brecciation and by faults containing synsedimentary fills. Gully filling probably occurred during reef growth, and younger reef has prograded over the gully fill. Blocks contain truncated former aragonite botryoidal cements, indicating early aragonite growth within the in situ reef. In contrast, former high-magnesian calcite rind cements post-date sedimentation within the gully. The morphology of cavern passages is controlled by reef facies variation, with narrower passages cut into the in situ reef and wider passages within the gully fill. Gully fills may also constitute more permeable zones in the subsurface.     

Freshwater control of ice-rafted debris in the last glacial period at Mono Lake, California, USA

The type section silts of the late Pleistocene Wilson Creek Formation at Mono Lake contain outsized clasts, dominantly well-rounded pebbles and cobbles of Sierran lithologies. Lithic grains > 425 μm show a similar pattern of variability as the > 10 mm clasts visible in the type section, with decreasing absolute abundance in southern and eastern outcrops. The largest concentrations of ice-rafted debris (IRD) occur at 67–57 ka and 46–32 ka, with strong millennial-scale variability, while little IRD is found during the last glacial maximum and deglaciation.Stratigraphic evidence for high lake level during high IRD intervals, and a lack of geomorphic evidence for coincidence of lake and glaciers, strongly suggests that rafting was by shore ice rather than icebergs. Correspondence of carbonate flux and IRD implies that both were mainly controlled by freshwater input, rather than disparate non-climatic controls. Conversely, the lack of IRD during the last glacial maximum and deglacial highstands may relate to secondary controls such as perennial ice cover or sediment supply. High IRD at Mono Lake corresponds to low glacial flour flux in Owens Lake, both correlative to high warm-season insolation. High-resolution, extra-basinal correlation of the millennial peaks awaits greatly improved age models for both records.     

The dispersion of fibrous amphiboles by glacial processes in the area surrounding Libby,Montana, USA

Mining operations began at a world-class vermiculite deposit at Vermiculite Mountain near Libby, Montana, circa 1920 and ended in 1990. Fibrous and asbestiform amphiboles intergrown with vermiculite ore are suspected to be a causative factor in an abnormally high number of cases of respiratory diseases in former mine and mill workers, and in residents of Libby. The question addressed in this report is whether some of the amphibole from Vermiculite Mountain could have been dispersed by Pleistocene glacial processes rather than by human activity after vermiculite mining began. The history of Pinedale glaciation in the Libby area provides a framework for estimating the presence and distribution of asbestiform amphiboles derived from Vermiculite Mountain and found in naturally occurring sediments of Glacial Lake Kootenai that underlie the Libby Valley area. There were two situations where sediments derived from Vermiculite Mountain were deposited into Glacial Lake Kootenai: (1) as lake-bottom sediments derived from meltwater flowing down Rainy Creek when the valley south of Vermiculite Mountain was free of ice but active ice still covered Vermiculite Mountain; and (2) as lake-bottom sediments eroded from the Rainy Creek outwash and re-deposited during a re-advance of the Purcell Trench Glacier lobe near Moyie Springs, Idaho.