Hasil untuk "Mineralogy"

A. A. Sanchez, A. A. Sanchez, M. P. Lau et al.

<p>Monitoring and treatment of contaminated mine water conventionally focuses on end-of-pipe assessment and remediation techniques, at the downstream outlet of mining sites after closure. Conversely, the initial stages of pollutant release and their pathways within abandoned mines have been largely overlooked. This study examines subsurface mining-affected anthropogenic structures and the dynamic hydrogeochemical loadings in terms of temporal increases in concentration and drainage pathways within them, revealing how variable subsurface flow activation impacts metal(loid) mobilization and opens novel direct mitigation options. We identified complex hydrological patterns through the mine (Reiche Zeche, Ore Mountains, Germany) in which percolation paths were dynamically connected to the drainage based on flow conditions. Using in-situ sensors, hydrogeochemical monitoring and stable water isotopes, we reveal a hydrodynamic regime in which episodic shifts in subsurface connectivity govern metal(loid) mobilization from localized storage zones, ultimately controlling solute export to surface waters. We use concentration–discharge (<span class="inline-formula"><i>C</i></span>–<span class="inline-formula"><i>Q</i></span>) relationships, the Pollution Load Index (PLI), and hydrological concepts to evaluate metal transport during the annual pattern of flow regimes. Our analyses of event-scale <span class="inline-formula"><i>C</i></span>–<span class="inline-formula"><i>Q</i></span> patterns reveal site- and element-specific shifts in flow path activation in a very short time. Low flow periods are often considered low risk for contaminant mobilization, yet contaminant hotspots within poorly connected hydrological zones can emerge during these times. The resulting high pollution potential and solute accumulation are governed by the sequence and timing of crossing or exceeding a connectivity or flow threshold, as described by fill-and-spill and lotic-lentic cycle concepts. Notably, Zn loads (in terms of flux) during low flow, pre-flush periods reached values up to six times higher than median values. Preceding the flushing events, geochemical and microbial-mediated metal leaching create the spatially distributed contaminant stock, remobilized during reconnection events. With a large proportion of heavy metal loads occurring during low flow and especially just before the high flow (flush) period, source-related, decentralized water treatment structures become much more feasible than end-of-pipe solutions that require higher throughput volumes and multi-element filtering. This work also highlights the need for event-sensitive monitoring and treatment strategy options that prioritize internal system behavior to mitigate pollution risk in abandoned mines and other caverned hydrological systems.</p>

Yu.V. Erokhin, M.V. Tsyganko, V.V. Khiller et al.

The object of study includes the copper slags of the Petropavlovsk plant, which had been operated since 1760 to 1822 in area of an eponymous settlement, which was further transformed to town of Severouralsk (Sverdlovsk region). The composition of samples was determined using scanning electron microscopy, inductively coupled plasma mass spectrometry and X-ray spectral analysis. The slags are composed of fayalite-hedenbergite aggregates with iscorite, magnetite, glass, metallic copper, bornite, digenite (?), sphalerite and galena. Secondary mineralization includes cuprite, brochantite, barite and atacamite. The slags are a result of smelting of sphalerite-bearing chalcopyrite ores, which were extracted at the nearby Tur’ya mines and other small ore occurrences located in the vicinity of the Petropavlovsk plant.

Péter Sipos, Ivett Kovács, Adrienn Tóth et al.

Although both ferromanganese and calcareous nodules are common in hydromorphic soils, their joint presence has rarely been evaluated. Studying their occurrence in soils forming toposequences may significantly contribute to a better understanding of their formation. We investigated the relationship between their formations in four soil profiles from a salt affected toposequence through mineralogical, micro-fabric, micro-chemical, morphological, and stable isotope analyses.Concentric ferromanganese nodules, representing the most developed nodules, were formed within the most intense groundwater fluctuation zone, whereas calcareous nodules showed a maximum in size just above the lowest groundwater level in each hydromorphic profile. Ferromanganese nodules formed slowly through matrix impregnation in the wet season under the effect of an intense groundwater fluctuation. Contrarily, calcareous nodules precipitated rapidly in cavities in the dry season following the decline of the groundwater table. Dissolution and re-precipitation were characteristic of calcareous nodules formed at shallower depths, suggesting the influence of the fluctuating groundwater on their formation, as well. The smectitic clay mineralogy of the soils affected drainage, promoting the development of a secondary groundwater fluctuation zone. Additionally, smectites provided a surface for the initial precipitation of Fe-oxyhydroxides, and facilitated the formation of cavities in the soil.In the ferromanganese nodules, calcite precipitated simultaneously with Fe-oxyhydroxides, whereas this was not the case for Fe (and Mn) oxyhydroxides observed within the calcareous nodules. Calcite particles in the ferromanganese nodules were covered by calcified microorganisms, suggesting a biogenic contribution to their formation. Such contributions could also play a role in the formation of the ferromanganese nodules in the lowest topographic position, presumably due to the continuous Fe supply.The formation of both ferromanganese and calcareous nodules is strongly correlated with groundwater flow in hydromorphic soils, although their primary formation periods differ. Their key properties provide valuable insights into groundwater flow characteristics and reflect microbial contributions to their formation.

B. I. Slobodian, M. V. Heichenko, A. Sh. Mienasova

The paper addresses practical aspects of the quality assurance and quality control (QA/QC) procedures’ application by duplicating historical drill holes and analytical studies during the preliminary exploration of the Polokhiv lithium deposit. These procedures are mandatory in the absence of core material and duplicate samples of historical holes and in the case of analytical studies. The procedures for selecting duplicate holes and the comparison methods are highlighted. Primary geological information, i.e. cores and duplicate samples, is missing for the most mineral deposits in Ukraine, which were explored during the Soviet period. Therefore, the only method to ensure and control the quality of the historical drilling is to duplicate the holes. The analysis and comparison of the duplicated wells was carried out according to the geological sections of the columns and according to the characteristics of the ore sections. A good convergence of the results of the historical drilling with the control one is shown. In general, according to the control results, the qualitative and quantitative indicators of lithium mineralization at the Polokhiv deposit were confirmed. Sample analysis and testing activities are performed by reputable ISO-accredited and lithium-certified laboratories. All analytical and other test results are supported by duly signed certificates and reports issued by the laboratories and accompanied by a statement of the methods used. This provides confidence in the reliability of the data obtained. Methodical approaches to the processing QA/QC results, which are accepted in international practice and their fundamental differences from those accepted in Ukraine, are presented. The results of the analytical studies form the core basis of mineral resource estimation, so the reliability of the results is critical. The results of the study are supported by numerous figures, graphs and tables. This significantly increases the informativeness of the presented results and improves their comprehension. The specified methodological approaches to solving the problems of quality of geological exploration works in Ukraine issues will improve and increase the reliability of exploration data of the mineral deposits in Ukraine.

V.A. Simonov, A.V. Kotlyarov, V.V. Sharygin et al.

Crystallization conditions of olivine in dunites of the Guli massif (Siberian Platform) are established using petrological, geochemical, mineralogical and thermobarogeochemical methods. The formation of olivine and chromite from picrite-meimechite ultramafic magmas was followed by the fractionation of clinopyroxenes from basaltic systems with the formation of residual high-alkaline melts. The calculations in PETROLOG and COMAGMAT programs based on the compositions of inclusions and minerals, as well as using olivine-spinel geothermometer, showed that a magmatic system, which was responsible for the formation of dunites of the Guli massif, evolved during the crystallization of olivine in a wide temperature range of 1520–1250 °C in a magma chamber at a depth of about 17 km.

Deepak Tapriyal, Foad Haeri, Dustin Crandall et al.

Abstract Carbon storage technology is primarily targeted in saline formations, which is a porous rock matrix filled with brine, sealed with a low permeability caprock. There are significant variations of CO2 wetting properties, typically reported in the literature as contact angle of CO2 and brine interacting with a rock material, suggesting that CO2 could become wetting under geostorage conditions and negatively impact containment effectiveness. Here, we performed the first controlled laboratory measurements of CO2‐brine contact angles on shale rocks from low permeability sealing formations with distinctive mineralogic properties—calcite‐rich, quartz‐rich, and dolomite‐rich. We targeted temperatures at 40° and 100°C, pressures at 8.3, 34.5, and 62.1 MPa, and salinity at 35,000 and 260,000 ppm. Results show no significant change in contact angle with mineralogy, temperature, pressure, salinity, and CO2 bubble size. We conclude that caprocks will remain water‐wet at geologic CO2 storage conditions and will not lose their capillary sealing capacity.

R.V. Zharkov, O.V. Bergal-Kuvikas, A.V. Degterev et al.

The paper presents summary data on the historical activity of Ivan Grozny volcano (Iturup Island, Kuril Islands), including the results of our own studies of solfatar-hydrothermal and eruptive activity of the volcano since 2004. Using modern analytical methods, we obtained data on the chemical composition of the volcano rocks and pyroclastics of the explosive eruption in 2012–2013. The composition of the volcano rocks varies from andesites to daciandesites of medium potassium series. Pyroclastics of the 2012–2013 eruption corresponds to andesites with medium potassium content in terms of bulk chemical composition. The material composition of glasses according to microprobe data is much more acidic. The obtained values are similar to the geochemical compositions of the products of the 1973 and 1989 eruptions. Taking into account the prevailing wind regime dominating in the central part of Iturup Island, the hazard from ash fall of probable future eruptions of Ivan Grozny volcano was assessed, indicating the predominant spread of ash in the north-western, western and south-western directions.

Dmitry Ganyushkin, Dmitry Bantcev, Ekaterina Derkach et al.

The glacier recession of the North-Chuya ridge, Altai, after the maximum of the Little Ice Age (LIA) is estimated based on remote sensing and in situ studies of the Bolshoi Maashei glacier. The glacier area decreased from 304.9 ± 23.49 km² at the LIA maximum to 140.24 ± 16.19 km² in 2000 and 120.02 ± 16.19 km² in 2021. The average equilibrium-line altitude (ELA) rise after the LIA was 207 m. The reduction of glaciers was caused by the warming trend, most rapid in the 1990s, and by the decrease in precipitation after the mid-1980s. The volume of glaciers decreased from approximately 16.5 km³ in the LIA maximum to 5.6–5.8 km³ by 2021. From the LIA maximum to 2022, the Bolshoi Maashei glacier decreased from 17.49 km² to 6.25 km², and the lower point rose from 2160 m to 2225 m. After the LIA, the glacial snout retreat was about 1 km. The fastest retreat of the glacier terminus was estimated in 2010–2022 as 14.0 m a⁻¹ on average. The glacier mass balance index was calculated, with the results showing a strong negative trend from the mid-1980s until now. Strong melt rates caused the increase in the area of the Maashei lake, which could lead to the weakening of its dam, and prepared for its failure in 2012. The current climatic tendencies are unfavorable for the glaciers.

W. Rush, W. Rush, J. Self-Trail et al.

<p>Eocene transient global warming events (hyperthermals) can provide insight into a future warmer world. While much research has focused on the Paleocene–Eocene Thermal Maximum (PETM), hyperthermals of a smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, namely the Eocene Thermal Maximum 2 (ETM2 and H2), in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil, and dinocyst biostratigraphy, as well as the recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment, utilizing clay mineralogy, marine palynology, <span class="inline-formula"><i>δ</i>¹⁸</span>O of biogenic calcite, and biomarker paleothermometry (TEX<span class="inline-formula">₈₆</span>). Paleotemperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of the warming does not correspond to the CIE of the ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors in addition to <span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span> forcing have influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to CIE warming. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very warm and (seasonally) stratified<span id="page1678"/> conditions, likely salinity-driven, across H2. The absence of evidence for strong perturbations in local hydrology and nutrient supply during ETM2 and H2, compared to the PETM, is consistent with the less extreme forcing and the warmer pre-event baseline, as well as the non-linear response in hydroclimates to greenhouse forcing.</p>

مسعود منجزی, حسام دهقانی, محمدرضا عجم زاده et al.

خطر احتمال وقوع حوادث و اثرات ناشی از آن همواره در هر پروژه وجود دارد. حوادث باعث ایجاد خسارات مالی، زمانی، جانی و ... می‌شود و از نظر روانی نیز برکارکنان و کارگران تاثیر منفی می‌گذارد. شناسایی و مدیریت عوامل موثر در حوادث برای جلوگیری و یا کاهش احتمال وقوع حادثه در پروژه‌ها لازم است. انفجار، یکی ازعملیاتی است که همواره خطر وقوع حادثه را به همراه دارد. در این تحقیق از بین تعداد زیادی عوامل موثر بر وقوع حوادث ناشی ازانفجار در عملیات معدنی و عمرانی، 13 عامل با نظر کارشناسان انتخاب و با رتبه‌بندی این عوامل، موثرترین عامل شناسایی شده است. برای نیل به این هدف از روش شبیه‌سازی مونت‌کارلو به‎همراه روش تصمیم‌گیری تحلیل سلسله مراتبی استفاده شد و رتبه‌بندی بر اساس نظر 15 نفر از کارشناسان خبره در این حوزه انجام گرفت. در نهایت با بررسی نتایج حاصل، سه عامل زمان نامناسب، کیفیت چال‎زنی و مهارت فردی، به عنوان مهم‌ترین عوامل ایجاد یک حادثه شناسایی شدند. هم‎چنین نتایج نشان داد که ویژگی‎های الگوی حفاری، نوع ماده منفجره و خصوصیات توده‌سنگ، کمترین تاثیر را بر وقوع یک حادثه دارند. مقایسه نتایج با عوامل ثبت‎شده برای حوادث واقعی، صحت نتایج حاصل از این تحقیق را تایید کرد.

Yu. S. Savchuk, A. V. Volkov

Research subject. Large and super-large orogenic gold deposits are widely distributed across all continents. This work was aimed at characterizing 24 giant facilities with reserves and resources of more than 500 tons of metal.Materials and methods. The work was based on the authors’ long-term fieldwork research into large and super-large gold deposits in Central Asia and the North-East of the Russian Arctic. In addition, an extensive bibliographic analysis of publications devoted to the largest gold deposits in the world was carried out.Results. A structural analysis showed that the formation of these deposits underwent two main stages. The early – subduction stage – was characterized by the development of gentle disturbances, such as thrusts, thrust nappies, viscous discontinuities and crushing zones, while the structural paragenesis included isocline lying folds and cleavage of the axial surface. The second stage – collision-transpression – began with the deformation of the previously formed structure into open and compressed folds with steep axial surfaces; as a result of further compression, a series of longitudinal discontinuous disturbances developed. An oblique collision of interacting plates lead to the development of shear transpression structural paragenesis, in which oblique cracks predominate. It is established that, in orogenic gold deposits, different stages were characterized by ore-controlling and ore-bearing structures. The binding of ore-bearing disturbances to different stages enabled isolation of deposits of various types that differ in structural and morphological features. The main characteristics of the identified types of deposits are provided. The formation of these deposits occurred during convergent processes, with the ore-controlling structures being a surface manifestation of the movement of block terrains along the surface of the detachment.Results. It is concluded that the surface of the detachment is not only a tectonic zone, but also a channel that provides a link between various deep levels of the subduction zone, from where ore-bearing hydrothermal fluids emerge.

J. J. Virtasalo, P. Österholm, A. T. Kotilainen et al.

<p>Rivers draining the acid sulfate soils of western Finland are known to deliver large amounts of trace metals with detrimental environmental consequences to the recipient estuaries in the eastern Gulf of Bothnia, northern Baltic Sea. However, the distribution of these metals in the coastal sea area and the relevant metal transport mechanisms have been less studied. This study investigates the spatial and temporal distribution of metals in sediments at nine sites in the Kvarken Archipelago, which is the recipient system of Laihianjoki and Sulvanjoki rivers that are impacted by acid sulfate soils. The contents of Cd, Co, Cu, La, Mn, Ni, and Zn increased in the cores during the 1960s and 1970s as a consequence of intensive artificial drainage of the acid sulfate soil landscape. Metal deposition has remained at high levels since the 1980s. The metal enrichment in sea floor sediments is currently visible at least 25&thinsp;km seaward from the river mouths. Comparison with sediment quality guidelines shows that the metal contents are very likely to cause detrimental effects on marine biota more than 12&thinsp;km out from the river mouths. The dynamic sedimentary environment of the shallow archipelago makes these sediments potential future sources of metals to the ecosystem. Finally, the strong association of metals and nutrients in the same sediment grain size class of 2–6&thinsp;<span class="inline-formula">µ</span>m suggests that the transformation of dissolved organic matter and metals to metal–organic aggregates at the river mouths is the key mechanism of seaward trace metal transport, in addition to co-precipitation with Mn oxyhydroxides identified in previous studies. The large share of terrestrial organic carbon in the total organic C in these sediments (interquartile range – 39&thinsp;%–48&thinsp;%) highlights the importance of riverine organic matter supply. These findings are important for the estimation of environmental risks and the management of biologically sensitive coastal sea ecosystems.</p>

Iris Alvarez-Ortíz, Esther M. Cruz-Gámez, José F. Lastra-Rivero et al.

El presente trabajo, a partir del estudio comparativo (textural, mineralógico y geoquímico) de las alteraciones en las rocas relacionadas a los yacimientos Júcaro y Yagruma (Formación Encrucijada), ofrece nueva información sobre las asociaciones minerales, particularidades de los procesos hidrotermales en estos depósitos y tipos de alteración. La alteración diagenética se encuentra relacionada con calcita, clorita, pirita, albita, cuarzo y zeolita, tiene carácter regional y se distribuye en toda la Formación, manifiesta una tendencia general a valores mayores de Na2O+CaO(adj) e inferiores en FeOt+MgO(adj), con índice de alteración (AI) entre 18 y 40. La hidrotermal se presenta sobre todo en basaltos más relacionados al yacimiento Júcaro (estratoligados) y a los sills sinvolcánicos de diabasas y basaltos del yacimiento Yagruma (zona de conductos); están presentes los valores más bajos de Na2O+CaO(adj) y mayores en FeOt+MgO(adj), con AI entre 26 y 52. El estudio reveló que la alteración hidrotermal es de baja intensidad, con la asociación de minerales típicos descritos para este proceso en los VMS (Volcanic-Massive Sulfide): clorita, calcita, pirita, cuarzo, sericita y epidota. Se corrobora que en ambos depósitos las alteraciones minerales son similares, solo que en Júcaro predomina clorita-calcita-pirita-sericita y en Yagruma clorita-pirita-calcita- sericita-epidota-cuarzo.

Mohammad Nurcholis, Dwi Fitri Yudiantoro, Darban Haryanto et al.

Activities of Old Lawu volcano in the past resulted in various soil development in the south side of the volcano, especially in the area of the east side of Wonogiri and west side of Ponorogo, that located in the Java island. This area is also famous as herbal crops producer for raw materials of traditional herbal medicine industry. The purpose of this research is to identify the process of exposure of the parent materials in the Old Lawu volcanic environment, the development of the soil parent materials, and the mineralogy variation.  Four Sites with different geological formation were observed, and each Site was selected for soil profile identification on the process of weathering of rocks.  All layers of each profile were then sampled and then sand fraction of the fine earth particle was determined for the mineral type using X-ray diffraction (XRD).  The results showed that materials of the Old Lawu volcanic activity that was of quaternary period had sized material of gravel, sand, silt, and clay. These materials were then exposed and undergone the process of weathering, transportation, and sedimentation according to the landform at that period. The development of soil as geoheritage in this region was influenced by the material size, landform, and settling process. There are four typologies of the environment on the soil development at the present study, there are: (1) Soil that developed in the inter-fluvial basins is characterized by thick soil and a clear horizon differentiation process as a feature of maturity stage of soil development. (2) Soil developed on the slope of hill area is characterized by shallow in solum and it is in early-stage soil development. (3) Soil developed on the volcanic materials that were transported and sedimented in the river environment is characterized by the thick soil above the layered fine size parent material with a variety of color. (4) Soil developed on the tertiary breccia showed early soil development.

I.A. Blinov, M.N. Ankushev, M.A. Rassomakhin et al.

The Novy Temir Fe occurrence with magnetite ores and copper mineralization is confined to the central part of the Kulikovsky serpentinite massif. Copper was mined here ig the Bronze and Early Iron ages. The primary Cu minerals are chalcopyrite and bornite. The secondary Cu minerals in oxidation zone include malachite, chrysocolla, azurite, and rare delafossite, covellite and chalcocite. In addition to copper minerals, the ores contain Ni and Co sulfides, arsenides and sulfarsenides (pentlandite, nickeline, cobaltite, unidentified Ni arsenide). The Ni, As and Co minerals in oxidation zone are rare, which is related to their rare primary occurrence.

Z. S. Nikiforova

Identification of indicators on placer gold allowed predicting formation types of primary sources of gold and their location in the East Siberian platform, overlapped by thick cover of Meso-Cenozoic deposits, where traditional methods of prospecting of gold deposits are ineffective. Each gold-ore formation is known to have the corresponding geological-structural control. Based on study of mineralogical-geochemical features of placer gold and regularity of its distribution, it has been determined that primary sources of low-sulfide gold-quartz, gold-copper-porphyritic, gold-ferruginous-quartzite, gold—rare-metal and gold-platinoid formations are confined to basement outcrops and their framings; gold-sulfide-quartz formation — to metasomatites, developed within the zones of deep faults, tracing the terrigenous-carbonate series; and manifestations of gold-silver formation are localized in the zones of intracontinental paleorifts. In general, identification of geological-structural location of specific types of gold sources favors more correct selection of prospecting methods for gold deposits in areas with thick sedimentary cover, and evaluating their potential.

ZHOU Jiao-hua, WANG Jian-yu, GU Ming-xin et al.

The Tangjiaping molybdenum deposit, discovered in 2006, is a large-scale porphyry molybdenum deposit. The studies of its basic geological work and metallogenic regularity have been completed by previous researchers with conventional geological methods. The results showed that the Tangjiaping molybdenum deposit belongs to the plutonic hypabyssal porphyry molybdenum deposit and the Tangjiaping granodiorite porphyry molybdenum mineralization acts as a mother rock mining area. However, there is no proof that the metallogenic regularity is from the perspective of mineralogy. From the mineralogy angle four kinds of typomorphic characteristics (chemical composition, trace elements, crystal structure, lattice parameters, structure etc.) of molybdenite, pyrite, quartz, and feldspar from the Tangjiaping molybdenum mine were studied using the separation of single mineral to chemical analysis, X-ray Diffraction and rock mineral identification technology. The results are: ① the mining area of molybdenite is 2H type, and the content of Re component is very low; the δ34S of molybdenite values is 3.0‰; sulfur isotopic changes in a small range close to the meteorite sulfur with the characteristics of deep source sulfur, indicating that the sulphur is derived from the granite porphyry. ② pyrite is rich in trace elements Mo, Co, Cu, poor Ni, Pb, Zn, in addition, low content of Ag and Au, S/Se is greater than 250000, which is consistent with the 'S/Se more than 250000-500000 area could not find gold' conclusion. ③ quartz is rich in Mo. δ18O in mineralization quartz veins are lower, showing a small amount of precipitation appeared in the hydrothermal alteration during the later mineralization. ④ feldspar is rich in Mo. Light rare earth (La-Nd) content is higher than that of rare earth (Sm-Ho) content and heavy rare earth (Er-Y) content. Potassium feldspar degree of ordering, triclinic and structure parameter values were higher. Potassium feldspar structure at low temperature, shows that the potassium feldspar in this area is triclinic symmetry microcline at low temperature conditions, and it is the product of hydrothermal activity; rock potash feldspathization characteristics of molybdenum mineralization is the most favorable. The Tangjiaping molybdenum deposit therefore belongs to the plutonic hypabyssal porphyry molybdenum deposit, and this paper has important significance for the summarization of porphyry type molybdenum ore metallogenic regularity and prospecting marks.

L. Pérez Cruz, J. Urrutia Fucugauchi

Results of a rock magnetic study of marine sediments from the Alfonso Basin, Bay of La Paz are used to investigate sediment sources and depositional environment in the southern Gulf of California during the Holocene. Radiocarbon dating provides stratigraphic control, with age for the core bottom sediments of 7597-7831 cal. yr B.P. Magnetic signal is dominated by fine-grained titanomagnetites, derived from the silicic volcanic units surrounding the Bay of La Paz. Magnetic mineralogy is relatively homogenous as seen in bulk magnetic properties of low-field susceptibility, remanent intensity and coercivity. Magnetic hysteresis loops show strong variable paramagnetic components; after paramagnetic correction loops show saturation at low fields and high saturation magnetization values. Plots of hysteresis parameter ratios for domain state show that samples group in the pseudo-single domain field, with mixtures of single and multi-domain particles. Magnetic susceptibility log shows relatively high frequency dependence factors, particularly for the Middle Holocene, suggesting contribution of fine-grained superparamagnetic minerals related to eolian deposition. The well-preserved laminated sequence indicates predominant anoxic conditions in the basin floor. Depositional environment had a dominant supply of pluvial detrital sediments and eolian fimaterial with less abundant biogenic input.

S. Pilet, J. Hernandez, B. Villemant

Lewis D. Ashwal, Bruce Cairncross