Dolomitic Rocks Research Articles

Mineralization processes involved in the formation of the Jinya Carlin-type Au deposit, northwestern Guangxi, China: Evidence from in situ trace element and S isotope geochemistry of Au-bearing zoned pyrite

The Jinya Au deposit is a representative example of the Carlin-type Au mineralization located within the “Golden Triangle” area of the Yunnan, Guizhou, and Guangxi provinces, China. The deposit is located within a fault zone at the contact between a carbonate platform and contains Au mineralization hosted by arsenopyrite and pyrite, with the latter providing evidence of changes in fluid composition during pyrite growth. Back-scattered electron (BSE) images indicate that this Au-bearing zoned pyrite can be divided into dark cores (Py1), lighter colored central sections (Py2), and dark rims (Py3). Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and secondary ion mass spectrometry (SIMS) analyses provide information on compositional and S isotopic variations within this pyrite. These data indicate that each pyrite zone is compositionally homogeneous, showing Co/Ni ratios of <1. Pyrite has δ34S values ranging from –8.54‰ to –4.05‰, with Py1, Py2, and Py3 having average δ34S values of –6.89‰, –5.60‰, and –5.99‰, respectively. These values are indicative of thermochemical sulfate reduction (TSR) processes. Mineralizing fluids that formed the deposit derived from a sedimentary source mixed with variable amounts of meteoric water (Py1 = 3%; Py2 = 32%; Py3 = 23%). The ore-forming fluids within the sedimentary basin metasomatized the Fe-rich dolomitic rocks of the forming the early pyrite cores (Py1). This fluid flow focused along the main fault zone, which at this time was permeable, allowing with an influx of meteoric water. This mixing process lowered the temperature of the system, causing the precipitation of large volumes of Au and As. This caused the formation of the light-colored and As- and Au-enriched central parts of the pyrite within the deposit (i.e., Py2). A subsequent decrease in the amount of meteoric water in the system led the formation of the darker Py3 rims that contain low concentrations of As and Au.

Study on the mechanism of reactive acid transport in fractured two-mineral carbonate rocks

Centering on the mechanism of reactive acid transport in fractured two-mineral carbonate rocks, this paper established a two-mineral model for fractured media with the extended finite element method based on the two-scale continuum model. A series of examples are presented with the numerical model, the numerical results show that the variation of volume fraction of dolomite, the surface reaction constant of dolomite as well as the diffusion coefficient can alter the dissolution morphology and dissolution performance. As the volume fraction of dolomite increases or surface reaction constant of dolomite decreases, the dissolution patterns would transform form wormhole to uniform dissolution. And the bedding distribution and porphyritic distribution of dolomite will cause a fragmentation phenomenon for the dissolution morphology. Besides, the bedded dolomite can bring about an increased acid consumption. It can be inferred that the reaction rate difference between limestone and dolomite is the key factor that affects the dissolution morphology, and the retarded acid with a low reaction rate in dolomite rocks does not perform well.

Towards in situ U–Pb dating of dolomite

Abstract. Recent U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has demonstrated that reasonable precision (3 %–10 %, 2σ) can be achieved for high-resolution dating of texturally distinct calcite phases. Absolute dating of dolomite, for which biostratigraphy and traditional dating techniques are very limited, remains challenging, although it may resolve many fundamental questions related to the timing of mineral-rock formation by syngenetic, diagenesis, hydrothermal, and epigenetic processes. In this study we explore the possibility of dating dolomitic rocks via recent LA-ICP-MS dating techniques developed for calcite. The in situ U–Pb dating was tested on a range of dolomitic rocks of various origins from the Cambrian to Pliocene age – all of which are from well-constrained stratigraphic sections in Israel. We present imaging and chemical characterization techniques that provide useful information on interpreting the resulting U–Pb ages and discuss the complexity of in situ dolomite dating in terms of textural features that may affect the results. Textural examinations indicate zonation and mixing of different phases at the sub-millimeter scale (&lt; 1 µm), and thus Tera–Wasserburg ages represent mixed dates of early diagenesis and some later epigenetic dolomitization event(s). We conclude that age mixing at the sub-millimeter scale is a major challenge in dolomite dating that needs to be further studied and note the importance of matrix-matched standards for reducing uncertainties of the dated material.

Controls on shale oil accumulation in alkaline lacustrine settings: Late Paleozoic Fengcheng Formation, northwestern Junggar Basin

Understanding the potential and controls on shale oil accumulation in alkaline lacustrine settings is essential to evaluate its contribution to global shale oil budget. Here we present a case study in the Late Paleozoic Fengcheng Formation of the Mahu Sag, northwestern Junggar Basin, China. Results show that the fine-grained sedimentary rocks in alkaline lacustrine settings are complex in lithology with mixed contribution from argillaceous rocks, dolomitic rocks, and pyroclastic rocks. The massive argillaceous dolomite (with alkali minerals) and pyroclastic rocks have the highest shale oil potential with average OSI values at 344.67 mg HC/g TOC and 124.65 mg HC/g TOC in the study area, respectively. In terms of controls on shale oil accumulation in these two rocks, both rock types have good hydrocarbon generation potential. Coupled with abundant fractures and various types of pores, large amounts of free hydrocarbons are retained. Therefore, the fine-grained sedimentary rocks in alkaline lacustrine settings host prospective shale oil potential and can serve as an important exploration field. This is generalized that shale oil potential in saline lacustrine settings is promising. The complex relationship between TOC and shale oil potential suggest that more studies are needed to precisely evaluate the shale oil potential with possibly new proxies and standards.

Transport of fine-grained sediment in oceanic currents: Holocene supply to sediment drifts around Flemish Cap by the Labrador Current

Hemipelagic sediment on the upper continental slope is commonly transported from the adjacent continental shelf, but along-slope transport of sediment by thermohaline currents may be an important source of sediment. One such case is Flemish Cap, a promontory of the eastern Canadian continental shelf protected from direct continental sediment supply by the deep-water Flemish Pass. Pleistocene sediment drifts around Flemish Cap accumulated glacially derived sediment from the north, transported by the Labrador Current. This study determines the changing sources and controls on the accumulation of bioturbated hemipelagic sediment through the Holocene. From a set of >150 box cores in water depths of 600–1200 m, some 30 push cores, up to 55 cm long, were studied in detail for stratigraphic variation in grain size, bulk geochemistry (XRF) and detrital petrology (granule petrology and qXRD). Criteria were developed to identify trawling disturbance. Two Holocene stratigraphic units, A) almost lacking ice-rafted detritus and B) with some coarse IRD and higher detrital carbonate, correlate with similar units farther north on the Labrador Shelf and Slope. Sedimentation rates based on 26 radiocarbon dates were ~ 0.02–0.03 m/ka over much of the region below the 800 m isobath, locally reaching 0.1 m/ka in some sediment drifts, but as low as 0.01 m/ka on southeastern Flemish Cap. Where sedimentation rate was >0.03 m/ka, the record of flow vigor from the sortable silt proxy since 6 ka shows three different zones of Labrador Current flow with different timing of changes in flow vigor. The mid to early Holocene record shows that the meltwater-driven inner Labrador Current extended across the entire study area. Where sedimentation rates are lower, periods of sediment bypassing are inferred and the sortable silt record varies little. Correlation with external drivers is hampered by century-scale core resolution and sparse dating control. The distinctive abundance of dolomite rock flour in hemipelagic sediments around Flemish Cap indicates important supply from Baffin Bay icebergs during the entire Holocene, diluted prior to 6 ka by sediment from Hudson Strait and some Labrador ice streams. Baffin Bay icebergs melt preferentially in the southern Labrador Sea, dropping their load, with the fine grained component transported to the south by the Labrador Current. Similar partitioning of coarse- from fine-grained components of IRD may take place on other high-latitude outer continental margins.

Surfactant-Aided Low-Salinity Waterflooding for Low-Temperature Carbonate Reservoirs

Summary Carbonate reservoirs tend to be oil-wet/mixed-wet and heterogeneous because of mineralogy and diagenesis. The objective of this study is to improve oil recovery in low-temperature dolomite reservoirs using low-salinity and surfactant-aided spontaneous imbibition. The low-salinity brine composition was optimized using ζ-potential measurements, contact-angle (CA) experiments, and a novel wettability-alteration measure. Significant wettability alteration was observed on dolomite rocks at a salinity of 2,500 ppm. We evaluated 37 surfactants by performing CA, interfacial-tension (IFT), and spontaneous-imbibition experiments. Three (quaternary ammonium) cationic and one (sulfonate) anionic surfactants showed significant wettability alteration and produced 43–63% of original oil in place (OOIP) by spontaneous imbibition. At a low temperature (35°C), oil recovery by low-salinity effect is small compared with that by wettability-altering surfactants. Coreflood tests were performed with a selected low-salinity cationic surfactant solution. A novel coreflood was proposed that modeled heterogeneity and dynamic imbibition into low-permeability regions. The results of the “heterogeneous” coreflood were consistent with that of spontaneous-imbibition tests. These experiments demonstrated that a combination of low-salinity brine and surfactants can make originally oil-wet dolomite rocks more water-wet and improve oil recovery from regions bypassed by waterflood at a low temperature of 35°C.

A New Model for Formation of Lacustrine Primary Dolomite by Subaqueous Hydrothermal Venting

AbstractAlmost 230 years after it was first described, formation of the mineral dolomite remains enigmatic. Dolomitic rocks are abundant throughout the geological record, and most are thought to form by replacement of limestone. Recent work has identified substantial volumes of high‐temperature primary dolomite deposited in Cretaceous lacustrine rift basins in Northern China. Here we provide the missing conceptual understanding of the fundamental processes that may have formed such dolomitic sediments by combining an analysis of the physics and chemistry of sub‐lacustrine hydrothermal systems with new data on the mineralogy and geochemistry of the dolomites. This novel mechanism accounts for systematic changes in the characteristics of the dolomite sequence that intimately link with the evolution of convection within the fault damage zone and venting of brine‐rich mixtures to the lake basin, and provides a new model to be tested against dolomitic sediments in high‐enthalpy settings such as rifting basins.

Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

The “Dolomite Problem” has been a controversy for over a century, owing to massive assemblages of low-temperature dolomite in ancient rocks with little dolomite forming today despite favorable geochemical conditions. Experiments show that microbes and their exopolymeric substances (EPS) nucleate dolomite. However, factors controlling ancient abundances of dolomite can still not be explained. To decode the enigma of ancient dolomite, we examined a modern dolomite forming environment, and found that a cyclic shift in microbial community between cyanobacteria and anoxygenic phototrophs creates EPS suited to dolomite precipitation. Specifically, EPS show an increased concentration of carboxylic functional groups as microbial composition cycles from cyanobacterial to anoxygenic phototroph driven communities at low-and high- salinity, respectively. Comparing these results to other low-T forming environments suggests that large turnover of organic material under anoxic conditions is an important driver of the process. Consequently, the shift in atmospheric oxygen throughout Earth’s history may explain important aspects of “The Dolomite Problem”. Our results provide new context for the interpretation of dolomite throughout Earth’s history.

Effects of freeze-thawing cycles on the physical and mechanical properties of basaltic and dolomitic rocks evaluated with a decay function model

Basalts from Corlu Karatepe and dolomite from Kirklareli subjected to freeze-thawing (F-T) cycles were studied, and the resulting changes were analyzed. The unweathered rocks and those subjected to 50 F-T cycles after every 10 cycles were tested for the water absorption by weight and volume, P-wave velocity, uniaxial compression strength, point load strength index, and Schmidt hardness values under laboratory conditions. In addition, the relationship between physical and mechanical properties was examined after every 10 cycles, and an exponential model was proposed for the mechanical properties. Two model-generated parameters, i.e., the decay constant and half-life value for each mechanical property, were used to evaluate the disintegration rate of the rocks under the cyclical action of F-T. The evaluation suggests that disintegration behavior was not comparable between the two types of rocks. However, the basalt samples had similar mechanical properties, which indicated a high degree of correlation.

Underground operations in Internatsionalny mine rock mass susceptible to gas dynamic phenomena

Starting from 2019 extraction of diamond ore reserves from Internatsionalnaya pipe by Internatsionalny mine of ALROSA is carried out in complex geological and geomechanical conditions. Geodynamic situation is complicated by a few gas dynamic phenomena of different nature and scale recorded. The investigation results on gas release dynamics from dolomite rock mass during drivage of Spiral decline, in a test site on level.-802/-820 m in case of prediction and prevention measures undertaken to combat gas dynamic phenomena are presented. The dynamic characteristics of gas in outburst-hazardous dolomite rocks are calculated by the initial velocity of gas release. Efficiency of the current outburst hazard prediction method using facility MIG-Ts1 is proved. At the same time, it is necessary to perform additional studies to collect sufficient statistics and reliable data on gas release and gas pressure during drivage operations in the mine. The authors appreciate participation of Deputy CEO of VostNII Science Center, Doctor of Engineering Sciences, Professor V. S. Zykov, Doctor of Engineering Sciences, Professor A. V. Dzhigrin, Director of Research Center for Applied Geomechanics and Convergent Technologies in Mining at NUST MISIS College of Mining, Doctor of Engineering Sciences, Professor of the Russian Academy of Sciences V. A. Eremenko and Director of VNIMI’s Kemerovo Division, Candidate of Engineering Sciences P. V. Grechishkin.

The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance

A recent storm (27th–30th October 2018), named Vaia, hit most part of the Northeast of Italy affecting the geomorphic aspect of almost all mountain catchments of the area. The event triggered new instabilities such as windthrows, landslides and debris flows. At present, few studies dealt with the analysis of the impact of a Large Infrequent Disturbance at large catchment scale. This work provides a focus on the Tegnas Torrent Basin (Belluno Province) and aims at detecting how, where, and how much this storm affected the basin. Moreover, it integrates two different approaches considering both the dynamic and static aspects of the sediment, via DEM of Difference (DoD) and Index of Connectivity (IC), respectively. The Tegnas sub-basins responded contrastingly: the Bordina (volcanic origin and covered by pastures and spruce forests) was mainly affected by windthrows (7% of the sub-basin area) and landslides (0.5%), while the Angheraz (outcropping dolomite rocks), was stricken only by debris flows (1.0%). Morphological changes were clear along the entire channel network, with predominant erosion in the steepest upstream parts (over 2 m of the channel elevation), and deposition in the lower main valley floor (over 3 m of the channel elevation). The IC analysis along the instabilities highlighted that the windthrows occurred mainly in areas of high connectivity, which may be important for future management strategies. Moreover, the proposed integrated approach, based on the combination IC-DoD, permitted a detailed identification of sediment routing and a contemporary estimation of erosion and deposition volumes generated by a high magnitude low-frequency event. Based on these results, cascading processes are expected and further analysis are required to fully consider the impact of a Large Infrequent Disturbance.

Forest Soil Colloids Enhance Delivery of Phosphorus Into a Diffusive Gradient in Thin Films (DGT) Sink

Phosphorus (P) is preferentially bound to colloids in soil. On the one hand, colloids may facilitate soil P leaching leading to a decrease of plant available P, but on the other hand they can carry P to plant roots, thus supporting the P uptake of plants. We tested the magnitude and the kinetics of P delivery by colloids into a P sink mimicking plant roots using the Diffusive Gradients in Thin-Films (DGT) technique. Colloids were extracted with water from three forest soils differing in parent material using a method based on dispersion and sedimentation. Freeze-dried colloids, the respective bulk soil, and the colloid-free extraction residue were sterilized and mixed with quartz sand and silt to an equal P basis. The mixtures were wetted and the diffusive fluxes of P into the DGTs were measured under sterile, water unsaturated conditions. The colloids extracted from a P-poor sandy podzolic soil were highly enriched in iron and organic matter compared to the bulk soil and delivered more P at a higher rate into the sink compared to bulk soil and the colloid-free soil extraction residue. However, colloidal P delivery into the sink was smaller than P release and transport from the bulk soil developed on dolomite rock, and with no difference for a soil with intermediate phosphorus-stocks developed from gneiss. Our results provide evidence that both the mobility of colloids and their P binding strength control their contribution to the plant available P-pool of soils. Overall, our findings highlight the relevance of colloids for P delivery to plant roots.

Occurrence states and potential influencing factors of shale oil in the Permian Fengcheng Formation of Mahu Sag, Junggar Basin

Taking the Permian Fengcheng Formation of the Mahu Sag of the Junggar Basin as an example, a systematic study of shale oil was carried out for the occurrence states and potential influencing factors by integrating various data including cores, well logging, X-ray diffraction, argon ion polishing and field emission-scanning electron microscopy (FE-SEM) observations, high-pressure mercury injection (HPMI), rock pyrolysis, and two-dimensional nuclear magnetic resonance (NMR) experiments. The shale oil of the Fengcheng Formation in the Mahu Sag mainly has two modes of occurrence:film-like adsorbed oil and infilled free oil. The main constrains for shale oil occurrence state include lithofacies association, mineral composition and reservoir space. Four lithofacies associations namely dolomitic shale, sandy shale-dolomitic siltstone, dolomite rock containing alkaline mineral-argillaceous siltstone, and siliceous dolomite rock-dolomitic siltstone were recognized in the shale reservoirs of the Fengcheng Formation. The mineral composition, reservoir space, micro-pore structure and shale oil occurrence states of different lithofacies appeared to be quite different. The contents of quartz, pyrite and organic matter were positively correlated with thermal cracking hydrocarbon (S2), and the contents of feldspar and dolomite showed a weak positive correlation with free hydrocarbon (S1). The free oil was mainly stored in mesopores and large pores, such as secondary dissolution pores and residual intergranular pores. The absorbed oil was mainly stored in mesopores and small pores, such as organic pores, intercrystallite pores and the surface of mineral particles. The mineral composition and pore-throat structure were dominant factors controlling the occurrence state of shale oil.

Dynamic stability analysis of rock tunnels subjected to impact loading with varying UCS

The present paper has been carried out to understand the effects of impact loading on the rock tunnels, constructed in different region corresponding to varying unconfined compressive strength (UCS), through finite element method. The UCS of rockmass has substantial role in the stability of rock tunnels under impact loading condition due to falling rocks or other objects. In the present study, Dolomite, Shale, Sandstone, Granite, Basalt, and Quartzite rocks have been taken into consideration for understanding of the effect of UCS that vary from 2.85 MPa to 207.03 MPa. The Mohr-Coulomb constitutive model has been considered in the present study for the nonlinear elastoplastic analysis for all the rocks surrounding the tunnel opening. The geometry and boundary conditions of the model remains constant throughout the analysis and missile has 100 kg of weight. The general hard contact has been assigned to incorporate the interaction between different parts of the model. The present study focuses on studying the deformations in the rock tunnel caused by impacting load due to missile for tunnels having different concrete grade, and steel grade. The broader range of rock strength depicts the strong relationship between the UCS of rock and the extent of damage produced under different impact loading conditions. The energy released during an impact loading simulation shows the variation of safety and serviceability of the rock tunnel.

Vibrations Induced by Mechanical Rock Excavation on R.C. Buildings in an Urban Area

The paper describes the numerical approach adopted to investigate the effects of vibrations induced on reinforced concrete (R.C.) buildings by the excavation works needed to bury an existing railway line crossing an urban area in the south of Italy. The construction works are carried out in dolomitic rocks, characterized by a high resistance to excavation. Therefore, they may have a great impact on the surrounding environment in terms of induced vibrations. The study is conducted through an uncoupled approach, investigating the dynamic response of the geotechnical system and the above-surface structure, separately. The impulse force equivalent to the dynamic action of a breaker hammer is used as input motion for 2D finite element (FE) geotechnical simulations of the wave propagation process occurring during the excavation. Then, the acceleration time histories obtained from the geotechnical analyses are adopted to study the dynamic performance of an “index” R.C. building, representing the most recurrent structural typology in the examined area, through a 3D FE model. The results show how the adoption of a mitigation strategy consisting in the execution of a preliminary vertical cut followed by a rock crushing treatment allows to significantly reduce the vibrations induced by the excavation processes on existing buildings.

CTAB adsorption onto dolomite in the presence of ionic liquid and deep eutectic solvent: Experimental and theoretical studies

Surfactant adsorption onto reservoir rock surfaces would undesirably influence its functionality from standpoint of enhanced oil recovery (EOR). This study investigates the performance of an imidazolium-based ionic liquid (IL) and a choline chloride-based deep eutectic solvent (DES) as additives to inhibit cetyltrimethylammonium bromide (CTAB) adsorption onto dolomite rock surface from experimental and theoretical perspectives. In the experimental part, static adsorption tests were conducted using UV–Vis spectrophotometer to find the adsorption density of the chemicals in both single (e.g. only CTAB) and combination modes (CTAB-IL and CTAB-DES). In addition, Langmuir and Freundlich isotherms were used to fit the experimental data and also to discern the adsorption patterns. In the theoretical part, interaction energies were determined using the XDLVO theory. The experimental results revealed lower adsorption of IL and DES in comparison with CTAB. Moreover, the addition of IL and DES, as potential adsorption inhibitors, effectively reduced CTAB adsorption where almost 73% reduction was observed when 1000 ppm of the IL was added to 2000 ppm of CTAB in seawater. From theoretical investigation, it was found that among different interaction energies, Lewis acid-base interaction was primarily dominant. The consistency between the experimental and theoretical results was further confirmed when the work of adhesion was calculated and compared for different chemicals investigated in this study.

Dolomite rock sand as fine aggregate replacement in construction activities: A comparative study

This paper attempts to study the use of dolomite rock sand (DRS) as fine aggregate replacement to crushed sand considering the increasing demand from construction industry and depleting crushed sand availability, in this regard DRS as fine aggregate (FA) replacement might be a sustainable option. Further- DRS was chosen for this study, considering its local availability in abundance in the region of Coimbatore and its transportation within the region proves to be cost effective in comparison with crushed sand. Concrete design mixes were prepared to achieve concrete grades of M20, M25 and M30 for FA replacement by 0, 10, 20 and 30% for each concrete grade. Tests on fresh and hardened concrete were conducted. The scope of the study also focused on durability aspects of concrete. The results of the study indicated that, for all concrete grades, 20–30% of FA replacement by DRS was found to be optimum, further comparative analysis was performed between control sample with crushed sand as fine aggregate and with crushed sand replaced by DRS in various % replacements. DRS showed better performance in concrete and was found to be a good alternative replacement for crushed sand. Also, Scanning Electron Microscope (SEM) analysis and Energy Dispersive X-Ray Analysis (EDX) was done to analyze the packing and chemical composition of the aggregates in concrete and suitable conclusions were drawn. Thus, this research in using DRS with the combinations of crushed sand in specific proportions has proven to be a sustainable option.

A combined multivariate approach analyzing geochemical data for knowledge discovery: The Vazante – Paracatu Zinc District, Minas Gerais, Brazil

The Vazante Group comprises a Proterozoic sequence of carbonate and siliciclastic rocks located in Minas Gerais, Brazil. It is the host of the Vazante-Paracatu Zinc District, including world-class hypogene zinc silicate deposits, in the southern part, and several Pb – Zn sulfide deposits in the northern part, all hosted in dolomitic rocks. A recent study revealed the occurrence of base metal sulfide mineralization that formed prior to the Brasiliano orogenic event in the siliciclastic rocks (Serra do Garrote Formation) that underlie these dolomite-hosted silicates and sulfide deposits. These siliciclastic rocks were considered as potential sources of elements for the hydrothermal fluids that formed the dolomite-hosted deposits, however there was little evidence of depletion of the source rocks during the orogenic event. In this paper, Random Forest is used in unsupervised mode along with t-distributed Stochastic Neighbor Embedding (t-SNE) and principal component analysis (PCA) over a lithogeochemical dataset of samples of the Serra do Garrote siliciclastic rocks collected throughout the basin to provide insights about processes related to the mineralizing system at the Vazante-Paracatu District.Multivariate analysis reveals that the Serra do Garrote Formation geochemical signature typical of pre-orogenic mineralization. This is characterized by PC1+ with association of Zn, Cd, Cu, Hg, In, V ± Sb, Se, Mo, Re, which corresponds to nearly 60% of the variance in the data. This suggest that these elements are related to a widespread and/or, long-living hydrothermal activity without an efficient mechanism to focus the metal-bearing fluids, causing basin-scale sub-economical Zn and related metal enrichment. Furthermore, PC2 and PC6 distinguish the zones with signatures interpreted to be related to depletion. PC2+ separates Cd-poor sphalerite, which is typical of the generation of sphalerite that occur in zones with textural evidence of remobilization. PC6+ with association of As, organic carbon and S, identified the zones where sphalerite was leached from the pyrite-rich layers. Areas with multivariate signatures of both pre-orogenic enrichment and syn-orogenic depletion in the source siliciclastic rocks include the Vazante-North Extension and Varginha zinc silicate deposits/occurrence, Ambrósia silicate zinc deposit and the Engenho Velho prospect. Preliminary exploration in this prospect revealed hydrothermal alteration typical of the zinc silicate deposits. This study is pioneering in applying these numerical models in possible source rocks to assist in identifying targets for exploration in basins.

PHYSICAL PARAMETERS, TENSILE AND COMPRESSIVE STRENGTH OF DOLOMITE ROCK SAMPLES: INFLUENCE OF GRAIN SIZE

The purpose of this paper is to investigate the strength, physical and engineering index parameters of selected dolomitic rocks with emphasis on grain size. For this purpose, three groups of dolomite from north western Iran, with the same mineral composition but different grain size, were selected; fine grain, medium grain and coarse grain. Three sets of laboratory experiments are performed on 32 samples: first; petrography tests for determining mineral composition and their percentage, and microstructure of rock containing grain size and grain size distribution, second; experiments to determine the physical properties of the rocks included density, compressional and shear wave velocity, and the third category of experiments included uniaxial compressive strength test, Brazilian tensile strength and point load strength. According to the results; there are significant positive correlation between grain size and uniaxial compressive strength (r = 0.89), point load strength (r = 0.58), Brazilian strength (r = 0.69), and average Young’s modulus (r = 0.64). Also, with increasing grain size, density decreases (r = –0.77). There is strong correlation between compressional wave velocity and shear velocity (r = 0.88). There are also a strong correlation among the uniaxial compressive strength, Brazilian tensile strength and point load strength.

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, China

ABSTRACT Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.