Hydrothermal alteration of the surface volcanic rocks at the Acoculco geothermal field,Mexico:a multi-parametric approach

The studies on hydrothermal alteration-induced eff ects in surface and subsurface rocks provide useful information in the characterization and exploitation of a geothermal reservoir.Generally,these studies are based on traditional,and reliable methods like petrography(primary and secondary minerals,and grade of alteration),and geochemistry(mobility of elements,changes in mass and concentration of elements,and fluid inclusions).Recently,apart from these established methods,some methods based on the geochemical(Chemical Index of Alteration,CIA;Weathering Index of Parkar,WIP;Loss on Ignition,LOI;and Sulfur,S)and rock magnetic properties(magnetic susceptibility,χlf;and percentage frequency-dependent susceptibility,χfd%)are also being applied in the identification of whether a rock is an altered or a fresh one.The Acoculco Geothermal Field(AGF),Mexico,is characterized by high temperature and very low permeability,and it is considered a promissory Enhanced Geothermal System.The following changes are observed in the rocks as a result of an increase in hydrothermal alteration:(1)an increase in CIA,LOI,and S values,and a decrease in WIP;(2)an increase in quartz and quartz polymorph minerals(silicification),and clay minerals(argillization);and(3)decrease inχlf values.At AGF,the most altered surface acid rocks are characterized by entirely quartz and its polymorphs,and clay minerals.The present study also indicates the applicability of the binary plots of major elements(felsic vs mafic component)and rock magnetic parameters(χlf vs.χfd%).The rock withχfd%value of 2-10 andχlf value<0.5×10^(-6)m^(3) kg^(-1)indicate the presence of single domain and stable single domain grains,which in turn suggests that it is an altered rock.These methods are simple to apply,rapid,reliable,and have the potential to become eff ective tools for the identifi cation of hydrothermally altered rocks during the initial stage of geothermal exploration.

Geochemical Characteristics and Significance of Major Elements, Trace Elements and REE in Mineralized Altered Rocks of Large-Scale Tsagaan Suvarga Cu-Mo Porphyry Deposit in Mongolia

The alteration types of the large-scale Tsagaan Suvarga Cu-Mo porphyry deposit mostly comprise stockwork silicification, argillization, quartz-sericite alteration, K-silicate alteration, and propylitization. The mineralized and altered zones from hydrothermal metallogenic center to the outside successively are Cu-bearing stockwork silicification zone, Cu-beating argillized zone, Cu-Mo-bearing quartz-sericite alteration zone, Cu-Mo-bearing K-silicate alteration zone, and pro- pylitization zone. The K-silicate alteration occurred in the early phase, quartz-sericite alteration in the medium phase, and argillization and carbonatization （calcite） in the later phase. Ore-bearing-altered rocks are significantly controlled by the structure and fissure zones of different scales, and NE- and NW-trending fissure zones could probably be the migration pathways of the porphyry hydrothermal system. Results in this study indicated that the less the concentrations of REE, LREE, and HREE and the more the extensive fractionation between LREE and HREE, the closer it is to the center circulatory hydrothermal ore-forming and the more extensive silicification. The exponential relationship between the fractionation of LREE and HREE and the intensity of silicification and K-silicate alteration was found in the Cu-Mo deposit studied. The negative Eu anomaly, normal Eu, positive Eu anomaly and obviously positive Eu anomaly are coincident with the enhancement of Na2O and K2O concentrations gradually, which indicated that Eu anomaly would be significantly controlled by the alkaline metasomatism of the circulatory hydrothermal ore-forming system. Therefore, such characteristics as the positive Eu anomaly, the obvious fractionation between LREE and HREE and their related special alteration lithofacies are suggested to be metallogenic prognostic and exploration indications for Tsagaan Suvarga-style porphyry Cu-Mo deposits in Mongolia and China.

Simulation of rock mass balance of two typical altered rocks in Wulong gold deposit of Liaoning, China

Wulong gold deposit hosted in granite and fine-grained d io rite dikes, is a typical gold deposit of quartz vein type. Granite and fine-gra ined diorite dikes in the ore deposit were altered by metallogenic fluid. Based on the altered zoning, the samples of petrochemistry were collected and major an d trace elements were analyzed systematically. Calculation of rock mass balance shows that the major elements (SiO 2, K 2O, P 2O 5, Fe 2O 3) in the two ty pical altered rocks are clearly added into the altered system, and that trace el ements (V, Y, Zr, Ni, Co, Sr) assume an increasing and decreasing tendencies in the altered granite and altered diorite dike, respectively. The fluid/rock ratio s are 30.17-181.00 and 115.44-692.67 respectively for altered granite and alte red diorite dike, they have profoundly effects on metallogenesis.

Estimate of Uniaxial Compressive Strength of Hydrothermally Altered Soft Rocks Based on Strength Index Tests

The purpose of this study was to clarify the relationships between results of index tests and uniaxial compressive strength (UCS) in hydrothermally altered soft rocks of the Upper Miocene, which are typical of the soft rock found in northeastern Hokkaido, Japan. Index tests were performed using point load testing machine and needle penetrometer with irregular lump specimens under forced-dry, forced-wet, and natural-moist states. The relationships between irregular lump point load strength (IPLS) index and UCS, and needle penetration (NP) index and UCS were “UCS = approximately 19 IPLS index” and “UCS = 0.848 (NP index)0.619”, respectively, in soft rocks with a UCS below 25 MPa. These relationships could be applied to on-site tests of rocks with natural moisture content. The UCS could be calculated from IPLS and NP tests on soft rocks only when UCS was below 25 MPa, using the equations obtained as a result of this study.

Enhancing the spontaneous imbibition rate of water in oil-wet dolomite rocks through boosting a wettability alteration process using carbonated smart brines

Most fractured carbonate oil reservoirs have oil-wet rocks.Therefore,the process of imbibing water from the fractures into the matrix is usually poor or basically does not exist due to negative capillary pressure.To achieve appropriate ultimate oil recovery in these reservoirs,a water-based enhanced oil recovery method must be capable of altering the wettability of matrix blocks.Previous studies showed that carbonated water can alter wettability of carbonate oil-wet rocks toward less oil-wet or neutral wettability conditions,but the degree of modification is not high enough to allow water to imbibe spontaneously into the matrix blocks at an effective rate.In this study,we manipulated carbonated brine chemistry to enhance its wettability alteration features and hence to improve water imbibition rate and ultimate oil recovery upon spontaneous imbibition in dolomite rocks.First,the contact angle and interfacial tension(IFT)of brine/crude oil systems were measured for several synthetic brine samples with different compositions.Thereafter,two solutions with a significant difference in WAI(wettability alteration index)but approximately equal brine/oil IFT were chosen for spontaneous imbibition experiments.In the next step,spontaneous imbibition experiments at ambient and high pressures were conducted to evaluate the ability of carbonated smart water in enhancing the spontaneous imbibition rate and ultimate oil recovery in dolomite rocks.Experimental results showed that an appropriate adjustment of the imbibition brine(i.e.,carbonated smart water)chemistry improves imbibition rate of carbonated water in oil-wet dolomite rocks as well as the ultimate oil recovery.

Multi-scale data joint inversion of minerals and porosity in altered igneous reservoirs—A case study in the South China Sea

There are abundant igneous gas reservoirs in the South China Sea with significant value of research,and lithology classification,mineral analysis and porosity inversion are important links in reservoir evaluation.However,affected by the diverse lithology,complicated mineral and widespread alteration,conventional logging lithology classification and mineral inversion become considerably difficult.At the same time,owing to the limitation of the wireline log response equation,the quantity and accuracy of minerals can hardly meet the exploration requirements of igneous formations.To overcome those issues,this study takes the South China Sea as an example,and combines multi-scale data such as micro rock slices,petrophysical experiments,wireline log and element cutting log to establish a set of joint inversion methods for minerals and porosity of altered igneous rocks.Specifically,we define the lithology and mineral characteristics through core slices and mineral data,and establish an igneous multi-mineral volumetric model.Then we determine element cutting log correction method based on core element data,and combine wireline log and corrected element cutting log to perform the lithology classification and joint inversion of minerals and porosity.However,it is always difficult to determine the elemental eigenvalues of different minerals in inversion.This paper uses multiple linear regression methods to solve this problem.Finally,an integrated inversion technique for altered igneous formations was developed.The results show that the corrected element cutting log are in good agreement with the core element data,and the mineral and porosity results obtained from the joint inversion based on the wireline log and corrected element cutting log are also in good agreement with the core data from X-ray diffraction.The results demonstrate that the inversion technique is applicable and this study provides a new direction for the mineral inversion research of altered igneous formations.

Hydrothermal Alteration of Basaltic Rocks at Eruptive Vents on Mount Cameroon Volcano, West Africa

The study of changes in rocks due to interaction with hydrothermal fluids at active volcanoes provides insights into wall rock alteration associated with ore deposits formed in the geological past. Therefore, the nature of mineral alteration and chemical changes experienced by wall rocks can be investigated at eruptive sites on active volcanoes and the results used to better constrain ore-forming processes. In this study, we investigated the alteration at eruptive sites at Mount Cameroon volcano. These eruptive vents lie along NE-SW-trending fissures that define the Mount Cameroon rift. The vents are surrounded by cones composed largely of pyroclastic materials and to a lesser extent lava. Fumaroles (volcanic gases) rising through the vents during and after the 1999 eruption have resulted in the alteration of the pyroclastic robble along the fissures and the inner walls of the cones. Consequently, altered basaltic materials are covered with thin films of reddish, yellowish to whitish secondary minerals. These coatings result from an interaction between the surfaces of the basaltic glass with volcanically-derived acidic fluids. Petrographic investigations and XRD analysis of the basalts have identified primary mineral phases, such as olivine, pyroxene (mainly augite) and feldspars. Alteration products revealed include ubiquitous silica as well as gypsum, magnetite, feldspar, alunite and jarosite. Jarosite occurrence indicates that SO2 is the primary volcanically-derived acid source involved in coating formation. High contents of sulfur identified in the basalts indicate that melts at Mount Cameroon can be sulfur saturated as backed by previous melt inclusion data. Whole rock geochemical analysis shows a gain in silica in the altered samples and this ties with the mass balance calculations although minor gains of Al2O3, , MgO, MnO, CaO and K2O are shown by some samples.

Petrophysical characteristics of rocks and sulfides from the SWIR hydrothermal field

Study of petrophysical properties of rocks in seafioor hydrothermal fields has great significance for inves- tigation of seafloor hydrothermal activities, especially for polymetallic sulfides prospecting. In the present study, based on the current experimental conditions, we conducted systematic experiments to measure the magnetic susceptibility, electrical resistivity, porosity, density, as well as acoustic wave velocity of seafloor rocks and sulfides. Subsequently, we measured the physical characteristics of hydrothermal sulfides, basalts and peridotites which were collected from newly discovered seafloor hydrothermal fields at 49.6°E, 50.5°E, 5 1°E, 63.5°E, and 63.9°E of the Southwest Indian Ridge （SWIR）. Previously available and newly collected data were combined to characterize the physical differences between polymetallic sulfides and rocks. We also discussed the impact of hydrothermal alteration on the bedrock and demonstrated how these petrophysical properties of rocks can help in geophysical prospecting of seafloor hydrothermal fields as indicators.

Mineralogical Zonation of Wall-Rock Alteration in Jiaodong Gold Province, North China

Mineralogical zonation of wall rock alterations in Mesozoic granitoids is summarized from 18 gold deposits in Jiaodong gold province on the Asian mobile continental margin of the west circum Pacific rim. This paper deals with wall rock alterations developed around gold mineralizations of the quartz vein type in granitoids and the wall rock alteration type in granitoid basement contacts along Mesozoic fault zones trending mostly NNE SSW and NE SW. Five alteration zones are distinguished from host rock to ore zone, namely the chloritization and sericitization zone, the hematitization rutilization and microclinization zone, the quartz sericitization zone, the pyrite quartz sericitization zone and the pyrite silicification zone. The former two are outer zones marked by incomplete alteration of first mafic and then felsic minerals of the granitoids, while the later three are inner zones marked by complete alteration of both mafic and felsic minerals of the granitoids leading to retrogressive sericitization and progressive silicification with participation of ore elements. The whole process proceeds under dynamometamorphism with high fugacity of volatiles. Wall rock alteration is the intermediate link between unaltered host rock and ore mineralization both in time and space. Development of the alteration zonation and its mineral composition controls genetic type of mineralization, size and grade of the deposit and location of the ore zones.

Significant Mercury Isotope Anomalies in Hydrothermal Altered Coals from the Huaibei Coalfield,China

Igneous intrusion into coal-bearing strata can cause an abrupt increase of temperature-pressure and trigger hydrothermal circulation at the igneous rock-coal contact zone. The conductive heat of intruded lnagma and convective migration of secondary hydrothermal fluids have been observed to significantly alter the concentrations and modes of occurrence of mercury （Hg） in thermally-intruded coals.

GEOCHEMICAL KINETIC SIMULATION ON WALL-ROCK ALTERATION ZONING IN THE BAOBAN GOLD FIELD, SOUTH CHINA

The Baoban gold field, located in the area of Hainan Island, South China, bears three gold de-posits: Tuwaishan, Baoban, and Erjia, which belong to the wall-rock alteration type. In terms of the petrography data, four alteration zones can be recognized from the ore-vein outwards: (1) the silicification zone; (2) the sericitization zone, (3) the chloritization zone; and (4) the calcitization zone. The geochemical kinetic simulation was carried out on sericitization and chloritization . Results of the study showed that the formation of the alteration zones is connected with the properties of hydrothermal fluids: the initial concentration of Al (O H)3, the diffusion coefficient and the velocity of fluid flow. The alteration zones in this field are the result of multiple interaction between hydrothermal fluids and wall rocks.

Characteristics of Element Migration in the Process of Wall-Rock Alteration in the Shibangou Gold Deposit,Western Henan

Abstract The Shibangou gold deposit in western Henan is associated with irregular quartz veinlets occurring in altered shear zones dissecting a dioritic intrusion. The altered shear zones are characterized by silicification, pyritization, sericilization, chloritization and K-feldspar alteration. Zoning of altered rocks adjacent to the Au-bearing quartz veins is obviously exhibited. Fine-grained sulphides and quartz veinlets of different ages and small-scale fissures are widely distributed in the central part of the altered zones. Major mineralization types in this gold deposit are Au-bearing quartz veinlets and altered rocks in the shear zones. Samples were collected from drilling cores according to the alteration zoning and mineralization type and all samples were analyzed for major and trace elements. Mass balance, volume change (fv=97.3–71.9%) and major element variation sequences are studied in terms of major elements. The changes of mobile components (SiO2, K2O, Fe2O3) and CaO) and immobile component (Al2O3) in the wall-rock alteration are discussed. The gold mineralization is associated with the enrichment of As, Ag, Hg and Pb and depletion of Cu and Zn. The study of compositional variation of altered rocks proves to be a very efficient method for defining the extent of wall-rock alteration, fluid activity and mineralization and enrichment.

A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasivelyaltered or metamorphosed and contain abundant secondary minerals such as albite, chlorite, epidote,prehnite, pumpellyite, actinolite, titanite, quartz, and/or calcite. They were denoted as spilitesor spilitic rocks in terms of their petrographic features and mineral assemblages. The metamorphicgrades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. Thisindicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area haveformed as a result of Caledonian regional metamorphism. We suggest that the previously denotedspilitic rocks or altered volcanic rocks should be re-denoted as metabasalts or metabasaltic rocks.The metamorphic grade of the volcanic rocks increases with their age: prehnite-pumpellyite faciesfor the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpellyite to lowergreenschist facies for the lower part of the Middle Ordovician volcanic rocks, and lower greenschistfacies for the Lower Ordovician volcanic rocks. The P-T conditions are estimated as T = 240 - 290deg C and P = 1.5-4.5 kbar for the lower part of the Middle Ordovician rocks, and T = approx 300 degC for the Lower Ordovician rocks. The variations of mineral assemblages occurring at differentdomains of the volcanic rocks were controlled by the variations of the effective bulk composition inthose domains during metamorphism. The geochemical characteristics of Mg-Al chromite in theMayaxueshan volcanic rocks are consistent with an origin of island arc environment.

FRACTURE MECHANICS CONDITIONS OF WALLROCK ALTERATION

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Spectrum of fluid/rock alteration in the IOCG system of eastern Gawler Craton

In this paper we present petrology and geochemistry from a range of metasomatically altered rocks within the Eastern Gawler Craton mineralised province of South Australia.The rocks represent four protolith types;granite,basalt,non-carbonate metasediment and calc-silicate metasediment.Alteration paragenesis varies in intensity and between the protolith types.I.The alteration paragenesis of granites has the first pervasive chlorite and the second filled hematite with two generations of chalcopyrite.After the

Petrographic Studies on Hydrothermally Altered Conglomerates at Naer Creek, Sichuan Province, China

Based on analysing the wall rock alterations and related rocks which were used as a hint for mineral deposits and as a basis of mechanism research of fluid rock interaction, according to the results of petrographic analysis, the authors of this paper consider that the hydrothermally altered conglomerates have the following features. First, the conglomerates studied can be divided into two groups: Group A consists mainly of the gravels of granite and basalt; Group B is mainly composed of the gravels of basalt, limestone, mudstone and granite. Second, three distinct hydrothermal alterations or diagenesis were distinguished in Group A, that is, pre depositional hydrothermal alteration of the volcanic gravels (saussuritzation chloritization, epidotization chloritization, albitization and sericitization), mechanical diagenesis and post diagenetic hydrothermal alteration (epidote chlorite and prehnite calcite association). Third, the Group B is subjected mainly to pre depositional hydrothermal alteration; the alteration types are the same as of Type A. Last, the origin of the post diagenetic hydrothermal alteration of Group A is presumed to be related to the ascending of thermal water along the Naer Creek fault.

Distributive characteristics of reservoirs and exploration potential associated with intrusive rocks of Yingcheng Formation in Yingtai rift depression, NE China

Petroleum geologists have paid great attentions to the volcanic reservoirs of Songliao Basin in NE Chi- na. There are plenty of subvolcanic rocks in the Songliao Basin accompanying the Early Cretaceous Yingeheng Formation. The logging data show the good reservoir potential of these intrusive rocks but the distribution char- acteristics and formation mechanisms of these reservoirs are not clearly understood. Based on the previous stud- ies by using coring, cuts and logging data of Yingtai rift depression, the reservoirs＇ characteristics of intrusive rocks are presented. There are two types of intrusive rocks namely the syenodiorite-porphyrite and diabase which occur as laccolith and/or sill, both having the characteristics of low gamma and high density with little primary porosity and permeability. The prevalent reservoir porosity is the secondary porosity, such as spongy/cavernous pore, tectonic fracture. The laboratory data of porosity of diabase can reach 6.7%, but the permeability is less than 0.6 x 10-3μm2, median pressure is high, indicating that the pore throat of this kind reservoir is small. The maximum logging porosity is about 12%. The change of porosity does not correlate to the buried depth. It is the major significant differences in the distributive characteristics compared to the normal sedimentary rock reservoirs. Most of intrusive rocks underwent alteration diagenesis whilst some were subjected to precipitation diagenesis. The spongy and cavernous pore can be formed during the alteration processes of plagioclase to illite and pyroxene to chlorite. The secondary porosity is greatly correlated with the alteration intensity of matrix, pla- gioclase and pyroxene. There are pyroxenes and more plagioclases in diabase, which cause the higher alteration intensity than the syenodiorite-porphyrites in the same acid fluid. So the porosity of diabase is higher than that of syenodiorite-porphyrites. The top or/and bottom part of intrusive rocks develop the higher porosity. Because those parts are easy to contact formation fluid, and the shrink fractures give the more surface for reaction be- tween fluid and rock. The porosity of intrusive rocks is same to the volcanic rocks in Yingtai rift depression and Xujiaweizi rift depression which bear the prolific gas. It suggests good reservoir potential. Intrusive rocks are hosted by the dark mudstone which indicates semi-deep and deep lake facies belt.

Effect of Groundwater Radiolysis on the Wall-rock Alteration of Uranium Ore Deposits

Reported for the first the in this paper are the results of simulating experimnts on theγ-irradiation-induced oxidation of Fe2+ under the physicochemical conditions tr= 200℃,P= 50 MPa,Eh= -0.1V, PH=7.2) similar to tfor under which moderate-low temperature hydrothermal uraniumdeposits are formed . Evdence shows that the effect of groundwater radiolysis m to be the majormechanism of wall-rock alteration (hematitizaton ) of hydrothermal uranium deposits.Moreover, adiscussion was made of possible dets of radiolyss of the waterrock system on wall-rock alterationsincluding argillization and decoloration of uranium ore deposits on the basis of the experimentalresults.

^(40)Ar-^(39)Ar dating and geological implication of auriferous altered rocks from the middle-deep section of Q875 gold-quartz vein in Xiaoqinling area, Henan,China

The 40Ar-39Ar age method is employed in this work to analyze the auriferous altered rocks from the middle-deep section of the Q875 gold-quartz vein in the Xiao-qinling area, and the results show that the main gold deposition of the Q875 occurred in ca. 128-126 Ma. As a typical gold-bearing quartz vein in this gold-rich area, the age data obtained from the Q875 also constrain on the metallogenic time of the lode gold deposits developed in the same geological settings. This geochronological study supplies new evidence for further understanding the timing of gold mineralization, the genesis of gold deposits and the geodynamic settings in Xiaoqinling area.

Wettability alteration and oil recovery by spontaneous imbibition of smart water and surfactants into carbonates

Naturally fractured carbonate reservoirs have very low oil recovery efficiency owing to their wettability and tightness of matrix.However,smart water can enhance oil recovery by changing the wettability of the carbonate rock surface from oilwet to water-wet,and the addition of surfactants can also change surface wettability.In the present study,the effects of a solution of modified seawater with some surfactants,namely C12 TAB,SDS,and TritonX-100(TX-100),on the wettability of carbonate rock were investigated through contact angle measurements.Oil recovery was studied using spontaneous imbibition tests at 25,70,and 90°C,followed by thermal gravity analysis to measure the amount of adsorbed material on the carbonate surface.The results indicated that Ca2+,Mg2+,and SO42-.ions may alter the carbonate rock wettability from oil-wet to water-wet,with further water wettability obtained at higher concentrations of the ions in modified seawater.Removal of NaCl from the imbibing fluid resulted in a reduced contact angle and significantly enhanced oil recovery.Low oil recoveries were obtained with modified seawater at 25 and 70°C,but once the temperature was increased to 90°C,the oil recovery in the spontaneous imbibition experiment increased dramatically.Application of smart water with C12 TAB surfactant at 0.1 wt%changed the contact angle from 161°to 52°and enhanced oil recovery to 72%,while the presence of the anionic surfactant SDS at 0.1 wt%in the smart water increased oil recovery to 64.5%.The TGA analysis results indicated that the adsorbed materials on the carbonate surface were minimal for the solution containing seawater with C12 TAB at 0.1 wt%(SW+CTAB(0.1 wt%)).Based on the experimental results,a mechanism was proposed for wettability alteration of carbonate rocks using smart water with SDS and C12 TAB surfactants.