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Notes: Abstract The strata-bound fluorite-(barite-)Pb-Zn ore deposits occurring within the more than 3,000 m thick Alpujárride Carbonate Formation of the Betic Cordillera, southern Spain, are linked to two defined stratigraphic positions (within the Anisian and at the Ladinian-Carnian transition) and to definite sedimentary contexts. These are highly restricted lagoons isolated from the open sea by calcarenitic barriers with noticeable development of algal mats in their inner margins. Preevaporitic deposition conditions are usually recognized. The ore-bearing horizons are located either at the transition from evaporitic to normal marine deposition (as in the case of the Anisian ore-bearing horizon) or at the transition from normal marine to evaporite deposition (as in the case of the uppermost Ladinian to lowermost Carnian ore-bearing horizon). These vertical facies changes correspond also to the transitions from predominantly terrigenous (continental and/or coastal) sedimentation to marine carbonate sedimentation (Anisian ore deposits) and vice versa (upper Ladinian-lower Carnian). It is proposed that the ore deposits in the Triassic Alpujárride Formation are controlled by certain facies associations in the lagoonal environment. The depositional environment determines the hydrologic regimes responsible for early diagenetic formation of fluorite and of the related ore minerals in a way similar to that of early-diagenetic dolomite. This hypothesis is consistent with field and petrographic observations and with available geochemical and itotopic data reported in other works. The present investigation intends to underline the role played by surface-linked ore formation processes, i.e., by ore-forming processes essentially linked to the environments in which sedimentation and early diagenesis takes place, in the genesis of numerous strata-bound Zn-Pb-(F-Ba) ore deposits in carbonate rocks.

Notes: Abstract A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with 87Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in δ 18O values and essentially no change in δ 13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower δ 13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in δ 18O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.

Notes: Abstract The Pataz region in the eastern part of the North Peruvian Department La Libertad hosts a number of important gold mining districts like La Lima, El Tingo, Pataz, Parcoy, and Buldibuyo. Economic gold mineralization occurs in quartz-sulfide veins at the margin of the calc-alkaline Pataz Batholith, that mainly consists of granites, granodiorites, and monzodiorites. The batholith is of Paleozoic age and cuts the Precambrian to Early Paleozoic low-grade metamorphic basement series. Its intrusion was controlled by a NNW-trending fault of regional importance. The gold-bearing veins are characterized by a two-stage sulfide mineralization. Bodies of massive pyrite and some arsenopyrite were formed in stage 1, and after subsequent fracturing they served as sites for deposition of gold, electrum, galena, sphalerite, and chalcopyrite. It is concluded that gold was transported as a AuCl 2 - -complex by oxidizing chloride solutions and deposited near older pyrite by micro-scale redox changes and a slight temperature decrease. Mineralogical, textural, geochemical, and microthermometric features are interpreted as a consequence of mineralization at considerable depth produced by a hydrothermal system linked with the emplacement of the Pataz Batholith. acteristics in order to outline a general physicochemical model of the hydrothermal ore-forming processes.

Notes: Abstract.  A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lowermost part of the carbonate sequence interacted with 87Sr-enriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in δ18O values and essentially no change in δ13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower δ13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in δ18O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.

Notes: Abstract The Punta del Cobre belt is located 15 km south of Copiapó, northern Chile. It comprises several Cu(-Fe)-Au deposits in the Punta del Cobre and Ladrillos districts, east of the Copiapó river, and the Ojancos Nuevo district, with the new Candelaria mine, and Las Pintadas district, west of the river. The mineralization in the Punta del Cobre belt is characterized by a simple hypogene mineral assemblage of chalcopyrite, pyrite, magnetite, and hematite. Average ore grades are 1.1 to 2% Cu, 0.2 to 0.6 g/t Au, and 2 to 8 g/t Ag. Massive magnetite occurs as veins and irregularly shaped bodies. The ore is spatially associated with alkali metasomatism and in particular with potassic alteration. The Cu(-Fe)-Au deposits are hosted mainly in volcanic rocks of the Punta del Cobre Formation (pre-upper Valanginian) that underlie Neocomian limestones of the Chañarcillo Group. This region experienced backarc basin formation in the Neocomian, uplift and granitoid intrusions in the middle Cretaceous, and eastward migration of the magmatic front of about 30 km between middle Cretaceous and Paleocene. To determine the timing of ore deposition and to reconstruct parts of the thermal history of the Punta del Cobre district, in the eastern part of the belt, we have obtained 40Ar/39Ar incremental-heating and Rb-Sr analyses of mineral and whole-rock samples. An 40Ar/39Ar incremental-heating experiment on hydrothermal biotite, formed synchronous with the Cu(-Fe)-Au mineralization, yielded an inverse isochron age of 114.9 ± 1.0 Ma (all errors reported at ±2σ), consistent with a Rb-Sr isochron of 116.8 ± 2.7 Ma calculated from 7 whole-rock samples. These data are interpreted to represent the age of potassic alteration that accompanies mineralization. Ore formation temperatures of 400 °C to 500 °C were previously estimated based on paragenetic relationships. Shearing at the Candelaria deposit occurred after ore deposition and before the main stage of batholith emplacement. Published K-Ar ages for the middle Cretaceous batholith near the Punta del Cobre belt range from 119 to 97 Ma. Our data suggest that the mineralization is related to the earlier stages of batholith emplacement. The biotite age spectrum indicates that the Punta del Cobre district was not affected by temperatures above ∼300 °C–350 °C, the closure temperature for argon in biotite, during the contact metamorphic overprint produced by later emplaced batholithic intrusions. Whole-rock 40Ar/39Ar ages are considerably younger; incremental-heating experiments yielded an inverse isochron age of 90.7 ± 1.2 Ma and weighted mean plateau ages of 89.8 ± 0.6 Ma and 89.5 ± 0.6 Ma. These samples are dominantly K-feldspar, for which we assume an argon closure temperature of ∼150 °C, thus they give the age of cooling below ∼150 °C–200 °C.

Notes: Abstract The aim of the present communication is to emphasize that some variations of the measured δ 13C and δ 18O values are apparent, and due to analytical interferences caused by the presence of sulfur and organosulfur compounds in the analyzed carbonates. This is particularly relevant for isotopic studies on carbonate-hosted mineral deposits, where the nearly ubiquitous association of the host carbonates with organic matter and sulfides can certainly affect the metallogenetic interpretations. In this work two methods were used to overcome the disturbing effects of sulfides and organic matter: (1) sample pretreatment following the method proposed by Charef and Sheppard (1984), combining the oxidation of organic matter with sodium hypochlorite and trapping of the sulfur species with silver phosphate; and (2) laser-based microprobe extraction. Apparent isotopic variations in sparry dolomite from a single hand sample of zebra ore from the MVT Zn-Pb deposit, San Vicente, central Peru, are as large as 6‰ δ 13C and 4‰ δ 18O. These variations are reduced to several tenths of a per mil when the samples are pretreated. A careful examination of the effects of treatment with NaOCl and/or Ag3PO4 in relation to the concentration of sulfide inclusions indicates that the main disturbing effects for δ 13C values are the presence of sulfur species and organic matter, whereas the δ 18O values are mainly affected by the presence of sulfides. Fine- and medium-grained replacement carbonates from MVT and other sediment-hosted base metal deposits are potentially the most affected during isotope analysis, due to the common presence of organic matter and sulfides. Using in situ laser microprobe techniques, it is possible to determine isotopic variations at a sub-millimeter scale. Our results show that laser extraction analysis allows a more precise sampling of the carbonate minerals, and minimizes contamination of the sample with sulfides and to some extent with intergrown organic matter. However, there is an isotopic shift associated with the laser extraction technique, of the order of 0.5–1‰ for δ 13C and δ 18O values.

Notes: A procedure for the wavelength-dispersive x-ray fluorescence determination of major, minor and trace elements in fused discs of base metal sulphide ores and of iron-manganese oxide ores is described. The objective was to obtain reasonable chemical analyses of ore samples of a wide compositional range. Owing to the scarcity of well characterized geochemical reference ore samples, calibration was done with synthetic glass disc standards. The concentration range and the association of the elements considered in the standards take into account the geochemistry of most ore types. A fusion method with a high dilution ratio (sample-to-lithium tetraborate ratio= 1 : 10) was used to reduce the matrix effects. This allows a direct comparison of the synthetic standards with unknown geological sample discs. Two different methods of data reduction of the measured intensities were tested: (1) intensity-based correction method for the determination of major, minor and trace elements and (2) peak/background ratio method for the determination of minor and trace elements. A critical evaluation of the reproducibility, as judged from duplicate analyses, and the accuracy, estimated by comparison with international reference materials, showed the procedure to be capable of obtaining reasonable chemical analyses for most elements of geological interest.