DESIGNING DACITE QUARRY DEVELOPING OPTIONS

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1 DESIGNING DACITE QUARRY DEVELOPING OPTIONS. URECHE I. 1, ONESCU D. 2, PAPP D. C Lafarge Romania Concrete and Aggregate Division, Bucharest, Romania 2. Belevion Ltd., Bucharest, Romania 3. Geological Institute of Romania Cluj-Napoca Branch, Romania ABSTRACT We report a coupled petrographic, geochemical, fluid inclusion and geophysical study of the Magura Sturzii Neogene dacite quarry, Bargau Mountains, East Carphatians. New geophysical investigations facilitated a first evaluation of a porphyry copper structure within the quarry, the establishment of a chronology for the emplacement of the magmatic body, and the delineation of the optimal sectors for mining. The geophysical surveying consists in: magnetic (total field intensity T), electric (IP and rezistivity ρ a ), and radiometric (global gamma ray lγ) investigations. Two relatively high magnetic anomalies sources, located in the middle north and south side of the quarry, are the main feature of the local magnetic field distribution. They correspond to fresh dacite and porphyry type mineralized dacite. High IP (95 mv/v,) relatively low ρ a (<250 Ωm), and low gamma ray intensities (16 imp./min) also pointed out the occurrence of a porphyry type system. The porphyry body core is surrounded by a polymictic breccia, and intruded by an andesite body, placed in central-south part of the surveying area, having good exploitation perspectives. 1. INTRODUCTION The aim of this study was to acquire, in addition to previous geological, geochemical, and fluid inclusions investigations, new geophysical data in order to design developing options for the Magura Sturzii Neogene dacite quarry, Bargau Mountains, Romania. At present, the dacite rocks extracted from the quarry are used for road and railways construction. Our complex investigations facilitated the evaluation of a porphyry copper structure within the quarry which was not mentioned before, the establishment of a chronology for the emplacement of the magmatic body, and the delineation of the optimal sectors for exploitation, preventing future mining operations in those directions with high risks (un-conform rocks, thick overburden, interbeded rocks and structural features). The results of the study have critical importance both for scientific community, allowing a better understanding of the evolution of the Neogene magmatic activity in the area, and for the decision makers in forecasting low operating costs quarry development, and for minimizing the environmental impact. The case study presented here is presently extended to other quarries concessions within the LAFARGE portfolio. 153

2. GENERAL GEOLOGY AND STUDY AREA The Bargau Mountains, which host the Magura Sturzii dacite quarry, belong to the Neogene subvolcanic zone of the Inner Carpathain arc.

2 2. GENERAL GEOLOGY AND STUDY AREA The Bargau Mountains, which host the Magura Sturzii dacite quarry, belong to the Neogene subvolcanic zone of the Inner Carpathain arc. Neogene magmatism was associated with the consumption of a small piece of ocean crust, attached to the Eastern European plate, beneath the ALCAPA (Alpine Carpathian Pannonian) and Tisia Getia continental blocks (Seghedi et al., 1998). The subduction zone is located on the Tisia Getia block close to the boundary with both the ALCAPA block and the East European plate, between the two volcanic segments: Oas Gutai in the north-west and Calimani Gurghiu Harghita in the south-east (Fig. 1). Fig. 1. Location sketch map of the Magura Sturzii dacite quarry In the Bargau area special geological and structural conditions resulted from the tectonic contact of the Rodna metamorphic massif with the Transcarpathian Flysch zone, delineated by the Somes Fault system. The host rocks of the magmatites are sedimentary deposits of the Transcarpathian Flysch. The main intrusive units consist of dacites, quartz garnet andesites, andesites, microdiorites, and basaltic andesites. The intrusions vary in volume and have a surface exposure from 1 km 2 to 20 km 2. The surface exposure of the Sturzii dacite intrusive unit is about 4.5 km 2, and its maximum elevation is 942 m. 3. PREVIOUS PETROGRAPHIC, GEOCHEMICAL AND FLUID INCLUSION STUDIES Prior to the present work, we have performed detailed petrographic, geochemical, and fluid inclusions studies on the Neogene magmatic rocks of the subvolcanic zone (e. g. Papp et al., 2003, 2005). Main results are summarized bellow. 154

3 The magmas that generated the magmatic rocks in the Bargau Mountains had a calck-alkaline character and show a complete differentiation trend from basaltic andesites to rhyolites. Transitional textures occur between subvolcanic and plutonic facies, and between hypabisal and volcanic-like facies. There is a relative high degree of crystallization and most of the rocks are porphyritic. Based on mineral compositional data, major and trace elements, as well as Sr and O isotope data two different series of rocks have been separated within the Bargau Mountains: one medium-k and another high-k (Papp et al., 2005). The Magura Sturzii dacites belong to the medium-k series, which is the oldest, being emplaced at about 10.6 Ma. The magmas of the medium-k series had a rapid ascent toward the surface, as proven by the presence of primary garnet bearing rocks (quartz garnet andesites, dacites), or by the sporadic occurrence of mafic cognate enclaves. The δ 18 O values of amphiboles vary from 4.2 to 5.4 (SMOW). The δ 18 O value measured on amphiboles from Magura Sturzii dacite is 4.3. The range of 87 Sr/ 86 Sr ratios is from to The decrease of the δ 18 O values as 87 Sr/ 86 Sr ratios and SiO 2 increase shows a progressive contamination of a mantle derived magma with a contaminant depleted in δ 18 O and enriched in 87 Sr/ 86 Sr (i. e. hydrothermally altered lower crustal rocks). The intrusive unit Sturzii consists of dacites, andesites, as well as contact and eruptive breccias. In the inner part of the unit hydrothermal alteration processes (silicification, argillization and propilitization) occur. The associated mineralization consists of pyrite and chalcopyrite either as small veins or they are disseminated within the rock. Amphyboles are the main mafic minerals within the Sturzii dacite. They are Ca-rich and (ferrotschermakite). Zoning is present within most of the amphibole crystals (Mg-richer rim and Fe-richer core). Biotite is present in association with amphiboles. Plagioclase feldspars are the main component of the rock. They form both phenocrysts and microlites in the matrix. Anorthite content varies between % (andesine). Plagioclase feldspar frequently show normal and oscillatory zoning, indicating modification of crystallization conditions (i. e. rapid cooling during the emplacement of the intrusive body). The presence of primary garnets is a special feature of these rocks. They represent 1-2 % of the rock volume. They form phenocrysts with subhedral or euhedral morphologies, mm in size, and have Almandine-rich composition (over 55 %). Pressure estimations for amphibole crystallization record significant differences between the pressure values corresponding to the core of the crystals (~780 MPa) and the pressure values 155

4 corresponding to the rim (~ 490 MPa). This finding clearly shows that decompression occurred during crystallization of amphiboles. The pressure estimates suggest mid-crustal depth of approximately km for amphibole crystallization. Fluid inclusion study (Papp et al., 2003) revealed the exclusively presence of aqueous fluids (H 2 O NaCl system). Homogenization, both to liquid and vapour, occurs between 120 and 540 o C. The general evolution of aqueous fluids is to decreasing salinity with decreasing temperature. Low salinities (from 6 to 1.4 wt.% NaCl eq.) indicate the presence of meteoric water. The occurrence of exploded fluid inclusions indicates decompression regime during magma uplift. Pressure decreasing, inferred from the chemical composition of amphiboles in Magura Sturrzii dacite, also suggest a decompression regime. High temperature, high salinity fluids are early, most probably magmatic, followed by a boiling event of the hydrothermal system, possibly related to a change of fluid pressure from litho- to hydrostatic and dilution by the meteoric fluids, at about 400 o C. The characteristics of the fluid inclusions suggested the tendency of the intrusions to evolve towards a porphyry copper system. This was the first indication of the presence of a porphyry copper structure within the Magura Sturzii dacite quarry. In order to evaluate the porphyry copper structure and to design dacite quarry developing options, additional geophysical investigations were performed. 4. GEOPHYSICAL INVESTIGATION 4.1. Methods The geophysical data were acquired and processed by Belevion Ltd. The geophysical surveying consists in: magnetic (total field intensity T), electric (IP and rezistivity ρ a ), and radiometric (global gamma ray lγ) investigations. The operated equipments were: EDA proton magnetometer, Scintrex electric equipment (receiver and transmitter), Garmin GPS, Leica topographic station, HR Geiger type radiometer, and Leica Disto laser distances equipment. Surface surveyed morphology was framed in the highest difficulty class (slopes angles between 27 and 49 o, young thickset forest, cliffs, detritus and fallen trees). The east quarry extension (0.075 km 2 ) was covered with 4.8 km of magnetic and gamma ray measurements and 2.0 km of electric measurements distributed on 3 investigation profiles (100 m depth). In addition, 6 drill holes/400 m (F1 to F7) were drilled for verifying the geophysical anomalies. 156

5 4.2 Results scale). The assumed interpretation keys based on in situ measurements are summarized in Table 1. Processed magnetic, radiometric and electric data are plotted as maps and cross-sections (1:2000 Fig.2. Magnetic profile, resistivity and chargeability, cross-sections in the Magura Sturzii dacite quarry Profile no.2. D dacite, Dpy mineralized dacite, Bc breccias, A andesite. 157

6 Magnetic images bring out the anomalies sources, as high, intermediate and low types. Two relatively high magnetic anomalies sources, located in central-north and south side are the main feature of the local magnetic field distribution. They correspond to propylitized and mineralized dacite in the central-north side and to intrusive andesite in the south side. The intrusive andesite was also intercepted in the F3, F4 and F5 drill holes. The high magnetic anomalies are cut by low horizontal gradient weak magnetic field, indicated a homogenous source with intermediate to low magnetic properties. Altered dacite (weak argilic alteration, partly silicified ± disseminated pyrite and polymictic explosion breccia, intercepted in F6 and F5 drill holes) is a magnetic low to intermediate source type. In the north side a well defined dipolar magnetic anomaly is produced by a high magnetized source: i. e. a porphyry copper structure with pyrite + chalcopyrite + pyrrhotite + magnetite. In the south side of the surveyed area, a low magnetic anomaly is characterized by intermediate magnetic horizontal gradients. The anomaly is ascribed to a sharp vertical contact (fault type), which put in contact an andesite type source with a low magnetic source type (i.e. polymictic breccia, altered dacite and mineralized dacite). The dacite was also intercepted in F2 drill hole. The electric survey (IP and resistivity measurements) was performed along three north-south parallel lines, on the east side of the quarry. The electric survey covers a 100 m depth as resolution. The second and third profiles (IP and ρ a line presented as cross-sections), showed the highest IP in the north (95mV/V) (Fig. 2). This is double by a relatively low ρ a (<250 Ωm). The source draws a classic mineralized body, belonging to a porphyry type system, as the magnetic data pointed as well. In the south side, high ρ a sources are disclosed, and there are extended in the depth. They overlap the high magnetic sources and show the occurrence of intrusive andesite. In the upper part of the cross-sections, high ρ a values are accompanied by low IP, showing a heterogeneous developing source. Global gamma ray data indicate the fresh and altered dacite subsurface extension, by intermediate intensities (20 imp./min.). The highest gamma ray intensity is located in the south side of the area and is possible to reflect a clay stone enclave in the dacite rock (30 imp./min). In the north side, the gamma ray intensities show the lowest intensities (16 imp/min.) The allocated source is mineralized dacite. 158

7 Table1 Summary of the geophysical survey in Sturzii dacite quarry Rock type IP ρ a χ Iγ Note Dacite (partly propylitized) LL H H I - H Andesite LL HH HH L Dacite (argillized, partly silicified ± py) I - L LL I - L I - H mds Dacite mineralized (porphyry type) HH I - L H I - L mds Clay stone thermally affected I H L HH Structural features I - L L L - IP chargeability; ρ a electric resistivity; χ - magnetic susceptibility; Iγ: - gamma ray intensity HH: highest, H: high; I: intermediate; L: low; LL: lowest mds magnetite destruction sources 5. CONCLUSIONS Magura Sturzii dacite quarry opened, in the east side, a classic immature porphyry copper structure, host by a dacite intrusion, with a inner stock-work (silicified in the depth and argilized in the upper part), with low magnetic properties, high IP signal and low to intermediate resistivities. The porphyry body core is surrounded by a polymictic breccia, intruded by an andesite body, placed in central-south part of the surveying area with good mining perspectives. The outer ring of the porphyry intrusive structure is represented by fresh and propylitic dacite (mined in the central, west and north side of the quarry). In the surveying area fresh and propylitic dacite are developing in the north part. In extreme south, breccia is outcropping. In the east extension of the quarry, un-conform rocks are developed as: altered and mineralized dacite and polymictic explosion breccia, which are in contact with andesite. Based on the previous geological, geochemical and fluid inclusion studies, as well as on the geophysical investigations, a possible model for the emplacement of the Magura Sturzii intrusive unit could be established. The interpretative model is presented in Fig

Fig. 3. Geological model of the Magura Sturzii dacite quarry The emplacement of the dacite, related to a decompression regime, was the first intrusive event.

8 Fig. 3. Geological model of the Magura Sturzii dacite quarry The emplacement of the dacite, related to a decompression regime, was the first intrusive event. The dacite intrusion locally and laterally is interstratified with schists and Flysch sedimentary deposits (clays). The emplacement of the andesite body represents the second intrusive event. Critical mechanical and chemical interactions with the pre-existing dacite occurred. In the north side and sporadically in the south side of the intrusive unit contact breccia (schist and dacite fragments) developed. The emplacement of the andesite body also fractured the pre-existing rocks and induced circulation of sulphur-rich hydrothermal fluids. The end result was the mineralization of the dacite and the development of a porphyry copper structure. REFERENCES Papp D.C., Tecce F., Frezzotti M. L and Ureche I., 2003, Microthermometric study of fluid inclusions in Neogene shallow intrusions from Inner Carpathian arc (Romania), J. Geochem. Expl., 78-79, Papp D.C., Ureche I., Seghedi I., Downes H., Dallai L., 2005, Petrogenesis of convergent-margin calcalkaline rocks and the significance of the low oxygen isotope ratios: the Rodna-Bargau Neogene subvolcanic area (East Carpathians), Geologica Carpathica, 56, 1, Seghedi I., Balintoni I., Szakacs A., 1998, Interplay of tectonics and Neogene post-collisonal magmatism in the Intra-Carpatathian region. Lythos, 45,

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