El Soldado- Boric

Geología mina
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  Boric, R., Holmgren, C. & Wilson, N.S.F. & Zentilli, M., 2002 - The Geology of theEl Soldado Manto Type Cu (Ag) Deposit, Central Chile; in Porter, T.M. (Ed.),Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, Volume 2; PGC Publishing, Adelaide, pp 163-184. 163 THE GEOLOGY OF THE EL SOLDADO MANTO TYPECu (Ag) DEPOSIT, CENTRAL CHILE 1 Ricardo Boric, 2 Carmen Holmgren, 3  Nicholas S. F. Wilson and 1 Marcos Zentilli 1  Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada, 2 Consultant, Santiago, Chile, 3  Energy and Environment, Geological Survey of Canada, Calgary, Alberta, Canada. Abstract - El Soldado is the largest (>200 Mt @ 1.4% Cu) of the known Cu manto-type deposits incentral Chile. It is strata-bound within a submarine, bimodal calc-alkaline basalt - rhyodacite unit of theLower Cretaceous Lo Prado Formation., which also contains marine carbonaceous shales and volcaniclasticsandstones. Although stratigraphically restricted, the clustered orebodies are mostly vein-like and discordant,controlled by a system of N-S to NNW faults formed within a transtensional zone (cymoid loop) of a sinistral,strike-slip brittle shear system. Individual orebodies are zoned, with an external and deeper zone of barren pyrite, followed inward by concentric zones with chalcopyrite-pyrite, chalcopyrite bornite, bornite-chalcocite,and a central zone of chalcocite ( ±  digenite ±  covellite) and abundant hematite. The deposit was formed intwo main phases: 1) a low-temperature, diagenetic phase during which framboidal pyrite developed inassociation with migrated petroleum, at ca. 130 to 120 Ma; 2) a high-temperature (>300 o C from fluidinclusions) hydrothermal phase at ca. 103 Ma, (coinciding with batholith emplacement), that deposited earlyhematite ( ±  magnetite), followed by chalcopyrite, bornite and chalcocite, mostly replacing pre-existing pyrite, with the excess Fe forming hematite. Gangue minerals are calcite, albite, k-feldspar and chlorite.The hydrothermal Cu mineralization is associated with an increase in Na and depletion in K in host rocks,although there are localised zones of K increase in bornite-chalcocite assemblages near structures. Isotopicstudies indicate that: a) the sulphur in diagenetic pyrite provided the bulk of the sulphur for Cu sulphides; b) petroleum was the source of carbon in bitumen and part of the carbonate; c) osmium in diagenetic pyritewas derived from the black shales; d) strontium in calcites was inherited from the Cretaceous arc lavas;e) oxygen isotopes in carbonates, and K-feldspar and atmospheric argon in K-feldspar plus the high salinityof fluid inclusions (21-26% NaCl equivalent) suggest a basinal connate-metamorphic brine was responsiblefor Cu transport, yet a (distal) magmatic component to the fluids cannot be ruled out. Introduction The El Soldado copper deposit (32 °  38’ Lat S; 71 °  04’ LongW; 500-1000 m above sea level.) is located in the CoastalCordillera of Central Chile, 120 Km northwest of the capitalcity Santiago, and 30 km from the Pacific coast (Figure 1).The total identified resources at El Soldado, comprising production plus reserves, is well over 200 million tonnes(metric tons) @ 1.35% Cu, including 70 million tonnes @1.8 % Cu mined out from rich zones (Contador and Glavic, Editors note: The El Soldado deposit has been included within theIron Oxide Copper-Gold (IOCG) family by a number authors in theexisting literature, although others would disagree. It does have significanthematite and specularite associated with the copper mineralising phase.However, it differs from many of the recognised members of the family(such as Olympic Dam and La Candelaria) in that there appears to bemuch less evidence for a magmatic contribution to the mineralisation, but instead a more definite link to basinal processes, perhaps driven by amagmatic heat engine. In fact it exhibits the influence of many of the processes normally associated with sediment hosted copper deposits suchas White Pine (USA) and the Kupferschiefer (Europe). This excellent,well reasoned paper which presents a series of carefully researchedobservations and cogent arguments for the formation of this deposit wasinvited for the reader to assimilate and ponder whether El Soldado is amember of, or is related to, the IOCG family of deposits. 2000), thus making it equivalent, in terms of total metalcontent, to a medium size porphyry copper deposit, yetsmaller than Mantos Blancos and Candelaria (eg. Maksaevand Zentilli, this volume).This paper offers an updated description of the geology, petrography, structure, ore and gangue mineralogy,hydrothermal alteration, and paragenesis of the El Soldadodeposit, incorporating previous information (eg. Holmgren,1987; Klohn et al., 1990), and adding new data accumulatedin the last decade during development and mining of neworebody clusters at the mine. We also incorporatesummaries of detailed studies developed during the lastdecade in a collaborative effort between the CMDLC staff,various consultants, and faculty and students at DalhousieUniversity, Halifax, Canada. Manto Type Copper Deposits Chile contains one of the largest copper concentrations onEarth and annually produces around 37% of the world’scopper (Camus and Dilles, 2001). Most of that productioncomes from the giant porphyry copper deposits of Cenozoicage, although a significant proportion is from Mesozoic  164 The Americas Figure 1: Location of El Soldado in central Chile, and in relation to other Chilean Cu and Fe deposits.  El Soldado, Chile - R. Boric et al.  165 Figure 2: View of the El Soldado camp on the steep western slope of a snow-capped 2300 m high range. The dotted linemarks the approximate boundary between the marine Lo Prado Formation and the sub-aerial andesitic lavasand red beds of the Veta Negra Formation. Also shown are the Morro open pit (op), waste dumps (wd) thetransport level for the underground operation (tl), the flotation plant (fp) and the leaching plant (lp).strata-bound copper (silver) and copper (gold-iron) deposits(Figure 1). These stratabound deposits are distributed inthe western part of the Andean orogen between 21 °  to 34 ° latitude S and the most significant are hosted by volcanicand volcano-sedimentary sequences of Jurassic to Lower Cretaceous age, which were accumulated in intra-arc basins.Among the strata-bound deposits, the copper (silver) family,also known as “mantos” or “manto-type” copper depositsin the literature, are the better understood. This family of deposits usually has a high copper grade (1.5-2% Cu),relatively low contents of silver (5-20 g/t) and are practicallydevoid of gold. They are hosted by Jurassic volcanic rocksin northern Chile (21 ° - 26 °  latitude S; eg. Mantos Blancos,Michilla) and by Early Cretaceous volcano sedimentarysequences (27 °  to 34 °  latitude S; eg. Punta del Cobre, ElSoldado). The most important mines of this subtype areMantos Blancos in northernmost Chile, and El Soldado, incentral Chile (Figure 1). The ore in these deposits is onlystrata-bound in a regional sense, meaning that they areepigenetic, and although the ores are restricted to certainstratigraphic units, in detail they are discordant; their orebodies are structurally controlled or follow relatively permeable horizons in the strata. Hypogene mineralogy isrelatively simple, and consists of bornite, chalcocite ( ± digenite), chalcopyrite, pyrite and hematite. The sulphideores tend to occur in reduced (probably sub-marine) stratarather than in oxidised (sub-aerial) strata, suggesting to mostauthors that oxi-reduction reactions were important in their genesis. Supergene metal enrichment related to surficialweathering, which has been significant in making some porphyry copper deposits economic, is not a main factor inthe mantos, although oxidised ores are exploited in many.Gangue minerals are relatively scarce and the effects of hydrothermal alteration are difficult to distinguish fromlow-grade regional metamorphic assemblages (eg. Sato,1984; Sillitoe, 1992).The genesis of these manto type deposits has been thesubject of much controversy (eg. Fontboté, 1990), with the proposed genetic models ranging from volcanogenicsyngenetic (eg. Ruiz, et al., 1965; Camus, 1980; Ruiz andPeebles, 1988), through hypotheses that call for fluidsliberated during low-grade metamorphism of the volcanic piles (eg. Sato, 1984: Westra, 1988; Sillitoe, 1992); fluidsdirectly derived from granitoid plutons (eg. Carter, 1961;Palacios, 1986; Klohn et al., 1990), to a combination of these last two mechanisms (eg. Fontboté, 1990). Thescarcity of effectively datable minerals has made absolutedating difficult, and the pronounced alkali metasomatismof the host rocks (Boric, 2002) complicates their interpretation. Nevertheless, available geochronologicaldata suggest that, in this family of deposits, ores wereemplaced at least 10 Ma after the deposition of the hoststrata, mainly during two metallogenic pulses, in the LateJurassic and in the Early Cretaceous (Munizaga et al., 1988;Boric et al., 1990; Tassinari et al., 1993; Vivallo andHenriquez, 1998; Wilson et al., submitted; Maksaev andZentilli, this volume). Geography and History The El Soldado mine (Figure 1) is located within 8 km of the Pan American Highway, railway links and major power grids. The mine is currently owned and operated by Cia.Minera Disputada de las Condes Ltda. (CMD), an affiliateof ExxonMobil Coal and Minerals Co. (EMCMC). ElSoldado’s primary operations (Figure 2) comprise an open  166 The Americas Figure 3: El Soldado in its regional stratigraphic setting. Note the large number of Cu deposits, mines and occurrenceshosted by the Lower Cretaceous Lo Prado Formation and the distal location of El Soldado with respect tooutcrops of the Cretaceous batholith. pit mine (the Morro pit), an underground mine, the El Cobresulphide flotation concentrator plant, and a leaching plantwith Solvent Extraction Electro Winning (SXEW).Concentrates are sent to the CMD Chagres smelter locatednorth of the Aconcagua River, southeast of El Soldado(Figure 1), or to the Ventanas port on the Pacific coast.The mine lies on the steep west flank of a 2300 m high,moderately vegetated range (brush and small deciduoustrees), in a zone of warm-temperate climate, with abundantwinter fog, irregular precipitation in the form of rain(~ 400 mm/year), and occasional snow during May toAugust (Figure 2). Temperatures range from –2 o C in winter to ~30 o C in summer, with a yearly median of ~15 o C(Fuenzalida, 1965).At El Soldado, mining of exposed high-grade pods beganin the eighteenth century, although the first modernexploitation did not commence until 1919, when ore grading between 7 and 15 % Cu was extracted from an undergroundmine. Since then, under a number of subsequent owners, production has been almost continuous, but was limited toless than 600 t/day until the late 1960s, when it wasincreased to 3300 t/day. Exxon Minerals, today EMCMC, 2024Km Fault (observed)Lo Prado Formation (Neocomian)Veta Negra Formation (Barremiam - Albian)Quaternary SedimentsGranodiorite Intrusive (Lower Cretaceous)Las Chilcas Formation (Aptian - Albian?)Ocoa Member Purehue Member Upper Member Lower Member Horqueta Formation (Upper Jurassic)Fault (inferred)Manto-type Cu DepositsVein-type Cu DepositsEl Soldado (Large)El Sauce (Medium) Mine MineOccurrenceOccurrence o 3230’ o 71   o 3240’ El Salado l   Salado CaquicitoLa IslaSanta FeVeta NegraVeta del AguaEl Cerrado l   errado La Victoriana a   Victoriana   El Soldado Quenes ~ La ComunaSanPedroGuayacanRusaEl Peumo Palqui El CarmenRegaloLos MaquisEl SauceLuisaLasGuiasChancletaFarellonLa Patagua a   Patagua Las Animas Las nimas    Amarilla   r   e   v   i  R  a  u g  i L  a  L Cabildo
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