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Papers by Marcos Zentilli
Fission-track re-evaluation of thermal inversion of the Scotian Margin: the need to consider the ... more Fission-track re-evaluation of thermal inversion of the Scotian Margin: the need to consider the presence of diagenetic apatite and drilling-mud contamination
Strontium Isotopes of minerals within the Carboniferous (Mississippian) Windsor Group of Atlanti... more Strontium Isotopes of minerals within the Carboniferous (Mississippian) Windsor Group of Atlantic Canada are different from Mississippian seawater values, and indicate a re-homogenization of the Rb-Sr system occurred at 300 Ma, when hot fluids circulated and formed various Pb-Zn (such as Gays River), and Ba mineral deposits.
The presence of zinc in hypogene copper sulphides at Chuquicamata has been known from chemical an... more The presence of zinc in hypogene copper sulphides at Chuquicamata has been known from chemical analyses and polished section descriptions for half a century, but it was only in the 1990s, when zinc interfered with innovative metallurgical processes that it was given any attention. Although it has never been recovered, zinc is probably as abundant as molybdenum. The background concentration in the deposit’s porphyry rocks (0.010% Zn) is an order of magnitude greater than similar granodiorites elsewhere. Zinc was introduced relatively late in the hydrothermal evolution of the deposit and is strongly structurally controled. Zinc occurs in sphalerite (ZnS) and in rare tennantite (Cu12 As4 S13 to Cu10 Zn2 As4 S13) in which it contains up to 8% Zn. Enargite (Cu3AsS4) contains up to 2% Zn but in general all other copper sulphides contain less than 0.6% Zn, the detection limit of the microprobe in these assemblages. There are two main styles of sphalerite occurrence: 1) crystals
Andean Metallogeny
Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebo... more Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40Ar/39Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40Ar/39Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40Ar/39Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district. Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40Ar/39Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40Ar/39Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y. The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartz-sericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40Ar/39Ar). The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105o ± 20oC, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40Ar/39Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody. ©2004 Society of Economic Geologists Special Publication 11, 2004, pp. 0000–0000 †Corresponding author: e-mail, vmaksaev@cec.uchile.cl
www.elsevier.com/locate/jenvrad
Previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in... more Previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in Chuqui-camata in a mineral assemblage including typically chalcocite-digenite (Cu1.85S1.12 and Cu1.99S1.01), covellite (CuS and Cu1.08S0.92), and sphalerite (up to 1.2 wt % Fe). Microscopic observations on samples from a central and a southern section, completed by scanning electronic microscope (SEM) backscattering electron imaging and electron microprobe analyses, suggest that all sphalerite in Chuquicamata is hypogene. A scenario, backed by observations in each step, that explains the formation of the peculiar “sphalerite rims ” is the following: (1) precipitation of chalcopyrite, typical of the early and main hydrothermal stages; (2) precipitation of sphalerite rimming chalcopyrite and in voids and in weakness sites during the late hydrothermal stage; (3) partial or total replacement of chalcopyrite by chalcocite-digenite during the late hydrothermal stage; and (4) formation of lame...
Nuclear Tracks and Radiation Measurements, 1993
Chemical Geology, 1984
ABSTRACT The abundances of U and Th in the Late Proterozoic calc-alkaline volcanic rocks of north... more ABSTRACT The abundances of U and Th in the Late Proterozoic calc-alkaline volcanic rocks of northwest Africa which have undergone low-grade metamorphism, are highly variable. Dacites and rhyolites have higher contents of these elements than basic rocks although there is no obvious correlation of U and Th with SiO2. In the acidic rocks, U and Th are closely related to K, whereas in the basic and most intermediate lavas, the two elements correlate with immobile incompatible elements such as Zr, Nb, La and Ce. In comparison with equivalent fresh Recent rocks, the acidic volcanic rocks have lower contents of U and Th. As expressed by the low U/K and Th/K ratios, U and Th in these rocks are also depleted relative to K. The depletion of U and Th, which is probably related to secondary processes, suggests that the acidic volcanic rocks from northwestern Africa may have been an important source of radioactive elements for concentration in deposits.
Tectonomagmatic Influences on Metallogeny and Hydrothermal Ore Deposits: A Tribute to Jeremy P. Richards (Volume I)
The supergiant Chuquicamata porphyry Cu-Mo deposit in northern Chile is truncated on its west sid... more The supergiant Chuquicamata porphyry Cu-Mo deposit in northern Chile is truncated on its west side by a N-S-trending regional fault (the West fault), juxtaposing its ore to a relatively barren granodiorite (Fortuna Igneous Complex). There has been much speculation about the fate of, and extensive exploration for, the “missing half” of the deposit. It has been proposed that the west side of the fault hides the ore at depth, or that it was uplifted and the ore eroded; however, regional geologic mapping suggests that the West fault had a postore left-lateral strike-slip displacement of ca. 35 km. Accordingly, exploration, so far unsuccessful, has been focused in an area 35 km south near the Loa River and the city of Calama. In 1989, the Mina Ministro Hales (MMH) deposit was unexpectedly discovered west of the fault, under thick gravels, only 7 km south of the main mine. A previous study at MMH had suggested that mineralization was as old as 39 Ma, hence its ores were correlated with de...
www.elsevier.com/locate/jenvrad
Fission-track re-evaluation of thermal inversion of the Scotian Margin: the need to consider the ... more Fission-track re-evaluation of thermal inversion of the Scotian Margin: the need to consider the presence of diagenetic apatite and drilling-mud contamination
Strontium Isotopes of minerals within the Carboniferous (Mississippian) Windsor Group of Atlanti... more Strontium Isotopes of minerals within the Carboniferous (Mississippian) Windsor Group of Atlantic Canada are different from Mississippian seawater values, and indicate a re-homogenization of the Rb-Sr system occurred at 300 Ma, when hot fluids circulated and formed various Pb-Zn (such as Gays River), and Ba mineral deposits.
The presence of zinc in hypogene copper sulphides at Chuquicamata has been known from chemical an... more The presence of zinc in hypogene copper sulphides at Chuquicamata has been known from chemical analyses and polished section descriptions for half a century, but it was only in the 1990s, when zinc interfered with innovative metallurgical processes that it was given any attention. Although it has never been recovered, zinc is probably as abundant as molybdenum. The background concentration in the deposit’s porphyry rocks (0.010% Zn) is an order of magnitude greater than similar granodiorites elsewhere. Zinc was introduced relatively late in the hydrothermal evolution of the deposit and is strongly structurally controled. Zinc occurs in sphalerite (ZnS) and in rare tennantite (Cu12 As4 S13 to Cu10 Zn2 As4 S13) in which it contains up to 8% Zn. Enargite (Cu3AsS4) contains up to 2% Zn but in general all other copper sulphides contain less than 0.6% Zn, the detection limit of the microprobe in these assemblages. There are two main styles of sphalerite occurrence: 1) crystals
Andean Metallogeny
Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebo... more Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40Ar/39Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40Ar/39Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40Ar/39Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district. Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40Ar/39Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40Ar/39Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y. The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartz-sericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40Ar/39Ar). The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105o ± 20oC, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40Ar/39Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody. ©2004 Society of Economic Geologists Special Publication 11, 2004, pp. 0000–0000 †Corresponding author: e-mail, vmaksaev@cec.uchile.cl
www.elsevier.com/locate/jenvrad
Previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in... more Previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in Chuqui-camata in a mineral assemblage including typically chalcocite-digenite (Cu1.85S1.12 and Cu1.99S1.01), covellite (CuS and Cu1.08S0.92), and sphalerite (up to 1.2 wt % Fe). Microscopic observations on samples from a central and a southern section, completed by scanning electronic microscope (SEM) backscattering electron imaging and electron microprobe analyses, suggest that all sphalerite in Chuquicamata is hypogene. A scenario, backed by observations in each step, that explains the formation of the peculiar “sphalerite rims ” is the following: (1) precipitation of chalcopyrite, typical of the early and main hydrothermal stages; (2) precipitation of sphalerite rimming chalcopyrite and in voids and in weakness sites during the late hydrothermal stage; (3) partial or total replacement of chalcopyrite by chalcocite-digenite during the late hydrothermal stage; and (4) formation of lame...
Nuclear Tracks and Radiation Measurements, 1993
Chemical Geology, 1984
ABSTRACT The abundances of U and Th in the Late Proterozoic calc-alkaline volcanic rocks of north... more ABSTRACT The abundances of U and Th in the Late Proterozoic calc-alkaline volcanic rocks of northwest Africa which have undergone low-grade metamorphism, are highly variable. Dacites and rhyolites have higher contents of these elements than basic rocks although there is no obvious correlation of U and Th with SiO2. In the acidic rocks, U and Th are closely related to K, whereas in the basic and most intermediate lavas, the two elements correlate with immobile incompatible elements such as Zr, Nb, La and Ce. In comparison with equivalent fresh Recent rocks, the acidic volcanic rocks have lower contents of U and Th. As expressed by the low U/K and Th/K ratios, U and Th in these rocks are also depleted relative to K. The depletion of U and Th, which is probably related to secondary processes, suggests that the acidic volcanic rocks from northwestern Africa may have been an important source of radioactive elements for concentration in deposits.
Tectonomagmatic Influences on Metallogeny and Hydrothermal Ore Deposits: A Tribute to Jeremy P. Richards (Volume I)
The supergiant Chuquicamata porphyry Cu-Mo deposit in northern Chile is truncated on its west sid... more The supergiant Chuquicamata porphyry Cu-Mo deposit in northern Chile is truncated on its west side by a N-S-trending regional fault (the West fault), juxtaposing its ore to a relatively barren granodiorite (Fortuna Igneous Complex). There has been much speculation about the fate of, and extensive exploration for, the “missing half” of the deposit. It has been proposed that the west side of the fault hides the ore at depth, or that it was uplifted and the ore eroded; however, regional geologic mapping suggests that the West fault had a postore left-lateral strike-slip displacement of ca. 35 km. Accordingly, exploration, so far unsuccessful, has been focused in an area 35 km south near the Loa River and the city of Calama. In 1989, the Mina Ministro Hales (MMH) deposit was unexpectedly discovered west of the fault, under thick gravels, only 7 km south of the main mine. A previous study at MMH had suggested that mineralization was as old as 39 Ma, hence its ores were correlated with de...
www.elsevier.com/locate/jenvrad