Late Magmatic Event Research Articles

Stitching together the Ungava Orogen, northern Quebec: geochronological (TIMS and ICP–MS) and geochemical constraints on late magmatic events

Late magmatic activity in the Ungava Orogen of northern Quebec is manifest as granitic dykes and small, rare plutons that crosscut all tectono-stratigraphic elements of the orogen. Conventional U–Pb geochronology (thermal ionization mass spectrometry (TIMS)) on one particularly important pluton that cuts all these domains (the Lac Duquet monzogranite) indicates its age of emplacement at 1742.2 ± 1.3 Ma. This undeformed and nonmetamorphosed pluton postdates the youngest structures in the orogen (D4 folds), thereby constraining the timing of the latest deformation to >1742 Ma. Laser ablation inductively coupled plasma–mass spectrometry (ICP–MS) on zircons from the same sample identified a large range in 207Pb/206Pb ages of inherited grains from 1.7 to 3.2 Ga, corresponding to the ages of the host rocks for the pluton. This high-K peraluminous monzogranite pluton contains moderate to high concentrations of large ion lithophile elements and fractionated and enriched light rare earth elements, similar in composition to the surrounding continental crust and to other crustally derived granites. Initial 87Sr/86Sr values of 0.7040–0.7051 and εNd ranging from −4.4 to −9.7 indicate incorporation of a significant amount of older material in the petrogenesis of the pluton. It is proposed that anatexis of the surrounding continental crust due to structural thickening during the waning stages of the Ungava orogeny resulted in the generation of the Lac Duquet pluton and was the source for its inherited zircons.

U–Pb geochronology of the Nain craton on the eastern margin of the Torngat Orogen, Labrador

New U–Pb geochronological results provide insight into the geological evolution of the northernmost Nain craton. A tonalitic orthogneiss, retrogressed from granulite to amphibolite facies, is interpreted to have crystallized at 2832 ± 3 Ma; zircon overgrowths, dated at 2769 ± 5 Ma, are interpreted as metamorphic in origin, whereas metamorphic monazite crystallized at 2692 ± 2 Ma. The crystallization age of this sample suggests it may be correlative with similar rocks of the Kanairiktok Plutonic Suite of the Hopedale block, whereas the age of the zircon overgrowths is synchronous with metamorphism in the Saglek block and may indicate a link at this time. A foliated tonalité crystallized at 2802 ± 2 Ma; titanite in this sample records deformation at 1745 ± 4 Ma; the emplacement age suggests a link with Hopedale block, whereas the titanite growth is clearly related to Torngat orogeny. Two foliated granites crystallized at 2587 ± 3 and 2564 ± 2 Ma, respectively, synchronous with late magmatic events recognized in the southern Nain craton. An Avayalik mafic dyke was emplaced at [Formula: see text] Ma, and subsequently metamorphosed at [Formula: see text] Ma. An intraboudin granitic pegmatite crystallized at 1780 ± 2 Ma, and titanite in a diorite formed at 1735 ± 3 Ma, recording the effects of deformation in the Komaktorvik shear zone during Torngat orogeny.

The epithermal gold-silver deposit of Choquelimpie, northern Chile

The Choquelimpie gold-silver deposit is located at an elevation of 4,860 m above sea level, 115 km due east of Arica in northern Chile. A 5,500-metric-ton/day open-pit mine-heap leach operation was brought on stream in 1988 by the Sociedad Contratual Minera Vilacollo, whose shareholders are Shell Chile, Westfield Minera, and Citibank.The deposit is located near the Arica bend of the South American continent, where the earth's crust has a thickness of more than 70 km. It lies in a belt of calc-alkalic to alkali-calcic Miocene to Recent volcanism which runs parallel to the Pacific coast and contains a series of volcanic-hosted precious metal deposits. These have been worked in Peru, Chile, and Bolivia since the time of the Spanish conquest, usually for Ag but also with locally important Au values. These deposits are characterized by Ag-Au-Pb-Zn.The volcano hosting the mineralization is a subalkalic stratovolcano of late Miocene age; a late magmatic event has been dated at 6.60 + or - 0.20 Ma. The volcano is located at the intersection of a major N 60 degrees E-trending lineament with a N 35 degrees E fault, active from the late Miocene to the Recent. The volcano consists of dacitic and andesitic volcanic flows and breccias and is intruded by dacite domes. Erosion of its central part has exposed a zone of hydrothermal alteration and mineralization centered upon an andesitic feldspar porphyry, which is probably a dome.Most of the mineralization worked until recently came from narrow argentiferous veins, but the present operation is centered on a series of hydrothermal breccia bodies emplaced in a N 60 degrees E-trending zone 2 km long.Peripheral to the mineralized zone, propylitic alteration exists, grading inward to a sericite-pyrite assemblage. Toward the breccia bodies this assemblage is overprinted by silicification associated with base metals, pyrite, kaolinite, and barite. The breccias themselves and some adjacent lithologies contain precious metal species including native Au and Ag, electrum, and argentite. A final stage of mineralization is represented by base and precious metals in veins which cut the breccias. Oxidation has produced supergene enrichment of Ag, but probably not of Au.Limited fluid inclusion data from structures peripheral to the breccia bodies indicate homogenization temperatures ranging from 213 degrees to 305 degrees C. Salinities are below 5 wt percent NaCl equiv. Samples from one location show evidence of boiling at a minimum depth estimated as 300 m below the paleowater table.Based on the available evidence, Choquelimpie is interpreted as a volcanic-hosted, acid sulfate, epithermal deposit. It is associated with an Ag-Au-Pb-Zn metallogenic province which contrasts with the Maricunga and El Indio Au-Ag-Cu-As provinces of central Chile.

Evolution of the Cornubian ore field, Southwest England; Part I, Batholith modeling and ore distribution

The Cornubian ore field of southwest England is associated with an S-type ilmenite series, high heat production (HHP), two-mica, tourmaline-bearing monzogranite batholith, which formed between 270 to 295 Ma. Base metal deposits are mainly in the form of steeply dipping lodes emplaced in the batholith roof and a hornfelsic sequence of upper Paleozoic mud rocks, sandstones, and mafic volcanics. The steep-sided batholith has a gently sloping base 10 to 15 km below the present surface. Three-dimensional modeling of gravity data indicates that it has a total volume of about 68,000 km 3 and is divisible into distinct western and eastern parts. Computer-aided evaluation of mineral production statistics shows that most of the Sn, W, Cu, and As production is located within the axial trace of the batholith, with the western end enriched in Sn, Cu, and Zn relative to the eastern end. By contrast, most of the Fe, As, Sb, F, Ba, and china clay production is located in the eastern end, while Pb and Ag production is concentrated in the country-rock-filled, south-facing embayments on the flank of the batholith. All significant wolframite production is from within the granite or within a 700-m distance of an observed or projected granite contact, whereas 90 to 100 percent of the Sn, Cu, and As production occurs within 1,500 m of a contact. By contrast 30 percent of the Zn production and over 80 percent of the Pb and Ag production is from distances of more than 1,500 m from a granite contact. Pre-, syn-, and postbatholith faults and extensional fractures exert a fundamental control on ore distribution, with most of the Sn-W-Cu-As-Zn ores hosted by fractures trending parallel to the axial trace of the batholith and most of the Pb, Sb, Ag, Ba, and fluorite mineralization hosted by faults trending approximately normal to the axial trace of the batholith. The nature of the sedimentary sequence also influenced metal distribution, with most of the Sn-Cu-As production coming from areas underlain by Devonian argillaceous rocks. An important feature of the granite, which has a bearing on its metallogenic potential, is its high internal heat production due to its high total content of U, Th, and K. Modeling of the cooling history of the granites reveals that temperatures in the deeper parts of the batholith were maintained close to, or above, the solidus for 10 to 20 Ma after initial emplacement at 280 to 295 Ma. Residual volatile and metal-enriched magmas trapped within the batholith provided source materials for younger magmatism (270-280 Ma) and the main episode of lode formation which was related to the late magmatic events. Continued high heat flow throughout the Mesozoic and Cainozoic promoted thermally driven ground-water convection and epithermal mineralization.