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Portovelo Ecuador
The Portovelo-Zaruma Mining Camp, Southwest Ecuador: Porphyry and Epithermal Environments
Gold Deposits of Southern Ecuador
SEG Newsletter 49 (April)
Metal Sources in Mineral Deposits and Crustal Rocks of Ecuador (1° N–4° S): A Lead Isotope Synthesis
Petrogenetic Evolution of Arc Magmatism Associated with Late Oligocene to Late Miocene Porphyry-Related Ore Deposits in Ecuador
SEG Newsletter 11 (October)
An Application of Electrical Resistivity Tomography To Investigate Heavy Metals Pathways
Geodynamic controls in the southernmost Northern Andes magmatic arc: Trace elements and Hf-O isotopic systematics in forearc detrital zircon
Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco
Gamma-enhancement of reflected light images: A rapid, effective tool for assessment of compositional heterogeneity in pyrite
INTERESTING PAPERS IN OTHER JOURNALS
SEG Newsletter 58 (July)
Optical image and microchemical analysis of gold grains from a weathered profile of the Minvoul greenstone belt, northern Gabon
SEG Newsletter 55 (October)
Tectonic Assembly of the Northern Andean Block
Abstract Based primarily on geologic field observations as recorded by numerous geoscientists over the last three decades, backed by more recent geochemical, seismic, gravity, magnetic, tomographic, and satellite-based techniques, an integrated synthesis and interpretation of the tectonic assembly of the entire Northern Andean Block (the Andes of Ecuador, Colombia, and Venezuela) is presented. Tectonic reconstruction is based on the identification and characterization of more than 30 distinct lithotectonic and morphostructural units (including terranes, terrane assemblages, physiographic domains, etc.) and their bounding suture and fault systems, which, based on geologic, geophysical, and dynamo-tectonic considerations, define four distinct tectonic realms representing the entire Northern Andean region. These include the Guiana Shield Realm (GSR), the Maracaibo subplate Realm (MSP), the Central Continental subplate Realm (CCSP), and the Western Tectonic Realm (WTR). The GSR provided the backstop for the progressive, accretionary continental growth of northwestern South America in the middle–late Proterozoic, in the middle Paleozoic, and finally during the Mesozoic-Cenozoic Northern Andean orogeny. Middle Cretaceous through Miocene time slices illustrate how, beginning in the Aptian, the sequential dextral-oblique accretion of the allochthonous oceanic WTR along the Pacific margin acted simultaneously with the northwest migration of the MSP (a detached segment of the Guiana Shield) into and over the Caribbean plate, exerting enormous transpression upon the CCSP trapped between them. Each tectonic realm contributed distinct tectonic mechanisms during Northern Andean “cause and response” orogenesis, and each realm records a unique internal deformational style, which in large part provides the basis for realm definition. Additionally, based on lithologic, geochemical, and paleomagnetic data and paleogeographic reconstructions, the intimate and complementary Mesozoic-Cenozoic history of the Northern Andean Block and the Caribbean plate are recognized. The migratory path of the Caribbean plate along the western and northern margin of the South American craton, as recorded by the accretionary history of the allochthonous WTR, has been instrumental in the modern-day configuration of the Northern Andean Block. Throughout this paper, the importance and contribution of underlying Proterozoic through middle Mesozoic geostructural elementsinthe development of Mesozoic-Cenozoic Northern Andean orogeny-phase tectonic configuration (structural style, uplift mechanisms, basin development, magmatism, etc.) are stressed. Additionally, the complex reality of Northern Andean Block assembly is contrasted with “classical” Central Andean “Cordilleran-type” orogenic models, and numerous differences are illustrated that render the application of typical Cordilleran-type models unacceptable. These differences are exemplified by the highly oblique collision/accretion/subduction tectonics of allochthonous oceanic terranes in the WTR, the detachment, migration and plis de fond –style of deformation in the MSP and the unique, transpressive pop-up of the Eastern Cordillerain the CCSP, all of which have no geologic analog in the Central Andes.
SEG Newsletter 51 (October)
SEG Newsletter 52 (January)
Abstract The Nambija gold district, southeastern Ecuador, consists of oxidized skarns developed mainly in volcaniclastic rocks of the Triassic Piuntza unit, which occurs as a 20-km-long, north-trending, contact-metamorphosed lens within the Jurassic Zamora batholith. High gold grades (10–30 g/t) are accompanied in most mines by very low Fe, Cu, Zn, and Pb sulfide contents. The skarn is constituted dominantly by massive brown garnet (mean Ad 38 ). Subordinate pyroxene-epidote skarn developed mainly at the margins of brown garnet skarn bodies. Mostly idiomorphic and more andraditic garnet (mean Ad 45 ) occurs in blue-green skarn formed as a later phase, in places with high porosity, at the transition with vugs and discontinuous dilational type I veins. The last garnet generations are mainly andraditic and occur largely as honey-yellow to red-brown clusters and cross-cutting bands (mean Ad 84 ). As typical for other skarns developed in volcaniclastic rocks, mineral zoning is poorly defined. The retrograde overprint is weakly developed, commonly fails to alter the prograde minerals, and is mainly recognized in mineral infilling of structurally controlled (N10°–60°E) vugs and up to several-centimeter-wide type I veins, as well as interstices in blue-green skarn. Retrograde minerals are milky quartz, K-feldspar, calcite, chlorite, and hematite, ±plagioclase, ±muscovite, plus minor amounts of pyrite, chalcopyrite, hematite, sphalerite, and gold. Vugs and type I veins are cut by thin (1–2-mm) throughgoing type II veins that show similar orientations and mineralogy. Native gold is associated with retrograde alteration, mainly in the irregular vugs and type I veins, and subordinately in interstitial spaces and throughgoing type II veins. It is not observed in sulfide-rich type III veins, which cut the previous vein generations. High-temperature (up to 500°C) and high-salinity (up to 60 wt % NaCl equiv) inclusions in pyroxene represent the best approximation of the fluid responsible for a significant part of the prograde skarn stage. Such a highly saline fluid is interpreted as the result of boiling of a moderately saline (~8–10 wt % NaCl equiv) magmatic fluid at temperatures of ~500°C. Moderate-to low-salinity fluid inclusions (20−2 wt % NaCl equiv) in paragenetically later garnet as well as in epidote and quartz from vugs and type I veins may represent later, slightly lower temperature (420° −350°C) trapping of similar moderately saline fluids with or without some degree of boiling and mixing. The similarity of salinities and homogenization temperatures in late garnet, epidote, and quartz fluid inclusions is consistent with the apparent continuum between the prograde and retrograde skarn stages, as illustrated by the general lack of prograde mineral alteration, even at the contacts with retrograde fillings. Gold deposition, together with that of small amounts of hematite, chalcopyrite, and pyrite, took place during fluid cooling in the retrograde skarn stages but not during the last retrograde alteration, as indicated by the absence of gold in the sulfide-rich type III veins. The abundance of gold-bearing samples with high hematite/sulfide ratios and generally low total sulfide contents suggests high oxygen fugacities during gold deposition. The northeast structural control of vugs and type I veins, compatible with regional northeast-striking structures, in part with a dilational character, suggests that skarn formation, including gold deposition in the retrograde stage, took place under conditions of tectonic stress. Minimum Re-Os ages of 145.92 ± 0.46 and 145.58 ± 0.45 Ma for molybdenite from type III veins are compatible with skarn formation and gold mineralization during Late Jurassic magmatism. A genetic relationship with felsic porphyry intrusions that cut the Jurassic Zamora batholith and crop out near several gold skarns is suggested by a published hornblende K-Ar age of 141 ± 5 Ma for a felsic porphyry in the northern part of the Nambija district. Furthermore, the minimum Re-Os ages of ~146 Ma are just slightly younger than the published K-Ar ages (154 ± 5, 157 ± 5 Ma) for the Pangui porphyry copper belt about 70 km north of Nambija.
SEG Newsletter 57 (April)
Abstract The Temascaltepec district is located in the southernmost part of the Mexican silver belt. It is made up of three major vein deposits. La Guitarra is the most outstanding and it is composed of six main veins. The veins trend from 120° to 90°, dipping steeply to the southwest and south, with a strike length of more than 3.5 km and a maximum vein thickness of about 15 m (avg 5 m). The veins are hosted by a late Laramide stock, which consists of monzogranite and dikes of leucogranite and granitic porphyry. Mining has been developed on six main underground levels, exposing mineralization for 400 m vertically and 1,200 m horizontally. Vein stratigraphy is divided into three main mineralization stages. Stage I occurs in a brecciated body which contains the highest concentration of base metal mineralization within the deposit, and whose mineral assemblages are gold free and silver poor. Stage II consists of four substages (A, B, C, and D), is characterized by repetitive silica banding and brecciation, and also contains the greatest volume of precious metal mineralization. Stage III is displayed as centimeter-wide veinlets, and is the highest grade stage in terms of precious metal mineralization. The stage I silver-mineral assemblage is scarce and is directly associated with the occurrence of bladed calcite. Pseudomorphic replacements by quartz of rhombohedral adularia crystals and bladed calcite are widespread in stages IIA and IIB. Evidence of boiling coincides with both high FeS content in stage I sphalerites (up to 0.25 mole fraction) and silver mineralization. Stages IIB and III show lower FeS content in sphalerites (up to 0.16 and 0.12 mole fraction, respectively). As sphalerite grain cores are usually iron free, and iron content grows toward the rim displaying oscillatory banding, it is inferred that a repetitive sequence of an a S2 increase culminated with silver mineral precipitation. A close association between electrum and Ag-Cu sulfosalts exists. The Ag content in electrum increases with elevation in stages IIB and III, from the crystal core to the border, and it is higher in stage III than in stage IIB. The Ag-bearing tetrahedrite-tennantites have strong compositional zonations between Ag-Cu, Zn-Fe, and Sb-As. Ag and Sb content increases with height, from the crystal core to the border, and from stage IIB to stage III. Such features similarly occur in Ag-Cu and Ag-Pb sulfosalts at any stage. Mineral pairs and compositions may be used as geothermometers, considering: (1) arsenic solubility in miargyrite, (2) stephanite-arsenostephanite breakdown at high temperatures, (3) proustite-xantho-conite and pyrargyrite-pyrostilpnite dimorphic changes, and (4) opal and chalcedony occurrence. The combinations of sulfide-sulfosalt geothermometers indicate a temperature decrease with elevation and time, within each stage: from >197° to ≥170°C in stage I, from 197° to ≥120°C in stage IIB, and from ≥240° to ≥180°C in stage III. Since opal and chalcedony were deposited before the mineralized bands in stage IIB, it is thought that the metallic mineral assemblage was associated with hotter fluids than those associated with the previous bands. The main ore stages (I, IIB, and III) display a similar evolving trend in mineral deposition, starting with the deposition of base metal sulfides and ending with the deposition of Ag-Au minerals. After base metal sulfide deposition, ore associations point to an a S2 increase with time in each stage. This is marked, for instance, in the thickest ore bands of stage IIB where enargite occurs as the latest mineral. However, the a S2 conditions for each stage, determined using mineral geother-mometers, comprise similar value ranges. The Sb/(Sb + As) ratio distribution in stage IIB Ag tetra-hedrites indicates the main direction of fluid flow to be southeast to northwest, and from the lowest levels to the upper. The Ag/Pb ratio suggests the existence of several feeder channels for hydrothermal fluids during stages IIA and IIB. The paragenetic sequence and mineral zonation in La Guitarra are similar to those described in some of the large low-sulfidation epithermal deposits in Mexico, especially Fres-nillo and Guanajuato in terms of the vein stratigraphy and complex depositional history. The most abundant silver-bearing minerals in La Guitarra are Ag tetrahedrites and proustite-pyragyrite.