Author: LUIS MANUEL ALVA VALDIVIA

A review of iron oxide transformations, rock magnetism and interpretation of magnetic anomalies: el Morro Mine (Brazil), a case study

LUIS MANUEL ALVA VALDIVIA HECTOR LOPEZ LOERA (2011)

"Se presenta una revisión de la mineralogía de los óxidos de hierro y su relación con las propiedades magnéticas. Se discuten los procesos magnéticos y modelos de una gran área alrededor de la mina El Morro, Brasil en el depósito alcalino-carbonatítico Jacupiranga. Propiedades magnéticas de las rocas y observaciones de minerales opacos sugieren que la titanomagnetita es la principal fase mineral. La anomalía magnética sugiere cuerpos mineralizados con una tendencia general este-oeste. El cuerpo mas grande esta en la parte norte del área y mide al menos 5 por 2 km. Todos los cuerpos están ubicados en un dominio aeromagnético de forma semi-elipsoidal y rumbo general noreste-suroeste. Se propone que la zona anómala continua hacia el norte y que el área de interés económico total puede extenderse aproximadamente 8 x 4 km en dirección norte-sur. Se modelaron dos perfiles con direcciones N-S y E-O y el mejor ajuste corresponde a varios grandes cuerpos irregulares. Los cuerpos mineralizados mas importantes están ubicados debajo del alto magnético hacia el norte; esta inferencia es apoyada por datos gravimétricos. Otros cuerpos están ubicados debajo del bajo magnético y se extienden hacia el sur. El mejor ajuste de la anomalía calculada a la observada se obtuvo usando principalmente la magnetización inducida."

"A review of iron oxide mineralogy and its relationship with rock magnetic properties is presented. Magnetic processes and models of a large area around the El Morro, Brazil mine in the Jacupiranga alkaline-carbonatitic deposit is discussed. Rock magnetic properties and opaque mineral observations suggest that titanomagnetite is the main magnetic phase. The magnetic anomaly suggests ore bodies of an east-west general strike. The largest body is in the northern part of the area. It measures at least 5 by 2 km. All bodies are located in an aeromagnetic domain of semi-ellipsoidal shape and northeast-southwest general strike. It is proposed that the anomalous zone is continuous to the north and that the whole area of economic interest may extend roughly 8x4 km in the north-south direction. We modeled two N-S and E-W profiles and the best fit corresponds to several large irregular bodies. The most important mineralized bodies are located beneath the northward continuation of the magnetic high; this inference is supported by a gravimetric data. Other bodies are located under the magnetic low and extend to the south. The best fit of calculated to observed anomaly is obtained by using mostly induced magnetization."

Article

Óxidos de hierro Magnetismo de rocas Modelado de anomalías magnéticas Complejo Jacupiranga Brasil CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA CIENCIAS DE LA TIERRA Y DEL ESPACIO GEOFÍSICA

Magnetic characteristics of fracture zones and constraints on the subsurface structure of the Colima Volcanic Complex, western Mexico

Héctor López Loera Jaime Humberto Urrutia Fucugauchi Luis Manuel Alva Valdivia (2010)

"Detailed magnetic anomaly surveys over the central and southern sector of the Colima rift, western Mexico, are used to investigate the subsurface structure and faults and/or fractures in the volcanic terrains formed by activity in the Colima volcanic complex (CVC). The CVC is located within the large north-south Colima rift in western Mexico. The Colima rift is a major active tectonic structure, trending perpendicular to the Middle America Trench and related to subduction of the Rivera and Cocos plates. Volcanic activity in the CVC has migrated southward toward the trench. Analyses of faults and recent deformation in the CVC and Colima rift are of major interest in volcano-tectonic studies and for hazard assessment. Structural analyses and fault mapping, however, are difficult because young volcanic and pyroclastic rocks obscure structural features and stratigraphy. Most of the southern Colima rift is covered by volcanic avalanches and volcaniclastic units, which have resulted in resurfacing of the volcanic terrains. Here we show that magnetic anomalies permit identification of faults and mapping of volcano-sedimentary and volcanic units. Total magnetic field measurements spaced every 0.5 km along 8 profiles, with an overall length of 284.5 km and covering the CVC sector of the Colima rift, have been obtained. We recognize fractures and fault zones of local and regional character from their characteristic magnetic anomaly response. Large mapped structures include the north-south Montitlan, northeast-southwest La Lumbre, and east-west La Escondida faults, which can be traced across the area from the magnetic profiles. Fault magnetic anomalies are modeled by lateral contrasts in terms of step models assuming thin dipping elongated zones along the fault planes. The study shows that faults in the CVC volcanic terrain can be investigated by magnetic surveying."

Article

System Sedimentary Andesites Collapse Geology Graben Hawaii CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA CIENCIAS DE LA TIERRA Y DEL ESPACIO GEOLOGÍA GEOLOGÍA

Estudio aeromagnético del complejo volcánico de Colima, occidente de México - implicaciones tectónicas y estructurales

HECTOR LOPEZ LOERA JAIME HUMBERTO URRUTIA FUCUGAUCHI LUIS MANUEL ALVA VALDIVIA (2011)

"Se reporta la interpretación cualitativa y cuantitativa de anomalías aeromagnéticas del complejo volcánico de Colima (CVC) y sus alrededores. La zona se caracteriza por anomalías magnéticas de distintas amplitudes y longitudes de onda asociadas con estructuras volcánicas y cuerpos intrusivos, incluyendo al CVC. Se identifican 24 dominios aeromagnéticos definidos por patrones característicos de anomalías en un área de aproximadamente 11,500 km2 alrededor del CVC. El Volcán de Colima y el Nevado de Colima se caracterizan por anomalías dipolares de gran amplitud. Se investiga la estructura profunda del complejo volcánico utilizando un modelo magnético dimensional 2¾ - D, encontrando que la fuente asociada a las anomalías de los volcanes se relaciona con una posible cámara magmática de forma alargada, con una longitud mayor a los 6.8 km en dirección N-S y espesor máximo de 5.6 km. En el Volcán de Colima, ésta se localiza a una profundidad de 4.8 km. Este cuerpo se extiende por una distancia del orden de 5.6 km hacia el sur del CVC con un espesor promedio del orden de 0.54 km. Bajo el volcán Nevado de Colima se interpreta una posible cámara magmática colapsada de forma alargada con una longitud en dirección N-S mayor a 5.6 km y con espesor máximo de 1.2 km con una profundidad del orden de los 5.2 km bajo el cráter del Nevado. Se documentan, por primera vez, nuevas estructuras geológicas asociadas a anomalías magnéticas sepultadas por productos volcánicos, comofallas y cuerpos intrusivos. En la mayor parte del mapa magnético reducido al polo, se observan anomalías monopolares de grandes dimensiones, amplitudes altas y longitudes de onda largas, asociadas a rocas intrusivas. A 47 km al NW de los volcanes Colima y Nevado de Colima se puede apreciar un cinturón formado por tres de estas anomalías. En la zona también existe una serie de anomalías magnéticas relacionadas con la presencia de yacimientos de mineral de hierro, como la mina El Encino ubicada a 48 km al SE y el área mineralizada de la Sierra de Manantlán a 42 km al ESE del volcán de Colima."

"Results of qualitative and quantitative interpretation of aeromagnetic anomalies over the Colima volcanic complex (CVC) and surrounding areas are presented. The area is characterized by magnetic anomalies of distinct amplitude and wavelength associated with volcanic structures and intrusive bodies, including the CVC. We identified 24 aeromagnetic domains in an area of 11,500 km2 around the CVC. We identify, for the first time, new geological structures buried under volcanic products, like faults and intrusive bodies. High amplitude magnetic dipolar anomalies characterize the volcanic structures of Colima and Nevado de Colima volcanoes. CVC deep structure was investigated by using a 2¾ D magnetic model, finding that the source is related to a possible magmatic chamber of elongated shape, >6.8 km long, in north-south strike with maximum thickness of 5.6 km, which is located at 4.8 m depth beneath the Colima volcano. The body extends to 5.6 km south of the Colima volcano, with average thickness of about 0.54 km. Under Nevado de Colima volcano, we interpreted a collapsed magmatic chamber, with elongated shape, >5.6 km long and 1.2 km of maximum thickness at a depth of around 5.2 km below the Nevado summit. In most of the reduced to the pole magnetic anomaly map, we observed large monopolar anomalies, high amplitude, and long wavelengths associated to the intrusive bodies. Three of these anomalies form a 47 km long belt to the NW of the CVC. There are several magnetic anomalies related to iron-ore deposits, i.e., El Encino mine (48 km SE of CVC) and Sierra de Manantlán (42 km ESE of CVC)."

Article

Anomalías aeromagnéticas Modelo aeromagnético Mapeo geológico Tectónica Complejo volcánico de Colima México CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA CIENCIAS DE LA TIERRA Y DEL ESPACIO GEOLOGÍA

Paleomagnetic pole positions and geomagnetic secular variation from the Cretaceous Ponta Grossa dike swarm (Brazil)

MIGUEL ANGEL CERVANTES SOLANO LUIS MANUEL ALVA VALDIVIA JUAN JULIO MORALES CONTRERAS FREDY RUBEN CEJUDO RUIZ HECTOR LOPEZ LOERA ANA MARIA SOLER ARECHALDE JAIME HUMBERTO URRUTIA FUCUGAUCHI (2015)

"A detailed paleomagnetic and rock magnetic investigation has been carried out on the Early Cretaceous Ponta Grossa dike swarm in southern Brazil. This formation seems an excellent target for paleomagnetic study. The dikes are widely distributed over a large area, easy to access, and they record faithfully the geomagnetic field at the time of the eruption. Most of them are fresh and have been dated by K-Ar and Ar-Ar. Thermomagnetic experiments (low-field versus temperature curves) suggest low-Ti titanonnagnetites as main remanence carriers, and their domain structure is characterised by a mixture of single-domain and multi-domain grains. Characteristic paleomagnetic directions are retrieved from 28 out of 29 sites (235 standard paleomagnetic cores). 17 sites show normal polarity, 10 sites show reverse polarity and one site shows an oblique direction, with negative inclination, separated 79 degrees of the mean normal directions. The reversal test is positive at the 95% confidence level which ensures that the secondary remanent magnetizations were successfully removed and the sampling adequately averaged the palaeosecular variation. The paleosecular variation parameters values obtained in this study correlate with those determined for the Cretaceous Normal Superchron, between 125 to 84 Ma. The new paleomagnetic pole position (222.0 degrees E, 88.1 degrees S, N = 24, K = 35.68, A(95) = 5.0 degrees) agrees well (within uncertainties) with the reference poles determined from Besse and Courtillot (2002), and disagree with those reported in previous studies. The new results should be considered for estimating the Cretaceous paleomagnetic poles for stable South America."

Article

Ponta Grossa Dike Swarm South America Paleomagnetism Cretaceous CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA CIENCIAS DE LA TIERRA Y DEL ESPACIO GEOQUÍMICA