Caracterización petro-calcográfica y estructural de Mina Buena Vista IX, yacimiento estratoligado CU-(AG)
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2020
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es
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Universidad Andrés Bello
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Licencia CC
Licencia CC
Resumen
Mina Buena Vista es un yacimiento estratoligado Cu-(Ag) categorizado
como pequeña minería ubicado en el sector Portales, distrito minero Cerro Negro,
Región de Valparaíso. Sus pertenencias mineras abarcan 34 Ha y se componen
de rocas volcanosedimentarias que afloran a modo de franjas de orientación
preferencial norte - sur. Al generar el mapeo geológico de dicha superficie, sus
respectivas litologías se pueden subdividir de oeste a este en: Areniscas
tobáceas, lutitas carbonosas y en parte carbonáceas, aglomerado, brechas y
lavas. Este paquete volcanosedimentario posee una relación de contacto
concordante, forman un homoclinal con leve manteo al SE y se asocian al
Miembro Ñilhue, techo de Formación Las Chilcas de edad Cretácica inferior.
Además, se encuentran intruidas por un cuerpo de dimensiones batolíticas, de
composición tonalítica a granodiorítica que aflora al este y norte de las
pertenencias mineras y en las cotas altas del Cerro Cruz de Piedra.
La faena minera es de tipo subterránea, se explota mediante el método
Room and Pilar y se compone de dos niveles que en conjunto suman 1500 m de
galerías. La mineralización ocurre como vetillas, relleno de poros y diseminada
alojándose en un horizonte litológico de 5 m de espesor compuesto por areniscas
tobáceas grises, lutitas carbonáticas con abundantes restos vegetales, mantos
lenticulares de chert y tobas líticas. Dicho material rocoso se extrae a razón de
2000 a 2500 Ton/mes y se encuentra mineralizado por sulfuros tales como;
bornita, calcosina, calcopirita y menores cantidades de esfalerita, covelina y fases
minerales de plata (no identificadas). Además, hay presencia de magnetita, pirita
cúbica y pirita framboidal.
En base al estudio microscópico de cortes petro-calcográficos se constata
que la roca caja ha sido sometida de forma local, pero pervasiva a una alteración
tipo potásica, compuesta por la paragénesis mineral feldespato potásico, cuarzo,
magnetita y asociada a un núcleo de alta ley compuesto por bornita y calcosina.
Dicha alteración además se asocia espacialmente con la estructura principal de
la faena, un dique microdiorítico de 130 m de largo por 8 m de ancho y orientación
N40°W/85°SW que intruye a través de una falla normal, la cual genera un salto de 10 m en la vertical desplazando el “manto” mineralizado desde la cota 1380 a
la 1370 m s.n.m. Además, se constata otra tendencia estructural de orientación
subperpendicular a la falla principal y actitud N35°E. Dichas estructuras también
son fallas normales y cortan el “manto” mineralizado escalonándolo y generando
descensos métricos entre las cotas 1390 a 1380 m s.n.m. A través de las
estructuras mencionadas también intruyen diques microdioríticos, pero de
menores dimensiones que el principal.
A la alteración hidrotermal tipo potásica se le sobrepone un segundo
evento de alteración tipo propilítico, caracterizado por la paragénesis epidota,
carbonato, actinolita, axinita, cuarzo y asociado a bornita + calcopirita + esfalerita
+ pirita que afecta de forma generalizada pero menos pervasivamente a la roca
caja. Se realiza también la zonación de los sulfuros asociados a la segunda
alteración, la cual comienza con un sector de buena ley, donde abunda bornita y
calcopirita compuesto por areniscas tobáceas, lutitas carbonosas y chert. Para
finalmente llegar a una zona distal de pirita y algo de calcopirita. En base a los
resultados mineralógicos se concluye que el deposito alcanzó una temperatura
máxima de 350 °C según diagrama de Corbett y Leach, (1998).
Gracias a comparaciones con trabajos de otros autores y a los resultados
aquí obtenidos, se concluye que Mina Buena Vista IX presenta la mayoría de las
características que definen a un Estratoligado Cu-(Ag) alojado en rocas del
Cretácico inferior entre las que destacan mineralización asociada a una
estructura principal, núcleo de alta ley compuesto de bornita más calcopirita, la
alteración propilítica característica y un horizonte litológico preferente para
mineralización.
Finalmente, se reporta el hallazgo de fases minerales de plata no
identificadas, casi milimétricas, exsueltas en calcosina. Dicha exsolución es poco
común, tampoco se hallaron estudios donde sea visible con microscopio óptico
como es el caso del presente trabajo. Se recomienda continuar esa línea
investigativa, con microscopia electrónica de barrido y análisis semicuantitativo
EDS para así determinar la composición y tipo de minerales exsueltos y poder
generar un aporte mayor al conocimiento en la génesis de este tipo de depósito.
Mina Buena Vista is a Cu-(Ag) stratabound site categorized as a smallscale mining located at Portales sector, in the Cerro Negro mining district of Valparaíso Region. Its mining properties cover 34 hectares and they are made up by volcano sedimentary rocks that emerge as north-south preferential orientation stripes. When the geological mapping of this particular surface is generated, its respective lithologies are subdivided into the following characterizations from west to east: tophaceous sandstones, carbonic and partly carbonaceous lutites, conglomerates, rifts and lava. This volcano sedimentary sequence is characterized by a concordant contact relationship and it creates an homoclinal with a slight dip to the south east; it is associated to the Miembro Ñilhue, Las Chilcas formation roof of the lower Cretaceous period. Besides, it is intruded by a body of batholitic dimensions, of tonalitic to granodioritic composition that emerges east and north of the mining site and in the high tops of Cerro Cruz de Piedra. It is an underground mine site exploited by means of the Room and Pillar method, and it is made up by two levels that altogether amount to 1500 m of galleries. Mineralization appears as veinlets, pore fillings and dissemination deployed into a 5 m thickness lithologic horizon consisting of grey tophaceous sandstones, carbonaceous lutites with abundant plant debris, chert lenticular mantles and lithic tuff. This particular rocky material is extracted at a rate of 2000 to 2500 ton/month and it is mineralized by sulphides such as; bornite, chalcocite, calchopyrite and lower amounts of sphalerite, covellite and silver mineral stages of silver (unidentified). In addition, magnetite, cubic and framboidal pyrite are also present. Based on the microscopic study of petrochalcographic cuts it is confirmed that the wall rock has been submitted in a local way but it is pervasive to a potassium type of alteration, composed of the potassium feldspar mineral paragenesis, quartz, magnetite and linked to a high grade core made up of bornite and chalcocite. In addition, this particular alteration is spatially associated to the main structure of the mining site, i.e., a 130 m long by 8 m wide microdioritic dyke facing N40°W/85°SW, intruding through a normal fault that generates a 10 m leap on the vertical that displaces the mineralized “mantle” from 1380 to 1370 m above sea level. Besides, another structural orientation sub-perpendicular to the main fault and preferential attitude of N35°E is confirmed. These structures are also regular faults and they cut the mineralized “mantle”, staggering it and generating metric declines between levels 1390 to 1380 m above sea level. This is noted when a NW – SE transect follow-up is carried out in the upper level of the mining site. There are also microdioritic dyke intrusions through the already mentioned structures, but they show lower dimensions compared to the main one. A second event of a propylitic type of alteration characterized by the paragenesis of epidote, carbonate, actinolite, axinite and quartz and linked to the bornite + chalcopyrite + sphalerite + pyrite is superimposed to the hydrothermal potassium type alteration. Zoning of sulphides associated to the second alteration is performed, which begins with a high grade sector where there are abundant bornite and chalcopyrite made up of tophaceous sandstones, carbonaceous lutites and chert. Finally, a distal zone of pyrite and some chalcopyrite is attained. Based on these particular mineralogical results, it is concluded that the deposit attained a maximum temperature of 350 °C according to the Corbett and Leach Diagram, (1998) and based on comparisons of other authors´ studies, it is determined that Mina Buena Vista IX shows most of the features that define a Cu- (Ag) stratabound site installed in the lower Cretaceous rocks. Lastly, the finding of almost millimetric silver mineral phases is reported; they are unidentified and exsolved in chalcocite. This particular exsolved solution is rather uncommon. As a matter of fact, no studies where silver minerals are visible under an optical microscope were found, as in the case of the present study. It is recommended to continue with this particular research line with the scanning electron microscopy and EDS semiquantitative analysis to be able to determine the composition and type of exsolved minerals, thus generating a greater contribution to the genesis of this kind of deposit.
Mina Buena Vista is a Cu-(Ag) stratabound site categorized as a smallscale mining located at Portales sector, in the Cerro Negro mining district of Valparaíso Region. Its mining properties cover 34 hectares and they are made up by volcano sedimentary rocks that emerge as north-south preferential orientation stripes. When the geological mapping of this particular surface is generated, its respective lithologies are subdivided into the following characterizations from west to east: tophaceous sandstones, carbonic and partly carbonaceous lutites, conglomerates, rifts and lava. This volcano sedimentary sequence is characterized by a concordant contact relationship and it creates an homoclinal with a slight dip to the south east; it is associated to the Miembro Ñilhue, Las Chilcas formation roof of the lower Cretaceous period. Besides, it is intruded by a body of batholitic dimensions, of tonalitic to granodioritic composition that emerges east and north of the mining site and in the high tops of Cerro Cruz de Piedra. It is an underground mine site exploited by means of the Room and Pillar method, and it is made up by two levels that altogether amount to 1500 m of galleries. Mineralization appears as veinlets, pore fillings and dissemination deployed into a 5 m thickness lithologic horizon consisting of grey tophaceous sandstones, carbonaceous lutites with abundant plant debris, chert lenticular mantles and lithic tuff. This particular rocky material is extracted at a rate of 2000 to 2500 ton/month and it is mineralized by sulphides such as; bornite, chalcocite, calchopyrite and lower amounts of sphalerite, covellite and silver mineral stages of silver (unidentified). In addition, magnetite, cubic and framboidal pyrite are also present. Based on the microscopic study of petrochalcographic cuts it is confirmed that the wall rock has been submitted in a local way but it is pervasive to a potassium type of alteration, composed of the potassium feldspar mineral paragenesis, quartz, magnetite and linked to a high grade core made up of bornite and chalcocite. In addition, this particular alteration is spatially associated to the main structure of the mining site, i.e., a 130 m long by 8 m wide microdioritic dyke facing N40°W/85°SW, intruding through a normal fault that generates a 10 m leap on the vertical that displaces the mineralized “mantle” from 1380 to 1370 m above sea level. Besides, another structural orientation sub-perpendicular to the main fault and preferential attitude of N35°E is confirmed. These structures are also regular faults and they cut the mineralized “mantle”, staggering it and generating metric declines between levels 1390 to 1380 m above sea level. This is noted when a NW – SE transect follow-up is carried out in the upper level of the mining site. There are also microdioritic dyke intrusions through the already mentioned structures, but they show lower dimensions compared to the main one. A second event of a propylitic type of alteration characterized by the paragenesis of epidote, carbonate, actinolite, axinite and quartz and linked to the bornite + chalcopyrite + sphalerite + pyrite is superimposed to the hydrothermal potassium type alteration. Zoning of sulphides associated to the second alteration is performed, which begins with a high grade sector where there are abundant bornite and chalcopyrite made up of tophaceous sandstones, carbonaceous lutites and chert. Finally, a distal zone of pyrite and some chalcopyrite is attained. Based on these particular mineralogical results, it is concluded that the deposit attained a maximum temperature of 350 °C according to the Corbett and Leach Diagram, (1998) and based on comparisons of other authors´ studies, it is determined that Mina Buena Vista IX shows most of the features that define a Cu- (Ag) stratabound site installed in the lower Cretaceous rocks. Lastly, the finding of almost millimetric silver mineral phases is reported; they are unidentified and exsolved in chalcocite. This particular exsolved solution is rather uncommon. As a matter of fact, no studies where silver minerals are visible under an optical microscope were found, as in the case of the present study. It is recommended to continue with this particular research line with the scanning electron microscopy and EDS semiquantitative analysis to be able to determine the composition and type of exsolved minerals, thus generating a greater contribution to the genesis of this kind of deposit.
Notas
Memoria (Geólogo)
Palabras clave
Yacimientos Estratoligados, Petrografía