Publication: El yacimiento de Zn-Pb de Castellanos (Cuba): un depósito sedimentarioexhalativo finijurásico-cretácico
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Publication Date
2023
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Sociedad Geológica Mexicana
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Abstract
El yacimiento de Zn-Pb de Castellanos, situado al noroeste de la isla de Cuba, encaja en un paquete de pizarras negras intercalado en la parte superior de una potente secuencia siliciclástica (>4km), constituida principalmente por areniscas
y limolitas de edad Jurásico Inferior y Medio (Formación San Cayetano). El yacimiento es estratiforme, se orienta en dirección NE-SO concordantemente con la estratificación de las pizarras encajantes y está constituido por dos bloques (el bloque Farallones al NE y el bloque Susana al SO) separados por una zona de falla con rumbo N120ºE; una zona de falla con la misma dirección limita también el bloque Farallones en su contacto NE. Este bloque se superpone a un complejo filoniano constituido por filones de pirita±calcopirita y cuarzo, de <10cm de espesor, y orientados también en dirección N120ºE. El cuerpo estratiforme principal está constituido por pirita, esfalerita y galena, con ganga de carbonatos (principalmente dolomita-ankerita y calcita), cuarzo, barita y, en menores proporciones sericita, biotita parcialmente alterada a clorita y apatito criptocristalino. Muestra una estructura zonal con una zona basal discontinua rica en pirita y una zona superior heterogénea en la que varían tanto las proporciones modales de sulfuros+carbonatos (localmente con cuarzo), como la abundancia relativa de esfalerita+galena respecto a pirita. En conjunto, presenta una textura groseramente bandeada en la que alternan irregularmente bandas ricas en pirita, bandas con esfalerita y galena, y bandas de dolomita-ankerita (±sericita, biotita-clorita y apatito criptocristalino), mejor definida hacia el SO del cuerpo mineral. En este sentido, y hacia el techo de la mineralización., tiende a aumentar la proporción modal de cuarzo y localmente de barita. De NE a SO también se observa una variación en la composición química de la esfalerita, la cual se hace progresivamente más rica en Zn, a la vez que cristaliza una generación más tardía de esfalerita extremadamente pobre en Fe (<1% en peso). En los límites de bloques, la mineralización está afectada por varios episodios de fracturación y brechificación durante los cuales precipitaron y/o se movilizaron diversas generaciones de calcita, barita, cuarzo y galena. En los episodios más tardíos de esta etapa precipita también calcopirita rellenando fracturas y huecos intersticiales entre los fragmentos de sulfuros previos. La interpretación genética de la mineralización de Castellanos que se propone está basada en la precipitación repetitiva (multifásica) de los sulfuros+carbonatos como consecuencia de la interacción de fluidos/salmueras mineralizantes ascendentes a lo largo de las fracturas que dieron lugar al complejo filoniano con los sedimentos ricos en materia orgánica depositados en el fondo de una subcuenca anóxica desarrollada en la cuenca de margen pasivo donde se depositó la Formación San Cayetano. Las salmueras fueron oxidantes, saturadas en sulfato, y se habrían generado en la plataforma carbonatada con evaporitas desarrollada en los márgenes de la cuenca durante el Jurásico Superior-Cretácico Inferior. La infiltración de estas salmueras a través de la potente secuencia de areniscas y limolitas aumentó su temperatura hasta 100-200ºC favoreciendo la disolución y movilización del Zn y Pb necesarios para formar la mineralización principal. La etapa de fracturación/brechificación facilitó también el ascenso de fluidos reductores equilibrados con los niveles de pizarras negras intercalados en la secuencia siliciclástica, que aportaron Au y Ba a la mineralización. En los últimos estadios, la fracturación permitió el ascenso de fluidos más profundos y calientes (~350ºC) que dieron lugar a la precipitación tardía de calcopirita.
The Castellanos Zn-Pb deposit in northwest Cuba is hosted by black shales intercalated in the upper part of a thick siliciclastic sequence (>4km) mainly made up of sandstones and siltstones of Lower to Middle Jurassic age (San Cayetano Formation). The ore deposit is tabular to wedge-shaped and stratiform, oriented NE-SW conformably with the stratification of the host black shales and appears splitted into two blocks (the Farallones block to the NE and the Susana block to the SW) by a N120Eoriented fault zone; a similarly oriented fault zone marks the NE contact of the Farallones block. This block overlies a stockwork zone made up of <10cm-thick, pyrite-chalcopyrite-quartz veins striking N120E too. The orebody consists of pyrite, sphalerite and galena, with carbonates (mainly dolomite-ankerite and calcite), quartz, and barite, as well as minor percentages of sericite, partly altered biotite and cryptocrystalline apatite. The internal structure is zoned and consists in a discontinuous, lower pyrite-rich level overlayed by a heterogeneous ore level with variable sulfide+carbonate (locally with quartz) modal proportions, as well as variable amounts of sphalerite+galena relative to pyrite. The upper level shows a broad banded texture defined by alternating pyrite-rich, sphalerite+galena and dolomite-ankerite (±sericite, partly altered biotite and cryptocrystalline apatite) bands, which becomes better defined towards the SW part of the orebody. Modal proportions of quartz and to a lesser extent barite increase too from NE to SW and from bottom to top, especially within the Susana block. An additional NE-SW trend is observed in the chemical composition of sphalerite, whose Fe content decreases southwestwards; this Fe-poor sphalerite coexists with a second generation of sphalerite containing even lower Fe (<1wt.%). Different events of fracturing and brecciation affected the sulfide ore near N120E oriented fault zones, which promoted precipitation and remobilization of several generations of calcite, barite, quartz and galena. The late events also allowed precipitation of chalcopyrite filling fractures and interstitial spaces among fragments of the previous sulfide ore. The proposed genetic interpretation of the Castellanos orebody is based on the repeated (multiphase) precipitation of sulfides+carbonates due to the interaction between ore-forming brines migrating upwards through the stockwork fractures and organic matter-rich sediments deposited in an anoxic sub-basin developed in the major passive-margin basin where the sandstones and siltstones of the San Cayetano Formation accumulated. The ore-forming brines were oxidizing, saturated in sulfate, and generated in the evaporite-bearing carbonate platform developed in the margins of the passive-margin basin from Upper Jurassic to Lower Cretaceous times. The infiltration of these brines through the thick siliciclastic sequence gave rise to a significant increase in temperature (up to 100-200ºC), thus promoting dissolution and remobilization of the Zn and Pb quantities necessary to form the main sulfide ore. The fracturing and brecciation stages enabled upward migration of reduced fluids equilibrated with black shales intercalated at different levels of the siciliclastic sequence, which, in turn, supplied Au and Ba to the ore. Late fracturing also allowed ascent of deep, hot fluids (~350ºC) capable of precipitating chalcopyrite.
The Castellanos Zn-Pb deposit in northwest Cuba is hosted by black shales intercalated in the upper part of a thick siliciclastic sequence (>4km) mainly made up of sandstones and siltstones of Lower to Middle Jurassic age (San Cayetano Formation). The ore deposit is tabular to wedge-shaped and stratiform, oriented NE-SW conformably with the stratification of the host black shales and appears splitted into two blocks (the Farallones block to the NE and the Susana block to the SW) by a N120Eoriented fault zone; a similarly oriented fault zone marks the NE contact of the Farallones block. This block overlies a stockwork zone made up of <10cm-thick, pyrite-chalcopyrite-quartz veins striking N120E too. The orebody consists of pyrite, sphalerite and galena, with carbonates (mainly dolomite-ankerite and calcite), quartz, and barite, as well as minor percentages of sericite, partly altered biotite and cryptocrystalline apatite. The internal structure is zoned and consists in a discontinuous, lower pyrite-rich level overlayed by a heterogeneous ore level with variable sulfide+carbonate (locally with quartz) modal proportions, as well as variable amounts of sphalerite+galena relative to pyrite. The upper level shows a broad banded texture defined by alternating pyrite-rich, sphalerite+galena and dolomite-ankerite (±sericite, partly altered biotite and cryptocrystalline apatite) bands, which becomes better defined towards the SW part of the orebody. Modal proportions of quartz and to a lesser extent barite increase too from NE to SW and from bottom to top, especially within the Susana block. An additional NE-SW trend is observed in the chemical composition of sphalerite, whose Fe content decreases southwestwards; this Fe-poor sphalerite coexists with a second generation of sphalerite containing even lower Fe (<1wt.%). Different events of fracturing and brecciation affected the sulfide ore near N120E oriented fault zones, which promoted precipitation and remobilization of several generations of calcite, barite, quartz and galena. The late events also allowed precipitation of chalcopyrite filling fractures and interstitial spaces among fragments of the previous sulfide ore. The proposed genetic interpretation of the Castellanos orebody is based on the repeated (multiphase) precipitation of sulfides+carbonates due to the interaction between ore-forming brines migrating upwards through the stockwork fractures and organic matter-rich sediments deposited in an anoxic sub-basin developed in the major passive-margin basin where the sandstones and siltstones of the San Cayetano Formation accumulated. The ore-forming brines were oxidizing, saturated in sulfate, and generated in the evaporite-bearing carbonate platform developed in the margins of the passive-margin basin from Upper Jurassic to Lower Cretaceous times. The infiltration of these brines through the thick siliciclastic sequence gave rise to a significant increase in temperature (up to 100-200ºC), thus promoting dissolution and remobilization of the Zn and Pb quantities necessary to form the main sulfide ore. The fracturing and brecciation stages enabled upward migration of reduced fluids equilibrated with black shales intercalated at different levels of the siciliclastic sequence, which, in turn, supplied Au and Ba to the ore. Late fracturing also allowed ascent of deep, hot fluids (~350ºC) capable of precipitating chalcopyrite.