Facies analysis of a 1.9 Ga, continental margin, back-arc, Felsic Caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen region, Sweden
Facies analysis of a 1.9 Ga, continental margin, back-arc, Felsic Caldera province with diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen region, Sweden
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DOI：
10.2113/gsecongeo.91.6.979
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发表时间：
1996-09-01
期刊：
影响因子：
--
通讯作者：
Christofferson, H
中科院分区：
文献类型：
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作者：
Allen, RL;Lundstrom, I;Christofferson, H
位于瑞典中南部的Bergslagen矿区1,000多年来一直是主要的金属生产地。在地质方面，Bergslagen是一个大型的，早元古代（主要是1.90-1.87 Ga），长英质岩浆区，主要是在波罗的海盾中的中到高变质等级的强烈矿化的一部分。该地区有各种各样的矿石存款类型，包括带状铁建造，磁铁矿钙硅酸盐夕卡岩，含锰夕卡岩和碳酸盐铁矿石，磷灰石铁矿石，层状和层控锌铅银（铜金）硫化物矿石，和W夕卡岩。大多数矿床产于热液蚀变火山岩及伴生的金属灰岩和矽卡岩中。本研究是一项面向野外的区域相分析，其基础是通过热液蚀变、变形和变质作用的强烈叠加识别原始火山和沉积特征。该地区被解释为一个伸展的，可能是回弧，活跃的大陆边缘岩浆区。它经历了强烈的岩浆活动、热穹隆和地壳伸展，随后是逐渐减弱的伸展作用、逐渐减弱的火山作用、热沉降、从伸展到挤压变形的逆转、区域变质作用和构造反转。火山序列包括近端（近喷口），中间（火山侧翼）和远端（火山边缘）相协会的流纹岩火山碎屑破火山口火山和下属英安岩流纹岩复合体的交指。非焊接到焊接不良的火山碎屑流和沉降单位，以及它们快速沉降的陆上和水下等价物是最丰富的火山相。次火山斑状侵入体和隐穹也很丰富。火山碎屑主要从浅水和陆上近端区域脱落到浅水和中等深水（风暴波底以上和以下）远端区域。大多数矿床形成于区域衰退火山阶段，并出现在由流纹质灰粉砂岩、石灰岩和玻璃结晶砂岩和角砾岩组成的中远端相组合中。这些矿化相组合主要沉积在晴天浪基面以下的水下环境中，但局部高于风暴浪基面。据解释，水深主要为10至500米，局部地区更深。虽然矿床出现在中间到远端相组合，许多发生在上面，或横向相邻，下沉的火山喷口复合体。这表明了几种矿石存款类型和这些火山中心的晚期岩浆阶段之间的空间，时间和可能的成因关系。除含磷灰石铁矿床外，尚不确定矿石是否与特定火山的特定岩浆热液事件有关。这是可能的，特别是对于在长英质火山岩中具有强烈局部下盘蚀变的贱金属硫化物矿床。然而，火山中心的一个重要作用可能是为区域对流地热系统提供强烈的正扰动（+/-岩浆热液输入）。主要的Zn-Pb-Ag-（Cu-Au）矿床跨越两种端元类型之间的范围：层状灰-粉砂岩海底矿床，与某些层状沉积物海底Zn-Pb-Ag矿床，特别是布罗肯希尔型矿床相似;层控火山相关石灰岩-矽卡岩海底交代矿床，更接近于长英质火山相关块状硫化物矿床。相模型都提出了。条带状铁建造被解释为化学悬浮沉积，韵律性地与流纹质灰粉砂岩互层。矽卡岩铁矿石是石灰岩中的层控矿石，包括变质沉积矿石和交代交代矿石。含磷灰石铁矿石与其他矿石的不同之处在于其形成于英安质侵入杂岩中，该杂岩可能是英安岩-流纹岩火山中心的根带。
The Bergslagen mining district in south-central Sweden has been a major metal producer for more than 1,000 years. In geologic terms, Bergslagen is the intensely mineralized part of a large, Early Proterozoic (mainly 1.90-1.87 Ga), felsic magmatic region of mainly medium to high metamorphic grade in the Baltic Shield. The district contains a diverse range of ore deposit types, including banded iron-formation, magnetite-calc-silicate skarn, manganiferous skarn- and carbonate-hosted iron ore, apatite-bearing iron ore, stratiform and strata-bound Zn-Pb-Ag-(Cu-Au) sulfide ores, and W skarn. Most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns.This study is a field-oriented, regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration, deformation, and metamorphism. The region is interpreted as an extensional, probably back-are, active continental margin magmatic region. It evolved through stages of intense magmatism, thermal doming, and crustal extension, followed by waning extension, waning volcanism, thermal subsidence, reversal from extension to compressional deformation, regional metamorphism, and structural inversion. The volcanic successions comprise interfingering proximal (near vent), medial (volcano flanks), and distal (volcano margin) facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite-rhyolite complexes. Nonwelded to poorly welded pyoclastic flow and fallout units, and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies. Subvolcanic porphyritic intrusions and cryptodomes are also abundant. The pyroclastic debris was shed from mainly shallow-water and subaerial proximal areas to shallow- and moderately deep-water (above and below storm wave base) distal areas.Most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash-siltstone, limestone, and vitric crystal sandstone and breccia. These mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base, but locally above the storm wave base. Water depths are interpreted to have been mainly 10 to 500 m, with local deeper areas. Although the ore deposits occur in medial to distal facies associations, many occur above, or laterally adjacent to, subsided volcanic vent complexes. This suggests a spatial, temporal, and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers. Except for the apatite-bearing iron deposits, it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes. This is possible, especially for the base metal sulfide deposits, which have intense localized footwall alteration in felsic volcanic rocks. However, an important role of the volcanic centers may have been to provide strong positive pertubations (+/- magmatic hydrothermal input) to the regional convective geothermal system. The main Zn-Pb-Ag-(Cu-Au) ore deposits span a range between two end-member types: stratiform ash-siltstone-hosted sea-floor deposits that have similarities to some stratiform sediment-hosted Zn-Pb-Ag deposits, especially the Broken Hill-type deposits; and strata-bound volcanic-associated limestone-skarn subsea-floor replacement deposits that more closely resemble felsic volcanic-associated massive sulfide deposits. Facies models are presented for both. The banded iron-formations are interpreted as chemical suspension deposits, rhythmically interbedded with rhyolitic ash-siltstone. The skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements. Apatite-bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite-rhyolite volcanic center.