Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits, 1. Lady Loretta Deposit, Queensland

Stratiform sediment hosted Zn–Pb–Ag deposits, often referred to as SEDEX deposits, represent an economically important class of ore, that have received relatively little attention in terms of defining lithochemical halos and geochemical vectors useful to exploration. This study concentrates on the Lady Loretta deposit which is a typical example of the class of Proterozoic SEDEX deposits in northern Australia. We examined the major and trace element chemistry of carbonate-bearing sediments surrounding the deposit and defined a series of halos which extend for several hundred metres across strike and up to 1.5 km along strike. The stratiform ore lens is surrounded by an inner sideritic halo [Carr, G.R., 1984. Primary geochemical and mineralogical dispersion in the vicinity of the Lady Loretta Zn–Pb–Ag deposit, North Queensland. J. Geochem. Expl. 22, 217–238], followed by an outer ankerite/ferroan dolomite halo which merges with low iron dolomitic sediments representative of the regional background compositions. Carbonate within the inner siderite halo varies in composition from siderite to pistomesite (Fe_0.6Mg_0.4CO₃), whereas carbonate in the outer ankerite halo varies from ferroan dolomite to ankerite (Ca_0.5Mg_0.3Fe_0.2CO₃). Element dispersion around the stratiform ore lens is variable with Pb, Cu, Ba and Sr showing very little dispersion (<50 m across strike), Zn and Fe showing moderate dispersion (<100 m) and Mn and Tl showing broad dispersion (<200 m). Within the siderite halo Cu, Mg and Na show marked depletion compared to the surrounding sediments. The magnitude of element dispersion and change in carbonate chemistry around the Lady Loretta orebody has enabled the development of three geochemical vectors applicable to exploration. Whole rock analyses are used to calculate the three vector quantities as follows: (1) SEDEX metal index = Zn + 100Pb + 100Tl; (2) SEDEX alteration index = (FeO + 10MnO)100/(FeO + 10MnO + MgO); (3) manganese content of dolomite: MnO_d = (MnO × 30.41)/CaO. All three vectors increase to ore both across strike and along strike. The manganese content of dolomite (MnO_d) exhibits the most systematic pattern increasing from background values of about 0.2 wt% to a maximum of around 0.6 wt% at the boundary between the ankerite and siderite halos. Siderite within the inner halo contains considerably more Mn with MnO values of 0.4 to 4.0 wt%. It is suggested here that the basket of indices defined at Lady Loretta (Zn, Tl, metal index, alteration index, MnO_d and MnO_s) is applicable in the exploration for stratiform Zn–Pb–Ag deposits in dolomite-rich sedimentary basins generally. The indices defined can firstly assist in the identification of sedimentary units favourable for SEDEX mineralisation, and secondly provide vectors along these units to ore. The alteration index and MnO_d, however, should only be used for exploration dolomitic sequences; they are not recommended for exploration in clastic sequences devoid of carbonates.     

新疆阿尔泰山南缘克兰盆地红墩SEDEX型铅锌矿

红墩铅锌矿床位于克兰裂谷盆地,形成于被动陆缘裂谷构造环境,具有SEDEx型铅锌矿床的地质特征,容矿岩性为一套巨厚的粉砂质、泥质碎屑岩建造,成矿作用与海相基性火山岩有关,矿体产状与地层走向一致,矿石具有明显的纹层状构造,在矿物共生组合上,表现为锌多铅少,不含铜,且闪锌矿为含铁较少的低温闪锌矿。     

沉积岩型铅锌矿床的成矿系统研究

沉积岩型铅锌矿床是铅锌矿床中重要类型之一。在系统论述沉积岩型铅锌矿床成矿背景、地球化学和矿床类型的基础上,分别阐明了沉积喷流型(SEDEX)、密西西比型(MVT)和砂岩型(SST)3个成矿系统的成矿要素、作用过程、矿床和异常组合以及成矿模式。对3个铅锌成矿系统作了全面对比,获得一些认识:(1)SST矿床的矿化表现为对构造和高孔隙度有利岩性而不是地层层位的依赖性;(2)云南会泽碳酸盐型铅锌矿(MVT)成矿物质除热卤水沉淀来源外,还有火山岩系的提供,属后生混合流体成矿作用;(3)沿古陆边缘同生断裂构造发生的海底热水喷流沉积作用为SEDEX型铅锌成矿奠定了物质基础;裂谷伸展期常有“双峰式”火山活动,提供部分矿源、热源、水源;同生断层长期活动提供热水循环通道和矿质沉积场所;(4)风化剥蚀的变质基底为三者提供了成矿物质,成矿热卤水在盆地中经过了演化过程。     

中国SEDEX型矿床成矿流体硼、硅、氦-氩同位素组成研究评述

SEDEX型矿床成矿流体的研究是矿床地球化学研究的重要课题之一。正确识别系统中不同的流体来源及其混合过程,是深刻理解SEDEX型矿床形成演化的关键。系统总结了国内几个典型的SEDEX型矿床同位素研究成果,认为B和Si同位素是根据SEDEX型矿床独特的矿物组合而提出的一种示踪方法,对矿床成因和沉积环境示踪效果理想;He、Ar同位素则因为在地壳和地幔储库中有极不相同的组成,是理想的幔源流体示踪剂。鉴于SEDEX型矿床含有电气石、黄铁矿、硅质岩等特殊的矿物与岩石组合,B、Si、He-Ar同位素可能更适合SEDEX型矿床矿化流体来源研究,并指出其理论发展的薄弱之处。     

Impact of fish farming on the distribution of phosphorus in sediments in the middle Adriatic area

During the last decade, intensive fish farming developed along the central Croatian coast, creating a need to study and evaluate its potential influence on unaffected sites. We considered phosphorus as an indicator of the influence of fish farming and investigated the distribution of phosphorus forms in sediment from several fish farms and marine areas of different trophic status in the middle Adriatic. Analyses of samples were performed with modified SEDEX techniques. Our results indicated that authigenic apatite phosphorus showed no significant differences among the investigated stations, while organic phosphorus concentrations reflected the trophic status of the station area. Below-cage sediment was characterized by enhanced fish debris phosphorus and low detrital apatite phosphorus concentrations, while sediment from an anthropogenically influenced bay showed the highest values of iron bound phosphorus species. Among the different P fractions, fish debris phosphorus proved to be the most sensitive indicator of the influence of fish farming on marine sediment.     

锡铁山矿床两类喷流沉积成因的铅锌矿体研究

锡铁山大型铅锌矿床发育有非层状和层状两种类型的铅锌矿体,通过对两类矿体产出地质条件与地球化学特征的研究,发现非层状矿体分布于代表喷口系统的网脉状蚀变带的上方附近,其中发育有大量的热水爆破角砾岩,而层状矿体分布于远离喷口的外侧,属于远端沉积的产物。两者具有相同的矿物组合和成矿元素组合,自非层状矿体→层状矿体,矿石品位下降,Zn/Pb比值增高,黄铁矿相对含量增加。揭示出非层状矿体不是层状矿体后期改造的产物,而是形成于喷流作用阶段。考虑到锡铁山矿床中规模巨大的喷口系统以及厚层状的喷流成因大理岩,推断锡铁山矿床及周边地区还有巨大的找矿潜力,找矿重点是层状铅锌矿床(体)。     

HYC (McArthur River) SEDEX deposit, Australia: First paleomagnetic results

Analysis of pyrrhotite-borne paleomagnetic data from four sites on each limb of a syncline through the supergiant HYC (McArthur River) SEDEX Zn–Pb deposit in Australia indicates that the mineralization is entirely postfolding, formed within 15 Myr after deposition at 1640 Ma of the host Barney Creek Formation, and thus is either late syndiagenetic or epigenetic in origin.     

青海虎头崖地区多金属矿床成因类型的厘定及找矿方向

祁漫塔格成矿带是青海省重要的铜多金属成矿区带,虎头崖地区铜铅锌矿床由虎头崖和迎庆沟两个相连矿区组成,近年来找矿取得了很大进展,但该区矿床成因类型尚有争议。通过详细的野外观察,从不同角度收集证据,厘定了矿床的成因类型:一是与印支期岩体、奥陶—志留系滩间山群、上石炭统缔敖组有关的热液型;二是与中元古界狼牙山组有关的喷流沉积-热液叠加改造型。同时,对矿床控矿因素进行了分析,并建立了找矿标志,以指导找矿。     

Hydrothermal Mineralization on the Mesoproterozoic Passive Continental Margins of China: A Case Study of the Langshan-Zha'ertaishan Belt,Inner Mongolia, China

Most ore-forming characteristics of the Langshan-Zha'ertaishan hydrothermal exhalation belt, which consists of the Dongshengmiao, Huogeqi, Tanyaokou and Jiashengpan large-superlarge Zn-Pb-Cu-Fe sulfide deposits, are most similar to those of Mesoproterozoic SEDEX-type provinces of the world. The characteristics include: (1) All deposits of this type in the belt occur in third-order fault-basins in the Langshan-Zha'ertaishan aulacogen along the northern margin of the North China Platform; (2) these deposits with all their orebodies hosted in the Mesoproterozoic impure dolomite-marble and carbonaceous phyllite (or schists) have an apparent stratabound nature; ores display laminated and banded structures, showing clear depositional features; (3) there is some evidence of syn-sedimentary faulting, which to a certain extent accounts for the temporal and spatial distribution and the size of the orebodies in all deposits and the formation of intrabed conglomerates and breccias; (4) they show lateral and vertical