Distribution and speciation of gold in biogenic and abiogenic calcium carbonates – Implications for the formation of gold anomalous calcrete
Distribution and speciation of gold in biogenic and abiogenic calcium carbonates – Implications for the formation of gold anomalous calcrete
复制标题
DOI：
10.1016/j.gca.2011.01.014
复制
发表时间：
2011-04
影响因子：
5
通讯作者：
F. Reith;B. Etschmann;R. C. Dart;D. Brewe;S. Vogt;A. S. Mumm;J. Brugger
中科院分区：
文献类型：
--
作者：
F. Reith;B. Etschmann;R. C. Dart;D. Brewe;S. Vogt;A. S. Mumm;J. Brugger
钙灰石(土壤成因碳酸钙)是澳大利亚半干旱和干旱地区化探金矿的重要采样介质，因其分布广泛，易于取样，钙(Ca)与Au呈强正相关，而与贱金属无相关性。在这项研究中，我们表明，Au异常钙质的形成可以通过驻留微生物的活动进行生物调节，而不可能简单地是非活动细胞被动成核或蒸发运输过程的结果。钙化的微体化石在南澳大利亚的Barns Au-Prospects中蕴藏着丰富的钙质矿物。这些微化石是实验室实验中形成的钙化细胞和生物膜的形态类似物，这些细菌培养物来自Barns勘探区金异常钙质砂。这些培养物沉淀出的碳酸钙主要由方解石组成，方解石是钙解石中的主要碳酸盐矿物。利用同步辐射显微X射线荧光(S-μ)图谱研究了活性细菌培养的碳酸钙盐中金、锌、钙等金属元素的分布。在μm尺度上，Au在这些钙碳酸盐中的分布是不均匀的，不同于贱金属的分布，因此模拟了这些金属在钙华中的空间分异。利用显微X射线吸收近边结构谱(μ-XANES)对活性细菌沉积的钙质碳酸盐中金的形态进行了测定，并与在金异常钙质中观察到的金的形态进行了比较。在Au“热点”中观察到金属Au，而在“热点”周围的晕中检测到离子Au。相反，在存在死亡细菌细胞或提高溶液pH或PCO2的情况下产生的沉淀物，即羟基磷灰石、紫柱石和球铁矿，不能反映钙质混凝土的矿物学。在球状陨石中，金的分布和形态是均匀的，不能再现活性细胞沉淀的钙质和碳酸钙中观察到的变化。通过递增干燥介质来增加溶液中相对于钙的过饱和度，在存在热致死细胞的情况下只产生X射线无定形沉淀物或羟基磷灰石。综上所述，本研究表明，生物成因的碳酸钙作用与金的沉淀作用相结合的活跃微生物过程有可能驱动金异常钙质的形成。
Calcrete (pedogenic Ca carbonate) is an important sampling medium for geochemical gold (Au) exploration in semi-arid and arid regions of Australia, because it is widespread, easy to sample and calcium (Ca) shows a strong positive correlation with Au, but not with base metals, in calcrete overlying buried Au mineralization. In this study we show that the formation of Au-anomalous calcrete can be biomediated through the activity of resident microorganisms, and may not simply be the result of passive nucleation on inactive cells or evapotransporative processes. Calcified microfossils are highly abundant in calcrete from the Barns Au-prospect in South Australia. These microfossils are morphological analogues of calcified cells and biofilms formed in laboratory experiments conducted with active bacterial cultures enriched from Au-anomalous calcareous sand from the Barns prospect. Calcium carbonates precipitated by these cultures consisted mostly of calcite, which is the main carbonate mineral in calcrete. Synchrotron micro-X-ray fluorescence (S-μXRF) mapping was used to assess the distribution of Au, Zn, Ca and other metals in Ca carbonates precipitated by active bacterial cultures. On a μm-scale the distribution of Au was heterogeneous in these Ca carbonates and differed from base metal distribution, thus mimicking the spatial separation of these metals observed in calcrete. The speciation of Au in Ca carbonates precipitated by active bacteria was measured using micro-X-ray absorption near edge structure spectroscopy (μ-XANES) and resembled that observed in Au-anomalous calcrete closely. While metallic Au was observed in Au ‘hotpots’, ionic Au was detected in the halo surrounding the ‘hotspot’. In contrast, the precipitates produced in the presence of dead bacterial cells or by raising solution pH or pCO2, i.e., hydroxylapatite, portlandite and vaterite, respectively, did not reflect the mineralogy of calcrete. Gold distribution and speciation in vaterite, formed by raising pCO2, were homogenous and did not reproduce the variation observed in calcrete and Ca carbonates precipitated by active cells. Increasing the supersaturation with respect to Ca in solution by incremental drying of the medium produced only X-ray amorphous precipitates, or hydroxylapatite in the presence heat-killed cells. In conclusion, this study shows that active microbial processes that combine biogenic Ca carbonatogenesis with Au precipitation are likely to drive the formation of Au-anomalous calcrete.