Measured and computed neutralization potentials from static tests of diverse rock types

Static tests typically are used in the initial stage of environmental assessments to predict the potential for acid mine drainage, and also are used to monitor waste disposal at active mining operations. The neutralization potential (NP) of 19 sedimentary, metamorphic, and igneous extrusive rocks was measured by the Sobek method to compare the results with those computed by using the quantitative mineralogy of the rocks and the NP values that were deemed to be appropriate for the individual minerals. Most of the computed NP values of the rocks are lower than the measured values, but a close relationship is evident. If the contents of carbonate minerals are low, the NP in non-ultramafic igneous rocks typically is controlled by the amount of Ca substitution in plagioclase, whereas in ultramafic rocks the NP contribution is dominated by the amount of olivine and serpentine. Compositional and NP analyses of plagioclase concentrates indicate that the NP systematically varies from <1 for the Na end-member to about 14 for compositions near that of the Ca end-member. Among several variables that can affect the Sobek NP values, the volume and normality of the acid added to the sample are well-known to influence the results; however, the temperature and duration of the acidification stage can also have a pronounced effect on the measured NP of some rocks, depending on their mineralogy. Fizz tests to estimate the carbonate content and the consequent acid addition are highly subjective, and the ‘no fizz’ rating is not to be taken literally. The NP results themselves provide a clear indication of whether a sample has been over-acidified or under-acidified in the test procedure.     

富挥发分岩浆补给对斑岩型铜-钼矿床形成的关键作用:以西藏尼木岗讲矿床为例

西藏冈底斯带中段的岗讲斑岩铜-钼矿床发育多期次侵入体,而成矿作用主要与其中一期岩体(流纹英安斑岩)密切相关。为探究其原因,本文对岗讲斑岩铜-钼矿床中发育的各期次侵入体进行了全岩主、微量元素分析,并重点研究各期次侵入体内部新鲜斑晶(黑云母、斜长石)和副矿物(锆石、磷灰石)的化学成分和结构特征。结果表明,矿区各期次侵入岩均属于高钾钙碱性系列,具有相近的锆石饱和温度,都来自较为氧化的岩浆。此外,相比其他期次侵入岩,主成矿期的流纹英安斑岩中的磷灰石具有较高的SO_3、Cl含量,较低的F含量;黑云母含有较高的Cl含量和较低的F含量;同时,斜长石发育反环带。这些证据表明,在主成矿期岩浆就位之前,存在富S、Cl的偏基性岩浆注入了深部岩浆房,并发生岩浆混合作用,这不仅导致了主成矿期斑岩体的就位,同时还诱发流体出溶进入浅部岩浆房,并最终形成岗讲斑岩铜-钼矿床。