Chemistry of sulfide deposites of ODP Site 106-649

The Snake Pit active hydrothermal field was discovered at 23°22'N on the Mid-Atlantic Ridge during ODP Leg 106. Among the ten holes drilled in the mound at the foot of an active chimney, only three (649B, 649F, and 649G) had substantial recovery, and produced cores of unconsolidated hydrothermal deposit made up of porous sulfide fragments with minor talc pellets and biological debris, and a few pieces of brassy massive sulfides. Eight representative samples from the 6.5-m-long core from Hole 649B were analyzed for bulk chemistry, both by XRF (major elements) and NAA (trace elements). Major elements average compositions show high Fe (36 wt%), S (37 wt%), and Cu (12 wt%) contents, and minor Zn (6.7 wt%), reflecting a mostly high-temperature deposit. Trace elements are characterized by a high Au content (600 ppb) which could express the maturity of the mound. Mineralogical assemblages show evidence of sequential precipitation, and absence of equilibrium. Major sulfide phases are pyrrhotite, pyrite, Fe, Cu sulfides, marcasite, and sphalerite. Three types of samples are distinguished on the basis of textures and mineral assemblages: type 1, rich in pyrrhotite, with approximately equivalent amounts of Cu, Fe sulfides, and sphalerite and minor pyrite; type 2, rich in Cu, Fe sulfides, which are cubic cubanite with exsolutions and rims of chalcopyrite; and type 3, essentially made up of sphalerite. Type 2 samples likely represent fragments of the inner chimney wall. The presence of talc intergrown with cubic cubanite/chalcopyrite in one big piece from Hole 649G is probably related to mixing of the hydrothermal fluid with seawater.蛇穴（Snake Pit）活动热液场于大西洋中脊23°22′N位置，在大洋钻探计划（Ocean Drilling Program, ODP）第106航次中被发现。研究人员在活动烟囱脚下的热液丘上钻取了10个钻孔，其中仅3个钻孔（649B、649F与649G）获得了可观的岩心回收率；所获取的松散热液沉积物岩心由多孔硫化物碎屑、少量滑石颗粒及生物残骸组成，并夹杂少量黄铜色块状硫化物标本。 研究人员对649B孔6.5米长岩心中的8个代表性样品开展了全岩化学分析，分析方法包括X射线荧光光谱法（X-ray Fluorescence, XRF，用于常量元素检测）与中子活化分析法（Neutron Activation Analysis, NAA，用于微量元素检测）。常量元素平均组成显示，样品富含铁（36 wt%）、硫（37 wt%）与铜（12 wt%），锌含量仅为6.7 wt%，反映该热液丘主体为高温沉积产物。微量元素以高金含量（600 ppb）为特征，该特征可反映热液丘的成熟度。 矿物组合特征显示出连续沉淀的证据，且未达到平衡状态。主要硫化物物相包括磁黄铁矿、黄铁矿、铁铜硫化物、白铁矿以及闪锌矿。根据结构构造与矿物组合，可将样品划分为3类：第1类以磁黄铁矿为主，铜铁硫化物、闪锌矿含量大致相当，仅含少量黄铁矿；第2类以铜铁硫化物为主，该类物相为具出溶结构与黄铜矿边部的立方型古巴矿；第3类则几乎完全由闪锌矿组成。第2类样品大概率代表了烟囱内壁的碎屑物质。在649G孔的一块大标本中发现了滑石与立方型古巴矿/黄铜矿共生的现象，该现象可能与热液流体与海水的混合作用有关。