山西北落峡铁矿床流体包裹体研究与矿床成因讨论

GENESIS OF THE BEILUOXIA SKARN IRON DEPOSIT IN SHANXI PROVINCE: FLUID INCLUSION EVIDENCE

北落峡铁矿床是太行山南段矽卡岩型铁矿成矿区域内一个比较重要的矿床。本文通过对北落峡铁矿床流体包裹体研究,讨论了该矿床成矿流体特征、性质、流体演化及铁矿床形成机制。认为成矿流体源自岩浆,流体组分以Na-Fe-Cl-SO_4-H_2O为主。成矿流体温度、盐度分别高达540—660℃和55—66wt.％Nacl。岩浆期后流体不混溶作用是造成成矿流体高盐度的原因。由于流体温度、盐度降低以及沸腾作用导致溶液中铁溶解度急剧下降并发生磁铁矿沉淀。

The Beiluoxia skarn iron deposit in Shanxi province is a replacement product of Middle Ordovician limestone adjacent to a Mesozoic diorite phacolith. The diorite intrusion has been strongly albitized towards the orebodies. Post magmatic hydrothermal and albitization fluids served as the main iron source for the Beiluoxia deposit. The major gangue minerals include early diopside and garnet and late epidote and calcite. Magnetite precipitated earlier than epilote but later than garnet. The fluid inclusions can be divided into three compositionally distinct groups: (A) liquid+gas+multiphase solids (540-660℃, with salinities of 50-70 NaCl wt% equiv.), (B) liquid+gas(+halite) (240-525℃, 1.6-42 NaCl wt% equiv.); (C) gas+liquid, with gas phase>50 (240-460℃). Type A mostly appears in diopside, whereas Type B and C mainly occur in calcite, garnet and epidote. All of these types of inclusions are traces of metallogenic fluids throughout the principal ore-forming process. The fluid inclusion evidence shows that the fluids belong to the Na-Ca-Cl-SO_4 system with abundant iron in the early stage. Type A fluid, having the highest homogenization temperature and containing great amounts of Fe, Na and Cl, is considered to be the main ore-forming fluid. This is further supported by the fact that, of all inclusions, only Type A ones contain small crystals of hematite. Magnetite precipitation resulted from rapid decrease in fluid temperature. Fluid inclusions in calacite coeval with magnetite show indications of fluid boiling.