内蒙古小东沟斑岩钼矿床地质特征及成因探讨

Geological features and origin of Xiaodonggou porphyry molybdenum deposit in Hexigten Banner,Inner Mongolia

小东沟早白垩世斑状黑云母花岗岩株侵位于上二叠统火山-沉积岩中,属富碱钙-碱性火成岩。钼矿化大多产在该岩株的顶部及其内、外接触带,呈细脉浸染状、条带状和团块状,最大矿化深度可达300m。钼矿石中的金属矿物有黄铁矿、辉钼矿、黄铜矿、闪锌矿和磁铁矿;非金属矿物有钾长石、石英和绢云母。从岩株中心向外,热液蚀变类型依次为硅化、钾化、云英岩化和青磐岩化,其中硅化和云英岩化与钼矿化具密切空间分布关系。据流体包裹体测试数据,可推测该矿床的形成温度为320～420℃,形成深度为1.56～2.85km。稳定同位素测定结果表明,钼矿石主要是以岩浆水为主的混源流体淀积的结果,硫和钼主要来自中酸性岩浆源。另外,低f(O2)和高f(S2)的弱酸性还原效应以及成矿流体"沸腾"作用均是导致辉钼矿沉淀的原因。小东沟矿床属斑岩型中高温热液钼矿床。

Located in the easternmost part of the Early Paleozoic tectono-magmatic belt within the Ondur Sum-Ongniud Banner on the northern edge of the North China Craton, the newly discovered Xiaodonggou deposit is a medium-size porphyry molybdenum deposit occurring in central-eastern Inner Mongolia. During the Mesozoic Yanshanian orogeny, intense tectonic and igneous activities resulted in large-scale granitoid magmatism in Xiaodonggou and its neighboring areas. The Xiaodonggou granitoid porphyry stock was emplaced in the volcano-sedimentary sequence of the Upper Permian Ranfangdi Formation, controlled by the NS-trending fault zone. With an outcrop area of 0.22 km^2, it consists mainly of porphyritic biotite granite and fine-grained biotite granite which have the same mineral assemblage. Rock-forming minerals are mainly quartz, microcline, microperthite and plagioclase associated with minor biotite and sericite. Accessory minerals comprise magnetite, apatite, zircon, sphene and allanite. The porphyry stock is petrochemically characterized by relatively high SiO2, K2O, Rb, U and Th, and obviously low CaO, Fe2O3 ＋ FeO, MgO, St, Ba, Nb, Ta and REE, thus belonging to siliconhigh, potassium-rich rock of calc-alkaline series. Molybdenum mineralization occurs exclusively within the Xiaodonggou porphyritic granitoid stock as veins, veinlets and disseminated blocks. Molybdenum ore consists of pyrite, chalcopyrite, molybdenite, sphalerite, pyrrhotite, scheelite, wolframite, K-feldspar, quartz, sericite, chlorite and epidote. As the most important economic molybdenum mineral, molybdenite occurs in the forms of impregnation, radioflakes, thin films, aggregated flakes and veinlets in veins. Re-Os isotopic data of six molybdenite separates define a correlation line corresponding to an age of （135.5 ± 1.5） Ma. Hydrothermal alterations in Xiaodonggou roughly show zonation of K feldspathization, greisenization, silicification, and propylitization from the porphyritic biotite granite stock outward. Of these alterations, silicification and greisenization are well developed and have intimate relationship with molybdenum mineralization. Fluid inclusion studies yield homogeneous temperatures from 260℃; to 480℃, with the main ore-forming stage ranging from 320℃ to 420℃. Gaseous inclusions are frequently coexistent with multiphase inclusions containing such daughter minerals as halite, sylvite, molybdenite and K feldspar. Intense boiling of the ore-forming fluid resulted in the de＇position of a great volume of molybdenite under the circumstances of medium to high temperature, relatively low f（O2） and f（S2） and weak acidity. Sulfur and oxygen isotopic data of the molybdenum ore indicate that the ore-forming fluid was dominated by magmatic water at the early ore-forming stage and characterized by mixed solution, with magmatic water as the major source at the main ore-forming stage. In contrast, the ore-bearing fluid was mainly heated meteoric water. Therefore, it is conceivable that sulfur and molybdenum were mostly derived from the magmatic source. Combined geochemical data, field geological observations and petrological evidence indicate that the Xiaodonggou deposit is a porphyry type hypothermal-mesothermal molybdenum deposit formed by Early Cretaceous intraplate granitoid magmatism on the northem margin of the North China Craton. In addition, the genetic model and mineral exploration criteria of the Xiaodonggou deposit can also be used as useful tools in the comprehensive evaluation of concealed molybdenum deposits on the northem margin of North China Craton.