滇西北北衙斑岩型金多金属矿床万硐山矿段构造控岩控矿作用机制

Mechanism of Rock- and Ore-controlling Structures in the Wandongshan Ore Block of the Beiya Super-large Porphyry-type Poly-metallic Gold Deposit, Northwestern Yunnan

滇西北北衙金多金属矿床是西南"三江"多金属成矿带新生代斑岩型矿床的典型代表,是近些年来取得找矿突破的超大型金多金属矿床。运用矿田地质力学的理论和方法,通过大比例尺构造剖面精测,进行典型控岩控矿构造力学性质的鉴定,分析构造对斑岩成岩和成矿作用的控制机制。认为北衙金多金属矿床万硐山矿段自燕山晚期–喜马拉雅早期以来主要经历了近EW向→近SN向→近EW向三期构造主压应力的转变,形成了典型的褶皱–断裂–节理控岩控矿构造系统。构造控岩控矿过程为:燕山晚期–喜马拉雅早期,在近EW向主压应力作用下,形成北衙矿区控岩控矿构造格架,为斑岩侵位和成矿作用发生提供了有利的成岩成矿构造环境,并伴有石英钠长斑岩的侵位;喜马拉雅中期,主压应力方向转变为近SN向,伴随构造活动发生石英正长斑岩的超浅成侵位和成岩作用,并发生斑岩型金成矿作用,在层间断裂破碎带、断裂带及节理构造内形成带状、"似层状"和脉状金矿体,在岩体内形成斑岩型浸染状金矿体,在斑岩与围岩接触带形成矽卡岩型金矿体;喜马拉雅晚期,本区再次经受近EW向挤压作用,主要对先期形成的斑岩和金矿体产生变形、变位,并造成风化剥蚀,成矿作用主要表现为氧化淋滤富集型金矿的形成。

The Beiya poly-metallic gold deposit is one of the typical Cenozoic porphyry-type deposits in the Sanjiang poly-metallic metallogenetic belt, northwestern Yunnan. In recent years, an important breakthrough has been achieved in the Beiya poly-metallic gold deposit prospecting, which made the Au reserves rank the super-large-scale class. Through precise measurement of the large scale structural sections, we identified the mechanical properties of typical rock- and ore-controlling structures and analyzed the mechanism of the structural control of porphyry emplacement and ore-formation. The main constructional compressive stress in the Beiya orefield transited from near EW-trending to near SN-trending and to near EW-trending, forming the typical folds-faults-joints rock- and ore-controlling structures system since Late Yanshanian. The basic folding structural framework formed under near EW-trending principal compressive stress during Late Yanshanian to Early Himalayan period provided favorable structural environment for the emplacement of the porphyries. The principal compressive stress turned to near SN-trending in Middle Himalayan, which was accompanied by the ultra-shallow intrusion of the quartz syenite porphyry and porphyry-type poly-metallic gold mineralization. The ore-bodies occurred in the interlayered faulted fracture zones, faults and joints which were distributed in belts type, veins type and layers type, and porphyry-type disseminated ore-bodies were formed in the porphyry, some skarn-type gold-rich ore-bodies were formed in the contact zones of porphyry and surrounding rock. The ore bodies are hosted in the interlayered faulted fracture zones, faults, and joints, occurring as banded, vein-type, and strata-form ores. Disseminated ores are located inside of the porphyry while the skarn-type ores are found in the contacts. During Late Himalayan, the nearly EW-trending compression resulted in the destruction of ore-bodies and porphyry, and to the subsequent uplift and denudation. The oxidizing-leaching-type ore-bodies were formed mainly in this period.