滇西北北衙金多金属矿田的成岩成矿作用：对印-亚碰撞造山过程的响应

Petrogenesis and metallogenesis of the Beiya gold- polymetallic ore district,northwestern Yunnan province,China: Responses to the Indo-Asian collisional processes

北衙金多金属矿田是藏东-金沙江-哀牢山新生代富碱斑岩成矿带中南段的代表性矿床之一,发育一个与富碱斑岩密切相关的金多金属成矿系统。本文较为系统地分析了矿田内的成岩成矿作用特征及其演化过程,并探讨其与印-亚碰撞造山过程的响应关系。富碱斑岩具有埃达克岩的地球化学亲合性,其源区是喜马拉雅期印-亚碰撞造山造成的软流圈向东挤出汇聚使大规模走滑断裂活化,诱发玄武质下地壳部分熔融的壳幔过渡层,成岩年龄可分为第一期(65～59Ma)、第二期(36～32Ma)、第三期(26～24Ma)和最晚期(3.8～3.6Ma)等4期,其中第二、三期与富碱岩带北段的两期岩浆集中活动时期基本吻合,形成的斑岩对金多金属成矿较为有利。区内金多金属矿床可划分为三个矿床类型和七个矿床亚类,即与喜马拉雅早—中期斑岩有关的金多金属矿床(Ⅰ),包括接触带夕卡岩型、斑岩型和热液充填型(及熔浆型)金多金属矿床;与喜马拉雅第三期斑岩有关的金多金属矿床(Ⅱ),包括爆破角砾岩型和叠加热液改造型金多金属矿床;以及与喜马拉雅期表生作用有关的风化堆积型金矿床(Ⅲ),包括古砂矿型和红色粘土型金矿床。Ⅰ、Ⅱ类型矿床受富碱斑岩及伴生的 NE 到 NNE 向断裂控制,赋存于富碱斑岩体内、内外接触带及其附近围岩的层间破碎带或构造裂隙带中,在成因和空间上与斑岩及隐爆角砾岩等密切有关。成矿物质和成矿流体主要来源于地幔,围岩地层只是提供了成矿的空间,不同类型的矿体之间呈"贯通式"的时间和空间关系,构成了一个统一的喜马拉雅期富碱斑岩-热液型金多金属成矿系统。先期形成矿床明显受后期岩浆热液的叠加改造,但矿化分布和成矿元素组合仍表现为以斑岩为中心,存在 CuAu(Mo)多金属→FeCuAuPbZn 多金属→AuPhZnAg 多金属的分带特征。从最早期含金铁矿床形成之后,原生金矿的次生富集和表生成矿作用就已开始,并形成不同成因类型的风化-堆积型金矿床。其中,古红色粘土型金矿床的成矿主要发生在始新世到渐新世,河-湖相古砂金矿床形成于23～5Ma 期间,红色粘土型(残坡积型)金矿床可从始新世一直延续至今。通过与区域斑岩成岩成矿演化时序的对比,提出与藏东-金沙江-哀牢山斑岩成矿带上的众多矿床一样,北衙矿田内的成岩成矿作也是喜马拉雅期印-亚陆陆碰撞造山带成岩成矿作用在东南缘构造转换带的远程效应,记录了印-亚大陆碰撞造山的详细过程。因而,该矿田深部及外围地区,仍存在巨大的找矿潜力。盐源-丽江断裂带可能也是一务与藏东-金沙江.哀牢山斑岩成矿带联系密切而又相对独立的富碱斑岩成矿带。

Beiya gold-polymetallic ore district found in northeastern Yunnan province in the last few years, located in the middlesouthern section of the eastern Xizang-Jinsha River - Ailao Mountain alkali-rich porphyry metallogenetic belt, is one of the typical gold-polymetallic metallogenesis system related to alkali-rich intrusion during the Himalayan epoch. Based on the previous researches and the extensive field investigations from the recent scientific programs, this article systematically studies the petrogenesis, metallogenesis evolving sequence in the ore district and their responses to the Indo-Asian collisional processes. The main intrusion rock types including quartz-albite porphyry, quartz-K-feldspar porphyry and biotite-K-feldspar porphyry with adakitic magma affinity, which intruded during the period of 65 to 59 Ma, 36 to 32 Ma, 26 to 24 Ma and 3.8 to 3.6 Ma respectively. The magmas of ore-bearing alkali-rich porphyries were derived from mixed melting of the crust-mantle transitional layer in eastern margins of the Tibet plateau, and genetically related to the asthemosphere eastern-extruding and the large-scale strike-slip faulting due to the Himalayanian Indo-Asian collisional systems. The duration of the first episode and the second episode are consisted with the two concentrative petrogenesis episodes approximately. Within the ore district, three types and seven subclasses of gold-polymetallic deposits have been recognized, there are （Ⅰ） porphyry copper-gold-polymetallic deposits related to the first and second stage of alkali-rich porphyry intrusion, including skarn-type gold-polymetallic deposits （including magma-type ）, porphyry-type copper-gold deposits and hydrothermal-type gold- polymetallic deposits, （Ⅱ） porphyry-related hydrothermal-type gold-polymetallic deposits due to the third stage of alkali-rich porphyry intrusion, including cryptoexplosive breccia-type gold-ferrous-lead- zinc-polymetallic deposits and hydrothermal superimposed-type gold-polymetallic deposits, and （Ⅲ） weathered-sedimentary-type deposits associated with the fluvial and lacustrine sedimentary rocks formed by the surface weathering and the laterization, including red clay-type gold deposits and palaeoplacer-type gold deposits. The Ⅰ-and Ⅱ-type deposits were controlled directly by the alkali-rich porphyry intrusion and NE-NNE-trending strike-slip faulting, and occur in the porphyry intrusions, the cryptoexplosive breccia pipes, the porphyry contacts and the porphyry outside contacts of the fractured zone and the intensive joint zone within middle Triassic limestone of Beiya Formation. The earlier deposits usually were overprinted by the later epithermal mineralization system associated with late magmatism, which formed either isolated, but spatially coexisted a high-grade giant deposit, and all provided the material sources for the palaeoplacer-type deposits. Correspondingly, ore- forming mineralization also changed from Cu-Au-（Mo） associations, Fe-Cu-Au-Pb-Zn associations to Au-Pb-Zn-Ag associations, the mineralization resources and fluid resources both come from the alkali-rich porphyries magmatism, and construct a porphyry- hydrothermal metallogenic system, the wall rocks only provide a space for the metal deposition. The secondary enrichment and supergenetic mineralization began at the time of the primary gold deposits occurrence, and brought up various weathered-sedimentary- type deposits. Among them, the palaeo red clay-type gold deposits formed during the period from Eocene to Oligocene, the palaeoplacer gold deposits formed during the period from 23 to 5Ma, and the modern red clay-type （including eluvium-type） gold deposits continued all along. Comparing to the regional evolution sequences, it is explained that the Beiya gold-polymetallic deposits be controlled by the evolution of the Indo-Asian collisional orogen and tectonomagmatism under the collision belt, which implies the same dynamic setting of paroxysmal mineralization of porphyry-type deposits to the eastern Xizang-Jinsha River-Ailao Mountain porphyry metallogenetic belt, displayed the long-distance effects in the structural transform zone of the main collisional orogenic setting since the Palaeocene. Episodically stress relaxation during tectonically transforming from transpressional（55 -40 Ma） to transtensional（24 -17 Ma） regimes probably caused multiple magmatic intrusions, which most likely result in the protraction of the hydrothermal system and superimprosed mineralization in the eastern Indo-Asian continental collision zone, and the duration from 36 to 32Ma is the main mineralization period of the porphyry- hydrothermal metallogenic system. There are great gold-polymetallic prospective reserves within the deep areas of ore district and surrounding regions. It is possible that the Yanyuan-Lijiang fault control a comparatively independent porphyry metallogenetic belt with close relationship to the Xizang-Jinsha River - Ailao Mountain porphyry metallogenetic belt.