老挝班康姆铜金矿成矿流体及成矿物质来源:H-O-He-Ar-C-S-Pb同位素证据

Sources of ore-forming fluids and metals of the Pangkuam Cu-Au deposit,Laos:evidence from H-O-He-Ar-C-S-Pb isotopes

老挝班康姆矿床是近年来在琅勃拉邦-黎府成矿带新发现的一个大型铜金矿床。该矿床矽卡岩与矿体主要赋存在安山岩中且缺乏矽卡岩分带,与典型矽卡岩矿床的地质特征存在一定的差别。因此,厘清班康姆铜金矿床的成矿流体、成矿物质来源及矿床成因机制是后续开展琅勃拉邦-黎府成矿带大型铜金矿床找矿勘探的基础。该矿床矿化阶段石英流体包裹体δD分布于-110‰~-90‰,δ^(18)O分布于-1.5‰~7.1‰,其中低δD的样品具有相对高的δ^(18)O值;黄铁矿流体包裹体的3He/4He为0.41~3.43Ra(大部分<1Ra),^(40)Ar/^(36)Ar为314.8~362.4。H-O及He-Ar同位素结果表明,班康姆矿床成矿流体来源于岩浆流体(至少部分来自地幔)与低δD的大气雨水的混合,雨水占更大的比例,且某些矿化流体的雨水端元在混合前经历了明显的水岩作用。除一件样品(BK64)的黄铁矿具有高的δ^(34)S(8.1‰)外,其余硫化物的δ^(34)S分布于-0.9‰~1.5‰,位于地幔硫的范围。共生硫化物对的硫同位素平衡分馏计算以及动力学分馏不支持高δ^(34)S(8.1‰)黄铁矿的硫来自从热液流体,可能来自围岩。热液方解石的δ^(13)C范围为-3.1‰~2.5‰,δ^(18)O变化于26.0‰~28.4‰,指示其碳来自矿区灰岩,而灰岩的溶解为热液摄取围岩的重硫提供了可能。矿石黄铁矿Pb同位素组成(^(206)Pb/^(204)Pb:17.9284~18.7756;^(207)Pb/^(204)Pb:15.5336~15.6651;^(208)Pb/^(204)Pb:37.9125~38.8090)位于黎府褶皱带和长山褶皱带晚二叠世—中三叠世大陆弧岩浆岩的Pb同位素范围,介于印支地块玄武岩和泰国-老挝S-型花岗岩及相关矿床的Pb同位素组成之间,指示班康姆矿床的Pb来自壳幔混合源。本文S-Pb-He-Ar同位素结果及区域Cu-Au成矿过程的岩石地化研究,表明班康姆矿床Cu、Au主要来自地幔。与典型矽卡岩Cu-Au矿床的S-Pb-H-O同位素及矽卡岩矿物流体包裹体盐度特征的对比,结合前人的火山气热液交代火山岩形成矽卡岩的实验结果,认为班康姆矽卡岩型Cu-Au矿床的形成机制为深部出溶的气相为主的含矿岩浆流体沿断裂上升到浅部交代安山岩或大理岩并经历了流体混合、沸腾及矿石沉淀等过程。

The newly discovered Pangkuam deposit in NW Laos is a large-size copper-gold deposit from the Luang Prabang-Loei metallogenic belt.Most of the skarn and orebodies are hosted in andesite and the zonations of skarn minerals,unlike as in typical skarn-type-deposits,are scarce inthe Pangkuam deposit.Thus,identifyingthe sources of ore-forming fluid and metals,and clarifying metallogenic process of the Pangkuam copper-gold deposit is essential for the subsequent exploration of large copper-gold deposits in the Luang Prabang-Loei metallogenic belt.The δD and δ^(18)O of fluid inclusions for quartz from copper-gold mineralization stages range from-110‰to-90‰and-1.5‰to 7.1‰respectively,and the lower δD values match the higher δ^(18)O.The ore-forming fluid enclosed in ore pyrite have 3He/4He of 0.41 Ra to 3.43 Ra(mostly<1 Ra)and ^(40)Ar/^(36)Ar of 314.8 to 362.4.The H-O-He-Ar isotope results suggest that the ore-forming fluid of the Pankuam deposit is derived from the mixing of magmatic fluid(part or in whole from the mantle)and meteoric water with low δD,and the latter have a larger proportion of mass in ore-forming fluid.Some meteoric water component of ore-forming fluid have obviously exchanged oxygen isotope with wallrock before mixing.Except for one sample with high δ^(34)S value(8.1‰),the rest δ^(34)S values of the sulfide(-0.9‰to 1.5‰)fall in the range of mantle sulfur isotopes compositions.The result of calculations of sulfur isotope equilibrium fractionation between pyrite and coexisting chalcopyrite from the sample No.BK64,and kinetic fractionation theory both show that the sulfur of pyrite with δ^(34)S of 8.1‰is notderived from hydrothermal fluid,likely from wallrock.The carbon isotope compositions of hydrothermal calcite(δ^(13)C:-3.1‰~2.5‰;δ^(18)O:26.0‰~28.4‰)indicate their carbon source from limestone in Pangkuam district.The limestone dissolution process proved a pathway for sedimentogenesis carbon and sulfur being taken into ore-forming fluids.The ^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb and ^(208)Pb/^(204)Pb of ore pyrites,ranging respectively within 17.9284~18.7756,15.5336~15.6651 and 37.9125~38.8090,are similar to that of Late Permian-Middle Triassic continental arc volcanic-intrusive rocks in Loei and Trungson fold belts,and lie between Indosinian basalts and S-type granite and granite-related deposits in Thailand and Laos,which suggest that Pb inPangkuam depositis derived from mantle and upper crust.According to the S-Pb-He-Ar isotope results and petrogeochemistry studies related to skarn-type Cu-Au deposits in Luang Prabang-Loei metallogenic belt,we conclude that Cu and Au of the Pangkuam depositare mainly derived from the mantle.On base of comparative studies of S-Pb-H-O isotopes and the salinity of fluid inclusions for skarn minerals between Pangkuam deposit and other typical skarn-type Cu-Au deposits,and the previous experiments results that the reaction between the volcanic gas hydrothermal fluids and andesite could form prograde metamorphic skarn under high-temperature conditions,we suggest that theore-bearing magmatic fluid of Pangkuam skarn-type Cu-Au deposit dominated by gas-phase rises along the fault to replace the andesite or marble,and undergoes multiple geological processes,such as fluid mixing,boiling and ore precipitation.