Water table movement and supergene enrichment at Spence porphyry copper deposit, northern Chile
Water table movement and supergene enrichment at Spence porphyry copper deposit, northern Chile
复制标题
智利北部斯彭斯斑岩铜矿床地下水位运动和表生富集
DOI：
10.1080/25726838.2019.1608703
复制
发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Shaw J
中科院分区：
文献类型：
--
作者：
Shaw J
在智利北方的中部安第斯山脉，表生富集作用是产生挖掘出来的斑岩铜矿的经济可提取金属品位的基础（Sillitoe和McKee，1996年）。当挖掘出的多氯二苯二恶英在近地表环境中经历氧化风化时，就会发生富集。硫化物（主要是黄铁矿[FeS 2]）被大气降水溶解产生硫酸，硫酸浸出铜（例如从黄铜矿[CuFeS 2]）并将其向下输送到溶液中。在地下水位以下的还原条件下，铜通过离子交换反应再沉淀，形成富含次生硫化物（如辉铜矿[Cu 2S]）的表生层（Sillitoe，2005年）。地下水位形成氧化和还原条件之间的边界，并决定了沥滤和富集可能发生的深度（Ague和Brimhall，1989年）。通过金属的水再分配进行表生富集需要大气降水;然而，智利北方目前是地球上最干旱的地区之一。先前的研究表明，智利北方PCD的表生富集在中新世中期停止，可能是由于干旱的增加（Sillitoe和McKee，1996年;库珀等人，2016年），尽管条件可能已经干燥了更长的时间（Clarke 2006）。我们的目标是了解Spence PCD中Cu淋溶与地下水位移动之间的关系，通过对存款淋滤盖层表生蚀变和风化矿物的测年，已发表的智利多氯联苯表生年代学是基于硫化物风化形成的表生明矾石[KAl 3（SO 4）2-（OH）6]的40 Ar/39 Ar测年（Sillitoe，2005年）。仅在地下水位以上的氧化条件下形成的赤铁矿[Fe 2 O3]的（U-Th）/He年龄可以随时间限制地下水位深度（库珀等人，2016年）。Spence的明矾石年龄表明富集发生在44至20 Ma之间（Rowland和Clark，2001年），尽管初步的一套赤铁矿（U-Th）/He年龄表明地下水位从矿物覆盖层底部下降到辉铜矿富集带顶部的最低位置，时间为11至2 Ma。然而，在斯宾塞的许多样品中，明矾石脉横切赤铁矿脉，这意味着
Supergene enrichment has been fundamental in producing the economically extractable metal grades of exhumed porphyry copper deposits (PCDs) in the Central Andes of northern Chile (Sillitoe and McKee 1996). Enrichment occurs when exhumed PCDs undergo oxidative weathering in the near-surface environment. Dissolution of sulphides (principally pyrite [FeS2]) by meteoric water produces sulphuric acid, which leaches Cu (eg from chalcopyrite [CuFeS2]) and transports it downward in solution. Under reducing conditions beneath the water table, Cu is reprecipitated through ion-exchange reactions, forming a supergene blanket enriched in secondary sulphides (eg chalcocite [Cu2S])(Sillitoe 2005). The water table forms the boundary between oxidising and reducing conditions and determines the depth to which leaching and enrichment can occur (Ague and Brimhall 1989). Supergene enrichment through aqueous redistribution of metals requires meteoric water; however, northern Chile is currently one of the driest regions on Earth. Previous research suggests that the supergene enrichment of PCDs in northern Chile ceased during the middle Miocene, possibly due to an increase in aridity (Sillitoe and McKee 1996; Cooper et al. 2016), although conditions may have been dry for much longer (Clarke 2006).We aim to understand the relationship between Cu leaching and water table movement at the Spence PCD, by dating supergene alteration and weathering minerals in the leached cap of the deposit. Published supergene chronologies for PCDs in Chile are based on 40Ar/39Ar dating of supergene alunite [KAl3 (SO4) 2-(OH) 6](Sillitoe 2005), formed through sulphide weathering.(U-Th)/He ages of hematite [Fe2O3], which only forms under oxidising conditions above the water table, can constrain water table depth through time (Cooper et al. 2016). Alunite ages for Spence suggest enrichment occurred from 44 to 20 Ma (Rowland and Clark 2001) although a preliminary suite of hematite (U-Th)/He dates suggest the water table dropped from the base of the post-mineral cover to its lowest position at the top of the chalcocite enrichment zone between 11 and 2 Ma. However, in many samples from Spence, veins of alunite cross-cut hematite veins, meaning the existing chronologies of