造山带背景铜镍矿床蚀变过程与水的来源——东天山黄山南矿床氢氧同位素的指示

造山带背景铜镍矿床以富水为主要特征, 但水(流体)在成矿中的作用以及其对岩石的改造过程仍不明确。本文以黄山南铜镍矿床为例, 通过辉橄岩和橄辉岩等超镁铁岩的蚀变矿物组合和H-O同位素变化规律, 限定蚀变过程与流体性质及来源。黄山南超镁铁岩原生矿物主要有尖晶石、橄榄石、斜方辉石、单斜辉石和少量填隙状角闪石、云母, 蚀变矿物有角闪石(浅闪石、阳起石、透闪石、普通角闪石以及镁闪石)、滑石、绿泥石和蛇纹石等。根据岩石结构与蚀变矿物比例, 将超镁铁岩分为弱蚀变、中等蚀变和强蚀变3类。蚀变矿物组合与角闪石成分指示超镁铁岩经历了高温蚀变阶段(>700℃), 形成了镁质闪石+滑石+绿泥石; 中温蚀变阶段(700~550℃)和低温蚀变阶段(< 550℃), 分别形成了钙质闪石+滑石+绿泥石+蛇纹石与滑石+碳酸盐+蛇纹石的矿物组合。蚀变岩石普遍以中低温蚀变为主, 可能与中低温阶段的叠加-改造作用相关。岩石随蚀变程度增加, Si、Na、K、Mn等主量元素和Rb、Ba等微量元素呈现明显降低趋势, 表明大多数元素在流体改造过程中从岩石中迁出, 说明蚀变过程为开放体系。中-强蚀变岩石中, 硫化物矿物边部的形态呈锯齿状, 但主体仍为磁黄铁矿-镍黄铁矿-黄铜矿的矿物组合, 指示蚀变过程可能造成了部分Ni、Cu、S元素的溶解与迁移, 但是改造程度相对有限。不同蚀变程度超镁铁岩均富集轻H同位素(δD: -86.6‰~-128.6‰)且O同位素变化较大(δ18O: 1.7‰~10.8‰)。虽然部分样品的H-O同位素接近岩浆水与大气降水的混合趋势线, 但是大多数强蚀变样品则呈现截然不同的趋势, 即δ18O值相似但δD值变化较大, 指示蚀变过程以岩浆水主导且经历了不同程度的去气作用。不同程度的岩浆去气作用可能与岩浆的结晶分异过程密切相关。因此, 本文认为黄山南矿床蚀变岩石为岩浆阶段"自蚀变"作用的产物。不同构造背景铜镍矿床的H-O同位素对比指示, 岩浆"自蚀变"作用在超镁铁侵入岩中普遍发生, 而造山带背景铜镍矿床的蚀变程度可能相对更高。

Alteration process and water origin of Ni-Cu deposits in orogenic belts: Insights from H and O isotopes of the Huangshannan deposit in East Tianshan

Magmatic Ni-Cu sulfide deposits in orogenic belts are characterized by a high proportion of hydrous mineral phases. However, the role of water (fluid) in the minimization process and its modification to the host rock remains unclear. This study investigates alteration processes and water origin of the Huangshannan Ni-Cu deposit in East Tianshan based on mineral assemblages and H-O isotopes of ultramafic rocks. The primary rock-forming minerals in the Huangshannan deposit mainly include spinel, olivine, orthopyroxene and clinopyroxene, with a small amount of interstitial hornblende and mica; the secondary minerals consist of amphibole (edenite, actinolite, tremolite, hornblende, and cummingtonite), talc, chlorite, serpentine and carbonates. In terms of textures and altered mineral assemblages, the Huangshannan rocks are classified into three altered degrees: low, medium, and severe. The secondary mineral assemblage and the amphibole composition reveal that the ultramafic rocks have experienced three stages of alteration: high-temperature (>700℃, forming cummingtonite+talc+chlorite), medium-temperature (700~550℃), and low-temperature (< 550℃). The medium and low-temperature alterations formed calcic amphibolite+talc+chlorite+serpentine and talc+carbonate+serpentine, respectively. Note that the medium-low temperature mineral assemblages are more common, probably due to the overprint and modification at medium-to-low temperatures stages. The contents of major elements (such as Si, Ca, Na, K, Mn) and some fluid-mobile elements (such as Rb and Ba) in whole-rock decrease with the increasing alteration degree, suggesting that these elements were carried out by fluids in an open system during alteration. The edge of sulfide minerals commonly occurs as a needled shape in the medium-to-severe altered rocks, but the sulfide phases, pyrrhotite+pentlandite+chalcopyrite, remain the same. This suggests that the dissolution and migration of chalcophile elements (like Ni, Cu and S) are limited. The ultramafic rocks with different alteration degrees are enriched in lighter H isotopes (δD, -86.6‰~-128.6‰) and have a wide δ18O variation of 1.7‰~10.8‰. Although the H-O isotopes of some rocks plot close to the mixing line between magmatic water and atmospheric precipitation, most samples counter this tendency. Most rocks have similar δ18O values but divergent δD values, suggesting that the fluids were mainly magmatic water that has undergone fluid exsolution (degassing) in varying degrees. The degree of degassing may strongly relate to the degree of magma fractionation. Thus, it is indicated that the alteration of the Huangshannan rocks resulted from "self-alteration" at the magmatic stage. The H-O isotopes comparison of Ni-Cu deposits from diverse tectonic settings suggests that "self-alteration" is widespread in ultramafic rocks; however, the deposits in orogenic belts may have a relatively higher alteration degree.