Chemical Composition and Genesis Implication of Garnet from the Laoshankou Fe-Cu-Au Deposit, the Northern Margin of East Junggar, NW China
Chemical Composition and Genesis Implication of Garnet from the Laoshankou Fe-Cu-Au Deposit, the Northern Margin of East Junggar, NW China
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DOI：
10.3390/min11030334
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发表时间：
2021-03
期刊：
影响因子：
2.5
通讯作者：
Pei Liang;Yu Zhang;Yuling Xie
中科院分区：
文献类型：
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作者：
Pei Liang;Yu Zhang;Yuling Xie
为了揭示东准噶尔北缘老山口铁铜金矿床不同石榴石的形成机制及其对流体演化的意义，本研究对三种类型的石榴石进行了详细研究。 （1）1型钙硅酸盐岩，形成于钙硅酸盐阶段（I阶段，预矿化阶段），被2型石榴石和磁铁矿取代，成分范围为Grs44~53Adr44~53，稀土元素总含量相对较低（8.14~32.8 ppm），LREE（轻稀土元素）明显贫化，具有明显的正Eu异常（1.36–9.61）。 (2)2型富铝钙铁矿，形成于角闪石-绿帘石-磁铁矿阶段的早期亚阶段(II期，磁铁矿主成矿阶段)，可分为2a型和2b型2个亚型。 2a 型石榴石表现出多合成孪晶和相对较窄的 Adr63–66Grs31–34 成分变化，并具有 HREE（重稀土元素）富集和正 Eu 异常 (3.22–3.69)。 2b 型石榴石具有 Adr55–77Grs21–43 的广泛成分变化，稀土元素含量相对较高 (49.1–124 ppm)，轻稀土元素明显贫化，且具有明显的正 Eu 异常 (2.11–4.61)。 (3) 与硫化物期(Ⅲ期，主要铜金矿化期)相关的3型钙铁榴石(Adr>91)与老山口其他类型石榴石不同，具有稀土总含量最低(1.66~91.1 ppm)、重稀土分布平坦、轻稀土富集、Eu正异常最强(3.31~45.48)的特点。稀土元素融入石榴石中很大程度上受外部因素控制，例如流体化学、pH、fO2 和水岩比及其晶体化学。 1型和2型石榴石主要遵循X2+（例如Ca2+）位点空位的产生，例如[X2+]−3VIII[]+1VIII[REE3+]+2VIII。 3型石榴石的REE3+取代机制是Na+-REE3+耦合取代，例如[X2+]−2VIII[X+]+1VIII[REE3+]+1VIII，没有评估位点空位的产生。 1型到3型石榴石的成分变化表明流体成分和物理化学条件存在显着差异，可用于追踪石榴石的流体-岩石相互作用和热液演化。 1型钙铝榴石是由岩浆流体在低水岩比、低O2和中性pH环境下，在近封闭系统中通过扩散交代作用形成的，优先掺入重稀土钙铝榴石中。由于流体在接近封闭系统条件下的长孔隙停留和持续渗透交代作用，高水岩比的流体具有增加的fO2、更积极地掺入Fe3+和REE的特征，并形成2型富铝钙铁矿。相比之下，3型石榴石是在具有最高fO2和水岩比的弱酸性环境下氧化岩浆流体形成的，并且在开放系统中受到外源高盐度和富钙流体的影响。因此，老山口矿床中不同类型、不同年代石榴石的地球化学特征可以提供流体演化的重要信息，揭示从钙硅酸盐阶段到硫化物阶段从中性岩浆流体到氧化岩浆流体的转变，并加入外部非岩浆富钙流体。上述证明的流体演化过程进一步表明，老山口矿床更倾向于是IOCG类（氧化铁-铜-金）矿床，而不是典型的矽卡岩矿床。
In order to reveal the formation mechanism of different garnets and its implications for the fluid evolution in the Laoshankou Fe-Cu-Au deposit in the northern margin of East Junggar (NW China), three types of garnet have been investigated in detail in this study. (1) Type 1 grossular, formed at Ca-silicate stage (stage I, the pre-mineralization stage), was replaced by Type 2 garnet and magnetite, and displays a compositional range of Grs44–53Adr44–53, which has relatively lower total REE (rare earth elements) contents (8.14–32.8 ppm) and markedly depleted LREE (light rare earth elements) with distinctive positive Eu anomaly (1.36–9.61). (2) Type 2 Al-rich andradite, formed at the early sub-stage of amphibole-epidote-magnetite stage (stage II, the main magnetite mineralization stage), can be divided into two sub-types, i.e., Type 2a and Type 2b. Type 2a garnets exhibit polysynthetic twinning and relatively narrow compositional variations of Adr63–66Grs31–34 with HREE-(heavy rare-earth elements) enrichment and positive Eu anomalies (3.22–3.69). Type 2b garnets own wide compositional variations of Adr55–77Grs21–43 with relatively higher REE contents (49.1–124 ppm), markedly depleted LREE and a distinctive positive Eu anomaly (2.11–4.61). (3) Type 3 andradite (Adr>91) associated with sulfide stage (stage III, the main copper-gold mineralization stage) is different from other types of garnets in Laoshankou, which are characterized by lowest total REE contents (1.66–91.1 ppm), flat HREE patterns, LREE-enrichment and the strongest positive Eu anomalies (3.31–45.48). Incorporation of REE into garnet is largely controlled by external factors, such as fluid chemistry, pH, ƒO2 and water-rock ratios as well as its crystal chemistry. Type 1 and 2 garnets mainly follow the creation of X2+ (e.g., Ca2+) site vacancy, e.g., [X2+]−3VIII[]+1VIII[REE3+]+2VIII. The REE3+ substitution mechanism for Type 3 garnet is the Na+-REE3+ coupled substitutions, e.g., [X2+]−2VIII[X+]+1VIII[REE3+]+1VIII, without the evaluation of the creation of site vacancy. The compositional variations from Type 1 to Type 3 garnet indicate significant differences of fluid compositions and physicochemical conditions, and can be used to trace the fluid–rock interaction and hydrothermal evolution of garnet. Type 1 grossular was formed by magmatic fluid under low water–rock ratios and ƒO2, and neutral pH environment by diffusion metasomatism in a nearly closed system with the preferential incorporation into the grossular of HREE. As the long fluid pore residence and continuing infiltration metasomatism under nearly closed-system conditions, fluids with high water/rock ratios were characterized by increased ƒO2, more active incorporation of Fe3+ and REE, and formed Type 2 Al-rich andradite. In contrast, Type 3 garnet formed by oxidizing magmatic fluid under a mildly acidic environment with highest ƒO2 and water–rock ratios, and was influenced by externally derived high salinity and Ca-rich fluids in an open system. Thus, the geochemical features of different types and generations of garnets in the Laoshankou deposit can provide important information of fluid evolution, revealing a transition from neutral magmatic fluid to oxidizing magmatic fluid with addition of external non-magmatic Ca-rich fluid from the Ca-silicate stage to the sulfide stage. The above proved the fluid evolution process further indicates that the Laoshankou deposit prefers to be an IOCG-like (iron oxide-copper-gold) deposit rather than a typical skarn deposit.