Petrogenesis of the late Tonian arc-related Um Balad gabbro-diorite complex (Egypt) and insight into its spatially related orogenic gold mineralization

The gabbro-diorite complex of the Um Balad prospect hosts lode gold mineralization. The complex is dated at 723 ± 4 Ma using the LA-ICP-MS U-Pb zircon method and is correlated with the late Tonian-early Cryogenian subduction-related magmatic stage during the evolution of the Arabian-Nubian Shield. The gabbro-diorite complex evolved through the crystallization of a calc-alkaline magma and the subduction signature of this magma is verified by primitive mantle-normalized trace element patterns that show enrichment in large ion lithophile elements, U, and Th relative to high field strength elements as well as negative Nb and Ti anomalies. The high La/Yb_cn (3.1–9.4) and Ta/Yb (0.12–0.41) ratios are consistent with a continental arc rather than an oceanic arc system. Amphibole chemistry indicates that this complex might have crystallized under a moderately oxidizing condition from a hydrous magma (>6 wt% water content) at temperature and pressure estimates of about 800°C and 3 kbar, respectively. Gold mineralization in the Um Balad prospect is confined to structurally controlled massive quartz±carbonate veins and surrounding alteration halos. Alteration in the prospect is represented by localized sericitization and carbonation as well as pervasive chlorite-sericite alteration. The alteration halos are characterized by enrichment in K and Rb and depletion in Ca and Sr compared to their host rocks. The veins of the prospect are related to lower order extensional fractures associated with the regional first order transpressional Najd Fault System. The high Fe contents of the gabbro-diorite complex represent a suitable chemical trap for gold through sulfidation of the host rocks. Supergene alteration resulted in the formation of goethite in association with atacamite and chrysocolla. Free mill gold is associated with these supergene phases, which were deposited in near neutral to slightly alkaline conditions.

乌姆巴拉德勘查靶区（Um Balad prospect）的辉长闪长岩杂岩体（gabbro-diorite complex）赋存脉金成矿作用（lode gold mineralization）。该杂岩体采用激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb定年法测得年龄为723 ± 4 Ma，并可与阿拉伯-努比亚地盾（Arabian-Nubian Shield）演化过程中的通纽纪晚期-成冰纪早期俯冲相关岩浆阶段相对比。该辉长闪长岩杂岩体由钙碱性岩浆（calc-alkaline magma）结晶演化而来，其俯冲地球化学标识可通过原始地幔标准化微量元素配分模式（primitive mantle-normalized trace element patterns）得到验证：该模式显示相对于高场强元素（high field strength elements），大离子亲石元素（large ion lithophile elements）、U以及Th发生富集，同时呈现Nb和Ti的负异常。较高的La/Yb_cn（3.1–9.4）与Ta/Yb（0.12–0.41）比值，指示其形成于大陆弧环境而非洋弧体系。角闪石化学成分（amphibole chemistry）分析显示，该杂岩体可能形成于中等氧化条件下的含水岩浆（hydrous magma，水含量＞6 wt%）中，结晶温度与压力分别约为800℃与3 kbar。乌姆巴拉德勘查靶区的金成矿作用严格受构造控制，赋存于受构造调控的块状石英±碳酸盐脉及其周边的蚀变晕（alteration halos）中。该靶区的蚀变作用表现为局部绢云母化（sericitization）与碳酸盐化（carbonation），以及广泛发育的绿泥石-绢云母蚀变（chlorite-sericite alteration）。相较于寄主岩石，蚀变晕具有K和Rb富集、Ca和Sr亏损的特征。该靶区的脉体与区域一级压扭性奈夫断裂系（Najd Fault System）相关的低级序伸展断裂具有成因联系。辉长闪长岩杂岩体的高Fe含量可通过寄主岩石的硫化作用，成为金的适宜化学捕集介质。表生蚀变（supergene alteration）作用形成了与氯铜矿（atacamite）、硅孔雀石（chrysocolla）伴生的针铁矿（goethite）。易选金（free mill gold）与这些表生矿物伴生，其沉积环境为近中性至弱碱性条件。