Geology of the Acropolis prospect, South Australia, constrained by high-precision CA-TIMS ages

Acropolis is an Fe-oxide–copper–gold prospect ∼20 km from Olympic Dam, South Australia, and marked by near-coincident gravity and magnetic anomalies. Prospective Fe-oxide–apatite ± sulfide veins occur in Mesoproterozoic and Paleoproterozoic volcanic and granitoid host units beneath unmineralised sedimentary formations. We have produced a geological map and history of the prospect using data from 16 diamond drill holes, including LA-ICPMS and high-precision CA-TIMS ages. The oldest unit is megacrystic granite of the Donington Suite (<i>ca</i> 1850 Ma). A non-conformity spanning <i>ca</i> 250 My separates the Donington Suite and felsic lavas and ignimbrites of the Gawler Range Volcanics (GRV; 1594.03 ± 0.68 Ma). The GRV were intruded by granite of the Hiltaba Suite (1594.88 ± 0.50 Ma) and felsic dykes (1593.88 ± 0.56 Ma; same age as the Roxby Downs Granite at Olympic Dam). The felsic dykes are weakly altered and lack Fe-oxide–apatite–sulfide veins, suggesting that they post-date the main hydrothermal event. If correct, this relationship implies that the main hydrothermal event at Acropolis was <i>ca</i> 1594 Ma and pre-dated the main hydrothermal event at Olympic Dam. The GRV at Acropolis are the same age as the GRV at Olympic Dam and <i>ca</i> 3–7 My older than the GRV exposed in the Gawler Ranges. The gravity and magnetic anomalies coincide with sections through the GRV, Hiltaba Suite and Donington Suite that contain abundant, wide, Fe-oxide veins. The GRV, Hiltaba Suite and Donington Suite are unconformably overlain by the Mesoproterozoic Pandurra Formation or Neoproterozoic Stuart Shelf sedimentary formations. The Pandurra Formation shows marked lateral variations in thickness related to paleotopography on the underlying units and post-Pandurra Formation pre-Neoproterozoic faults. The Stuart Shelf sedimentary formations have uniform thicknesses.KEY POINTSFe-oxide–apatite ± sulfide veins are hosted by the Gawler Range Volcanics (1594.03 ± 0.68 Ma), the Hiltaba Suite granite (1594.88 ± 0.50 Ma) and Donington Suite granite (<i>ca</i> 1850 Ma).The age of felsic dykes (1593.88 ± 0.56 Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was <i>ca</i> 1594 Ma.The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and <i>ca</i> 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges. Fe-oxide–apatite ± sulfide veins are hosted by the Gawler Range Volcanics (1594.03 ± 0.68 Ma), the Hiltaba Suite granite (1594.88 ± 0.50 Ma) and Donington Suite granite (<i>ca</i> 1850 Ma). The age of felsic dykes (1593.88 ± 0.56 Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was <i>ca</i> 1594 Ma. The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and <i>ca</i> 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges.

阿克罗波利斯（Acropolis）是一处位于南澳大利亚奥林匹克坝（Olympic Dam）约20公里处的铁氧化物-铜-金型矿点，其标志性特征为近乎重合的重力异常与磁异常。潜在的铁氧化物-磷灰石±硫化物脉体赋存于未矿化沉积地层之下的中元古代（Mesoproterozoic）与古元古代（Paleoproterozoic）火山岩及花岗岩类寄主岩体中。研究团队依托16个金刚石钻孔的数据（包括激光剥蚀电感耦合等离子体质谱（LA-ICPMS）与高精度化学稀释热电离质谱（CA-TIMS）年龄数据），编制了该矿点的地质图与地质演化历史。

该矿点最古老的地质单元为唐宁顿套（Donington Suite）的巨晶花岗岩，形成时代约为1850 Ma。一道跨度约2.5亿年的不整合面分隔了唐宁顿套岩体与高勒岭火山岩（Gawler Range Volcanics, GRV）的长英质熔岩及熔结凝灰岩，高勒岭火山岩的形成年龄为1594.03 ± 0.68 Ma。高勒岭火山岩被希尔塔巴套（Hiltaba Suite）花岗岩（1594.88 ± 0.50 Ma）与长英质岩脉（1593.88 ± 0.56 Ma，与奥林匹克坝的罗克斯比唐宁花岗岩（Roxby Downs Granite）时代一致）侵入。

该长英质岩脉蚀变程度较弱且不含铁氧化物-磷灰石-硫化物脉体，表明其形成晚于区域主要热液事件。若该关系成立，则可推断阿克罗波利斯矿点的主要热液事件时代约为1594 Ma，早于奥林匹克坝的主要热液事件。阿克罗波利斯矿点的高勒岭火山岩与奥林匹克坝的高勒岭火山岩时代一致，且比出露于高勒岭（Gawler Ranges）的高勒岭火山岩老约3~7 Ma。

区域重力与磁异常与赋存大量宽幅铁氧化物脉体的高勒岭火山岩、希尔塔巴套花岗岩及唐宁顿套岩体的剖面相重合。高勒岭火山岩、希尔塔巴套花岗岩及唐宁顿套岩体被中元古代潘杜拉组（Pandurra Formation）或新元古代（Neoproterozoic）斯图特陆架（Stuart Shelf）沉积地层不整合覆盖。潘杜拉组的厚度存在显著横向变化，这与下伏岩体的古地形及潘杜拉组形成后、新元古代之前的断层活动相关。斯图特陆架沉积地层的厚度均一。

### 核心要点
铁氧化物-磷灰石±硫化物脉体赋存于高勒岭火山岩（1594.03 ± 0.68 Ma）、希尔塔巴套花岗岩（1594.88 ± 0.50 Ma）及唐宁顿套花岗岩（约1850 Ma）中。被解释为成矿后形成的长英质岩脉（1593.88 ± 0.56 Ma）的年龄数据表明，阿克罗波利斯矿点的主要热液事件时代约为1594 Ma。阿克罗波利斯矿点的高勒岭火山岩与奥林匹克坝的高勒岭火山岩时代一致，且比出露于高勒岭的高勒岭火山岩老约3~7 Ma。铁氧化物-磷灰石±硫化物脉体赋存于高勒岭火山岩（1594.03 ± 0.68 Ma）、希尔塔巴套花岗岩（1594.88 ± 0.50 Ma）及唐宁顿套花岗岩（约1850 Ma）中。被解释为成矿后形成的长英质岩脉（1593.88 ± 0.56 Ma）的年龄数据表明，阿克罗波利斯矿点的主要热液事件时代约为1594 Ma。阿克罗波利斯矿点的高勒岭火山岩与奥林匹克坝的高勒岭火山岩时代一致，且比出露于高勒岭的高勒岭火山岩老约3~7 Ma。