Mesoproterozoic rift sedimentation, fluid events and uranium prospectivity in the Cariewerloo Basin, Gawler Craton, South Australia

The Cariewerloo Basin formed in the Mesoproterozoic following assembly of the Gawler Craton, South Australia, and was filled by arenaceous redbeds of the Pandurra Formation. While previous regional-scale work reveals a basin with similar size and sedimentary fill to the Proterozoic Athabasca and Kombolgie basins that host unconformity-related uranium deposits, few details of the Cariewerloo Basin are known. In this study, stratigraphy, petrography, lithogeochemistry, stable isotope geochemistry and ⁴⁰Ar/³⁹Ar geochronology are integrated to clarify the depositional history of the Pandurra Formation, and to assess fluid events in the basin that could be linked to the formation of uranium deposits. In the study area, the Pandurra Formation was deposited in two eastward-thickening packages that terminate at faulted basement uplifts, interpreted as half-grabens that formed in a continental rift system as the eastern Gawler Craton underwent extension. Deposition occurred between 1575 Ma (latest Hiltaba Suite age) and <i>ca</i> 1490 Ma, the ⁴⁰Ar/³⁹Ar age of diagenetic illite in the basal Pandurra. Diagenesis involving fluids having δ¹⁸O and δ²H values between –2.1 and 3.6‰, and between –66 and –8‰, respectively, occurred at around 150°C. Protracted diagenesis preferentially occurred in the upper Pandurra Formation based on petrography and Pearce Element Ratios that show complete replacement of detrital lithic and feldspathic grains by diagenetic phyllosilicates, and younger ⁴⁰Ar/³⁹Ar ages between <i>ca</i> 1330 and 1200 Ma that record fluid events later into basin history. Conversely, the basal Pandurra Formation shows better preservation of detrital grains, and older ⁴⁰Ar/³⁹Ar ages around 1450 Ma that suggest these strata became closed to fluid flow earlier in basin history. Although, based on O-isotope ratios, fluid–rock interaction did not occur in the Cariewerloo Basin to the same extent as that in the Athabasca or Kombolgie basins, it is possible that a uranium deposit formed where the upper Pandurra Formation was in contact with metasedimentary basement units outside the present basin margins.

卡里沃卢盆地（Cariewerloo Basin）形成于南澳大利亚高勒克拉通（Gawler Craton）拼合后的中元古代，其内充填有潘杜拉组（Pandurra Formation）的砂质红层。既往区域尺度研究显示，该盆地的规模与沉积充填特征与产有不整合相关型铀矿床的中元古代阿萨巴斯卡盆地（Athabasca Basin）及科姆博尔吉盆地（Kombolgie Basin）相似，但目前对卡里沃卢盆地的细节认知仍较为匮乏。
本研究整合地层学、岩石学、岩石地球化学、稳定同位素地球化学及⁴⁰Ar/³⁹Ar年代学方法，旨在阐明潘杜拉组的沉积演化历史，并评估盆地内可能与铀矿床形成相关的流体事件。
研究区范围内，潘杜拉组以两个向东增厚的沉积序列产出，终止于断裂控制的基底隆起带，被解释为大陆裂谷系统中形成的半地堑——彼时高勒克拉通东部正处于伸展构造环境。沉积作用发生于1575 Ma（希尔塔巴套（Hiltaba Suite）的最新年龄）至约1490 Ma之间，该年龄值取自潘杜拉组底部成岩伊利石的⁴⁰Ar/³⁹Ar测年结果。
成岩流体的δ¹⁸O与δ²H值分别介于-2.1‰~3.6‰与-66‰~-8‰之间，成岩作用发生温度约为150℃。持续时间较长的成岩作用优先发育于潘杜拉组上部：基于岩石学观察与皮尔斯元素比值（Pearce Element Ratios）分析，碎屑岩屑与长石颗粒已被成岩层状硅酸盐完全交代，且其⁴⁰Ar/³⁹Ar测年结果为约1330~1200 Ma的较年轻年龄，记录了盆地演化后期的流体事件。反之，潘杜拉组底部保留了更完好的碎屑颗粒，其⁴⁰Ar/³⁹Ar测年结果为约1450 Ma的较老年龄，表明该层位在盆地演化早期即已封闭，不再受流体活动影响。
尽管基于氧同位素比值判断，卡里沃卢盆地内的流体-岩石相互作用程度未及阿萨巴斯卡盆地或科姆博尔吉盆地，但仍存在铀矿床形成的可能性：即潘杜拉组上部与现今盆地边界之外的变沉积基底岩系接触的部位。