Diamonds from the Santo Antônio River (Delfinópolis, Minas Gerais): probable relationship with the Canastra-3 Kimberlite

Abstract The study identifies the Canastra-3 Kimberlite magnetic anomaly as the likely primary source of the alluvial diamonds recovered by "garimpeiros" in the Santo Antônio River basin (Delfinópolis, southwestern Minas Gerais). This conclusion is based on cumulative geophysical, hydrographic, metallogenical and mineral geochemistry evidences. The study area is located within fertile ground in the border of the São Francisco craton, close to other diamond primary sources and secondary deposits. This kimberlitic target is the only known in the Santo Antônio River basin. In addition, the known mineralized gravels of this river, worked in the past by "garimpeiros", have evidence of a short transport (angular pebbles and blocks), further evidence of a nearby source. The original data collected in the "Minas Gerais Aerogeophysical Survey Program" was processed and analyzed with the Euler Deconvolution method, implemented in software Oasis Montaj. With the exception of the Canastra-3 body anomaly, all others in the study were classified as non-kimberlitic. Recent sampling work on the weathered top of the Canastra-3 Kimberlite recovered indicator minerals, notably a high proportion of pyrope garnets of the G-10 type, which is unusual among the kimberlites of the region.摘要 本研究认定，卡纳斯特拉-3号（Canastra-3）金伯利岩（Kimberlite）磁异常（magnetic anomaly）乃是巴西米纳斯吉拉斯州西南部德尔菲诺波利斯境内圣安东尼奥河流域内，由嘎里姆佩罗斯采矿者（garimpeiros）采获的冲积钻石的疑似主要原生矿源。该结论基于累积的地球物理、水文、成矿学与矿物地球化学证据。研究区域位于圣弗朗西斯科克拉通（São Francisco craton）边界内的成矿有利地带，毗邻其他钻石原生矿床与次生矿床。该金伯利岩靶区为圣安东尼奥河流域内目前已知的唯一此类靶区。此外，该河流域过往由garimpeiros开采的已知矿化砾石，显示出搬运距离较短的特征（砾石与岩块均呈棱角状），进一步佐证了附近存在原生矿源。本研究对"米纳斯吉拉斯州航空地球物理调查计划"所采集的原始数据进行了处理与分析，采用了在Oasis Montaj软件中实现的欧拉反褶积法（Euler Deconvolution）。除卡纳斯特拉-3号岩体异常外，研究区内其余所有异常均被归类为非金伯利岩异常。近期针对卡纳斯特拉-3号金伯利岩风化表层的采样工作采获了指示矿物，其中G-10型镁铝榴石（pyrope garnets）占比极高——这在该区域的金伯利岩中实属罕见。