苏北地区辉绿岩床群岩石成因及原生金刚石成矿潜力

Petrogenesis and Primary Diamond Mineralization Potential of Dolerite Sills in Northern Jiangsu Region

摘要

苏北地区位于华北克拉通东南缘,是潜在的优质原生金刚石找矿靶区.毗邻苏北的栏杆地区辉绿岩中虽已发现大量I_b型原生金刚石,但苏北地区的金刚石勘探工作仍无重大突破.辉绿岩的岩石成因缺乏系统研究,原生金刚石成矿潜力尚未深入探讨,限制了进一步的勘探部署.本研究系统分析了苏北地区两个含金刚石岩体和四个具有勘探潜力岩体的地球化学特征,并与栏杆地区的老寨山辉绿岩进行对比.结果表明:含金刚石和不含金刚石的辉绿岩地球化学特征无显著差异,均为拉斑系列亚碱性玄武岩,形成于大陆板内环境;两类岩体普遍富集大离子亲石元素(如Rb、Ba、La、Eu)和高场强元素(如Nb、U、Nd、Ti);部分含金刚石的辉绿岩可能经历了轻微地壳混染,但混染程度较低;源区性质指向过渡型地幔,稀土和微量元素特征介于OIB(洋岛玄武岩)和E-MORB(富集型洋中脊玄武岩)之间,推测为地幔柱成因,此类辉绿岩的源区深度通常为60~140km,与I_b型金刚石的稳定深度相符,表明苏北地区辉绿岩在理论上具备作为金刚石载体的条件;辉绿岩中寻找原生金刚石矿床仍面临挑战,结合团队先前研究发现,此类I_b型原生金刚石在地幔中存留时间相对较短,处于初生阶段即被带出金刚石稳定区而停止生长,总体经济价值有限.后续勘探重点应转向金伯利岩或钾镁煌斑岩类的目标岩体.

Abstract

The northern Jiangsu region, located on the southeastern margin of the North China craton, is a potential target for primary diamond exploration. Although numerous I_b-type diamonds have been discovered in the dolerite of the Langan area, Anhui, no significant breakthrough has been made in diamond exploration in northern Jiangsu. The petrogenesis of these dolerites and their primary diamond mineralization remain poorly understood, which hampers further exploration. This study systematically analyzed the geochemical characteristics of two diamond-bearing and four potentially diamond-bearing dolerite samples from northern Jiangsu, and compared them with the diamond-bearing dolerite from Laozhaishan in the Langgan area. The results show that there are no significant geochemical differences between diamond-bearing and non-diamond-bearing dolerites. Both are tholeiitic sub-alkaline basalts formed in an intraplate setting, enriched in large-ion lithophile elements (e.g., Rb, Ba, La, Eu) and high-field-strength elements (e.g., Nb, U, Nd, Ti). Some diamond-bearing dolerites may have experienced minor crustal contamination, but its effect is limited. The source characteristics indicate a transitional mantle origin, with rare-earth and trace-element patterns intermediate between OIB and E-MORB, suggesting a plume-related origin. The estimated source depth is typically 60-140 km, consistent with the stability depth of I_b-type diamonds, suggesting that the dolerite in northern Jiangsu could serve as a suitable host rock for diamonds. However, the search for primary diamond deposits in dolerite remains challenging. Based on previous research, I_b-type primary diamonds have a relatively short residence time in the mantle. Once they are transported out of the diamond stability field during their early formation, their growth ceases, resulting in limited economic value. Future exploration should therefore more on locating kimberlites or lamproites.

关键词

苏北地区 / 辉绿岩床群 / 新元古代 / 原生金刚石矿 / 成矿潜力

Key words

northern Jiangsu region / dolerite sills swarms / Neoproterozoic / primary diamond deposit / mineralization potential

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基金资助

江苏省地质局科技创新项目(2023KY07)

江苏省地质局科技创新项目(2021KY09)

江苏省自然科学基金项目(BK20242019)

江苏省地质局地质勘查项目(苏地矿发[2023]77号)

江苏省地质局地质勘查项目(苏地矿发[2022]83号)