Mineralium Deposita ( IF 4.4 ) Pub Date : 2023-10-25 , DOI: 10.1007/s00126-023-01220-9 Enzio Garayp , Hartwig E. Frimmel
Evidence is presented that the metaconglomerate-hosted Jacobina gold deposits in Brazil represent paleoplacers that became partly remobilized during later metamorphic overprint analogous to Witwatersrand-type deposits elsewhere, notably in South Africa. This includes strong lithological and sedimentological control on the gold, presence of detrital minerals with gold inclusions, and detrital gold particles. Detrital, synsedimentary, and post-depositional pyrite types can be differentiated. Whereas the first two types can be linked to gold accumulation, the latter was associated with gold dispersion. Synsedimentary pyrite has the highest Au content, from which elevated Au concentrations in Archean rivers can be inferred. The nature and extent of post-depositional alteration, mainly in the course of the Paleoproterozoic Orogeny, distinguishes the Jacobina deposits from other Witwatersrand-type gold deposits. Phase equilibria and Zr-in-rutile thermometry indicate peak metamorphic temperatures of ca. 600 °C. Both Mg-chlorite and Fe-chlorite formed in disequilibrium at approximately 280–340 °C during retrograde metamorphism. An igneous signature in the chemistry of some of the tourmaline and the remobilization of gold associated with Fe-oxides, near intrusive rocks, point at a local magmatic influence on the post-depositional mineralization stage. Whether magmatic hydrothermal fluids added Au to the system at that stage remains to be determined. Remobilization during regional metamorphism was insufficient to form substantial ore bodies but led to purification of the initially detrital gold particles that now contain relatively little Ag and Cu and lack Hg.
中文翻译:
巴西 Jacobina 超砾岩金矿床的修正古砂矿模型
有证据表明,巴西的超砾岩中的 Jacobina 金矿床代表了古砂矿,这些矿床在后来的变质覆盖过程中被部分重新激活,类似于其他地方(尤其是南非)的 Witwatersrand 型矿床。这包括对金的强烈岩性和沉积学控制、含有金包裹体的碎屑矿物和碎屑金颗粒的存在。可以区分碎屑、同沉积和沉积后黄铁矿类型。前两种类型可能与黄金积累有关,而后者则与黄金分散有关。同沉积黄铁矿的金含量最高,由此可以推断太古宙河流中的金浓度较高。沉积后蚀变的性质和程度(主要是在古元古代造山运动过程中)将 Jacobina 矿床与其他 Witwatersrand 型金矿床区分开来。相平衡和金红石中 Zr 测温表明峰值变质温度约为 100 ℃。 600℃。镁亚氯酸盐和铁亚氯酸盐均在逆变质过程中于大约 280-340 °C 的不平衡状态下形成。一些电气石的化学特征以及与侵入岩附近的铁氧化物相关的金的再活化的火成岩特征,表明局部岩浆对沉积后矿化阶段的影响。岩浆热液是否在该阶段将金添加到系统中仍有待确定。区域变质作用期间的再活化不足以形成大量矿体,但导致了最初碎屑金颗粒的纯化,这些颗粒现在含有相对较少的银和铜,并且缺乏汞。