Local and Regional Mass Transfer During Thrusting, Veining, and Boudinage in the Genesis of the Giant Shale-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

Abstract

Multiscale structural and geochemical studies have been applied to understand the genesis of the Paracatu deposit in Brazil, a shallow-dipping, bulk-tonnage, vein-style low-grade Au(-Ag-Pb-Zn) orebody hosted in ~1000 Ma black phyllites of the Paracatu Formation, associated with intense shearing accompanying thrusting during the ~680 Ma Brasiliano orogeny. Massive to laminated quartz-sulfide-carbonate veins and associated alteration and Au mineralization formed early in the deformation history, and these veins were boudinaged during subsequent progressive increase in shear strain. Geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100-m scales with respect to Ti, Al, Zr, V, and rare earth elements, including a footwall with protolith attributes similar to those of the ore. Oxygen, hydrogen, and sulfur isotopes do not reveal a distinctive external fluid source; rather, they reflect fluid-rock equilibration with the host phyllites during greenschist facies regional metamorphism. Detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that Si, Ca, and Sr were removed and Al, K, and Ti were residually concentrated during formation of synboudinage dark selvages. This process of mass transfer occurred at scales more local than the processes responsible for ore genesis.Despite the local processes of chemical equilibration and redistribution, many of the main ore components appear externally derived at scales broader than the orebody. Gold, As, Ag, Sb, Bi and, to some extent, Pb and Zn show a strong spatial and statistical correlation with boudinaged quartz-sulfide ± carbonate veins, and these are surrounded by a broader (10-50-m vertical scale) halo of enriched K, Ba, and volatiles (loss on ignition), with depleted Na and Sr. The majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins, and gold-bearing pyrite shows mineral chemistry consistent with a hydrothermal origin. The main population of large veins locally preserve preboudinage, complex internal vein textures, including sulfide-rich wall-rock laminae similar to laminated fault-fill veins in many orogenic-style vein deposits. These larger veins contain most of the gold in the Paracatu resource. Metals were most likely sourced distal to the Paracatu Formation and were precipitated in sheeted shear or tensile veins during the early stages of the Brasiliano thrusting. The oblate strain recorded by the boudinaged, mineralized veins is anomalous and distinctive to Paracatu at km scales, even though the thrusting is regional. Although the orebody may have been physically dislocated by thrusting from the place where the early veins formed, the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or, perhaps, a spatially restricted competency contrast within the stratigraphy, which contributed to the localization of the present mineralization during the thrusting.

Application of Data Analytics Techniques to Establish Geometallurgical Relationships to Bond Work Index at the Paracutu Mine, Minas Gerais, Brazil

“…The deposit is structurally situated on the hanging wall of a thrust fault within a regional East verging fold-thrust belt system [24,25]. The Paracatu orebody is a kilometer-scale structure that is oriented sub-parallel to the stratigraphy, defining a tabular lens-shaped structure striking north-north-west (NNW) and moderately dipping towards the south-west (SW) [26] (Figures 2 and 3). The orebody is 5 km long, 2 km wide and up to 140 m thick.…”

“…The sedimentary protolith represents a relatively homogenous lithology composed of deformed carbonaceous siltstones and shales metamorphosed to lower greenschist facies. Common metamorphic mineral assemblages at Paracatu include sericite, quartz, ilmenite, plagioclase, trace carbonates and sulfides [26]. Weak carbonate alteration is dominated by siderite, dolomite and ankerite assemblages [28,29].…”

“…Bedding is relatively well preserved in low-strain zones, where it is subparallel to the main foliation. However, strong structural transposition in high-strain zones makes it difficult to recognize primary sedimentary features [26]. The mineralized and non-mineralized phyllites are also differentiated by geochemical signature.…”

“…Histograms of the remaining elements were studied to select those having an appropriate granularity and availability of data within the detection range. Finally, these elements were cross-referenced with those established from geochemical studies at Paracatu [26,30]. After pre-processing a 20-element subcomposition of geochemical variables was retained for data analysis ( Table 2).…”

“…Pb is a low field strength element that can be mobilized during weathering processes. The Cr-V-Al cluster represents a group of immobile elements in the phyllite [26,30]. Similarly, the other two clusters can be explained by mineralization and hydrothermal alteration at Paracatu [26].…”