Red Clay Type Gold Deposits in China

Red clay type gold deposits,located in the south of China,are situated not only in orogenic belts,but also in inner cratons,where climate is tropical-subtropical with clear arid and humid.The lateritic weathering crust often can be divided into five zones,including topsoil,siliceous duricrust zone,multi-color zone(or red clay zone in some deposits),pallid zone and saprolite zone from surface to the base rock,several of which are absent in some deposits.The base rocks are composed mainly of carbonate rocks with minor clastic rocks,intermediate-basic volcanic rocks and intermediate-acid and alkalic intrusions.The orebodies are mainly located in the multi-color zone with part of them in the pallid and saprolite zones.The ore sources include orebodies of Carlin-type gold deposits and porphyry gold deposits,as well as gold-rich base rocks.The red clay type gold deposits experienced early-stage endogenic gold mineralization and laterization during the Tertiary and Quaternary.The areas with endogenic gold deposits,especially Carlin-type gold deposits and porphyry gold deposits in karst depressions on the plateau,structual erosional platforms in the middle-lower mountains,and intermountain basins in southern China are well worth studying to trace red clay type gold deposits.

Geotechnical Properties of Some Clay Deposits in Some Parts of Southwestern Nigeria in Relation to Its Engineering Implications on Constructions

Twenty-two clay samples from Oduna,Okada,and Etsako clay deposits in Southwestern Nigeria were subjected to Geotechnical analysis to evaluate their impact on engineering,especially on construction.The results revealed that the clays deposits were majorly fine-grained texture(<60%),with Plasticity ranging from 23%to 121%,Liquid Limit(25%~205%)having plasticity of medium to very high with a specific gravity from 1.93 to 2.58.Using the American association of state highway and transportation office(AASHTO)and Unified system classification scheme(USCS),the samples fell within A-7-6(clayey soil)indicating a subgrade that is fair to poor and within the CL-CH category of fine-grained soil with medium to very high plasticity.Compaction having maximum dry density(MDD)to optimum moisture content(OMC)ranges from 0.94 g/cm^(3)~1.68 g/cm^(3)to 11.9%~44.5%,Triaxial result with cohesion between 0.33~35 and shear strength from 44~120 and California bearing ratio for unsoaked bottom 7.52~40,top 4.82~39.18 and soaked bottom 2.89~30.41,top 4.21~33.53.The geotechnical properties of the clay deposits do not meet the standard requirement hence the implication in engineering might be susceptibility to construction failures.

Development and Evolution of the Size of Polygonal Fracture Systems during Fluid-Solid Separation in Clay-Rich Deposits

In continental and oceanic conditions, clay-rich deposits are characterised by the development of polygonal fracture systems（PFS）. PFS can increase the vertical permeability of clay-rich deposits（mean permeability ≤10-16 m2） and are pathways for fluids. On continents, the width of PFS ranges from centimeters to hundreds of meters, while in oceanic contexts they are up to a few kilometers large. These structures are linked to water-solid separation during deposition, consolidation and complete fluid squeeze of the clay horizon. During the last few decades, modeling of melt migration in partially molten plastic rocks led to rigorous quantifications of two-phase flows with a particular emphasis on 2D and 3D induced flow structures. The numerical modeling shows that the melt migrates on distances almost equal to a few times the compaction length L that depends on permeability and viscosity. Consequently, polygonal structures in partially molten plastic rocks are resulted from the melt-rock separation and their sizes are proportional to L. Applying these results to fluid-solid separation in clay-rich horizons, we show that（1） centimetric to kilometric PFS are resulted from the dramatic increase of L during compaction and（2）, this process involves agglomerates with 100 μm to 1 mm size.