Indicators of decratonic gold mineralization in the North China Craton

Abstract

The North China Craton (NCC) hosts the world's most typical example of “decratonic” gold deposits including lode, porphyry, cryptoexplosive breccia, and skarn subtypes. The lithospheric structure in the ore fields show “triple-junctions” of basement microblocks, zones of lithospheric thinning, and “hotspots” with abnormal crustal heat flow. The basement rocks are dominantly composed of trondhjemite-tonalite-granodiorite (TTG) suite and amphibolites. The intermediate-felsic calc-alkaline plutons with crust-mantle mixed origin and formed under high oxygen fugacity, volatile-rich and moderately fractionated conditions, together with dyke swarms of diverse compositions are indicators for the gold mineralization. The mantle under the gold field shows metasomatic or juvenile “EM2” or “EM2 + EM1” characteristics. The gold deposits are closely associated with craton destruction, heterogenous reactivation, large-scale magmatic intrusion and eruption, and upwelling of voluminous ore-bearing fluids within a transitional tectonic regime. The presence of mantle-derived mafic rocks, mafic microgranular enclaves (MME) and amygdaloidal bodies, minerals rich in Te, Ni, Cr, Mg, C and N, He and stable isotopes with mantle signature, among other features, indicate that the ore-forming metals and fluids have mixed mantle and deep crustal origin. Gold deposits coexisting or closely associated with mafic dykes and crust-mantle-derived stocks indicate that the upwelling of ore-forming fluid and localization of the ore-materials were facilitated at least partly by the intrusion of deep magma. The spatial distributions of altered rocks with alkali-feldspar-hematite zone distal to the orebody (barren) and pyrite-phyllite (sericite) zone proximal to the ore (ore-bearing zone) indicate a marked physico-chemical interface at the final stage of evolution of the ore-forming fluid. The presence of wide alteration zone indicates that the ore-controlling faults were in a compressive stress state, whereas narrow alteration zone is generally associated with a tensile stress state. The cataclastic, poikilitic, interstitial and zonal textures displayed by the various stages of ore minerals are related to the impact of alternating local tectonic stress fields in the faults, as well as fault-valve mechanism. Intense gold mineralization is typically associated with the development of polymetallic sulfide minerals and complex mineral assemblages. The major gold mineralization is also commonly related with pyrite with xenomorphic or complex forms, rich in As, Se, and Te, with large unit cells, wide X-ray diffraction peaks, and high P/(P + N) values corresponding to thermoelectric conductive type. Quartz grains formed in the main gold mineralization stage are generally fine grained and irregular, occasionally occurring as grain clusters, and showing complex thermoluminescence patterns with both high and low temperature peaks. Multiple rhythmic zones in quartz, pyrite or other ore minerals with various impurities in between the different zones indicate the pulses of ore fluid flow toward the upper part of the orebody. In contrast, minerals with few or no zones may indicate an ore-fluid reservoir typically in the middle to lower parts of orebody.