Genesis of the Baiyun pyrophyllite deposit in the central Taihang Mountain, China: Implications for gold mineralization in wall rocks

摘要

Genesis of pyrophyllite alteration can provide important clues for ore prospecting due to its various relationships with metallic mineralization. The Baiyun pyrophyllite deposit in the central Taihang Mountain (CTM) of the North China Craton (NCC) provides a natural case to gain insights into the genesis of pyrophyllite alteration and its relationships with gold mineralization in the wall rocks. The pyrophyllite ores primarily occur as subparallel-sheeted veins in fractures of the late Archean meta-volcanic rocks, and locally as filling fractures within the meta-intrusions. Phyllic alteration halos around the pyrophyllite orebody decline outwards, where gold mineralization develops as disseminated aggregates in honeycomb-like pyrites. Pyrophyllite ores are mostly dominated by 1TC-type pyrophyllite, sericite and quartz with minor kaolinite, illite and albite as identified by XRD analyses. Zircon grains from the granitic porphyry in the ore field define a weight mean U-Pb age of 130 +/- 1 Ma (MSWD = 1.8, n = 19), and those from the sericite quartz schist yield an upper intercepted U-Pb age of 2551 +/- 8 Ma. Zircon grains from the pyrophyllite ore yield an upper intercepted U-Pb age of 2566 +/- 11 Ma and a lower intercepted age of 136 +/- 7 Ma (MSWD = 0.99; n = 20), reflecting a Mesozoic magmatic-hydrothermal event, which could possibly be triggered by the lithospheric destruction of the NCC. LogfO(2)-pH diagram suggests the fluids responsible for pyrophyllite alteration are extremely acidic (pH = 2.20) and under high oxygen fugacity (>Delta HM + 2.14). Calculated H-O isotopic compositions (delta D-H2O = -34 to -20 parts per thousand; delta O-H2O = 3.4 to 7.5 parts per thousand, 270 degrees C; delta O-H2O = 0.2 parts per thousand similar to 4.3 parts per thousand, 200 degrees C) of pyrophyllite ores display typical features of volatile-dominated fluids with minor impact of meteoric water. Metal concentrations of pyrophyllite ores decrease with increasing contents of SiO2, suggesting the fluids generating pyrophyllite alteration are not conductive to gold mineralization. The results herein indicate that the Baiyun pyrophyllite deposit, formed as magma degassing in deep, are similar to the common root zones of advanced argillic lithocaps (barren shoulders) from porphyry environments and probably be the signs for erosion of epithermal deposits at shallow crustal depths.