The Formation Of Incarnadining Alteration And K-Feldspar Megacrysts:Their Significance For The Mineralization In The Shangzhuang Gold Deposit,Jiaodong Peninsula

The Jiaodong peninsula is the most important gold producing area of China.Gold deposits are related to the Mesozoic magmatic activities.The wall-rock suffered hydrothermal alteration widely,such as beresitization,pyritization,silicification and incarnadining alteration.It is important to study on the gold host rocks and wall rock alteration closely associated with gold mineralization for exploring on the precipitation mechanism of gold.Shangzhuang gold deposit is located in the north of the Zhao-Ye gold-deposit belt of Jiaodong Peninsular.The main ore bodies controlled by the Wang'ershan fault occurrence near the contact zone between Linglong gneissic granite and Guojialing porphyritic granodiorite.Wall-rock alteration includes incarnadining alteration,sericitization and silicification alteration in the order from far and near to the ore body.The intensity of incarnadining alteration plays a critical role in the enrichment of gold mineralization.In this research,we try to understand the mechanism of incamiding alteration by investigating the K-feldspar megacrysts beared in host rocks.The occurrence of incarnadining alteration displays large areal distribution and disseminated distribution along minute fractures in Shangzhuang gold mining area.Through geologic investigation in the field and optical microscope and SEM observation,it is found that the albitization and sericitization of plagioclase limited in the core of the grain and the highly porous albite grains.Submicron size hematites precipited in the interfaces of feldspars.Using SUPCRT92,we calculated equilibrium constants to construct an activity diagram that relates fluid composition to albite,sericite and K-feldspar in equilibrium at 200MPa and evaluated the process of feldspar replacement.The complex associations between the fluid-feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and the hematite precipitation within orthoclase pores at the replacement front results in the red coloration.The Shangzhuang rock contains 30vol%K-feldspar crystals,which are 2×4?3×6cm commonly.There are many mineral inclusions within the megacrysts which weak alignment in local areas,such as quartz and plagioclase.Sawtooth zoning of Ba megacrysts were not seen in the SEM images.The bulk composition of granodiorite bearing K-feldspar megacrysts is similar to those of less megacrysts.Natural and experimental studies of rocks with appropriate composition consistently indicate that K-feldspar is one of the last phases to nucleate and crystallize during the system cooling.The common mineral association of K-feldspar,plagioclase and albite were produced by the exsolution and subsolidus re-equilibration of granitic rocks with hydrothermal fluids.The multiple episodes of reheating kept the temperature of system at?400?,small crystals dissolve in melts to feed growing megacrysts as the system cooling.The Precambrian Jiaodong metamorphic formation and Shangzhuang granite both contain high Au in composition.Repeated pulses of granitic magma emplacement during pluton construction provided the energy to sustain the system temperature at 300?400?,which is the range of activating temperature of Au element.The Au enriched in the fluid would precipitate in the proper region as the system cooling gradually and the changes of pH and the compositions of fluid.