Zoning of mineralization in hypogene porphyry copper deposits: Insight from comb microfractures within quartz-chalcopyrite veins in the Hongshan porphyry Cu deposit, western Yunnan, SW China

摘要

The origin of zonal mineralization in porphyry copper deposit is important for understanding the mineralization processes. We propose a new, modified "orthomagmatic" genetic model for mineralization zoning in hypogene porphyry copper deposits. This new model is based on the features and formation mechanism of comb microfractures in quartz-chalcopyrite veins within pyrite vein from the Hongshan porphyry copper deposit in Zhongdian County, western Yunnan Province, SW-China. The main evidence for this model is volume expansion related to crystallization of chalcopyrite, magnetite and K-metasoma-tism in the deposit.Comb microfractures are well developed in quartz-chalcopyrite veins and are present as comb-quartz veinlets consisting of a zone of central longitudinal quartz overprinted by laterally grown quartz combs. Chalcopyrite fragments lie perpendicular to the central quartz veinlet and were dismembered by the quartz combs. The combed microfractures are typical tensional hydrofractures. The formation of the comb microfractures is related to volume expansion that was induced by crystallization of chalcopyrite from a chalcopyrite melt that resulted in the subsequent increase of volumetric pressure in the confined residual silica melt. The formation mechanism of the comb microfractures, including volume expansion induced by crystallization, increases volumetric pressure, hydrofracturing and fluid expulsion, and was the most likely process for zoning of minerals in hypogene porphyry copper deposits.Fabrics in the veins and veinlets are consistent with overpressuring and injection and are common structures that are directly related to volumetric pressure and crystallization of chalcopyrite and magnetite and K-metasomatism in hypogene porphyry copper deposits. The volume expansion ratio of chalcopyrite mineral to melt and that of magnetite mineral to melt are approximately 19 vol.% and 20 vol.%, respectively. The volume expansion rate of a monomolecular lattice is ≥8 vol.% for orthoclase replacing plagioclase. We suggest that zoning of mineralization in hypogene porphyry copper deposits is mainly related to hydrofracturing, migration and distribution of ore-forming fluids, including (1) the enclosure and formation of a mush core, (2) concentration of ore-forming fluids in the orthoclase shell, (3) K-metasomatism and pressure building, (4) hydrofracturing and migration of ore-forming fluids, and (5) prior crystallization of magnetite and chalcopyrite that will expel residual fluids upwards and outwards due to the change in volumetric pressure.