The Machangqing copper-molybdenum deposits occur in the Shanjian fold belt, Yunnan Province, China. Two types of ores are distinguished: (1) Cu-Mo quartz veinlets in magnetite-series granite porphyry; and (2) Cu-Mo skarns occurring at the contact between the Ordovician sedimentary sequence and the granite porphyry. With decreasing temperature and a(K+)/a(H+) of hydrothermal fluids initially in equilibrium with K-feldspar, the following alteration patterns developed within the porphyry, from the center outwards: silicification, K-silicate, phyllic and argillic alteration. The paragenetic sequence of alteration minerals observed in the Cu-Mo skarns resulted from decreasing temperature and/or increasing a(Ca2+)/a(H+)(2) of the hydrothermal fluids initially in equilibrium with grandite garnet. Fluid inclusions in quartz suggest boiling during the mineralization. The mineralization temperatures based on filling temperatures and salinities of quartz are in the following ranges: about 265 degrees to 400 degrees C and 5.0 to 14.6 wt.% NaCl eq. for the Cu-Mo veinlets; and 200 degrees to 500 degrees C and 10.2 to 42.0 wt.% NaCl eq, for the Cu-Mo skarns. As is evident from log fo(2)-pH diagrams, ores of the early stage of mineralization in the Cu-Mo skarns, characterized by the assemblage magnetite + pyrite + rare pyrrhotite + K-feldspar + quartz, were deposited from highly alkaline and high temperature fluids. With decreasing temperature and fo(2), the pH of the ore fluids was shifted towards slightly alkaline to neutral, with the resultant formation of the main stage ores, characterized by the assemblage chalcopyrite + pyrite +molybdenite + sphalerite +K-feldspar +sericite (muscovite) +epidote +quartz. Very minor amounts of ore minerals, including matildite, bismuthinite and electrum, are associated with a late stage of ore formation.
In the case of the Cu-Mo veinlets, it can be stated roughly that both fs(2) and fo(2) conditions were in the stability held of pyrite, with pH of the ore fluids buffered by the assemblage sericite + K-feldspar +quart +/- calcite. K-Ar age determinations were made on the granite porphyry, biotite phenocrysts and hydrothermal biotite in the Cu-Mo skarns, giving ages of 42.5 to 34.6Ma, 52.3Ma, and 39.2 to 26.4Ma, respectively.
It is concluded that the Cu-Mo mineralization at Machangqing shows a close spatial and temporal association with the Himalayan felsic magmatism of the magnetite-series type.