KAIHUI YANG and XUANXUE MO China University of Geosciences, Beijing, China
The Laochang mine, located in the Sanjiang area of southwestern Yunnan Province of China, is a volcanogenic massive Pb-Zn-Cu sulfide deposit. It is hosted by a pyroclastic sequence of Early Carboniferous age, namely the lower member of the Yiliu Group, which was intruded by syenite and rhyolite porphyry. The volcanic suite is basaltic or basaltic-andesitic in composition, has alkaline affinities, and is associated with the initial rifting along the margin of the Baoshan-Shanstate continental microplate (Yang, 1989). The ores are typically Pb>Zn>Cu in composition, averaging 3.82% Pb, 3.51% Zn, from 0.01% to 1.85% Cu (0.107% av.), a high average content of Ag (113 g/t) and recoverable amounts of Au, Tl, Gd, Ga, Ge, In. A massive sulfide zone, in which lenticular and stratiform orebodies are localized in a sulfide-carbonate-silica shale unit or sulfidic tuffite horizon that usually overlies coarse-grained pyroclastic rocks, is underlain by a stringer sulfide zone developed in a subvolcanic intrusion or volcanic sequence. The sulfide mineral assemblages, from the top to the bottom of the deposit, are: realgar + orpiment -~ galena + sphalerite + pyrite — chalcopyrite + pyrite + pyrrhotite + arsenopyrite. Oxide and sulfate minerals occur rarely in the massive ores. The massive ores are generally fine-grained and banded, but coarse-grained ore, colloform and framboidal texture, and growth-zoned pyrite and sphalerite are common. Sedimentary features such as graded bedding, brecciated and deformed ore clasts, and chaotically emplaced blocks, are well preserved, indicating that the massive orebodies have undergone slumping and displacement. Alteration is weak in the hanging-wall rocks, but strong in the stringer zone. Two distinctive alteration zones have been identified: the upper zone, characterized by an assemblage of quartz, carbonates, chlorite, albite, zeolite and sericite, and the lower zone containing diopside, tremo-lite, actinolite, garnet, clinozoisite, and epidote.
The general features of the Laochang deposit are in accord with the descriptive model of VMS deposits (Lydon, 1984). However, it falls into neither the Zn-Cu group nor the Zn-Pb-Cu group chemically (Franklin et al., 1981). The depositional setting of the Laochang deposit is quite different from Cyprus or Besshi-type massive sulfide deposits. Laochang is hosted by alkaline lavas that were erupted in a submarine continental rift. Although some features of the Laochang deposit are similar to those of Kuroko-type deposits, there are remarkable differences in the alteration mineral assemblage, the proportion of ore-forming elements (lead enriched) and the amounts of oxides and sul-fates. These suggest corresponding differences in ore fluid composition and the associated ore-forming environment. Thus, the authors suggest that the Laochang deposit may be a distinct type of VMS deposit.