云南澜沧老厂火山成因块状铅锌铜硫化物矿床的基本特征及其成因类型

MAIN FEATURES AND GENETICAL TYPE OF THE LAOCHANG VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, YUNNAN PROVINCE

云南澜沧老厂矿床形成于滇西南昌宁-孟连晚古生代裂谷早期,是一个在我国具有重要典型意义的火山成因块状多金属硫化物矿床。本文基于作者的工作,并结合前人的一些研究成果,较系统地总结了老厂矿床的容矿火山岩系、矿床结构、矿体形态、矿石结构构造、矿物组合、蚀变分带等方面的特征,进而通过与火山成因块状硫化物矿床的描述性模式及其主要类型的对比,探讨老厂矿床的典型性。

The Laochang Mine, located in southwestern Yunnan Province of P. R. China, is a volcanogenic massive Pb-Zn-Cu sulfide deposit in the Sanjiang Area. It is hosted by the pyroclastic sequence of Lower Carboniferous age, the lower member of the Yiliu group which was intruded by syenite and rhyolite-porphyry. It has been shown that the volcanic suite, basaltic or basalt-andesitic in composition, has alkaline affinities and was associated with the initial rifting along the margin of the Baoshan-Shanstate(in Burma) continental microplate.The ores are typically Pb>Zn>Cu in composition, with 4.26%Pb, 3.25% Zn on average, from 0.5% to 1.86% Cu, a high content of Ag (119 g/t on average) and recoverable amounts of Au, Tl, Gd, Ga, Ge, In. A massive sulfide zone, in which lenticular or stratiform orebodies are localized in a su-lfide-carbonate-silica shale unit or sulfidic tuffite horizon that usually overlies the coarse-grained pyroclastic rocks, is underlain by a stringer sulfide zone developed in subvolcanic intrusion or volcanic sequence. The sulfides grade from the top to the bottom of the deposit as realgar + orpiment→galena + sphalerite + pyrite→chalcopyrite + pyrite + pyrrhotite + arsenopyrite. Oxides and sulf ates occur scarely in the messive ores. The massive ores are generally fine-grained and banded, but coarse-grained, colloform and fra-mboided texture, and the growth zoning of pyrite or sphalerite are easily observed. The sedimentary features such as graded bedding, brecciated and plastically deformed ore clasts, 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, tre-molite, actinolite, clinozoisite, epidote, garnet.The general features of the Laochang deposit are in accord with the discriptive model of VMS deposits (Lydon, 1984). However, it falls into neither Zn-Cu group nor Zn-Pb-Cu group( Franklin et al, 1981), chemically. The Laochang deposit is considerably distinct from Cyprus-or Besshi-type massive sulfide deposits. Although some features of the Laochang deposit are close to those of Kuroko-type deposits, remarkable differences exist in the mineral assemblage of alteration, the proportion of ore-forming elements, the composition of the volcanic host, the amounts of oxides and sulfates, as well as tectonic setting, perhaps suggesting corresponding differences in ore fluid composition and the associated ore-forming environment. Thus, the authors suggest that the Laochang deposit should be a distinct type of VMS deposits.