Characteristics of the Laochang Volcanogenic Massive Sulfide Deposit, Southwestern Yunnan, China

Abstract 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.
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Summary: Regional and orientation geochemical stream sediment surveys were carried out by the Quebec Department of Energy and Resources in southwestern Quebec, over a part of the Mont-Laurier Terrane of the Grenville Province. The area contains some small stratiform zinc deposits. The response of Zn mineralization to geochemical surveys is weak; the known mineralization is not defined by anomalous samples and consistent patterns. This study uses a technique that increases the geochemical signature of...
Publication: Exploration & Mining Geology
Author(s): CLAUDE BELLEHUMEUR, MICHEL JEBRAK
Issue: 1
Volume: 2
Year: 1993
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Summary: The Corbet deposit is one of 17 proximal Cu-Zn massive sulfide deposits located within the Noranda Cauldron which occurs within the core of the Noranda Volcanic Complex. The deposit is located at the top of the Flavrian Formation, the lowermost formation of the 3000 m thick Mine Sequence; the deposit has yielded 2.75 M tonnes of ore grading 3% Cu, 0.96% Zn, 2.06 g/t Ag and 1.0 g/t Au. The multi-lens Corbet deposit formed within a 250 m wide crater that occurs within a localized breccia pile....
Publication: Exploration & Mining Geology
Author(s): H.L. GIBSON, D.H. WATKINSON
Issue: 1
Volume: 2
Year: 1993
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Summary: Electromagnetic surveys were conducted over the Yava sandstone-hosted lead deposit in Nova Scotia to evaluate the usefulness of such surveys for detecting and mapping sedimentary-hosted metallic ore deposits. Both transient and multifrequency systems were used for that purpose. The deposit consists of three zones of disseminated galena which also contain minor amounts of pyrite and sphalerite filling the interstices of the sandstone grains. Each zone overlies a basement depression and is...
Publication: Exploration & Mining Geology
Author(s): AJIT K. SINHA
Issue: 1
Volume: 2
Year: 1993
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Summary: The process of arriving at the size and grade of: (a) a newly-discovered body of mineralization, (b) a potential ore resource, (c) an orebody proposed for mining, or (d) the ore reserves in a mine, requires fundamental geological, and geometrical determinations as well as physical and chemical analyses. This paper outlines geological and geometrical features considered critical, in the authors' experience, in realistic reserve determination. These are discussed under the headings "The Four...
Publication: Exploration & Mining Geology
Author(s): OWEN OWENS, W.P. ARMSTRONG
Issue: 1
Volume: 2
Year: 1993
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Summary: The Jasper Gold mine in northern Saskatchewan produced a total mill feed of 140 127 tonnes at an average grade of 19.0 g/t Au. Both diamond drilling and chip samples from underground workings returned extremely high values, up to 12 851 g/t (375 oz/T) Au over 0.10 meter from a chip sample. In the presence of such extremely high grades, the accurate estimation of reserves is a major problem.
In the first part of this study, production data from the mine were used to evaluate the results of a...
Publication: Exploration & Mining Geology
Author(s): CHRIS M. HEALEY
Issue: 1
Volume: 2
Year: 1993
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Summary: Yidun Island Arc in Sanjiang Region is one of the important Kuroko-type volcano-genie massive sulfide deposits (VMS) districts in southwestern China. Tectonically, the region is situated in the eastern portion of the Tethys orogenic belt, and is a late Triassic extensional island-arc. The island-arc underwent earlier arc construction, intra-arc rifting, later arc construction and back-arc spreading during its evolution and formed a complete trench-arc-basin system, i.e., the Ganzi-Litang...
Publication: Exploration & Mining Geology
Author(s): ZENGQIAN HOU
Issue: 1
Volume: 2
Year: 1993
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Summary: The East Bull Lake gabbro-anorthosite intrusion is one of several Early Proterozoic layered mafic intrusions occurring along the Southern Province - Superior Province boundary in the Elliot Lake - Sudbury region, Ontario. The base of the East Bull Lake intrusion comprises abundant plagioclase cumulates (Leucogabbronorite Zone) hosting two distinct styles of sulfide mineralization: (1) disseminated magmatic sulfides, and (2) disseminated to massive, structurally controlled sulfides. Both types...
Publication: Exploration & Mining Geology
Author(s): D.C. PECK, R.S. JAMES, P.T. CHUBB
Issue: 1
Volume: 2
Year: 1993
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Summary: The prospective Tulks volcanic belt hosts five significant massive sulfide deposits, Tulks Prospect, Tulks East, Jack's Pond, Daniel's Pond and Victoria Mine, as well as numerous showings and indications throughout its 70 km outcrop length. Regionally, the 498 Ma sequence consists dominantly of felsic pyroclastic rocks and flows with interbedded mafic volcanic units and finegrained sediments, commonly graphitic. Contemporaneous granitoid rocks occur in the south half of the belt. Rocks...
Publication: Exploration & Mining Geology
Author(s): C.B. McKENZIE, D.W. DESNOYERS, D. BARBOUR
Issue: 1
Volume: 2
Year: 1993
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