La'erma Au-Se deposit, Luqu Co., Gannan, Gansu, Chinai

Regional Level Types
La'erma Au-Se deposit	Deposit
Luqu Co.	County
Gannan	Prefecture
Gansu	Province
China	Country

Carlin-like gold deposit, hosted in Cambrian-Ordovician carbonaceous and siliceous slates, and related to Mesozoic diorite dykes. The deposit also contains minor uranium ores.

Gold mineralization at the La’erma and Qiongmo Au–Se deposits are hosted by the siliceous formation of the Cambrian Taiyangding Group, which consists of the Yaxiang, Xige, and La’erma Formations, which have distinct stratigraphic, lithologic, and structural characteristics. The up to 1000 m-thick siliceous formation, which was formed by submarine exhalative sedimentation, consists mainly of a series of carbonaceous cherts and slates with minor carbonate lens. The siliceous formation is characterized by the enrichment of organic carbon. There are no significant igneous rocks outcrops, but there are some quartz diorite, granodiorite, and dacite porphyry dikes, which are several hundred meters in length, and 1 to 2 m-thick, that intruded the Lower Cambrian strata along faults.
The La’erma and Qiongmo Au–Se deposits consist of over 100 ore bodies, most of which are located in the La’erma deposit to the west. The discovered orebodies are stratabound and lenticular in shape, and are distributed along faults and fracture zones that are E−W-trending and mostly NW-dipping with dip angles of 60°–80°. The orebodies also follow the bedding planes, lithological contacts and anticlinal fold hinges. The west-central part of the La’erma deposit (lines 91–121) is the main mineralized section, which is approximately 1250 m long and 200–320 m wide. The lengths of the orebodies in the La’erma deposit range from 40 m to 450 m and are 1.0–38.05 m in width with vertical extents of 80–200 m. The Au grade of these orebodies ranges from 1.00 to 25.00 g/t with the highest grade being 66 g/t. Orebodies in Qiongmo deposit are 60–330 m in length, with a thickness of 1.0–8.49 m with vertical extents of 40–430 m, carrying 1.0–6.14 g/t Au. The orebodies are large and the mineralization is concentrated where the fracture converges to the west, and they are small where the fractures spread out to the east.
Three types of ores have been identified based on their host rocks: chert type, slate type, and dacite porphyry type. The chert type, which can be divided into layered chert ore and silicified ore, is the best mineralized ore, containing 75%–80% of the total ore with gold grades between 1.0 and 7.0 g/t. However, the slate type represents the most exposed orebodies with relatively low gold grades of 2.0–5.0 g/t. Deposits exhibit a sequence of brecciated, stockwork, veined, and disseminated structures, replacing each other from the mineralization center towards the outer areas.
Stage I comprises pyrite and quartz with minor stibnite. Stage II is represented by the association of pyrite–marcasite–quartz––native gold. In stage Ⅱ, the native gold commonly coexists with stibnite. There are a large of sulfides, including chalcopyrite, marcasite, bornite, pyrrhotite, polydymite, sphalerite, and galena. Stage III mainly includes stibnite–selenides–native gold–quartz–baryte. This stage is important as it involves the formation of selenides, such as tiemannite, clausthalite, antimonselite, kullerudite, and an unnamed Ni3As3S3Se mineral phase. In this stage, the native gold occurring in droplets or anhedral coexists with stibnite and tiemannite. Selenides, such as tiemannite, antimonselite, clausthalite, and kullerudite, often coexist with sulfides like stibnite and famatinite in baryte and quartz. In the late stage Ⅲ, subhedral–anhedral cinnabar, realgar, and orpiment occur in a baryte–quartz vein. Stage IV is dominated by quartz–baryte–dickite. Stages II and III are the main metallogenic stages.

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References