Summary:
The Jiama Project is located within the Gangdese Yanshanian epi-continental arc on the central southern portion of the Gangdese-Nianqing Tanggula Terrain. At the tenement scale, rock types are typically passive epi-continental clastics and carbonates. The stratigraphy is dominated by marbles and limestones of the Upper Jurassic Duodigou Formation and sandstones, slates and hornfels belonging to the Lower Cretaceous Linbuzong Formation. Although some mafic and intermediate to felsic dykes have been mapped in outcrop and drill core, the intrusive granitic body responsible for the wide spread contact metamorphism and copper-polymetallic mineralisation is yet to be identified. A number of thrust faults and shears concentrated between major geological contacts and a large detachment fault (Gliding Nappe Fault) have also been observed. Locally bedrock units are overlain by unconsolidated Quaternary colluvium and alluvium.
The Jiama deposit is a structurally controlled stratiform skarn-type copper-polymetallic system. The majority of high grade mineralisation is associated with shear zone contacts between the Duodigou and Linbuzong formations and shear related folding. The zone of mineralisation within fault hosted skarn alteration measures kilometres in both strike and dip and remains open at depth to the northeast.
Mineralisation is also associated with granite porphyry dykes intruding the ‘Duodigou Marble’ and observed within the overlying hornfels of the Linbuzong Formation.
Although both deposit types are of lower grade than that of the skarn, the hornfels style of mineralisation may potentially be of further economic value in the future.
Three types of copper-polymetallic mineralisation are recognised in the project area, these include skarn, hornfels and porphyry hosted deposits.
High grade copper-polymetallic mineralisation is associated with skarn type alteration within shear zone contacts between the Duodigou and Linbuzong formations. This stratiform fault zone is tabular to lenticular in shape, has a variable dip, strikes west-northwest and hosts the primary lode.
The mineralised body itself has a 60° near surface dip angle which flattens to an average dip of 10° with depth. Thicknesses vary between 2 m to 240 m, averaging 33 m, its strike length is approximately 2,400 m and its length down dip ranges between 150 to 1,900 m. It is primarily divided into two zones of mineralisation; the Niumatang Zone and the Jiama Zone. A number of smaller, lenticular shaped skarn-type mineralised bodies have also been identified below and to the southeast of the primary body, however they are generally discontinuous and of limited strike (<200 m) and thickness.
Copper mineralisation shares associations with chalcopyrite, bornite and chalcocite and is hosted by either thin veinlets, as disseminated sulphide crystals or as massive sulphide zones.
Hornfels hosted mineralisation is of lower grade than that of the skarn type deposits and occurs predominantly as disseminated sulphides. Mineralisation is fine grained and consists of copper in association with chalcopyrite, bornite and molybdenum. No massive sulphide zones or veining has been recognised within the texturally massive and highly fractures rock mass. Earlier observations of copper associated with pyrite and pyrrhotite coating fracture planes has been attributed to secondary enrichment.
The mineralised body shows no preferential orientation, it is shallow, tabular shaped and approximately 1,200 m in length, 1,000 m wide and generally 10 to 50 m thick, although a maximum thickness of 826 m was intercepted in drillhole ZK3216. It pinches out in the westsouthwest and in general is thinner towards both the west-northwest and east-southeast.
Mineralisation in porphyritic granodiorite and monzogranite is characterised by molybdenum with lesser amounts of chalcopyrite and bornite. Sulphides are medium to coarse grained and confined to a thin horizontal pipe shaped body with a maximum known thickness of 476 m. The porphyritic host lies beneath the Duodigou Formation and appears intruded into the sheared contact between the two basement units.