Brittle origins for disseminated gold mineralization in mylonite: Gaocun gold deposit, Hetai goldfield, Guangdong Province, South China

Gold mineralization in the Hetai goldfield in South China is almost entirel y confined to a series of steeply dipping ductile shear zones generated dur ing Hercynian Indosinian orogenesis of Sinian host rocks. Approximately hal f of the mineralization is in the form of disseminations in the mylonite an d the remainder was clearly emplaced as a result of brittle deformation in veinlets and as sulfide cement to brecciated mesoscopic quartz veins. Heavi ly influenced by the presence of disseminated mineralization, all publicati ons on Hetai determine a major role for ductile deformation in the introduc tion of gold into the deposit. The brittle deformation is typically regarde d as redistributing, within the host structures, the gold initially introdu ced as a result of ductile deformation mechanisms. However, textures of the sulfides associated with;he gold indicate an entirely post ductile deforma tion timing;as they display none of the features attributed to ductile defo rmation. During the evident brittle deformation phase, competence contrasts at the microscopic scale allowed fracturing to focus on the quartz-sericit e domain boundaries of the mylonite foliation, thus permitting the introduc tion of auriferous fluid into the mylonite to create disseminated mineraliz ation. The control exerted by preexisting structures is further reflected i n the restriction of disseminated mineralization to the quartz domains, pre sumably because the sericite domains accommodated the deformation in a more distributed fashion. Several other lints of evidence support an origin of the Hetai deposit in a n upper crustal regime. A similar style of disseminated mineralization, to that seen in the mylonite, surrounds microveinlet terminations in the brecc iated quartz veins, as well as overprinting pseudotachylite, which is assoc iated with cataelasite. There are also large volumes of unmineralized mylon ite indicating no connection between the intensity of strain and gold grade . We conclude that, in structurally controlled deposits, the textural categ ory of disseminated gold miner alization can be generated either under enti rely ductile or entirely brittle conditions, and that microtextural evidenc e has to be carefully analyzed to determine the mode of formation of a depo sit.