The Miwah high–sulphidation epithermal Au–Ag deposit, Aceh, Indonesia: Geology and spatial relationships of gold with associated metals and structures

Abstract

Geological, mineralogical, and whole–rock geochemical studies of the Plio–Pleistocene Miwah high–sulphidation Au–Ag deposit in Aceh, Indonesia, revealed evidence of a complex and dynamic interplay between structural development and mineralisation. The mineralisation occurs in a zone of predominantly phreatic breccias measuring 1.1 km long by 400 m wide and 200–350 m deep within a fault-bounded calc–alkaline andesite ridge. Within this zone, low-grade disseminated Au-Ag has superimposed narrow, high-grade Au-Ag zones. Hydrothermal alteration and mineralisation in three mineralised centres, the Miwah Bluff in the west and the West Block M and East Block M in the east, are controlled by steeply dipping (>60°) and northerly trending pre- to syn-mineral feeders. This alteration displays an outward sequence of vuggy silica I, alunite + silica (advanced argillic), and kaolinite + illite (argillic); a distal chlorite + smectite (propylitic) zone may precede or constitute the end product of the above alteration. Late silica flooding converted parts of the vuggy silica I and the alunite + quartz zone into variably sulphide-rich massive silica and vuggy silica II containing pyrophyllite ± alunite, respectively. Penecontemporaneous with the massive silicification, main-stage mineralisation initially deposited intergrowths of pyrite + enargite ± quartz, followed by scattered enargite and pyrite, and finally, rapidly precipitated Ag-bearing native gold dendrites in vugs, with lesser amounts of sulphides. The crack–seal texture of the host rocks, paragenetic variation in precious- and base-metal mineral species, and selective dissolution, fracturing, and shearing of most of the metallic and gangue minerals are interpreted to reflect transient pulses of fluid flows and mineralisation within zones of structural deformation.High–grade Au zones and associated metals formed clusters along the feeders, whereas low–grade disseminations of the same metals were spread across the deposit. These metals are zoned along the feeders from apical Au + Ag to deep Cu + As; Sb, Te, and Sn are variably distributed. Laterally, the zonation is from Au + Ag in the feeders, through Cu, As, Sb, Bi, Te, and Sn and finally to Pb or Zn in the periphery. While these bulk-rock metal abundances indicate the efficacy of certain physico-chemical conditions of the fluids to accumulate the metals, their ratios point to an upward paleo–flow path of the fluids along the northerly striking faults and related fractures. Combining all of the above data, we propose that Miwah developed in fault-fracture networks beneath paleo–solfataras in a volcanic crater where the ascending acidic magmatic gas or gas + fluids (vapours) transformed the phreatic breccia host rocks first into vuggy silica and advanced argillic rocks, which were subsequently resilicified and mineralised into Au- and base-metal-rich massive silica during the depressurisation and condensation of the gas + fluid phases. Mineralisation processes terminated with limited Ag-enrichment by late-stage fluids in the upper segment of the deposit and in deep fractures.