Abstract:

In the 1990s，the Boka gold deposit was first found in paleo-mesoproterozoic strata in Dongchuan district，Yunnan Province.The genesis of the deposit is still controversial.In order to determine the characteristics of its ore genesis，this study examine the genetic mineralogical characteristics of pyrites of different generations from the Boka gold deposit using optical microscope（OM），powder and in-situ X-ray diffraction（XRD），electron microprobe analyses（EPMA）.The results show that pyrites from the deposit can be divided into three generations.Py1 has undeveloped zoning structure and the smallest lattice constant of the three.It has enrichment in S and depletion in Fe，low As，Co，Ni.It formed in a magmatic hydrothermal environment.In Py2 repair growth and zoning structure are developed very well and the lattice constant is the largest.The enrichment in S is lower than in Py1，it has depletion of Fe，high As，Co，Ni.Its deposition was related to mixed magmatic and underground thermal brines.Py3 is crushed and granulated，with minor euhedral crystals formed by dynamic metamorphic recrystallization.It has enrichment of S and depletion in Fe.The degree of S enrichment is significantly lower than Py1 or Py2.The lattice constant and As，Co，Ni contents are between Py1 and Py2.These results indicate the deposit originally formed in an environments similar to Py2 in the Indosinian-early Yanshanian and subsequently underwent a deformation-metamorphic stage in Himalayanian.Among them，the single magmatic hydrothermal ore-forming fluid in magmatic hydrothermal stage has been evolved the double-sources of those fluids in the mixture stage.This study shows that the Boka deposit is a magmatic hydrothermal type gold deposit which was transformed by dynamic shear deformation and metamorphism that resulted in further enrichment of the orebodies.

Key words: genetic mineralogy, pyrite, gold deposit, Boka, Yunnan Province