Growth Pattern and Its Indication of Spheroidal Nano-Micro Crystal Aggregates of Pyrite in the Baiyunpu Pb-Zn Polymetallic Deposit, Central Hunan

The Baiyunpu deposit lies in the southwest plunging Dachengshan anticline in central Hunan, which is a large Pb-Zn polymetallic deposit. The orebodies were surrounded by the Qiziqiao Formation limestone in the Middle Devonian, and its geological occurrence is consistent with the wall rocks. A large number of spheroidal pyrite aggregates are found unevenly distributed in the ores. The spheroidal aggregates are made up of kernels and concentric rings. The kernels are composed of approximately epigranular pyrite nanocrystals, while the rings are composed of accumulated pyrite microcrystals growing along the radial direction. The spheroidal pyrite aggregate and its outer zones can be divided into five areas（A–E）. The results of electron probe micro analysis（EPMA） show that from the zone A1 to B, Co/Ni 〈1, the sum of Co and Ni is 0.08%–0.26%, S/Fe increases from 2.06 to 2.15. While from the zone C to E, Ni cannot be detected and S/Fe decreases from 2.22 to 2.08. Powder X-ray diffraction（XRD） analysis in the micro zone shows obvious crystalline characteristics in the aggregates. Moving from the inside outwards, the maximum diffraction peak intensity of the（111） and（220） crystal planes of pyrite increases, and the crystallinity improves. The degree of change in the（111） plane is the most prominent. Considering the theory of crystal growth along with the geologic features of the depositional environment where the spheroidal pyrite aggregates developed, we confirm that the spheroidal aggregates are the result of nano-micro crystalline gathering and growth occurring by the following sequence of processes：nano-crystalline nucleation and growth, gathering into a ball, oriented growth of microcrystals, continuous accumulation, and adjustment of grain boundaries. The formation of the spheroidal pyrite aggregates in the late Qiziqiao Formation of the Middle Devonian occurred in a neutral to weak alkaline and reductive sedimentary environment in the normal oxygen-rich shallowwater carbonate platform edge. The variations in the S/Fe ratio and crystallisation characteristics indicate that during pyrite crystal growth, the sulphur fugacity was high locally and rose constantly, the degree of supersaturation decreased locally and the growth environment was stable relatively.

CONSTRUCTION OF AN INDEX SYSTEM FOR SUSTAINABILITY ASSESSMENT IN SMALLHOLDER FARMING SYSTEMS

Smallholder farming systems are important for global food security,but these faces multiple environmental challenges hindering sustainable development.Although sustainable smallholder agriculture issues have been widely discussed and addressed by scientists globally,harmonized approaches in evaluating sustainability are still lacking.This paper proposes a five-step process for constructing a sustainability assessment method for smallholder farming systems,namely definition of system boundary&functional unit,indicator selection,indicator weighting,indicator conversion,and indicator aggregation.The paper summarizes the state-of-art progresses in agricultural sustainability assessment at different stages,and systematically discussed the benefits and limitations of weighting and aggregation methods.Overall,this evaluation process should be useful by providing rational and comprehensive results for quantifying the sustainability of smallholder farming systems,and will contribute to practice by providing decision-makers with directions for improving sustainable strategies.