Experimental Research on the Formation Conditions of the Cassiterite-Sulphide Deposits in the Dachang Tin Ore Field

Abstract

According to the characters of mineral distribution in the deposits, the mineralization system of the deposits is divided into several simple systems, each of which has been examined experimentally.

The following minerals have been synthesized experimentally: cassiterite (Ct), monoclinic-pyrrhotite (Mpo), hexagonal-pyrrhotite (Hpo), arsenopyrite (Asp), mar-matite (Mmt), jamesonite (Ja), zinkenite (Zin), herzenbergite (Hzb), pyrite (Py), galena (Ga), stibnite (Stb), anhydrite (Ah), β-hexagonal herzenbergite (Ber), marca-site (Ms), quartz (Q), calcite (Cal), etc.

We counder that: 1) The formation of cassiterite is closely related to monoclinic-pyrrhotite, so good results should be achieved in exploration by the use of magnetic prospecting if the disturbance caused by other magnetic minerals is eliminated. 2) Because the formation temperature of Po-Po+Py-Py falls gradually and the sulphur fugacity rises progressively, the temperature and the sulphur fugacity are the main factors affecting the zoning pattern of minerals in the Dachang cassiterite-sulphide deposit. 3) In the ore-forming hydrothermal solutions, there are elements to form sulphide and sulphate minerals, and there exist the same conditions of sulphur fugacity and temperature which are the main factors to develop large amounts of Ja in the cassiterite-sulphide deposits. 4) After the experiment, the concentrations of metal ions in the medium solution approximately equal the lowest ore-forming boundary concentrations of these principal metal ions in the ore-forming fluids of the ore field. The ore-forming hydrothermal solution is a Sn-Fe-Pb-Sb-S-As-Si-H₂O system high in sulphur and moderately rich in iron. The medium solution is a mixture of alkali metal chlorides and alkali metal fluorides. Ore-forming oxygen fugacity is roughly 10^-28–10^-40 atm. (350–250°C). 5) It is considered that the metal element zoning with “copper and zinc near the granite, cassiterite farther away” and “cassiterite at the upper part, copper at the lower and zinc at the middle” in the ore field is due to the variation in oxygen fugacity and the difference in sulphophile and lithophile tendencies of the ore-forming metal elements.

From the results of our experiment and the geological features of the deposit, we believe that the biotite granite is the main source of the ore-forming material and metallogenic thermal fluids, whereas part of the sulphur, lead, and antimony is from the wall rocks.