Physiochemical Restrictions of Mineral Zoning of Sediment-Hosted Stratiform Copper Deposit in SW China

Abstract

The Chuxiong basin, located in southwest China, is well known as a mineralization area of red-bed type copper deposits in China. These deposits are characterized by mineral zoning, which is especially true for the Dayao deposits. The mineral zoning is consistent for both horizontal and vertical zoning; from the base (center) of the ore body to the top (outermost), the mineral zones are from hematite, chalcocite, chalcocite + bornite, and bornite + chalcopyrite to pyrite. We studied the mineral zoning in detail using a thermodynamic phase diagram method, such as log O 2 -log S 2 , pH-log O 2 , and pH-Eh, and discussed the constraints on the order of the minerals precipitation under different physiochemical conditions. It is indicated that changes in temperature have little effect on pH and Eh in the formation of minerals. S 2− is stable only below 473 K, and the forming temperature of chalcocite must be below 473 K. In this paper, we also explain the mineral zoning formation mechanism and propose that the main controlling factor of mineral zoning is pH. Because this mineral zoning is widespread in sediment-hosted deposits, studies on this mechanism can considerably promote better understanding of the genesis of ore deposits in order to guide the exploration.

Cu and Au Mineralization of the Tolparovo Ore Occurrence: Evidence for the Formation of Redbed Copper Occurrences in Neoproterozoic Deposits of the Southern Urals

“…Mineral formation probably occurred with a sequential change in oxidative conditions to alkaline ones. The calculations of the physicochemical parameters of ore formation in stratiform copper deposits show that the deposition of the pyrite → chalcopyrite → bornite → chalcocite association should be accompanied by a change in the pH of the medium from acidic to alkaline [19]. The initial oxidative conditions in the Tolparovo ore occurrence can be indicated by the presence of barite in the mineralized veins, where Cu directly correlates with Ba contents (see Figure 4).…”

“…At this stage, pyrite, chalcopyrite, auricuprid, bornite, digenite, sphalerite, tetrahedrite-(Zn), galena, wittichenite, hessite, and empressite were deposited in association with dolomite, chlorite, and barite (Figure 11). Mineral deposition occurred at a high sulfur fugacity, typical of bornite-containing ores [19,84,85], equal to 10 −7.6 −10 −5.8 . The tellurium fugacity was 10 −10.7 −10 −7.8 .…”

“…In the compared ore zones, the common geochemical features of host rocks involve their enrichment in Cu, As, Ba, and Zn. In the Aktash ore zone, at the contact with the dike complex, six copper ore occurrences are localized in Middle Riphean sandstones (see Figure 1a, ore occurrences are indicated by numbers [18][19][20][21][22][23] with Cu (up to 6.2 wt %), As and Sb (up to 1.0 wt %), Ba (up to 0.5 wt %), Zn (up to 0.3 wt %), and Au (up to 1.0 ppm). An enrichment of ore zones with the same elements, in particular Cu and As, suggests that their additional source in the Tolparovo ore occurrence could be postmagmatic fluids.…”

“…According to the study of thermodynamic equilibria in redbed copper deposits [19], the process of mineral formation is mainly controlled by the pH of the medium and is weakly dependent on the temperature. At the same time, mineralogical zonality is found in many redbed copper deposits, consisting in the sequential deposition of Fe and Cu sulfides from the edge parts of the ore zones to the central ones in the following order: pyrite → chalcopyrite → bornite → chalcocite [8,14,16,19]. In the Tolparovo ore occurrence, as well as in other ore occurrences of the Bashkirian uplift, the zonal deposition of ore minerals is either absent or rather weakly expressed.…”

“…Minerals 2024, 14, x FOR PEER REVIEW 2 of 33 20% of the world's copper reserves [12]. One significant group of stratiform deposits is represented by the so-called redbed copper deposits, which are widely represented globally [13][14][15][16][17][18][19][20][21].…”