Effects of host rock mineralogical composition and sedimentary facies on development of geochemical halos in Shahmirzad Pb-Zn deposits, central Alborz, Iran

Abstract

Strata-bound carbonate hosted PbZn deposits or Mississippi Valley-type (MVT) deposits represent an economically important class of ore that received much attention in terms of defining effects of host rock petrography and sedimentary facies on development of geochemical halos. However there are few works on relation between sedimentary facies and MVT mineralizations in Iran. The Cretaceous carbonate rocks in the central Alborz (CA), northern Iran, host numerous small PbZn deposits and occurrences. The Shahmirzad PbZn deposits, located in the south of CA, geographically can be split into two groups, the north east (NEDs) and north west (NWDs) deposits. The Upper Cretaceous sequence in the NWDs area is composed of massive light gray limestone without any signs of later changes such as dolomitization and silicification. Additionally, three principal micro-facies are present: skeletal mudstone/wackestone (F1), biolithoclastic packstone (F2) and allochthonous bioclastic floutstone (F3). In contrast, the Cretaceous sequence hosting the NEDs is composed of massive brown limestone with six different types of dolomite (Rd1, Rd2, Rd3, Rd4, Cd1 and Cd2, the two last types are related to ore mineralization and open space filling) distinguished based on crystal size, distribution, and crystal boundary shape. The sedimentary succession in the NEDs indicates that the sedimentation took place in inner, mid and outer ramp environments, resulting in three different carbonate facies. In this area 15 carbonate facies were identified. The permeability and mineral compositions of these areas have obviously influenced the quantity of major and minor elements such as Fe (in average 0.24% in the NWDs and 0.12% in the NEDs), Si (in average 1.86% in the NWDs and 0.44% in the NEDs), Al (in average 0.50% in the NWDs and 0.13% in the NEDs), K (in average 0.24% in the NWDs and 0.06% in the NEDs), Sr (in average 685ppm in the NWDs and 204ppm in the NEDs), Na (in average 0.03% in the NWDs and 0.01% in the NEDs), Zn (in average 46ppm in the NWDs and 188ppm in the NEDs) and Pb (in average 3.3ppm in the NWDs and 83ppm in the NEDs). Not only are minor and major elements affected by changes in host rock mineral composition, but REEs decreased in concentration in dolomitic host rocks of NEDs. Some of the noticeable differences between the NWDs and the NEDs REEs data are i.e. La (in average 3.52ppm in the NWDs and 1.47ppm in the NEDs), Ce (in average 5.20ppm in the NWDs and 2.21ppm in the NEDs) and Eu (in average 0.13ppm in the NWDs and 0.03ppm in the NEDs). The combination of these data and host rock characteristics indicate that host rock compositions and textures effectively controlled these variations. We believe that perhaps the lack of dolomitization and low permeable facies in the NWDs precluded migration of diagenetic fluids and hydrothermal solutions in the carbonate host rocks. On the other hand, the presence of broad dolomitization and permeable facies in the NEDs carbonate host rocks may create appropriate conditions for diagenetic and hydrothermal fluid migration and interaction with the host rocks.