HEAVY METALS DISTRIBUTION IN THE MACONI MINING ZONE, QUERETARO

The Maconi Mining District in northeast Queretaro State, central Mexico, has been exploited for zinc, copper, lead and some silver and gold since the late 1500´s. This ore deposits is a polymetallic skarn type deposit hosted in upper Cretaceous calcareous rocks. Currently, there are more than one million of tons of mine waste material deposited in five mine tailings impoundments. The main ore minerals in the ore deposit are: galena (PbS), chalcopyrite (CuFeS), sphalerite (ZnS), and abundant pyrite (FeS) with some arsenic (As). The topography of the area is mountainous with high relief from top of the mountains to bottom of rivers (more than 1,000 m difference in topography). So, the risk of high mobility of heavy metals into the environment posses a real treat to local population. We present preliminary results of chemical analyses in mine tailings, sediments, soil, surface water and one water spring (groundwater). Results show that the mine tailings have concentrations of As = 1.631 mg/kg, Cu = 35 mg/kg, Pb = 22 mg/kg and Zn = 605 mg/kg. River sediments have concentrations of of As = 149 to2,582 mg/kg, Cu varies from 45 to 146 mg/kg, Pb varies from 69 to 117 mg/kg and Zn from 216 to 1,030 mg/kg. The land for agricultural purpose is relatively small (local consumption only), buit the soil on the small agricultural spots have As from 32 to 117 mg/kg, Cu from 10 to 22 mg/kg, Pb from 14 to 37 mg/kg and Zn from 45 to 87 mg/kg. With respect to water we got the following results: (1) water from a tailing impoundment has the following concentrations: As = 0.59 mg/l, Cu = 0.22 mg/l, Pb 0.017 mg/l and no Zn reported; (2) water from a creek directly affected by the tailings impoundment: As = 1.41mg/l, Cu = 0.01 mg/l, Pb = 0.02 mg/l y Zn = 0.05 mg/l; (3) surface water from Maconi River: As = 0.05 mg/l, Cu = 0.01 mg/l, Pb <0.017 mg/l, and Zn = 0.01 mg/l; and finally for (4) the groundwater (spring-water) we have: As <0.049 mg/l, Cu = 0.02 mg/l, Pb <0.017 mg/l, and Zn = 0.04 mg/l.

We also present some discussion on the possible geochemical factors controlling the release, transport and mobility of heavy metals in the environment.