Ore zoning and tetrahedrite compositional variation at Orcopampa, Peru

Abstract

The Orcopampa silver mining district is located in southern Peru, about 150 km north-northwest of the city of Arequipa. Argentian tetrahedrite, the main ore mineral, was deposited in large normal faults cutting Tertiary volcanic rocks. Ore distribution and compositional variations of tetrahedrite in the Santiago Norte-Split, Manto, and Calera veins were studied in sufficient detail to clearly define zoning patterns.In the Santiago Norte-Split vein the ore grade mineralization forms a single, continuous and horizontally elongated ore shoot, 500 m long and 200 m in vertical extent. Contouring of average silver grades of ore blocks using a 10-oz-Ag/ton cut-off grade shows that this ore shoot actually consists of a sinuous ore band which is only 25 to 75 m wide but over 700 m long. The 200-m vertical interval of the ore shoot results from undulations of the ore band within the near-vertical vein structure.In the Manto vein there are three major ore shoots 300 to 400 m apart along strike and with vertical extents of about 350 m. These ore shoots are connected at their bases by two small ore shoots that correspond to a relatively narrow subhorizontal ore band which is locally below ore grade.The main ore shoot in the Calera vein plunges about 45 degrees to the southwest, has a vertical extent of 300 m, and is 300 m long on the lowest developed level. Using a 10-oz-Ag/ton cutoff grade it is shown to consist of an antiformal ore band with 20- to 30-m-wide limbs and a very high grade 100- X 150-m crest.Orcopampa tetrahedrite is generally high in antimony and in zinc, and most of its compositional variation can be expressed with the atomic ratio X Ag = 100 Ag/(Ag + Cu). This ratio typically varies up to 4 units for different parts of a crystal (or crust) and for separate samples from a single locality. However, for the veins as a whole, this ratio varies systematically between 2 and 40.Changes in tetrahedrite composition within the Santiago Norte-Split and Manto veins are highlighted by X Ag isopleths of 12.5 and 18.5, separating low, medium, and high values of this zoning index. For the Santiago Norte-Split vein these isopleths faithfully mimic the ore band inferred on the basis of ore grades, with lower silver contents in tetrahedrite at the base and higher silver contents in tetrahedrite at the top of the ore band. For the Manto vein the X Ag isopleths demonstrate that the three major ore shoots are located where the ore band rises and descends steeply, tightly doubling upon itself. Thus, the Manto ore band is 25 to 125 m wide and 2,500 m long in the 1,000 m of vein strike length studied.The mode of formation of the tetrahedrite ore bands remains speculative. Possibly many pulses of an ascending hydrothermal fluid precipitated tetrahedrite of progressively changing composition in response to temperature, pressure, or chemical gradients (fractional crystallization). If the permeability at the time of mineralization was roughly uniform, the resulting ore band would be subhorizontal as in the Santiago Norte-Split vein. Major vertical ore shoots like those of the Manto vein would form in areas of enhanced permeability. Alternatively, tetrahedrite with systematically changing composition precipitated in response to temperature and chemical diffusion gradients between two contrasting hydrothermal reservoirs, one above and the other below the ore band.The patterns of ore distribution and tetrahedrite zoning in the Orcopampa veins can be used to significantly aid mineral exploration and development. Moreover, a preliminary investigation shows that, under favorable circumstances, the multiple microprobe spot analyses used for this investigation can be substituted by routine on-site wet chemical procedures that are generally faster and less costly.