PETROLOGY OF ORE-RELATED INTRUSIVE ROCKS IN MAHERABAD PORPHYRY CU-AU PROSPECT AREA, EAST OF IRAN

The Maherabad gold-rich porphyry copper prospect area is located in the eastern part of Lut block, east of Iran. This is the first porphyry Cu-Au prospecting area which is discovered in eastern Iran. Based on early stage of exploration, Cu is between 179- 6830 ppm (ave. 3200 ppm) and Au is up to 1000 ppb (ave. 570 ppb). Fifteen mineralization-related intrusive rocks range (Upper Eocene) in composition from diorite to monzonite have been distinguished. Monzonitic porphyries had major role in Cu-Au mineralization.

The ore-bearing porphyries are I-type, metaluminous, high-K calc-alkaline to shoshonite intrusive rocks which were formed in island arc setting. These rocks are characterized by average of SiO₂> 59 wt%, Al₂O₃ > 15 wt%, MgO< 2 wt%, Na₂O> 3 wt%, Sr> 870 ppm, Y< 18 ppm, Yb< 1.90 ppm, Sr/Y> 55, moderate LREE, relatively low HREE and enrichment LILE (Sr, Cs, Rb, K and Ba) relative to HFSE (Nb, Ta, Ti, Hf and Zr). They are chemically similar to some adakites, but their chemical signatures differ in some ways from normal adakites, including higher K₂O contents and K₂O/Na₂O ratios and lower Mg#, (La/Yb)_N and (Ce/Yb)_N in Maherabad rocks. Maherabad intrusive rocks are the first K-rich adakites that can be related with subduction zone. Partial melting of mantle hybridized by hydrous, silica-rich slab-derived melts or/and input of enriched mantle-derived ultra-potassic magmas during or prior to the formation and migration of adakitic melts could be explain their high K₂O contents and K₂O/Na₂O ratios. Low Mg# values and relatively low MgO, Cr and Ni contents imply limited interaction between adakite-like magma and mantle wedge peridotite. Source modeling indicates that high-degree of partial melting (relatively up to 50%) of a basaltic garnet-bearing (lower than 10%) amphibolite to amphibolite lacking plagioclase as a residual or source mineral can explain most of the moderate to low Y and Yb contents, low (La/Yb)_N, high Sr/Y ratios and lack of negative anomaly of Eu in the rocks of the district. The geochemical signature of the adakites within the granitoid rocks represents a characteristic guide for further exploration for copper porphyry-type ore deposit in Eastern Iran.