Efrén Pérez Segura - Centro de Geociencias ::.. UNAM

Abstract
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The state of Sonora is the main producer of metals, minerals and industrial rocks in
Mexico. Main production includes copper, molybdenum, gold, graphite, baryte,
wollastonite and limestone for cement. Porphyry copper, skarns (W, Fe and base metal), as
well as epithermal and mesothermal gold deposits are the main types of ore deposits in
Sonora. The present work deals about metallogeny of the La Esperanza skarn deposits in
Sonora, which contains Ni and Co in economic ranges. La Esperanza is located to the north
of Sierra Santo Niño (SSN) in Central Sonora. The SSN is a horst with an altitude between
400 and 1800 m.a.s.l. and it is bordered by the grabens or semigrabens of the Yaqui and
Bacanora rivers. The general geology of the SSN is characterized by a detritic-carbonated
paleozoic sequence, ranging from Carboniferous to Permian ages. These paleozoic series
occurs like roof pendants over intrusive laramidic granitoids, sometimes the paleozoic
sediments overthrust the andesitic volcanic rocks of the Upper Cretaceous-Lower Tertiary
Tarahumara Formation. Molase sedimentary deposits, similar to the Tertiary Báucarit
Group, and related volcanic rocks fill up the Yaqui and Bacanora rivers tectonic bassins.
The granitoid assemblage, named in the present work as the Moras-Novillo-Bacanora
Batholith (MNBB), is probably a multiple intrusions complex batholith made of tonalitesgranodiorites-granites
bearing biotite and hornblende or, sometimes pyroxene. The San
Lucas Porphyry (SLP) is a hypabisal or subvolcanic stock related to the magmatic activity
of the MNBB; it has monzonitic to quartz sienitic compositio n and it is genetic and
spatially related to the La Esperanza skarns. Major elements geochemistry for the MNBB
shows the following ranges: SiO 2 55.1 to 71.1 %, Al2O3 14.8 to 16.9 %, Fe2O3, 2.3 to 7.4
%, MgO 0 to 4.8 %, CaO 1.6 to 7.5 %, Na2O 3.2 to 4.9 % a nd K2O 2.1 to 4.0 %. In Harker
diagrams we can observe a systematic depletion for Al2O3, CaO, MgO, Fe2O3, TiO 2 and
P2O5 with increasing SiO 2. Otherwise, trace elements of the MNBB show positive
correlations for Sr/Rb, Sr/Ni and Sr/Cr, according with the differentiation rocks degree. The
major elements ranges for the SLP are as follow: SiO2 63.1 to 67.9 %, Al2O3 15.3 to 16.0
%, Fe2O3 1.1 to 4.3 %, MgO 0.0 to 2.0 %, CaO 1.8 to 2.6 %, Na2O 3.9 to 5.3 % and K2O
3.9 to 6.8 %; the SLP becomes poorer in MgO, TiO 2 and P2O5 with increasing SiO2. The
REE pattern indicates enrichment in LREE and depletion in HREE for the MNBB and SLP.
Sr and Nd isotopic data let us calculating 87 Sr/ 86 Sri of 0.7066 and an ?Ndi of –4.6 for the
BMNB and a 87 Sr/ 86 Sri of 0.7070 and ?Ndi of –6.1 for the SLP; while model ages related to
depleted mantle (TDM) are respectively 1.05 and 0.97 Ga. These isotopic data are similar
to other granitoids in Central Sonora and are interpreted as a result of continental crust
contamination. U-Pb and Ar/Ar geochronologic data on the MNBB and SLP indicate a
magmatic pulse trough 90 Ma not reported before in Central Sonora; and another magmatic
stage trough 62 Ma. These ages let us to propose that paleozoic platform overthrusted the
andesitic rocks of Tarahumara Formation between 100-90 Ma, since granitoids are intrusive
in both sedimentary and volcanic rocks. Geochemistry characteristics of the MNBB and
SLP, mainly related to REE behavior and Y, Sr/Y, YbN y (La/Yb)N values, situate these
rocks in the adakitic field, originated from magma compatible with garnet-hornblende
restite, partially contaminated by continental crust and the mantle edge. The La Esperanza
ore bodies are constituted by some skarns, developed on the SLP and paleozoic limestones
contact. Los Rieles, El Barranco, El Tigre and La Reyna are the main identified bodies.
Economic exploration is poor, however preliminary resources can be estimated on 1 Mt